Status of ground-based gamma-ray astronomy
SStatus of ground-based gamma-ray astronomy
Marianne Lemoine-Goumard ∗ CENBG, CNRS-IN2P3, Université de Bordeaux19 Chemin du Solarium, CS 10120, F-33175 GRADIGNAN CedexE-mail: [email protected]
This article is the write-up of a rapporteur talk given at the 34th ICRC in The Hague, Netherlands.It attempts to review the results and developments presented at the conference and associated tothe vibrant field of ground-based gamma-ray astronomy. In total, it aims to give an overview ofthe 19 gamma-ray sessions, 84 talks and 176 posters presented at the 34th ICRC on this topic.New technical advances and projects will be described with an emphasis given on the cosmic-rayrelated studies of the Universe.
The 34th International Cosmic Ray Conference,30 July- 6 August, 2015The Hague, The Netherlands ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ a r X i v : . [ a s t r o - ph . H E ] O c t round-based gamma-ray astronomy Marianne Lemoine-Goumard
Very high-energy (VHE) electromagnetic radiation reaches Earth from a large part of the Cos-mos, carrying crucial and unique information about the most energetic phenomena in the Universe.Yet, it has only been in the last 25 years that we have had instruments to "observe" this radiation.The situation changed with the development of imaging air Cherenkov telescopes (IACTs) and par-ticle detectors, which have now matured to open a new window for exploration of the high-energyUniverse. These 2 different strategies to detect gamma rays are very complementary: IACTs havea small field of view, a small duty cycle but very good angular and energy resolution, while particledetectors have a much larger field of view and duty cycle but poor angular resolution in comparisonto the IACTs. These two types of detectors are also extremely complementary with (and benefitfrom) gamma-ray satellites such as
Fermi -LAT which is described in an accompanying article giv-ing a status report on space-based gamma-ray astronomy [1].
1. Status of the different experiments
Currently, six main experiments are operational : the IACTs VERITAS, MAGIC, FACT andH.E.S.S. and the particle detectors HAWC, and Tibet AS γ . On the same site in Tibet, data-takingwith ARGO-YBJ concluded after 5 years of observations in February 2013 but some summaryresults were presented at this ICRC. VERITAS, at the Fred Lawrence Whipple Observatory in southern Arizona has 4 telescopesof 12m diameter and cameras, covering a FoV of 3.5 ◦ [2]. Since inauguration, the array has un-dergone two major hardware upgrades. The first, in 2009, relocated one of the telescopes to bettersymmetrize the array, while the second, over 2011-2012, replaced the L2 trigger system and in-strumented each of the cameras with new, high efficiency PMTs. Additionally, VERITAS collectdata under bright moonlight conditions using two non-standard techniques, one with reduced high Figure 1:
Kifune plot (named after T. Kifune, who first showed a similar plot at the 1995 ICRC in Rome),showing the number of sources detected over time for various wavebands [11]. round-based gamma-ray astronomy Marianne Lemoine-Goumard voltage PMT settings and the other with UV filters placed over the PMTs. The utility of addingobservation time under these conditions was demonstrated by the detection of a flare state in a BLLac object, 1ES 1727+502, under bright moonlight conditions [3].MAGIC is a system of two 17m diameter IACTs located at 2200m at La Palma on the CanariesIsland. Between summer 2011 and 2012 the telescopes went through a major upgrade, carried outin two stages [4]. In the first part of the upgrade, in summer 2011, the readout systems of bothtelescopes were upgraded. In summer 2012, the second stage of the upgrade followed with anexchange of the camera of the MAGIC-I telescope to a uniformly pixellized one. Finally, duringWinter 2013-2014, after several years of development, a new system (sum-trigger) was imple-mented for stereoscopic observations. The energy threshold with this sum-trigger is 35 GeV.Located on the same site as MAGIC, the FACT telescope consists of a 1440-pixel G-APDcamera at the focus of one of the original HEGRA telescopes [5]. Since Summer 2012, FACT isoperated remotely (without the need of a data-taking crew on site). Even more important, duringmore than three years of operation of FACT, G-APDs have proven to be very reliable. The resultspresented at this ICRC, in particular the spectrum of the Crab nebula in excellent agreement withother IACTs [6], are a clear proof of the reliability, stability and performance of this system, show-ing that they are an alternative and excellent solution for the future.H.E.S.S. is originally an array of 4 12m diameter telescopes. Since 2012, a fifth telescope of28m located at the center of the array is operational. This allows to lower the energy thresholddown to 30 GeV at zenith. Since the camera electronics of CT1-4 are much older than the oneof CT5, an upgrade is being carried out since summer 2015 with a full completion planned for2016. The goals of this upgrade are threefold: reducing the dead time of the cameras, improvingthe overall performance of the array and reducing the system failure rate related to aging [7].
