DETERMINING THE NATURE OF THE BRIGHTEST SERENDIPITOUS NUSTAR SOURCE

In our systematic search for serendipitous sources (serendips) in the NuSTARdata, we have found a very bright (7.5 mCrab in the 210 keV band) serendip(J0923 s1) near the edge of one of the NuSTAR fields of view. The source hasbeen previously detected in the soft Xray band, but its nature is unknown.Based on its Xray spectrum, its optical counterpart, and its longtermvariability (as seen by MAXI), it must be Galactic, and, while some of itsproperties are suggestive of a magnetic Cataclysmic Variable, other propertiescause problems with this interpretation. One interesting possibility is that itmay be an accreting black hole with a high temperature disk. We propose NuSTARand XMMNewton observations to determine the nature of J0923 s1. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]