science with agile -...

Science with AGILEM. Tavani*, G. Barbiellini1, A. Argan**, N. Auricchio*, P. Caraveo*,

A. Chen**, V. Cocco§, E. Costa^, G. Di Cocco*, G. Fedel1^ M. Feroci1,M. Fiorini*, T. Froysland§, M. Galli", F Gianotti*, A. Giuliani*,C. Labanti*, I. Lapshov^1, P. Liparin, F. Longo**, E. Massaron,

S. Mereghetti*, E. Morelli*, A. Morselli§, A. Pellizzoni*, F Perotti*,P. Picozza§, C. Pittori§, C. Pontoni§§, M. Prest§§, M. Rapisarda^, E. Rossi*,

A. Rubini1, P. Soffitta1, M. Trifoglio*, E. Vallazza*, S. Vercellone* andD. Zanellon

*IFC-CNR, V. Bassini 15, 201 33 Milano - Italyt Univ. di Trieste and INFN, V. Padriciano 99, Trieste - Italy

**IFC-CNR Milano, and CIFS, Villa Gualino, Viale Settimio Severo 63, 10133 Torino - Italy*ITESRE-CNR, V. Gobetti 101, 40129 Bologna - Italy

*Univ. Roma-Hand INFN, V. Ric. Scientifica 1, 00133 Roma - Italy^IAS-CNR, V Fosso del Cavaliere, 00133 Roma - Italy

\\ENEA, Sez. Bologna, Italy. Fisica Univ. Roma-I and INFN, P.le Aldo Mow 2, 001 85 Roma - Italy

wC/mv. di Ferrara and INFN, V del Paradiso 12, 44100 Ferrara - Italy^INFN Trieste, and CIFS, Villa Gualino, Viale Settimio Severo 63, 10133 Torino - Italy

, Sez. Roma, Italy

Abstract. AGILE is an ASI gamma-ray astrophysics space Mission which will operate in the30 MeV - 30 GeV with imaging capabilities also in the 10^0 keV range. Primary scientific goalsinclude the study of AGNs, gamma-ray bursts, Galactic sources, unidentified gamma-ray sources,diffuse Galactic and extragalactic gamma-ray emission, high-precision timing studies, and QuantumGravity testing. AGILE will be the only Mission entirely dedicated to source detection above 30MeV during the period 2003-2006.

THE AGILE MISSION

AGILE is a Small Scientific Mission dedicated to high-energy astrophysics supported bythe Italian Space Agency and scientifically developed in CNR and INFN laboratories.AGILE is currently in Phase C, and planned to start operations during the year 2003.The AGILE instrument is highly innovative and designed to detect and image photonsin the 30 MeV-50 GeV and 10-40 keV energy bands. AGILE is characterized by anexcellent spatial resolution and timing capability, and by an unprecedently large field ofview covering ~ 1/5 of the entire sky at energies above 30 MeV.

The AGILE spacecraft will be of the MITA class (total satellite weight of ~ 230 kg)to be launched in a low-background equatorial orbit of height near 550 km. AGILEscientific data will be of great relevance also for joint studies of high-energy sources withother scientific satellites and ground-based facilities for radio/optical/TeV observations.

CP587, GAMMA 2001: Gamma-Ray Astrophysics 2001, edited by S. Ritz et al.© 2001 American Institute of Physics 0-7354-0027-X/01/$18.00

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Downloaded 07 Feb 2005 to 137.138.4.170. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

THE INSTRUMENT

The AGILE scientific instrument is based on an innovative design allowing the simulta-neous detection of hard X-rays and gamma-rays with unprecedented imaging and tim-ing capabilities [23, 24]. The instrument consists of two imaging detectors: (1) Super-AGILE (SA) and, (2) the Gamma-Ray Imaging Detector (GRID) made of a SiliconTracker and a Mini-Calorimeter. The Mini-Calorimeter is also capable of independentlydetect transient events. A description of the instrument can be found in Ref. [4]. Physicalconstraints on the instrument are tight in terms of absorbed power ( < 70 W), volumeand mass (~ 80 kg). We summarize here the main instrument characteristics.

