Low state hard x-ray observation of Cyg X-1

Details Low state hard x-ray observation of Cyg X-1 Low state hard x-ray observation of Cyg X-1

Aug 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991aipc..232..370b&link_type=abstract

Gamma-ray line astrophysics. AIP Conference Proceedings, Volume 232, pp. 370-372 (1991).

Astronomy and Astrophysics

Astrophysics

X-Ray Sources, X-Ray Bursts, Black Holes, X-Ray

Scientific paper

We report a ``super-low'' state observation of the black hole candidate Cygnus X-1 in the energy range 15-120 keV. The data were obtained with the ``POKER'' experiment, designed to perform high sensitivity observations of cosmic sources in the hard X-Ray range (15-120 keV). The telescope consisted of an array of three high pressure Xenon Multiwire Proportional Counters (MWPC) with a total sensitive area of 7,500 cm2. The detectors were filled at a pressure of 2.6 bar with a mixture of Xe/Argon/Isobutane to provide an efficiency greater than 20% for photon energies from 15 keV to 110 keV. The MWPC spectral resolution was 13% at 60 keV. The fields of view of the three MWPCs were coaligned and limited by means of hexagonal copper collimators with an aperture of 5.0 degree FWHM. In order to provide imaging capability to the telescope one of the MWPC was equipped with two co-rotating Rotation Modulation Collimators, developed at AIT, and modulating a geometrical area of 1,600 cm2 of the detector. The telescope was launched from the Milo Base, Sicily (Italy), on 1985 August 5, and scanning and pointed observations were carried out on the Crab Nebula, A0535+26, MCG 8-11-11, NGC 4151, Cygnus X-1 and Her X-1. The Cygnus X-1 and Crab photon spectra are well described by single power laws, with photon indeces of α=1.87 and α=2.17 and intensities of 2.57×10-3 and 2.32×10-3 ph cm-2 s-1 keV-1 at 50 keV, respectively. The low hard X-ray emission from Cyg X-1 confirms that during the POKER observation the source was in a ``low'' state as also supported by EXOSAT data collected at lower energies on August 12.

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