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The sigma mission on the granat satellite
Adv. Space Res. Vol. 10, No. 2, pp. (2)223—(2)232, 1990 Printed in Great Britain. All rights reserved.
0273-1177/90 $0.00 +50 Copyright © 1989 COSPAR
THE SIGMA MISSION ON THE GRANAT SATELLITE J. P. Roques,’~’J. Pau1,’~P. Mandrou* and F. Lebrun** *
Centre d’Etude Spatiale des Rayonnements, BP 4346, 31029 Toulouse
Cedex, France **
Centre d’Etudes Nucleaires de Saclay, 91191 Gif-sur- Yvette Cedex, France
ABSTRACT The soviet spacecraft GRANAT, to be launched in july 1989, will carry seven experiments dedicated to the hard X-ray/Low energy gamma ray astronomy. After a brief description of the mission and of the overall flight hardware, we shall focus on the SIGMA experiment. This french telescope designed to obtain high sensitivity images of the sky in the 30 - 1300 key energy range will be extensively described. The main scientific objectives of the instrument will also be discuss. THE GRANAT MISSION The soviet GRANAT satellite is entirely devoted to high-energy astronomy. This five-ton satellite features an Astron-type platform already used in the VEGA Mission.The launch is now planned for july 1989 and an operational life time in excess of 18 months is foreseen. The orbit chosen is highly excentric (2000 - 200 000 km) with a perigee fixed in the southern hemisphere. The period of this orbit is 4.05 days but because of operational constraints and especially passage through the Earth radiation belts (below 60 000 km) the useful time per orbit is 3 days. This satellite is 3 axis stabilized : one axis is pointed towards the sun another is fixed with a star tracker. The GRANAT payload is made of seven instruments whose characteristics are in table 1. DESCRIPTION OF THE SIGMA EXPERIMENT The SIGMA experiment was first proposed to C.N.E.S. (Centre National d’Etudes Spatiales, the french space agency) in june 1981 by two french laboratories (Centre d’Etude Spatiale des Rayonnements in Toulouse and Service d’Astrophysique in Saclay). It is now engaged in the frame work of a French-Soviet joint program as the main payload of GRANAT. The high energy astronomy results which have been obtained until now have shown the necessity to build experiments with both a good angular resolution and a large field of view. This is why the main goal of the SIGMA mission is to perform accurate images of the sky in the hard X-Ray/low energy gamma-ray domain, which is almost unexplored. In order to fullf ill these objectives, SIGMA has been designed using coded mask imaging techniques. The instrument is thus constructed with the association of a coded mask (built with elements having a transparency of 0 or 1) and a position sensitive detector which receives the flux from the sky modulated by the mask. If M is an array representing the mask transmission, and S is an array representing the object in the field of view of the imaging device, then the image P recorded on the position sensitive detector is such as P=M*S+B where B represents the background distribution.
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J. P. Roques et a!.
TABLE 1 The GRANAT payload
INSTRUMENT
MAIN OBJECTIVES
CHARACTERISTICS
SIGMA (French)
GAMMA-RAY IMAGING, SPECTRAL AND TIME ANALYSIS OF LOCALIZED SOURCES
30 keV-l.3 MeV FIELD OF VIEW 4°45’ x 4’20’
TELESCOPE WITH CODED MASK USING A GAMMA RAY CAMERA
ART-P (Soviet)
X-RAY IMAGING, SPECTRAL AND TIME ANALYSIS OF LOCALIZED SOURCES
3 keV-lOO keV FOV : l.8°xl.8°
TELESCOPE WITH CODED MASK USING FOUR INDEPENDENT DETECTORS
ART-S (Soviet)
SPECTRAL AND TIME ANALYSIS OF RELATIVELY BRIGHT LOCALIZED SOURCES
3 keV-150 key FOV 2° x 2°
SPECTROSCOPE WITH FOUR PROPORTIONAL COUNTER
PHEBUS (French)
LOCALIZATION SPECTRAL AND TIME ANALYSIS OF BURSTS
100 keV-lOO keV
SIX DETECTORS AROUND THE PERIPHERY OF THE SPACECRAFT TO COVER THE ENTIRE FOV
KONUS (Soviet)
LOCALIZATION, SPECTRAL, AND TIME ANALYSIS OF BURST
20 keV-
SEVEN DETECTORS DISTRIBUTED AROUND THE SPACECRAFT
TOURNESOL (Soviet)
OPTICAL DETECTION OF BURST. SPECTRAL ANALYSIS OF A PARTICULAR SOURCE, ANALYSIS OF BURST
2 keV- 20 MeV X-RAY FOV : 6° x 6° FOV IN VISIBLE LIGHT : 5°x 5°
FOUR PROPORTIONAL COUNTERS AND TWO OPTICAL DETECTORS. THE SYSTEM IS GIMBALMOUNTED.
WATCH (Danish)
MONITORING OF PERSISTENT X-RAY SOURCES. X-RAY BURST DETECTION AND LOCALIZATION
6 key -180 keV FOV 4 it
FOUR INSTRUMENTS USING A ROTATING COLLIMATOR CONCEPT
2 MeV
DESCRIPTION
If there is an array G such that M*G=5 Then the reconstructed objet W is expressed by W
~ * G = (M =O+B*G =
*
0
+
B)
*
G
Some class of arrays such that M *G = 5 exists. For SIGMA we have choosen Uniformely Redondant Arrays /1/ (these URA’s represents a special case of quadratic residue matrices when their dimensions are given by two prime numbers differing by 2 /2/. INSTRUMENTAL CONCEPT A schematic view of the instrument is shown in Figure 1. The main elements are: 1/ A position-sensitive detector based on the Anger camera principle /3/ it consists of a Nal (tl) cristal 1.25-cm thick analysed throught an optical glass 1.25-cm thick by 61 hexagonal photomultiplier tubes inserted in a carbon fiber honeycomb structure. The energy resolution and position accuracy of this detector are shown figure 2 and figure 3. 2/ A 1.5 cm thick tungsten mask consisting of 49 x 53 elements built on the basis of a 29 x 31 URA. The size of these elements is 9.4 x 9.4 mm and the mask is mounted 2.5 m away from the detection plane.
