GAMMA-LIGHT: High-Energy Astrophysics above 10 MeV
Aldo Morselli, Andrea Argan, Guido Barbiellini, Walter Bonvicini, Andrea Bulgarelli, Martina Cardillo, Andrew Chen, Paolo Coppi, Anna Maria Di Giorgio, Immacolata Donnarumma, Ettore Del Monte, Valentina Fioretti, Marcello Galli, Manuela Giusti, Attilio Ferrari, Fabio Fuschino, Paolo Giommi, Andrea Giuliani, Claudio Labanti, Paolo Lipari, Francesco Longo, Martino Marisaldi, Sergio Molinari, Carlos Muñoz, Torsten Neubert, Piotr Orleanski, Josep M. Paredes, M. Ángeles Pérez-García, Giovanni Piano, Piergiorgio Picozza, Maura Pilia, Carlotta Pittori, Gianluca Pucella, Sabina Sabatini, Edoardo Striani, Marco Tavani, Alessio Trois, Andrea Vacchi, Stefano Vercellone, Francesco Verrecchia, Valerio Vittorini, Andrzej Zdziarski
GGAMMA-LIGHT: High-Energy Astrophysics above 10 MeV
Aldo Morselli a , Andrea Argan b , Guido Barbiellini c,d , Walter Bonvicini c , Andrea Bulgarelli e , Martina Cardillo f ,Andrew Chen g , Paolo Coppi h , Anna Maria Di Giorgio i , Immacolata Donnarumma i , Ettore Del Monte i , ValentinaFioretti e , Marcello Galli j , Manuela Giusti i , Attilio Ferrari n , Fabio Fuschino e , Paolo Giommi o , Andrea Giuliani p ,Claudio Labanti e , Paolo Lipari o , Francesco Longo c,d , Martino Marisaldi i , Sergio Molinari i , Carlos Mu˜noz p , TorstenNeubert q , Piotr Orlea´nski r , Josep M. Paredes s , M. ´Angeles P´erez-Garc´ıa v , Giovanni Piano i , Piergiorgio Picozza a,f ,Maura Pilia w , Carlotta Pittori u , Gianluca Pucella , Sabina Sabatini i , Edoardo Striani i,f,a , Marco Tavani , AlessioTrois x , Andrea Vacchi c , Stefano Vercellone i , Francesco Verrecchia u , Valerio Vittorini i , Andrzej Zdziarski i a INFN Roma Tor Vergata b INAF Roma, Italy c INFN Trieste, Italy d Univ. of Trieste, Italy e INAF-IASF Bologna, Italy f Univ. di Roma ”Tor Vergata” g INAF-IASF Milano, Italy h Yale Univ., USA i INAF-IAPS Roma, Italy j ENEA Bologna, Italy k Univ. Torino, Italy l ASDC Frascati, Italy m INAF-IASF Milano, Italy n Univ. Torino, Italy o INFN and Univ. di Roma ”La Sapienza” p Universidad Autonoma de Madrid and IFT-UAM / CSIC, Spain q DTU Space, Denmark r SRC PAS, Poland s Univ. Barcelona, Spain t University of Salamanca and IUFFyM, Salamanca, Spain u INAF OAR and ASDC v ENEA Frascati, Italy w ASTRON, The Netherlands x INAF OAC, Cagliari, Italy y INAF-IASF Palermo, Italy z NCAC, Poland)
Abstract
The energy range between 10 and 50 MeV is an experimentally very di ffi cult range and remained uncovered since thetime of COMPTEL. Here we propose a possible mission to cover this energy range. Keywords:
1. Introduction
High-energy phenomena in the cosmos, and in partic-ular processes leading to the emission of gamma- rays inthe energy range 10 MeV - 100 GeV, play a very specialrole in the understanding of our Universe. This energyrange is indeed associated with non-thermal phenomenaand challenging particle acceleration processes. TheUniverse can be thought as a context where fundamentalphysics, relativistic processes, strong gravity regimes, and plasma instabilities can be explored in a way thatis not possible to reproduce in our laboratories. High-energy astrophysics and atmospheric plasma physics areindeed not esoteric subjects, but are strongly linked withour daily life. Understanding cosmic high-energy pro-cesses has a large impact on our theories and laborato-ries applications. The technology involved in detectinggamma-rays is challenging and drives our ability to de-velop improved instruments for a large variety of appli-
