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Optical Faulkes Telescope monitoring of MAXI J0556-332 caught its rise into outburst

ATel #8517; David M. Russell (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project and Astrophysics Research Institute, LJMU)
on 8 Jan 2016; 17:28 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

Subjects: Optical, X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 8530, 8854

MAXI J0556-332 is a neutron star X-ray binary with a possible orbital period of ~ 16.4 hours (Cornelisse et al. 2012). It was first detected on 2011 January 11 (ATel #3102) and recently, almost exactly 5 years later a new outburst has been reported by MAXI on 2016 January 6 (ATel #8513). The source is one of the few transient Z-sources, has a strongly heated neutron star during quiescence, and has been suggested to lie at a large distance of ~45 kpc (Homan et al. 2014).

We have been monitoring MAXI J0556-332 at optical wavelengths since 2011 with the 2-m Faulkes Telescopes (at Haleakala on Maui, Hawaii and Siding Spring, Australia) in the Bessell I-band filter (exposure times are 200 sec). The source is generally detected in quiescence, at a magnitude during 2015 of I ~ 18.6 +- 0.1 where the error represents the scatter in the data points. The magnitudes are calibrated using several USNO-B1/NOMAD catalogue stars in the field of view (these values may suffer from systematic errors of several tenths of a magnitude due to uncertainties in the USNO-B1/NOMAD field star magnitudes).

We detect a slight brightening on 2015 December 29 (MJD 57385.4): I = 18.32 +- 0.08, in itself only barely (a few sigma) above the scatter in quiescence. Our next pointing on 2016 January 2 (MJD 57389.3) gave I = 17.83 +- 0.06, a much more obvious brightening. We consequently made observations in three Bessell filters on January 7 (MJD 57394.6) and derive V = 17.41 +- 0.05, R = 17.70 +- 0.04, I = 17.17 +- 0.05. By January 7 the source was therefore ~1.4 mag above quiescence, having brightened at a mean rate of ~0.13 mag/day since December 29. The initial rise probably started around December 29, and must have began after our previous observation on 2015 December 19 (MJD 57375.6) when the magnitude was I = 18.55 +- 0.06, consistent with quiescence. The optical flux of the source was rising for at least 8 days before it brightened above the X-ray detection sensitivity limit of MAXI, highlighting the benefit of optical, as well as X-ray, monitoring of these transients. During the 2011 outburst the source reached a peak magnitude of I ~ 16.4 (see ATel #3116 and our light curves linked below). MAXI J0556-332 may therefore continue to brighten over the next days-weeks; multiwavelength observations are encouraged during the rise.

The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008). This work makes use of observations from the Las Cumbres Observatory Global Telescope Network.

Faulkes Telescope light curve of MAXI J0556-332