Physics The on-ground calibration of AGILE satellite and the study of astrophysical sources

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1 Scientifica Acta 2, No. 2, (2008) Physics The on-ground calibration of AGILE satellite and the study of astrophysical sources Fabrizio Boffelli Dipartimento di Fisica Nucleare e Teorica, Università di Pavia, Via Bassi 6, Pavia, Italy Istituto Nazionale di Fisica Nucleare, INFN Pavia, Via A. Bassi 6, Pavia, Italy fabrizio.boffelli@pv.infn.it Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF/IASF Milano, Via E. Bassini 15, Milano, Italy fabrizio@iasf-milano.inaf.it AGILE is an Italian Space Agency mission dedicated to the observation of the gamma-ray Universe. The AGILE very innovative instrumentation combines for the first time a gamma-ray imager together with a hard X-ray imager. It is sensitive simultaneously to photons in the ranges 30 MeV - 50 GeV and kev. AGILE s GRID (Gamma Ray Imaging Detector) was calibrated at the INFN National Laboratories in Frascati. A beam of gamma-ray photons in the energy range MeV was produced by Bremsstrahlung of electrons and tagged by a dedicated set-up in the Beam Test Facility, based on the measurement with silicon strip detectors of the electron trajectory in a magnetic field. The comparison with Montecarlo simulations allowed to calculate PSF and Effective Area. AGILE was successfuly launched on April 23, AGILE provides crucial data for the study of AGN, GRBs, pulsars, unidentified gamma-ray sources, Galactic compact objects, supernova remnants, TeV sources and fundamental physics by microsecond timing. 1 Introduction Gamma-ray astrophysics above 100 MeV is an exciting field of astronomical sciences that received a strong impulse in recent years. Detecting cosmic gamma-ray emission in the energy range from a few tens of MeV to a few tens of GeV is possible only from space instrumentation, and in the past 20 years several space missions confronted the challenge of detecting cosmic gamma-rays. Gamma-ray emission from cosmic sources at these energies is intrinsically non-thermal and provides a diagnostic of particle acceleration and radiation processes in extreme conditions. AGILE is expected to substantially advance our knowledge in several research areas including the study of Active Galactic Nuclei and massive black holes, Gamma-Ray Bursts (GRBs), the unidentified gammaray sources, Galactic transient and steady compact objects, isolated and binary pulsars, pulsar wind nebulae (PWNae), supernova remnants, TeV sources, and the Galactic Center while mapping the overall gammaray emission from our Galaxy. Furthermore, the fast AGILE electronic readout and data processing (resulting in detector deadtimes smaller than 200 µsec) allow to perform, for the first time, a systematic search for sub-millisecond gamma-ray/hard X-ray transients that are of interest for both Galactic compact objects (searching outburst durations comparable with the dynamical timescale of 1M compact objects) and quantum gravity studies in extragalactic sources. Calibration of the AGILE Scientific Instrument is essential for the interpretation of its results during the space mission. The instrument effective area, its angular resolution and its energy resolution must be determined through a number of calibration runs aimed at establishing the instrument performance by exposing it to beams of photons, protons and electrons of known energy and direction. The calibration of the AGILE Gamma-Ray Imaging Detector (GRID) will be carried out at the BTF (Beam Test Facility) of LNF (Laboratori Nazionali di Frascati) of INFN.

