SIGMA: The hard X-ray and soft gamma-ray telescope on board the GRANAT space observatory

Adv. Space Res.Vol. 11, No.8, pp. (8)289-(8)302, 1991 Printed in Great Britain. All rights reserved.

0273-4177/91 $0.00 + .50 Copyright © 1991 COSPAR

SIGMA: THE HARD X-RAY AND SOFT GAMMA-RAY TELESCOPE ON BOARD THE GRANAT SPACE OBSERVATORY J. Pau1,~P. Mandrou,** J. Ba11et,~’M. Cantin,* J. P. Chabaud,** B. Cordier,* M. Ehanno,** A. Goldwurm,* A. Lambert,* J. Landé,~ P. Laurent,* F. Lebrun,* J. P. Leray,* B. Ména,** M. Niel,** J. P. Roques,** G. Rouaix,** L. Salotti,* P. Souleile** and G. Vedrenne** *Se~iced’Astrophysique, Centre d’Etudes Nucléaires de Saclay, 91191 Gif-surYvette, France **Cent,.e d’Etude Spatiale des Rayonnements, 9, Avenue du Colonel Roche, BP 4346, 31029 Toulouse Cedex, France

ABSTRACT

The SIGMA telescope, the largest French scientific spacepayload ever launched, is one of the main devices aboard the Soviet astronomy satellite GRANAT, successfully launched on December 1, 1989 from Baikonour, USSR. This high-energy space—telescope of unprecedented size, has been designed to produce high-resolution images of the hard X-ray and soft gamma-ray sky, in the energy range from 35 Key to 1.3 MeV. After a comprehensive description of the instrument, a report is given on the most relevant characteristics of the telescope, including preliminary results from in-flight calibrations performed in the course of bright source observations. INTRODUCTION Since the early times of high-energy astronomy, satellite and balloon telescopes operating in the hard X-ray and soft gamma—ray domain, where focusing techniques become totally impracticable, have used the combination of wide—angle (few degrees) collimation and on/off source chopping to yield source fluxes and locations. Due to the poor angular

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resolution (typically one degree) of these techniques, firm identification of newly discovered high-energy celestial sources with known astronomical objects was possible only in the case where the emission has a clear time signature. At the beginning of the previous decade, it was recognized that one possible means of improving existing hard X-ray and soft gamma-ray telescopes is the incorporation of the codedaperture technique to actually image celestial sources. The primary advantage of such a technique is to maintain the angular resolution of a single pinhole camera, while increasing the overall effective area of the instrument. Moreover, the coded—mask principle includes the simultaneous measurement of the sky plus dectector background: systematic effects due to temporal variations in the background are minimized. This paper reports on the French SIGMA telescope, the cornerstone of the French—Soviet space-astronomy cooperation program, the first coded—aperture telescope sensitive to radiation in the energy range from 35 Key to 1.3 MeV to be operated in space. DESCRIPTION OF THE SIGMA TELESCOPE The SIGMA telescope, first proposed in June 1981, was constructed by two French laboratories (Service d’Astrophysique at Saclay, and Centre d’Etude Spatiale des Rayonnements at Toulouse), both under contract to CNES*. This hard X—ray and soft gamma—ray instrument of unprecedented size (weighing about one ton, it measures 3.50 m high and the diameter at the base is 1.20 m) features a coded mask, a position—sensitive detector (PSD), active and passive shielding devices, and the needful service modules. A schematic view of the instrument is shown in Figure 1. Instrumental concept The coded aperture is located 2.5 m from the PSD; it is an array of 49 x 53 square elements, whose basic pattern is a 29 x 31 Uniformly Redundant Array (URA), which is known to have ideal imaging properties /1/. The opaque 1.5 cm thick tungsten mask elements are bonded to a honeycomb plate that supports and stiffens the mask assembly without hindering the transparency of the open mask elements. The dimensions of the URA mask cell (9.4 mm x 9.4 mm) impose the following key properties of the telescope: the maximum sensitivity rectangular field of view: 4.3 deg. x 4.7 deg., surrounded by a wider field of -

*

Centre

Agency.

National

d’Etudes

Spatiales,

the

French

Space

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SIGMA

CODED MASK SQUARES OP TUNGSTEN SHIELDING (TIN. LEAD,TANTALUM)

HEAT

SHIELD

PLASTIC SCINTILLATOR ANTI-COINCIDENCE CIRCUIT OPTICAL HEAD ELLAR TECTOR

GAMMA CAMERA PHOTO •

MULTIPLIER MODULE

ELECTRONICS

Fig.