The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, located at Sierra Ne-gra, Mexico at 4100 m a.s.l., is sensitive to gamma rays in the energy range of 100 GeV to 100TeV [8]. HAWC consists of an array of 300 water Cherenkov detectors (WCDs) and a predictedpoint source sensitivity of 10 times that of its predecessor Milagro. The full HAWC array was com-pleted in March 2015, but science operations already started in August 2013 with a partially-builtarray. As said above, one of the real advantage of particle detectors such as HAWC is their largefield of view and high duty cycle.Two other large particle detector arrays are located at very high altitude, 4300 m a.s.l., inYangbajing (Tibet, China): ARGO-YBJ and Tibet AS γ . ARGO-YBJ consists of a single layerof resistive plate chambers completely covering an area of 100 ×
110 m. It has been operatedstably for 5.3 years, with an average duty cycle of 86% and for a total effective time of 1670.45days [9]. The Tibet AS γ air shower array, also at Yangbajing, is still operationnal [10]. It consistsof 789 closely-scintillator detectors covering an area of 36900 m2. It is in operation since 19993 round-based gamma-ray astronomy Marianne Lemoine-Goumard
Figure 2:
The TeV source catalog in Galactic coordinates as of summer 2015, courtesy of TeVCat. but a recent upgrade with a muon detector array will allow to reduce the background by selectinggamma-like events. Simulations demonstrate that using the full-scale MD array will enable to re-ject background cosmic-ray events by ∼
2. The Galactic sky
A lot can happen in our violent neighborhood. Massive stars will end their lives in a terribleexplosion called supernova which can lead to the formation of a pulsar or a black hole and a shockthat propagates in the interstellar medium called a supernova remnant. All these different sourcescan accelerate particles to very high energy. In turn, these particles will emit gamma rays that canbe observed by ground-based experiments.
The Galactic plane survey is certainly the investigation that provides the best general view ofour Galaxy. Thanks to its location in the southern hemisphere, HESS has a privileged access to4 round-based gamma-ray astronomy
Marianne Lemoine-Goumard
Figure 3:
TS map for the H.E.S.S. Galactic Plane Survey [12]. Identifiers for sources that have beendescribed in publications or announced at conferences are included. a very large part of the Galaxy. Roughly 2800 hours of high-quality observations of the Galacticdisk are available in the Galactic longitude range 250 to 65 degrees and Galactic latitude range b < ∼ ∼
2% Crab nebula point-source sensitivity) survey of the Milky Way in TeV gamma-rays. The HESS Galactic Plane Survey(HGPS) has revealed a diverse population of cosmic accelerators in the Galaxy, from which 77very-high-energy (E > 0.1 TeV) γ -ray sources have been compiled thanks to a new pipeline devel-oped in this context for both source detection and characterization (morphology and spectrum) [12].Out of the 77 sources, 16 are new sources that were previously unknown or unpublished. The dis-tribution of the TeV sources along the Galactic Plane is presented in Figure 3.With its large field of view and location at 19 ◦ N latitude, HAWC is surveying the GalacticPlane from high Galactic longitudes down to near the Galactic Center. Due to the limited angu-lar resolution of HAWC, a major challenge analyzing the emission from the inner Galaxy regionwhere source confusion is frequent, is to deconvolute and identify sources. To do so, a likelihoodframework has been developed to simultaneously fit the positions and fluxes of multiple sources,allowing to determine the number of sources in a region of interest (ROI). During this ICRC, ananalysis of the inner Galaxy region of l ∈ [+15 ◦ , +50 ◦ ] and b ∈ [–4 ◦ , +4 ◦ ] using the data taken witha partially-completed HAWC array has been presented, showing ten source candidates identifiedwith > σ post-trials, eight of them having tentative TeV associations [13]. The high sensitivityof HAWC in comparison to Milagro for such study and the consistency with the H.E.S.S. obser-vations on the same region is very well highlighted in Figure 4. Most sources are either pulsarwind nebula (PWN) candidates or unidentified sources (UNID). In the end, it is very likely that adominant fraction of the sources detected by HAWC will be PWNe, as it is already the case withTeV telescopes. 5 round-based gamma-ray astronomy Marianne Lemoine-Goumard
Figure 4:
Comparison of the significance maps of the inner Galaxy region for 8 years of Milagro data(Top), 150 days of HAWC-250 data (Middle), the H.E.S.S. Galactic Plane survey presented at the last ICRC(Bottom).
