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Swift J1753.5-0127 has returned to the hard state

ATel #8782; A. W. Shaw (U. Southampton), J. A. Tomsick, M. Clavel (SSL/UCB), D. Altamirano, P. A. Charles (U. Southampton), F. Fuerst (SRL/Caltech), P. Gandhi (U. Southampton), F. Rahoui (ESO and Harvard) and D. J. Walton (JPL/Caltech)
on 6 Mar 2016; 16:34 UT
Credential Certification: Aarran Shaw (A.Shaw@soton.ac.uk)

Subjects: X-ray, Binary, Black Hole, Transient

We report on new X-ray observations of the unusual black hole candidate Swift J1753.5-0127, which was discovered to be in a low luminosity soft state in March 2015 (Shaw et al. 2016, MNRAS, arXiv:1602.05816). After a brief recovery to pre-transition flux levels, the source has once again declined in the 15-50 keV hard X-ray band as seen by Swift-BAT. A similar response is seen by MAXI-GSC. However, from the BAT and MAXI measurements, it is unclear whether the source is in the soft or hard state and whether the flux is steadily declining or not.

We requested ToO observations with Swift-XRT which commenced on February 25 2016 with a 2ks pointing and continued on February 28, March 1 and March 6. The spectra are all well constrained with an absorbed power law with index Γ=1.58±0.08, 1.72±0.09, 1.73±0.06 and 1.62±0.08, respectively. The unabsorbed 2-10 keV flux was calculated to be (1.46±0.06)E-10, (0.91±0.05)E-10, (1.45±0.06)E-10 and (1.51±0.08)E-10 erg cm-2 s-1 on each day. We therefore conclude that Swift J1753.5-0127 has returned to the hard state. The 2-10 keV unabsorbed flux is a factor ~2-3 lower than that observed by Tomsick et al. (2015, ApJ, 808, 85), on 2014 April 5, during a previous low flux period.

Further ToO observations with Swift have been scheduled and multi-wavelength (particularly optical and radio) observations are encouraged.

We would like to thank the Swift team for promptly scheduling these observations.