In this contribution I will review the status of γ-astronomy at energies larger than ∼ 30 MeV from satellites. The measurements of the instruments aboard the Compton Gamma Ray Observatory have given a tremendous boost to this field of research, with γ-rays observed from a wide range of galactic and extra-galactic sources. The missions planned for the near future will be briefly summarized

Lipari, P.

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IL NUOVO CIMENTO Vol. 24 C, N. 4-5 Luglio-Ottobre 2001Gamma astronomy from space(∗)P. LipariINFN and Dipartimento di Fisica, Universita` di Roma I - Italy(ricevuto il 5 Dicembre 2000; approvato il 12 Febbraio 2001)Summary.— In this contribution I will review the status of γ-astronomy at energieslarger than ∼ 30 MeV from satellites. The measurements of the instruments aboardthe Compton Gamma Ray Observatory have given a tremendous boost to this fieldof research, with γ-rays observed from a wide range of galactic and extra-galacticsources. The missions planned for the near future will be briefly summarized.PACS 98.70.Rz – Gamma-ray sources; gamma-ray bursts.PACS 01.30.Cc – Conference proceedings.1. – IntroductionFour “messengers” tell us about the Universe outside the solar system: i) light (pho-tons), ii) cosmic rays, iii) neutrinos and iv) gravity waves. Light has been historicallythe “cornerstone” of astrophysics, with an expansion from the optical band to new wave-bands. The study of the shortest wavelength (or highest energy) has received a dramaticboost in the last decade, with the most important contribution obtained with satelliteexperiments, and γ-astronomy has emerged as a a mature, vital, and highly interestingfield (for recent reviews see [1-4]).High energy photons (Eγ >∼ few MeV) do not penetrate deeply in the atmosphere,therefore their direct detection is only possible from space with detectors placed onsatellites. This of course put serious limitations on the area, weight, and power of thedetectors, and only relatively small ones are possible. This limits the measurements toenergy not too high, when the γ flux is still large. At larger energies, when photons cangenerate a large electromagnetic showers, it is possible to have detectors at ground level,as Cherenkov telescopes or Air Shower arrays. In this contribution I will only considerdetectors in space.(∗) Paper presented at the Chacaltaya Meeting on Cosmic Ray Physics, La Paz, Bolivia,July 23-27, 2000.c© Societa` Italiana di Fisica 617618 P. LIPARI+90-90-180+180Third EGRET CatalogE > 100 MeVActive Galactic NucleiUnidentified EGRET SourcesPulsarsLMCSolar FLareFig. 1. – Point sources detected by EGRET at > 100 MeV (Third EGRET Catalog) [6].2. – The results of EGRETThe most significant development in this decade has been the launch in 1991 of theCompton Gamma Ray Observatory (CGRO) with aboard its four instruments. Thesatellite remained active until June 2000 when it was deorbited.The four instruments aboard the CGRO were: EGRET a spark chamber plus calo-rimeter instrument with sensitivity in the energy interval 30 MeV to 30 GeV; COMPTEL,a Compton telescope in the interval 0.1 to 30 MeV; BATSE, an omnidirectional X-rayand soft γ-ray “burst” detector consisting of large NaI scintillators sensitive to 30 keVto 2 MeV photons (with smaller spectroscopic NaI crystals for measurements up to 110MeV); and OSSE, consisting of Nai-CsI phoswiches detecting photons of 0.1 to 10 MeV.The highest energy detector EGRET (detecting photons in the energy range ∼30 MeV-30 GeV) has produced a wealth of astrophysical results. Prior to EGRET,two other successful satellite experiments, SAS-2 and COS-B, pioneered the field of highenergy γ-rays, and were able to make some of the first maps of the γ-ray sky and producethe first γ-ray source catalogs [5].The γ-ray sky in the 30 MeV to 30 GeV range comprises both diffuse radiation aswell as point sources. The diffuse radiation constitutes the bulk of the received photons,and is found to have a Galactic component arising from cosmic-ray interactions with