Perspectives on Gamma-Ray Burst Physics and Cosmology with Next Generation Facilities
Weimin Yuan, Lorenzo Amati, John K. Cannizzo, Bertrand Cordier, Neil Gehrels, Giancarlo Ghirlanda, Diego Götz, Nicolas Produit, Yulei Qiu, Jianchao Sun, Nial R. Tanvir, Jianyan Wei, Chen Zhang
mmy journal manuscript No. (will be inserted by the editor)
Perspectives on Gamma-Ray Burst Physics andCosmology with Next Generation Facilities
Weimin Yuan · Lorenzo Amati · JohnK. Cannizzo · Bertrand Cordier · NeilGehrels · Giancarlo Ghirlanda · DiegoG¨otz · Nicolas Produit · Yulei Qiu · Jianchao Sun · Nial R. Tanvir · JianyanWei · Chen Zhang
Received: date / Accepted: dateW. YuanKey Laboratory of Space Astronomy and Technology, National Astronomical ObservatoriesChinese Academy of Sciences, Datun Rd 20A, Beijing 100012 ChinaE-mail: [email protected]. AmatiINAF - IASF Bologna, via P. Gobetti 10140129 Bologna, ItalyE-mail: [email protected]. CannizzoCRESST/Joint Center for AstrophysicsUniv. of Maryland, Baltimore CountyBaltimore, MD 21250, USAE-mail: [email protected]. CordierCEA-Irfu, Service d’AstrophysiqueF-91191 Gif-sur-Yvette, FranceE-mail: [email protected]. GehrelsAstroparticle Physics DivisionNASA/Goddard Space Flight CenterGreenbelt, MD 20771, USAE-mail: [email protected]. GhirlandaIstituto Nazionale di Astrofisica INAF - Osservatorio Astronomico di BreraVia E. Bianchi 46 - I 23807 Merate (LC), ItalyE-mail: [email protected]. G¨otzCEA-Irfu, Service d’AstrophysiqueF-91191 Gif-sur-Yvette, FranceE-mail: [email protected]. SunInstitute of High-Energy Physics, Chinese Academy of Sciences,Yuquan Lu 19B, Shijingshan District, Beijing, ChinaE-mail: [email protected] a r X i v : . [ a s t r o - ph . H E ] J un W. Yuan, et al.
Abstract
High-redshift Gamma-Ray Bursts (GRBs) beyond redshift ∼ γ -ray polarization experiment POLAR is also introduced. Keywords
Gamma-ray bursts · high-redshift · Gamma-ray · X-ray · instrumentation N. R. TanvirUniversity of Leicester, Dept. of Physics and Astronomy,University Road, Leicester, LE1 7RH, United KingdomE-mail: [email protected]. ProduitISDC, Chemin d’ Ecogia 16, CH-1290 Versoix, SwitzerlandE-mail: [email protected]. Qiu · J. Wei · C. ZhangKey Laboratory of Space Astronomy and Technology, National Astronomical ObservatoriesChinese Academy of Sciences, Datun Rd 20A, Beijing 100012 ChinaE-mail: [email protected]; [email protected]; [email protected] Bursts: Next Generation Facilities 3
Understanding the structure formation and the first stars around the epochof reionization is a major driver for the next generation of space- and ground-based astronomical facilities. Being the brightest objects in the Universe,Gamma-Ray Bursts (GRBs) beyond redshift ∼ . − . ∼ ∼
10 and beyond. This calls for the next generation of high-energywide-field instruments with unprecedented sensitivity one order of magnitudeor more higher than Swift/BAT. The proposed THESEUS and Einstein Probemissions (and others alike), which are based on novel technology of X-ray fo-cusing, are promising concepts to achieve these goals. On the other hand,follow-up observations in multi-wavebands, particularly in the near- and mid-infrared, of the afterglows of high-redshift GRBs are key to identify their hostgalaxies, to measure the redshifts, and to enable detailed astrophysical andcosmological studies. This can be facilitated by the advent of the ground- andspace-based large telescopes at multi-wavelengths currently being under con-struction, such as the 30-m class optical/near-IR telescopes, Square KilometerArray (SKA) in the radio, and the space-borne WFIRST and JWST mis-sions in the optical/near-IR/mid-IR. Last, complementary to the explorationof the early Universe, future extensive measurement of the X-ray and γ -raypolarization of GRBs, expected to be achieved by POLAR will open up a newdimension in understanding the radiation region geometry and radiation mech-anism of GRB. This chapter describes future experiments that are expectedto advance this exciting field, both currently being built and being proposed,including SVOM (Section 2; contributed by B. Cordier, D. G¨otz, Y. Qiu & J.Wei), POLAR (Section 3; J.C. Sun & N. Produit), WFIRST & JWST (Sec-tion 4; N. Gehrels & J.K. Cannizzo), TMT, GMT & E-ELT (Section 5; N. T.Tanvir), SKA (Section 6; G. Ghirlanda), Einstein Probe (Section 7; W. Yuan& C. Zhang), and THESEUS (Section 8; L. Amati). W. Yuan, et al.
The success of the Swift mission for catching GRBs and other transients illus-trates the benefit of its unique combination of space agility, fast communica-tion with the ground, multi-wavelength observing capability, and long lifetime.These capabilities have permitted the detection of nearly 1000 GRBs and themeasure of nearly 300 redshifts, have offered a new look at GRB progenitorsand have led to several important discoveries, like the existence of GRBs atthe epoch of the re-ionization of the universe. The aim of SVOM (Space-basedmulti-band astronomical Variable Objects Monitor) is to continue the explo-ration of the transient universe with a set of space-based multi-wavelength in-struments, following the way opened by Swift. SVOM is a space mission devel-oped in cooperation between the Chinese National Space Agency (CNSA), theChinese Academy of Science (CAS) and the French Space Agency (CNES). Themission features a medium size satellite, a set of space and ground instrumentsdesigned to detect, locate and follow-up GRBs of all kinds, a anti-Sun pointingstrategy allowing the immediate follow-up of SVOM GRBs with ground basedtelescopes, and a fast data transmission to the ground. The satellite (see Figure1) carries two wide field high energy instruments: a coded-mask gamma-rayimager called ECLAIRs, and a gamma-ray spectrometer called GRM, and twonarrow field telescopes that can measure the evolution of the afterglow after aslew of the satellite: an X-ray telescope called MXT and an optical telescopecalled VT. The ground segment includes additional instrumentation: a wideangle optical camera (GWAC) monitoring the field of view of ECLAIRs inreal time during part of the orbit, and two 1-meter class robotic follow-uptelescopes (the GFTs). SVOM has some unique features: an energy thresholdof ECLAIRs at 4 keV enabling the detection of faint soft GRBs (e.g. XRFsand high-redshift GRBs); a good match in sensitivity between the X-ray andoptical space telescopes which permits the detection of most GRB afterglowswith both telescopes; and a set of optical instruments on the ground dedicatedto the mission. The mission has recently been confirmed by the Chinese andFrench space agencies for a launch in 2021, and it has entered in an activephase of development. We present an overview of the scientific objectives ofSVOM in the next sub-section, and a brief description of the instrumentalaspects of the mission in the following ones.2.1 Scientific objectivesThe main science goal of SVOM is the study of cosmic transients detected inhard X-rays and in the optical domain. While the mission has been designedfor the study of GRBs, it is also well suited for the study of other types of high-energy transients like Tidal Disruption Events (TDEs), Active Galactic Nuclei(AGNs), or galactic X-ray binaries and magnetars. For this type of sources,SVOM is both a ”discovery machine”, with wide field instruments that surveya significant fraction of the sky (ECLAIRs, GRM and GWAC), and a ”follow- amma-Ray Bursts: Next Generation Facilities 5
Fig. 1
Schematic showing the SVOM spacecraft with its multi-wavelength space payload.It consists of two wide-field instruments: ECLAIRs and the Gamma-Ray Monitor (GRM)for the observation of the GRB prompt emission and two narrow field instruments: theMicro channel X-ray Telescope (MXT) and the Visible Telescope (VT) for the observationof the GRB afterglow emission. Right: Space and ground instruments join to enable a uniquecoverage in time and wavelength. up machine”, with fast pointing telescopes in space and on the ground (MXT,VT, and GFTs) that provide a multi-wavelength follow-up of different kindsof sources, with good sensitivity and a high duty cycle. The follow-up can betriggered by the satellite itself or from the ground, upon reception of a requestfor target of opportunity observations (ToO). In this section we concentrateon the objectives of SVOM for GRBs. Through GRB population simulationswe have evaluated a GRB detection rate of 70–80 GRBs/yr for ECLAIRs and90 GRBs/yr for GRM. One essential goal of SVOM is to provide GRBs witha redshift measurement. The redshift is required to measure the energeticsof the burst and the epoch at which the GRB occurred in the history of theuniverse. Four elements in the design of the mission concur to facilitate themeasure of the redshift for SVOM GRBs: a near anti-solar pointing duringmost of the orbit ensuring that SVOM GRBs can be quickly observed withground based telescopes, a good sensitivity of the on-board optical telescopepermitting the rapid identification of high-z candidates (which are not detectedat visible wavelengths), Near Infra-Red follow-up on the ground to look for theafterglows of dark/high redshift GRBs, and agreements with the community topromote the optical spectroscopy of SVOM GRBs using large telescopes. Withthis strategy we expect to measure the redshift of more than 50% of SVOMGRBs, constructing a sample that could potentially be more representativeof the intrinsic GRB population than the Swift sample. One drawback ofthis pointing strategy is that it avoids the galactic plane. Galactic sourcescan nevertheless be observed during target of opportunity pointings. In thefollowing, we discuss a few selected topics about GRBs for which SVOM maybring decisive progress.
