During the past week, NuSTAR observed CL Pismis 17 3, hitherto known as an isolated Be star, that shows suspiciously strong X-ray emission above 3 keV. This is part of a campaign to probe such anomalously X-ray bright Be stars for hidden accreting companions, like a neutron star, or possibly a white dwarf. The NuSTAR data is crucial to determine if the system is undergoing active mass transfer, as the hard X-ray spectrum is a reliable probe for Comptonised emission, as expected from an accreting neutron star. In such a case, the system would join a growing list of Be X-ray binaries, that are known to be accreting persistently at a stable lower luminosity, instead of being punctuated by very luminous outbursts as they typically exhibit. This not only allows us to sharpen constraints on the X-ray luminosity function of High Mass X-ray Binaries, which are crucial to understand our Galactic star formation history, but also provides us with a test case to study persistent but tenuous accretion mechanisms.

Author: Aafia Ansar Mohideen (Research Scientist, Dr. Karl Remeis Sternwarte Astronomical Institute, Germany)