Discovery of Soft Spectral Component and Transient 22.7s Quasi Periodic Oscillations of SAX J2103.5+4545

摘要

XMM-Newton observed SAX J2103.5+4545 on January 6, 2003, while RXTE wasmonitoring the source. Using RXTE-PCA dataset between December 3, 2002 andJanuary 29, 2003, the spin period and average spin-up rate during theXMM-Newton observations were found to be $354.7940pm0.0008$ s and$(7.4pm0.9)imes10^{-13}$Hz s$^{-1}$ respectively. In the power spectrum ofthe 0.9-11 keV EPIC-PN lightcurve, we found quasi periodic oscillations around0.044 Hz (22.7 s) with an rms fractional amplitude $sim $6.6 %. We interpretedthis QPO feature as the Keplerian motion of inhomogenuities through the innerdisk. In the X-ray spectrum, in addition to the power law component with highenergy cutoff and $sim6.4$ keV fluorescent iron emission line (Baykal et al.,2002), we discovered a soft component consistent with a blackbody emission with${m{kT}}sim1.9$keV. The pulse phase spectroscopy of the source revealed thatthe blackbody flux peaked at the peak of the pulse with an emission radius$sim 0.3$ km, suggesting the polar cap on the neutron star surface as thesource of blackbody emission. The flux of the iron emission line at $sim 6.42$keV was shown to peak at the off-pulse phase, supporting the idea that thisfeature arises from fluorescent emission of the circumstellar material aroundthe neutron star rather than the hot region in the vicinity of the neutron starpolar cap.