X-ray variability in the Seyfert galaxy Markarian 618.

Abstract

We report 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. The 0.1-10 keV X-ray spectrum is adequately explained by a simple power law with the photon index {GAMMA} = 1.8^+0.7^_-0.5_( 90 per cent confidence), similar to that observed from various Seyfert galaxies with the line-of-sight absorption consistent with the 21-cm value. We find evidence for rapid variability on a time-scale of 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 per cent for a delay of 600 s for the hard flux as compared to the soft flux. The observed variability and the bolometric luminosity of the nuclear source in Mrk 618 limit the mass of the putative super- massive black hole to (0.8-2.0) x 10^7^ M_sun_. An accretion rate of ~0.2 M_sun_ yr^-1^ is required to power the source with an efficiency of 0.1. 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 scatterer in the range (3.3-16.7) X 10^10^ cm^-3^ for plasma temperatures in the 100-511 keV range.