THE X-RAY-SPECTRUM OF CYGNUS X-1

The first moderate-resolution (FWHM approximately 160 eV) X-ray spectrum of a galactic black hole candidate in the 1-11 keV band has been obtained with the BBXRT experiment aboard Astro 1. There is a broad excess of flux in the 6-7 keV band above a simple power-law continuum which is consistent with that expected from reflection of the observed continuum from a disk of neutral material surrounding the central source. The edge at 7.1 keV due to photoelectric absorption by neutral Fe is deeper than that predicted for material of solar abundances, which suggests an overabundance of Fe in the source. There is little evidence for any emission features due to Fe with the possible exception of a narrow line at 6.4 keV with an equivalent width of 13 +/- 11 eV. Previous, lower resolution observations and subsequent model calculations suggested that the excess flux in the Fe K band emission seen from Cyg X-1 is produced by fluorescence from a relativistic accretion disk around the black hole. The upper limits for representative models of this kind are about 60 eV, which is a factor of about 2 lower than predictions. Pronounced intensity dips were seen, especially at low energies, during the 1251 s observation. These events are modeled as variations in the material absorbing an unchanging continuum shape. As reported by previous observers, the flux at the lowest energies is more than that predicted by a simple model of variation in the absorbing column. Several more complicated models, including one in which the low-energy excess is caused by dust scattering in the interstellar medium, are consistent with the data. The absorption during the intervals outside the dips is 5.8 x 10(21) hydrogen atoms cm-2, which is consistent with estimates based on optical and UV measurements.