CODED MASK SQUARES OF TUNGSTEN SHIELDING (TIN, LEAD, TANTALUM)
HEAT SHIELD
PLASTIC SCINTILLATOR ANTI-COINCIDENCE CIRCUIT OPTICAL HEAD ELLAR TECTOR
GAMMA CAMERA PHOTO MULTIPLIER MODULE
ELECTRONICS
Fig. i. Schematic view of the Sigma telescope.
I II.!
_____
3 ~ ~
____
___
2
___
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~
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~4
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___
--
7E1.1E+03
____
~
~‘~‘ -
Deg.-->
99.3E+04
12.OE+E14
-.
0
12.1E+0E
1.11
I
20.1E÷03
~4°~E02
Fig. 5. Simulation of the galactic centre, 30—50 key, 10 days observation time.
X
SIGMA on the GRANAT Satellite
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All these caracteristics impose the key proFerties detection sensitive area : 797 cm maximum sensitivity field of view 4~.45’ x 4°.20’ intrinsic resolving power : 13 arc mm.
of telescope
3/ An active CsI (tl) anticoincidence shield surrounds the detector and limits its field of view to about 1 sr. This shielding consists of 31 independant cristals, each of them being analysed by two photomultiplier tubes.The seven cristals of the botton anticoincidence are 4 cm thick. The lateral shield is made of two rings of 6 scintillators. The cristal of the lower ring are 4 cm thick, those of the upper ring of 3 cm thick. The lateral area of this shielding is 19200 cm A thin (5 mm) plastic scintillator detector, covering the field of view, vetoes incoming charged particles. A passive graded shield (0.5 mm of Pb, 0.1 mm of Ta, 0.4 mm of Sn) is wrapped around the tube holding the mask. This shielding significantly reduces the low—energy induced background. Sensitivity The anticipated sensitivity of the SIGMA telescope depends on both the efficiency of the detector and the background. While the sensitivity of the detector can be simply derived from actual measurements, a realistic estimate of the background requires detailed computations. Taking in account all the expected background contributions : diffuse celestial, spallations, secondary neutrons and photons induced within the spacecraft ; the expected background spectrum has been calculated. Then it is possible to deduce the continuum sensitivity which corresponds to 3 a signification in one day or ten days observation (figure 4). For the line sensitivity it is necessary to take in account the energy resolution of the detector (figure 2). The resulting 3 a spnsitivities for ten days observation are 2 l0’~ ph/cm2.s for 122 key, 2 l0 ph/cm2.s for 511 key and 2.5 l0~ ph/cm2.s for 847 key line. Here we must point out an important feature of SIGMA for y ray lines detection : thanks to the coded mask imaging technique the measurements of the background and the signal are made simultaneously. This will allow a very secure detection of lines. SIGMA OPERATING MODES SIGMA can operate in any one of several modes, which will be selected on the basis of the scientific objectives assigned to the telescope at a given time. A/ Spectral imaging mode In this mode the telescope will perform the following functions simultaneously.
C\uhl
101
112
1I~ 0 Ik~V)
Fig. 2. Energy resolution of the position sensitive detector.
J. P. Roques ci a!.
(2)228
101
io2 E (keV)
Fig. 3. Accuracy of the position detection system. Fine imaging : Four high resolution images, 248 x 232 pixels, are made in four adjacent energy bands. The thresolds of these energy channels are adjustable. This set of four images may be repeated N times, N being a number between 1 and 12. For a typical 24 hours observation, a set of fine images will have an exposure of 4 hours and the telescope will record a total of 24 high resolution images. Spectral imaging : 95 images of 124 x 126 pixels are made in 95 energy bands varying logarithmically in width. This set of images is repeated N/2 times
(N is the same value as defined above). For a typical observation this set of images have an exposure time of eight hours and SIGMA will record 285 images of 124 x 116 pixels. Slow variability : The counting rate in four energy channels, the same four energy bands than those defined for fine imaging, is recorded every four seconds. High resolution spectrum : 1024 channels spectral analysis, of the photon detected by the position sensitive detector is performed. This analysis is made every 30 mm. for a typical observation (i.e. 16 times per spectral imaging format). Attitude drift : The attitude drift of the spacecraft is measured with a stellar sensor. This measure is recorded every four seconds with an accuracy of 15 arc seconds. y Ray burst : The gamma ray burst functions, the anticoincidence shield are enabled. 2
~
io~
CRAB
CYG
both on the imaging system and
I
Xl
CAN A
_______
~
-
11_i 101
81111
1
I 102 ENERGY t~~V>
Fig. 4. Sensitivity for 1 day and 10 days observation time.