thelocal interstellar gas and radiation, as well as an almost uniformly distributed componentbelieved to originate outside the Galaxy [7]. The average spectrum of the extragalacticdiffuse emission is well-described by a single power-law with an index −(2.10 ± 0.03) inthe 30 MeV to 30 GeV range. The precise origin of the extragalactic diffuse emission isnot well-known, and possibly includes both diffuse origin as well as contributions fromunresolved point sources, for example unresolved blazars.The point sources detected by EGRET above 100 MeV are shown in fig. 1. Severaltypes of sources are present as listed in table I. In addition to these sources EGRET hasalso detected 5 γ-ray bursts, the X-ray binary Cen X-3, and 1 solar flare.It is interesting to note that more than 60% of the EGRET sources are unidentified,with no firmly established counterparts at other wavelengths. Some of these sources haveGAMMA ASTRONOMY FROM SPACE 619Table I. – Sources in the Third EGRET (3EG) Catalog.Source Class NumberBlazars 67 (94)∗Pulsars 6Unidentified 170Normal Galaxy 1 (LMC)Radio Galaxy 1 (Cen A)* 27 AGN have been marginally identified.remained unidentified since the first surveys of the γ-ray sky with the COS-B satellite [5],and are one of the outstanding mysteries of the EGRET mission.The majority of the sources away from the galactic plane have been identified as activegalactic nuclei (AGN), essentially all of them of the “blazar” class. AGNs are currentlyunderstood as objects where the gravitational energy of material accreting on a massiveblack hole powers the emission of jets where plasma is emitted with relativistic speed.The blazars are those objects where one of the jets points close to the direction of theline of sight. A review on the blazars observed by EGRET may be found in [8].Gamma-ray emission models for blazars are generally divided into two broad classes:leptonic and hadronic. A method to discriminate between the two could naturally be thedetection of neutrinos.3. – Gamma-ray burstsOne of the most exciting results in high energy astrophysics is undoubtedly the de-tection of γ-ray bursts (GRB). These events, first recorded by the VELA satellites in1967, are bright transients in the γ-ray sky, with a typical duration of few seconds. Morethan 2500 bursts have been recorded by BATSE [9], they showed a distribution per-fectly isotropic, but dishomogeneous, pointing to a cosmological distribution. Five ofthe BATSE bursts were also detected by EGRET [10], demonstrating that the emissionextends to high energy.The relatively poor angular resolution of BATSE of a few degrees has made the detec-tion of GRB counterparts (i.e. sources at optical or radio wavelengths) difficult. The lackof counterparts has hindered efforts to pin down the distance scale of GRBs. In 1997, amajor breakthrough was achieved with the first detection of optical counterparts. Thiswork was accomplished by the Beppo/SAX [11] satellite detector in conjunction withpowerful optical telescopes, both on the ground (Keck) and in space (HST). By using acombination of wide and narrow-field X-ray telescopes, Beppo/SAX can determine thepositions of some bursts with an accuracy of several arc minutes. This excellent local-ization allowed the detection of fading optical counterparts for several dozen bursts [12].Redshift values have been determined for approximately 15 counterparts; they indicatethat the correlated bursts are cosmological in origin (typical z ∼ 1). For reviews of theafterglow see [13]. The typical inferred energy outputs of the bursts range between 1051and 1053 ergs (assuming isotropic emission). For one event, GRB990123, it has also beenpossible to obtain a contemporaneous optical detection with the telescope ROTSE-I [14]that can slew in an automated fashion upon receiving an alert from the GRB Coor-dinate network. At its peak brightness, the optical magnitude was 8.95 which, whencombined with the measured redshift value of 1.60, meant that this burst was the most620 P. LIPARIluminous object ever detected. GRB990123 