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Two years of successful operations of the coded-mask telescope SIGMA for hard X-ray and soft γ-ray astronomy
Acta A s t r o . a u t i c a Vol. 30, pp. 2 6 1 - 2 6 9 , 1993
0 0 9 4 - 5 7 6 5 / 9 3 $ 6 . 0 0 + 0.00 P e r g a m o n Press Ltd
Printed in Great Britain
TWO
YEARS
OF
SUCCESSFUL OPERATIONS OF THE CODED-MASK TELESCOPE FOR HARD X-RAY AND SOFT y-RAY ASTRONOMY
Service
9, Avenue
SIGMA
Jacques Paul and Frangois Lebrun d'Astrophysique, Centre d'Etudes de Saclay 91191 Gif-sur-Yvette Cedex, France
Pierre Mandrou and Jean-Pierre Roques Centre d'Etude Spatiale des Rayonnements du Colonel Roche BP 4346, 31029 Toulouse Cedex, Abstract
The
SIGMA
France
Telescope
At the beginning of the previous decade, it was recognized that a possible means of improving existing hard X-ray and yray telescopes, where focusing techniques are totally impracticable, is the incorporation of the c o d e d - a p e r t u r e 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 and detector background, systematic effects due to temporal variations in the b a c k g r o u n d are removed.
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 the Centre National d'Etudes Spatiales (CNES), the French Space Agency. This hard X-ray and soft y-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.
This paper reports on the French SIGMA telescope, the first c o d e d - a p e r t u r e telescope sensitive to radiation in the energy range from 35 KeV to 1.3 MeV to be operated in space. The SIGMA telescope is one of the main devices on board the astronomy satellite GRANAT, successfully launched on December i, 1989 from Baikonour, Kazakhstan. After a comprehensive description of the instrument, a report is given on the most relevant inferences which can be drawn from two years of successful inorbit operations, in order to better define the next generation of y-ray instruments.
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. 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 key properties of the telescope, such as the m a x i m u m sensitivity field (4.3 deg. x 4.7 deg.), surrounded by a wider field of decreasing sensitivity, the tQ-
Instrumental
Copyright © 1992 by the IAF. All rights reserved 261
concept
262
43rd M F Congress
tal detection area, which is the 794 cm 2 central rectangular zone of the PSD whose size matches the basic 29 × 31 mask pattern, the intrinsic anuular resolution (13 arc min. ) , the point-sourG~ location accuracy, which is less than 2 arc min. taking into account the PSD coding element size. The PSD design is based on the principle of the Anger cameras used in nuclear medicine. S c i n t i l l a t i o n flashes of light produced in a 57 cm diameter and 1.25 cm thick NaI(TI) crystal by the photon induced electrons are detected by at least seven of the 61 hexagonal photomultiplier tubes (PMTs), mounted within a carbon-fiber honeycomb-structure, and optically coupled to the crystal via a 1.25 cm thick pyrex disk. Two separate calibration devices, both including a 24~Amradioactive source embedded within a plastic scintillator viewed by two PMTs, are m o u n t e d well above the PSD. Each 2'~Am d i s i n t e g r a t i o n giving a 60 keV photon also releases an particle, which is immediately a b s o r b e d by the scintillator and thus d e t e c t e d 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 anticoincidence shield, made of 31 independent CsI(TI) 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 u p p e r ring are 3 cm thick. The lateral area of this anticoincidence well reaches
19,200 cm 2. 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.i mm tantalum and 0.4 mm tin) is wrapped around the tube holding the mask in order to prevent the entrance of off-axis low-energy y rays contributing to the background. The service
modules
Three major 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 the management of all experiment subsystems, the arrangement of the SIGMA telescope data flow throughout the mass memory, the management of the t e l e c o m m a n d and telemetry links with the spacecraft, and 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. The on-board data recording system is a 128 megabit bubble memory, where all scientific and e n g i n e e r i n g 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 d o w n l i n k e d to the ground station at a rate of 64 kilobits s "~. Since the counting rate within the total detection area is in excess of 400 counts s "~, the full data stream, including for each event position, energy and time informations, is such
43rd ~ F Congress
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, w h i c h are selected on the basis of the scientific objectives assigned to the telescope at a given time. Two small optical telescopes are m o u n t e d in a direction parallel 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 min.) is much less accurate than the actual resolution which the SIGMA telescope requires. Every 4 sec., the optical head data, once p r o c e s s e d by the on-board computer, yields on-line estimates of the spacecraft attitude drifts (with an accuracy of the order of i0 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 m e a s u r e d by the PSD.
SIGMA In-orbit
behavior
Since the successful launch of the satellite into its nominal h i g h l y - e c c e n t r i c 4-day orbit (perigee : 2000 km, apogee : 200 000 km), more than two years of in-flight operations have been achieved, leading to a precise estimate of the in-orbit p e r f o r m a n c e s of the instrument and of its long-term evolution.