Preprint submitted to Elsevier July 24, 2018 a r X i v : . [ a s t r o - ph . I M ] J un ations. GAMMA-LIGHT is a Small Mission whichaims at an unprecedented advance of our knowledgein many sectors of astrophysical and Earth studies re-search. The Mission will open a new observational win-dow in the low-energy gamma-ray range 10-50 MeV,and is configured to make substantial advances com-pared with the previous and current gamma-ray exper-iments (AGILE [1] and Fermi [2]). The improvementis based on an exquisite angular resolution achievedby GAMMA-LIGHT using state-of-the-art Silicon tech-nology with innovative data acquisition. Despite the re-cent important results and progress, AGILE and Fermiare leaving crucial unresolved issues in the energy win-dow 10-100 MeV. GAMMA-LIGHT will address all as-trophysics issues left open by the current generation ofinstruments. In particular, the breakthrough angular res-olution in the energy range 100 MeV - 1 GeV is cru-cial to resolve patchy and complex features of di ff usesources in the Galaxy as well as increasing the pointsource sensitivity. This proposal addresses scientifictopics of great interest to the community, with particularemphasis on multifrequency correlation studies involv-ing radio, optical, IR, X-ray, soft gamma-ray and TeVemission. At the end of this decade several new ob-servatories will be operational including LOFAR, SKA,ALMA, HAWK, CTA. GAMMA-LIGHT will ”fill thevacuum” in the 10 MeV-10 GeV band, and will provideinvaluable data for the understanding of cosmic and ter-restrial high-energy sources.
2. Astrophysics Objectives of GAMMA-LIGHT
Many crucial scientific issues are left unsolved by thecurrent generation of gamma-ray instruments (AGILE,Fermi). They constitute the main GAMMA-LIGHT sci-entific objectives :1. more sensitive search of Dark Matter gamma-raysignatures in the Galaxy and in particular in the GalacticCenter region;2. completely resolving the Galactic Center region ingamma-rays: Sgr A*, GeV and TeV sources, nebulae,compact sources, SNRs;3. resolving the di ff use emission in the Galactic planein relation with cosmic-ray propagation, star forming re-gions in the Galactic plane; extending the cosmic-raypropagation and emission properties of the ”Fermi bub-bles” to the lowest energies below 100 MeV;4. resolving spatially and spectrally SNRs and ad-dressing the origin and propagation of cosmic- rays withunprecedented accuracy;5. polarization studies of gamma-ray sources; Figure 1: GAMMA-LIGHT scheme. On the top there is the scientificinstrument showing the external AC system, the Silicon Tracker, andthe Calorimeter.
6. detection of soft gamma-ray pulsars in the range10-100 MeV, and pulsar wind nebulae studies;7. detection of compact objects, microquasars, rel-ativistic jets in the range 10 MeV - 1 GeV resolvingthe issue of hadronic vs. leptonic jets for a variety ofsources (e.g., Cyg X-3);8. detection and localization of transients and exoticsources with much improved sensitivity; detection ofCrab Nebula gamma-ray flares with excellent sensitivitydown to 10 MeV;9. blazar studies down to 10 MeV, excellent position-ing resolving source confusion;10. GRB excellent capability in the range 10 MeV -5 GeV; sub-millisecond timing capability in the range0.3-100 MeV.
