Since June 2008 two gamma-ray satellite experiments, AGILE and Fermi-LAT, have been in contemporaneous operation. Their results allow the study of gamma-ray astrophysics from space to enter in a new era. Starting from the heritage of the EGRET experiment, the science objectives of these two experiments are reviewed. The role of the technological improvement in such achievements is also briefly introduced

Barbiellini, G.

Longo, F.

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DOI 10.1393/ncc/i2009-10424-6Colloquia: IFAE 2009IL NUOVO CIMENTO Vol. 32 C, N. 3-4 Maggio-Agosto 2009Recent results in High Energy Gamma-ray AstrophysicsG. Barbiellini(1)(2) and F. Longo(1)(2)(1) Dipartimento di Fisica, Universita` di Trieste - Trieste, Italy(2) INFN, Sezione di Trieste - Trieste, Italy(ricevuto il 19 Settembre 2009; pubblicato online il 12 Ottobre 2009)Summary. — Since June 2008 two gamma-ray satellite experiments, AGILE andFermi-LAT, have been in contemporaneous operation. Their results allow the studyof gamma-ray astrophysics from space to enter in a new era. Starting from theheritage of the EGRET experiment, the science objectives of these two experimentsare reviewed. The role of the technological improvement in such achievements isalso briefly introduced.PACS 95.55.Ka – X- and γ-ray telescopes and instrumentation.1. – Introduction to High Energy Gamma-ray Astrophysics main questionsThe study of the sky in the energy band E > 10MeV to E ∼ 100GeV is commonlycalled High Energy Gamma-ray Astrophysics. In this energy band, some of the mostenergetic phenomena of the Universe happen. The processes at play in the high energysources are characterized by a wide-band non-thermal emission. Observing the sourcesfrom radio to gamma-rays helps to clarify their nature and their environments. Moreoverfundamental physics questions could be answered using the information provided bydistant high energy sources. The two approaches are indeed complementary in the sensethe second one needs the first to avoid potential source issues, while the first one needsthe second one to address potential implications of the observations. In this sense thestudy of high energy astrophysics will clarify questions such as the origin of CosmicRays in Supernova Remnants, the composition and the velocity of the ExtragalacticJets in powerful Active Galactic Nuclei, the behaviour of elecromagnetic fields in stronggravitation field typical of Pulsars, the magnetic reconnection process in Solar Flaresor the nature of the misterious Gamma-ray Bursts. Furthermore fundamental questionsas the possible evidence of dark matter annihilation in the center of the Galaxy or theviolation of Lorentz invariance. The same complementary approach is reflected on thecollaborations behind the current generation of high energy gamma-ray experiments.The main experiments are built and managed by researchers sharing both high energyparticle physics and astrophysics expertise [1].c© Societa` Italiana di Fisica 1920 G. BARBIELLINI and F. LONGO+90-90-180+180Third EGRET CatalogE > 100 MeVActive Galactic NucleiUnidentified EGRET SourcesPulsarsLMCSolar FLareFig. 1. – Map of source locations for the third EGRET catalogue [6], shown in Galacticcoordinates.2. – The EGRET heritageThe actual study of the high-energy sky relies on the great discoveries of the EnergeticGamma Ray Experiment Telescope (EGRET) (for a recent review see [2]). EGRET wasthe high-energy instrument on the Compton Observatory, covering the energy range20MeV to 30GeV [3]. EGRET was built as a typical pair production telescope. Asystem able to identify the gamma-ray interaction in the presence of a huge backgroundof charged particles and measure the gamma-ray arrival time, arrival direction, andenergy. This was done by means of multidetector system, composed of an AnticoincidenceSystem, a Spark chamber tracker system with 28 thin tantalum sheets, a scintillator-based Time-Of-Flight detector and Sodium Iodine Calorimeter [2].In its nine-year lifetime EGRET observed the entire high-energy gamma-ray sky. Thebrightest source is the Milky Way, particularly toward the inner part of the Galaxy, asa result of the interactions of high-energy cosmic rays with the interstellar matter andradiation fields. Individual gamma-ray sources appear as excesses above the modeleddiffuse emission [4]. The EGRET analysis process used a maximum likelihood methodto