Frequency-resolved spectroscopy of XB1323-619 using XMM-Newton data: detection of a reflection region in the disc

Abstract

We present the frequency-resolved energy spectra (FRS) of the low-mass X-ray binary dipper XB1323-619 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 disc. XB1323-619 is an Atoll source and a type-I burster. 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 of 0.002-0.04 and 0.07-0.3 Hz. We attribute this component to the accretion disc 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 disc-blackbody emission. We detect a different form of FRS for the higher frequency ranges of 0.9-6 and 8-30 Hz which is modelled best with a power law and a Gaussian emission line at 6.4(-0.3)(+0.2) keV with an equivalent width of 1.6(-1.2)(+0.4) and 1.3(-0.9)(+0.7) 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 disc regions. The conventional spectrum of the source also shows a weak broad emission line around 6.6 keV. In addition, we find that the 0.9-6 Hz frequency band shows two quasi-periodic oscillation (QPO) peaks at 1.4(-0.2)(+1.0) and 2.8(-0.2)(+0.2) Hz at about 2.8 sigma-3.1 sigma confidence level. These are consistent with the previously detected similar to 1 Hz QPO from this source. 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 disc with a maximum size of 4.7 x 10(9) cm and a minimum size of 1.6 x 10(8) cm calculated using light travel time considerations and our frequency-resolved spectra.