W. Yuan, et al.
In order to get a better understanding of the GRB phenomenon SVOM was de-signed to detect all kinds of GRBs, and to provide extensive multi-wavelengthobservations of the prompt GRB and its afterglow. In addition to classicallong GRBs that will be detected by the two wide field instruments (ECLAIRsand GRM), SVOM will routinely detect short bursts with its gamma-ray mon-itor and soft GRBs (XRFs or highly redshifted GRBs) with ECLAIRs. Theability to detect soft GRBs with a spectral energy distribution (SED) peak-ing below 20-30 keV will favour the detection of X-ray flashes. As shown byBeppoSAX and HETE-2 (Heise et al. 2001; Sakamoto et al. 2004), these faintGRBs can only be detected if they are close enough and if their spectral energydistribution peaks at low energies (because faint GRBs with a SED peakingat high energies radiate too few photons to be detected). The detection ofX-Ray Flashes in the local universe (z < The instrument suite of SVOM will provide good multi-wavelength coverage ofGRBs. For those occurring in the field of view of GWAC, the prompt emissionwill be measured from 1 eV to 5 MeV, with the GWACs, ECLAIRs, andthe GRM. The prompt optical emission may also be observed by one GFT forGRBs lasting longer than 40 seconds. GRB afterglows will be observed with thetwo narrow field instruments on-board the satellite and with the ground follow-up telescopes on Earth. For some GRBs, SVOM will provide a very completeview of the phenomenon and its evolution, hopefully bringing new insight intothe complex physics at work in these events. A lesson learned from Swift is thata few well observed GRBs may crucially improve our understanding of GRBphysics, as was the case for GRB 130427, a bright nearby burst detected bySwift, Fermi and various optical and radio telescopes on the ground (Maselliet al. 2014; Perley et al. 2014). The physical processes at work within the jetremain not well understood even after the observation of hundreds of GRBs.Comprehensive discussions of the theoretical challenges connected with theunderstanding of the prompt GRB emission can be found in Kumar and Zhang(2015) and Zhang (2014). The authors show that several crucial questionsconnected to the physics of the ultra-relativistic jet and its interaction with thesurrounding medium remain unanswered, like the nature and content of the jet(is the energy stored in the baryons or in magnetic fields?), the mechanisms ofparticle acceleration, the micro physics and the dominant radiation processes,the importance of the reverse shock, the role of pairs, etc. Performing multi-wavelength observations during the prompt emission and the early afterglow,SVOM will provide key observations to understand the physics of relativisticjets. GRBs detected with SVOM will also benefit from contemporaneous or amma-Ray Bursts: Next Generation Facilities 7 follow-up observations with a novel generation of powerful instruments, likeCTA (the Cerenkov Telescope Array) for very high energy photons, LSST(Large Synoptic Survey Telescope) for optical transients associated with on-axis and off-axis GRBs, and the precursors of SKA (the Square KilometerArray) in radio.
Their extreme luminosity of GRBs permits their detection in hard X-rays upto very high redshifts, possibly beyond z = 10 and the spectroscopy of theoptical afterglow provides the redshift of the burst and a tomographic visionof the line of sight to the burst. With Swift and the measure of about 300redshifts, GRBs are providing new diagnostics of the distant Universe. Whenthe signal to noise ratio (SNR) of the optical spectrum of the afterglow issufficient, we get detailed information on the circumburst medium, on the gasand dust in the host galaxy, on the intergalactic medium and the interveningsystems. With a smaller SNR, the measure of the redshift allows locatingthe time of the explosion in the history of the universe and reconstructingthe history of the GRB formation rate, which traces the formation rate ofmassive stars. There is of course a special interest in very distant GRBs (z > W. Yuan, et al.
ECLAIRs (see Figure 2) is the instrument on-board the satellite that willdetect and locate the GRBs. ECLAIRs is made of four parts: a pixellateddetection plane (1024 cm ) with its readout electronics, a coded mask, a shielddefining a field of view of 2 steradians (89 ◦ × ◦ ), and a processing unit incharge of detecting and locating transient sources. The detection plane is madeof 200 modules of 32 CdTe detectors each, for a total of 6400 detectors of size4 × × ∼ (cid:48) for a source at the limit of detection). The instrument features a countrate trigger and an image trigger, like Swift. These triggers are computed, fromthe photon data, in several energy bands and on time-scales ranging from 10ms to several minutes. Our simulations show that ECLAIRs will detect 70-80GRBs/yr. GRM consists of a set of three detection modules. Each of them is madeof a scintillating crystal (sodium iodide), a photomultiplier and its readoutelectronics. Each detector has a surface area of 200 cm and a thickness of 1.5cm. One piece of plastic scintillator in front of NaI(Tl) is used to distinguishlow energy electrons from normal X-rays. The three modules are pointed atdifferent directions to form a total field of view of 2 sr, within which rough (ofthe order of 10 degrees radius) localization of transient sources can be achievedon-board. The energy range of the GRM is 15-5000 keV, extending the energyrange of ECLAIRs towards high energies to measure E P eak for a large fractionof SVOM GRBs. We expect that GRM will detect >
90 GRBs/yr. GRM willhave a good sensitivity to short/hard GRBs, like the GBM of Fermi. GRMcan generate on-board GRM-only triggers, taking use of only GRM detectors.Such triggers with localization information will be transferred to ECLAIRs fortrigger enhancement on the short GRBs, and to ground facilities (e.g. GWAC,GW experiments) for joint observations. A calibration detector containing one amma-Ray Bursts: Next Generation Facilities 9
Fig. 2
Schematics showing the different sub-systems of ECLAIRs except the data pro-cessing unit in charge of the GRB detection and localisation. Right: Laboratory spectralmeasurements performed on a detector module prototype (a 32 CdTe pixel matrix) withradioactive sources (241Am). The red vertical line corresponds to the expected low energythreshold of the ECLAIRs camera [6]. radioactive
Am isotope is installed on the edge of each detection module, forthe purpose of gain monitoring and energy calibration. In addition, a particlemonitor auxiliary to GRM can generate South Atlantic Anomaly alerts andhelp protecting the detection modules.
MXT (see Figure 3) is a light and compact focusing X-ray telescope designedto observe and measure the properties of the GRB X-ray afterglow after aslew of the satellite. The telescope will implement a novel technique to focusX-rays, based on micropore optics arranged in a lobster eye geometry. The useof micropore optics instead of classical X-ray electro-formed mirrors permitsa significant reduction of the size and weight of the telescope, fitting on amedium size satellite like SVOM. The optics has a diameter of 24 cm and afocal length of 1 meter (G¨otz et al. 2014). MXT will make use of the radiationhard pn-CCD detector developed for the DUO mission and adopted in a largerversion for the eROSITA mission (Meidinger et al. 2014; Predehl et al. 2014).The MXT detector is made of 256 x 256 Si pixels of 75 µ m side and has anexpected energy resolution of 75 eV at 1 keV. MXT will be operated in theenergy range of 0.2-10 keV, will have an effective area of 45 cm at 1 keV,and a field of view of 64 ×
64 arc minutes. Despite the smaller effective areawith respect to XRT on board Swift at 1 keV (about 120 cm ) MXT will beable to fully characterize the GRBs light curves (as shown in Figure 3). Infact with a sensitivity of 7 × − erg cm − s − in 10 s, MXT will detectthe afterglows of more than 90% of SVOM GRBs. Indeed its sensitivity iswell adapted to early GRB afterglow observations, and its PSF of about 4 arcmin will allow to significantly reduce the ECLAIRs error boxes. Simulations Fig. 3
Schematics showing the different sub-systems of the MXT. Right: XRT measured(blue and red points) versus MXT simulated (green points) light curve of GRB 050730, aGRB with a median flux in the Swift/XRT afterglow database. based on the Swift-XRT database show that the localization accuracy for theMXT is ∼
13” for 50% of the bursts, 10 minutes after the trigger (statisticaluncertainty only).
The Visible Telescope (VT) is a dedicated optical follow-up telescope on boardthe SVOM satellite. Its main purpose is to detect and observe the optical af-terglows of gamma-ray bursts localized by ECLAIRs. It is a Ritchey-Chretientelescope (see Figure 4) with a diameter of 40 cm and an f-ratio of 9. Its lim-iting magnitude is about 22.5 (M V ) for an integration time of 300 seconds VTis designed to maximize the detection efficiency of GRB’s optical afterglows.Instead of a filter wheel, a dichroic beam splitter is used to divide the lightinto two channels, in which the GRB afterglow can be observed simultane-ously. Their wavelength ranges are from 0.4 µ m to 0.65 µ m (blue channel) andfrom 0.65 µ m to 1 µ m (red channel). Each channel is equipped with a 2k ×
2k CCD detector. While the CCD for the blue channel is a normal thinnedback-illuminated one, a deep-depleted one is adopted for the red channel toobtain high sensitivity at long wavelength. The Quantum Efficiency (QE) ofthe red-channel CCD at 0.9 µ m is over 50%, which enables VT to have thecapability of detecting GRBs with the redshift larger than 6.5. The field ofview of VT is about 26 (cid:48) × (cid:48) , which can cover the error box of ECLAIRs inmost cases. Both CCDs have a pixel size of 13.5 µ m × µ m, correspondingto spatial resolutions of 0.77 arc second. This ensures the GRB positioningaccuracy to be greatly improved by VT from several arc minutes (ECLAIRs)and tens of arc-seconds (MXT) to a level of sub-arc second. amma-Ray Bursts: Next Generation Facilities 11 In order to promptly provide the GRB alerts with the sub-arc second ac-curacy, VT will do some data processing on board. After a GRB has beenlocalized by the co-aligned MXT, lists of sources are extracted from the VTsub-images whose centres and sizes are determined by the GRB positions andthe corresponding error boxes provided by MXT. The lists are immediatelydownlinked to the ground through the VHF network. Then, the ground soft-ware will make finding charts with these lists (see Figure 4) and search theoptical counterparts of the GRB by comparing the lists with the existing cat-alogues. If a counterpart is identified, an alert will be then produced anddistributed to world-wide astronomical community, which is useful for trig-gering large ground-based telescopes to measure the redshifts of the GRBsby spectroscopy. VT is expected to do a good job on detecting high-redshiftGRBs. The confirmed high-redshift GRBs are rare in the Swift era, in contrastto a theoretical prediction of a fraction of more than 10% (Salvaterra 2015).This is probably due to the fact that for most Swift GRBs the early-timeoptical imaging follow-up is not deep enough for a quick identification andsome faint GRBs cannot be spectroscopically observed in time by the largeground-based telescopes. This passive situation will be significantly improvedby SVOM, due to the high sensitivity of VT, in particular at long wavelength,and the prompt optical-counterpart alerts. Additionally, the anti-solar point-ing strategy of SVOM allows GRBs to be spectroscopically observed by largeground-based telescopes at the early time of the bursts. Consequently, morehigh-redshift GRBs are expected to be identified in the SVOM era. VT is alsoused to support the platform to achieve the required high pointing stability.A Fine Guidance Sensor (FGS) is mounted on the VT focal plane to measurerelative image motions. Its images are processed in real time by a specializeddata processing unit to get the centroid positions of several stars brighter thanthe magnitude of 15 (M V ). The results are sent at a frequency of 1Hz to theplatform to improve the pointing stability, which enables VT to have a goodperformance in a long exposure time.2.3 Ground Based InstrumentsThe ground follow-up instruments constitute an important part of the mission.Three instruments are developed for the follow-up of SVOM GRBs: a wideangle camera that surveys a significant fraction of the sky for transients, andtwo robotic telescopes. In addition to these dedicated instruments, the SVOMcollaboration will seek agreements with various existing telescopes or networkswilling to contribute to the follow-up of SVOM GRBs. GWAC (see Figure 5) provides a unique way to survey a large field of view foroptical transients. The instrument will monitor 63% of ECLAIRs field of view,looking for optical transients occurring before, during and after GRBs. GWAC
Fig. 4
Schematic showing the different sub-systems of the VT. Right: a simulated imageof the VT and its associated finding chart.