compare probabilities of fitting a given region of the sky with and without a source [5].The most complete analysis of the sky by the EGRET team was the third EGRET cata-logue [6]. Figure 1 shows the 3EG results on gamma-ray sources, with the source locationsshown in Galactic coordinates. Among the 271 3EG gamma-ray sources, EGRET de-tected 94 sources with probable or possible association with the class of Active GalacticNuclei known as blazars, 5 pulsars, the Large Magellanic Cloud detected as an extendedgamma-ray source, one solar flare and 170 sources, well over half the total, with noidentification with known astrophysical objects [2].EGRET detected also few gamma-ray bursts (GRBs) above 30MeV. The EGRETphotons from GRBs were seen coincident with low-energy gamma-ray emission for atleast 4 bursts among the brightest GRBS seen by BATSE [7]. In addition to the prompthigh-energy emission, delayed emission from some GRBs was detected by EGRET [8].RECENT RESULTS IN HIGH ENERGY GAMMA-RAY ASTROPHYSICS 21In another remarkable event [9], the EGRET calorimeter detected an additional delayedspectral component.The main EGRET results and the challenges left to the current generation could besummarized in the following main four topics.– The brightest persistent sources detected in high energy gamma-rays are pulsars,rapidly rotating neutron stars in high magnetic fields. Definetely more exposureand more precise timing with respect to EGRET is needed to detect in gamma-raysmore known radio pulsars and to discover new pulsars.– Many of the bright sources away from the Galactic plane are highly variable typesof Active Galactic Nuclei in the so-called blazar class. To understand the process atplay in these sources a simultaneous mutiwavelength observation campaign mightbe useful. A larger field of view with respect to EGRET will let the new gamma-rayexperiments to detect more flaring blazars.– Most of the EGRET sources are not associated with known objects. A betterangular resolution is definetely needed to clearly associate the gamma-ray excesswith sources known at other wavelengths.– A larger field of view, a wider energy coverage and a better timing will be usefulto understand the nature of the high energy emission from GRBs.The new generation of high-energy space missions has to address some of the funda-mental issues that were left open (or unresolved) by EGRET. It is important to makesubstantial progress in the instrumentation and mission concept to achieve the followinggoals: 1) improving the gamma-ray angular resolution near 100MeV by at least a factorof 2-3 compared to EGRET; 2) obtaining the largest possible field of view (FOV) at100MeV reaching 2.5-3 sr; 3) drastically reducing the deadtime for gamma-ray detectionfrom the EGRET value of 100ms; 4) obtaining broad-band spectral information possi-bly including a simultaneous detection capability in the MeV and X-ray energy ranges;5) carrying out a rapid quicklook analysis of the gamma-ray data and a fast disseminationof results and alerts; 6) stimulating efficient multifrequency programs [10].3. – AGILEThe AGILE scientific payload is made of three detectors surrounded by an Anticoin-cidence system, all combined into one integrated instrument with broad-band detectionand imaging capabilities [10]. In particular the Gamma-Ray Imaging Detector (GRID)is sensitive in the energy range ∼ 30MeV–50GeV, and consists of a Silicon-TungstenTracker, a Cesium Iodide Calorimeter, and an Anticoincidence system. The GRID isdesigned to achieve an optimal angular resolution, a very large field-of-view, and a sensi-tivity comparable to that of EGRET for sources within 10-20 degree from the main axisdirection.The map of the high significant AGILE sources is shown in fig. 2. This map is theresult of the significance-limited catalog that includes only sources above 4 σ extractedfrom the sample of AGILE detections obtained with a conservative data analysis ap-proach [11].AGILE fullfilled in its 2 years of operation most of the challenges that EGRETleft open. Here we report some of the most important results made by the AGILEteam.22 G. BARBIELLINI and F. LONGOFig. 2. – The First AGILE-GRID Catalog of high-confidence sources [11].