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optical identification of soft γ-ray sources discovered by the “SIGMA” telescope
Adv. Space Res. Vol. 11, No.8, pp. (8)83-(8)84, 1991 Printed in Great Britain. All right reserved.
0273-1177(91 $0.00 + .50 Copyright © 1991 COSPAR
A PROGRAM FOR X-RAY/OPTICAL IDENTIFICATION OF SOFT ?-RAY SOURCES DISCOVERED BY THE “SIGMA” TELESCOPE P. A. Caraveo,* G. F. Bignami,* S. Mereghetti,* J. Pau1,’~ A. Goldwurm,** L. Vigroux,** P. Mandrou,*** G. Vedrenne*** and J. P. Roques~~ *Jstj~toFisica Cosmica, Milano, Italy **Se~iced’Astrophysique, CEA Saclay, France ***CEsR, Toulouse, France
The aim of this programme is to use the ESO 3.5m class telescopes to search for the optical counterparts ofhard X-ray/soft y-ray sources (including y-ray bursts and transient events) discovered by the SIGMA telescope on board the GRANAT satellite (successfully launched on Dec.lSt, 1989), also exploiting the soft X-ray data of the all sky survey to be performed by the ROSAT satellite (successfully launched on June 2nd, 1990). It will be the first time that coordination between on going high-energy space missions, such as SIGMA and ROSAT, and ground based telescopes is implemented on a programmed long-term basis. SIGMA The SIGMA telescope (Paul et al., this issue) represents the first breakthrough in one of the last unexplored wavelength regions in astronomy. It consists of a gamma camera/coded mask telescope system separated by 2.5 m, with imaging capability yielding a source localization accuracy of few arcmins within a field of view of 4.3°x4.7° and a limiting sensitivity in the tens of milliCrab region. The energy rangejoes from 35 keY to 1.3 MeV, and operations, planned for 2 years, will be based on l0~sec. pointed observations of the 3-axis stabilized telescope.
The SIGMA observations will yield results on both known X-ray sources, extending our knowledge of their spectrum, and on new sources, galactic and extragalactic. In addition, the mission will study the y-rav bursts occuring during its operating lifetime, and few of them have already been observed. Thanks to the dimension of the fully coded field of view, several bursts will be observed through the imaging coded mask system, thus yielding, for the first time, an immediate localization with an accuracy
of a few arcminutes and allowing a rapid coordination with ground based optical observations. It is worth noting that the rectangular telescope field of view features a central area, in which the telescope is at its maximum, surrounded by a wide field of decreasing sensitivity (the half-sensitivity boundary is a 10.6°x 11.4°rectangle) within which sources can still be positioned within several arcmins, depending on their intensity.
(8)83
(8)84
P. A. Caraveo Ct aL
ROSAT In parallel to the SIGMA higher energy observations, the ROSAT mission will work in the soft (.1-2 keY) X-ray domain, performing first a sky survey and then a sequence ofpointed observations. The authors have organized a collaboration between the two missions for exploiting the
ROSAT survey data on the basis of the SIGMA results. A Memorandum of Understanding on this subject has been recently signed by the SIGMA and ROSAT team leaders as well as by the ESO Key Project P.I.( G.F.B.) This should result in an improvement of both source positioning and knowledge of spectral shape, rendering much more interesting and meaningful the search for the optical counterpart. Based on the previous experience in optical studies of y/X-ray
sources, the addition ofthe ROSAT data will be crucial to the success ofthis project.
SIGMA—*ROSAT--.~Optical
~
~::c::g~
~
~ESOIMILANO~ ~
OPTICAL (ESO, Key Project) The search for an ontical identification of newly discovered sources, which is the most challenging, albeit thepotentially most rewarding, part of the programme, is the subject of a Key Project recently approved by the European Southern Observatory for their 3.5m class telescopes. The strategy here will consist to take first CCD images of the source region in two colours, compare the images with all existing catalogues, hopefully find potential candidates and then resort to MOS (Multiple Object Spectrometer) spectra. Another useful dimension for identification is time variability on both long and short term timescales, as in the case of AGNs and X-/gamma-ray pulsars in our Galaxy. Polarimetry, also possible on a complete source field, is yet anothertool for identifying a high-energy, non thermal candidate among normal field stars. This type of work has already resulted in the past few years in the successful study of several X-y ray source regions, carried out both in imaging/photometry, spectroscopy, polarimetry and time variability. The first observing run at the NT~is foreseen for January 1991. By that time we hope to be able to follow the strategy outlined above searching the optical counterparts of the first SIGMA/ROSAT serendipitous sources.