The number of TeV sources in this class is still growing and the recent detection by MAGIC ofthe PWN 3C58 is a very good example [14]. This PWN is the faintest detected at TeV energies andthe least energetic. Thanks to the large number of PWNe detected by TeV experiments, populationsstudies are now being carried out. Interestingly, using a simple modeling, it is possible to describethe trends and scatter of the present PWN population. For instance, all PWNe candidates detectedat TeV energies are powered by relatively young pulsars with high spin down power. However,variations between the different systems are also clearly visible and could be due to the differencesin the surrounding medium or different starting conditions [15].We cannot present PWNe without saying a word on the most famous one: the Crab Nebula. Itis now well known that strong flares have been detected at GeV energies by AGILE and
Fermi -LAT.At TeV energies, the Crab nebula seems to be much more quiet and no variability was detectedso far by the different experiments. Being one of the best studied objects, the precision of themeasurements obtained by the different experiments is now extremely impressive. For instance,the spectral measurements obtained by the MAGIC collaboration cover more than three decadesin energy [25]. Modeling these data is now a real challenge : simple models (constant magneticfield or spherical symmetry) are unable to reproduce the current TeV data. On the same source, theH.E.S.S. collaboration successfully presented results from the full hybrid H.E.S.S. array, applying amethod which combines monoscopic and stereoscopic events into one overall analysis [17]. Theseresults are important in two respects: this lowers the energy threshold and increases the statisticsto constrain even further theoretical models on PWNe, and it offers groundwork for the futureCherenkov Telescope Array (CTA) which will also be a hybrid system. Finally, still on the Crabnebula, search for continuous gamma-ray emission above 100 TeV has been performed by the TibetAS array [10]. No significant excess was found, and the upper limit obtained above 140 TeV ismost constraining until now.Geminga is an important high energy source. With its location at approximately 250 pc fromus, it is one of the closest known middle aged pulsars. The relative proximity of Geminga raises aninteresting possibility, namely that the high-energy particles accelerated by the PWN, most likelyelectrons and positrons, may be at the root of the explanation of the "positron excess". And indeed,6 round-based gamma-ray astronomy
Marianne Lemoine-Goumard some authors [18] have used the detection of an extended source spatially consistent with Gemingaby Milagro and have shown that this source could be the evidence for the production, accelerationand escape of electrons with energies up to 100 TeV. HAWC-250 also sees an excess of 6.3 σ at thelocation of the Geminga pulsar using an additional 3 ◦ smoothing [19]. This new result thereforeconfirms Milagro’s observation. In addition, the slightly lower significance obtained by HAWC incomparison to Milagro, despite a larger dataset, favors a spectrum harder than the Crab althougha Crab-like spectrum cannot be ruled out. It should be noted that no detection was reported byMAGIC and VERITAS despite intense observations but such large sources are extremely hard todetect with small field of view IACTs [20, 21]. Two analysis techniques are being implementedby VERITAS which will help in the study of spatially extended gamma-ray emission which mayemanate from this region. These techniques are progressing well and should be ready to analyzethe VERITAS Geminga data in the near future.This directly leads us to the topic of pulsars which are powering these bright and numerousPWNe. The search for pulsed emission from the Crab pulsar in very-high energy gamma rays has along history. The MAGIC telescope was the first to detect pulsed emission above 25 GeV [22].Some time later, observations with VERITAS revealed pulsed gamma-ray emissions at energiesabove 100 GeV [23]. During this ICRC, VERITAS presented an updated energy spectrum thatextends beyond 400 GeV [24]. On its side, still on the Crab, the MAGIC collaboration is nowseeing significant pulsed emission above 1 TeV as can be seen Figure in 5 [25]. These new resultsare a real challenge for pulsar models. It is impossible to reach such high energies with synchro-curvature which would support the inverse Compton mechanism. In addition, to avoid fast coolingdue to the large magnetic fields in the magnetosphere, the emission region must be far from thesurface of the neutron star, in the vicinity of the termination of the magnetosphere. However, keep-ing a pulsed signal through inverse Compton radiation at such large distance from the pulsar is nottrivial. Being one of the best studied objects, the Crab is still providing new surprises.A second detection of pulsed emission from a pulsar, Vela, was recently announced by H.E.S.S.down to energies of 20 GeV thanks to the use of H.E.S.S.II data in monoscopic mode [26]. How-ever, a deep observation campaign will be needed to investigate and constrain the maximum energyof the detected gamma-ray photons.
Pulsars are powering PWNe but they also can play the role of compact object in gamma-raybinaries such as PSR B1259 −
63 that represents the only case in this specific class of systems forwhich the nature of the compact object, a neutron star with a spin period of 48 ms, has been es-tablished. Although the new H.E.S.S.II data does not provide any significant evidence of an abruptincrease of the VHE emission similar to the HE case with
Fermi -LAT, the new 2014 observationsnevertheless reveal a relatively high source flux state at TeV energies ∼
50 days after periastron [27].The case of LS I +61 ◦
303 is extremely intriguing. The flux from this gamma-ray binary variesstrongly with the orbital period of 26.5 days. The maximum VHE flux is found around apastron7 round-based gamma-ray astronomy
Marianne Lemoine-Goumard
Figure 5:
Spectral energy distributions of the two peaks P1 and P2 of the Crab pulsed emission as measuredwith
Fermi -LAT (below 100 GeV, open circles) and MAGIC (above 70 GeV, filled circles) [25]. The MAGICflux of the Crab nebula is also plotted in blue for comparison. at a level typically corresponding to 10 – 15% of the Crab Nebula flux (>300 GeV). During re-cent VERITAS observations, relatively short (day scale), bright TeV flares were observed from thesource around apastron in two orbital cycles (October and November 2014). Both cases exhibitedpeak fluxes above 25% of the Crab nebula flux (>300 GeV), making these the brightest VHE flaresever detected from this source. Different scenarii are proposed to understand the origin of theseflares, depending on the nature of the compact object: a microquasar scenario or a pulsar binaryscenario. Further multi-wavelength observations of LS I +61 ◦
303 are therefore necessary to fullyunderstand the varying TeV emission from the source and determine the nature of the compactobject.