– Searching for pulsed signals from 35 potentially interesting radio pulsars, orderedaccording to Fγ ∝√E˙d−2 and for which contemporary or recent radio data wereavailable, AGILE detected three new top-ranking nearby and Vela-like pulsarswith good confidence both through timing and spatial analysis. Among themthe remarkable PSR B1509-58 with a magnetic field in excess of 1013 Gauss,PSR J2229+6114 providing a reliable identification for the previously unidentifiedEGRET source 3EG 2227+6122 and the powerful millisecond pulsar B1821-24, inthe globular cluster M28 [12].– AGILE made several multiwavelength campaigns. One of the most interestingobjects studied by this means by AGILE is the blazar 3C 454.3. One simultaneouscampaign was carried out in November 2007 with AGILE, INTEGRAL, Swift, theWEBT Consortium, and the optical-NIR telescope REM. AGILE detect significantday-by-day variability of the gamma-ray emission. A correlation analysis based onthe entire data set is consistent with no time-lags between the gamma-ray and theoptical flux variations [13].– AGILE performed an extensive observation campaign on the Galactic region host-ing the Carina nebula and the remarkable colliding wind binary Eta Carinae (EtaCar) during the period 2007 July to 2009 January. AGILE detected a gamma-ray source consistent with the position of Eta Car, providing the long sought firstdetection above 100MeV of a colliding wind binary [14].– GRB 080514B was the first gamma-ray burst (GRB), since the time of EGRET, forwhich individual photons of energy above several tens of MeV have been detectedwith a pair-conversion tracker telescope. The hard X-ray emission observed bythe X-imager of AGILE lasted about 7 s, while there is evidence that the emissionabove 30MeV extends for a longer duration (at least 13 s) [15].RECENT RESULTS IN HIGH ENERGY GAMMA-RAY ASTROPHYSICS 2390 0270180180+90−90Unassociated AGN PulsarX−ray binary Globular clusterFig. 3. – Location of the LAT Bright Sources [17].4. – Fermi-LATThe Large Area Telescope (LAT) [16] on board the Fermi Mission is a pair-conversiontelescope with a precision tracker and calorimeter, each consisting of a 4 × 4 array of16 modules, a segmented anticoincidence detector that covers the tracker array, and aprogrammable trigger and data acquisition system. Each tracker module has a verticalstack of 18 x, y tracking planes, including two layers (x and y) of single-sided siliconstrip detectors and high-Z converter material (tungsten) per tray. Every calorimetermodule has 96 CsI(Tl) crystals, arranged in an 8 layer hodoscopic configuration with atotal depth of 8.6 radiation lengths, giving both longitudinal and transverse informationabout the energy deposition pattern. The calorimeter’s depth and segmentation enablethe high-energy reach of the LAT and contribute significantly to background rejection.The aspect ratio of the tracker (height/width) is 0.4, allowing a large field-of-view (2.4 sr)and ensuring that most pair-conversion showers initiated in the tracker will pass into thecalorimeter for energy measurement [16].Figure 3 reports the initial results for energies above 100MeV for the 205 most sig-nificant (statistical significance greater than ∼ 10-σ) γ-ray sources in the early missionLAT data. These are the best-characterized and best-localized point-like (i.e., spatiallyunresolved) γ-ray sources [17].During its first months of operation the LAT team made several important discov-eries not only fullfilling EGRET challenges, but opening really a new era in gamma-rayastrophysics. Here we report only some of them.– Although there are more than 1800 known radio pulsars, until recently, only sevenwere observed to pulse in gamma-rays and these were all discovered at other wave-lengths. The Fermi Large Area Telescope made it possible to pinpoint neutron starsthrough their gamma-ray pulsations. LAT reported the detection of 16 gamma-ray pulsars in blind frequency searches. Most of these pulsars are coincident withpreviously unidentified gamma-ray sources, and many are associated with super-24 G. BARBIELLINI and F. LONGOnova remnants. Direct detection of gamma-ray pulsars enables studies of emissionmechanisms, population statistics, and the energetics of pulsar wind nebulae andsupernova remnants [18].