X-ray variability of active galactic nuclei in the soft and hard X-ray bands

Abstract

We investigate the X-ray variability characteristics of hard X-ray-selected active galactic nuclei (AGNs)-based on Swift/Burst Alert Telescope-in the soft X-ray band using the Rossi X-ray Timing Explorer/All-Sky Monitor (RXTE/ASM) data. The uncertainties involved in the individual dwell measurements of the ASM are critically examined, and a method is developed to combine a large number of dwells with appropriate error propagation to derive long-duration flux measurements (greater than 10 days). We also provide a general prescription to estimate the errors in variability derived from rms values from unequally spaced data. Though the derived variability for individual sources is not of very high significance, we find that, in general, the soft X-ray variability is higher than that in hard X-rays, and the variability strengths decrease with energy for the diverse classes of AGNs. We also examine the strength of variability as a function of the break timescale in the power density spectrum (derived from the estimated mass and bolometric luminosity of the sources) and find that the data are consistent with the idea of higher variability at timescales longer than the break timescale.