Fig. 5
First prototype of one GWAC module. The final system will be composed by 9modules. Right: the figure shows the discovery space of an instrument dedicated to shorttime-scale optical transients (Wozniak et al. 2009). will also have its own trigger system, providing alerts to the world. GWAC isa complex system: the heart of the system is a set of 36 wide angle cameraswith a diameter of 18 cm and a focal length of 22 cm, together these camerascover a field of view of 5000 sq. deg. They use 4k ×
4k CCD detectors, sen-sitive in the range of wavelength 500-800 nm. These cameras reach a limitingmagnitude V=16 (5 σ ) in a typical 10 second exposure. This set of camerasis completed by two 60 cm robotic telescopes. equipped with EMCCD cam-eras. These telescopes will provide multicolour photometry of the transientsdiscovered by GWAC with a temporal resolution ≤ amma-Ray Bursts: Next Generation Facilities 13 The ground follow-up telescopes have two main goals. Firstly, they measurethe photometric evolution of the optical afterglow in the first minutes afterthe trigger in a broad range of visible and NIR wavelengths, with a tempo-ral resolution of few seconds. Secondly, when an afterglow is detected, theyprovide its position with arc second precision within 5 minutes of the trigger.Some essential features of the GFTs are their field of view, their size, and theirsensitivity in the near infrared. The field of view ( ∼
30 arc minutes) enablesobserving quickly the entire error boxes of ECLAIRs. The size, typically 1meter, allows the detection of all visible (i.e. non-dark) afterglows at the con-dition to arrive within few minutes after the trigger (Akerlof and Swan 2007;Klotz et al. 2009). Finally, the near infrared sensitivity permits the detectionof high-z GRBs and GRBs extinct by dust, whose afterglow are obscured inthe visible domain (Greiner et al. 2008). GFTs are especially useful for thestudy of the early afterglow during the slew of the satellite, and for the rapididentification of the optical afterglow in various cases: when SVOM cannotslew to the burst or when the slew is delayed due to pointing constraints, andwhen the optical afterglow is only visible in the NIR. One telescope is locatedin China at Xinglong Observatory and the other one will be located in Mexicoat San Pedro M´artir.2.4 The SystemIn order to facilitate measuring the redshifts of GRBs detected with ECLAIRs,the instruments of SVOM will be pointed close to the anti-solar direction. Mostof the year the optical axis of the SVOM instruments will be pointed at about45 ◦ from the anti-solar direction. This pointing is interleaved with avoidanceperiods during which the satellite passes away from the Sco X-1 source and thegalactic plane. This strategy ensures that SVOM GRBs will be in the nighthemisphere and quickly observable from the ground by large telescopes. Moredetails on the SVOM pointing strategy, can be found in Cordier et al. (2008).As soon as a GRB will be located, its coordinates and its main characteris-tics will be sent to the ground within seconds with a VHF antenna. The VHFsignal will be received by one of the ∼
40 ground stations distributed aroundthe Earth below the orbit. The data will then be relayed to the Operation Cen-ter, which will send SVOM alerts to the internet via the GCN and VO Eventnetworks (http://gcn.gsfc.nasa.gov/), and to the ground instruments GWACand the GFTs. SVOM can also perform target of opportunity observations(with MXT and VT for instance), with a delay of few hours, which dependson the availability of uplink communication with the satellite.SVOM will try to select high-z GRB candidates by analysing MXT andVT data, and multi-band photometry data from GFTs. If a GRB is detectedby MXT in the soft X-rays, but not by VT in the optical band, it will beselected as a candidate of high-z or an optically dark GRB. Ground telescopes with NIR capabilities are encouraged to follow up these candidates as soonas possible to try to measure their redshift. GFTs will be able to measurephotometric redshifts by observing GRB afterglows with multiple filters fromthe visible to the NIR bands.
SVOM will be a highly versatile astronomy satellite, with built-in multi-wavelength capabilities, autonomous re-pointing and dedicated ground follow-up. With its peculiar pointing strategy and the low energy detection threshold,SVOM is expected to improve the number of GRBs detected at high redshift,and hence to contribute to the use of GRBs as probes of the young Universe.Beyond the GRB studies emphasized here, SVOM will bring new observa-tions about all types of high energy transients, in particular those of extra-galactic origin (TDEs, AGNs, etc.). amma-Ray Bursts: Next Generation Facilities 15
Until recent years, only a few polarization measurement results on Gamma-Ray Burst (GRB) prompt emissions have been published (Yonetoku et al.2011), while in the same time more and more hard X-ray/Gamma-ray po-larization measurement instruments have been put into operation or underdevelopment. All these efforts will accumulate a larger observation databaseon GRB’s polarization.POLAR (Produit et al. 2005) is a novel space-borne Compton polarimeterdedicated for the precise measurement of the polarization of GRB’s promptemission, which is expected to be on-board the Chinese space laboratory“Tiangong-2 (TG-2)” with a scheduled launch date in late 2016. The futuredetailed measurement of the polarization of GRBs will lead to a better under-standing of its radiation region geometry and emission mechanisms. POLARwill contribute greatly to two aspects on the polarization observation of GRBs.One is its high sensitivity which is minimum detectable polarization (MDP).The MDP of POLAR can be as low as ∼
10% (Suarez 2010). The other oneis its high observation statistics. ∼
50 GRBs/year are expected to be detectedby POLAR. This will provide significant results for restricting the radiationmechanism of GRBs.3.1 Design of POLAR detectorPOLAR is composed of the polarization detector (OBOX) and electronic cabi-net (IBOX). OBOX consists of 25 detector modular units (DMU). Each DMUis composed of 64 low-Z material plastic scintillator (EJ-248M) bars, read-outby a flat-panel multi-anode photomultipliers H8500 and ASIC front-end elec-tronics. The CAD view of DMU and OBOX are shown in Figure 6
Fig. 6
Detector design of POLAR. Left: Detector modular unit; Right: polarization detectorOBOX6 W. Yuan, et al.
The linear polarization degree and polarization direction of the polarizedGamma-rays can be reconstructed by POLAR through Compton scatteringeffect. In order to improve the Compton scattering efficiency, the low-Z mate-rial plastic scintillator has been selected as the detection material of POLARas it is more stable chemically and mechanically.3.2 Detection principleThe main interaction between the hard X-rays/Gamma-rays within the energyrange 50 keV ∼
500 keV and plastic scintillator material is Compton scattering.The differential cross-section is given by the Klein-Nishina formula for thepolarized Compton scattering interaction process: dσ p dΩ = r ε [ ε + ε − − θ cos η ]= r ε [ ε + ε − − sin θ + sin θ cos(2( η + π , (1)where r is the classic radius of an electron and ε =E’/E is the ratio of theenergy of the scattered photon and the energy of the incident photon, θ is theCompton scattering angle and η is the scattering azimuth angle.An incident photon interacts with one of the 1600 PS bars in POLAR throughCompton scattering effect; the recoil electron will be absorbed by the PS barand its deposited energy will be detected by POLAR and readout by the fol-lowing electronics. The scattered photon will interact with next PS bars untilbeing absorbed or escaping the detector array. For the unpolarized Gamma-rays, the scattering azimuth angle is isotropic, while for the polarized Gamma-rays, the scattering azimuth angle distribution is related to the incident pho-tons’ polarization degree and polarization angle. Therefore, the polarizationproperty of the incident Gamma-rays can be reconstructed by analyzing thedistribution of the Compton scattering azimuth angles. Figure 7 shows the de-tection principle of POLAR.In Figure 7, P is the polarization vector of the incident Gamma-rays, θ γ and ϕ γ describe the incident photons’ direction, e − is the recoil electron, ξ is theprojection of the Compton scattering azimuth angle in the detector plane.3.3 Physics performance study with Monte-Carlo methodThe performance of POLAR has been studied using the Monte-Carlo simu-lation method. The GEANT4 toolkit is used for the simulations. The Band amma-Ray Bursts: Next Generation Facilities 17 Fig. 7
Detection principle of POLAR by Compton scattering effect (Band et al. 1993) function is used to create the spectrum of an GRB. Typ-ically, the standard GRB spectrum ( α =-1.0, β =-2.5 and E peak =200 keV) isused for the study. Figure 8 shows the triggered events on the 1600 detectionchannels of POLAR with different GRB incident angles. Fig. 8
Triggering pattern of POLAR detector with standard incident GRB photons. Left: θ γ =0 ◦ , ϕ γ =0 ◦ ; Right: θ γ =45 ◦ , ϕ γ =0 ◦ For different incident angles of GRBs, the reconstructed modulation factor byPOLAR can be quite different. Figure 9 shows the modulation factor measuredby POLAR as a function of the incident angle of GRB when the GRB photonsare 100% polarized. More simulation results are discussed else where (Suarez2010; Xiong et al. 2009; Sun 2012). ∼
140 keV. Thebeam of ID11 is horizontally polarized. The size of the beam used for POLARcalibration is 0.5 × with original beam intensity ∼ phs/s, whichwas reduced to about 10 ∼ phs/s with absorber to avoid the data pileupin the electronics.During the ESRF beam test, four different beam energies were used, i.e., 140keV, 110 keV, 80 keV and 60 keV. For each beam energy, three different beamincident angles (0 ◦ , 30 ◦ and 60 ◦ ) were chosen, where 0 ◦ is on-axis, 30 ◦ and60 ◦ are off-axis. Figure 10 shows the very preliminary analysis results of thereconstructed Compton scattering azimuth angle distributions (modulationcurves) with 140 keV beam. When fitting the modulation curves, the calcu-lated modulation factors for 90 ◦ and 0 ◦ polarized beam are 40.23% ± ± (deg) γ θ ( % ) µ Modulation factor (neighbor bars included), threshold = 5 keV (deg) γ θ ( % ) µ Modulation factor (neighbor bars excluded), threshold = 5 keV
Fig. 9
Simulated modulation factor (100% polarized) as a function of the incident angle θ γ of GRB; ϕ γ is set to 0 ◦ for simplicity. Left: neighboring bars included; Right: neighboringbars excludedamma-Ray Bursts: Next Generation Facilities 19 to the Monte-Carlo simulation result of 40.90%, which verifies the accuracy ofthe model for the Monte-Carlo simulations.3.5 Summary and discussionsFor GRB prompt emissions, the most interesting energy range is about 100keV ∼
200 keV, within which POLAR has its best performance. The Monte-Carlo simulation results show that for the GRBs with total energy fluence F total larger than 3 × − erg · cm − , the minimum detectable polarization(MDP) of POLAR can reach below ∼ ◦ ( F total ≥ − erg · cm − ). The main technical properties of POLAR are summarized in Ta-ble 1. The calibration test results show that POLAR performs as expected onthe polarization measurement. After eliminating all kinds of system effects ofthe instrument during the data analysis procedure and optimising the Monte-Carlo simulation model, the final measurement results and simulation resultsare quite similar. The calibration results as well as the optimised instrumentworking parameters will be used in the further Monte-Carlo simulations andreal in-orbit operations.POLAR has also undergone and passed the space qualification tests, suchas thermal cycling, vibration, shock and thermal vacuum, etc. These testscan verify the reliability of POLAR instrument and guarantee its long-termoperation and observation. Fig. 10
Modulation curve measured during the ESRF beam tests. Left: the polarizationdirection of the beam relative to POLAR detector is 90 ◦ ; Right: the polarization directionof the beam relative to POLAR detector is 0 ◦ Table 1
Main technical properties of POLARNo. Property Performance1 Detector material Plastic scintillator (EJ-248M)2 Yearly detectable GRBs ∼
503 GRB localization accuracy ≤ ◦ ( F total ≥ − erg · cm − )4 Detection energy range 50 keV ∼
500 keV5 Field of view ± ◦ ×± ◦ <
10% ( F total ≥ × − erg · cm − )amma-Ray Bursts: Next Generation Facilities 21 WFIRST and
JWST
WFIRST4.1.1 InstrumentationWFIRST consists of two instruments, a Wide-Field Instrument (WFI) and acoronagraph.The WFI will be capable of imaging and spectroscopy over thousands ofsquare degrees and monitoring of SNe and microlensing fields. It has sensitiv-ity in the 0 . − µ m bandpass and a 0.28 deg field-of-view (FOV), about ahundred times that of JWST . The WFI has 18 H4RG detectors (288 Mpixels)and utilizes 6 filter imaging. The wide field instrument includes two channels,a wide field channel and an integral field unit (IFU) spectrograph channel. Thewide field channel includes three mirrors and a filter/grism wheel to providean imaging mode covering 0 . − . µ m and a spectroscopy mode covering1 . − . µ m. The IFU channel uses an image slicer and spectrograph toprovide individual spectra of each 0.15 arcsec wide strip covering the 0 . − . µ m over a 3 . × .