– LAT made the first simultaneous observations that cover the optical, X-ray, andhigh-energy gamma-ray bands of the BL Lac object PKS 2155-304. The gamma-ray bands were observed for 11 days, between 2008 August 25 and 2008 September6, jointly with the Fermi Gamma-ray Space Telescope and the HESS atmosphericCherenkov array, providing the first simultaneous MeV-TeV spectral energy dis-tribution (SED) with the new generation of gamma-ray telescopes. The ATOMtelescope and the RXTE and Swift observatories provided optical and X-ray cov-erage of the low-energy component over the same time period. The object wasclose to the lowest archival X-ray and very high energy (VHE; > 100GeV) state,whereas the optical flux was much higher. The light curves show relatively little(30%) variability overall when compared to past flaring episodes, but this campaignfound a clear optical/VHE correlation and evidence for a correlation of the X-rayswith the high-energy spectral index [19].– Energetic young pulsars and expanding Supernova remnants (SNR) are the mostvisible remains after massive stars, ending their lives, explode in core-collapse super-novae. The Fermi Gamma-Ray Space Telescope has unveiled a radio quiet pulsarlocated near the center of the compact synchrotron nebula inside the supernovaremnant CTA 1. The pulsar, discovered through its gamma-ray pulsations, has aperiod of 316.86 milliseconds and a period derivative of 3.614× 10−13 seconds persecond. Its characteristic age of 104 years is comparable to that estimated for theSNR [20].– Gamma-ray bursts (GRBs) are highly energetic explosions signaling the death ofmassive stars in distant galaxies. The Gamma-ray Burst Monitor and Large AreaTelescope onboard the Fermi Observatory together record GRBs over a broad en-ergy range spanning about 7 decades of gamma-ray energy. In September 2008,Fermi observed the exceptionally luminous GRB 080916C, with the largest appar-ent energy release yet measured. The high-energy gamma-rays are observed to startlater and persist longer than the lower energy photons. A simple spectral form fitsthe entire GRB spectrum, providing strong constraints on emission models. Theknown distance of the burst enables placing lower limits on the bulk Lorentz factorof the outflow and on the quantum gravity mass [21].5. – Conclusions: the importance of the technological developmentBoth AGILE and Fermi -LAT, unlike the previous generation instruments, such asEGRET, do not require high voltages nor gas operations. Both instruments containas the central detector a Silicon-Tungsten Tracker characterized by a very fine spatialresolution (obtained by a special arrangement of Silicon microstrip detectors and analogsignal storage and processing), a very small deadtime for gamma-ray detection, and thecapability of self-triggering allowing the achievement of a large field of view crucial forthe detection of the variable gamma-ray sky.RECENT RESULTS IN HIGH ENERGY GAMMA-RAY ASTROPHYSICS 25REFERENCES[1] Longo F., Nuovo Cimento B, 123 (2008) 918.[2] Thompson D. J., Rep. Prog. Phys., 71 (2008) 116901.[3] Fichtel C. E. et al., Proceedings of the 18th International Cosmic Ray Conference,Bangalore, India, Aug. 22-Sept. 3, 1983, Conference Papers, Vol. 8 (A85-22801 09-93) (TataInstitute of Fundamental Research, Bombay) 1983, pp. 19-22.[4] Hunter S. D. et al., Astrophys. J., 481 (1997) 205.[5] Mattox J. R. et al., Astrophys. J., 461 (1996) 396.[6] Hartman R. C. et al., Astrophys. J. Suppl. Ser., 123 (1999) 79.[7] Dingus B. L., Proceedings of Gamma-Ray Burst and Afterglow Astronomy 2001: AWorkshop Celebrating the First Year of the HETE Mission, American Inst. Physics Conf.Proc., Vol. 662 (2003) 240.[8] Hurley K. et al., Nature, 372 (1994) 652.[9] Gonza´lez M. M. et al., Nature, 424 (2003) 749.[10] Tavani M. et al., Astron. Astrophys., 502 (2009) 995.[11] Pittori C. et al., to be published in Astron. Astrophys., eprint arXiv:0902.2959.[12] Pellizzoni A. et al., Astrophys. J. Lett., 695 (2009) L115.[13] Vercellone S. et al., Astrophys. J., 690 (2009) 1018.[14] Tavani M. et al., Astrophys. J. Lett., 698 (2009) L142.[15] Giuliani A. et al., Astron. Astrophys., 491 (2008) L25.[16] Atwood W. B. et al., Astrophys. J., 697 (2009) 1071.[17] Abdo A. A. et al., Astrophys. J. Suppl., 183 (2009) 46.[18] Abdo A. A. et al., Science, 325 (2009) 840.[19] Aharonian F. et al., Astrophys. J. Lett., 696 (2009) L150.[20] Abdo A. A. et al., Science, 322 (2008) 1218.[21] Abdo A. 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Recent results in High Energy Gamma-ray Astrophysics

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Recent results in High Energy Gamma-ray Astrophysics

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