The neutron star low mass X-ray binary MAXI J0556-332 is close to quiescence

ATel #14195; M. C. Baglio, D. M. Russell, D. M. Bramich, P. Saikia (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 18 Nov 2020; 13:48 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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On April 11 2020 (MJD 58950), enhanced X-ray flux at the position of the neutron star low-mass X-ray binary MAXI J0556-332 was detected by the MAXI/GSC nova alert system (ATel #13628). The source has been in outburst since. A corresponding optical brightening was reported in ATel #13631, thanks to observations performed with the iTelescope.Net T17 0.43m telescope in Siding Spring, Australia. According to ATel #13631, the source reached a magnitude of R = 18.5+/-0.3 on April 13, 2020 (MJD 58952), when the rise of the outburst was still ongoing.

Here we report on the results of an optical monitoring campaign of the outburst performed with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes. We monitored the target since the start of its fourth outburst using the optical B, V, R, I filters. Magnitudes have been extracted using the real-time data analysis pipeline, the ''X-ray Binary New Early Warning System (XB-NEWS; see Russell et al. 2019 and Goodwin et al. 2020 for details).

According to our monitoring, the peak of the outburst was reached in the optical between April 28 (MJD 58967) and May 25 (MJD 58993), depending on the filter, with the following magnitudes at the peak: B= 18.23 +/- 0.01, V= 18.19 +/- 0.01 (MJD 58967); R = 18.26 +/- 0.02 (MJD 58993); I = 17.95 +/- 0.03 (MJD 58987). These peak magnitudes are almost identical to those of the previous outburst in 2016 (ATel #8854) but fainter than the first outburst in 2011 (ATel #3104; #3116). From May 25, 2020 (MJD 58993) to October 3, 2020 (MJD 59125), the flux level was maintained at an almost constant level in all bands, with average magnitudes: B=18.57+/-0.01, V= 18.51+/-0.01, R=18.50 +/- 0.01, I=18.35+/-0.01, and a standard deviation of 0.2-0.3 mag in all bands. After October 3, the outburst started to decline, and it is now at its lowest flux level since the beginning of the outburst: B=19.54 +/- 0.03, V=19.26 +/- 0.06, R=19.46 +/- 0.03, I=19.26 +/- 0.06 on November 10, 2020 (MJD 59163).
This last I-band magnitude value in particular is close to the quiescent level of the source according to our continuous monitoring performed with LCO in the last few years (on average, I=19.39+/-0.01 between MJD 58000 and 58800).
In the UV, recent Swift/UVOT observations resulted in the following Vega magnitudes: uvw2=20.29+/-0.15 on Nov. 9 (MJD 59162); u=19.87+/-0.19 on Nov. 11 (MJD 59164); uvm2 >19.75 on Nov. 13 (MJD 59166), uvw1 > 20.18 on Nov. 14 (MJD 59167).

We therefore suggest that the source has now returned to quiescence. Further observations are encouraged to confirm this result, especially X-ray observations to monitor the cooling of the neutron star over the following months; the neutron star in MAXI J0556-332 was strongly heated after the first outburst (Homan et al. 2014). The LCO observations are part of an on-going monitoring campaign of ~ 50 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.

Light curve of MAXI J0556-332