SIGMA on the GRANAT Satellite
(2)229
B/ Time variability imaging
This mode is the same as the fine imaging mode except that the function spectral imaging is replaced by time variability : in this mode we record images of 124 x 116 pixels between two commandable threholds. One image have an exposure time of N x 60 mn /48 (i.e. the minimum exposure time is 75 seconds). Thus for a typical observation SIGMA will record a film of 285 images in a given energy band. C/ Photon by photon mode This mode allows the recording of the coordinates and the energy of all detected photons. With this mode we will be able to reconstruct 248 x 232 pixels images in 256 energy channels. The time is also recorded every 63 photons with an accuracy of 1 ms. This storage is interrupted when the on-board memory is full. The capacity of this memory, 128 Mbits, allows the recording of 5.2 10 photons and, if we assume a counting rate of 300 count/s the observation will have a duration of five hours. D/ Fast variability In this mode, no imaging is performed, the experiment records only the energy (in 128 channels) and the date (1 ms accuracy) of each detected photons. E/ y ray burst detection We shall describe two differents detection system for analysing gamma ray bursts. Burst detection in the imaging system
The detection of bursts in the field of view of the telescope will give rise to a rapid localization (l’4O” accuracy) after image processing ;
this
localization will be accomplished independantly of other satellite experiment. The gamma ray burst mode is triggered by four independant circuits which combine two energy bands with two integration time (0.25 and 2 s.). The two energy bands are contiguous and commandable. The background is measured in 64 s. and the trigger occurs if one of the four circuits detects a counting rate in excess of more than 9 a. In this case, the position and the energy of 32256 photons are recorded. 28224 after the trigger signal and 4032 before. The time is recorded every 63 photons with 1 ms accuracy. 2 in the range 30-100 key and The of the this l08 erg/cm 2 l0 sensitivity erg/cm in 100system keV-l is MeV energy range for an integration time of 0.25 s. The number of events expected by year in the field of view is
between 2 and 20. Burst detection with the anticoincidence shield
This mode 2 takes advantage of the large area of the anticoincidence shield (19200 cm ). This mode is fully discribed in reference /4/, only the main features are discussed here. Temporal analysis : the time history of a burst event is recorded with 8 ms accuracy during 30 s. A time to spill function records the time every 32 photons with a 8 s accuracy. This will allow the study of fine time structure of burst onsets, the total capacity of this time to spill function is of 12288 values. Spectral analysis : The spectral analysis is performed between 100 keV to 7.5 MeV during 10 mm. These spectra are with a various temporal accuracy (from 125 ms to 30 s.) and various spectral resolutions (from 128 to 25 energy channels). Sensitivity : The expected sensitivity in the 100 keV-l MeV energy range for a 0.25 s. integration time is lO~ erg/cm2. The number of bursts which should be detected is 3000 in 4 it sr per year for lOOt live time, using the Log N -Log S
curve.
J. P. Roques ci a!.
(2)230
DATA PROCESSING AND COMPUTER SIMULATION
During the study and the realisation of the SIGMA telescope an extensive work has been made concerning the data processing of such an imaging instrument. This work has been made with both the help of a numerical simulation program of the experiment and the realisation of a laboratory model of the imaging system /5/, /6/. All these studies have been made in two purposes : the first was the optimization of the imaging system configuration, and the second was the preparation of the scientific data analysis. Here we shall just summarize the main results we obtained. Non uniformity of the detector The detector of SIGMA is a position sensitive detector of the Anger camera variety. This kind of camera suffers from inherent localised aberrations of the detector sensitivity and linearity. The resultant variation of counts in the image pixels due to this effect is typically about 10% of the mean value. The reconstruction of the image being a cross correlation method amplifies this variation leading to an important loss of the SNR ratio. A correction of the detector picture is thus needed before any reconstruction of the sky. This correction can be made with the help of suitable calibration matrices which can be obtained on board or in the laboratories. This method and the constraints linked to its use are described in /7/. It has been prouved that this method is sufficiently efficient to allow correct deconvolution to be performed even in the case of the failure of one of the photomultiplier tube camera. Fast deconvolution The basic reconstruction method of the sky map from the detector image p consists of a convolution product 2: W = Pmultiplications * G. The reconstruction of the x m2 and additions matrix method involves (where w n, by m this are the dimensions ofn the matrices) which become very big for large matrices and then impracticable for a systematic use. However, using the mathematical properties of the IJRA matrices, it is possible to reduce substantially the computational effort required. The method we have developped /8/ accomplishes the reconstruction of 248 x 232 matrix in 5 seconds on a ND 570 computer (Norsk Data) which is a 32 bits 3.3 Mips computer. Cleaning algorithm Another problem which has been studied and solved is that of sources located outside the field of view of the telescope,which projects only a partial mask shadow on the detector plane. In this case the coded mask properties are not conserved (i.e. the cross correlations with other projections are non zero) and a simple deconvolution creates ghost images. A cleaning algorithm has been performed to remove these ghosts. It works with an iterative algorithm which is able to recognize and locate partially coded sources and then substract its contribution on the detector matrix /5/. This algorithm makes possible the use of the partially coded field of view. The simulated image of the galactic center region show the result obtained with this algorithm (figure 5). Improvement of the resolving power of the instrument To obtain the sky map, the matrix recorded on the detector is according to the finely sampled balanced correlation /9/. The spread function of this cross correlation method is basically pyramid with the FWMM equal to the width of one mask element, 13’ 20’ square which defines the separating power.
reconstructed simple point a square i.e. 13’ 20” x
This square corresponds to a 8 x 8 pixels square on the image, as in the case of SIGMA the size of a mask cell is subdivided in 8 x 8 pixels on the detector. Two methods have been studied which deconvolve the reconstructed image from the pyramid introduced by the use of the cross correlation methods /10/, /11/. The method is also able to deconvolve from the additional blurring
SIGMA on the GRANAT Satellite
which comes from the spatial
resolution
of the detector.