Supernova remnant represent another important type of objects in our Galaxy and one of theprime candidates for the acceleration of cosmic rays. However, the identification of TeV sources asSNRs is absolutely not trivial and entirely relies on their shell-like appearance and a TeV morphol-ogy matching their shell-like counterparts in radio and non-thermal X-rays. Using the increaseddata set of the Galactic Plane Survey, the HESS collaboration has reported the detection and identi-fication of one new shell-type SNR candidate [29], HESS J1534 − − − round-based gamma-ray astronomy Marianne Lemoine-Goumard
Figure 6:
Left: Gamma-ray excess map of IC 443 as viewed by VERITAS. Black contours are at 3, 6, 9 σ level. Locations of OH maser emission are represented by the red crosses while the position of the PWNCXOU J061705.3+222127 is indicated by a green diamond. White contours show the radio emission. Right:Counts map of the region of IC443 using 83 months of Fermi -LAT Pass 8 data, PSF2+PSF3 events from 5to 500 GeV. The image has been smoothed with a Gaussian kernal radius of 9 arcminutes. particles accelerated in this remnant. For what concern RX J1713.7 − Fermi -LAT sees a very similar morphology above 5GeV, as shown in Figure 6. The spectral analysis that is going on in different regions of the remnantseems to show a harder spectrum in the North East (where there is no interaction with the molec-ular cloud) which is extremely important to constrain and probe the environmental dependence ofcosmic-ray diffusion.
The Galactic center (GC) region also hosts several potential cosmic-ray accelerators. Indeed,the hard spectrum of the ridge emission, revealed by H.E.S.S. in 2006, and its spatial correlationwith the local gas density suggest that the emission detected was due to collisions of multi-TeVcosmic rays with the dense clouds of interstellar gas present in this region. Now thanks to a total of260 hours, the H.E.S.S. collaboration has performed a thorough analysis showing that ∼ half of GCridge emission is distributed like dense gas tracers over a projected distance of 140 pc and fadesbeyond [34]. An additional large scale emission that does not correlate with dense gas tracers, and9 round-based gamma-ray astronomy Marianne Lemoine-Goumard
Figure 7:
Top Panels: New VHE g-ray images of the GC region as seen by H.E.S.S., in Galactic coordinatesand smoothed with the H.E.S.S. PSF [34]. Left panel: gamma-ray significance map. Right panel: same mapafter subtraction of the two point sources G0.9+0.1 and HESS J1745 − could be the result of unresolved sources and/or a gas component in a diffuse phase not seen bygas tracers, is also required as can be seen in Figure 7. Finally, a new source is detected, dubbedHESS J1746 − − ∼
15 pc isdetected. It could be the signature of a radial gradient of CRs in the central molecular zone (CMZ)that is expected if they are accelerated by the SMBH itself. Interestingly, the measured CR densityprofile in this region appears to be in a quite good agreement with a 1/r profile. It is remarkablethat such a dependence is expected in the case of an accelerator located in the inner 10 pc regionof GC continuously injecting particles for more than 10 years. Finally, the energy spectrum of thediffuse gamma-ray emission, extracted from a ring centered at the GC up to a radius of 60 pc iswell described by a pure power-law with a photon index 2.3 up to 40 TeV, without any indicationof energy cut-off or break. The hadronic origin of the diffuse VHE emission implies that gammarays result from the decay of neutral pions produced by relativistic protons (at least 2.8 PeV at 68%confidence level) interacting with the interstellar gas. This is the first robust identification of a VHEcosmic hadronic accelerator operating as a PeVatron [35] !At higher energy (E >
350 GeV) and longitudes comprised between 25 ◦ and 100 ◦ , more thanfive years of ARGO-YBJ data have been used to study the diffuse gamma-rays from the Galacticplane. A spectral analysis has been carried out, showing an energy spectrum slightly softer than10 round-based gamma-ray astronomy Marianne Lemoine-Goumard that of the
Fermi -LAT Galactic diffuse emission model but consistent at 1 σ level. On the otherhand, the TeV flux averaged over the Cygnus region 65 ◦ < l < ◦ shows a marginal evidence ofa harder spectrum, indicating the possible presence of young cosmic rays coming from a nearbysource [9]. More data, especially with HAWC, would be extremely useful to confirm this result. As a final point on Galactic sources and moving to the extragalactic sky, the Large MagellanicCloud, a dwarf satellite galaxy of our Milky Way, is extremely instructive and encouraging for thefuture. Indeed, using a total of 210 hours, H.E.S.S. detected three extremely energetic objects ofdifferent type within the LMC [38]: • the PWN N 157B, a counterpart to the Crab nebula but with an electron acceleration effi-ciency is five times lower, • the SNR N 132D, one of the oldest VHE gamma-ray emitting SNRs, • the superbubble 30 Dor C, thus putting into light a new class of VHE emitters.It is the first time in a galaxy outside the Milky Way, that individual sources of very high energygamma-rays can be resolved. Interestingly, the unique object SN 1987A is, surprisingly, not de-tected, which constrains the theoretical framework of particle acceleration in very young supernovaremnants. These results open a new window at TeV energies and we can expect further discoverieswith more sensitive surveys of the LMC in gamma-rays, for instance with the Cherenkov TelescopeArray.
