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X-ray/UV re-brightening of Swift J1753.5-0127 observed with Swift

ATel #10081; A. K. H. Kong (NTHU/Oxford)
on 16 Feb 2017; 08:19 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

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

Referred to by ATel #: 10097, 10110, 10114, 10118, 10288, 10325, 10562, 10664

We report on a Swift follow-up observation of the X-ray transient Swift J1753.5-0127 during its recent re-brightening in the optical (ATel #10075). After 11 years of an X-ray outburst, the source has been on its way to quiescence in 2016 November as indicated by X-ray, optical, and radio observations (ATel #9708,#9735,#9739,#9741,#9758,#9765). In particular, the last few optical observations taken in 2016 November indicate that the source is about 4 magnitudes fainter comparing to the outburst, and no X-ray emission is detected with Swift. Since then, the source was not observable because of the Sun. Since 2017 Jan 30, the source has re-brightened in the optical back to the outburst level (ATel #10075).

We observed Swift J1753.5-0127 with Swift XRT on 2017 Feb 16 01:43 UT for about 1.9 ks. The source is clearly detected with a count rate of 0.83 c/s. This is a factor of ~300 brighter than the last Swift XRT upper limit observation taken on 2016 Nov 7 (ATel #9735). The X-ray spectrum can be fit with an absorbed power-law model with a photon index of 1.59+/-0.16 and nH=(2.4+/-0.8)e21 cm^-2. The unabsorbed 0.3-10 keV flux is 6e-11 ergs/cm^2/s. This is about a factor of 3 lower than the flux level during the last outburst observation on 2016 June 8 (ATel #9735). The spectrum is also comparable to the hard X-ray state of the source (e.g., ATel #8782).

Swift J1753.5-0127 was also observed with Swift/UVOT in the u filter. The source is clearly seen with u=16.85 +/- 0.05 (stat) +/- 0.02 (sys) in the Vega system.

This observation confirms the return of the hard X-ray outburst of Swift J1753.5-0127. As discussed in ATel #10075, this is somewhat unusual in terms of the variability. Multi-wavelength follow-up observations are encouraged to investigate the nature of this re-brightening.

We would like to thank the Swift team for scheduling this observation.