The unusual accretion state of the black hole candidate Swift J1753.5-0127
Abstract
The transient Swift J1753.5-0127 (hereafter J1753) is a short-period, (candidate) black hole X-ray binary (BHB) in the Galactic halo. J1753 is unusual in that it did not return to quiescence after the initial outburst faded, instead it has remained in the low-hard state. A remarkable feature of J1753 is the presence of a number of extended X-ray dips in the Swift-BAT light curve, which appear to be related to a very short term spectral softening of the source. In March 2015, the Swift-BAT count rate dropped to 0.001 cts/s, indicative of another state transition. Swift XRT observations revealed a very soft spectrum, well constrained with a multicolor disc model plus an additional powerlaw component with photon index Γ=4.28. However, the inner disc temperature of ∼0.35keV was reminiscent of a hard-state disc, despite the 0.6-10keV flux being dominated by the soft (0.6-2keV) band. This is unlike any accretion state seen previously in BHBs. We present here the results of an extensive, multi-wavelength observing campaign of J1753 in this apparently new state, including contemporaneous XMM-Newton and NuSTAR pointings. This gives us unrivalled spectral coverage up to 79keV, providing us with the clearest picture yet of the accretion disc of this unusual source.
- Publication:
-
The Extremes of Black Hole Accretion
- Pub Date:
- July 2015
- Bibcode:
- 2015ebha.confE..62S