3. The extragalactic sky
The two last years since the previous ICRC have been very active for gamma-ray blazars,AGNs with jets pointing close to the line of sight, resulting in several new TeV detections of veryinteresting objects: S3 0218+357, PKS 1441+25, RGB J2243+203, and S3 1227+25. With thesenew detections, blazars are now detected at VHE out to an extreme redshift of z = 0.944, whileuntil recently the farthest sources observed in this energy range were 3C 279 (z = 0.536), KUV00311-1938 (z > 0.506) and PKS1424+240 (z > 0.6).
Observations of farther sources in VHE gamma-rays are difficult due to the strong absorptionin the interaction with the background radiation field originating from starlight emission and itsre-processing by interstellar medium integrated over cosmic history, called the extragalactic back-ground light (EBL). VHE gamma-rays interact with IR to UV photons via electron-positron pairproduction, resulting in an attenuated observed flux and effectively creating a gamma-ray "hori-zon". In this context, the recent detection of PKS 1441+25 by MAGIC is extremely interestingsince it is the most distant blazar detected by TeV experiments with its redshift of z=0.939 [39].Immediately after the MAGIC discovery in April 2015, VERITAS initiated a ToO observationcampaign. A total of 15 hours of good-quality observations was acquired during a 1-week period,11 round-based gamma-ray astronomy
Marianne Lemoine-Goumard resulting in the detection of a very soft spectrum excess of ∼
400 events [40]. Thanks to its highredshift and using the spectrum derived from these observations, preliminary Extragalactic Back-ground Light (EBL) constraints for this flare are highly competitive in comparison to previousmeasurements using a sample of sources, as can be seen in Figure 8.
Figure 8:
Specific intensity of the EBL. Galaxy counts and direct measurements are extracted from theliterature and represented by upward- and downward-going arrows, respectively. The model-independentresult by [41] is shown with gray points, while the model-dependent results based on the EBL intensity by[42] and [43] are shown with filled gray and black regions, respectively (1 σ confidence level). It is clear from the example of PKS 1441+25 that, to maximize the detection chance of highredshift blazar, the observations are often triggered by a high state observed in lower energy ranges.In particular,
Fermi -LAT scanning the whole sky in GeV range can provide alerts of high energyfluxes and spectral shape. It was the case for QSO B0218+357, also known as S3 0218+35, a blazarlocated at the redshift of 0.944. QSO B0218+357 is one of only two objects with a measuredgravitational lensing effect in GeV energy range. In 2012, it went through a series of outburstsregistered by the
Fermi -LAT instrument. The statistical analysis of the light curve autocorrelationfunction led to a measurement of time delay between the direct and the lensed components of 11.46 ± Fermi -LAT in July 2014.Unfortunately, the first component was not visible by MAGIC due to the full moon period but theycould catch the second (lensed) component. It is therefore the first gravitationally lensed blazardetected at the VHE energies. Interestingly, the emission is very bright despite the attenuation atsuch redshift which can be used to put constraints on the EBL.