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This method leads
to an increase of the separating power where value become about 10’ arc for the most unfavourable value of the spatial resolution (i.e. at low energy),
an to 5’ for the most favourable
case.
Maximum entropy The application of the maximum entropy method has also been applied to the SIGMA experiment configuration ; results and description can be found in /12/ and references therein. SCIENTIFIC OBJECTIVES Astronomical observations in the spectral range of SIGMA should provide the
only way to explore the sites where the largest energy transferts occur, and to study the violent processes which determine the dynamics and evolution of stars and galaxies. It is beyond the scope of such a paper to detail all the objectives of such a mission. However, we shall illustrate the scientific capabilities of the SIGMA mission with the following examples
The galactic
center
Thanks to the gamma-ray spectrometer onboard the HEAO-3 satellite
,
the
presence of a variable electron-positron annihilation line emission from the
center of our Galaxy is now well admitted. However, a detailed picture of the 511 keV line emission is still lacking, resulting in a very confuse situation with regard to the origin of this emission. SIGMA with both its high resolution imaging capabilities and the energy resolution of its detector should enable a better understanding of the origin of this
emission. The galactic center sources Concerning the emission of localised sources irs this region the situation is also quite confuse since observations with a good sensitivity, and a sufficient angular resolution are still lacking in the energy range of SIGMA. In order to provide an estimate of the SIGMA capabilities to resolve this complex region, a simulation of galactic center has been performed
/13/. This simulation includes a model of the diffuse emission and a set of point sources. - Concerning the diffuse emission we used the lOOi.tm IRAS map normalized in the region of interest to the integral galactic y-ray emission. - The simulation sources was done using the results of /14/, /15/, /16/, /17/ with ad-hoc extrapolation of their spectra. The sky map obtained after deconvolution of this simulation is shown in figure 5. The simulation has been made in the 30-50 keV energy band for a 10 days observation. It can
be seen that, within the adopted assumptions, the diffuse emission is still visible. Concerning the point sources, the improvement given by highresolution imaging is obvious. Active galaxies nuclei
The spectra of active galaxies nuclei (AGN) have a mean spectral index of -0.6 with a break in the MeV region. Thus they emit the most important fraction of their energy in the spectral domain accessible with SIGMA. In addition it seems very important to search for a possible e°e annihilation feature in the AGN’s spectra. This line may be induced by the following process : photon-photon interaction associated to high energy photons produces electron positron pairs which, after cooling, annihilate giving a 511 keV line emission. AGN’s are also highly variable. All these studies are accessible with SIGMA, thus an important extragalactic program is foreseen. As an example, with the anticipated SIGMA sensitivity, the variability of NGC 4151 can be studied with 5 mn time resolution with 5o signification in the 50 keV-100 keV energy range. SN1987A Obviously, the supernovae SN1987A will be observed by SIGMA. Now, after a number of observations, a consistent model is beginning to emerge. For example, the theoretical prediction of the keV (Co flux 2.s. /18/.122for theline end intensities of 1989. This decay) a the flux sensitivity of 8 1O~ ph/cm is well give above of the SIGMA experiment. Search for lines and also study of the continuum will be possible in very good conditions with the SIGMA telescope.
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J. P. Roques et a!. CONCLUSION
The expected performances of SIGMA experiment, its high sensitivity and high angular resolution, should provide a number of new and exciting results in the field of hard X-ray/y-ray astronomy. REFERENCES /1/ E.E. Fenimore, and T.M. Cannon, Appl.Opt. 17, 337 (1978) /2/ D. Calabro, and J.K. Wolf, Inf. Control 11, 537, (1968) /3/ H.O. Anger, B.A. Trans 5, 311, (1966) /4/ A. Guerry, M. Jouret, J.P. Rogues, Ph. Laudet, P. Mandrou, M. Niel, J. Paul, Adv. spa. Res Vol 6, n°4, 103 (1986) /5/ J.P. Rogues, 3ème nN28l3 (1983)
cycle
Thesis,
Université
Paul Sabatier Toulouse,
/6/ Ph. Laudet, Thesis n° 218, Université Paul Sabatier, Toulouse III (1987) /7/ Ph. Laudet, and J.P. Rogues, Nuci. Inst. and Meth. in Phys.Res. A267, 212, (1988) /8/ S.E. Woosley, P.P. Pinto, Aip Conf., Vol 170, 98, (1987) /9/ E.E Fenimore, and
T.M. Cannon, Appl.Opt. 20, 1858 (1981)
/10/ Ph. Laudet, and J.P. Rogues, Applied Optics Vol 27, N~2O, 4226, (1988) /11/ Ph. Laudet, and J.P. Rogues, Applied Optics, in press (1989) /12/ A. Guerry, These de Doctorat, Université Paul Sabatier Toulouse N° 370, (1988) /13/ Ph. Laudet, J.P. Rogues, A. Guerry, P. Mandrou, F. Lebrun, 3. Paul, Adv Space Res. Vol 6, NN4, 161, (1986) /14/ A. Levine et al., Astrophys. 3. Suppl. 54, 581 (1984) /15/ R.J. Proctor, G.K. Skinner, and A.P. Wilimore, Mon.Not.R.Astron.Soc. 185, 745 (1978). /16/ E.G. Cruddace, G. Fritz, S. Shulman, H. Friedman, 3. Mckee, and M. Johnson, Astrophys. 3., lett. 222, L95 (1978) /17/ M.G. Watson, R. Willingale, J.E. Grindlay, and P. Hertz, Astrophys. 3. 250, 142 (1981). /18/ F.K. Knight, W.N. Johnson, J.D. Kurfess and M.S. Strickman, Astrophys. 3. 290, 557 (1985)