1. Exploded

view of the

SIGMA telescope

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decreasing sensitivity such that the half-maximum sensitivity boundary is a 10.9 deg. x 11.5 deg. rectangle, 2 central the total zone detection the size 794 cm rectangular of the area: PSD whose matches the basic 29 x 31 mask pattern, the intrinsic angular resolution: 13 arc mm., the point—source location accuracy: less than 2 arc mm. taking into account the PSD coding element size (1.175 mm x 1.175 mm). —

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—

The PSD design is based on the principle of the Anger cameras used in nuclear medicine. Scintillation flashes of light produced in a 57 cm diameter and 1.25 cm thick NaI(T1) crystal by the photon-induced electrons are detected by at least seven of the 61 hexagonal photomultiplier tubes (PNT5), mounted within a carbon-~fiberhoneycomb—structure, and optically coupled to the crystal via a 1.25 cm thick pyrex disk. The position of the centroid of the scintillation flash is derived from an analog comparison of the electric pulses produced by the PMTs having detected the scintillation flash. Two separate

calibration devices, both including a 241Am radioactive source embedded within a plastic scintiliator viewed by two PMTs, are mounted well above the PSD. Each 24i~ desintegration giving a 60 key photon also releases an alpha particle, which is immediately absorbed by the plastic scintillator and thus detected by the optically coupled PMTs. These tagged calibration—source photons enable continuous control of the gains of each of the 61 PMTs of the PSD. A thick active ánticoincidence shield, made of 31 independent CsI(Tl) crystal blocks, each optically coupled to 2 PMTs, surrounds the PSD and limits its field of view to about 1 Sr. The bottom shield consists of seven blocks 4 cm thick; the lateral shield is made of two rings of 12 blocks, the crystals of the lower ring are 4 cm thick, while those of the upper ring are 3 cm thick. The lateral area of this anticoincidence well reaches 19,200 cm2. A thin (5 mm) plastic scintillator, mounted within a diffusive box viewed by 4 PMTs, is located on top of the active shield well to veto the incoming charged particles. A passive graded shield (0.5 mm lead, 0.1 mm tantalum and 0.4 mm tin) is wrapped around the tube holding the mask in order to minimize the low—energy induced background. The service modules Three essential service modules (on—board computer, mass memory, star tracker) also belong to the experimental device. The whole telescope is regulated by the on-board computer, whose main functions are:

SIGMA

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the management of all experiment subsystems, including switch—on and switch—off procedures, the arrangement of the SIGMA telescope data flow throughout the mass memory; this function is also performred for other devices aboard the GRANAT spacecraft: the French gamma—ray burst detector array (the PHEBUS experiment) and two Soviet X—ray experiments (the ART-P and ART-S telescopes),, which all share the SIGMA mass memory, the management of the telecommand and telemetry links with the spacecraft, the on-line treatment of the stellar tracker data, to derive the spacecraft attitude drifts and to perform on-line corrections of the SIGMA data. —

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The on-board data recording system is a 128 megabits bubble memory, where all scientific and engineering data are recorded. The on—board computer software is also recorded within the memory, in such a manner that program instructions can be amended from ground via the telecommand link. When the telemetry link between the ground and the GRANAT spacecraft is activated, the whole content of the memory is downlinked to the ground station at a rate of 64 kilobits/sec. Two small optical telescopes are mounted in a parallel direction to the SIGMA axis. The main purpose of this optical head, operating as a star tracker, is to estimate the 3-axis components of the spacecraft attitude drifts, an unavoidable function since the pointing stability of the GRANAT spacecraft (of the order of 40 arc mm.) is much less accurate than the actual resolution which the SIGMA telescope requires. Each 4 seconds, the optical head data, once processed by the on—board computer, yields on—line estimates of the spacecraft attitude drifts (with an accuracy of the order of 10 arc sec.). For each photon recorded within the total detection area, the attitude drift estimates are converted into corrections applied to the event position coordinates as measured by the PSD*. SIGMA OPERATING MODES Since the counting rate within the total detection area is in excess of 400 counts/sec, the full data stream, including for each event position, energy and time informations, is such that the 128 megabit on-board memory would be entirely filled in only a few hours. In order to adjust the capacity of the memory to extended observation periods, SIGMA can operate in any one of several data—compression modes, which to the cyclical pattern of the URA mask, the attitude drift do not introduce any loss at the periphery of the images since the correction process also operates cyclically. *

Owing

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are selected on the basis of the scientific assigned to the telescope at a given time.

