Optical non-detection of MAXI J1957+032 with LCO

ATel #15466; M. C. Baglio, D. M. Russell, K. Alabarta, P. Saikia (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 24 Jun 2022; 11:22 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The faint X-ray transient MAXI J1957+032 was observed to undergo a new outburst on June 18 2022 (MJD 59748.7) by MAXI (ATel #15440). The outburst was then confirmed by NICER observations, thanks to which pulsations at 314 Hz have been detected (ATel #15444), which gave a confirmation to the accreting millisecond pulsar nature of the source. Moreover, NICER observations performed between 19 and 21 June 2022 (MJD 59749.0 - 59751.6; ATel#15456) allowed to detect a binary orbital period of 3653.47 s (~1 hour).
The optical counterpart was observed on June 20 and 21 2022 (MJD 59750.6 and 59751.1) with magnitudes g'=19.04+/-0.04, r'=18.92+/-0.04, i'=18.59+/-0.06 and g'=18.98+/-0.05, r'=18.69+/-0.06, i'=18.75+/-0.07, respectively (ATel #15448).
A decline of the X-ray flux was detected between June 19 and 21 2022 (MJD 59749.1 - 59751.6) from NICER observations (ATel #15456), then confirmed by Swift on 22 June (MJD 59752.1; ATel #15457). A non-detection at X-ray frequencies was then reported in ATel #15460 from Swift observations performed on June 23 (MJD 59753.1).
MeerKAT observations at radio frequencies also reported a non-detection on June 21 (MJD 59751.9), with a 3-sigma flux density upper limit of 48 microJy (ATel #15462).

We have been monitoring the source since near the beginning of the outburst (ATel #15448) with the 1m telescopes of the Las Cumbres Observatory (LCO) network, as part of a monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008).
According to our monitoring, the peak of the outburst at optical frequencies was reached around June 21 2022 (MJD 59751.1, 2.4 days after the first X-ray detection of the outburst), with magnitudes g'=18.98+/-0.05, r'=18.69+/-0.06, i'=18.75+/-0.07, as reported in ATel #15448. After that date, the flux started to decay, with magnitudes g'=19.15+/-0.12, r'=19.17+/-0.14, i'=19.38+/-0.15 on MJD 59751.4.

The last observations obtained with the same strategy with LCO on June 23 (MJD 59753.6) and 24 (MJD 59754.1) resulted in non-detections, with the following 3-sigma upper limits: g'>22.55, r'>22.10, i'>21.58, y>17.90.
This likely indicates that the source is back to quiescence, only 4.9 days after the first X-ray detection of the outburst, which is similar to the duration of previous outbursts of the same source (Mata Sanchez et al. 2017).