Swift J1753.5-0127 no longer detected by Swift

ATel #9735; A. W. Shaw (U. Alberta), J. A. Tomsick (SSL/UCB), A. Bahramian (Michigan State), P. Gandhi (U. Southampton), D. M. Russell (NYU Abu Dhabi), P. A. Charles (IAC & U. Southampton)
on 8 Nov 2016; 17:39 UT
Credential Certification: Aarran Shaw (A.Shaw@soton.ac.uk)

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We report on new X-ray observations of the candidate black hole X-ray binary Swift J1753.5-0127, which began its outburst in June 2005 (ATel #546) and has remained active for >11 years. Swift-BAT and MAXI-GSC have seen a slow decline in X-ray flux over the last few years but more recently, the Faulkes Telescope Project has shown a more rapid drop at optical wavelengths, indicating that the source may finally be declining to quiescence (ATel #9708).

We requested ToO observations of Swift J1753.5-0127 with Swift-XRT on 2016 November 6 and 7 before the source became Sun constrained. No X-ray source is detected at the position of Swift J1753.5-0127 in either exposure. We derive upper limits (90% CL) of 5.65E-03 and 2.83E-03 counts s^-1 on November 6 and 7, respectively. This corresponds to an upper limit on the unabsorbed 0.6-10keV flux of 3.13E-13 and 1.57E-13 erg cm^-2 s^-1 for the respective exposures, assuming a power law index Γ=1.7 (ATel #8782) and n_H=2E21 cm^-2 (Froning et al. 2014). The last X-ray detection of Swift J1753.5-0127 was on 2016 June 8 and measured a count rate of 3.75 counts s^-1. Assuming the same spectral parameters as above, this corresponds to an unabsorbed 0.6-10keV flux of 1.92E-10 erg cm^-2 s^-1, ~3 orders of magnitude brighter than the upper limits detailed above.

Swift J1753.5-0127 was also observed with Swift UVOT in the UVM2 (November 6) and UVW1 (November 7) filters. As in Swift-XRT, the source is not detected. We determine the 3-sigma upper limit (Vega) magnitudes to be UVM2>20.40 and UVW1>20.16.

These non-detections by Swift's pointed instruments are consistent with the decline seen by Swift-BAT, MAXI-GSC and the Faulkes Telescope Project and we conclude that Swift J1753.5-0127 is entering a quiescent state for the first time since the source's discovery.

Though the source will soon become Sun-constrained, we encourage multi-wavelength follow-up, particularly at optical wavelengths.

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