Optical brightening of SAX J1808.4-3658 with no X-ray detection - precursor to new outburst?

ATel #12964; David M. Russell (NYU Abu Dhabi), Adelle J. Goodwin (Monash University), Duncan K. Galloway (Monash University), Jean J. M. in 't Zand (SRON), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Maria Cristina Baglio (NYU Abu Dhabi)
on 30 Jul 2019; 12:00 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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The millisecond pulsar low mass X-ray binary and recurrent transient, SAX J1808.4-3658 last had a reported outburst in April-May 2015 (ATel #7364, #7371, #7376, #7379, #7380, #7469; Sanna et al. 2017; Di Salvo et al. 2019). SAX J1808.4-3658 has been observed in outburst approximately every 4 years since it was discovered in 1998, and as such is expected to go into outburst this year. We have been monitoring SAX J1808.4-3658 with the 2-m Faulkes Telescope South (at Siding Spring, Australia) since 2008 (see Elebert et al. 2009; Tudor et al. 2017) and more recently with the Las Cumbres Observatory (LCO) 1-m robotic telescopes.

From 18 i'-band images taken in 2019 (7 February to 22 July), the mean magnitude is i' = 19.78, with a standard deviation of 0.19 mag (this includes some flux from a few faint stars within the aperture in this crowded field). The most recent magnitudes indicate a flux enhancement at the position of the X-ray binary:
25 July (MJD 58689.48): i' = 19.50 +- 0.05
25 July (MJD 58689.82): i' = 18.95 +- 0.07
27 July (MJD 58691.99): i' = 19.09 +- 0.11
28 July (MJD 58692.99): i' = 18.99 +- 0.08; V = 19.45 +- 0.12
30 July (MJD 58694.18): i' = 18.81 +- 0.05; V = 19.09 +- 0.07

The optical i'-band magnitude over the last six days is 0.7-1.0 mag brighter than the quiescent level so far in 2019. In particular in the last 1.2 days it has brightened by 0.36 +- 0.14 mag in V-band and 0.18 +- 0.09 mag in i'-band. Each of the last 4 i'-band points is ~4-5 sigma brighter than the quiescent variability. This could signify the beginning stages of a new outburst, or an optical precursor (e.g. Bernardini et al. 2016; Russell et al. 2019) to an imminent outburst. Alternatively, this could represent a brief optical flare from low-level accretion activity, in which case the source may return to quiescence. The optical light curve is linked below.

We observed the source with the Neil Gehrels Swift Observatory (Swift) X-Ray Telescope (XRT) on 21, 23, 25, 27, and 29 July and there was no significant detection in the 0.1-10 keV energy band for the 0.5 ks observations, with an upper limit of 0.05 count/s. We will continue to monitor the source for X-ray activity over the coming week with Swift XRT to confirm if this optical brightening is a precursor to outburst.

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