0273-1177/90 $0.00 +50 Copyright © 1989 COSPAR
THE SIGMA MISSION ON THE GRANAT SATELLITE J. P. Roques,’~’J. Pau1,’~P. Mandrou* and F. Lebrun** *
Centre d’Etude Spatiale des Rayonnements, BP 4346, 31029 Toulouse
Cedex, France **
Centre d’Etudes Nucleaires de Saclay, 91191 Gif-sur- Yvette Cedex, France
ABSTRACT The soviet spacecraft GRANAT, to be launched in july 1989, will carry seven experiments dedicated to the hard X-ray/Low energy gamma ray astronomy. After a brief description of the mission and of the overall flight hardware, we shall focus on the SIGMA experiment. This french telescope designed to obtain high sensitivity images of the sky in the 30 - 1300 key energy range will be extensively described. The main scientific objectives of the instrument will also be discuss. THE GRANAT MISSION The soviet GRANAT satellite is entirely devoted to high-energy astronomy. This five-ton satellite features an Astron-type platform already used in the VEGA Mission.The launch is now planned for july 1989 and an operational life time in excess of 18 months is foreseen. The orbit chosen is highly excentric (2000 - 200 000 km) with a perigee fixed in the southern hemisphere. The period of this orbit is 4.05 days but because of operational constraints and especially passage through the Earth radiation belts (below 60 000 km) the useful time per orbit is 3 days. This satellite is 3 axis stabilized : one axis is pointed towards the sun another is fixed with a star tracker. The GRANAT payload is made of seven instruments whose characteristics are in table 1. DESCRIPTION OF THE SIGMA EXPERIMENT The SIGMA experiment was first proposed to C.N.E.S. (Centre National d’Etudes Spatiales, the french space agency) in june 1981 by two french laboratories (Centre d’Etude Spatiale des Rayonnements in Toulouse and Service d’Astrophysique in Saclay). It is now engaged in the frame work of a French-Soviet joint program as the main payload of GRANAT. The high energy astronomy results which have been obtained until now have shown the necessity to build experiments with both a good angular resolution and a large field of view. This is why the main goal of the SIGMA mission is to perform accurate images of the sky in the hard X-Ray/low energy gamma-ray domain, which is almost unexplored. In order to fullf ill these objectives, SIGMA has been designed using coded mask imaging techniques. The instrument is thus constructed with the association of a coded mask (built with elements having a transparency of 0 or 1) and a position sensitive detector which receives the flux from the sky modulated by the mask. If M is an array representing the mask transmission, and S is an array representing the object in the field of view of the imaging device, then the image P recorded on the position sensitive detector is such as P=M*S+B where B represents the background distribution.
(2)224
J. P. Roques et a!.
TABLE 1 The GRANAT payload
INSTRUMENT
MAIN OBJECTIVES
CHARACTERISTICS
SIGMA (French)
GAMMA-RAY IMAGING, SPECTRAL AND TIME ANALYSIS OF LOCALIZED SOURCES
30 keV-l.3 MeV FIELD OF VIEW 4°45’ x 4’20’
TELESCOPE WITH CODED MASK USING A GAMMA RAY CAMERA
ART-P (Soviet)
X-RAY IMAGING, SPECTRAL AND TIME ANALYSIS OF LOCALIZED SOURCES
3 keV-lOO keV FOV : l.8°xl.8°
TELESCOPE WITH CODED MASK USING FOUR INDEPENDENT DETECTORS
ART-S (Soviet)
SPECTRAL AND TIME ANALYSIS OF RELATIVELY BRIGHT LOCALIZED SOURCES
3 keV-150 key FOV 2° x 2°
SPECTROSCOPE WITH FOUR PROPORTIONAL COUNTER
PHEBUS (French)
LOCALIZATION SPECTRAL AND TIME ANALYSIS OF BURSTS
100 keV-lOO keV
SIX DETECTORS AROUND THE PERIPHERY OF THE SPACECRAFT TO COVER THE ENTIRE FOV
KONUS (Soviet)
LOCALIZATION, SPECTRAL, AND TIME ANALYSIS OF BURST
20 keV-
SEVEN DETECTORS DISTRIBUTED AROUND THE SPACECRAFT
TOURNESOL (Soviet)
OPTICAL DETECTION OF BURST. SPECTRAL ANALYSIS OF A PARTICULAR SOURCE, ANALYSIS OF BURST
2 keV- 20 MeV X-RAY FOV : 6° x 6° FOV IN VISIBLE LIGHT : 5°x 5°
FOUR PROPORTIONAL COUNTERS AND TWO OPTICAL DETECTORS. THE SYSTEM IS GIMBALMOUNTED.
WATCH (Danish)
MONITORING OF PERSISTENT X-RAY SOURCES. X-RAY BURST DETECTION AND LOCALIZATION
6 key -180 keV FOV 4 it
FOUR INSTRUMENTS USING A ROTATING COLLIMATOR CONCEPT
2 MeV
DESCRIPTION
If there is an array G such that M*G=5 Then the reconstructed objet W is expressed by W
~ * G = (M =O+B*G =
*
0
+
B)
*
G
Some class of arrays such that M *G = 5 exists. For SIGMA we have choosen Uniformely Redondant Arrays /1/ (these URA’s represents a special case of quadratic residue matrices when their dimensions are given by two prime numbers differing by 2 /2/. INSTRUMENTAL CONCEPT A schematic view of the instrument is shown in Figure 1. The main elements are: 1/ A position-sensitive detector based on the Anger camera principle /3/ it consists of a Nal (tl) cristal 1.25-cm thick analysed throught an optical glass 1.25-cm thick by 61 hexagonal photomultiplier tubes inserted in a carbon fiber honeycomb structure. The energy resolution and position accuracy of this detector are shown figure 2 and figure 3. 2/ A 1.5 cm thick tungsten mask consisting of 49 x 53 elements built on the basis of a 29 x 31 URA. The size of these elements is 9.4 x 9.4 mm and the mask is mounted 2.5 m away from the detection plane.