15 arcsec FOV.The coronagraph can image ice and gas giant exoplanets as well as debrisdisks. It has a 400 − ≤ − contrast, and a 100 msecinner working angle (at 400 nm). The coronagraph instrument includes animaging mode and a spectroscopic mode to perform exoplanet direct imagingand spectroscopic characterization of planets and debris disks around nearbystars. has a 2.4 m wide-field IR telescope (0 . − µ m) and an exoplanetimaging coronagraph instrument (400 − J = 27 AB to study darkenergy (Figure 11) weak lensing and baryon acoustic oscillations, (ii) monitora few square degrees for dark energy SN Ia studies, (iii) perform microlensingobservations of the galactic bulge for an exoplanet census, and (iv) undertakedirect imaging observations of nearby exoplanets with a pathfinder corona-graph. The mission will have a robust and well-funded guest observer programfor 25% of the observing time. With a < WFIRST will be a powerful tool for time domain astronomy and forcoordinated observations with gravitational wave experiments. Gravitationalwave events produced by mergers of nearby binary neutron stars (LIGO-Virgo,cf. Kanner et al. 2012) or extragalactic supermassive black hole binaries (
LISA )will produce electromagnetic radiation that
WFIRST can observe. H4RG is a hybrid CMOS (Complimentary Metal-Oxide-Semiconductor) 4K ×
4K opticalimager made by Teledyne Scientific & Imaging.2 W. Yuan, et al.
Fig. 11
Dark energy techniques used by
WFIRST (Spergel et al. 2015).
JWST4.2.1 Instrumentation
The Integrated Science Instrument Module (ISIM) will house the four maininstruments that will detect light from distant stars and galaxies, and planetsorbiting other stars. The ISIM will include:(i) the Near-Infrared Camera, or NIRCam (University of Arizona),(ii) the Near-Infrared Spectrograph, or NIRSpec (ESA, with componentsprovided by NASA/GSFC),(iii) the Mid-Infrared Instrument, or MIRI (European Consortium, Euro-pean Space Agency (ESA), and NASA Jet Propulsion Laboratory),and (iv) the Fine Guidance Sensor/ Near InfraRed Imager and SlitlessSpectrograph, or FGS/NIRISS (Canadian Space Agency).The 6.5 m primary mirror, a gold-coated beryllium reflector, will have acollecting area about five times that of
HST . JWST will be oriented towardsnear-IR astronomy, but with sensitivity also in orange and red light, as well asthe mid-IR. Its coverage will span 0 . − µ m. The motivation for an emphasison near-IR to mid-IR is three-fold: high − z objects have their visible emissionsshifted into the IR, cold objects such as debris disks and planets emit moststrongly in the IR, and this band has been difficult to study from the groundor by previous space missions. JWST will operate near the Earth-Sun L2 Lagrange point, ∼ . × mbeyond the Earth. Objects near this point can orbit the Sun synchronouslywith the Earth, allowing the telescope to remain at a roughly constant dis-tance, using a single sunshield to block heat and light from the Sun and Earth. amma-Ray Bursts: Next Generation Facilities 23 This will keep the temperature of the spacecraft below 50 K which is necessaryfor IR observations.Launch is scheduled for 2018 on an Ariane 5 rocket. Its nominal missionlength is five years, with a goal of ten years. ’s scientific mission has four main components: (i) to search for lightfrom the first stars and galaxies, (ii) to study the formation and evolutionof galaxies, (iii) to understand the formation of stars and planetary systems,and (iv) to study planetary systems and the origins of life. This science wil befacilitated by
JWST ’s near-IR capabilities.
JWST is expected to see the very first galaxies, forming just a few 100 Myrafter the Big Bang. IR observations allow the study of objects and regions ofspace which would be obscured by gas and dust in the visible, such as themolecular clouds where stars are born, the circumstellar disks that give rise toplanets, and the cores of active galaxies. Relatively cool objects emit primarilyin the IR. Most objects cooler than stars are better studied in the IR, includingISM clouds, brown dwarfs, planets – both in our own and other solar systems,and comets and Kuiper belt objects. In addition,
JWST will be a valuabletool in high − z GRB follow-up (Figure 12) thanks to its ability to see back toearly time.
Fig. 12
Detectability of high − z GRB afterglows versus observer time t − T α wavelength ( solid blue curves ). JWST /NIRSpec detection threshold ( dottedred curves ) are shown adopting a spectral resolution λ/∆λ = 5000, a signal to noise ratioof 5 per spectral resolution element, and an exposure time of 0 . t − T z = 5, 7, 9, 11, 13, and 15, respectively (top to bottom).4 W. Yuan, et al. JWST and
WFIRST will bring in a new era of NIR and MIR astrophysics.
JWST will provide very deep ( J = 29) observations with multiple instruments. WFIRST will provide wide-field (1000’s of square degrees) deep ( J = 27)surveys. For both missions GRB follow-up science will be an integral part ofscience programs; JWST will have a 2 d TOO response time, while
WFIRST will have a < amma-Ray Bursts: Next Generation Facilities 25 HST and
JWST for many observations (e.g., Gilmozzi and Spyromilio2007). A summary of the basic vital statistics for each telescope is given inTable 2.
Table 2
Summary of planned 30 m class telescope characteristicsTelescope Location Altitude Diameter Mirror technology(m) (m)GMT Las Campanas 2550 24.5 7 monolithic mirrors, common focusTMT Mauna Kea 4050 30 Segmented primaryE-ELT Cerro Armazones 3060 39.3 Segmented primary
A major driver for construction of these facilities is the exploration of earlystructure formation, during and before the epoch of reionization. This pushestowards optimisation in the near infra-red, since the Gunn-Peterson troughrenders the universe opaque at rest-frame wavelengths below 1215˚A, whichcorresponds to λ > µ m at z ∼ >
7. In recognition of this, all three telescopeconsortia have prioritised building of intermediate resolution, integral fieldspectrographs with nIR capability; namely HARMONI on the E-ELT, IRISon the TMT and GMTIFS on the GMT. HARMONI and IRIS, indeed, havebeen selected as first light instruments. With spectral resolutions of R ∼ Gamma-ray bursts are extremely bright, and hence visible in principle to veryhigh redshifts. This, combined with their very broad spectral energy distri-butions, and association with massive star death, has long been recognised as making them potentially very powerful probes of the early universe (Lamb andReichart 2000; Tanvir and Jakobsson 2007). They trace early star formation,pin-point the positions of primeval galaxies, and provide luminous backlightsfor absorption spectroscopy (Chornock et al. 2013; Sparre et al. 2014; Hartooget al. 2015).To-date, the highest spectroscopic redshift for a GRB is z = 8 . z ≈ . z GRBs detected by
Swift have not been recognisedas such, and those that have been recognised have usually lacked the high-S/Ndata to fully exploit their potential.The next generation telescopes combine near infra-red optimisation withexquisite point-source sensitivity, thanks to their large collecting area and AO-assisted high spatial resolution. Thus afterglows for which only redshifts couldbe obtained previously, will in the future be used to study abundances, neu-tral hydrogen, dust and molecular content in even very faint hosts. All of theseproperties are extremely difficult to study by other means. This potential isillustrated graphically in Figure 13, which shows a simulated E-ELT spectrumof a typical GRB afterglow at z = 8 . z afterglows would be of con-siderable importance in understanding the sources driving reionization. Theformer provides the distribution of opacities, and hence escape fractions forionizing radiation, along the lines of sight to massive stars in the early universe(Chen et al. 2007; Fynbo et al. 2009), while the latter would allow us to mapthe progress of reionization and its variation from field to field (McQuinn et al.2008).Other transients that may be studied by 30 m class telescopes include su-pernovae of all types, particularly superluminous supernovae that, althoughrare, are bright enough to be studied in detail at z >
6. This class of eventmay include pair-instability supernovae, the likely end-point of many massivepopulation III stars (Heger et al. 2003).