objective

The spectral-imaging mode (Sfl. This is the most frequently adopted operation mode. In the case of a typical observation period, during which the on—board memory is shared together with the Soviet GRANAT telescopes, the following data are simultaneously recorded: 6 series of 4 fine images (232 x 248 pixels) in 4 adjacent energy intervals, 3 series of 96 coarse images (116 x 124 pixels) including 95 images in adjacent energy bands varying roughly logarithmically in width over the whole SIGMA energy domain, plus one monitoring image of the whole PSD, comprising only 241~ source photon induced events; their integration time (about 8 hours for a one day observation) is twice that of the fine images, 42 to 48 monitoring 1024 channel spectra of all photons recorded within the total detection area*, one sequence of counting rate measurements within the total detection area, recorded every 4 sec. in the 4 fine image energy intervals, one sequence of attitude drift measurements, as derived every 4 sec. by the on—board computer from the stellar sensor data. —

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The variability-imaging mode (Vfl. Same as the SI mode, except that the 3 series of 96 coarse images are replaced by a series of 288 consecutive coarse images recorded within a commandable energy window. The photon by photon mode (Ph). For each photon detected within the total detection area, the position coordinates (as measured by the PSD in a 232 x 248 matrix), and the energy deposited (in 256 energy channels) are recorded. Time measurements are also recorded every 63 events with a 1 ms accuracy, as well as series of attitude drift measurements, as derived every 4 sec. by the on—board computer from the stellar sensor data. In such a mode, the capacity of the memory enables the storage of 5.2 x 106 photons, i.e. less than 4 hours of effective observation. The fast variability mode (VR’i. For each photon detected within the total detection area, only the energy deposit (in 128 energy channels) and the event datation (with a 1 ms accuracy) are recorded. In such a mode, the capacity of the memory enables of the order of 3 hours of effective observation. In order to enable in—flight calibration, 241~ source photon induced events are not vetoed, contrary to the case of fine and coarse images, except the special case of monitoring images. *

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The burst-imaging mode. This mode operates in parallel with one of both imaging modes. At the PSD level, 4 counting-rate measurements are performed simultaneously, combining the results of two measurements with distinct integration times (0.25 and 2 sec.) within two commandable adjacent energy windows. If one of the 4 measurements exceeds by more than 9 a its mean value (as measured over 64 sec.), the burst— imaging mode is triggered. For 32,256 photons detected within the total detection area (28,224 after the trigger event, 4,032 before), the position coordinates (as measured by the PSD in a 232 x 248 matrix), and the energy deposit (in 256 energy channels) are recorded. Time measurements are also recorded every 63 events with an accuracy of 1 ms. The burst-anticoincidence mode. This mode, described in detail in reference /2/, takes advantage of the large area of the lateral anticoincidence shield. As the burst-imaging mode, it also operates in parallel with one of both imaging modes. IN-FLIGHT PERFORMANCE Since the successful launch of the satellite into its nominal highly eccentric orbit, and after the necessary outgasing period of the various subsystems of the SIGMA telescope, more than two months of in—flight operations were required either to complete the telescope adjustment (a quite difficult task, taking into account the 131 PMTs of both PSD and active-shield devices) and to evaluate the inf light background. At the end of this tuning period, several Crab Nebula observations were performed to assess the in— f light performance of the telescope. In-orbit background Like the other GRANAT devices, SIGMA operates only when the spacecraft altitude is higher than 70,000 kin, i.e. outside the external radiation belts. During these 3 working days (over a total of 4 days for the whole orbit), the background is rather stable, except the first 10 hours during which a decreasing background excess is apparent in the low—energy channels, induced by de—activation of short—period radioactive nuclei. After the initial few orbits (i.e. after a few plunges into the proton belts where most of the activation occurs), the background counting rate over the whole SIGMA energy domain quickly reached a value of the order of 450 counts/sec. Afterwards, with the exception of sudden bursts induced by solar events, a slow increase of the background counting rate during the first months (up to 500 counts/sec) has been noticed. This has been followed by a significant decrease down to 430 counts/sec. at the present time.