M 30-R
In-orbit
263
backaround
The analysis of the background spectrum, which is continuously measured in the form of 1024 channel monitoring spectra, has shown that the majority of the background photons are of local origin, as prompt y rays from nuclear interactions of energetic particles and y rays and B particles from shortperiod and long-period radioactive species, induced by radiation-belt and solar-flare particles. A fraction of the background counts originates also from non-coded celestial photons, due to the lack of opacity of the passive shield. After the initial few orbits (i.e. after a few plunges into the proton belts), the background counting rate over the whole SIGMA energy domain quickly reached a value of the order of 450 counts s -~. 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 s -~) has been noticed. This has been followed by a significant decrease down to 440 counts s -I at the present time. Such a long-term variation results from the evolution of the altitude of the orbit perigee. After a brief period of falling (from 2000 km down to i000 km), the perigee is continuously rising, 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 radioactive nuclei whose decay contributes substantially to the overall in-flight background. The increase in perigee is such that two years after launch, the perigee altitude is
264
43rd IAF Congress
of the order of 20 000 km, leading to a background countingrate down to 440 counts s -~.
noticed regarding the spatial distribution of the background over the PSD.
Like the other GRANAT devices, SIGMA operates only when the spacecraft altitude is higher than 70 000 km, i.e. outside the external radiation belts. During these 3 working days (out a total of 4 days for the whole orbit), the background is rather stable, except during the first six months of the mission, where a decreasing b a c k g r o u n d excess was apparent in the low-energy channels, when the satellite was emerging from the belts.
In our usual data analysis procedure, the recorded images are first corrected in order to remove spatial non-uniformities which are in part intrinsic to the d e t e c t o r and in part due to a structured background, and sky images are reconstructed by using standard deconvolution techniques. All the results o b t a i n e d so far confirm that the whole SIGMA concept is fully certified and demonstrate the aptitude of a coded-mask telescope to perform accurate images of the sky in the hard X-ray and soft y-ray domain.
In-orbit
imaaina
performances
The final quality of the images p r o d u c e d by the telescope depends on both flight instrument characteristics, and ground data p r o c e s s i n g abilities. The PSD design being that of a classical A n g e r camera, it suffers from inherent aberrations of the d e t e c t o r linearity, inducing a ±10% m o d u l a t i o n in the case of a uniform exposure. Moreover, the in-flight b a c k g r o u n d is not u n i f o r m l y spatially distributed over the PSD and a perceptible center-to-rim b a c k g r o u n d counting-rate d e c r e a s e is observed. Since the image reconstruction is based on a cross-correlation method, both detector and background induced non-uniformities lead to serious signalto-noise ratio losses. The mandatory correction of both effects requires calibration matrices derived either from ground c a l i b r a t i o n s and from inflight observations of blank fields. With the exception of features induced by solar events, the b a c k g r o u n d structure is r a t h e r stable : during two years of in-flight operations, only m i n o r changes have been
The most straightforward result that can be derived from observations of the brightest celestial sources concerns the p o i n t - s o u r c e localization accuracy of the telescope. Both Cygnus X-I and the Crab Nebula are d e t e c t e d within a one arc min. radius error circle, a result in close agreement with the intrinsic properties of the telescope. In addition, the apparent sharpness of the corresponding excesses in the images proves that the on-board image correction from spacecraft drifts, one of the most critical c o m p o n e n t involved in the image a c q u i s i t i o n process, works quite perfectly. In-orbit
sensitivity
In addition to the usual factors which govern the sensitivity of any detector (as the background and the sensitive area), The SIGMA sensitivity combines also the effects of the c o d e d - m a s k technique. Because of the mask, a source illuminates only half of the PSD (the other half measures the background) ; the effective area is then redu-
43rd ~ F Congress
ced to about 300 cm 2 at i00 keY. Moreover, the actual sensitivity of a coded-mask imaging-principle telescope is also function of the spatial resolution of the PSD. In-flight estimates of the PSD spatial resolution yield 5.4 mm (HWHM) around i00 keV, significantly worse than ground measurements (3.5 mm). Such a PSD spatial-resolution loss (noticeable only below 200 keV), results from a residual light noise induced in the large single PSD crystal by the high rate of charge particle events. The main consequence is a reduction of the overall contrast in the image and some sensitivity is lost due to the imaging capacities. For broad-band observations in a typical i0" s exposure, the 3 o sensitivity of SIGMA at i00 keY is 10 -5 photon cm-2 s-1 keY -x. In-orbit
eneruv measurement
Ground calibrations performed at the launch base before the flight, have been used to derive the SIGMA energy calibration -- the keY/channel relationship - and the instrument energy resolution (16% at 60 keY, 12% at 122 keV and 8% at 511 keV). In-orbit estimates of both energy resolution and energy calibration have been derived from a deep analysis of the background s p e c t r u m ~. At the beginning of the mission, spectral features resulting from the t r a p p e d - r a d i a t i o n induced radioactivity were detected when the satellite was emerging from the belts. The most conspicuous one, possibly due to the decay of x23I (induced by spallation within the PSD crystal) is centered just b e l o w 200 keV. It can be adequately fitted as a sharp line whose width is compatible with ground estimates, indica-
265
ting that the detector energy resolution around 200 keV was not significantly modified in orbit. Two persistent outstanding features are always detected in the spectra. The one centered at ~ 60 keV results mainly from the on-board 2'XAm calibration sources, the other seen at about 480 keV has been interpreted as a mixture of several cosmic-ray induced background lines, including that resulting from the 511 keV e ÷ e- annihilation photons. Taking as a reference the energy of the centroid of these two features, it is possible to monitor the long-term variation of the SIGMA keY/channel relationship. Its most reliable absolute calibrations were performed during the few days which follow strong solar proton events. Identified line features were then detected in the background spectra, such as a group of lines seen beyond 700 keV, which results mainly from the decay of 2°'Bi induced by solar protons in the passive shield.