2 142 Scientifica Acta 2, No. 2 (2008) 2 The AGILE satellite 2.1 The AGILE mission The space program AGILE (Astro-rivelatore Gamma a Immagini Leggero) is a high-energy astrophysics Mission supported by the Italian Space Agency (ASI) with scientific and programmatic participation by INAF, INFN, CNR, ENEA and several Italian universities [1 4]. The main industrial contractors include Carlo Gavazzi Space, Thales-Alenia-Space (formerly Laben), Rheinmetall Italia (formerly Oerlikon- Contraves), Telespazio, Galileo Avionica, and Mipot. The main scientific goal of the AGILE program is to provide a powerful and cost-effective mission with excellent imaging capability simultaneously in the 30 MeV-50 GeV and kev energy ranges with a large field of view that is unprecedented in highenergy astrophysics space missions. AGILE was successfully launched by the Indian PSLV-C8 rocket from the Sriharikota base on April 23, The AGILE instrument design is very innovative and based on solid state Silicon detector technology and state-of-the-art electronics and readout systems developed in Italian laboratories [5 7]. The instrument is light ( 100 kg) and very compact; the total satellite mass is about 350 kg. 2.2 The scientific instrument Super AGILE ST AC (a) (b) MCAL Fig. 1: (a) The integrated AGILE satellite in its final configuration being covered by the thermal blanket. The total satellite mass is equal to 350 kg. (b) The AGILE scientific instrument showing the hard X-ray imager, the gamma-ray Tracker, and Calorimeter. The Anticoincidence system is partially displayed, and no lateral electronic boards and harness are shown for simplicity. The AGILE instrument "core" is approximately a cube of about 60 cm size and of weight approximately equal to 100 kg. The AGILE scientific payload is made of three detectors surrounded by an Anticoincidence system, all combined into one integrated instrument with broad-band detection and imaging capabilities. A dedicated Data Handling system completes the instrument. Fig. 1(a) shows the integrated AGILE satellite and Fig. 1(b) the schematic view of the instrument. We summarize here the main characteristics of the instrument: several papers describe the individual detectors in detail [8 11]. The Gamma-Ray Imaging Detector (GRID) is sensitive in the energy range 30 MeV - 50 GeV, and consists of a Silicon-Tungsten Tracker, a Cesium Iodide Calorimeter, and an Anticoincidence system. The GRID trigger logic and data acquisition system (based on Anticoincidence, Tracker and Mini-Calorimeter

3 Scientifica Acta 2, No. 2 (2008) 143 information) allow an efficient background discrimination and inclined photon acceptance [12, 13]. The GRID is designed to achieve an optimal angular resolution (source location accuracy 6 12 for intense sources), a very large field-of-view ( 2.5 sr), and a sensitivity comparable to that of EGRET (AGILE s predecessor [14]) for sources within degree from the main axis direction (and substantially better for larger off-axis angles). The hard X-ray Imager (Super-AGILE) is a unique feature of the AGILE instrument. The imager is placed on top of the gamma-ray detector and is sensitive in the kev band. A Mini-Calorimeter operating in the "burst mode" is the third AGILE detector. It is part of the GRID, but also capable of independently detecting GRBs and other transients in the 350 kev - 50 MeV energy range with excellent timing capabilities. 2.3 The Silicon Tracker The silicon tracker provides the γ-ray imaging. It is based on the photon conversion into e + -e pair, with the subsequent detection and reconstruction of the e + and e tracks and interaction vertex, allowing to determine the photon energy and direction[9]. It is basically a telescope made of 12 squared trays of cm 2 area and 1.9 cm spaced out. Each tray is composed of 2 Si microstrip detectors, configured to provide orthogonal x-y coordinates, and a Tungsten layer 245 µm thick (except for the last 2 trays), that acts as photon converter with 0.8 X 0 in total. Each Silicon detector layer is made of 4 4 tiles of area cm 2 and thickness 410 µm, with 768 strips each one. The readout system is based on interleaved "readout" and "floating" strips, providing 384 readout channels with pitch equal to 242 µm for each tile. The Si tiles are arranged in groups of 4 with their strips electrically connected together, obtaining a total of 1536 readout strips for each Si layer (x and y oriented). By mean of analogic readout, a spatial resolution as good as 40 µm is achieved. The total number of readout channel of the GRID is The on-ground calibration of AGILE Calibration of any astronomical instrument is essential to the interpretation of its results of in-flight observations. This must be achieved through simulations, since it is not possible to learn every type of data for a very long time; in other words, it is not possible to perfectly recreate the real situation in which the satellite will operate when it is launched. The goal of calibrating AGILE is to estimate the instrument response function by means of exposure to gamma-ray beams. The ideal beam has to provide a flux of photons with properties (energy and direction) that are known to an accuracy much better than the resolving power of the instrument to be calibrated. 3.1 GRID calibration using the "tagged" γ beam at BTF The calibration of the GRID has been performed using a "tagged" γ beam prepared at Beam Test Facility (BTF) of INFN Frascati Laboratory (LNF) (Fig. 2) [15]. In the used setup some fixed energy e beam bunches, extracted from the LNF accelerator (DAΦNE) at a rate of 49 Hz, are sent on a multiple Si fixed target, made of two couples of Si single face detector with 410 µm thickness and cm 2 area. Each detector has 384 strips with pitch of 228 µm. This target has the double function to give a signal for every crossing e beam bunch and to cause the emission of photons through Bremsstrahlung mechanism (see Fig. 3(a)). The electrons of the beam are then bended using a magnetic dipole and sent to a beam dump. The electrons that produce photons by Bremsstrahlung lose a part of their momentum, and their curvature radius inside the magnet becomes smaller. Hence they are deflected on the "tagging" detector (made of Si microstrip tiles) placed inside the bending magnet. The position of their impact on the "tagging" detector depends on the curvature radius, and hence on the residual energy of the electron, from which the

