SAX J1808.4-3658 is back to the quiescent level

ATel #13162; Maria Cristina Baglio, David M. Russell (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Payaswini Saikia (NYU Abu Dhabi)
on 3 Oct 2019; 15:47 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The accreting millisecond X-ray pulsar SAX J1808.4-3658 started its current outburst on 2019 August 6th (MJD 58701; ATel#12964, ATel#12993). After reaching its peak on August 13th (MJD 58708), the outburst started its decay (ATel#13077) and entered the flaring state on August 24th (MJD 58719; ATel#13077, ATel#13103).

We report on the continuous monitoring of the outburst with the Las Cumbres Observatory (LCO) 1- and 2-m (Faulkes) telescopes (i', R, V, B bands). According to our monitoring, the outburst remained in the flaring state for approximately one month, until September 26th (MJD 58752). During this period, several flaring episodes were observed, the last of which peaked on September 24th (MJD 58750) and reached a magnitude i'=18.17+/-0.03.

On September 29th (MJD 58755) the flux started decreasing, and the outburst reached its faintest magnitude since its beginning (i'=19.81+/-0.18). The last observed magnitudes are:

October 1st, MJD 58757.1: i'=19.51+/-0.13; R=19.18+/-0.13.
October 2st, MJD 58758.1: i'=20.12+/-0.29; R=19.29+/-0.15; V=20.00+/-0.26.

The light curve is linked below. The latest i'-band magnitude is consistent with the quiescent level that the source had before the start of the outburst (i'=19.99+/-0.15 on July 18th, MJD 58682). This suggests that the source has returned to quiescence. However, possible reflares could happen, similarly to what the source did at the end of its 2015 outburst. We will continue monitoring the source to confirm the end of the outburst. Note that the above magnitudes may include some flux from nearby faint blended stars at these low flux levels.

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 Faulkes/LCO light curves of SAX J1808.4-3658