Observing transients with 30 m class telescopes offers great promise, but alsopresents particular challenges. Some challenges are for the wider astronomicalcommunity, in particular it is obviously essential that we maintain a capabilityto discover transient sources to feed into down-stream follow-up. In the 2020snew sources of transient discovery should be operating, notably the Large Syn-optic Survey Telescope (LSST), the advanced generation gravitational wavedetectors (e.g., aLIGO, aVirgo), and the SVOM satellite. However, of theseonly SVOM will be capable of detecting high redshift sources, specificallygamma-ray bursts (GRBs) and related high-energy transients. Since SVOM’s amma-Ray Bursts: Next Generation Facilities 27
Fig. 13
Simulated spectrum (black solid curve) of typical GRB afterglow around the Ly- α line, as it would be seen in a 30 min exposure with E-ELT. The high signal-to-noiseallows precise determination of neutral hydrogen column in both the host and interveningintergalactic medium, together with abundances for many metal species (the simulation isof 1% of Solar metallicity). Attempts to fit the damping wing with a host HI only modelis shown by the red dashed curve, and a 100% neutral IGM only model as a green dashedcurve; both provide poor fits. This shows that with such S/N it is possible to decomposethe host HI column density and IGM neutral fraction. capabilities are broadly similar to those of Swift we still only expect a modestrate of high- z bursts to be detected. Another challenge for transient follow-up,particularly of GRBs, is that their rapid time-evolution means that observa-tions should ideally be done on a time-scale of hours after trigger. With the30 m telescopes being restricted to just two geographical regions of the Earth,this limits the potential for follow-up, and, especially if one also considersweather and technical down-time, strongly motivates the continued operationof smaller telescopes as part of a transient follow-up network.Several other challenges are at least more within the control of the con-sortia building the 30 m class telescopes. First and foremost is the provisionof suitable imaging and spectroscopic capability. Unlike some other areas ofastronomy, wide field is not key here, but good spatial resolution and (simul-taneous) wide wavelength coverage is important. Since speed is of the essence,operational efficiency is also very important, both to realise rapid responsesto trigger requests (quick instrument/mode changes, active optics reconfigur-ing, target acquisition, etc.), and also to expedite quick-look reduction andtransmission of data to observers (or their software agents) allowing further follow-up to be initiated with minimal delay. These capabilities require at-tention even in the design phase, for example to ensure that suitable datacalibration products will exist to support the desired quick-look pipeline pro-cessing.5.3 Host galaxiesIn addition to the transients themselves, the 30 m class telescopes will pro-vide unprecedented insights into their host galaxies. In general, high- z galax-ies found in Lyman-break surveys are compact (e.g., Curtis-Lake et al. 2014),typically a few tenths of an arcsec, and hence only crudely resolved at HST res-olution in the nIR. GRB hosts seem to be even fainter (Tanvir et al. 2012), andprobably smaller, but with with spatial resolutions as good as ∼ .
04 arcsecfor E-ELT, significant detail will be resolvable, allowing us to see probe thephysical conditions leading to early star formation.5.4 ConclusionsWithin ten years, if these ambitious plans come to fruition, the astronomicalcommunity will have access to three 30 m class telescopes. Thanks to theirhuge grasp, adaptive optics capability, and near-infrared optimisation, thesefacilities will revolutionise our view of early structure formation in and beforethe era of reionization. Observations of transients and their host galaxies hasa particularly important role to play, as they provide a route to studying inremarkable detail the physical nature of the very small star-forming galaxiesthat are thought to have dominated star formation at that time. amma-Ray Bursts: Next Generation Facilities 29 γ –ray event. Currently, ∼
95% of the bursts detected bythe γ –ray detector (BAT – Burst Alert Telescope – 15–150 keV) are detectedon board by the X–Ray Telescope (XRT; 0.1–3 keV) and ∼
75% of these arealso detected at optical wavelengths.Until the last two years, the detection rate of GRBs in the radio wasaround ∼ ergs − − at 8.4GHz while the few detected short GRBs and SN/GRBs area factor 100 and 10 less luminous, respectively. Regarding the distance scaleof radio detected GRBs there seems to be no preference: both low redshift(e.g. GRB 980425 - Kulkarni et al. (1998)) and high redshift (GRB 090423 -Chandra et al. (2010)) events have been detected in the radio band.In those events with multi epoch detections (i.e. radio light curve) a typicalpeak luminosity of ∼ × erg s − Hz − is reached after about 10–20 days (3-6 days in the source rest frame) since the burst trigger. Typical post peak decayindex is of order unity. Figure 14 shows the radio light curve (top panel) ofGRB 030329, one of the GRBs with the densest/longest monitoring in the radioband so far. The light curve shows a peak at ∼
15 days at 8.5 GHz and slightlylater ( ∼
45 days) at 5 GHz, a typical behaviour of the radio light curve. Withinthe standard afterglow model where the radiation is produced by synchrotronemission at the external shock (produced by the deceleration of the jet bythe interstellar medium), the broad band spectral energy distribution (SED)can be described by a characteristic peak and at least three break frequencies:the self–asborption frequency ν sa below which the spectrum is steep (withtypical slopes ν − / ), the electron minimum frequency ν m corresponding tothe minimum energy ( γ m ) of the relativistically shock accelerated electronsand the cooling frequency ν c which corresponds to the minimum energy of theelectrons that cool on dynamical timescales. The typical ordering is ν sa < ν m < GRB SKA
Davide Burlon
Figure 1:
Radio light curves at 4.9 and 8.5 GHz (top panels) and spectral indices (bottom panels) for thewell-sampled, long-lasting GRBs 970508, 980703 and 030329 (Granot & van der Horst 2014). The spectralindex α (where F ν ∝ ν α ) between 4.9 and 8.5 GHz varies significantly due to the spectral evolution andscintillation effects. The dashed lines in the bottom panels indicate spectral indices of 2, 1 / − . only be detected if robotic telescopes are on target fast enough. However the peak of this emissionmoves to lower frequencies over time and can be probed at radio frequencies on a time-scale ofhours to days (Kulkarni et al. 1999). Multi-frequency observations of this reverse shock providecrucial information regarding the particle content of the jet and its internal magnetic field (e.g.Perley et al. 2014; Anderson et al. 2014, and references therein).Another way in which radio observations play a key role in GRB studies is by providingevidence for the relativistic expansion of the jet. The source size and its evolution can be estimatedeither directly by means of Very Long Baseline Interferometry (VLBI), or indirectly by utilisingthe effects of interstellar scintillation. The former method has only been possible for one nearbyGRB (Taylor et al. 2004), and the source size measurements have been combined with its lightcurves to better constrain the physical parameters (Granot et al. 2005; Mesler & Pihlström 2013).Indirect source size measurements are possible because of scintillation due to the local interstellarmedium, which modulates the radio flux (Goodman 1997). While the angular size of the blastwave is initially smaller than the characteristic angular scale for interstellar scintillation, it growswith time until it eventually exceeds this scale and the modulations quench. This can be utilised indetermining the source size and the expansion speed of the blast wave, which impose constraints onthe modelling of GRB afterglows (Frail et al. 1997). The future possibilities of VLBI observationsof GRBs in the SKA era are discussed elsewhere in this book; see the Chapter by Paragi et al.,“Very Long Baseline Interferometry with the SKA”, in proceedings of “Advancing Astrophysics4 GRB SKA
Davide Burlon
Figure 1:
Radio light curves at 4.9 and 8.5 GHz (top panels) and spectral indices (bottom panels) for thewell-sampled, long-lasting GRBs 970508, 980703 and 030329 (Granot & van der Horst 2014). The spectralindex α (where F ν ∝ ν α ) between 4.9 and 8.5 GHz varies significantly due to the spectral evolution andscintillation effects. The dashed lines in the bottom panels indicate spectral indices of 2, 1 / − . only be detected if robotic telescopes are on target fast enough. However the peak of this emissionmoves to lower frequencies over time and can be probed at radio frequencies on a time-scale ofhours to days (Kulkarni et al. 1999). Multi-frequency observations of this reverse shock providecrucial information regarding the particle content of the jet and its internal magnetic field (e.g.Perley et al. 2014; Anderson et al. 2014, and references therein).Another way in which radio observations play a key role in GRB studies is by providingevidence for the relativistic expansion of the jet. The source size and its evolution can be estimatedeither directly by means of Very Long Baseline Interferometry (VLBI), or indirectly by utilisingthe effects of interstellar scintillation. The former method has only been possible for one nearbyGRB (Taylor et al. 2004), and the source size measurements have been combined with its lightcurves to better constrain the physical parameters (Granot et al. 2005; Mesler & Pihlström 2013).Indirect source size measurements are possible because of scintillation due to the local interstellarmedium, which modulates the radio flux (Goodman 1997). While the angular size of the blastwave is initially smaller than the characteristic angular scale for interstellar scintillation, it growswith time until it eventually exceeds this scale and the modulations quench. This can be utilised indetermining the source size and the expansion speed of the blast wave, which impose constraints onthe modelling of GRB afterglows (Frail et al. 1997). The future possibilities of VLBI observationsof GRBs in the SKA era are discussed elsewhere in this book; see the Chapter by Paragi et al.,“Very Long Baseline Interferometry with the SKA”, in proceedings of “Advancing Astrophysics4 Time (days after burst)
Fig. 14
Radio light curve (top panel) at 8.5GHz (black symbols) and 5GHz (red symbols)of GRB 030329. Evolution of the spectral index (computed between 5 and 8.5 GHz) as afunction of time (bottom panel). Dashed lines in the bottom panel show reference slopesof -2/3, 1/3 and 2 (from bottom to top). Figure adapted from Granot and van der Horst(2014). ν c at relatively early times (days–weeks) and ν m < ν sa < ν c at late times. Theevolution of these frequencies and of the peak flux with time depends on the jetdynamics. The standard afterglow model (e.g. van Eerten et al. (2012)) linksthe spectrum (dependent on the the micro physical parameters describing theshock - i.e. electron energy index, fraction of energy in relativistic electronsand in magnetic field) and the jet dynamics (dependent on the macro physicalparameters, i.e. the kinetic energy of the jet and the density of the externalmedium).The radio band, therefore, is affected by ν sa and ν m and their relativeposition. The appearance of a peak in the radio light curve is produced bythe passage across the observing band of these frequencies. In the example ofGRB 030329 shown in Figure 14 the bottom panel shows that the spectrum amma-Ray Bursts: Next Generation Facilities 31 is self absorbed at early times (with index -2 typical of the SED below ν sa )and becomes softer at later times reaching a slope -0.6 which can be usedto infer the electron energy power law slope, p ∼ . γ –ray event. In this respect, radio observationsare unique in probing the transition of the blast wave to the non–relativisticregime (i.e. trans–relativistic phase). In particular, during this phase estimatesof the outflow parameters such as the kinetic energy (Frail et al. 2001; Bergeret al. 2004; Frail et al. 2005; Shivvers and Berger 2011) do not depend on rel-ativistic effects and jet aperture (if spherical symmetry is reached) differentlyfrom the same parameters´estimates performed during the early relativisticphase (although radio calorimetry is not assumptions free - e.g. Eichler andWaxman (2005)).Another unique feature of the radio band is the possibility to study thebroad band SED of dark bursts (see e.g. Melandri et al. (2012) for a recentcompilation of dark bursts properties): by combining multifrequency radioobservations with X–ray data it is possible to model the time evolution of theSED and infer, for bursts without optical detection, an estimate of the opticalextinction. One of the best cases, GRB 051022 (Rol et al. 2007), turned outto be highly extinguished ( ∼ z = 0 .
