1 SUPERAGILE: THE X-RAY MONITOR OF THE AGILE GAMMA-RAY MISSION M. Feroci1, E. Costa1, E. Del Monte1, I. Lapshov1, M. Mastropietro1, E. Morelli2, M. Rapisarda3, A. Rubini1, P. Soffitta1, 1 G. Barbiellini4, F. Longo4, M. Prest4, E. Vallazza4, 0 A. Argan5, S. Mereghetti5, M. Tavani5, S. Vercellone5, and A. Morselli6 0 2 1Istituto di Astrofisica Spaziale, CNR, Rome, Italy 2 Istituto TESRE, CNR, Bologna,Italy n 3ENEA - Frascati, Italy a J 4INFN - Sezione di Trieste, Italy 5Istituto di Fisica Cosmica ”G. Occhialini”, CNR, Milan, Italy 0 1 6Univ. Roma ”Tor Vergata” and INFN - Sezione di Roma 2, Italy 1 v 4 5 ABSTRACT 2003inanequatorial∼100minutesorbit,foranom- 1 1 inal lifetime of 2 years. It will use the ASI base in 0 Malindi as a ground station. The satellite mass will 1 SuperAGILE is the X-ray stage of the AGILE be about 200 kg (∼65 kg of payload) and the power 0 gamma-ray mission. It is devoted to monitor X-ray available to the payload is about 65 W. The scien- / (10-40 keV) sources with a sensitivity better than tifictelemetrywillbeabletotransmitapproximately h 10 mCrabs in 50 ks and to detect X-ray transients 300 Mbit ofdata to groundat eachpassageoverthe p - in a field of view of 1.8 sr, well matched to that of Malindi ground station. o the gamma-ray tracker, with few arc-minutes posi- r tion resolution and better than 5 µs timing resolu- TheAGILEpayload(Mereghettietal. 2000)iscom- t s tion. SuperAGILE is designed to exploit one ad- posed of a Si tracker, containing 14 planes of Si mi- a ditional layer of four Si microstrip detectors placed crostrip detectors (121 µm pitch), interleaved with : v on top of the AGILE tracker, and a system of four tungstenlayersusedasaphotonpairconverter(each Xi mutually orthogonal one-dimensional coded masks layeris0.07radiationlength-X0). Atthebottomof to encode the X-ray sky. The total geometric area the Si tracker a mini-calorimeter - two planes of CsI ar is 1444 cm2. Low noise electronics based on ASIC bars, for a total on-axis radiation length of 1.5 X0 technology composes the front-end read out. We - is in charge of the total absorption of the created presentherethe instrumentalandastrophysicalper- pairs. The same detector, however, can be used for formances of SuperAGILE as derived by analytical independent detection and triggering of high energy calculations, Monte Carlo simulations and experi- (300 keV - 200 MeV) transient events. The hard X- mental tests on a prototype of the silicon microstrip ray section, SuperAGILE, is located on top of the detector and front-end electronics. Si-tracker. Allthe abovepartsaresurroundedby an anticoincidence made of a 6-mmthick (5-mmon the top shield), segmented plastic scintillator. Key words: X-rays; Instrumentation. 1. INTRODUCTION 2. THE SUPERAGILE ASSEMBLY AGILE(“AstrorivelatoreGammaadImmaginiLEg- SuperAGILE is basically composed of a Detection gero”,Tavanietal. 2000andBarbiellinietal. 2000) Plane (DP), a Collimator equipped with a Coded is the firstmissionof the ProgramofSmall Missions Mask,aFront-EndElectronicsandanInterfaceElec- of the Italian Space Agency (ASI). The main goal tronics (SAIE).Table 1resumesthe mainSuperAG- of AGILE is to monitor the gamma-ray sky in the ILE characteristics and Figure 1 shows its appear- energy range between 30 MeV and 50 GeV, with a ance(seealsoSoffittaetal. 2000foramoreextensive largefieldofview(∼3sr),goodsensitivity,goodan- description). gular resolution and good timing (dead time lower than 100 µs for the gamma-ray detector). The DP is composed of 4 detection units (DUs), placed on a single Al honeycomb plane support, so AGILE is scheduled for launch by the beginning of that two of them sample the X-direction and the 2 other two are devoted to the Y-direction. Each DU is composed of 4 Si microstrip tiles, bonded in pairs so that the effective length of each strip is approx- imately 19 cm. They are read-out through a set Table 1. Characteristics of SuperAGILE of IDE AS-XAA1 chips, based on ASIC technology, 12 for each of the DUs. The collimator (present Detector Type Silicon Strip baseline: 500 µm thick Carbon Fiber field separa- Basic Detection Unit 4 Si tiles, tors, coated