Rapid X-ray variability of the Seyfert galaxy MCG -6-30-15.

Abstract

Strong and continuous variability was detected in the X-ray emission of the Seyfert I galaxy MCG-6-30-15 during a 9 hr EXOSAT observation in July 1984. Changes in the 2-6 keV flux appear quasi-sinusoidal with a modulation depth of 40-50 percent and peak-to-trough time-scales as short as 1500 s. Although less well determined, due to a lower counting rate and to filter cycling, correlated changes are also seen in the EXOSAT low-energy telescope, covering the energy band about 0.1-1.5 keV. No significant lag or lead of greater than about 1000 s is seen between the medium- and low-energy variations. A simple power law plus low-energy cut-off does not given an acceptable fit to the time-averaged spectrum, unless the line-of-sight absorbing column is surprisingly low; a separate soft spectral component, thermal or power law, being strongly preferred.