34) with anextraordinary energetic budget (exceeding 10 erg). Its emission has beendetected up to the GeV bad by Fermi and broadband SED modelling hasproduced different interpretations (e.g. Maselli et al. (2014)). The radio bandoffered the opportunity to interpret the early time radio flare as due to thereverse shock (also observed in the optical - Verstrand et al. 2013) suggestinga low density environment (Laskar et al. 2013; Perley et al. 2014). However,additional radio data (van der Horst et al. 2014) suggested also a possible alternative interpretation of a double jet component. This bursts is one of thebest examples where the combination of multi wavelength data allow us tobreak the degeneracy among the free parameters of the standard model whileit still represents a challenge for the standard model itself (Maselli et al. 2014).Radio afterglow observations offer in principle the unique opportunity tomeasure the jet expansion and constrain the GRB size. This can be done (i)directly through imaging as for (the only case) GRB 030329 which was ob-served to expand over a couple of months thus indicating an apparent velocity3–5 the speed of light (Taylor et al. 2004; Pihlstr¨om et al. 2007; Mesler et al.2012) or (ii) indirectly through radio scintillation (i.e. from the measurementof the time it is quenched when the projected source size is larger than thegalactic ISM inhomogenities causing it).At very late times, when the afterglow emission has faded below the hostgalaxy, radio observations allows us to derive an independent estimate of thestar formation in GRB hosts thus providing an estimate of the dust obscura-tion. Systematic search of radio host emission provided several upper limits atthe level of 10–100 µ Jy and only few detections at redshift z < (cid:28) (cid:12) yr − . One major source of uncertainty in these estimates is the unknownhost radio spectrum, which requires multifrequency observations.Radio hostsobservations will shed light on the class of dark GRBs ( ∼ . The dashed lines are the 5 σ Fig. 15
Contour levels (68% and 95%) of the distribution of the population of GRBscorresponding to the time when the peak of the afterglow is reached and the correspondingflux. Four characteristic frequencies corresponding to the SKA1-MID (REF) are shown withdifferent colors. The dashed lines show the expected continuum sensitivity limits at 5 σ fora 12h exposure. sensitivity limits obtained for a 12h exposure. Figure 15 shows that the char-acteristic timescale of the peak of the afterglow light curve moves to longertimes at lower frequencies. Current estimates with the SKA1-MID sensitivitylimits (Burlon et al. 2015) show that it will be possible to detect most of thepopulation of GRBs at relatively large frequencies (from 4 to 9.2 GHz, blueand red curves in Figure 15). Still the self absorption dominating the lowestfrequencies will make accessible with the SKA-1 roughly half of the bursts at1.4GHz (black curves) and almost make it tough to detect any GRB at 200MHz (green curves). These estimates only describe the power of the radio ob-servations to study the radio afterglow emission of GRBs. In general the followup campaigns of GRBs will strongly benefit from the increased sensitivitiesof present and forthcoming radio facilities provided that the detection rateof GRBs in the γ –ray band (i.e. the triggers) is maintained/increased withcurrent/future GRB detectors (from Swift and Fermi now to SVOM in thefuture).The high sensitivity of the SKA will make it possible to perform late timeradio observations of the bursts when they have transitioned to the non– relativistic regime. We have computed (Ghirlanda et al. 2013; Burlon et al.2015) the expected flux density at the non–relativistic transition and com-pared to the SKA1-MID expected limits. Only a handful of objects reach thesensitivity limit of current facilities (a few µ Jy at best). We foresee that thefull SKA will routinely observe a significant fraction (15–25%) of the wholeGRB afterglow population at late times.One challenging aspect of GRBs in the forthcoming era of extended radiofacilities and in the final SKA era will the possibility to access their parentpopulation. We know that GRBs are jetted sources with a typical openingangle of a few degree. We detect only those bursts whose jet is closely alignedwith our line of sight. This means that for burst that we detect in γ –raysthere are hundreds of events that point their jet elsewhere. Depending on thejet structure these events can still be detected as slow transients in wide fieldsurveys. Orphan afterglow properties have been studied in the literature Rossiet al. (2008); Ghirlanda et al. (2014, 2015). Despite their large number noconvincing evidence of a radio orphan GRB has been reported so far. Thisis most likely due to the sensitivity limits of past radio surveys. Forthcomingradio surveys like the VAST/ASKAP (operating at 1.4 GHz) or the MeerKATor EVLA (operating at 8.4 GHz) could detect 3 × − and 3 × − OA deg − yr − , respectively. The deeper SKA survey, reaching the µ Jy flux limit, coulddetect up to 0.2-1.5 OA deg − yr − (Ghirlanda et al. 2014; Metzger et al.2015).The SKA era will be transformational for the study of GRBs. The SKAwill access close to 100% of the GRBs detected in γ –rays and will substan-tially contribute to the multi wavelength follow up of these sources. Radioobservations will extend the timescale of follow up to very late times when theafterglow emission at other frequencies will be already undetectable. We willget an unprecedented insight into the true energy budget of GRBs and it willbecome possible to probe both the macrophysics of the ambient medium (e.g.the density profile) and the microphysics of the shocks Finally, it will becomepossible to detect the so far elusive class of orphan afterglows that should ap-pear as slowly evolving transients detectable with the SKA on weekly basis inits wide field survey. amma-Ray Bursts: Next Generation Facilities 35 Time-domain astronomy will see its golden era towards the end of this decadewith the advent of major wide-field facilities across the electromagnetic spec-trum and in the multi-messenger realms including gravitational wave and neu-trinos. In the X-ray regime, the driving science calls for new generation instru-ments with high sensitivity, good angular resolution (a few arc-minutes or less)and a large sky coverage (field of view of order of thousands square degrees).These requirements can be fulfilled by wide-field X-ray focusing optics—theemerging lobster-eye Micro-Pore Optics (MPO), as focusing imaging resultsin enormously enhanced gain in signal to noise, and thus high detecting sen-sitivity. The Einstein Probe (EP), which is a candidate mission of priority ofthe Chinese Academy of Sciences with an intended launch date around 2020,is based on this lobster-eye MPO technology. Its aim is to monitor a largefraction of the whole sky at high cadences with sensitivity in X-ray at leastone order of magnitude deeper than the most sensitive all-sky monitoring typeinstruments ever built.Among various types of faint transients that EP is expected to discover,high-redshift gamma-ray bursts are of particular interest since, as the mostluminous objects in the Universe at their peaks, they provide a unique toolto study the early Universe beyond redshifts 6 and up to ∼
20. During thisepoch the first luminous objects as Pop-III stars (as well as Pop-II stars) areexpected to form and start to re-ionise the Universe out of the dark ages(Ciardi and Ferrara 2005). Whilst synthesizing metals they quickly evolvedand exploded, and thereby polluted their environment. The detection of thesestars individually would be extremely difficult or almost impossible, however,even with the next generation of space observatories like JWST. The only wayof observing Pop-III stars in action is to detect their explosive deaths. Theyhave been predicted to produce a GRB-like event (Bromm and Loeb 2006;Wang et al. 2012). By using high-redshift GRBs as beacons we could identifyand probe the regions where the first stars and their remnants (the first blackholes) were formed.7.1 Micro-pore optics X-ray focusing technologyX-ray focusing instruments rely on multiple smooth refection surfaces almostparallel to incident X-rays, which can be arranged in several different config-urations. One is Lobster-eye optics (Angel 1979), which mimics the imagingprinciple of the eyes of lobsters as shown in Figure 16. Incoming light is re-flected off the walls of many tiny square pores arranged on a sphere and pointedtowards the cocentric spherical center. The reflection surfaces are configuredorthogonal to each other without a specific optical axis, and thus the FOV canin principle subtend the entire solid angle of 4 π . In contrast, the conventionalWolter-I optics (Wolter 1952) (used for Chandra, XMM-Newton, Swift/XTR, Fig. 16 etc.) has an tubular, rotationally symmetric elliptical or parabolic reflectionsurface followed by a hyperbolic surface, which is impossible to achieve a FOVlarger than one degree.Lobster-eye telescopes can be constructed using novel X-ray focusing de-vice, the so-called Micro-Pore Optics (MPO) (Fraser et al. 1993). This tech-nology integrates millions of square pores as shown in Figure 17 with sizes ofabout tens of micrometers on a very thin ( ∼ amma-Ray Bursts: Next Generation Facilities 37 Fig. 17
MPO device (left) and a close-up under microscope (right). The pore size is of 20micrometers.
