Soft X-Ray Absorption Due to the Photo-Ionized Plasma in the Low-Mass X-Ray Binary EXO 0748-676 Observed with XMM-Newton Observatory

We analyzed an effect of the photo-ionization on the continuum spectra using the XMM-Newton archive data of the low-mass X-ray binary EXO 0748-676. We found that the ionization degree of the circumstellar plasma, which modifies the soft X-ray spectrum through absorption, changes largely in association with the X-ray bursts and the dips. The burst light curve below ∼1keV is found to have a major contribution from the persistent emission, which temporary increased during the burst due to a reduction of the opacity of the absorber. We utilized a correlation diagram of the count rates (0.5-1.0keV vs. 2.0-4.0keV) to study the effect of photo-ionization during the dips. We found that the correlation can be much better reproduced if we take into account the photo-ionization of the absorber. No partial covering absorption was required, except for the residual flux in the deepest dips.