CODED MASK SQUARES OF TUNGSTEN SHIELDING (TIN, LEAD, TANTALUM)
HEAT SHIELD
PLASTIC SCINTILLATOR ANTI-COINCIDENCE CIRCUIT OPTICAL HEAD ELLAR TECTOR
GAMMA CAMERA PHOTO MULTIPLIER MODULE
ELECTRONICS
Fig. i. Schematic view of the Sigma telescope.
I II.!
_____
3 ~ ~
____
___
2
___
7?.SE+04 SS.GE+04
PU
~
5 /
~
E
+
~4
9E.EiE+03
___
--
7E1.1E+03
____
~
~‘~‘ -
Deg.-->
99.3E+04
12.OE+E14
-.
0
12.1E+0E
1.11
I
20.1E÷03
~4°~E02
Fig. 5. Simulation of the galactic centre, 30—50 key, 10 days observation time.
X
SIGMA on the GRANAT Satellite
(2)227
All these caracteristics impose the key proFerties detection sensitive area : 797 cm maximum sensitivity field of view 4~.45’ x 4°.20’ intrinsic resolving power : 13 arc mm.
of telescope
3/ An active CsI (tl) anticoincidence shield surrounds the detector and limits its field of view to about 1 sr. This shielding consists of 31 independant cristals, each of them being analysed by two photomultiplier tubes.The seven cristals of the botton anticoincidence are 4 cm thick. The lateral shield is made of two rings of 6 scintillators. The cristal of the lower ring are 4 cm thick, those of the upper ring of 3 cm thick. The lateral area of this shielding is 19200 cm A thin (5 mm) plastic scintillator detector, covering the field of view, vetoes incoming charged particles. A passive graded shield (0.5 mm of Pb, 0.1 mm of Ta, 0.4 mm of Sn) is wrapped around the tube holding the mask. This shielding significantly reduces the low—energy induced background. Sensitivity The anticipated sensitivity of the SIGMA telescope depends on both the efficiency of the detector and the background. While the sensitivity of the detector can be simply derived from actual measurements, a realistic estimate of the background requires detailed computations. Taking in account all the expected background contributions : diffuse celestial, spallations, secondary neutrons and photons induced within the spacecraft ; the expected background spectrum has been calculated. Then it is possible to deduce the continuum sensitivity which corresponds to 3 a signification in one day or ten days observation (figure 4). For the line sensitivity it is necessary to take in account the energy resolution of the detector (figure 2). The resulting 3 a spnsitivities for ten days observation are 2 l0’~ ph/cm2.s for 122 key, 2 l0 ph/cm2.s for 511 key and 2.5 l0~ ph/cm2.s for 847 key line. Here we must point out an important feature of SIGMA for y ray lines detection : thanks to the coded mask imaging technique the measurements of the background and the signal are made simultaneously. This will allow a very secure detection of lines. SIGMA OPERATING MODES SIGMA can operate in any one of several modes, which will be selected on the basis of the scientific objectives assigned to the telescope at a given time. A/ Spectral imaging mode In this mode the telescope will perform the following functions simultaneously.
C\uhl
101
112
1I~ 0 Ik~V)
Fig. 2. Energy resolution of the position sensitive detector.
J. P. Roques ci a!.
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101
io2 E (keV)
Fig. 3. Accuracy of the position detection system. Fine imaging : Four high resolution images, 248 x 232 pixels, are made in four adjacent energy bands. The thresolds of these energy channels are adjustable. This set of four images may be repeated N times, N being a number between 1 and 12. For a typical 24 hours observation, a set of fine images will have an exposure of 4 hours and the telescope will record a total of 24 high resolution images. Spectral imaging : 95 images of 124 x 126 pixels are made in 95 energy bands varying logarithmically in width. This set of images is repeated N/2 times
(N is the same value as defined above). For a typical observation this set of images have an exposure time of eight hours and SIGMA will record 285 images of 124 x 116 pixels. Slow variability : The counting rate in four energy channels, the same four energy bands than those defined for fine imaging, is recorded every four seconds. High resolution spectrum : 1024 channels spectral analysis, of the photon detected by the position sensitive detector is performed. This analysis is made every 30 mm. for a typical observation (i.e. 16 times per spectral imaging format). Attitude drift : The attitude drift of the spacecraft is measured with a stellar sensor. This measure is recorded every four seconds with an accuracy of 15 arc seconds. y Ray burst : The gamma ray burst functions, the anticoincidence shield are enabled. 2
~
io~
CRAB
CYG
both on the imaging system and
I
Xl
CAN A
_______
~
-
11_i 101
81111
1
I 102 ENERGY t~~V>
Fig. 4. Sensitivity for 1 day and 10 days observation time.