3. The instrument
A scheme of GAMMA-LIGHT can be seen in fig-ure 1. The gamma-ray Tracker is the heart of the pay-load and it is made of 40 planes of silicon strip detectorsorganized in 41 trays without tungsten converter. Eachtray is configured as follows: two layers of 25 Silicontiles each (except trays 1 and 41 which have a singlelayer) organized in 5 ladders composed of 5 tiles bondedtogether. Each tray is made of a 1 cm core of aluminumhoneycomb covered on both sides by a 0.5 mm thickCarbon fiber layer. The resulting tray height is 1.1 cmand the total Tracker height is slightly above 50 cm. TheTracker is a compact, low-power 153.600 channel de-tector with self-triggering capability, fast timing possi-bility and full analog readout. The active element is asingle-sided, AC-coupled, 410 µ m thick, 9.5x9.5 cm Silicon strip detector with microstrips of 121 µ m pitch,2 igure 2: Point Spread Function (PSF, 68% containment radius) of theGAMMA-LIGHT gamma-ray (GRID) imager (in red color) obtainedby extensive GEANT-4 simulations which assume an incidence angleof 30 ◦ , Silicon strip analog readout, and Kalman filter analysis of par-ticle tracks. For comparison, we show the Fermi-LAT Pass7V6 PSF(total LAT: blue curve; front-LAT: black color) and the AGILE PSF(in gray color). alternate strip readout pitch of 242 µ m with one floatingstrip, and polysilicon resistors for the bias.The Calorimeter (CAL) is made of CsI elements eachof dimensions: height = ff ective area is shown in figure 3 and the sensitivity for48 hr (solar time) observation is shown in figure 4. Fora 2-day observation, sensitivies up to GeV energies arebackground dominated. At higher energies, sensitivitiesare photon limited: here we show the limit sensitivitiesassuming at least N = ffi ciency of 0.6 similar to AGILE’s (checkedwith real data). Fermi-LAT is assumed to be in sky-scanning mode with an overall exposure e ffi ciency persingle source of 0.16 (as checked with real data). Fi-nal data acquisition e ffi ciencies are assumed to be equalto 0.6; they take into account background rejection in aLEO orbit and on-board trigger logic and ground dataprocessing as deduced from Fermi and AGILE. Figure 3: E ff ective area for the GAMMA-LIGHT GRID at 30 de-gree o ff -axis (in red color). For comparison, we also show the ef-fective areas of AGILE at 30 degree o ff -axis (in gray color), Fermi-LAT-front Pass7 V6 at normal incidence (total: blue color; front-LAT:black color), and COMPTEL’s (in purple). Trigger logic e ffi ciencyand background rejection have been taken into account.Figure 4: Point source (5-sigma) sensitivity for 48 hr (solar time)observation at 30 ◦ o ff -axis of the GAMMA-LIGHT GRID imager(in red color). Also shown are the Fermi-LAT Pass7V6 sensitivity(total-LAT: black color; front-LAT: blue color) and AGILE’s sensitiv-ity (gray) for the same duration.
4. Dark Matter in our Galaxy
The nature of Dark Matter (DM) is still a mystery.Gamma-ray emission from our Galaxy may reveal theexistence of certain types of DM, by means of the pro-duction of secondary γ -rays after the annihilation (ordecay) of the DM particle candidates [3].The importance of GAMMA-LIGHT for Dark Mattersearches can be seen in figures 5 and 6 where the di ff er-ential γ -ray energy spectra per annihilation of WeaklyInteracting Massive Particle (WIMP) are plotted [4].As one can see the bulk of the emission even for highWIMP masses is in the energy range 5 MeV - 100 MeV.In the Fermi-LAT analysis of the Galactic Center the3 igure 5: Di ff erential γ -ray energy spectra per annihilation for a fixedannihilation channel (b bar) and for a few sample values of WIMPmasses [4]. For comparison we also show the emissivity, with anarbitrarily rescaled normalization, from the interaction of primarieswith the interstellar medium. The solid lines are the total yields, whilethe dashed lines are components not due to π decays. di ff use γ -ray backgrounds and discrete sources, as wemodel them today, can account for the large majority ofthe detected γ -ray emission from the Galactic Center.Nevertheless a residual emission is left, not accountedfor by the above models of standard astrophysical phe-nomena [5], [6], [7].So in the inner region of the Milky Way a better an-gular resolution with respect to AGILE and Fermi isneeded in the 5 MeV - 100 MeV energy range in or-der to disentangle the possible DM contribution fromthe di ff use background and the point sources contribu-tion (see for example [8]).Let us finally remark that decaying DM can produce adetectable line in the Gamma-Light energy range [9]. Inprinciple, detectability is expected to be large in the veryGalactic Center since hadronic emission models for thisregion are predicting a fall down about 100 MeV (seeFig. 