The Soviet orbital observatory GRANAT was launched in December 1989 and
was operational till November 1998. One of the main instruments
of the observatory was the French-Soviet hard X-ray coded mask telescope SIGMA
(Paul et al.1 1991, Adv.Space Res., 11, 279). It was the first
space telescope that used coded aperture technique for reconstruction of
sky images in hard X-rays (35-1300 keV). The angular
resolution of the telescope was approximately 12' and the accuracy of a source
localization is approximately 2-3'.
SIGMA discovered numerous interesting hard X-ray sources including
GRS 1758-258, which is located
only 40' from bright soft X-ray source GX 5-1. It
detected hard X-ray flux from X-ray burster A1742-294, which is very
near to bright black hole binary 1E1740.7-2942. SIGMA set an upper
limit on the hard X-ray flux of from the central supermassive black hole in
our Galaxy.
During the period 1990-1998 SIGMA observed more that one quarter of the sky
with sensitivity better than 100 mCrab. The Galactic Center region
had the deepest exposure ( approximately 9 million sec), with the
sensitivity to a source discovery (S/N > ~ 5) or approximately
10 mCrab.
A list of all detected sources with references to publications
on them is presented in the paper of Revnivtsev et al. 2004, Astr. Lett. v.6.
In these survey images (40-100 keV) all performed observations are
averaged together. Transient sources that were discovered by
SIGMA may not visible in the averaged image.
This survey has some features that users should
keep in mind. The SIGMA telescope is a complicated instrument and
is strongly dominated by the accuracy of the background subtraction.
The presence of a very bright source in the field of view of the telescope
sometimes cannot be fully accounted for and as a result of it some 'ghost'
sources can appear. Such features can be seen in the regions near
very bright sources like Crab Nebula, Cyg X-1, Nova Per 1992,
Nova Mus 1991, Nova Oph 1993, and in the Galactic Center region.
In addition to its nominal field of view (~17x17 deg)
located around the optical axis of the telescope, SIGMA had another
window of relatively high transparency of its shield,
approximately 20-30°; apart from the optical axis.
Becuase of this a very bright sources like Cyg X-1 can
cause non zero illumination of the SIGMA
detector if they are located approximately 20-30°; from the optical axis.
The ring-like features caused by this effect, can be seen around Cyg X-1,
and Nova Per 1992.
The count rate of detected sources (or upper limits)
can be roughly translated into mCrab using the fact that
that Crab nebula gives the count rate approximately 2.8e-3 cnts/s in the units, provided in 'flux' maps Provenance: High Energy Astrophysics Department,
Space Research Institute, Moscow, Russia; CEA, Centre d'Etudes de
Saclay Orme des Merisiers, France; Centre d'Etude Spatiale
des Rayonnements, Toulouse, France; Fédération de
Recherche Astroparticule et Cosmologie Université de Paris, France. This is a service of NASA HEASARC.