S I G M A i m a u i n u capabilities
SIGMA is best at studying complex regions where source confusion was a major problem for previous collimated instruments. Imaging observations may be also more reliable because the source can be "seen" on the image and the background is m e a s u r e d at the same time and the same pointing as the source. Source
confusio~
Even if the known hard-X ray and soft y-ray sources are not particularly numerous, their d i s t r i b u t i o n is not uniform, and many of the most intriguing candidates belong to rather complex regions. A typical example of
43rd IAF Congress
266
such a s i t u a t i o n is the discovery by SIGMA of GRS 1758-258, one the h a r d e s t X-ray and soft y-ray source in the v i c i n i t y of the G a l a c t i c C e n t e r (GC), in the course of the first GC GRANAT survey on Spring 1990 2 Located 40 arc min. away from the well known b r i g h t LowMass X-ray b i n a r y (LMXB) GX 5-1, GRS 1758-258 exhibits a very hard s p e c t r u m and d o m i n a t e s the flux of this region above 30 keY. A re-examination of the data from p r e v i o u s c o d e d - m a s k imaging t e l e s c o p e s o p e r a t i n g in the 3 to 30 keV band (XRT aboard S p a c e l a b 2 and TTM aboard the Kvant m o d u l e of the MIR station) has r e v e a l e d a hard source close to the S I G M A position, leading to the i d e n t i f i c a t i o n of GRS 1758-258 w i t h a weak source detected in the EXOSAT images 3 Due to the p r o x i m i t y of GRS 1758-258 and GX 5-1, previous m e a s u r e m e n t s at high energies of non-imaging instruments have been attributed to the well known LMXB. With SIGMA, it is clear that b e y o n d 40 keY, the whole e m i s s i o n should be attributed to GRS 1758-258 (Fig. i). The i m a g i n g q u a l i t i e s of the t e l e s c o p e were then d e t e r m i n a n t not only to d i s e n t a n g l e a severe source confusion problem but also to e n a b l e the firm identif i c a t i o n of the source. Attainment
o~ r e l i a b l e
spectra
Since the SIGMA telescope can o b s e r v e in 95 c o n s e c u t i v e energy bands, in each of which an image of the sky is derived, It is p o s s i b l e to obtain the count s p e c t r u m of a source indep e n d e n t l y of any other emissions w h i c h o r i g i n a t e s from its surrounding, as e.g. from other s o u r c e s w i t h i n the field of view and from d i f f u s e processes. As for o t h e r d e v i c e s o p e r a t i n g in
3
4 S p r i n g - F a l l ' , 1990' 4 0 - 1 1 0 keV
~-. \
t/ GX5-1
-!
0-
GRS1758-258
--9[-
\, ~. - 2
f
[ I
~
L
33
6
Figure i. C o n t o u r plot image of the GX 5-1 region derived in the 40 to Ii0 keV band from all the data c o l l e c t e d during the two 1990 SIGMA GC surveys. The contours are in statistical significance b e g i n n i n g at the 3 a level and spaced by 1 a.
the soft y-ray domain, which is d o m i n a t e d by Compton scattering, a p r e c i s e k n o w l e d g e of the energy r e s p o n s e function of the PSD is r e q u i r e d to derive the actual source p h o t o n spectrum. Estimates of the SIGMA energy response are b a s e d on the ground calibrations p e r f o r m e d in the launch base just before the flight, substantiated by the results of a precise M o n t e - C a r l o s i m u l a t i o n 4 to interpret the ground measurements, which depend s t r o n g l y on the c a l i b r a t i o n environment. One remarkable feature of the c o d e d - m a s k technique is that it improves the energy response of the d e t e c t o r : the photons which interact primarily in the NaI crystal give interaction locations d i s t r i b u t e d a c c o r d i n g to the m a s k pattern, w h i l e those which e x p e r i e n c e s c a t t e r i n g processes prior to their d e t e c t i o n
43rd M F Congress
in the crystal give almost uniformly d i s t r i b u t e d interaction locations. The effect of the spatial decon~olution on the energy response function is to suppress most of the contribution of the photons scattered in other part of the experiment before their interaction in the detector. This mainly reduces the b a c k s c a t t e r e d peak, and the lower is the backscattered peak, the better is the spectral response. Note that backscattering is p a r t i c u l a r l y important in the case of the SIGMA experiment, since the PSD involves low Z substances known as efficient scattering media as the thick pyrex disk bounded to the NaI crystal, and mainly the silicon resin coating, widely spread as shock absorber around the 61 PMTs coupled to the crystal via the pyrex disk.
267
spectrum shows an excess around 500 keV. In fact, a detailed analysis of the count spectrum recorded during the observation demonstrates that this excess is compatible with a line emission centered at 481 ± 22 keV, as schown on Fig. 2. This feature is varying during the observation, being clearly present in the last 13 hours and practically absent in the first part of the observation 5.
. . . . . .
I
.
.
.
.
