X-Ray Luminosity and Spectral Variability of Hard X-Ray--selected Active Galactic Nuclei

Abstract

One hundred sixty-one observations archived in the EXOSAT data base are analyzed in order to study time and spectral variability of all 30 sion-line active galactic nuclei (AGNs) in the Piccinotti sample. For each source long-term (from days to months) and short-term (from minutes to hours) luminosity variations were sought using the soft (LE: 0.05-2 keV) and hard (ME: 1-8 keV) light curves available. Our results show that flux variability is an extremely common property of AGNs. All objects but one which were observed at least twice in the ME show signs of variation. More than 70% of those observed at least twice in the LE also show signs of variability. Moreover, time variations on the short time scale were also significantly detected in several sources. Fast variability was observed in 12 of 29 AGNs with available light curves in the 1-8 keV X-ray region. For three of these objects, rapid flux changes also occurred in the soft band. Spectral variability was also investigated for all the 14 sources that showed simultaneous long time variations in the ME and LE. Analysis of the ratio between the count rates in 6-10 keV and 3-6 keV bands as function of the 3-6 keV flux point out that half the objects in the subsample changed their spectral shape with the flux. When the soft part of the spectrum is taken into account, the number of AGNs characterized by spectral variability increases. In fact, variations of the ME/LE ratio with the hard and soft fluxes are present in 63% of the objects studied. In contrast to previously reported results for BL Lac objects, AGNs do not show a unique spectral behavior, although the prevailing trend appears to be a softening of the spectrum (ME alone and/or LE+ME) with increasing intensity.