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Pan-STARRS1 observations of the very faint X-ray transient XTE J1728-295 during quiescence

ATel #3011; A. K.H. Kong (NTHU, Taiwan), P. A. Price (Princeton), E. Magnier, K. Chambers, N. Kaiser, J. Morgan, W. Burgett, J. Heasley, W. Sweeney, C. Waters, H. Flewelling, J. Tonry (IfA, Hawaii)
on 6 Nov 2010; 05:20 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

Subjects: Optical, Binary, Black Hole, Neutron Star, Transient

We report a non-detection of the quiescent optical counterpart of the very faint X-ray transient XTE J1728-295 currently in the decaying stage of its recent outburst (ATel #2823, #2991, #2997) in the Pan-STARRS1 (PS1) 3Pi sky survey. During the survey, the field of XTE J1728-295 was observed with the r-band filter on 2010 June 5, nearly 3 months before the recent outburst (ATel #2823, #2824). The proposed optical counterpart (ATel #2827, #2870, #2997) was not seen in the 40-sec r-band image. Within 30 arcsec of the source, the faintest catalogued 3Pi object has a r-magnitude of about 21 (AB mag). We also checked images taken with other filters (g, i, z, and y) and the counterpart was not detectable. We conclude that the proposed optical counterpart has a r-magnitude of > 21 during quiescence. For comparison, all other sources except star 3 (the proposed optical counterpart) described in ATel #2827 were seen in the PS1 images with similar magnitudes. We therefore suggest that star 3 is the optical counterpart of XTE J1728-295 because of its dramatic variability between outburst and quiescence.

This discovery was enabled using the PS1 System operated by the PS1 Science Consortium (PS1SC) and its member institutions, http://www.ps1sc.org/PS1_System_ATel.htm . We would like to thank the PS1 telescope operators for their support of PS1 observations.