,
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183
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C r e d i b i l i t v of spectral features Even in the absence of source confusion problem, because the source is seen on the image and the background is measured at the same time and the same pointing as the source, imaging observations are also essential to assess the credibility of the spectral features which may be detected in the source spectrum. The discovery by SIGMA of a positron annihilation emission line in the soft ¥-ray emission from Nova Muscae illustrates the unique contribution of the coded-mask technique in such a case of a well isolated source. Nova Muscae is a new X-ray transient discovered simultaneously on January 8, 1991, by the watch X-ray burst detector aboard the GRANAT satellite and by the GINGA all sky X-ray monitor (IAU Circular N ° 5161). The source was observed by the SIGMA telescope four time in January, 1991. On Jan. 20, the source
.
.
.
.
.
.
.
.
.
.
.
.
182
!
18 ~
EnsrgW (keU) Figure 2. Photon spectrum of Nova Muscae of the last 13.3 hours of the Jan. 20 observation. The solid line represent the best fit model which includes a power-law spectrum plus a gaussian line.
A contour plot image of the Nova Muscae field of view, in the energy range 430-530 keV, is shown on Fig. 3. An excess of 5.1 a is located close (about 3 arc min.) to the ESO optical position (IAU Circular N°5165). Beyond any doubt, the fact that the source is clearly detected at the right position in the image derived in the energy interval where the line spectral feature is observed, substan-
43rd ~ F Congress
268
tially strengthens the overall confidence in the finding. This is p a r t i c u l a r l y obvious in this case, since the line is detected in a spectral region where outstanding features are detected in the background spectrum. Contrary to the on/off source chopping, in which background lines may easily pollute the source spectrum, the coded-mask technique is quite insensitive to the existence of background lines, whose counts, which cannot be d i s t r i b u t e d over the PSD with a pattern compatible with that induced by an actual source in the field of view, only contribute to increase the background level at these energies.
1 1 h~.4m
1 1 h l Em
-69.8
-78,8 1 I h36m
I 1 h24m
Figure 3. Contour plot images of the field of view of the Jan. 20 observation, in the energy range 430-530 keY. The contour level are in unit of statistical significance b e g i n n i n g at the 1 a level and spaced by 1 a. The Nova M u s c a e optical position is indicated by a cross.
Concludinu
remarks
The main outcomes of the present analysis of the first two years of successful in-orbit SIGMA operations can be summarized as follows :
with reuard to the orbit, it should be f i r s t stressed that all the well known advantages of the high eccentric orbit (HEO)r as e.g. the capability to continuously observe short-term variable phenomenons, take an important role in the quality of the scientific returns of the mission. Note also that the typical HEO provided by the PROTON booster offers additional advantages as a very high eccentricity, allowing (i) a very long observing time outside the radiation belts and (ii) a relative small fraction of the observing time disturbed by the disintegration of short-period radio-active nuclei induced by radiation-belt particles. The HEO provided by the PROTON booster also feature a high inclination, which permits long periods of satellite visibility from a single northern hemisphere ground station, and which minimizes the amount of radiation doses experienced during each plunge into the proton belts. In this context, the continuous increase in perigee of the GRANAT orbit, leading to a reduction of the in-flight background, clearly demonstrates that a substantial raise of perigee (at least to an altitude of the order of I0 000 km) just after the injection into the transfer orbit appears as mandatory for a future mission. with regard to the instrument, it should be first emphasized that the SIGMA concept, based on a static coded-mask aperture operating in the hard X-ray and soft y-ray regime, is fully certified, at least in the rather stable background environment experienced on the GRANAT HEO. The unique capabilities of such a concept have been demonstrated, as e.g. the c a p a b i l i t y to disentangle confuse regions and to perform re-
43rd ~ F Congress
liable spectral measurements. Note however that due to the PSD design, based on the principle of the Anger camera, the highenergy sensitivity of the PSD is limited by the thickness of the NaI crystal which cannot exceed 15 mm. Note also that some reduction of sensitivity were encountered in the low-energy regime, resulting from a deterioration of the PSD spatial resolution induced by the high rate of charge particle events in the large single crystal of the PSD. All such conclusions prompt some obvious remarks suitable for future missions :
269
due to the increase in sensitivity, confusion problems will also affect other attractive targets as e.g. the nearest cluster of galaxies, iii) the coded-mask techniques appear as the most efficient way to perform the mandatory high angular resolution observations of the hard X-ray and soft y-ray sky, provided that the PSD spatial resolution is adapted to the mask element size ; in order to avoid in-orbit sensitivity losses, the PSD design should favor a segmented approach.
References i) the quality of the scientific return of any y-ray mission depends chiefly on the right choice of the orbit, the best choice being an inclined HEO whose perigee has been raised just after injection, ii) spectral measurements are interesting only if the emitting sources are clearly identified, so that good imaging properties are m a n d a t o r y ; this is particularly the case in confuse regions as the GC vicinity, but
1 2 3
4
5
Leray, J.P. e t al. 1991, Proc. of 22 nd Int. Cosmic Ray Conf., Dublin 1991, 2~ 495. Sunyaev, R. e t al. 1991, Astron. Astrophys., 247, L29. Skinner, G.K. 1991, GammaraY Line Astrophysics, ed. Ph. Durouchoux and N. Prantzos, (New-York : AIP), 358. Barret, D. and Laurent, P. 1991, Nucl. Inst. and Meth., A307, 512. Goldwurm, A. et al. 1991, Ap.J., 389, L38.