A Silicon Tracker (ST), consisting of 14 detection planes, is devoted to the detectionand imaging of high-energy photons above ~ 20 MeV [4, 5, 6]. Particle track recon-struction is based on the floating strip readout of the analog (deposited charge) signalfrom Si microstrips of pitch of 121 /urn (see Fig. 1). The total number of channels is~ 43,000. The ST spatial resolution is excellent, reaching ~ 40^m for a variety of in-cidence angles [6, 5]. Fast low-power electronics allows reaching very short gamma-raydetection deadtimes of order of 100 ^/s. The ST on-axis effective area near 100 MeV is~500 cm2.

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FIGURE 1. The AGILE Tracker will make a crucial use of the analog signal from the Si microstrips.An AGILE beamtest was carried out in August, 2000 at the CERN Tl 1 beamline (East Hall, CERN PS).A photon beam (of energy range ~ 30 - 500 MeV) was produced by Bremsstrahlung of electrons ofmomentum ranging from 0.15 to 1 GeV/c hitting a thin lead target. The electron beam was deviated bya magnet spectrometer, and tagged by two delay wire chambers and a lead glass calorimeter. A typicalgamma-ray photon event detected by 4 Silicon detectors spaced with lead converters (each of ~0.07radiation length) is shown on the right. The histograms represent the charge collected on the readout stripsconfigured with the baseline AGILE tracker layout (Silicon microstrip pitch of 121 jam, for a floating stripreadout system of 242pm pitch). The spatial resolution achieved by this readout configuration is excellent(below 40 fjm for a wide range of photon incidence angles). Figure and data from Ref. [4, 5,6, 12].

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Super-AGILE (SA) consists of an additional plane of Si detectors placed on top ofthe Si Tracker, and of an ultra-light coded mask system [9, 15]. SA is aimed at detectinghard X-rays in the energy range between 10 and 40 keV. Imaging capabilities are quitegood (pixel size of ~ 6 arcmin) for an on-axis (5 — a) sensitivity of ~ 5 mCrab (1-dayintegration time).

The Mini-Calorimeter (MCAL) is made of two layers of CsI(Tl) bars with inde-pendent fast readout [1, 11]. MCAL supports both the ST particle energy reconstruc-tion (as part of GRID), and the independent detection of photons in the energy range~ 0.3 -100 MeV.

All AGILE active detectors are surrounded by an Anticoincidence (AC) Systemconsisting of a top plastic scintillator plane and 12 lateral planes [17].

The AGILE Data Handling System (DH) provides the on-board data processing forthe GRID, SA and MCAL events [25]. A DH essential task is the implementation of aGRB Search Procedure to be carried out for a large variety of trigger timescales (from< 1ms to tens of seconds). Super-AGILE can also quickly image fast transients, and a

fast communication channel for GRB alerts is currently envisioned.Table 1 summarizes the scientific performance of the AGILE detectors.

SCIENTIFIC PERFORMANCE

Fig. 2 shows the AGILE-GRID effective area, and Fig. 4 shows a typical GRID errorbox for a weak off-axis AGN (~ 30°). In both figures, we emphasize a comparison withEGRET capabilities.

10000AGILE, EGRET & COMPTEL EFFECTIVE AREAS

41 AGILE on axis* AGILE 50 deg... EGRET on cm

,v EGRET 4Q degI n COMPTEL on axis

10' Z 10°ENERGY (MeV)

FIGURE 2. Effective area as a function of photon energy for the AGILE-GRID, EGRET [27], andCOMPTEL [20]. GRID simulations results from Ref. [8].

