X-ray fading and optical/X-ray flaring in the current faint outburst of MAXI J0556-332

ATel #8854; David M. Russell, Silviu-Marian Udrescu (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project and Astrophysics Research Institute, LJMU)
on 22 Mar 2016; 18:17 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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The neutron star X-ray binary transient, MAXI J0556-332 began a new outburst at the start of this year (ATel #8513, #8517). Since 7 January (MJD 57394) we have been monitoring the optical activity of the source regularly in Bessel B, V, R, I filters with the 2-m Faulkes Telescopes and the 1-m Las Cumbres Observatory Global Telescope (LCOGT) network telescopes. Here, we report on LCOGT and Swift monitoring of this outburst.

We have acquired Faulkes/LCOGT pointings every 1-3 days since 7 January. The optical flux peaked near the start of the outburst on 7-8 January, at magnitudes B = 18.2, V = 18.0, R = 17.9, I = 17.9. We used field stars from the AAVSO Photometric All-Sky Survey (APASS survey; Henden et al. 2012) to flux calibrate the field, which is a significant improvement (smaller systematic error) over previous calibration using USNO/NOMAD (ATel #3116, #8517). B and V magnitudes of several APASS stars were used, and for R and I we converted APASS g', r', i' magnitudes to Bessel R, I using the conversions in Jordi et al. (2006).

Since the initial optical peak of the outburst, several dips (see also ATel #8530) and faint flares lasting a few days have been observed, which reduced in amplitude after ~ 2 February (MJD ~ 57420). The mean optical flux has started to decay, but only by ~ 0.4 mag since 2 February. Our latest pointing on 18 March (MJD 57465) had the source at I = 18.6 +- 0.2, about 0.8 mag above the I-band quiescent level of I ~ 19.4. The peak of the 2016 outburst was about 1 magnitude fainter than the peak of the 2011 outburst (see link to our light curves below).

Pointed Swift observations of MAXI J0556-332 have been made every 3 - 4 days since 10 January (MJD 57397). We built the X-ray light curve using the Swift-XRT data products generator at the UK Swift Science Data Centre (Evans et al. 2007). The maximum 2-10 keV XRT (WT mode) X-ray flux of 7.7e-10 erg/cm^2/s was measured on 13 January (MJD 57400), converting count rate to flux using NASA's WebPIMMS. The X-ray flux has declined since by a factor of 4-5 by 15 March (MJD 57462; see below link to light curves) with a few flares and dips during the decay.

Further multi-wavelength observations are encouraged during the decay of this rare transient Z-source (Homan et al. 2014). 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 and data supplied by the UK Swift Science Data Centre at the University of Leicester.

Light curves of MAXI J0556-332