4 144 Scientifica Acta 2, No. 2 (2008) Fig. 2: The BTF experimental setup where AGILE s GRID was calibrated. non interacting electrons Bremsstrahlung photon bending magnet Si target impact of reduced momentum electron tagging detector BTF e beam (a) (b) Fig. 3: (a) Schematic draw of the tagged γ beam realized at BTF of INFN Frascati laboratory to calibrate GRID. (b) The whole BTF experimental apparatus and AGILE Payload simulated by Montecarlo. Bremsstrahlung photon energy can be determined. The complete AGILE satellite was mounted on a specially designed mechanical support (MGSE), allowing to set any position and direction of the instrument with respect to the tagged photon beam. A beam of gamma-ray photons in the energy range MeV was produced by Bremsstrahlung of electrons and tagged by the dedicated set-up. About tagged events was accumulated for several incidence directions and instrument configurations. Both the GRID spectral and PSF response were carefully studied and compared with results of extensive Montecarlo simulations [16]. These took place through the use of GEANT 3.21, and took into consideration the whole BTF experimental apparatus (with tagging system structures) and AGILE payload (Fig. 3(b)). Furthermore, the leptonic background was studied by using the direct electrons and positrons beams interacting with the AGILE GRID for different geometries. A sequence of runs was obtained for both direct incidence on the instrument as well as for

5 Scientifica Acta 2, No. 2 (2008) 145 events originating by interactions with the spacecraft. 3.2 Measurement of the GRID angular resolution and Effective Area One of the most relevant measures performed during the calibration [16] is the Point Spread Function (PSF), defined as the angular aperture of the cone containing the 68 % of the reconstructed γ tracks, around the "true" direction given by the tagged γ beam. The PSF value depends on the γ direction relative to the telescope axis (defined by θ and φ angles), as shown in Fig. 4. For on-axis γs (θ = 0 ) the PSF is about 2.5. Fig. 4: PSF measurement. For each pair of θ and φ angles (with respect to the GRID axis) the circles represent the angular aperture of the cone containing the 68 % of the reconstructed γs, both for BTF data (red) and MC simulation (blue). All the scales are in degrees. θ = 0 o - Filter F4 Eff. Area (cm 2 ) Gamma Limbo Single Particle Total Energy (MeV) Fig. 5: Effective area depending on the energy, for an on-axis observation (θ = 0 ) with F4 filter. An interesting result [16] has been obtained for the calculation of the Effective Area. For the determination of the Effective Area it is important to know the filters (i.e. the algorithms giving θ, φ direction, energy

6 146 Scientifica Acta 2, No. 2 (2008) and type of events reconstructed in Silicon Tracker [17]) behaviour, depending on the energy used. The effective area indeed changes in relation to the filters taken into account. An exemple is shown in Fig. 5. The obtained estimated results are (with energy greater than 100 MeV): 500 cm 2 for θ = 0 ; 400 cm 2 for θ = 30 ; 350 cm 2 for θ = 50 ; the relative error is ±10%. 4 Study of astrophysical sources The AGILE program is motivated by very specific scientific requirements and goals. The essential point, that permeates the whole mission from its conception, is to provide a very effective gamma-ray space instrument with excellent detection and imaging capabilities both in the gamma-ray and hard X-ray energy ranges. The very stringent mission constraints (satellite and instrument volume, weight, cost, and optimized ground segment) determined, from the very beginning of the mission development, a specific optimization strategy. The AGILE mission challenge lays into an optimal gamma-ray/hard X-ray detection capability (together with excellent timing resolution in the energy band near 1 MeV) with a very light ( 100 kg) instrument. The AGILE instrument has been therefore designed and developed 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 for both the gamma-ray imager (2.5 sr, that is 1/4 of the entire sky for each pointing, i.e., FOV 5 times larger than that of EGRET) and the hard X-ray imager (1 sr); excellent timing capability, with overall photon absolute time tagging uncertainty of 2 µs coupled with very small deadtimes for gamma-ray detection ( 200 µs for the Si-Tracker and 20 µs for each of the individual CsI bars); a good sensitivity for pointlike gamma-ray and hard X-ray sources. Depending on exposure and background, after a 1-year program the flux sensitivity threshold reached values of (10 20) 10 8 photons cm 2 s 1 at energies above 100 MeV. The hard X-ray imager sensitivity is between 15 and 50 mcrab at 20 kev for a 1-day exposure over a 1 sr field of view. good sensitivity to photons in the energy range MeV, with an effective area above 200 cm 2 at the 30 MeV; a rapid response to gamma-ray transients and gamma-ray bursts, obtained by a special quicklook analysis program and coordinated ground-based and space observations; accurate localization ( 2 3 arcmins) of GRBs and other transient events obtained by the GRID-SA combination (for typical hard X-ray transient fluxes above 1 Crab); the expected GRB detection rate for AGILE is 1 2 per month; long-timescale continuous monitoring ( 2 3 weeks) of gamma-ray and hard X-ray sources; satellite repointing within 1 day after special alerts. The simultaneous hard X-ray and gamma-ray observations represent an innovative approach to the study of high-energy sources. We summarize here the main AGILE s scientific objectives: Active Galactic Nuclei. For the first time, simultaneous monitoring of tens of potentially gammaray emitting AGNs during each pointing will be possible. Several outstanding issues concerning the