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TABLE 1. AGILE Detectors' CapabilitiesGamma-Ray Imaging Detector (GRID)Energy Range 30 MeV - 50 GeVField of view ~ 3 srEffective Area (on-axis, at 400 MeV) ~ 540cm2

Effective Area (50-60° off-axis, at 400 MeV) ~ 320 cm2

Angular Resolution (68% cont. radius, 1 GeV) 36 arcminSource Location Accuracy (for S/N > 10) ~5-20 arcminEnergy Resolution (with MCAL, at 400 MeV) AE/E~ 1Deadtime ~ lOO^sAbsolute Timing Accuracy____________~ 2^i/s_______Mini-Calorimeter (MCAL)Energy RangeEnergy ResolutionEffective Area (at 300-900 keV)Effective Area (at 1-10 MeV)Effective Area (at 10-100 MeV)Deadtime (single Csl bar)Absolute Timing Accuracy

250 keV-200 MeV-IMeV~ 100cm2

-500cm2

- 1000cm2

Super-AGILE (SA)Energy Range 10-40keVField of view (Full Width at Zero Sens.) 107° x 68°Sensitivity (5a in 1 day) ~ 5 mCrabAngular Resolution (Pixel Size) 6 arcminSource Location Accuracy (for S/N~ 10) —1-3 arcminEnergy Resolution AE <4 keVDeadtime (single "daisy-chain" unit) < 5^/sAbsolute Timing Accuracy < 5^s

Gamma-Ray Astrophysics with the GRID

The GRID has been designed to obtain:• excellent imaging capability in the energy range 100 MeV-50 GeV, improving

the EGRET angular resolution by a factor of 2;• a very large field-of-view, allowing simultaneous coverage of ~ 1/5 of the entire

sky per each pointing (FOV larger by a factor of ~5 than that of EGRET);• excellent timing capability, with absolute time tagging of uncertainty near 1 /us

and very small deadtimes (~ 100,1/s for the Si-Tracker and ~ 20//s for each of theindividual Csl bars, see Fig. 7);

• a good sensitivity for point sources, comparable to that of EGRET for on-axissources, and substantially better for off-axis sources;

• excellent sensitivity to photons in the energy range ~30-100 MeV, with aneffective area above 200 cm2 at 30 MeV;

• a very rapid response to gamma-ray transients and gamma-ray bursts, ob-tained by a special Quicklook Analysis program and coordinated ground-based andspace observations.

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xlQ-4 -

20 30Energy (keV)

20 30Energy (keV)

FIGURE 3. Left Panel: Super-AGILE simulated sensitivity (solid data points: 5a, dashed data points:3a) for a 50 ksec integration and Crab-like spectrum for the combined four Si-detectors, in units of photonscm~2 s"1 keV"1. Right Panel: simulated sensitivity (solid data points: 5—a, dashed data points: 3a) inmCrab units. Simulation results from Refs. [9, 15]

Super-AGILE

An imaging coded mask detector system (Super-AGILE) in addition to the GRID willprovide a unique tool for the study of high-energy sources. The Super-AGILE FOV isplanned to be ~ 0.8 sr. Super-AGILE can provide important information including:

• source detection and spectral information in the energy range ~10-40 keV tobe obtained simultaneously with gamma-ray data (5 mCrab sensitivity at 15 keV(5 a) for a 50 ksec integration time);

• accurate localization (~l-2 arcmins) of GRBs and other transient events (fortypical transient fluxes above ~1 Crab); the expected GRB detection rate is ~ 1 — 2per month;

• excellent timing, with absolute time tagging uncertainty and deadtime near 5{is foreach of the 16 independent readout units of the Super-AGILE Si-detector;

• long-timescale monitoring (~2 weeks) of hard X-ray sources;• hard X-ray response to gamma-ray transients detected by the GRID, obtain-

able by slight repointings of the AGILE spacecraft (if necessary) to include thegamma-ray flaring source in the Super-AGILE FOV.