0 0 9 4 - 5 7 6 5 / 9 3 $ 6 . 0 0 + 0.00 P e r g a m o n Press Ltd
Printed in Great Britain
TWO
YEARS
OF
SUCCESSFUL OPERATIONS OF THE CODED-MASK TELESCOPE FOR HARD X-RAY AND SOFT y-RAY ASTRONOMY
Service
9, Avenue
SIGMA
Jacques Paul and Frangois Lebrun d'Astrophysique, Centre d'Etudes de Saclay 91191 Gif-sur-Yvette Cedex, France
Pierre Mandrou and Jean-Pierre Roques Centre d'Etude Spatiale des Rayonnements du Colonel Roche BP 4346, 31029 Toulouse Cedex, Abstract
The
SIGMA
France
Telescope
At the beginning of the previous decade, it was recognized that a possible means of improving existing hard X-ray and yray telescopes, where focusing techniques are totally impracticable, is the incorporation of the c o d e d - a p e r t u r e 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 and detector background, systematic effects due to temporal variations in the b a c k g r o u n d are removed.
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 the Centre National d'Etudes Spatiales (CNES), the French Space Agency. This hard X-ray and soft y-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.
This paper reports on the French SIGMA telescope, the first c o d e d - a p e r t u r e telescope sensitive to radiation in the energy range from 35 KeV to 1.3 MeV to be operated in space. The SIGMA telescope is one of the main devices on board the astronomy satellite GRANAT, successfully launched on December i, 1989 from Baikonour, Kazakhstan. After a comprehensive description of the instrument, a report is given on the most relevant inferences which can be drawn from two years of successful inorbit operations, in order to better define the next generation of y-ray instruments.
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. 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 key properties of the telescope, such as the m a x i m u m sensitivity field (4.3 deg. x 4.7 deg.), surrounded by a wider field of decreasing sensitivity, the tQ-
Instrumental
Copyright © 1992 by the IAF. All rights reserved 261
concept
262
43rd M F Congress
tal detection area, which is the 794 cm 2 central rectangular zone of the PSD whose size matches the basic 29 × 31 mask pattern, the intrinsic anuular resolution (13 arc min. ) , the point-sourG~ location accuracy, which is less than 2 arc min. taking into account the PSD coding element size. The PSD design is based on the principle of the Anger cameras used in nuclear medicine. S c i n t i l l a t i o n flashes of light produced in a 57 cm diameter and 1.25 cm thick NaI(TI) crystal by the photon induced electrons are detected by at least seven of the 61 hexagonal photomultiplier tubes (PMTs), mounted within a carbon-fiber honeycomb-structure, and optically coupled to the crystal via a 1.25 cm thick pyrex disk. Two separate calibration devices, both including a 24~Amradioactive source embedded within a plastic scintillator viewed by two PMTs, are m o u n t e d well above the PSD. Each 2'~Am d i s i n t e g r a t i o n giving a 60 keV photon also releases an particle, which is immediately a b s o r b e d by the scintillator and thus d e t e c t e d 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 anticoincidence shield, made of 31 independent CsI(TI) 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 u p p e r ring are 3 cm thick. The lateral area of this anticoincidence well reaches
19,200 cm 2. 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.i mm tantalum and 0.4 mm tin) is wrapped around the tube holding the mask in order to prevent the entrance of off-axis low-energy y rays contributing to the background. The service
modules
Three major 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 the management of all experiment subsystems, the arrangement of the SIGMA telescope data flow throughout the mass memory, the management of the t e l e c o m m a n d and telemetry links with the spacecraft, and 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. The on-board data recording system is a 128 megabit bubble memory, where all scientific and e n g i n e e r i n g 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 d o w n l i n k e d to the ground station at a rate of 64 kilobits s "~. Since the counting rate within the total detection area is in excess of 400 counts s "~, the full data stream, including for each event position, energy and time informations, is such
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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, w h i c h are selected on the basis of the scientific objectives assigned to the telescope at a given time. Two small optical telescopes are m o u n t e d in a direction parallel 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 min.) is much less accurate than the actual resolution which the SIGMA telescope requires. Every 4 sec., the optical head data, once p r o c e s s e d by the on-board computer, yields on-line estimates of the spacecraft attitude drifts (with an accuracy of the order of i0 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 m e a s u r e d by the PSD.
SIGMA In-orbit
behavior
Since the successful launch of the satellite into its nominal h i g h l y - e c c e n t r i c 4-day orbit (perigee : 2000 km, apogee : 200 000 km), more than two years of in-flight operations have been achieved, leading to a precise estimate of the in-orbit p e r f o r m a n c e s of the instrument and of its long-term evolution.
