Big Blue Bump and transient active regions in Seyfert galaxies

An important feature of the EUV spectrum (known as the Big Blue Bump, hereafter BBB) in Seyfert galaxies is the narrow range in its cutoff energy E-c from source to source, even though the luminosity changes by 4 orders of magnitude. Here we show that if the BBB is due to accretion disk emission, then in order to account for this ''universality'' in the value of E-c, the emission mechanism is probably optically thin bremsstrahlung. In addition, we demonstrate that the two-phase model with active regions localized on the surface of the cold disk is consistent with this constraint if the active regions are very compact and are highly transient, i.e., they evolve faster than one dynamical timescale.