X-ray variability of AGNs in the soft and the hard X-ray bands

TL;DR

This study analyzes the X-ray variability of AGNs across soft and hard bands, revealing that variability is generally higher in soft X-rays and decreases with energy, with variability strength linked to time scales and source properties.

Contribution

It introduces a new method for combining ASM dwell data to measure long-term flux variability and provides a prescription for error estimation in unevenly spaced data.

Findings

Soft X-ray variability exceeds hard X-ray variability.

Variability strength decreases with increasing energy.

Higher variability observed at time scales longer than the break time scale.

Abstract

We investigate the X-ray variability characteristics of hard X-ray selected AGNs (based on Swift/BAT data) in the soft X-ray band using the RXTE/ASM data. The uncertainties involved in the individual dwell measurements of 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 are not of very high significance, we find that, in general, the soft X-ray variability is higher than those in hard X-rays and the variability strengths decrease with energy for the diverse classes of AGN. We also examine the strength of variability as a function of the break time scale 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 time scales longer than the break time scale.