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Such a long-term variation results from the evolution of the altitude of the orbit perigee: after a brief period of falling (from 2,000 km down to 1,000 1cm), the perigee is continuously rising (up to 3,000 km at the time of writing), in such a manner that, from orbit to orbit, the time spent within the most active fraction of the radiation belts becomes more and more reduced. So it is with the production of activation-induced short-period and long-period radioactive nuclei, whose decay contributes substantially to the overall in-flight background. Since the increase in perigee will continue (two years after launch, a perigee altitude of the order of 20,000 km is expected), the background counting—rate decrease should also continue, possibly down to the pre-launch predicted rate value of 320 counts/sec. Outcome of the first Crab Nebula images Work is still in progress to analyse the whole in-flight calibration material. However, the preliminary results yet available, confirm the aptitude of the telescope to perform accurate images of the sky in the hard X—ray and soft gamma— ray regime: even if the first SIGMA images, early obtained from the Crab Nebula observations (see Figure 2), resulted from a rather crude derivation of the data recorded by the PSD, their patent qualities indicate that the whole SIGMA concept is fully certified. In particular, their apparent sharpness proves that the on—board image corrections from spacecraft drifts, one of the most critical component involved in the image acquisition process, works quite perfectly. The most straightforward result that can be derived from these first images concerns the point—source localization accuracy of the telescope: the Crab Nebula is detected within a 2 arc mm. radius error box, a result in close agreement with the intrinsic properties of the telescope. In-flight sensitivity in the imaging modes The final quality of the images produced by the SIGMA telescope depends on both flight instrument characteristics and ground data processing abilities. A typical problem which involves both flight hardware and ground software relates to the non-uniformity of the PSD. The PSD design being that of a classical Anger camera, it suffers from inherent aberrations of the detector linearity, inducing a ±10%modulation in the case of a uniform exposure. Moreover, the in-flight background which results mainly from the decay of activation-induced radio-active nuclei, from unshielded diffuse celestial photons, and from cosmic—ray interactions, is not uniformly spatially distributed over the PSD: a perceptible center—to—rim background counting—rate decrease is observed, especially in the high—energy regime.

CRAB

NEBULA

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Fig. 2. Image of the 4.3 deg. x 4.7 deg. field of view containing the Crab Nebula, crudely derived from the fine images (232 x 248 pixels) acquired by SIGMA in. the energy interval 40-120 key during the first Crab Nebula observation.

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the image reconstruction is based on a cross— correlation method, both detector and background induced non-uniformities lead to serious signal—to—noise ratio losses. The mandatory correction of both effects requires Since

calibration

matrices

derived

either

from ground

calibrations

or from in-flight data. Under these conditions, detailed conclusions are waiting for dependable image deconvolution processes, including a precise knowledge of the spatial distribution of the PSD background in the full SIGMA energy domain. Therefore only preliminary estimates can be derived with regard to the telescope sensitivity in the imaging modes. First, the rather nominal background rate we observe (of the order of 450 counts/sec, to be compared with the expected rate of 320 counts/sec) leaves hope that the SIGMA sensitivity should not significantly differ from ground calibration estimates. However, the effective sensitive area of a coded-mask imaging—principle telescope is also function of the spatial resolution of the PSD. Preliminary in-flight -estimates of the spatial resolution yield 6 mm around 60 key and 4 mm around 120 key, to be compared with ground estimates, respectively 4 mm and 3 mm. Such a resolution loss, which may result from low-amplitude high-frequency noise oscillations at the PSD level, makes questionable the sensitivity evaluation as derived from ground calibration only, even if this resolution deviation does not manifest beyond 200 key. Since the Crab Nebula is recognized as a standard reference within the high-energy astronomy field, a more reliable estimate of the telescope sensitivity could be derived from the Crab Nebula observation data. In addition, in the case of strong sources like the Crab Nebula, the quality of the background corrections has a less sensitive impact on the goodness of the results. Figure 3 presents a preliminary estimate of the effective imaging sensitive area of the telescope, with the Crab Nebula as a brightness reference. The reference spectrum (also shown in Figure 3) is that measured by McConnell et p1. /3/ in the course of a Crab Nebula observation also performed with an imaging telescope. It is worth noting that in the energy intervall 40-120 key, the Crab Nebula is detected at the 30 a level in an effective observation time of 21 hours. Energy resolution Ground calibrations performed in the launch base just before the flight, have indicated that the PSD energy resolution is 16% at 60 Key, 12% at 122 Key, 8% at 511 KeV. However, it should be expected that as for the spatial resolution, the energy resolution is also disturbed in the low energy channels. Indeed, the analysis of the 60 key line induced by the on—board calibration sources, which is clearly detected in the PSD monitoring spectra, indicates that at low energy

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3. Sensitivity (at the 3 a level for a one day observation) of the SIGMA telescope in 4 energy bands, as derived from the Crab Nebula observations. The solid line is the Crab Nebula spectrum measured by McConnell et p1. /3/. Fig.

the in-flight PSD energy resolution is significantly worse than ground calibration measurements. In order to determine if the same tendency observed for the spatial resolution holds for the energy resolution (i.e. the fact that the resolution deviation does not manifest beyond 200 key), work is in progress to analyse the 511 key background line also clearly visible in the PSD monitoring spectra. However, this line is severely blended With several activation-induced lines, in such a way that only quite sophisticated treatments may yield reliable conclusions.

