SuperAGILE: the X-ray Monitor of the AGILE gamma-ray Mission

4 Pages·0.29 MB·English

by  M. Feroci

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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
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2 1Istituto di Astrofisica Spaziale, CNR, Rome, Italy
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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

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v
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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