The combination of simultaneous hard X-ray and gamma-ray data will provide aformidable combination for the study of high-energy sources. Given the sensitivitiesof the GRID and Super-AGILE, simultaneous hard X-ray/gamma-ray information isanticipated to be obtainable for: (1) GRBs, (2) blazars with strong X-ray continuum

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Source Location Accuracy - AGN 95% Contours

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FIGURE 4. A comparison of simulated 99% contour levels for GRID (solid curve) and Super-AGILE(square) positioning of a relatively weak off-axis AGN, with what obtained by EGRET (dotted curve).We assumed a 1-week effective exposure time for a gamma-ray source of flux above 100 MeV equal to30 x 10~8ph.cm~2s~1 positioned at ~ 28 degrees off-axis for AGILE and at ~ 17 degrees off-axis forEGRET. Simulations results from Refs. [15, 28].

emission such as 3C 273 and Mk 501, (3) Galactic jet-sources with favorable geometries,(4) unidentified gamma-ray sources.

IMAGING AND TIMING OF GAMMA-RAY SOURCES

Fig. 5 shows a typical AGILE pointing, and Fig. 6 shows a comparison of intensity mapsobtained by a single 2-week viewing period of the anti-center region of the Galacticplane. Relatively bright AGNs and Galactic sources flaring in the gamma-ray energyrange above a flux of 10~6phcm~2 s"1 can be detected within a few days by the AGILEQuicklook Analysis. We conservatively estimate that for a 3-year mission AGILE ispotentially able to detect a number of gamma-ray flaring AGNs larger by a factor ofseveral compared to that obtained by EGRET during its 6-year operative life (see Fig. 8).Furthermore, the large FOV will favor the detection of fast transients such as gamma-ray bursts. Taking into account the high-energy distribution of GRB emission above 30MeV, we conservatively estimate that ~1 GRB/month can be detected and imaged in thegamma-ray range by the GRID and Super-AGILE.

The existence of a large number of variable gamma-ray sources (extragalactic andnear the Galactic plane, e.g., [13]) makes necessary a reliable program for quick re-sponse to transients. Quicklook Analysis of gamma-ray data will be a crucial task tobe carried out by the AGILE Team. Prompt communication of gamma-ray transients(typically requiring 1-3 days to be detected with high confidence for sources above10~6phcm~2s~1) is planned. Detection of short timescale (seconds/minutes/hours)transients (GRBs, SGRs, solar flares and other bursting events) is possible in the gamma-

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nga Pulsar

FIGURE 5. Comparison between a typical GRID pointing centered at the blazar 3C 279 region (areawithin the solid line circle of radius equal to 60°) and an EGRET pointing of the same source region (areawithin the dashed line of radius equal to 25°).

FIGURE 6. Left Panel: EGRET intensity map (photons energy above 100 MeV) of the GRO ViewingPeriod (VP) n. 1 of the field containing the Geminga and Crab pulsars (from the EGRET public database).Right Panel: AGILE simulated intensity map of the same sky region (above 100 MeV) assuming allsources at their average flux reported by the 3rd EGRET Catalog (from Ref. [28]). The variable nature ofseveral of these sources is clear from their absence in the left panel giving the GRO VP 1 pointing.In bothcases a 2-week total pointing duration was assumed. Intensity map scale in units of photons cm~2s~1 sr~!.

ray range. A primary responsibility of the AGILE Team will be to provide accurate po-sitioning of transients, and to alert the community through dedicated channels.

After a 1-year all-sky pointing program, we expect the AGILE average exposure fora generic source to be larger by a factor of ~ 4 compared to what obtained by EGRETduring the same time period. Therefore, AGILE average sensitivity for a generic gamma-ray source above the Galactic plane is expected to be better than EGRET by a factor ~ 2(see Ref. [16]). Deep exposures for selected sky regions can be obtained by a program

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with repeated overlapping pointings. For selected regions, AGILE can then achieve asensitivity larger than EGRET by a factor of ~ 4 — 5 at the completion of its program,reaching a minimum detactable flux near 5 x 10~8 phcm~2s~1. This capability can beparticularly important to study a selected list of persistent gamma-ray sources.