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TABLE 1 The SIGMA Observations Date

Target

Duration

Remarks

March 11 March 12 March 14 March 14 March 16 March 18 March 20 March 23 March 23 March 24 March 27 April 4 April 5 April 8 April 9 April 12 April 13 April 14 April 16 April 17 April 18 May 18 May 19 May 23 May 23 May 25 May 27 June 4 June 5 June 6 June 8 June 9 June 10 June 13 June 14 June 17 June 18

2CG 195+04 Crab Nebula Crab Nebula Crab Nebula PSR 1957+20 Cygnus X-3 Galactic Center Cygnus X-1 Cygnus X-l Galactic Center H 1657—416 GX 1+4 SMC X-1 Galactic Center A 0535+26 GX 5-1 3C 390.3 MCG 8—11—11 NCG 8—11—11 GX 5-1 14 82 2A 1219+305 NGC 7172 SN 1987a SN l987a 14 87 SN 1987 PKS 2155-304 M 87 14 87 14 87 NGC 4593 M 87 3C 279 3C 273 Cygnus X-l Cygnus X-3

23 30 3 42 6 40 30 3 29 27 30 29 30 29 29 24 23 28 23 4 22 3 35 3 24 6 20 11 8 22 22 22 22 5 29 8 24

in SI mode in SI mode in yR mode in SI mode safe mode break (1) strong solar flare no drift correction in yR mode in SI mode in SI mode in SI mode in SI mode in SI mode in SI mode in SI mode one PSD PMT of f in SI mode in SI mode strong solar flare safe mode break (1) in SI mode in Ph mode in SI mode in yR mode strong solar flare safe mode break (1) strong solar flare safe mode break (1) safe mode break (1) in SI mod~ in SI mode in SI mode in SI mode safe mode break (1) in SI mode safe mode break (1) no drift correction

Note

to

hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours hours

Table 1

(1) A safe mode break occurs as soon as the on—board software detects an abnormal process which may destroy vital data registered within the memory (as e.g. the onboard software codes). Several reasons may trigger the safe

mode

procedure,

including

electronic pertubations.

solar—flare

induced

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THE

aL

OBSERVING PROGRAM

The GRANAT spacecraft operations are organized in such a that the SIGMA telescope works for more than 4 orbits per month. Since spacecraft attitude shifts can be performed only when the telemetry link between the ground station and the spacecraft is activated (i.e. 3 to 4 times per orbit), a typical SIGMA observation duration ranges from 24 to 36 hours. The telescope observing program, established on a monthly basis, results from arm agreement between French and Soviet scientists, under the control of a French—Soviet Scientific Committee. manner

To ensure the largest flexibility, the monthly program is definitely frozen only one month in advance. In addition, changes to proposals can be submitted at the last moment if an outstanding target of opportunity turns up. Table 1 lists all observations already achieved from the end of the tuning phase. CONCLUSIONS Thanks to the remarkable performances of the Soviet and French space communities, the SIGMA telescope aboard the soviet GRANAT spacecraft has begun a valuable series of observations. The preliminary results yet available, confirming the aptitude of the telescope to perform images of the sky in the hard X-ray and soft gamma-ray regime with an unprecedented accuracy, appear already as a significant breakthrough in the field of high-energy astronomy. ACKNOWLEDGMENTS The authors are grateful to~the SIGMA Project Group of the CNES Toulouse Space Centre for their paramount contribution to the successful issue of the SIGMA mission. We gratefully acknowledge the personnel of the Lavotchine Space Company, of the Babakin Space Centre, of the Baikonour Space Centre and of the Evpatoria Ground Station for their unfailing support. We are grateful to our Soviet colleagues at the Space Research Institute in Moscow who have contributed to bring the SIGMA telescope into play. REFERENCES /1/ E.E. Fenimore and T.M. Cannon, ADplied Optics, 17, 337, (1978). /2/ A. Guerry, N. Jouret, J.P. Rogues, P. Laudet, P. Mandrou, M. Niel, J. Paul, Adv. Sp. Res.~, Vol 6, N° 4, 103 (1986). -

/3/ M.L. McConnell, P.P. Dunphy, D.J. Forrest, E.L. Chupp, A. Owens, Ap. J., 321, 543 (1987).