Hard x-ray emission from galactic black hole candidates

This dissertation describes observations of high-energy (10 keV-1 MeV) X-rays emitted by three objects in our Galaxy which may be black holes in binary star systems. The X-rays are believed to be produced by viscous heating of matter flowing into the hole. All of the observations were made with an array of inorganic scintillation counters on the HEAO-1 satellite in 1977 and 1978. The best-known of these sources, Cygnus X-1, is very intense (> 3 x 1037 erg/sec) and fairly close (2.5 kpc), and thus easy to study in detail. The energy spectrum shape in the 10-300 keV band did not differ in three observations six months apart, although the intensity did. The average spectrum shows weak evidence for a broad emission line produced by positron annihilation. There is also evidence for a periodic 5.6 day intensity variation, which was previously unknown for photon energy greater than 10 keV. This means that if the modulation is produced by absorbing matter, it must be ionized. No spectral variation was observed on time scales from minutes to months, but it is present on scales of a few seconds. A power spectrum analysis of the rapid X-ray variability shows that a broad range of time scales is in operation, with the longest a few seconds. This can be interpreted as the effect of large-scale instabilities in the accretion disk which modulate the flow of mass into the central, hot region. The X-ray emission of Circinus X-1 was weaker in 1977-78 than it was in other measurements in 1971-75. The spectrum also became a more steeply falling function of energy. The good correlation between the 10-200 keV and 3-6 keV intensitites rules out intensity modulation caused by intervening matter. Two very intense one-minute flares with very different energy spectra were observed about three minutes apart