The X-Ray Spectrum of Markarian 509 Observed by EXOSAT

Abstract

Mrk 509 was observed with Exosat at three different epochs during October and November 1983. Variability of about 30 percent was apparent both in the medium-energy (1-10 keV) and in the low-energy (less than 2 kev) X-ray bands between the different observations, on a time scale of 10 days. No correlation between the two energy bands was observed. The 1-10-keV spectrum of the source is best fitted by a power law with energy index 0.8. In addition, a fluorescence iron line is required to fit the data obtained on Nov. 3, 1983. The best-fit energy of the line is 6.31 + or - 0.21 keV (source frame) and the equivalent width 316 + or - 100 eV. No low-energy interstellar absorption is observed in the medium-energy spectra. When the low-energy photometric data are included in the spectral analysis, the single-power-law model requires an absorption density considerably smaller than the Galactic column density in the direction of the source: this can be fitted only through a variable soft X-ray component (VSXC). If a thermal spectrum is assumed for VSXC, a temperature of about 70 eV can be inferred from the data. The VSXC and the presence of the iron fluorescence line, have interesting implications on the source model: a limit on the size of the soft X-ray emission region of 10 to the 16th cm can be derived, and the fluorescent material must be hot, with a temperature in the range 50-80 million K, and have a flat spatial distribution.