AGILE detectors will have optimal timing capabilities. The on-board GPS system canreach an absolute time tagging precision for individual photons near 2 /us. Depending onthe detectors hardware and electronics, absolute time tagging can achieve values near1 — 2jiis for the Si-Tracker, and 3 — 4/us for the individual detecting units of the Mini-Calorimeter and Super- AGILE.

Furthermore, instrumental deadtimes will be unprecedently small for gamma-ray de-tection. The GRID deadtime will be < lOO^s (improving by three orders of magnitudethe performance of previous spark-chamber detectors such as EGRET). The deadtimeof MCAL single Csl bars is near 20^s, and that of single Super- AGILE readout unitsis ~ 5^s. Taking into account the segmentation of the electronic readout of MCAL andSuper- AGILE detectors (32 MCAL elements and 16 Super- AGILE elements) the effec-tive deadtimes will be much less than those for the individual units.

Fig. 7 shows the AGILE timing performance compared to other gamma-ray missions.Fast AGILE timing will, for the first time, allow investigations and searches for sub-millisecond transients in the gamma-ray energy range.

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FIGURE 7. Left Panel: Absolute time-tagging uncertainty (T) of AGILE and previous gamma-raydetectors. Right Panel Instumental deadtimes (T) for the AGILE detectors and previous gamma-rayinstruments.

SCIENTIFIC OBJECTIVES

We summarize here the main AGILE'S scientific objectives.• Active Galactic Nuclei. Simultaneous monitoring of a large number of AGNs per

pointing will be possible. Several outstanding issues concerning the mechanism of AGNgamma-ray production and activity can be addressed by AGILE including: (1) the studyof transient vs. low-level gamma-ray emission and duty-cycles; (2) the relationship be-tween the gamma-ray variability and the radio-optical-X-ray-TeV emission; (3) the cor-relation between relativistic radio plasmoid ejections and gamma-ray flares; (4) hard

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X-ray/gamma-ray correlations. A program for joint AGILE and ground-based monitor-ing observations is being planned. On the average, AGILE will achieve deep exposuresof AGNs and substantially improve our knowledge on the low-level emission as wellas detecting flares. We conservatively estimate that for a 3-year program AGILE willdetect a number of AGNs ~ 3 times larger than that of EGRET (Fig. 8). Super-AGILEwill monitor, for the first time, simultaneous AGN emission in the gamma-ray and hardX-ray ranges.

• Gamma-Ray Bursts. About ten GRBs were detected by the EGRET spark chamberduring ~ 7 years of operations [19] (see also Fig. 8). This number was limited by theEGRET FOV and sensitivity and, from what we know today, not by the GRB emissionmechanism normally producing gamma-rays above 100 MeV (Ref. [21]). The GRIDdetection rate of GRBs is expected to be a factor of ~ 5 larger than that of EGRET,i.e., >5-10 events/year). The small GRID deadtime (~ 103 times smaller than that ofEGRET) allows a better study of the initial phase of GRB pulses (for which EGRETresponse was in many cases inadequate). The remarkable discovery of 'delayed' gamma-ray emission up to ~ 20 GeV from GRB 940217 [14] is of great importance to modelprompt and afterglow acceleration processes. AGILE is expected to be highly efficientin detecting photons above 10 GeV because of limited backsplashing. Super-AGILEwill be able to locate GRBs within a few arcminutes, and will systematically studythe interplay between hard X-ray and gamma-ray emissions. Special emphasis is givento fast timing allowing the detection of sub-millisecond GRB pulses independentlydetectable by the Si-Tracker, MCAL and Super-AGILE.

GRBs Detected by EGRET_____

Jl0 10 20 30 40

Off-axis angle [degree]

FIGURE 8. Left Panel: The histogram shows the distribution of sin (a) (with a the off-axis angle) forthe AGNs detected by EGRET and the solid curve gives the number of AGNs detectable by AGILEduring a 3-year Mission lifetime [7]. Right Panel: Off-axis angle distribution for the EGRET detectionsof gamma-ray bursts.