The blazar Markarian 501 is among the brightest X-ray and TeV sources in the sky, andamong the few sources whose radio to VHE spectral energy distributions can be characterizedby current instruments by means of relatively short observations (minutes to hours). In 2013, an12 round-based gamma-ray astronomy
Marianne Lemoine-Goumard extensive multi-instrument campaign involving the participation of
Fermi -LAT, MAGIC, VERI-TAS, F-GAMMA,
Swift , GASP-WEBT, and other collaborations/groups and instruments was or-ganized [44]. It provided the most detailed temporal and energy coverage on Mrk 501 to date.This observing campaign included, for the first time, observations with the Nuclear StereoscopicTelescope Array (NuSTAR), which is a satellite mission launched in June 2012. NuSTAR providesunprecedented sensitivity in the hard X-ray range 3 – 79 keV, which, together with MAGIC andVERITAS observations, is crucial to probe the highest energy electrons in Mrk 501. A significantcorrelation between the X-ray fluxes (NuSTAR and Swift/XRT) and the VHE emissions (MAGICand VERITAS) was detected. Interestingly, a large fraction of the MAGIC data were affected bysand from the Saharan desert, in particular during the flaring activity. These data have been cor-rected using the atmospheric information from the LIDAR facility that is operational at the MAGICsite on an event by event basis. It is the first time that LIDAR information is used to produce aphysics result with Cherenkov telescope data taken during adverse atmospheric conditions.Obviously, with an instantaneous field of view of 2 sr, particle detectors like ARGO-YBJand HAWC can survey two-thirds of the sky every day. These unprecedented observational ca-pabilities allow to continuously scan the highly variable extragalactic gamma-ray sky providingcomplementary measurements during a MWL campaign. Multi-wavelength observations of Mrk421 over 4.5 years, from 2008 August to 2013 February, were presented by the ARGO-YBJ col-laboration [45]. According to the observed light curves, ten states (including seven large flares,two quiescent phases and one outburst) were selected and systematically analyzed. The underlyingphysical mechanisms responsible for different states may be related to the acceleration process orto variations of the ambient medium. ARGO-YBJ is no longer operational but the deployment ofHAWC is now complete and first flux light curves, binned in week-long intervals, were presented atthis ICRC for the TeV-emitting blazars Markarian 421 and 501 [46]. On both sources, indicationsof gamma-ray flare observations were shown, demonstrating that a water Cherenkov detector canmonitor TeV-scale variability of extragalactic sources on weekly time scales.
Blazars, which have been discussed in detail above, are the most numerous class of extra-galactic objects discovered at VHE. However, there is a growing evidence that blazars are not theonly extragalactic objects capable of VHE emission. With the detection of M 87, Cen A, IC 310,Per A and PKS 0625-354 [47] nearby radio galaxies (RGs) seem to constitute a new class of VHEsources. RGs are active galaxies with their relativistic jets oriented at intermediate to larger viewingangles with respect to the line of sight. As a result of larger inclinations, the observed non-thermalemission produced within the innermost parts of the jets is not amplified by relativistic beamingand hence different emission components, typically not present in observed blazar spectra, may be-come prominent. Therefore, since RGs are considered as blazars observed at larger viewing angles,modeling of such sources provides an independent check of blazar models. In addition, gamma-rayobservations of RGs may reveal some non-standard processes possibly related to the production ofhigh energy photons and particles within active nuclei and extended lobes. And third, increasingthe sample of gamma-ray RGs will enable to understand the contribution of nearby non-blazarAGN to the extragalactic gamma-ray background. These sources are therefore of very high interestand the example of IC 310 is excellent in this respect. Using radio data, the angle of the jet could13 round-based gamma-ray astronomy
Marianne Lemoine-Goumard
Figure 9:
Zoom of the MAGIC light curve of the flare of IC 310 calculated above 300 GeV into the timerange 00:57 (MJD 56244.04) to 01:40 (MJD 56244.07) [48]. The black lines show exponential fits to thelight curve to the rising and decay phases of the peak. The blue line shows the fit corresponding to theslowest doubling time necessary to explain the rising part of the flare at confidence level of 95%. be constrained between 10 and 20 ◦ which means that this is not a blazar. For the range of orien-tation angles inferred from radio observations, the Doppler factor is constrained to a value smallerthan 6. However, the MAGIC collaboration detected very strong variability from this source witha flux doubling time of 4.88 min for the rising phase (see Figure 9). The variability of 4.8 minconstrains the radius of the emission region to be ∼
20% of the event horizon: a very high valueof the Doppler factor is thus required to avoid the absorption of the gamma rays due to interactionswith low-energy synchrotron photons. In summary, trying to interpret the data in the frame of theshock-in-jet model meets difficulties. The MAGIC collaboration suggests that the emission wouldbe associated with pulsar-like particle acceleration by the electric field across a magnetospheric gapat the base of the radio jet [48]. The emission would then be produced by either inverse-Comptonscattering on a background photon field, or by curvature radiation. To finally answer which kind ofmechanism is responsible for ultra-rapid flux variability events, more observations are needed, e.g.,with higher sensitivity as provided by the Cherenkov Telescope Array as this potentially allows tomeasure even faster flares as well as rapid flux variations from other, e.g., fainter AGN. Further-more observing such events with simultaneous multi-wavelength coverage can help to constrainemission models.
