Swift J174510.8-262411 exhibits X-ray properties of a black hole transient in the hard state

ATel #4393; J. A. Tomsick (SSL/UC Berkeley), M. DelSanto (INAF/IAPS, Italy), T. Belloni (INAF/OAB, Italy)
on 19 Sep 2012; 07:15 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

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We carried out a spectral and timing analysis of Swift/XRT data from an observation of the new X-ray transient Swift J174510.8-262411 (ATELs 4380, 4381, 4383, 4385, and 4388). The Swift/XRT observation began on 2012 September 18 at 19.06 h UT, and an exposure of 797 s was obtained (ObsID 00533836003). XRT was operated in Window Timing mode, and count rate was 129 c/s (full band) and 106 c/s in the 0.6-10 keV band.

We fitted the 0.6-10 keV energy spectrum with three models: a powerlaw, a disk-blackbody (diskbb), and a diskbb+powerlaw (all with absorption, using the "wilm" abundances in XSPEC). The reduced chi2 values for these three models are 1.18 (714 dof), 1.77 (714 dof), and 1.06 (712) for the three models, respectively. We focus on the third (diskbb+powerlaw) model since it gives the best fit. The parameters (with 90% confidence uncertainties) are a column density of (1.70+/-0.04)e22 cm-2, a photon index of 1.53+/-0.02, and a very low diskbb temperature of <0.1 keV. The unabsorbed 0.6-10 keV flux of the powerlaw component, which dominates the spectrum, is 1.27e-8 erg/cm2/s.

Although this observation is close to the limit where pile-up can impact the spectrum, we made a second spectrum after removing the counts from the inner two pixels. In this case, the spectrum was not significantly different. For example, the power-law photon index is 1.57+/-0.03.

The 0.0078-142 Hz power spectrum is consistent with band-limited noise, which we fit with a zero-centered Lorentzian, and a QPO. The QPO frequency is 0.250+/-0.003 Hz and its width is 0.022+/-0.014 Hz. The fractional rms of the zero-centered Lorentzian is 22.8+/-0.8%, and the fractional rms of the QPO is 10.7+/-1.3%.

The properties described above, especially the power-law photon index of 1.5-1.6 and the QPO, are typical of the properties of BH transients in the hard state.