• Diffuse Galactic emission. The AGILE good angular resolution and large averageexposure will further improve our knowledge of cosmic ray origin, propagation, inter-action and emission processes. We also note that a joint study of Galactic gamma-rayemission from MeV to TeV energies is possible by special programs involving AGILEand new-generation TeV observatories of improved angular resolution.

• Gamma-ray pulsars. AGILE will contribute to the study of gamma-ray pulsars inseveral ways: (1) searching for pulsed gamma-ray emission from the ~ 30 new youngpulsars recently discovered in the Galactic plane (Ref. [10]); (2) improving photon statis-

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tics for gamma-ray period searches; (3) detecting possible secular fluctuations of thegamma-ray emission from neutron star magnetospheres; (4) studying unpulsed gamma-ray emission from plerions in supernova remnants and searching for time variability ofpulsar wind/nebula interactions, e.g., as in the Crab nebula.

• Search for non-blazar gamma-ray variable sources in the Galactic plane, a newclass of unidentified gamma-ray sources such as the mysterious GRO Jl 838-04 [22] andthe variable 2CG 135+1.

• Galactic sources, new transients. A large number of gamma-ray sources near theGalactic plane are unidentified and can be monitored on timescales of months/years.Also Galactic X-ray sources (such as Cyg X-l, Cyg X-3, GRS 1915+10, GRO J1655-40 and others) can produce detectable gamma-ray emission for favorable source statesand geometries, and a TOO program is planned to follow-up new discoveries of micro-quasars.

• Fundamental Physics: Quantum Gravity. AGILE detectors are suited for Quan-tum Gravity studies [26]. The existence of sub-millisecond GRB pulses lasting hun-dreds of microseconds [2] opens the way to study QG delay propagation effects with theAGILE detectors. If these ultra-short GRB pulses originate at cosmological distances,sensitivity to the Planck's mass can be reached [26].

REFERENCES

1. Auricchio N. et al., these Proceedings (2001)2. Bhat C.L., et al., Nature, 359, 217 (1992)3. Barbiellini G. et al., Proceedings of the 5th Compton Symposium, AIP Conf. Proceedings, ed. M.

McConnell, 2001, Vol. 510, p. 7504. Barbiellini G. et al., these Proceedings (2001a)5. Barbiellini G. et al., these Proceedings (2001b)6. Barbiellini G. et al., MM, submitted (2001)7. Chen A. et al., in preparation (2001)8. Cocco V., Longo F. and Tavani M., these Proceedings (2001)9. Costa E. et al., MM, in preparation (2001)10. D'Amico N., these Proceedings (2001)11. Di Cocco G. et al., MM, in preparation (2001)12. Fedel G., Laurea Dissertation, University of Trieste (2000)13. Hartman R.C. et al., ApJS, 123, 79 (1999)14. Hurley K. etal, Nature, 372, 652 (1994)15. Lapshov I. et al., these Proceedings (2001)16. Pellizzoni A. et al., these Proceedings (2001)17. Perotti F. et al., MM, in preparation (2001)18. Pittori C. et al., these Proceedings (2001)19. Schneid E.J. et al.,in AIP Conf. Proc. no. 384, p.253 (1996)20. Schoenfelder V. et al., ApJS, 86, 657 (1993)21. Tavani M., Phys. Rev. Letters, 76, 3478 (1996)22. Tavani M., et al., ApJ, 479, L109 (1997)23. Tavani M. et al., Proceedings of the 5th Compton Symposium, AIP Conf. Proceedings, ed. M.

McConnell, 2001, Vol. 510, p. 74624. Tavani M. et al., these Proceedings (2001)25. Tavani M. et al., in preparation (2001)26. Tavani, M., in preparation (2001).27. Thompson D.J. et al., ApJS, 86, 629 (1993)28. Vercellone S. et al., these Proceedings (2001)

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