X-ray variability in the Seyfert galaxy Markarian 618

Abstract

X-ray observations of the Seyfert type I galaxy Markarian 618 using the low-energy (LE) and the medium-energy (ME) detectors of the Exosat observatory are reported. The 0.1-10 keV X-ray spectrum is adequately explained by a simple power law with a photon index of 1.8 +0.7/-0.5 (90-percent confidence), similar to that observed from various Seyfert galaxies, with the line-of-sight absorption consistent with the 21-cm value. Evidence is found for rapid variability on a time scale of about 1000 s in both the LE and the ME data. A positive correlation is indicated between the soft and hard X-ray light curves at a maximum confidence level of 97.5 percent for a delay of 600 s for the hard flux as compared to the soft flux.. The observed lag of the hard X-rays when interpreted in terms of thermal Compton models leads to an estimate of the density of the scatter in the range (3.3-16.7) x 10 ¹⁰ cm^-3 for plasma temperatures in the 100-511 keV range.