Swift J1357.2-0933 is close to quiescence
ATel #13120; Saarah F. Pirbhoy, David M. Russell, Maria Cristina Baglio (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Payaswini Saikia (NYU Abu Dhabi)
on 23 Sep 2019; 10:44 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)
Subjects: Optical, Black Hole, Transient
The black hole candidate X-ray binary, Swift J1357.2-0933 has been in outburst since 2019 March 20 (MJD 58562) (ATel #12796, #12801, #12803). Similarly to what happened during its 2017 outburst (Stiele & Kong 2018 ApJ 852 34, Beri et al. 2019 MNRAS 485 3064, Paice et al. 2019 arXiv:1905.09784v1), the target remained in the hard state during the whole duration of the outburst, showing a simple power-law spectrum in the hard X-rays (ATel #12821) and optical dips (ATel #12867). Here we report long term optical monitoring with the 2-m Faulkes Telescopes North (at Haleakala Observatory, Maui, Hawaii, USA) and South (at Siding Spring Observatory, Australia) and the 1-m Las Cumbres Observatory (LCO) network.
After peaking on MJD 58597 (2019 April 24) at a magnitude of I = 16.58 +- 0.01, it started to fade. We measure a decrease in brightness of 0.0035 mag/day between MJD 58597 (2019 April 24) and 58673 (2019 July 9) in I-band. Around MJD 58673 (2019 July 9) it started fading more rapidly, at a rate of 0.074 mag/day. We find evidence for a re-flare during the decay around MJD 58700 (2019 August 5), peaking at I = 17.83 +- 0.05, g = 18.58 +- 0.02, z = 18.23 +- 0.05, r = 18.39 +- 0.02 mag. After this re-flare the source faded more rapidly towards quiescence and since MJD 58709 (2019 August 14), the source has been fainter than 19 magnitude in i-band. The pre-outburst quiescence magnitude was I = 20-21 mag. It is therefore close to quiescence and the latest magnitudes are I = 19.28 +- 0.26, g = 21.24 +- 0.33 mag on MJD 58733 (2019 September 7). From now, the source is not visible from the ground until the end of December 2019. Multi-wavelength observations are encouraged at this time to study the source near quiescence.
The LCO observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.
Optical LCO light curves of Swift J1357.2-0933
