Long-term variability of AGN at hard X-rays

Abstract

Aims. Variability at all observed wavelengths is a distinctive property of active galactic nuclei (AGN). Hard X-rays provide us with a view of the innermost regions of AGN, mostly unbiased by absorption along the line of sight. Characterizing the intrinsic hard X-ray variability of a large AGN sample and comparing it to the results obtained at lower X-ray energies can significantly contribute to our understanding of the mechanisms underlying the high-energy radiation. Methods. Swift/BAT provides us with the unique opportunity to follow, on time scales of days to years and with regular sampling, the 14-195 keV emission of the largest AGN sample available up to date for this kind of investigation. As a continuation of an early work using the first 9 months of BAT data, we study the amplitude of the variations and their dependence on subclass and on energy, for a sample of 110 radio quiet and radio loud AGN selected from the BAT 58-month survey. Results. About 80% of the AGN in the sample are found to exhibit significant variability on month-to-year time scales. In particular, radio loud sources are the most variable, and Seyfert 1.5−2 galaxies are slightly more variable than Seyfert 1, while absorbed and unabsorbed objects show similar timing properties. The amplitude of the variations and their energy dependence are incompatible with variability being driven at hard X-rays by changes in the absorption column density. In general, the variations in the 14-24 and 35-100 keV bands are correlated well, suggesting a common origin to the variability across the BAT energy band. However, radio quiet AGN display on average 10% larger variations at 14-24 keV than at 35-100 keV, and a softer-when-brighter behavior for most of the Seyfert galaxies with detectable spectral variability on a time scale of a month. In addition, sources with harder spectra are found to be more variable than softer ones, unlike what it is observed below 10 keV. These properties are generally consistent with a variable, in flux and shape, power law continuum, pivoting at energies > ∼ 50 keV, to which a constant reflection component is superposed. When the same time scales are considered, the timing properties of AGN at hard X-rays are comparable to those at lower energies, with at least some of the differences possibly ascribable to components contributing differently in the two energy domains (e.g., reflection, absorption).

X-ray spectral variability of Seyfert 2 galaxies

“…In the 14−195 keV energy band, Soldi et al (2014) estimated a variability amplitude of 25 [22][23][24][25][26][27][28]% using data from the Swift/BAT 58-month survey.…”

“…We did not find variations between the observations. Variations in this source in the 14-195 keV energy band were studied by Soldi et al (2014) using data from the Swift/BAT B.23. NGC 6300 NGC 6300 is a barred spiral galaxy, whose Seyfert 2 classification at optical frequencies was derived from the data reported in Phillips et al (1983).…”

“…In fact, this kind of variation has already been reported in the literature for objects included in the present work from intrinsic flux variations indicating changes in the nuclear continuum (Isobe et al 2005) or because they needed to set free the normalization of the power law for a proper fit to the data (LaMassa et al 2011). Also at higher energies, Soldi et al (2014) studied the long-term variability of 110 AGN selected from the BAT 58-month survey and argue in favor of a variable nuclear continuum plus a constant reflection component. Their result is independent of the classification of the objects, which includes Seyferts, NLSy1s, radio galaxies, and quasars.…”

“…Soldi et al (2014) report an amplitude of the intrinsic variability of 20 [16−24]% in the 14−195 keV energy band using data from the Swift/BAT 58-month survey.…”

“…The analysis of light curves from the Swift/BAT 58-month survey by Soldi et al (2014) showed a small variability amplitude of 11 [10−12]% in the 14−195 keV energy band.…”