M 30-R
In-orbit
263
backaround
The analysis of the background spectrum, which is continuously measured in the form of 1024 channel monitoring spectra, has shown that the majority of the background photons are of local origin, as prompt y rays from nuclear interactions of energetic particles and y rays and B particles from shortperiod and long-period radioactive species, induced by radiation-belt and solar-flare particles. A fraction of the background counts originates also from non-coded celestial photons, due to the lack of opacity of the passive shield. After the initial few orbits (i.e. after a few plunges into the proton belts), the background counting rate over the whole SIGMA energy domain quickly reached a value of the order of 450 counts s -~. 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 s -~) has been noticed. This has been followed by a significant decrease down to 440 counts s -I at the present time. Such a long-term variation results from the evolution of the altitude of the orbit perigee. After a brief period of falling (from 2000 km down to i000 km), the perigee is continuously rising, 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 radioactive nuclei whose decay contributes substantially to the overall in-flight background. The increase in perigee is such that two years after launch, the perigee altitude is
264
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of the order of 20 000 km, leading to a background countingrate down to 440 counts s -~.
noticed regarding the spatial distribution of the background over the PSD.
Like the other GRANAT devices, SIGMA operates only when the spacecraft altitude is higher than 70 000 km, i.e. outside the external radiation belts. During these 3 working days (out a total of 4 days for the whole orbit), the background is rather stable, except during the first six months of the mission, where a decreasing b a c k g r o u n d excess was apparent in the low-energy channels, when the satellite was emerging from the belts.
In our usual data analysis procedure, the recorded images are first corrected in order to remove spatial non-uniformities which are in part intrinsic to the d e t e c t o r and in part due to a structured background, and sky images are reconstructed by using standard deconvolution techniques. All the results o b t a i n e d so far confirm that the whole SIGMA concept is fully certified and demonstrate the aptitude of a coded-mask telescope to perform accurate images of the sky in the hard X-ray and soft y-ray domain.
In-orbit
imaaina
performances
The final quality of the images p r o d u c e d by the telescope depends on both flight instrument characteristics, and ground data p r o c e s s i n g abilities. The PSD design being that of a classical A n g e r camera, it suffers from inherent aberrations of the d e t e c t o r linearity, inducing a ±10% m o d u l a t i o n in the case of a uniform exposure. Moreover, the in-flight b a c k g r o u n d is not u n i f o r m l y spatially distributed over the PSD and a perceptible center-to-rim b a c k g r o u n d counting-rate d e c r e a s e is observed. Since the image reconstruction is based on a cross-correlation method, both detector and background induced non-uniformities lead to serious signalto-noise ratio losses. The mandatory correction of both effects requires calibration matrices derived either from ground c a l i b r a t i o n s and from inflight observations of blank fields. With the exception of features induced by solar events, the b a c k g r o u n d structure is r a t h e r stable : during two years of in-flight operations, only m i n o r changes have been
The most straightforward result that can be derived from observations of the brightest celestial sources concerns the p o i n t - s o u r c e localization accuracy of the telescope. Both Cygnus X-I and the Crab Nebula are d e t e c t e d within a one arc min. radius error circle, a result in close agreement with the intrinsic properties of the telescope. In addition, the apparent sharpness of the corresponding excesses in the images proves that the on-board image correction from spacecraft drifts, one of the most critical c o m p o n e n t involved in the image a c q u i s i t i o n process, works quite perfectly. In-orbit
sensitivity
In addition to the usual factors which govern the sensitivity of any detector (as the background and the sensitive area), The SIGMA sensitivity combines also the effects of the c o d e d - m a s k technique. Because of the mask, a source illuminates only half of the PSD (the other half measures the background) ; the effective area is then redu-
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ced to about 300 cm 2 at i00 keY. Moreover, the actual sensitivity of a coded-mask imaging-principle telescope is also function of the spatial resolution of the PSD. In-flight estimates of the PSD spatial resolution yield 5.4 mm (HWHM) around i00 keV, significantly worse than ground measurements (3.5 mm). Such a PSD spatial-resolution loss (noticeable only below 200 keV), results from a residual light noise induced in the large single PSD crystal by the high rate of charge particle events. The main consequence is a reduction of the overall contrast in the image and some sensitivity is lost due to the imaging capacities. For broad-band observations in a typical i0" s exposure, the 3 o sensitivity of SIGMA at i00 keY is 10 -5 photon cm-2 s-1 keY -x. In-orbit
eneruv measurement
Ground calibrations performed at the launch base before the flight, have been used to derive the SIGMA energy calibration -- the keY/channel relationship - and the instrument energy resolution (16% at 60 keY, 12% at 122 keV and 8% at 511 keV). In-orbit estimates of both energy resolution and energy calibration have been derived from a deep analysis of the background s p e c t r u m ~. At the beginning of the mission, spectral features resulting from the t r a p p e d - r a d i a t i o n induced radioactivity were detected when the satellite was emerging from the belts. The most conspicuous one, possibly due to the decay of x23I (induced by spallation within the PSD crystal) is centered just b e l o w 200 keV. It can be adequately fitted as a sharp line whose width is compatible with ground estimates, indica-
265
ting that the detector energy resolution around 200 keV was not significantly modified in orbit. Two persistent outstanding features are always detected in the spectra. The one centered at ~ 60 keV results mainly from the on-board 2'XAm calibration sources, the other seen at about 480 keV has been interpreted as a mixture of several cosmic-ray induced background lines, including that resulting from the 511 keV e ÷ e- annihilation photons. Taking as a reference the energy of the centroid of these two features, it is possible to monitor the long-term variation of the SIGMA keY/channel relationship. Its most reliable absolute calibrations were performed during the few days which follow strong solar proton events. Identified line features were then detected in the background spectra, such as a group of lines seen beyond 700 keV, which results mainly from the decay of 2°'Bi induced by solar protons in the passive shield.
