Short-term spectral variations during X-ray flares in black hole candidates and AGNs

Abstract

It has been pointed out that stellar black hole candidates (BHCs) and AGNs, especially Seyfert galaxies, have similar properties in X-ray time variations and energy spectra. We, however, have no crucial evidence that physical processes of these properties are the same. Here, is presented another common feature; spectral hardening during X-ray flares in these objects. Ginga data have revealed that X-ray “shots” in BHCs in the hard state have softer energy spectra then average spectra, and the spectra suddenly harden after the peak intensities of the shots. Using ASCA data of two Seyfert galaxies, MCG-6-30-15 and NGC 7314, it has been also found that spectra during bright “flares” are soft and become harder as the flares progress. The time variations in BHCs can be well explained by density fluctuations on the optical thin branch of an ADAF. The obtained similarity in the spectral evolution during the X-ray shots/flares and luminosity of these AGNs imply that the time variations in AGNs come from density fluctuations on the optical thick branch of an ADAF.