Gamma-ray bursts (GRBs) are the most luminous, highly-relativistic light sources knownand may be generated during the collapse of a massive star (long GRBs) or via a merger event(short GRBs). They emit light across the electromagnetic spectrum, including at GeV energies andhence provide key targets for VHE gamma-ray detectors. And, indeed, a recent observation of the
Fermi -LAT has established that GRBs produce photons in the very-high-energy (VHE, > 100 GeV)regime, when a 95.3 GeV photon (or 128 GeV when corrected for redshift) was detected in GRB14 round-based gamma-ray astronomy
Marianne Lemoine-Goumard
Swift -detected GRBs in the HAWC field of view duringHAWC-111 were shown. None of the GRBs is significant above 3 σ when accounting for trial fac-tors [51]. On the other side, the lower energy threshold of IACTs such as MAGIC and HESS is alsoan excellent advantage for the detection of GRBs, since it reduces the flux attenuation by pair pro-duction with the lower energy (optical/IR) photons of the EBL. Due to their limited field of view,IACTs must be equipped with a very fast repointing of the telescopes. For the case of H.E.S.S., inorder to minimize this delay, two major improvements have been made for the additional telescope(CT5) over the original 4 telescopes. Firstly the telescope drive system of CT5 is significantlyupdated over that of the original H.E.S.S. system, and is able to perform a full rotation of the tele-scope (180 ◦ in azimuth) in ∼
110 seconds. Additionally CT5 is permitted to point in reverse-mode,allowing the telescope to slew through zenith, resulting in significantly faster repointing for someGRBs, where otherwise a large azimuthal slew would be required. In addition to this rapid slewinga fully automatic target of opportunity (ToO) observation system has been implemented within theH.E.S.S. and MAGIC arrays, in this case receiving triggers from the GCN system [52, 53]. In thelast two years, the upgrade of the MAGIC system and an improved GRB observation procedure hasmade possible follow-up of GRBs within 100s after the event onset. The preliminary data analysisof 13 GRBs observed with this improved automatic procedure did not allow the detection of anysignificant signal, however, these new developments open a new phase in the study of GRBs.
4. The future
As discussed in Section 1, major upgrades to all of the current generation of IACTs haverecently been completed. For the future, new facilities are now in the prototyping stage and aredeveloped to offer more statistics, better quality of photon reconstruction and new "type" of photonsreconstructed (going to the lower or higher energy range).
One of the best solutions covering these 3 requirements is the Cherenkov Telescope Array(CTA). CTA is an array of about 50–100 Cherenkov telescopes per site at two sites in the southernand the northern hemispheres, thus allowing full-sky coverage. At the time of writing, formal nego-tiations on two sites (Paranal in Chile and la Palma in the Canaries Islands in Spain) are underway.The large number of telescopes will come in three size classes: Large Size Telescopes (LSTs) sensi-tive to the low energy showers (below 200 GeV) [54], Medium Size Telescopes (MSTs) increasingthe effective area within the CTA core energy range (between 100 GeV and 10 TeV) [55] andSmall Size Telescopes (SSTs, for CTA South only) [56] spread out over several km to catch the15 round-based gamma-ray astronomy Marianne Lemoine-Goumard
Figure 10:
Differential sensitivity for a point-like g-ray source of the CTA-N and CTA-S candidate arrays(50 hours of observation, N/S pointing average) [57] together with the current MAGIC and VERITAS (50hours) and and future Fermi-LAT (over 10 years of operation) and HAWC (1 and 5 years) attained sensitiv-ities. rare events at the highest energies of the electromagnetic spectrum (up to ∼
300 TeV). The currentdesign foresees 4 LSTs, 25 Davies Cotton (DC) MSTs, and 70 SSTs for CTA South which offersa privileged location to observe the Galactic Plane and Galactic Center over the full VHE range.The southern array may be even augmented with a proposed extension of up to 25 Schwarzschild-Couder (SC) MSTs. The northern site, with 4 LSTs and 15 MSTs, is expected to complement it.This very large number of telescopes improves the sensitivity (see Figure 10) and the energy cover-age by at least an order of magnitude compared to existing VHE instruments, as well as the angularresolution and the interval of energy detectable. The CTA collaboration now comprises more than1200 members from 31 countries and the very large number of contributions at this year’s ICRC isa direct sign that CTA is very well advanced.Prototypes are being assembled and the simulations and data analysis package is also movingforward. The science that will be available thanks to the excellent performances of CTA is huge: • with its large field of view, CTA will be an excellent experiment for surveys, • with its fast slewing CTA will be ideal for transients, • with its high energy coverage CTA will be excellent for PeVatron search, • with its excellent angular resolution CTA will be excellent for morphological studies, • with its excellent energy resolution CTA will be excellent for line search, • with its energy coverage down to 20 GeV CTA will be excellent for cosmology.16 round-based gamma-ray astronomy Marianne Lemoine-Goumard