was brighter than the brightest quasar byseveral orders of magnitude. Assuming isotropic emission, the inferred energy release ofGRB990123 exceeds 1054 ergs.A wide range of theoretical models have been proposed to explain gamma-ray bursts.The basic difficulty is to construct a physical mechanism that can produce and extractthe intense high energy emission we observe. The general picture calls for a cataclysmicevent which produces a relativistic fireball of material with Lorentz factors approaching1000 [15]. The relativistic material escapes from the region of high energy density alonga collimated jet. High energy radiation results when the jet collides with nearby ambientmaterial. The nature of the original cataclysmic event is not fully understood. Generallyfavored pictures include merging of neutron stars and hypernovae (“failed supernovae”of heavy stars) [16].All we know about GRBs has been obtained from the electromagnetic radiation theyproduce, but there are also speculations that they are the source of the highest energycosmic rays [17] and that they produce a detectable neutrino flux [18]. We can expectGRB research to remain exciting for years to come.4. – Future missionsIn this section we will briefly discuss the missions of γ-astronomy that are plannedfor the near future.4.1. INTEGRAL. – INTEGRAL [19] is a medium-size ESA mission with contributionfrom Russia (the launcher) and NASA, with the launch programmed for April 22, 2002.The detector will cover the “low energy” range Eγ = 15 keV–10 MeV. The instrumentwill have very good energy resolution, that is needed for the detection of nuclear lines, andalso very good angular resolution (12 arcminutes). The Integral payload consists of twomain gamma-ray instruments the Spectrometer SPI (SPectrometer on INTEGRAL), andthe Imager IBIS (Imager on Board the Integral Satellite). Each of them has both spectraland angular resolution, but they are differently optimised in order to complement eachother and to achieve overall excellent performance. These instruments are supportedby two monitor instruments which will provide complementary observations at X-rayand optical energy bands: The X-ray monitor JEM-X, will detect X-rays in the energyrange Eγ = 3 keV–35 keV, while the Optical Monitoring Camera OMC will detectphotons in the V band around 550 nm. The three main instruments of INTEGRAL(Spectrometer, Imager and X-ray detectors) share a common principle of operation:they are all coded-mask telescopes. The spectrometer will use an array of 19 hexagonalhigh-purity germanium detectors cooled to 85 K, and a hexagonal coded aperture maskis located 1.7 m above the detection plane in order to image large regions of the sky(fully coded field of view = 16 degrees) with a relatively modest angular resolution of 2degrees and an angular resolution of 2.2 keV (FWHM) at 1.33 MeV.The Imager IBIS is optimized for angular resolution. This in a coded mask telescopeis limited by the spatial resolution of the detector array (since diffraction is negligible atgamma-ray wavelengths).4.2. HETE-2 . – The High Energy Transient Explorer [20] is a small scientific satellitedesigned to detect and localize gamma-ray bursts. The detector was launched from aPegasus rocket in November of 2000. The coordinates of GRBs detected by HETE willbe distributed to interested ground-based observers within seconds of burst detection,GAMMA ASTRONOMY FROM SPACE 621allowing detailed observations of the initial phases of GRBs. The “2” in the name ofthe mission is a remainder of the sad story of the HETE-1 satellite that was launchedon November 4, 1996. The rocket failed to release the satellite, that died for lack ofsolar power within a day. HETE-2 is the successor quickly built with spare hardwarefrom the first satellite. The instruments of HETE will be: one gamma-ray detector (0.5–400 keV) and two X-ray detectors 2–25 keV and 0.5–10 keV (coded-aperture imagers).The expected angular resolution will be ∼ 10 arc-minutes for a “typical” burst, and aslow as 10 arc-seconds for the most