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Rapid optical decay of the neutron star transient MAXI J0556-332

ATel #8530; Ruolan Jin, Albert K. H. Kong (National Tsing Hua University, Taiwan)
on 11 Jan 2016; 11:37 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

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

Referred to by ATel #: 8854

We observed the neutron star X-ray transient MAXI J0556-332 currently in outburst with the 1m telescope at the Lulin Observatory in Taiwan. The observations were taken on 2016 January 9 (MJD 57396.66) and 10 (MJD 57397.67) with the SDSS g and r-band filters. The source is clearly seen in all images with exposure time ranging from 180s to 300s. By comparing with several comparison stars in the AAVSO Photometric All-Sky Survey (APASS), we obtained g = 18.74 +/- 0.06 and r = 19.10 +/- 0.06 on January 9, and g = 18.73 +/- 0.17 on January 10 (with thin cloud). On January 7, the source was at V = 17.41 and R = 17.7 (ATel #8517). This strongly suggests that the source is now about 1 magnitude fainter. A Swift/UVOT observation with the uvw2 filter was taken simultaneously with our Lulin observation on January 10. MAXI J0556-332 is detected with a significance of 16 sigma. The uvw2-band magnitude (in AB system) is 19.07 +/- 0.07 (stat) +/- 0.03 (sys).

We checked the recent MAXI/GSC data and the source also shows a slight decrease in X-ray flux back to the level at the start of the X-ray outburst on January 6 (ATel #8513). Both optical and X-ray observations indicate that the outburst of MAXI J0556-332 may start to decay, or the source is undergoing a more complicated evolution. Multi-wavelength observations are encouraged.

The Lulin Observatory is operated by the Graduate Institute of Astronomy in National Central University, Taiwan.