Swift XRT Timing Observations of the Black-Hole Binary SWIFTJ1753.5-0127: Disk-Diluted Fluctuations in the Outburst Peak

摘要

After a careful analysis of the instrumental effects on the Poisson noise todemonstrate the feasibility of detailed stochastic variability studies with theSwift X-Ray Telescope (XRT), we analyze the variability of the black hole X-raybinary SWIFT J1753.5-0127 in all XRT observations during 2005-2010. We presentthe evolution of the power spectral components along the outburst in two energybands: soft (0.5-2 keV) and hard (2-10 keV), and in the hard band find resultsconsistent with those from the Rossi X-ray Timing Explorer (RXTE). Theadvantage of the XRT is that we can also explore the soft band not covered byRXTE. The source has previously been suggested to host an accretion diskextending down to close to the black hole in the low hard state, and to showlow frequency variability in the soft band intrinsic to this disk. Our resultsare consistent with this, with at low intensities stronger low-frequencyvariability in the soft than in the hard band. From our analysis we are able topresent the first measurements of the soft band variability in the peak of theoutburst and find it to be less variable than the hard band, especially at highfrequencies, opposite to what is seen at low intensity. Both results can beexplained within the framework of a simple two emission-region model where thehot flow is more variable in the peak of the outburst and the disk is morevariable at low intensities.