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

Abstract

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...