Optical observations of the 2022 outburst of the Accreting Millisecond X-ray Pulsar SAX J1808.4-3658

ATel #15603; M. C. Baglio, D. M. Russell, K. Alabarta, P. Saikia, D. M. Bramich (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 9 Sep 2022; 09:42 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 has undergone a total of 9 outbursts since its discovery and up until August 19 2022 (MJD 59810). The last outburst was reported in 2019, and, according to the optical monitoring performed with the Las Cumbres Observatory (LCO) network of telescopes, the source was back to quiescence at optical wavelengths on October 2nd, 2019 (MJD 58758.1). The recurrence of outbursts for this source seems to follow a quadratic phenomenological model (Goodwin et al. 2020), with the next outburst predicted to happen at the end of 2023.

Unexpectedly, on August 19 2022 (MJD 59810.1) the MAXI/GSC nova alert system triggered on the onset of a new outburst from SAX J1808.4-3658 (ATel #15563). The outburst was then confirmed by NICER (ATel #15559) observations; radio detections of the outburst were also reported from MeerKAT (ATel #15584).

We report on optical observations of the current outburst performed with the 1m and 2m telescopes of the LCO network, as part of a monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008). LCO images are processed and reduced, and magnitudes are extracted and calibrated using a real-time data analysis pipeline, the "X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details).

According to our monitoring, after the end of the 2019 outburst, the system remained in quiescence at optical wavelengths for almost 3 years, with an average magnitude of i'=20.36 +/-0.01. Following the X-ray detection of the latest outburst (ATel #15559, #15563), we intensified our optical monitoring, and on MJD 59812.5-9 we measured the following magnitudes: zs=16.46+/-0.01, i'=16.60+/-0.01, r'=16.74+/-0.01, R=16.55+/-0.01, V=16.83+/-0.01, g'=16.86+/-0.01, B=17.10+/-0.01. These levels are consistent with the maximum brightness level reached during the 2019 outburst (i'=16.61+/-0.01 on MJD 58710.7), indicating that the outburst was likely to be near its peak at the time of the X-ray detection.
Prior to the outburst, we observed an enhanced flux of the source already on MJD 59799.6, with i'=18.41+/-0.05, which is significantly brighter with respect to the previous observation on MJD 59785.5 with magnitude i'=20.23+/-0.05 (consistent with the quiescent level to within 2sigma). It is therefore likely that the start of the outburst rise happened between MJD 59785.5 and 59799.6, 11-25 days before the first X-ray detection was reported.

After reaching its peak, the current outburst started its decay (at a rate of 0.16 mag/day in i' band), reaching its faintest magnitudes on MJD 59829.5: i'=19.24+/-0.08, r'=19.06+/-0.11, R=19.12+/-0.16, V=19.20+/-0.15, g'=19.06+/-0.09, B=19.21+/-0.18. The most recent observations (MJD 59829.8) however indicate a sudden increase in flux, with magnitudes i'=18.45+/-0.17, r'=18.79+/-0.16, g'=18.66+/-0.12, possibly indicating the start of the reflaring period that typically characterizes the last phase of all outbursts of SAX J1808.4-3658 (see e.g. ATel #13103).

The monitoring with LCO at optical wavelengths is ongoing. Multiwavelength observations are encouraged during the reflaring period.
We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical light curve of SAX J1808.4-3658