Multiple reflares in SAX J1808.4- 3658 during the outburst decline according to optical observations

ATel #13103; Maria Cristina Baglio, David M. Russell (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 12 Sep 2019; 20:44 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The accreting millisecond X-ray pulsar SAX J1808.4-3658 has been confirmed in its current outburst state on 2019 August 6th (MJD 58701) thanks to joint optical and X-ray observations (ATel#12993), following the first detection of optical brightening on July 30th (MJD 58694; ATel#12964).
According to NICER observations, the outburst reached its peak on August 13th (MJD 58708), after which it started to decay (ATel#13077). On August 24th (MJD 58719) the source entered its flaring state, starting to alternate between lower (~ 10^32 erg/s) and higher (~ 10^33 erg/s) luminosity periods on ~ days timescale (ATel#13077). On September 3rd (MJD 58729), a bright reflare was detected with NICER (ATel#13077) which lasted for approximately 1 day.

We have been monitoring the source during its outburst since the first detection of an optical brightening using the Las Cumbres Observatory (LCO) 1- and 2-m (Faulkes) telescopes (i', R, V, B bands). According to our observations, the peak of the optical outburst was detected on MJD 58706, i.e. August 11th, reaching a magnitude of i'=16.5. After this, the outburst started its decay. During the outburst decline, the source entered the flaring state, and experienced at least four episodes of flaring at optical frequencies, on August 30th (MJD 58725), September 2nd (MJD 58728), September 4th (MJD 58730; the latter likely coincident with the one detected by NICER according to ATel#13077) and is now in its fourth reflare. During these flaring episodes, the source reached a peak magnitude of i'=17.8, and a faintest magnitude of i'=18.6. The flares are visible at all wavelengths (i', R, V, B bands). The last reported magnitudes (MJD 58736) according to our monitoring are:

B= 18.47 +/- 0.25
V=18.4 +/- 0.23
R= 17.98 +/- 0.19
i'=18.11 +/- 0.11.

This flaring state might continue during the next few weeks. Further observations are encouraged.

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