Optical observations of the Accreting Millisecond X-ray Pulsar MAXI J1957+032 with LCO

ATel #15448; M. C. Baglio, D. M. Russell, K. Alabarta, P. Saikia (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 21 Jun 2022; 08:35 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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MAXI J1957+032 is an X-ray transient, discovered by the Gas Slit Camera (GSC) aboard Monitor of All Sky X-ray image (MAXI) during a faint outburst on May 11, 2015 (MJD 57153; ATel #7504). The source underwent a total of 4 very short (<5 days) outbursts in the two years following its discovery (ATel #7504, #8143, #8529, #9565; see also Mata Sanchez et al. 2017; Beri et al. 2019). The last outburst was reported in October 2016; the source has likely been in quiescence since then.
Based on the short duration of the outbursts, and on the similarity with other accreting millisecond X-ray pulsars (AMXPs), the compact object was suggested to be a neutron star (Mata Sanchez et al. 2017).
The optical counterpart was detected during the outbursts with different instruments (see Mata Sanchez et al. 2017), with magnitudes ranging from r~20 (4 days after the first X-ray detection) to r~18.5 (2.4 days after the first X-ray detection).

A new outburst from MAXI J1957+032 was detected with MAXI on June 18, 2022 (MJD 59748.7; ATel #15440). A day later, NICER detected coherent pulsations in the 0.3-10.0 keV energy range at 313.6 Hz, suggesting that the source is an AMXP (ATel #15444). Swift/UVOT observations detected the UV counterpart of the source during the new outburst, with magnitude UVW2 = 20.37 +/- 0.08 (ATel #15446).

We observed MAXI J1957+032 with the 1m telescopes of the Las Cumbres Observatory network, as part of a monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008). 1 pointing, 200s integration time each, in the g', r', i' and Y optical bands was obtained on June 20 (MJD 59750.6, i.e.1.9 days after the first X-ray detection of the outburst). We performed aperture photometry; magnitudes were calibrated against the Pan-STARRS catalog (AB magnitudes). The target is detected in g', r', i' bands, with the following AB magnitudes:

g'=19.04+/-0.04
r'= 18.92+/-0.04
i'= 18.59+/-0.06

In Y band, we only obtained a 3sigma upper limit of Y>18.2 (Vega).
The system was then observed again a few hours later, on Jun 21, 2022(MJD 59751.1, i.e. 2.4 days after the first X-ray detection), with the same strategy. We obtained the following AB magnitudes:

g'=18.98+/-0.05
r'= 18.69+/-0.06
i'= 18.75+/-0.07,

and a 3sigma upper limit of 17.4 mag in Y band (Vega).

Therefore, the source seems to be still brightening in r' band, while it is getting fainter in i' band, suggesting a change of color while the outburst evolves, or the presence of short timescale variability. The source has not entered the rapid decay stage, yet.
By comparing with previous outbursts observations, when a magnitude of r'~18.5 was reached ~2.4 days after the first X-ray detection of the outburst (Mata Sanchez et al. 2017), it is possible that the peak still has to be reached.

We will continue to monitor the source with LCO.
Multiwavelength observations are encouraged.

Optical finding chart of MAXI J1957+032