An investigation of the origin of soft X-ray excess emission from ARK 564 and MRK 1044

Abstract

We investigate the origin of the soft X-ray excess emission from Ark 564 and Mrk 1044. Based on a long XMM-Newton observation of Ark 564, we find clear evidence for time delays such that the variations in the 4-10 kev band lag behind that in the 0.2-0.5 kev band by 1768 ± 122 s. The full-band power density spectrum (PDS) of Ark 564 has a break at ~ 1.2 × 1^-3 Hz with power-law indices of ~ 1 and ~ 3 below and above the break. The hard (3-10 kev) band PDS is stronger and flatter than that in the soft (0.2-0.5) band. Based on a short XMM-Newton observation of Mrk 1044, we find no correlation between the 0.2-0.3 and 5-10 kev bands at zero lag. These observations imply that the soft excess is not the reprocessed hard X-ray emission. The EPIC-pn spectrum of Ark 564 is best described by a complex model consisting of optically thick Comptonization in a cool plasma for the soft excess and a steep power law, modified by two warm absorber media as inferred from the RGS data. The smeared wind and optically thick Comptonization models both describe the spectrum of Mrk 1044 satisfactorily, but the ionized reflection model requires extreme parameters. The data suggest two component coronas-a cool, optically thick corona for the soft excess and a hot corona for the power-law component. The existence of a break in the soft band PDS suggests a compact cool corona that can either be an ionized surface of the inner disk or an inner optically thick region coupled to a truncated disk.