2 of [10]).GAMMA-LIGHT can resolve the Galactic Center(GC) region and similar complex regions of the plane.The GC is indeed one of the most di ffi cult regions toobserve in high-energy gamma-rays. Optical emissionis heavily obscured by dust, and both the concentra-tion of point sources and the concentration of cloudsand di ff use emission enhancements is very high, a factthat complicates both the analysis and source identifi-cations. Towards the GC (and the anti-center) the ro- Figure 6: Di ff erential energy spectra per annihilation like in figure 5for a few sample annihilation channels and a fixed WIMP mass (200GeV). tational velocity of the Galaxy is entirely transverse,no longer allowing the distance of the interstellar gasto be determined through radio line shifts. In addi-tion, the column density of atomic hydrogen can be verylarge. Nevertheless, important progress has been madeby AGILE and Fermi. The most surprising discoveryhas been the observation of large, well-defined bub-bles / lobes above and below the Galactic plane extend-ing 50 ◦ above and below the GC with a width of about40 degrees in longitude [11]. These lobes have a uni-form surface brightness with sharp edges, neither limb-brightened nor centrally-brightened, and are nearly co-incident with a similar haze discovered in WMAP dataand later confirmed by Planck, suggesting a commonorigin, most likely inverse Compton emission of high-energy electrons scattering o ff the microwave photonsproducing gamma-rays or escaping hadrons. The lep-tonic or hadronic hypotheses would imply an injectionof high-energy particles in the past ∼
10 Myr. Possiblemechanisms include an accretion event onto the centralblack hole, a nuclear starburst, or the accumulation ofevents from a precessing jet. Each of these scenariosposes a number of problems. GAMMA-LIGHT will de-termine the morphology and spectral properties below 1GeV of the GC region and Fermi-bubbles with unprece-dented accuracy, and will contribute to resolving the is-sue of the nature of this emission.An example of the di ffi culty in the analysis of the GCregion is the search for the gamma-ray counterpart to4he super-massive black hole at the center of the galaxy,Sgr A*. At TeV energies, HESS has found a strongpoint source within 10 arcminutes of Sgr A*. However,source of the TeV emission may be either Sgr A* itself,or a nearby plerion discovered within the central fewarcseconds, or a putative ”black hole plerion” producedby the wind from Sgr A*, or the di ff use 10 pc regionsurrounding Sgr A*. An analysis of 25 months of Fermidata by [12] found 4 new sources within a 10 ◦ x10 ◦ re-gion around Sgr A* in addition to the 19 already listedin the first Fermi Source Catalog (1FGL). The sourcecoincident with both the HESS source and the positionof Sgr A*, 1FGL J1745.62900, shows no variability ineither GeV or TeV energies, while the GeV spectrumindicates that the emission mechanism must be distinctfrom that of the TeV emission. Much higher angularresolution is needed to distinguish among the variousscenarios, and GAMMA-LIGHT is the ideal instrumentto resolve these issues. Figure 7: The gamma-ray spectrum of one of the most prominentgamma-ray emitting SNRs showing a clear signature of neutral piondecay, W44. The Fermi (green triangles) and AGILE (red dia-monds) data are shown. The spectral modeling involves a compo-nent due to the neutral pion decay (yellow line) plus a componentdue to Bremsstrahlung (blue line). The red curve shows the expectedGAMMA-LIGHT sensitivity for a 1-year e ff ective time integration.