with a 75 µm Gold layer) is mounted 19 cm x 19 cm on the same tray supporting the DP, and in turn Total geometric Area 1444 cm2 supports the 4 orthogonal, one-dimensional coded Nominal Energy Range 10-40 keV masks. The coded masks have been designed based On-axis Effective Area 320 cm2 (13 keV) on an Hadamardsequence, with a 50% coveringfac- Detector Strip Size 121 µm tor. They will be manufactured either in Gold or Detector Thickness 410 µm in Tungsten, 100 µm thick. The SAIE is in charge Energy Resolution (FWHM) ∼3-4 keV of interfacing SuperAGILE with the AGILE Data Timing Accuracy ∼5 µs Handling System, allowing an event-by-event trans- Collimator Materials 75 µm Gold-coated missionwithbetterthan5µstimingresolution. The Carbon Fiber energy information will be provided in the extended Mask Size 1444 cm2 energy range between 1 and 64 keV, with 64 chan- Mask-Detector Distance 14 cm nels, to allow for a finer threshold calibration at Mask Transparency 50% lowenergies,andexploitforcalibrationpurposesthe Tungsten fluorescences at ∼58 keV. Mask Material Gold or Tungsten Mask Thickness 100 µm Mask Element Size 242 µm The combined capabilities of the SAIE and the AG- ◦ ◦ Field of View (FWZR) 107 x 68 ILEDataHandling(seealsoMorsellietal. 2000)will On-axis Angular Resolution 5.9 arcmin allowthetransmissiontogroundofarelativelylarge Source Location Accuracy ∼1-2 arcmin setofscientifichousekeepingdata,includingrateme- for bright sources ters and detector images. In particular, the AGILE Data Handling will be able to perform a continuous automatic search for transient events (e.g., gamma- raybursts)ontimescalesfrom1msto100s. Oncea transient event is triggered onboard, the Data Han- dling will be able to provide attitude-corrected sky images for it, determining the location of the tran- sient source on the sky. The possibility to distribute in almost real time the coordinates of a transient event through a fast link (e.g., TDRSS or similar) is currently under study. 3. LABORATORY TESTS ON PROTOTYPES The most critical items for the SuperAGILE design were the signal-to-noise ratio at low energies (i.e., around 10 keV) and the power consumption. For this reasonextensivelaboratorytests havebeenper- formed on the ASIC chips planned to be used as front-end electronics: the XA1.3 chip, precursor of the XAA1 chip, especially developed by IDE AS for SuperAGILE. The results of these tests (see Del Monteetal. 2000foradetaileddiscussion)showthat theelectronicnoisecanbereducedtolessthan4keV (FWHM) and the power consumption to less than 1 mW per channel (SuperAGILE includes 6144 inde- pendent electronic channels). The same tests have shown a critical dependence of several chip charac- teristics (gain, offset, strip address and others) from Figure 1. Schematic view of the SuperAGILE struc- the temperature. ture 3 5 10 30 5 20 0 1 40 5 5 520 5 60 40 5 0 20 350 10 5 Figure 2. Effective area at 13 keV of 2 crossed Su- Figure 3. 50 ks sensitivity of one SuperAGILE de- perAGILE detector units. Numbers give the area in tector unit, as a function of the off-axis angle, in the cm2 for the individual detector. energy range 10-40 keV. 4. SENSITIVITY we show the typical energy spectrum of the X-ray binary pulsar Her X-1, as observed by BeppoSAX near the maximum of the 35-day cycle (Dal Fiume We studied the sensitivity and expected astrophysi- et al., 1998),showing that SuperAGILE will be able calperformancesby meansofanalyticalcalculations to provideaccurateenergyspectra forthis andsimi- andMonteCarlosimulations. InFigure2wepresent larsources,alsowhenitgoestoitsperiodicminimum the map of the SuperAGILE effective area for two (a factor ∼3 fainter). As a result of the long point- individual DUs over their field of view (FOV), for a ings (∼2 weeks) driven by the gamma-ray tracker, monochromatic 13.1 keV photon beam (this energy SuperAGILE will point the same source(s) for the corresponds to the peak in the area vs. energy rela- samelongtime,thusprovidingbothaccurateenergy tion). Theareaoftwoorthogonalunits is presented, spectra over such a long integration time as well as showingtheoverlapoftheirFOV,providinganeffec- a monitoring of the