SIGMA on the GRANAT Satellite
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B/ Time variability imaging
This mode is the same as the fine imaging mode except that the function spectral imaging is replaced by time variability : in this mode we record images of 124 x 116 pixels between two commandable threholds. One image have an exposure time of N x 60 mn /48 (i.e. the minimum exposure time is 75 seconds). Thus for a typical observation SIGMA will record a film of 285 images in a given energy band. C/ Photon by photon mode This mode allows the recording of the coordinates and the energy of all detected photons. With this mode we will be able to reconstruct 248 x 232 pixels images in 256 energy channels. The time is also recorded every 63 photons with an accuracy of 1 ms. This storage is interrupted when the on-board memory is full. The capacity of this memory, 128 Mbits, allows the recording of 5.2 10 photons and, if we assume a counting rate of 300 count/s the observation will have a duration of five hours. D/ Fast variability In this mode, no imaging is performed, the experiment records only the energy (in 128 channels) and the date (1 ms accuracy) of each detected photons. E/ y ray burst detection We shall describe two differents detection system for analysing gamma ray bursts. Burst detection in the imaging system
The detection of bursts in the field of view of the telescope will give rise to a rapid localization (l’4O” accuracy) after image processing ;
this
localization will be accomplished independantly of other satellite experiment. The gamma ray burst mode is triggered by four independant circuits which combine two energy bands with two integration time (0.25 and 2 s.). The two energy bands are contiguous and commandable. The background is measured in 64 s. and the trigger occurs if one of the four circuits detects a counting rate in excess of more than 9 a. In this case, the position and the energy of 32256 photons are recorded. 28224 after the trigger signal and 4032 before. The time is recorded every 63 photons with 1 ms accuracy. 2 in the range 30-100 key and The of the this l08 erg/cm 2 l0 sensitivity erg/cm in 100system keV-l is MeV energy range for an integration time of 0.25 s. The number of events expected by year in the field of view is
between 2 and 20. Burst detection with the anticoincidence shield
This mode 2 takes advantage of the large area of the anticoincidence shield (19200 cm ). This mode is fully discribed in reference /4/, only the main features are discussed here. Temporal analysis : the time history of a burst event is recorded with 8 ms accuracy during 30 s. A time to spill function records the time every 32 photons with a 8 s accuracy. This will allow the study of fine time structure of burst onsets, the total capacity of this time to spill function is of 12288 values. Spectral analysis : The spectral analysis is performed between 100 keV to 7.5 MeV during 10 mm. These spectra are with a various temporal accuracy (from 125 ms to 30 s.) and various spectral resolutions (from 128 to 25 energy channels). Sensitivity : The expected sensitivity in the 100 keV-l MeV energy range for a 0.25 s. integration time is lO~ erg/cm2. The number of bursts which should be detected is 3000 in 4 it sr per year for lOOt live time, using the Log N -Log S
curve.
J. P. Roques ci a!.
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DATA PROCESSING AND COMPUTER SIMULATION
During the study and the realisation of the SIGMA telescope an extensive work has been made concerning the data processing of such an imaging instrument. This work has been made with both the help of a numerical simulation program of the experiment and the realisation of a laboratory model of the imaging system /5/, /6/. All these studies have been made in two purposes : the first was the optimization of the imaging system configuration, and the second was the preparation of the scientific data analysis. Here we shall just summarize the main results we obtained. Non uniformity of the detector The detector of SIGMA is a position sensitive detector of the Anger camera variety. This kind of camera suffers from inherent localised aberrations of the detector sensitivity and linearity. The resultant variation of counts in the image pixels due to this effect is typically about 10% of the mean value. The reconstruction of the image being a cross correlation method amplifies this variation leading to an important loss of the SNR ratio. A correction of the detector picture is thus needed before any reconstruction of the sky. This correction can be made with the help of suitable calibration matrices which can be obtained on board or in the laboratories. This method and the constraints linked to its use are described in /7/. It has been prouved that this method is sufficiently efficient to allow correct deconvolution to be performed even in the case of the failure of one of the photomultiplier tube camera. Fast deconvolution The basic reconstruction method of the sky map from the detector image p consists of a convolution product 2: W = Pmultiplications * G. The reconstruction of the x m2 and additions matrix method involves (where w n, by m this are the dimensions ofn the matrices) which become very big for large matrices and then impracticable for a systematic use. However, using the mathematical properties of the IJRA matrices, it is possible to reduce substantially the computational effort required. The method we have developped /8/ accomplishes the reconstruction of 248 x 232 matrix in 5 seconds on a ND 570 computer (Norsk Data) which is a 32 bits 3.3 Mips computer. Cleaning algorithm Another problem which has been studied and solved is that of sources located outside the field of view of the telescope,which projects only a partial mask shadow on the detector plane. In this case the coded mask properties are not conserved (i.e. the cross correlations with other projections are non zero) and a simple deconvolution creates ghost images. A cleaning algorithm has been performed to remove these ghosts. It works with an iterative algorithm which is able to recognize and locate partially coded sources and then substract its contribution on the detector matrix /5/. This algorithm makes possible the use of the partially coded field of view. The simulated image of the galactic center region show the result obtained with this algorithm (figure 5). Improvement of the resolving power of the instrument To obtain the sky map, the matrix recorded on the detector is according to the finely sampled balanced correlation /9/. The spread function of this cross correlation method is basically pyramid with the FWMM equal to the width of one mask element, 13’ 20’ square which defines the separating power.
reconstructed simple point a square i.e. 13’ 20” x
This square corresponds to a 8 x 8 pixels square on the image, as in the case of SIGMA the size of a mask cell is subdivided in 8 x 8 pixels on the detector. Two methods have been studied which deconvolve the reconstructed image from the pyramid introduced by the use of the cross correlation methods /10/, /11/. The method is also able to deconvolve from the additional blurring
SIGMA on the GRANAT Satellite
which comes from the spatial
resolution
of the detector.
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This method leads
to an increase of the separating power where value become about 10’ arc for the most unfavourable value of the spatial resolution (i.e. at low energy),
an to 5’ for the most favourable
case.