S I G M A i m a u i n u capabilities
SIGMA is best at studying complex regions where source confusion was a major problem for previous collimated instruments. Imaging observations may be also more reliable because the source can be "seen" on the image and the background is m e a s u r e d at the same time and the same pointing as the source. Source
confusio~
Even if the known hard-X ray and soft y-ray sources are not particularly numerous, their d i s t r i b u t i o n is not uniform, and many of the most intriguing candidates belong to rather complex regions. A typical example of
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such a s i t u a t i o n is the discovery by SIGMA of GRS 1758-258, one the h a r d e s t X-ray and soft y-ray source in the v i c i n i t y of the G a l a c t i c C e n t e r (GC), in the course of the first GC GRANAT survey on Spring 1990 2 Located 40 arc min. away from the well known b r i g h t LowMass X-ray b i n a r y (LMXB) GX 5-1, GRS 1758-258 exhibits a very hard s p e c t r u m and d o m i n a t e s the flux of this region above 30 keY. A re-examination of the data from p r e v i o u s c o d e d - m a s k imaging t e l e s c o p e s o p e r a t i n g in the 3 to 30 keV band (XRT aboard S p a c e l a b 2 and TTM aboard the Kvant m o d u l e of the MIR station) has r e v e a l e d a hard source close to the S I G M A position, leading to the i d e n t i f i c a t i o n of GRS 1758-258 w i t h a weak source detected in the EXOSAT images 3 Due to the p r o x i m i t y of GRS 1758-258 and GX 5-1, previous m e a s u r e m e n t s at high energies of non-imaging instruments have been attributed to the well known LMXB. With SIGMA, it is clear that b e y o n d 40 keY, the whole e m i s s i o n should be attributed to GRS 1758-258 (Fig. i). The i m a g i n g q u a l i t i e s of the t e l e s c o p e were then d e t e r m i n a n t not only to d i s e n t a n g l e a severe source confusion problem but also to e n a b l e the firm identif i c a t i o n of the source. Attainment
o~ r e l i a b l e
spectra
Since the SIGMA telescope can o b s e r v e in 95 c o n s e c u t i v e energy bands, in each of which an image of the sky is derived, It is p o s s i b l e to obtain the count s p e c t r u m of a source indep e n d e n t l y of any other emissions w h i c h o r i g i n a t e s from its surrounding, as e.g. from other s o u r c e s w i t h i n the field of view and from d i f f u s e processes. As for o t h e r d e v i c e s o p e r a t i n g in
3
4 S p r i n g - F a l l ' , 1990' 4 0 - 1 1 0 keV
~-. \
t/ GX5-1
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GRS1758-258
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6
Figure i. C o n t o u r plot image of the GX 5-1 region derived in the 40 to Ii0 keV band from all the data c o l l e c t e d during the two 1990 SIGMA GC surveys. The contours are in statistical significance b e g i n n i n g at the 3 a level and spaced by 1 a.
the soft y-ray domain, which is d o m i n a t e d by Compton scattering, a p r e c i s e k n o w l e d g e of the energy r e s p o n s e function of the PSD is r e q u i r e d to derive the actual source p h o t o n spectrum. Estimates of the SIGMA energy response are b a s e d on the ground calibrations p e r f o r m e d in the launch base just before the flight, substantiated by the results of a precise M o n t e - C a r l o s i m u l a t i o n 4 to interpret the ground measurements, which depend s t r o n g l y on the c a l i b r a t i o n environment. One remarkable feature of the c o d e d - m a s k technique is that it improves the energy response of the d e t e c t o r : the photons which interact primarily in the NaI crystal give interaction locations d i s t r i b u t e d a c c o r d i n g to the m a s k pattern, w h i l e those which e x p e r i e n c e s c a t t e r i n g processes prior to their d e t e c t i o n
43rd M F Congress
in the crystal give almost uniformly d i s t r i b u t e d interaction locations. The effect of the spatial decon~olution on the energy response function is to suppress most of the contribution of the photons scattered in other part of the experiment before their interaction in the detector. This mainly reduces the b a c k s c a t t e r e d peak, and the lower is the backscattered peak, the better is the spectral response. Note that backscattering is p a r t i c u l a r l y important in the case of the SIGMA experiment, since the PSD involves low Z substances known as efficient scattering media as the thick pyrex disk bounded to the NaI crystal, and mainly the silicon resin coating, widely spread as shock absorber around the 61 PMTs coupled to the crystal via the pyrex disk.
267
spectrum shows an excess around 500 keV. In fact, a detailed analysis of the count spectrum recorded during the observation demonstrates that this excess is compatible with a line emission centered at 481 ± 22 keV, as schown on Fig. 2. This feature is varying during the observation, being clearly present in the last 13 hours and practically absent in the first part of the observation 5.
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.
.
.
.
