Frequency Resolved Spectroscopy of EXO 0748-676 in Persistent Emission

ATel #2097; S. Balman (METU)
on 24 Jun 2009; 14:59 UT
Credential Certification: Solen Balman (solen@astroa.physics.metu.edu.tr)

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I report on the frequency resolved spectroscopy (FRS) of the low-mass X-ray binary EXO 0748-676, using data taken with the EPIC PN (timing or imaging mode) data on the XMM-Newton Observatory. The frequency resolved spectra in a low state (Lx about 1.2e36 erg/s) show that the spectra are frequency independent and best described by a simple power-law with a iron line at about 5 keV. The vicinity of the 45 Hz signal reported in bursts from this source by Villarreal & Strohmayer (2004, ApJL 614, L121) shows the above spectra during persistent emission. In a high state of the source (Lx about 7e36 erg/s) the frequency resolved spectra indicate a low temperature disk blackbody component (0.13-0.19 keV) which disappears with higher frequency. The second component is either a Comptonized plasma emission or a power law model and the spectra gets harder with increasing frequency as expected from the FRS of Z-sources (see Gilfanov et al. 2003 A&A 410, 217). In persistent emission vicinity of the 45 Hz shows only the second component. As a result, frequency resolved spectra indicate that during persistent episodes the emission in the vicinity of 45 Hz arises from the boundary layer, which may suggest that this period, detected by summing power spectra from early in the decay of several bursts, is a variability within the boundary layer. This is in support of the recent ATEL#2094 by Galloway & Chakrabarty anouncing 552Hz burst oscillations from this source.