powerful ones; the solid angle covered ∼ 1.5–2 str.Sophisticated on-board processing software will allow the location to be calculated onboard in real time, and ground post-burst analyses will provide refined localization. Inaddition to the study of GRBs, the HETE instruments will conduct a survey of the X-raysky, and study X-ray transients (sensitivity of few milliCrab in a day’s observation).On orbit, the HETE spacecraft will always point in the anti-solar direction for optimalexposure of the solar panels to the Sun. As a result, the HETE instruments monitor a∼ 2 steradian field centered roughly on the ecliptic. During the course of a year, HETEwill survey a region of the sky along the ecliptic which covers about 60% of the celestialsphere. Because of the anti-solar orientation of HETE, ground observers will always knowapproximately where HETE is observing. In addition, all bursts detected by HETE willbe at least 120 degrees from the Sun and, therefore, in prime position for observations byground-based optical observers. The scientific instruments operate during orbit twilightand night, when the Earth is not blocking their view.When a GRB is detected by HETE, a summary of the collected burst data is sentto a series of listen-only ground stations distributed around the equator. These dataare forwarded to MIT, where they are distributed to ground observers via the GRBCoordinates Network (GCN). Subscribers to GCN or visitors to the GCN web site canreceive notification of HETE GRB coordinates within seconds of burst onset.4.3. SWIFT . – Swift [21] will be the next satellite dedicated to the study of GRB’safter HETE-2, with a launch programmed for 2003. Three instruments will be on board:i) a γ-ray camera, ii) an X-ray telescope, iii) an UV and optical telescope.The strategy of the SWIFT mission will be unique. After the detection of a burstwith the γ-ray cameras, that will be approximately 5 times more sensitive than BATSEand have an angular resolution of ∼ few arc-minutes, Swift will have the unique ability torotate in orbit and repoint its gamma X-ray telescope, and ultraviolet/optical telescopeat the GRB with a delay of only 20–70 seconds. The combination of a smaller field of viewand a better sensitivity should result (depending on the extrapolation of the logN -logScurve) to an event rate of order 1/day. If an X-ray afterglow is observed (and this shouldhappen for around 1/3 of the events, that is for ∼ 100 burst/year), the X-ray telescopeshould be able to give a new position with a resolution ∼ 2.5 arc-seconds (sharp core of1′′) within ∼ 10 seconds, allowing the ground-based instruments to start the study.4.4. AGILE . – The AGILE γ-ray astronomy satellite [22] has been selected as thefirst Small Scientific Mission of the Italian Space Agency. With a launch in 2003, AGILEwill provide an important new tool for high-energy astrophysics in the 30 MeV–50 GeVrange before GLAST. Despite the much smaller weight and dimensions, the scientific per-formances of AGILE are comparable to those of EGRET. The AGILE scientific payloadis based on the state-of-the-art and reliably developed technology of solid-state silicondetectors. The instrument is very light (∼ 60 kg) and effective in detecting and monitor-ing gamma-ray sources (30 MeV–50 GeV) within a large field of view. The instrument622 P. LIPARIFig. 2. – Artist’s conception of GLAST, the next major satellite γ-ray telescope [24]. Themain GLAST instrument will consist of interleaved Si-strip/absorber layers for tracking and animaging CsI calorimeter for energy and position measurement.consists of a silicon-tungsten tracker, a cesium iodide mini-calorimeter (1.5 radiationlengths), an anticoincidence system made of segmented plastic scintillators, fast readoutelectronics and processing units. In constrast with old generation instruments such asEGRET, AGILE does not require gas operations and/or refilling, and does not requirehigh-voltages. Spectral information will be obtained from multiple scattering of createdpairs in the silicon planes (for