Frequency Resolved Spectroscopy of XB 1323-619 and EXO 0748-676

Abstract

We present the frequency resolved energy spectra (FRS) of the low-mass X-ray binary dipper XB 1323-619 and EXO 0748-676 using archival XMM-Newton observations. The FRS of XB 1323-619 is well described by a single blackbody component with kT in a range 1.2-1.6 keV. We attribute this to an existing boundary layer, the likely source of the soft emission, supported by radiation pressure. We detect a different form of FRS for the frequency ranges 0.9-6 Hz and 8-30 Hz which is modeled best with a power law and a gaussian emission line at 6.5(-0.1)(+0.1) keV. EXO 0748-676 reveals different variable regions found in different states with spectral hardening as the frequency increases showing similar FRS behavior to Z-sources. A disk blackbody emission component of 0.15 keV is found in the high state and it disappears at higher frequencies as expected. In the low state FRS (0.05-50 Hz) is independent of Fourier frequency and shows a fluorescence Fe line at 4.3-5.5 keV with large equivalent width.