A simulated point spread function (PSF) of a Lobster-eye telescope isshown in Figure 18. The cross-like feature is caused by complex reflection pro-cesses of X-ray photons across the square pore. The incoming X-rays are splitinto four beams corresponding respectively to no reflection (straight through,forming background), single reflection (cross arms) or double reflections off
Fig. 18
Simulated PSF at 1 keV of a Lobster-eye telescope with a focal length of 375 mm. adjacent walls of the pore (focal spot). The FoV of the optical arrangementindicated in Figure 16 is only limited by the size of the optics (the number ofMPO pieces) or the size of the detector. The PSF remains almost unchangedover the entire FOV without vignetting of the effective area. Such a wide-fieldlobster-eye telescope provides the technological basis of the next generationwide-field X-ray monitors to detect faint and short-lived phenomena like high-redshift Gamma-Ray Bursts, distant X-ray novae and tidal disruption events.7.2 Einstein ProbeThe Einstein Probe is a candidate mission of priority in the Advanced StudyPhase of the Space Science Pilot Programme of the Chinese Academy of Sci-ences (CAS), with an intended launch date around 2020. It will discover andcharacterise high-energy transients and monitor variable objects in the softX-ray band with unprecedented sensitivity. Its primary scientific goals are to:(1) reveal quiescent black holes at almost all astrophysical mass scales andstudy how matter falls onto them by detecting transient X-ray flares, particu-larly stars being tidally-disrupted by otherwise dormant massive black holes atgalactic centres; (2) discover the X-ray photonic counterparts of gravitational-wave transients found with the next generation of gravitational-wave detectors amma-Ray Bursts: Next Generation Facilities 39
FXT: 1×1 square degreeWXT 1,2,3: 20×20 square degree eachWXT 7,8: 20×30 square degree eachWXT 4,5,6: 20×20 square degree each
Fig. 19 (left) Layout of WXT modules and FXT. (right) Sketch of the field of view ofWXT (not to scale). [Figures adopted from Yuan et al. (2015)]. and precisely locate them; (3) carry out systematic and sensitive surveys ofhigh-energy transients, to discover faint X-ray transients of various types, suchas high-redshift GRBs, supernova shock breakout, and previously unknowntransients.The payload of EP consists of a Wide-field X-ray Telescope (WXT) witha field-of-view of 60 ×
60 deg in the nominal 0.5–4 keV band, and a Follow-upX-ray telescope (FXT) with a larger light-collecting power than WXT, bothbased on the lobster-eye MPO technology. In addition, EP is equipped witha fast alert telemetry system, in order to trigger multi-wavelength follow-upobservations from the worldwide community. The nominal mission lifetime isthree years with a goal of five years. For a more detailed description of thescientific goals and instrumentation of EP, please refer to Yuan et al. (2015).The WXT telescope consists of eight modules, with FXT mounted at thecenter (Figure 19). Table 3 lists the main specifications of both WXT and FXT.The eight WXT modules make up a spherical array mosaicked by 441 concen-tric MPO pieces, each of 40mmx40mm in size. The total FoV subtends a solidangle of 60 ×
60 deg ( ∼ ×
420 mm in total. As a baseline de-tector, gas detectors based on GEM (Gas electron multiplier) are currentlybeing developed. The effective area and sensitivity curve of WXT are shownin Figure 20. FXT is a narrow field-of-view ( ∼ ∼
80 cm at 1 keV). FXT has an aperture size of about 240 mm,which is mosaicked by 6 × Table 3
Specifications of WXT and FXTParameters WXT FXTNumber of modules 8 1Field-of-view 60 ◦ × ◦ ◦ × ◦ Focal length (mm) 375 1,400Angular resolution FWHM (arcmin) < < ) 8 @0.7 keV 80 @1 keVSensitivity (erg s − cm − @1,000 s) ∼ × − ∼ × − Energy (keV)0.5 1 1.5 2 2.5 3 3.5 4 4.5 ) E ff e c t i v e A r ea ( c m Total AreaFocal+Arm AreaFocal Area
Fig. 20 (left) Simulated effective area of EP/WXT with a GEM detector, for the centralfocal spot (green), central plus the cruciform arms (red), and total (black; plus unfocusedX-rays as diffuse background). The MPO arrays are coated with Iridium, and have sur-face roughness of ∼ σ =0.85 arcmin. Xenon gas and a window of a 40 nm-thick Si N foil coated with 30 nm-thick Aluminum are used for the detector. [Figure adopted from Zhao et al. (2014)]. (right)Detecting sensitivity as a function of accumulative exposure time (assuming a source spec-trum of an absorbed power-law with a photon index of Γ = 2 and 3, respectively). ∼
97 minutes each) almost the entire night sky willbe sampled. The pointing directions are shifted by about 1 degree per dayto compensate the daily movement of the Sun on the sky. In this way, theentire sky will be covered within half-a-year’s operation. Once a transientsource is detected with WXT and is classified and triggered by the processingand alerting system onboard, the satellite will slew to a new position to enablepointed follow-up observations of the new source with FXT. Meanwhile, WXTcontinues to monitor the new sky region centering the position of the transient.The alert data of transients are expected to be downlinked within one minuteor so via the fast telemetry system, for which the French VHF system isconsidered. amma-Ray Bursts: Next Generation Facilities 41
Fig. 21 (left) Layout of the Einstein Probe satellite. (right) Illustration of the field-of-viewand pointed observations in one orbit. [Figures adopted from Yuan et al. (2015)]. z ∼ −
30 GRBs at z > − z >
8) per year (Wu et al., in preparation), and will thus delivera considerable sample of high-redshift GRBs with which to study the earlyUniverse within its nominal lifetime of three years. ∼
10 and their associa-tion with explosive death of massive stars and star forming regions, Gamma–Ray Bursts (GRBs) are unique and powerful tools for investigating the earlyUniverse: SFR evolution, physics of re-ionization, galaxies metallicity evolutionand luminosity function, first generation (pop III) stars.The European community played a fundamental role in the enormousprogress in the field of GRBs in the last 15–20 years (BeppoSAX, HETE–2,Swift, AGILE, Fermi, plus enormous efforts in optical IR and radio follow-up)In 2012, two European proposals for ESA Call for Small mission dedicatedto GRBs and all-sky monitoring: GAME (led by Italy, SDD-based cameras +CZT-based camera + scintillator based detectors) and A–STAR (led by UK,lobster-eye telescopes + CdTe detectors). Following all this unique esperienceand efforts, a White Paper on GRBs as probes of the early Universe submit-ted in response to ESA Call for science theme for next L2/L3 missions (Amatiet al. 2013) was very well considered by ESA.The THESEUS proposal described below is a result and follow–on of all thisunique esperience and efforts by the European GRB community, reinforcingalso the collaboration with extra–European (e.g., USA) GRB communitiesand opening itself to tight colaboration with the cosmology and transientsastrophysical communities.8.2 The THESEUS mission conceptThe Transient High Energy Sky and Early Universe Surveyor (THESEUS) isa mission concept a mission concept that will be submitted to ESA in 2016by a large international collaboration in response to the Call for next M5 mis-sion within the Cosmic Vision Programme. The primary scientific goals of themission are linked to the following Cosmic Vision themes: 4.1 Early Universe,4.2 The Universe taking shape, and 4.3 The evolving violent Universe.As detailed below, the main goal of THESEUS is fully exploiting GRBs ascosmological probes, thus providing a fundamental and unique step forwardin our understanding of the early Universe. More in general, THESEUS wouldvastly increase the discovery space of several classes of high energy transientphenomena over the entire cosmic history. amma-Ray Bursts: Next Generation Facilities 43
Fig. 22
Example of the complementarity and uniqueness of the THESEUS mission w/r toother cosmological measurements. (Credits: Caltech/NASA and the THESEUS Collabora-tion)
This is achieved via a unique payload providing an unprecedented com-bination of: (i) wide and deep sky monitoring in a broad energy band (0.3keV–20 MeV); (ii) focusing capabilities in the soft X-ray band granting largegrasp and high angular resolution; and 3) on board near-IR capabilities forimmediate transient identification and first redshift estimate.8.3 Scientific goalsThe main scientific goals of THESUES can be summarized as follows. (a) Exploring the Early Universe (cosmic dawn and reionizationera) by unveiling the Gamma–Ray Burst (GRBs) population in thefirst billion years , namely to perform unprecedented studies of the star for-mation history up to z ∼ environment, and was radiation from massive stars its primary driver? Howdid cosmic chemical evolution proceed as a function of time and environment?;to investigate the properties of the early galaxies and what was the galaxiesglobal star formation in the re–ionization era, and to investigate the dark en-ergy properties and evolution. The complementarity of THESEUS under thisrespect with other “cosmology” mission investigating the CMB or the largescale structure of the Universe is illustrated in Figure 22. (b) Performing an unprecedented deep survey of the soft X- raytransient Universe in order to: Fill the present gap in the discovery spaceof new classes of transients events, thus providing unexpected phenomena anddiscoveries; Provide a fundamental step forward in the comprehension of thephysics of various classes of Galactic and extra–Galactic transients, like, e.g.:tidal disruption events TDE, magnetars /SGRs, SN shock break–out, Soft X–ray Transients SFXTS, thermonuclear bursts from accreting neutron stars, No-vae, dwarf novae, stellar flares, AGNs / Blazars); Provide real time trigger andaccurate ( ∼ ∼ Additional science . By satisfying the requirements coming from theabove main science drivers, the THESEUS payload will also automatically becapable to perform excellent secondary and observatory science, e.g.: unprece-dented insights in the physics and progenitors of GRBs and their connectionwith peculiar core-collapse SNe; substantially increased detection rate andcharacterization of subenergetic GRBs and X–Ray Flashes; IR survey andguest observer possibilities, thus allowing a strong community involvement;survey capabilities of transient phenomena similar to the Large Synoptic Sur-vey Telescope (LSST) in the optical: a remarkable scientific synergy can beanticipated.8.4 Payload and mission profileThe scientific goals which come from a full exploration of the early Universerequires the detection of a factor ten more GRBs (about 100) in the first bil-lion years of the Universe (z > ∼ ∼ <
2, in order to allowefficient counterpart detection, on-board spectroscopy and redshift measure- amma-Ray Bursts: Next Generation Facilities 45
Table 4
Main characteristics of the THESEUS/SXI instrument (Credits: P. O’Brien, J.Osborne, D. Willingale and the THESEUS Collaboration)
Table 5
Main characteristics of the THESEUS/XGS instrument (Credits: F. Fuschino, C.Labanti, M. Marisaldi and the THESEUS Collaboration) ment and optical and IR follow-up observations. Such performances can bestbe obtained by including in the payload a monitor based on the lobster–eyetelescope technique, capable of focusing soft X–rays in the 0.3–6 keV energyband over a large FOV. Such instrumentation has been under developmentfor several years at the University of Leicester, has an high TRL level (e.g.,BepiColombo) and can perform an all–sky survey in the soft X–rays with anunprecedented combination of FOV, source location accuracy ( <
1) and sen-sitivity thus addressing both main science goals of the mission. An onboard
Table 6
Main characteristics of the THESEUS/IRT instrument (Credits: A.J. Castro-Tirado, V. Reglero, D. Gotz and the THESEUS Collaboration) infrared telescope of the 0.5–1m class is also needed, together with spacecraftfast slewing capability (e.g., 30/min), in order to provide prompt identifica-tion of the GRB optical/IR counterpart, refinement of the position down to afew arcs (thus enabling follow-up with the largest ground and space observa-tories), on–board redshift determination and spectroscopy of the counterpartand of the host galaxy. The telescope may also be used for multiple observa-tory and survey science goals. Finally, the inclusion in the payload of a broadfield of view hard X–ray detection system covering the same survey FOV asthe lobster–eye telescopes and extending the energy band from few keV upto several MeV will increase significantly the capabilities of the mission. Asthe lobster-eye telescopes can be triggered by several classes of transient phe-nomena (e.g., flare stars, X–ray bursts, etc), the hard X–ray detection systemprovides an efficient means to identify true GRBs and detect other transientsources (e.g., short GRBs). The joint data from the three instruments willcharacterize transients in terms of luminosity, spectra and timing propertiesover a broad energy band, thus getting fundamental insights into their physics.Based on the above, the THESEUS payload consists of the instrumentsshortly described below.