5. SNRs and Di ff use ”Cocoons” in the Galaxy: Ori-gin and Propagation of CRs Although a fair number of Supernova Remnants(SNRs) have been detected and recently studied by AG-ILE and Fermi, a number of outstanding issues regard-ing the origin of cosmic-rays (CRs) remain to be ad-dressed. Today a few dozen SNRs are known to emitgamma rays: the study of these objects together withtheir non-thermal properties (e.g., radio and X-ray emis-sion) is the topic of many investigations in CR physics and particle acceleration mechanisms (e.g., [13] , [14],[15]). However it is often quite di ffi cult to distinguish,for these objects, the di ff erent components that con-tribute to the gamma-ray spectrum (for energies greaterthan ∼
10 MeV up to tens of TeV, the only emission pro-cesses expected to product emission are the decay ofneutral pions produced in p-p scattering, inverse Comp-ton on low energy photons and Bremsstrahlung). Themodels so far produced to explain the SNR gamma-rayemission show that the knowledge of the spectrum atlow energies (below 100 MeV) is crucial for this pur-pose. In fact, the rapid fall of the gamma-ray spec-trum at energies less than 100 MeV is the most sig-nificant feature in the spectrum which may allow todiscriminate hadronic vs. leptonic emission. ElectronBremsstrahlung can provide an opposite behavior. Untilnow, this analysis was possible only for very few SNRs(e.g., [16], [17]). GAMMA-LIGHT will be able to re-solve the complex morphology of the SNR gamma- rayemission. It will provide invaluable information for adetailed modelling of CR acceleration and propagation.
6. Polarization Studies
GAMMA-LIGHT can greatly contribute to deter-mine gamma-ray polarization for intense sources. Theabsence of high-Z converters in the gamma-ray Trackermakes possible the measurement of the pair productionplane and angles with good accuracy. A method to de-termine the polarization direction of high energy (50MeV-30 GeV) linearly polarized gamma-rays was an-alyzed and discussed in [18]. The polarization infor-mation is contained in the azimuthal distribution of thecreated pair. The GAMMA-LIGHT Tracker will havean excellent angular resolution above a few hundreds ofMeV to determine the aperture angle of positrons andelectrons. This is an exciting possibility in the study ofintense gamma-ray sources.
7. Terrestrial Gamma-Ray Flashes
Terrestrial Gamma-Ray Flashes (TGFs) (see [19] fora recent review) are one of the most intriguing phe-nomena in the geophysical sciences and the manifes-tation of the highest energy natural particle accelera-tors on Earth. TGFs are millisecond time-scale burstsof gamma-rays produced above thunderstorms and as-sociated to lightning activity. Although several obser-vations are available and a general picture of this phe-nomenon based on Bremsstrahlung by relativistic run-away electrons produced in thunderstorms strong elec-tric field is commonly accepted, there are many points5 igure 8: Timeline schedule versus the energy range covered bypresent and future detectors in X and gamma-ray astrophysics. which remain obscure. Among the outstanding issues,we mention here the highest achievable energies (whichtranslates into the maximum voltage drop that can beestablished within thunderclouds) the connection withlightning and cloud microphysics, and the global andlocal TGF occurrence rate. There are currently three ac-tive space instruments capable of TGF detection: AG-ILE, RHESSI, and Fermi GBM. AGILE in particularhave shown that the TGF energy spectrum extends wellabove 40 MeV ([20] up to 100 MeV [21] with a spectralshape which is di ffi cult to reconcile with current pro-duction models. TGFs are now established as one ofthe coupling mechanisms between lower and upper at-mosphere. Since TGFs appear to be a much more com-mon phenomenon than previously expected [22], it isimportant to assess the impact of these phenomena onthe physical / chemical state of the atmosphere, and onthe climate. This is especially true if a large fraction isemitted at high energy, as suggested by the recent AG-ILE observations, since high-energy particles can havea significant role in aerosol nucleation and ultimatelyin cloud formation [23]. The study of TGFs and en-ergetic radiation from thunderstorms has now entereda golden age. Future observational breakthrough willcome with the Atmosphere-Space Interaction Monitor(ASIM), the ESA mission for the study of TGFs andTransient Luminous Events (TLEs), and by the CNESmicro-satellite TARANIS, expected to be launched in2014 and 2015 respectively. However, none of the forth-coming missions have detection capabilities above 40MeV or imaging capabilities above 10 MeV, where thememory of the electric field orientation at the source re-gion is carried o ff by gamma-ray photons.
8. Conclusion
For X-ray and gamma-ray experiments the time ofoperation versus energy range is shown in figure 8. Notethat GAMMA-LIGHT will cover an interval not cov-ered by any other experiments. Note also the numberof other experiments at other frequencies that will allowextensive multifrequency studies.
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