Long-term variability of AGN at hard X-rays

Aims. Variability at all observed wavelengths is a distinctive pr operty 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 intrinsi c hard X-ray variability of a large AGN sample and comparing it to the results obtained at lower X-ray energies can significantly contr ibute 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 a 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 on the first 9 months of BAT data, we study the amplitude of the variations, and their dependence on sub-class 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 signific ant variability on months to years time scales, radio loud sources being the most variable, and Seyfert 1.5-2 galaxies being 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 of the absorption column density. In general, the variations in the 14‐24 and 35‐100 keV bands are well correlated, suggesting a common origin of 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 month time scale. I n addition, sources with harder spectra are found to be more variable than softer ones, opposite to what it is observed below 10 keV. These properties are generally consistent with a variable, in fl ux and shape, power law continuum, pivoting at energies& 50 keV, to which a constant reflection component is superpose d. 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, absorpt ion).