7 Scientifica Acta 2, No. 2 (2008) 147 mechanism of AGN gamma-ray production and activity can be addressed by AGILE including: (1) the study of transients vs. low-level gamma-ray emission and duty cycles [18]; (2) the relationship between the gamma-ray variability and the radio-optical-x-ray-tev emission; (3) the possible correlation between relativistic radio plasmoid ejections and gamma-ray flares; (4) hard X-ray/gamma-ray correlations. A program for joint AGILE and ground-based monitoring observations is being planned. On the average, AGILE will achieve deep exposures of AGNs and substantially improve our knowledge on the low-level emission as well as detecting flares. We conservatively estimated that for a 3-year program AGILE will detect a number of AGNs 2-3 times larger than that of EGRET. Super-AGILE will monitor, for the first time, simultaneously AGN emission in the gamma-ray and hard X-ray ranges. Gamma ray-bursts. A few GRBs were detected by the EGRET spark chamber during 7 years of operations. This number was limited by the EGRET FOV and sensitivity and not by the GRB emission mechanism. Owing to a larger FOV, the GRB detection rate by the AGILE-GRID is expected to be at least a factor 5 larger than that of EGRET, i.e., 2 5 events/year). Furthermore, the small GRID deadtime ( 500 times smaller than that of EGRET) allows a better study of the initial phase of GRB pulses (for which EGRET response was in many cases inadequate). The remarkable discovery of delayed gamma-ray emission up to 20 GeV from GRB [19] is of great importance to model burst acceleration processes. AGILE is expected to be highly efficient in detecting photons above 10 GeV because of limited backsplashing. The hard X-imager Super-AGILE will be able to locate GRBs within a few arcminutes, and will systematically study the interplay between hard X-ray and gamma-ray emissions. Special emphasis is given to fast timing allowing the detection of sub-millisecond GRB pulses independently detectable by the Si-Tracker, Mini-Calorimeter and Super-AGILE. Diffuse Galactic and extragalactic emission. The AGILE good angular resolution and large average exposure further improves our knowledge of cosmic ray origin, propagation, interaction and emission processes. A detailed gamma-ray imaging of individual molecular cloud complexes is possible. We also note that a joint study of gamma-ray emission from MeV to TeV energies is possible by special programs involving AGILE and new-generation TeV observatories of improved angular resolution. Gamma-ray pulsars and PWNae. AGILE will contribute to the study of gamma-ray pulsars (PSRs) in several ways: (1) improving timing and lightcurves of known gamma-ray PSRs; (2) improving photon statistics for blind gamma-ray period searches of pulsar candidates; (3) studying unpulsed gamma-ray emission from plerions in supernova remnants and searching for time variability of pulsar wind/nebula interactions, e.g., as in the Crab nebula and in the Galactic sources recently discovered in the TeV range [20]. Particularly interesting for AGILE are the 30 new young PSRs discovered in the Galactic plane by the Parkes survey [21]. Search for non-blazar gamma-ray variable sources in the galactic plane, currently a new class of unidentified gamma-ray sources such as GRO J [22]. Compact Galactic sources, micro-quasars, new transients. A large number of gamma-ray sources near the Galactic plane are unidentified, and sources such as 2CG 135+1/LS I can be monitored on timescales of month/years. Cyg X-1 is also monitored and gamma-ray emission above 30 MeV will be intensively searched. Also galactic X-ray jet sources (such as Cyg X-3, GRS , GRO J and others) can produce detectable gamma-ray emission for favorable jet geometries, and a TOO program is planned to follow-up new discoveries of micro-quasars. Supernova Remnants (SNRs). Several possible gamma-ray source-snr associations were proposed based on EGRET data [23]. However, none are decisive. High-resolution imaging of SNRs