energies less than ∼ 500 MeV) and by using the energydeposited in the tracker and the mini-calorimeter. The instrument is designed to achievean optimal angular resolution (source location accuracy ∼ 5′–20′ for intense sources),a very large field of view (>∼ 2 str), and a sensitivity comparable to that of EGRETfor on-axis (and substantially better for off-axis) point sources. AGILE will also carryan imaging hard X-ray detector to simultaneously monitor in the 10–40 keV range thesources observed in the central part of the gamma-ray field of view.4.5. GLAST . – The single most important new γ-ray telescope to be flown in spacethe next decade will be GLAST [23, 24], a state-of-the-art detector, based on severaltechniques developed for experimental particle physics, such as Si-strip tracking and CsIcalorimetry. With a very wide field of view, and a suitable pointing strategy, GLASTwill scan the entire sky on every orbit, offering unparalleled coverage of transient γ-rayGAMMA ASTRONOMY FROM SPACE 623phenomena, such as AGN flares and gamma-ray bursts. GLAST will have substantiallyimproved characteristics (angular resolution, energy resolution, energy range, etc.) rela-tive to its predecessor, EGRET. The resulting improvement in sensitivity of GLAST willenable the detection of up to two orders of magnitude more sources (e.g., approximately3000–4000 AGN). An artist’s conception of GLAST is shown in fig. 2. The current plancalls for a launch sometime in 2005.4.6. AMS . – The Anti Matter Spectrometer AMS [25] is scheduled for a three-yearmission on board the International Space Station Alpha (ISSA) from 2003 to 2006. AMShas as primary mission the search for cosmic ray antinuclei as well as the search for darkmatter studying anomalies in CR spectra and composition (e.g., e+, p¯), but has also aninteresting sensitivity as a high energy (0.3 to 100 GeV) gamma-ray detector.5. – ConclusionsThe entire field of astrophysics using very high energy particles (γ-rays, cosmic rays,and neutrinos) is in a state of rapid developement, stimulated by the exciting resultsobtained in the last few years. Using γ-rays, we are probing remarkable and unexpectedphenomena in objects such as active galaxies and gamma-ray bursts, we are also searchingfor the origins of the cosmic radiation. For the future, there is an expanding interest inthis field, and next-generation experiments in space and on the ground will greatly expandour discovery horizon.REFERENCES[1] Mukherjee R., astro-ph/0009369.[2] Catanese M. and Weekes T. C., astro-ph 9906501 (1999).[3] Ong R. A., Phys. Rep., 305 (1998) 95; astro-ph/0003014 (2000).[4] Hoffman C. M. et al., Rev. Mod. Phys., 71 (1999) 897.[5] Swanenburg B. N. et al., Astrophys. J., 243 (1981) L69.[6] Hartman R. C. et al., Astrophys. J., 123 (1999) 79.[7] Sreekumar P. et al., Astrophys. J., 494 (1998) 523.[8] Hartman R. C. et al., AIP Conf. Proc., 410 (1997) 307.[9] Paciesas W. S. et al., “The 4th BATSE Gamma-ray Burst Catalog” astro-ph/9903205.[10] Dingus B. L., Catelli J. R., Schneid E. J., AIP Conf. Proc., 428 (1998) 349.[11] Frontera F. et al., astro-ph/9802078.[12] Hurley K., Astron. Astrophys. Suppl., 138 (1999) 395–607.[13] Klose S., astro-ph/0001008; Galama T. J. astro-ph/0001465.[14] Akerlof C. et al., Nature, 398 (1999) 400.[15] Meszaros P. and Rees M., Astrophys. J., 476 (1997) 232.[16] MacFadyen A. I. and Woosley S. E., Astrophys. J., 524 (1999) 262.[17] See, for example: Waxman E., Phys. Rev. Lett., 75 (1995) 386.[18] See, for example: Halzen F. and Hooper D. W., Astrophys. J., 527 (1999) L93.[19] http://astro.estec.esa.nl/SA-general/Projects/Integral/integral.html[20] http://space.mit.edu/HETE/Welcome.html[21] http://swift.sonoma.edu/[22] http://www.ifctr.mi.cnr.it/Agile/[23] http://glastproject.gsfc.nasa.gov/[24] Gehrels N. and Michelson P., Astropart. Phys., 11 (1999) 277.[25] http://ams.cern.ch/AMS/ams homepage.html

Gamma astronomy from space

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Gamma astronomy from space

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