Maximum entropy The application of the maximum entropy method has also been applied to the SIGMA experiment configuration ; results and description can be found in /12/ and references therein. SCIENTIFIC OBJECTIVES Astronomical observations in the spectral range of SIGMA should provide the
only way to explore the sites where the largest energy transferts occur, and to study the violent processes which determine the dynamics and evolution of stars and galaxies. It is beyond the scope of such a paper to detail all the objectives of such a mission. However, we shall illustrate the scientific capabilities of the SIGMA mission with the following examples
The galactic
center
Thanks to the gamma-ray spectrometer onboard the HEAO-3 satellite
,
the
presence of a variable electron-positron annihilation line emission from the
center of our Galaxy is now well admitted. However, a detailed picture of the 511 keV line emission is still lacking, resulting in a very confuse situation with regard to the origin of this emission. SIGMA with both its high resolution imaging capabilities and the energy resolution of its detector should enable a better understanding of the origin of this
emission. The galactic center sources Concerning the emission of localised sources irs this region the situation is also quite confuse since observations with a good sensitivity, and a sufficient angular resolution are still lacking in the energy range of SIGMA. In order to provide an estimate of the SIGMA capabilities to resolve this complex region, a simulation of galactic center has been performed
/13/. This simulation includes a model of the diffuse emission and a set of point sources. - Concerning the diffuse emission we used the lOOi.tm IRAS map normalized in the region of interest to the integral galactic y-ray emission. - The simulation sources was done using the results of /14/, /15/, /16/, /17/ with ad-hoc extrapolation of their spectra. The sky map obtained after deconvolution of this simulation is shown in figure 5. The simulation has been made in the 30-50 keV energy band for a 10 days observation. It can
be seen that, within the adopted assumptions, the diffuse emission is still visible. Concerning the point sources, the improvement given by highresolution imaging is obvious. Active galaxies nuclei
The spectra of active galaxies nuclei (AGN) have a mean spectral index of -0.6 with a break in the MeV region. Thus they emit the most important fraction of their energy in the spectral domain accessible with SIGMA. In addition it seems very important to search for a possible e°e annihilation feature in the AGN’s spectra. This line may be induced by the following process : photon-photon interaction associated to high energy photons produces electron positron pairs which, after cooling, annihilate giving a 511 keV line emission. AGN’s are also highly variable. All these studies are accessible with SIGMA, thus an important extragalactic program is foreseen. As an example, with the anticipated SIGMA sensitivity, the variability of NGC 4151 can be studied with 5 mn time resolution with 5o signification in the 50 keV-100 keV energy range. SN1987A Obviously, the supernovae SN1987A will be observed by SIGMA. Now, after a number of observations, a consistent model is beginning to emerge. For example, the theoretical prediction of the keV (Co flux 2.s. /18/.122for theline end intensities of 1989. This decay) a the flux sensitivity of 8 1O~ ph/cm is well give above of the SIGMA experiment. Search for lines and also study of the continuum will be possible in very good conditions with the SIGMA telescope.
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J. P. Roques et a!. CONCLUSION
The expected performances of SIGMA experiment, its high sensitivity and high angular resolution, should provide a number of new and exciting results in the field of hard X-ray/y-ray astronomy. REFERENCES /1/ E.E. Fenimore, and T.M. Cannon, Appl.Opt. 17, 337 (1978) /2/ D. Calabro, and J.K. Wolf, Inf. Control 11, 537, (1968) /3/ H.O. Anger, B.A. Trans 5, 311, (1966) /4/ A. Guerry, M. Jouret, J.P. Rogues, Ph. Laudet, P. Mandrou, M. Niel, J. Paul, Adv. spa. Res Vol 6, n°4, 103 (1986) /5/ J.P. Rogues, 3ème nN28l3 (1983)
cycle
Thesis,
Université
Paul Sabatier Toulouse,
/6/ Ph. Laudet, Thesis n° 218, Université Paul Sabatier, Toulouse III (1987) /7/ Ph. Laudet, and J.P. Rogues, Nuci. Inst. and Meth. in Phys.Res. A267, 212, (1988) /8/ S.E. Woosley, P.P. Pinto, Aip Conf., Vol 170, 98, (1987) /9/ E.E Fenimore, and
T.M. Cannon, Appl.Opt. 20, 1858 (1981)
/10/ Ph. Laudet, and J.P. Rogues, Applied Optics Vol 27, N~2O, 4226, (1988) /11/ Ph. Laudet, and J.P. Rogues, Applied Optics, in press (1989) /12/ A. Guerry, These de Doctorat, Université Paul Sabatier Toulouse N° 370, (1988) /13/ Ph. Laudet, J.P. Rogues, A. Guerry, P. Mandrou, F. Lebrun, 3. Paul, Adv Space Res. Vol 6, NN4, 161, (1986) /14/ A. Levine et al., Astrophys. 3. Suppl. 54, 581 (1984) /15/ R.J. Proctor, G.K. Skinner, and A.P. Wilimore, Mon.Not.R.Astron.Soc. 185, 745 (1978). /16/ E.G. Cruddace, G. Fritz, S. Shulman, H. Friedman, 3. Mckee, and M. Johnson, Astrophys. 3., lett. 222, L95 (1978) /17/ M.G. Watson, R. Willingale, J.E. Grindlay, and P. Hertz, Astrophys. 3. 250, 142 (1981). /18/ F.K. Knight, W.N. Johnson, J.D. Kurfess and M.S. Strickman, Astrophys. 3. 290, 557 (1985)