fluxes and energy spectra over tive bi-dimensionalsource locationcapability within much shorter timescales. In Figure 4 we also show ◦ ◦ the central60 x60 partofthe FOV,inadditionto the observed energy spectra (in a flare and in a dip the one-dimensional location capability for the fur- state) of the Galactic microquasar GRS 1915+105 ther∼20◦. Themaximumeffectiveareaatthecenter (Feroci et al. 1999). Also in this case the Super- of the FOV is ∼80 cm2, thus giving a maximum of AGILE sensitivity is perfectly suited to allow both 2 320cm whenthefourunitsareconsideredtogether. spectralandtemporalvariabilitystudies. Itisworth noticing that the galactic microquasars are among InFigure3the5-σsensitivityispresentedforsources the primary targets also for the gamma-ray tracker with Crab-like energy spectra, for an integration andwilllikelybe pointedseveraltimes overtheAG- time of 50 ks, over the full 10-40 keV energy range. ILE lifetime. The sensitivity is presented as a function of the source location within the field of view, along the In Figure 5 we show the SuperAGILE capability in two directions. It is worth noticing the wide central studying fast hard X-ray transients, as the short re- part of the FOV with slowly variable sensitivity, al- current bursts from the Soft Gamma-ray Repeaters lowing for an excellent use of most of the FOV for (e.g., Aptekar et al. 2000). The plot clearly shows source monitoring purposes. thatalsoin250msSuperAGILEcanprovidedetailed energyspectraofsuchevents. Thewidefieldofview of SuperAGILE is very well suited for a monitoring 5. EXPECTED ASTROPHYSICAL of the activity of these sources,that are mostly con- PERFORMANCES centrated towards the galactic center, thus allowing to trigger pointed observations when they undergo periods of intense bursting activity. Furthermore, Inthissectionwepresenttheexpectedperformances the giantflares fromthese sources(e.g., Feroci et al. ofSuperAGILEforfewinterestingclassesofhardX- 2000)arevery goodcandidates for emissionof rapid raysources. InFigure4 weshowthe sensitivity(5-σ and intense flashes ofgamma-raysand SuperAGILE in 50 ks) as a function of energy. On the same plot canprovidethe Trackerwithanaccuratepositionof 4 Konus 980607a GRS 1915+105 (Flare) Konus 981031a GRS 1915+105 (Dip) Figure4. On-axissensitivityof SuperAGILE(5-σ in 50 ks) compared to the typical energy spectra of the two Galactic sources HerX-1(from Dal Fiumeet al. Figure 5. On-axis sensitivity of SuperAGILE (5- 1998) and GRS 1915+105 (from Feroci et al. 1999). σ in 250 ms) compared to two short bursts from the soft gamma-ray repeater SGR 1900+14 (Aptekar et a possible new soft gamma-ray repeater manifesting al. 2000) itself with a giant flare, as the source of the 1979 th March 5 event did. Finally,Figure6showsthesourcelocationcapability ofSuperAGILEforgamma-raybursts. Theplotsim- ulates the sky image of the SuperAGILE detection of GRB 980425, a relatively weak and soft gamma- ray burst in the BeppoSAX sample. Although the ◦ simulation assumes an event at 15 off-axis in both X and Y directions, the detection is highly signifi- cant and allows for a very good source location de- termination,ascanbe seenbythe right-handpanels where the sky image is zoomed-in. This capability motivated the set-up of the onboard triggering and sourcelocalizationsystemforSuperAGILE(seeSec. 2). REFERENCES Aptekar, R., et al. 2000,astro-ph/0004402 Barbiellini, G., et al. 2000, Proc. Fifth Compton Symposium, AIP 510, p.750 Dal Fiume, D., et al. 1998, A&A 329, L41 Del Monte, E., et al. 2000, Proc. SPIE conference 4140 Feroci, M., et al. 1999, A&A 351, 985 Figure 6. Simulation of the detection of the rela- Feroci, M., et al. 2000, ApJ in press (astro- tivelyweakandsoftgamma-rayburstGRB980425at ph/0010494) ◦ 15 off-set in both X and Y directions. The left pan- Mereghetti, S., et al. 2000, These proceedings els showhowtheeventis detectedintheskyimagein Morselli,A.,etal.2000,Proc.SPIEconference4140 the two orthogonal directions. The right panels show Soffitta,P.,etal.2000,Proc.SPIEconference4140 an enlargement of the same images. Tavani, M., et al. 2000,Proc. Fifth Compton Sym- posium, AIP 510, p.746