0273-1177(91 $0.00 + .50 Copyright © 1991 COSPAR
A PROGRAM FOR X-RAY/OPTICAL IDENTIFICATION OF SOFT ?-RAY SOURCES DISCOVERED BY THE “SIGMA” TELESCOPE P. A. Caraveo,* G. F. Bignami,* S. Mereghetti,* J. Pau1,’~ A. Goldwurm,** L. Vigroux,** P. Mandrou,*** G. Vedrenne*** and J. P. Roques~~ *Jstj~toFisica Cosmica, Milano, Italy **Se~iced’Astrophysique, CEA Saclay, France ***CEsR, Toulouse, France
The aim of this programme is to use the ESO 3.5m class telescopes to search for the optical counterparts ofhard X-ray/soft y-ray sources (including y-ray bursts and transient events) discovered by the SIGMA telescope on board the GRANAT satellite (successfully launched on Dec.lSt, 1989), also exploiting the soft X-ray data of the all sky survey to be performed by the ROSAT satellite (successfully launched on June 2nd, 1990). It will be the first time that coordination between on going high-energy space missions, such as SIGMA and ROSAT, and ground based telescopes is implemented on a programmed long-term basis. SIGMA The SIGMA telescope (Paul et al., this issue) represents the first breakthrough in one of the last unexplored wavelength regions in astronomy. It consists of a gamma camera/coded mask telescope system separated by 2.5 m, with imaging capability yielding a source localization accuracy of few arcmins within a field of view of 4.3°x4.7° and a limiting sensitivity in the tens of milliCrab region. The energy rangejoes from 35 keY to 1.3 MeV, and operations, planned for 2 years, will be based on l0~sec. pointed observations of the 3-axis stabilized telescope.
The SIGMA observations will yield results on both known X-ray sources, extending our knowledge of their spectrum, and on new sources, galactic and extragalactic. In addition, the mission will study the y-rav bursts occuring during its operating lifetime, and few of them have already been observed. Thanks to the dimension of the fully coded field of view, several bursts will be observed through the imaging coded mask system, thus yielding, for the first time, an immediate localization with an accuracy
of a few arcminutes and allowing a rapid coordination with ground based optical observations. It is worth noting that the rectangular telescope field of view features a central area, in which the telescope is at its maximum, surrounded by a wide field of decreasing sensitivity (the half-sensitivity boundary is a 10.6°x 11.4°rectangle) within which sources can still be positioned within several arcmins, depending on their intensity.
(8)83
(8)84
P. A. Caraveo Ct aL
ROSAT In parallel to the SIGMA higher energy observations, the ROSAT mission will work in the soft (.1-2 keY) X-ray domain, performing first a sky survey and then a sequence ofpointed observations. The authors have organized a collaboration between the two missions for exploiting the
ROSAT survey data on the basis of the SIGMA results. A Memorandum of Understanding on this subject has been recently signed by the SIGMA and ROSAT team leaders as well as by the ESO Key Project P.I.( G.F.B.) This should result in an improvement of both source positioning and knowledge of spectral shape, rendering much more interesting and meaningful the search for the optical counterpart. Based on the previous experience in optical studies of y/X-ray
sources, the addition ofthe ROSAT data will be crucial to the success ofthis project.
SIGMA—*ROSAT--.~Optical
~
~::c::g~
~
~ESOIMILANO~ ~
OPTICAL (ESO, Key Project) The search for an ontical identification of newly discovered sources, which is the most challenging, albeit thepotentially most rewarding, part of the programme, is the subject of a Key Project recently approved by the European Southern Observatory for their 3.5m class telescopes. The strategy here will consist to take first CCD images of the source region in two colours, compare the images with all existing catalogues, hopefully find potential candidates and then resort to MOS (Multiple Object Spectrometer) spectra. Another useful dimension for identification is time variability on both long and short term timescales, as in the case of AGNs and X-/gamma-ray pulsars in our Galaxy. Polarimetry, also possible on a complete source field, is yet anothertool for identifying a high-energy, non thermal candidate among normal field stars. This type of work has already resulted in the past few years in the successful study of several X-y ray source regions, carried out both in imaging/photometry, spectroscopy, polarimetry and time variability. The first observing run at the NT~is foreseen for January 1991. By that time we hope to be able to follow the strategy outlined above searching the optical counterparts of the first SIGMA/ROSAT serendipitous sources.