8 148 Scientifica Acta 2, No. 2 (2008) in the gamma-ray range can provide the missing information to decide between leptonic and hadronic models of SNR emission above 100 MeV. Fundamental Physics: Quantum Gravity. AGILE detectors are suited for Quantum Gravity (QG) studies. The existence of sub-millisecond GRB pulses lasting hundreds of microseconds [24] opens the way to study QG delay propagation effects by AGILE detectors. Particularly important is the AGILE Mini-Calorimeter with photon-by-photon independent readout for each of the 30 CsI bars of small deadtime ( 20 µs) and absolute timing resolution ( 3 µs). Energy dependent time delays near 100 µs for ultra-short GRB pulses in the energy range MeV can be detected. If these GRB ultra-short pulses originate at cosmological distances, sensitivity to the Planck s mass can be reached by AGILE. About some of these themes (in addition to those regarding AGILE s instrumentation and software) articles by AGILE Team have been recently published, some have to appear, others have been submitted or will be soon and others are in preparation. Many ATEL (Astronomical TELegrams) have been posted, based on recent detections and observations of AGILE. See [25 53]. The AGILE Science Program overlaps and is complementary to those of many other high-energy space Missions (INTEGRAL, RXTE, XMM-Newton, Chandra, SWIFT, Suzaku, GLAST) and ground-based instrumentation (radio telescopes, optical observatories, TeV observatories). The AGILE Science Program potentially involves a large astronomy and astrophysics community and emphasizes a quick reaction to transients and a rapid communication of crucial data allowing fast follow-up observations. References [1] M. Tavani et al., AGILE Phase A Report (1998). [2] M. Tavani et al. in: M. McConnel, Proceedings of the 5th Compton Symposium, AIP Conference Proceedings 510, 746, (2000). [3] M. Tavani et al., Texas in Tuscany, XXI Symposium on Relativistic Astrophysics, Florence, Italy, 9-13 December 2002, 183, (2003). [4] M. Tavani, F. Boffelli et al., Nuclear Instruments and Methods A 588, 52, (2008). [5] G. Barbiellini et al., Nuclear Physics B 43, 253, (1995). [6] G. Barbiellini et al., Nuclear Instruments and Methods 354, 547, (1995). [7] A. Bakaldin et al., Astroparticle Physics 109, (1997). [8] F. Perotti et al., Nuclear Instruments and Methods A 556, 228, (2005). [9] M. Prest et al., Nuclear Instruments and Methods A 501, 280, (2003). [10] M. Feroci M. et al., Nuclear Instruments and Methods A 581, 728, (2007). [11] C. Labanti et al., in: Proceedings of SPIE, , (2006). [12] M. Tavani, F. Boffelli, submitted to Nuclear Instruments and Methods, (2008). [13] A. Argan, F. Boffelli et al., to be submitted to Astronomy & Astrophysics, (2008). [14] R. C. Hartman et al., Astrophysical Journal Series , (1999). [15] M. Tavani et al., Nuclear Instruments and Methods A 572, 474, (2007). [16] F. Boffelli et al., in preparation. [17] A. Bulgarelli, F. Boffelli et al. in preparation. [18] S. Vercellone et al., Montly Notices of the Royal Astronomical Society 353, 890, (2004). [19] K. Hurley et al., Nature 372,652, [20] F. Aharonian et al., Astrophysical Journal 636, 777, (2006). [21] M. Kramer et al., Montly Notices of the Royal Astronomical Society 342, 1299, (2003). [22] M. Tavani et al., Astrophysical Journal 479, L109, (1997). [23] S.J. Sturner et al., Astronomy & Astrophysics 293, 17, (1995). [24] C.L. Bhat et al., Nature 359, 217, (1992). [25] F. Longo, F. Boffelli et al. in: Proceedings of the Santa Fe, AIP Conference Proceedings, 2007, 1000, 523, (2008).

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