Soft X–ray Imager (SXI) : a set of Lobster Eye (0.3–6 keV) telescopescovering a total FOV of 1 sr field with 0.5–1 arcmin source location accuracy.Each module is a focusing wide field lobster eye telescope based on the opticalprinciples described in previous sections. The optics aperture is 290 ×
290 mm formed by an array of 7 × ×
40 mm and are mounted on a spherical frame with radius ofcurvature 600 mm (2 times the focal length of 300 mm). The open apertureprovided by each plate is 38 ×
38 mm ; the outer dimension of the optics frameis 320 ×
320 mm . The focal plane of each SXI module is a spherical surface amma-Ray Bursts: Next Generation Facilities 47 of radius of curvature 600 mm situatedat a distance of 300 mm (the focallength) from the optics aperture. The detectors for each module comprise a2 × ×
61 mm ; the detectors aretilted to approximate to the spherical focal surface. InfraRed Telescope (IRT) : a 70 cm class near-infrared (up to 2 microns)telescope (IRT) with imaging and moderate spectral capabilities. The telescope(optics and tube assembly) will be made of SiC, a material that has beenused in other space missions (such as Gaia, Herschel, Sentinel 2 and SPICA).Simulations using a 0.7 m aperture Cassegrain space borne NIR telescope(with a 0.23 m secondary mirror), using a Teledyne Hawaii-2RG 2048x2048pixels detector (18 m/pixels, resulting in 0.3 arcsec/pix plate scale) show that,for a 22.5 (H) point like source in a single 300 s exposure one could expect aSNR of 6. In order to achieve such performances the telescope needs to becooled at 240 (+/- 3) K by passive means, conductive and radiative insulations.Regarding the instrument, the optics box needs to be cooled to 190 (+/- 5)K to by a two stage cooler for which the first stage will cool the optics andthe second stage (the cold end) will cool the IR detector itself to 95 (+/-10)K: this allows the detector dark current to be kept at an acceptable level. Themechanical envelope of IRT is a cylinder with 80 cm diameter and 180 cmheight. A sun-shield is placed on top of the telescope baffle for IRT straylightprotection. The thermal hardware is compossed by a pulse tube cooling theDetector and FEE electronics and a set of thermal straps extracting the heatfrom the electronic boxes and camera optics coupled to a radiator located onthe spacecraft structure. The overall telescope mass is 112.6 kg and the totalpower supply is 95W.
X–Gamma–rays Spectrometer (XGS) : non-imaging spectrometer (XGS)based on SDD+CsI, covering the same FOV than the Lobster telescope ex-tending its energy band up to 20 MeV. The XGS consiosts of 25 modulesmade of scintillator bars optically coupled to an array of Silicon Drift De-tectors (SDD) PhotoDiodes (PD) tightly packaged to each other. Both SDD-PDs and scintillator detect X- and gamma-rays. The top SDD-PD, facing theX-/gamma-ray entrance window, is operated both as X-ray detector for lowenergy X-ray photons interacting in Silicon and as a read-out system of thescintillation light resulting from X-/gamma-ray interactions in the scintillator.The bottom SDD-PD at the other extreme of the crystal bar operates onlyas a read-out system for the scintillations. The discrimination between energylosses in Si and CsI is based on the different shape of charge pulses resultingfrom X-ray interactions in Si or from the collection of the scintillation lightthanks to their different timing properties (Marisaldi et al. 2005). Each bar ismade of scintillating crystal 5 × ×
30 mm in size. Each extreme of the bar iscovered with a PD for the read-out of the scintillation light, while the othersides of the bar are wrapped with a light reflecting material convoying thescintillation light towards the PDs. The scintillator material is CsI(Tl) peak-ing its light emission at about 560 nm. The PD is realized with the technique Fig. 23
Schematic view of the payload and satellite in THESEUS: the central green andwhite tube contains the IRT detectors, optics and buffle; the red modules are the SXI; theblack–pink modules are the XGS. (Credits: F. Fuschino, C. Labanti and the THESEUSCollaboration) of Silicon Drift Detectors with an active area of 5 × so matching thescintillator cross section.The main instruments characteristics are summerized in Tables 4, 5 and 6,and a possible payload accomodation sketch is shown in Figure 23. Sensitivitycurves of the SXI and XGS are jointly shown in Figure 24. Examples of ex-pected performances in terms of GRB detection rate as a function of redshiftand rate of different classes of transients are shown in Figure 25 and Table 7,respectively.8.5 Mission profile and consortiumThe proposed mission profile includes an an onboard data handling (OBDH)system capable of detecting, identifying and localizing likely transients in theSXI and XGS FOV; the capability of promptly (within a few tens of secondsat most) transmitting to ground the trigger time and position of GRBs (andother transients of interest); and a spacecraft slewing capability of 30/min).The baseline launcher / orbit configuration is a launch with Vega to a low incli-nation low Earth orbit (LEO, ∼
600 km, < amma-Ray Bursts: Next Generation Facilities 49 Fig. 24
Sensitivity of the SXI (black curves) and XGS (red) vs. integration time. The solidcurves assume a source column density of 5 × cm − (i.e. well out of the Galactic planeand very little intrinsic absorption). The dotted curves assume a source column density of10 cm − (significant intrinsic absorption). The black dots are the peak fluxes for SwiftBAT GRBs plotted against T90/2. The flux in the soft band 0.3-10 keV was estimated usingthe T90 BAT spectral fit including the absorption from the XRT spectral fit. The red dotsare those GRBs for which T90/2 is less than 1 second. The green dots are the initial fluxesand times since trigger at the start of the Swift XRT GRB light-curves. The horizontallines indicate the duration of the first time bin in the XRT light-curve. The various shadedregions illustrate variability and flux regions for different types of transients and variablesources. (Credits: D. Willingale, P. O’Brien, J. Osborne and the THESEUS Collaboration) allowing the exploitation of the Earths magnetic field for spacecraft fast slew-ing and facilitating the prompt transmission of transients trigger and positionsto ground. The basic observing strategyASI antenna in Malindi and, as an op-tion, the brazilian antenna in Alcantara were proposed as ground stations. Thebasic observing strategy is based on alternating anti–sun and ∼ polar point-ing, a compromise between maximum sky coverage, optimization of follow–upobservations from the ground, instruments requirements (thermal, etc.). Con-sidered prompt downlink options include: NASA/TDRSS, ESA/EDRS, WHFnetwork, IRIDIUM network, ORBCOMM. MOC and SOC were proposed tobe managed by ESA, while the SDC was proposed to be managed by ASI(ASDC).The total payload mass of THESEUS, including all contingencies, was esti-mated to be ∼
350 kg, and the total spacecraft dry mass about 1000 kg (powerabout 230 W and 800 W, respectively). The foreseen telemetry budget is fully
Fig. 25
The annual rate of GRBs predicted for THESEUS SXI (red) compared to Swift(blue). The upper scale shows the age of the Universe. For Swift the actual number of knownredshifts is approximately one third that plotted and none were determined on board (theblue curve has been linearly scaled upwards to match the total Swift trigger rate). ForTHESEUS the red region uses the simulations from Ghirlanda et al. (2015) and adopts theinstrument sensitivity for the SXI. (Credits: G. Ghirlanda, R. Salvaterra and the THESEUSCollaboration)
Table 7
Theseus detection rates for different astrophysical transients and variables (Cred-its: J. Osborne, P. O’Brien, D. Willingale and the THESEUS Collaboration)amma-Ray Bursts: Next Generation Facilities 51 compatible with the capabilities of the X–band, which will be the standard fornext ESA M missions.The THESEUS payload consortium for the ESA/M4 proposal is organizedas follow. The SXI will be responsibility of the UK (led by the University ofLeicester), the XGS will be responsibility of Italy (led by INAF, the Univer-sity of Ferrara and INFN), and the IRT will be the responsibility of Spain (aconsortium led by IAA-CSIC including UV, INTA, UGR and UMA) for thecamera and ESA for the optics. The core consortium includes also Poland (ledby CBK) and Denmark (led by DTU) for payload data handling hardware andsoftware. Czech Repubblic (led by CTU) is also involved for contributions tothe SXI, as well as Slovenia (SPACE-Sl) for communications. France (CNES,CEA) is available to provide the SVOM network of VHF antennae and asso-ciated control center. A junior international contribution is foreseen by USA(NASA) for the TDRSS system and contributions to the XGS and IRT cam-era. Finally, Hungary and Ireland have declared its interest in investigatingtheir contributions to different payload components during assessment phase.
Acknowledgements
The authors are grateful to the International Space Science Instituteof Beijing (ISSI-Beijing), its executive director Prof. M. Falanga and all the staff for hostingand funding the workshop “Gamma-Ray Bursts: a tool to Explore the Young Universe” heldin Beijing from April 10 to 15 2015. W. Yuan and C. Zhang thank R. Willingale, J.P. Osborneand P. O’Brien for their contribution to the EP project and acknowledge support of the“Strategic Priority Research Program on Space Science” (Grant number No. XDA04061100)of the Chinese Academy of Sciences. B. Cordier and D. G¨otz acknowledge financial supportof the UnivEarthS Labex program at Sorbonne Paris Cit´e (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). Sun acknowledges support from the 973 program 2014CB845802 andNSFC 11503028.
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