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Frequency Resolved Spectroscopy of XB 1323-619 Using XMM-Newton data: Detection of a Reflection Region in the Disk

We present the frequency resolved energy spectra (FRS) of the low-mass X-ray binary dipper XB 1323-619 during persistent emission in four different frequency bands using an archival XMM-Newton observation. FRS method helps to probe the inner zones of an accretion disk. We find that the FRS is well described by a single blackbody component with kT in a range 1.0-1.4 keV responsible for the source variability in the frequency ranges 0.002-0.04 Hz, and 0.07-0.3 Hz. We attribute this component to the accretion disk and possibly emission from an existing boundary layer supported by radiation pressure. The appearance of the blackbody component in the lower frequency ranges and disappearance towards the higher frequencies suggests that it may also be a disk-blackbody emission. We detect a different form of FRS for the higher frequency ranges 0.9-6 Hz and 8-30 Hz which is modeled best with a power-law and a Gaussian emission line at 6.4$^{+0.2}_{-0.3}$ keV with an equivalent width of 1.6$^{+0.4}_{-1.2}$ keV and 1.3$^{+0.7}_{-0.9}$ keV for the two frequency ranges, respectively. This iron fluorescence line detected in the higher frequency ranges of spectra shows the existence of reflection in this system within the inner disk regions. In addition, we find that the 0.9-6 Hz frequency band shows two QPO peaks at 1.4$^{+1.0}_{-0.2}$ Hz and 2.8$^{+0.2}_{-0.2}$ Hz at about 2.8-3.1 $σ$ confidence level. These are consistent with the previously detected $\sim$ 1 Hz QPO from this source (Jonker et al. 1999). We believe they relate to the reflection phenomenon. The emission from the reflection region, being a variable spectral component in this system, originates from the inner regions of the disk with a maximum size of 4.7$\times 10^9$ cm and a minimum size of 1.6$\times 10^8$ cm calculated using light travel time considerations and our frequency resolved spectra.