A cooling accretion disk in the black hole candidate SWIFT J1753.5-0127

ATel #1066; J. M. Miller, E. Rykoff (University of Michigan)
on 3 May 2007; 19:22 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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SWIFT J1753.5-0127 belongs to an interesting class of black hole candidates that appear to remain in the low/hard state during their outbursts. The well-known black hole XTE J1118+480 is part of this class. SWIFT J1753.5-0127 is more special still, in that it has remained active in the low/hard state since the onset of its outburst in 2005 (see Palmer et al. 2005, ATEL #546).

The low column density along the line of sight to SWIFT J1753.5-0127 makes it possible to detect the blackbody-like spectrum from the accretion disk, even when it is faint and cool. Miller, Homan, and Miniutti (2006) detected such a cool (kT ~ 0.2 keV) disk during the late decay of the initial outburst of this source. They found that for a plausible range of parameters, the cool disk is consistent with remaining at the innermost stable circular orbit around the compact object. Subsequent work on the black hole candidate XTE J1817-330 by Rykoff et al. (2007) has shown that such cool thermal components are indeed disks, since they follow a clear T^4 trend, even across large factors in flux and across state transitions.

In order to further investigate disks in the low/hard state, we analyzed Swift observations of SWIFT J1753.5-0127 during its 2005 outburst (observations spanning MJD 53562-53605). Fitting a simple disk blackbody plus power-law model, a cool disk is required at the 3 sigma level or higher in 15 of the early Swift observations when the transient was still bright. Measured disk temperatures are found to span 0.28-0.37 keV. Combining these temperature and flux points with that obtained from XMM-Newton at a later time, it is clear that flux is positively correlated with temperature. The data are not of sufficient quality to permit very strong statements, but the trend seen is broadly consistent with the T^4 relation expected for a blackbody with a constant emitting area.

This finding supports a growing body of evidence that disks do not immediately or automatically recede when black holes transition into the low/hard state, and suggests that advective disks or flows take hold at lower accretion rates than is marked by the state transition.

A plot of unabsorbed disk flux versus temperature with 90% confidence errors is available at:
http://www.astro.lsa.umich.edu/~jonmm/1753.pdf

We thank Ian Hoover for help and insights.