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C r e d i b i l i t v of spectral features Even in the absence of source confusion problem, because the source is seen on the image and the background is measured at the same time and the same pointing as the source, imaging observations are also essential to assess the credibility of the spectral features which may be detected in the source spectrum. The discovery by SIGMA of a positron annihilation emission line in the soft ¥-ray emission from Nova Muscae illustrates the unique contribution of the coded-mask technique in such a case of a well isolated source. Nova Muscae is a new X-ray transient discovered simultaneously on January 8, 1991, by the watch X-ray burst detector aboard the GRANAT satellite and by the GINGA all sky X-ray monitor (IAU Circular N ° 5161). The source was observed by the SIGMA telescope four time in January, 1991. On Jan. 20, the source
.
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.
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.
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.
182
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18 ~
EnsrgW (keU) Figure 2. Photon spectrum of Nova Muscae of the last 13.3 hours of the Jan. 20 observation. The solid line represent the best fit model which includes a power-law spectrum plus a gaussian line.
A contour plot image of the Nova Muscae field of view, in the energy range 430-530 keV, is shown on Fig. 3. An excess of 5.1 a is located close (about 3 arc min.) to the ESO optical position (IAU Circular N°5165). Beyond any doubt, the fact that the source is clearly detected at the right position in the image derived in the energy interval where the line spectral feature is observed, substan-
43rd ~ F Congress
268
tially strengthens the overall confidence in the finding. This is p a r t i c u l a r l y obvious in this case, since the line is detected in a spectral region where outstanding features are detected in the background spectrum. Contrary to the on/off source chopping, in which background lines may easily pollute the source spectrum, the coded-mask technique is quite insensitive to the existence of background lines, whose counts, which cannot be d i s t r i b u t e d over the PSD with a pattern compatible with that induced by an actual source in the field of view, only contribute to increase the background level at these energies.
1 1 h~.4m
1 1 h l Em
-69.8
-78,8 1 I h36m
I 1 h24m
Figure 3. Contour plot images of the field of view of the Jan. 20 observation, in the energy range 430-530 keY. The contour level are in unit of statistical significance b e g i n n i n g at the 1 a level and spaced by 1 a. The Nova M u s c a e optical position is indicated by a cross.
Concludinu
remarks
The main outcomes of the present analysis of the first two years of successful in-orbit SIGMA operations can be summarized as follows :
with reuard to the orbit, it should be f i r s t stressed that all the well known advantages of the high eccentric orbit (HEO)r as e.g. the capability to continuously observe short-term variable phenomenons, take an important role in the quality of the scientific returns of the mission. Note also that the typical HEO provided by the PROTON booster offers additional advantages as a very high eccentricity, allowing (i) a very long observing time outside the radiation belts and (ii) a relative small fraction of the observing time disturbed by the disintegration of short-period radio-active nuclei induced by radiation-belt particles. The HEO provided by the PROTON booster also feature a high inclination, which permits long periods of satellite visibility from a single northern hemisphere ground station, and which minimizes the amount of radiation doses experienced during each plunge into the proton belts. In this context, the continuous increase in perigee of the GRANAT orbit, leading to a reduction of the in-flight background, clearly demonstrates that a substantial raise of perigee (at least to an altitude of the order of I0 000 km) just after the injection into the transfer orbit appears as mandatory for a future mission. with regard to the instrument, it should be first emphasized that the SIGMA concept, based on a static coded-mask aperture operating in the hard X-ray and soft y-ray regime, is fully certified, at least in the rather stable background environment experienced on the GRANAT HEO. The unique capabilities of such a concept have been demonstrated, as e.g. the c a p a b i l i t y to disentangle confuse regions and to perform re-
43rd ~ F Congress
liable spectral measurements. Note however that due to the PSD design, based on the principle of the Anger camera, the highenergy sensitivity of the PSD is limited by the thickness of the NaI crystal which cannot exceed 15 mm. Note also that some reduction of sensitivity were encountered in the low-energy regime, resulting from a deterioration of the PSD spatial resolution induced by the high rate of charge particle events in the large single crystal of the PSD. All such conclusions prompt some obvious remarks suitable for future missions :
269
due to the increase in sensitivity, confusion problems will also affect other attractive targets as e.g. the nearest cluster of galaxies, iii) the coded-mask techniques appear as the most efficient way to perform the mandatory high angular resolution observations of the hard X-ray and soft y-ray sky, provided that the PSD spatial resolution is adapted to the mask element size ; in order to avoid in-orbit sensitivity losses, the PSD design should favor a segmented approach.
References i) the quality of the scientific return of any y-ray mission depends chiefly on the right choice of the orbit, the best choice being an inclined HEO whose perigee has been raised just after injection, ii) spectral measurements are interesting only if the emitting sources are clearly identified, so that good imaging properties are m a n d a t o r y ; this is particularly the case in confuse regions as the GC vicinity, but
1 2 3
4
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Leray, J.P. e t al. 1991, Proc. of 22 nd Int. Cosmic Ray Conf., Dublin 1991, 2~ 495. Sunyaev, R. e t al. 1991, Astron. Astrophys., 247, L29. Skinner, G.K. 1991, GammaraY Line Astrophysics, ed. Ph. Durouchoux and N. Prantzos, (New-York : AIP), 358. Barret, D. and Laurent, P. 1991, Nucl. Inst. and Meth., A307, 512. Goldwurm, A. et al. 1991, Ap.J., 389, L38.