CTA is not the only way to go in the future. The Large High Altitude Air Shower Observatory(LHAASO) plans to build a hybrid extensive air shower (EAS) array at an altitude of 4410 m a.s.l.in Sichuan province, aiming for very high energy gamma ray astronomy and study of cosmic rayswith energies in 10 – 10 eV [58]. LHAASO consists of an extensive air shower array coveringan area of 1 km (KM2A), 75000 m water Cherenkov detector array (WCDA), an in-filled showercore detector array (SCDA) and 12 wide-field air Cherenkov/fluorescence telescopes. An integralsensitivity of 1% Crab unit can be reached at 3 TeV and 50 TeV as seen in Figure 11 (Left) incomparison to the sensitivity curves of other detectors. Thanks to its large area and its high capa-bility of background rejection, LHAASO can reach a sensitivity at energies above 30 TeV that issignificantly higher than that of current instruments (and even 5 times higher than that of CTA),offering the possibility to monitor the gamma-ray sky up to PeV energies with an unprecedentedsensitivity. In Figure 11 (Right), the sensitivity of LHAASO is compared with the extrapolation offluxes of six known SNRs. Four out of these six SNRs (Tycho, CasA, SNR G106.3+2.7 and W51)have fluxes higher than the LHAASO one-year sensitivity [59].It seems clear that projects using Cherenkov telescopes or using particle detector are very com-plementary since each of them are exploring different aspects of the gamma ray emission. Below10 TeV, observing a single source, a Cherenkov telescope array as CTA has a higher sensitivitycompared to particle detectors like HAWC and LHAASO. Thanks to the better angular and energyresolution, a Cherenkov telescope can perform detailed morphological and spectral analyses. Par-ticle detectors, however, can monitor a source throughout the year and, thanks to their large field ofview, have a much bigger chance to catch unpredictable transient events like flares. In addition, theenergy coverage of LHAASO and CTA is obviously very different. Prototype detector arrays ofabout 1% LHAASO are designed and built at the Yangbajing Cosmic Ray Observatory and used tocoincidentally measure cosmic rays with the ARGO-YBJ experiment. The LHAASO field prepa-ration will start in 2015 and detector construction is expected to start in 2016 and finish in 2020.According to the construction schedule, 1/4 of LHAASO will be put into operation and producephysical data in 2018.Abother on-going project is called M@TE (Monitoring at TeV Energies). Its goal is to builda SiPM camera for one of the telescope and join the blazar monitoring to better catch the risingand falling edge of a flare and perform more continuous monitorings. In Mexico, there are twotelescope mounts available. Other possible sites, to close the gaps even more, are India, Japanand Hawaii. Given the dropping prices of the photosensors, these extensions for the long-termmonitoring have become affordable small-scale instruments [60].Finally, MACHETE [61], is a concept proposing to build an array of two non-steerable tele-scopes with a FOV of 5 ×
60 sq.deg. oriented along the meridian. With such configuration, roughlyhalf of the sky drifts through the FOV in a year. The sensitivity that MACHETE would achieveafter 5 years of operation for every source in this half of the sky is comparable to the sensitivitythat a current IACT achieves for a specific source after a 50 h devoted observation. In addition, forsources observable in a single night, it reaches a sensitivity of 8% Crab which is perfect to triggerother telescopes. 17 round-based gamma-ray astronomy
Marianne Lemoine-Goumard
Figure 11:
Left: Sensitivity of LHAASO to a Crab-like point gamma ray source compared to other experi-ments. The Crab spectrum measured by ARGO-YBJ from 300 GeV to 20 TeV extrapolated to 1 PeV is usedas a reference flux. Right: integral spectra of the six SNRs in the LHAASO field of view extrapolated to 1PeV. The solid lines represent the measured spectra, while the extrapolations are shown by dotted lines.
5. Concluding remarks
Every ICRC over the last decade has seen dramatic advances in ground-based gamma-rayastronomy, and this meeting was no exception. All collaborations have produced a wealth of ex-citing results. One cannot summarize easily (and objectively) the 300 contributions that have beenpresented during this week since the number of discoveries that have been announced is quite im-pressive: first detection of a superbubble (in the LMC), first evidence of the detection of a PeVatronat the Galactic Center, first morphological study of a SNR interacting with a molecular cloud, firstdetection of pulsed emission from the Crab pulsar above 1 TeV, first gravitationally lensed blazardetected at the VHE energies, detection of the most distant blazar at TeV energies with a redshift ofz=0.939... These beautiful results are an excellent demonstration that VHE phenomena are ubiq-uitous throughout the Universe. But many of the results have raised new questions which requiremore and better data for a deeper understanding of the underlying phenomena. Figure 1 very wellshows that that the only limiting factor in our field is not the number of sources but only the sen-sitivity of the instruments. New projects are therefore crucial and are already very well advanced,in particular CTA and LHAASO. By the next ICRC, the deployment of the instruments for these 2projects will have started, marking another important step in the development of the field. I lookforward to seeing you all there.
Acknowledgments
I would like to thank the organizers for the invitation to give a rapporteur talk at the 34th ICRC.I also want to thank all the collaborations for making the presentations available beforehand andfor their support during this ICRC. Thank you to all the TeV colleagues for sharing and explaining18 round-based gamma-ray astronomy
Marianne Lemoine-Goumard all your great results before, during and after the talks. I really appreciated the discussion that wehad during this week.
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