THE X-RAY SPECTRAL VARIABILITY OF MARKARIAN-766

Analysis results from ASCA and ROSAT observations of the narrow-line S eyfert 1 galaxy Mrk 766 are reported. In the ASCA observation, we obse rved rapid variability with a doubling timescale of 1000 s. A spectral variability event was observed in which the spectrum softened and har dened above and below similar to 1 keV, respectively, as the flux incr eased. The spectra could be modeled with five components: an absorbed power law, warm absorber, iron Kcl line, and soft excess component flu x. The spectral variability resulted from a highly significant change in the intrinsic photon index from Gamma similar to 1.6 to similar to 2.0, an increase in the warm absorber ionization, and a marginally sig nificant decrease in the soft component normalization. A similar to 10 0 eV equivalent width narrow iron K alpha line was detected in the hig h-state spectrum. Spectral hardening during flux increases was observe d in three ROSAT observations. The change in intrinsic photon index an d disappearance of the soft excess component in tile ASCA spectra can be explained as a transition from a first-order pair-reprocessed spect rum to a pair cascade brought about by a sudden increase in the inject ed electron Lorentz factor. The change in the ionization of the warm a bsorber, though model dependent, could correspond to the increase in f lux at the oxygen edges resulting from the spectral index change. The ROSAT spectral variability can be interpreted by variable intensity ha rd power law and a relatively nonvarying soft component, possibly prim ary disk emission. These results are compared with those reported from other narrow-line Seyfert 1 galaxies.