Swift X-ray and UV observations of six Gaia binaries supposedly containing a neutron star

Abstract

Recent observations have led to the discovery of numerous optically selected binaries containing an undetected component with mass consistent with a compact object (neutron star (NS) or white dwarf). Using the Neil Gehrels Swift Observatory, we carried out X-ray and UV observations of a small sample of these binaries. Four systems are wide (with an orbital period P$>300 d$), and were chosen because of their small distance (d$<$250 pc) and because the mass of the collapsed component favors a NS. Two others are compact systems (P$<0.9 d$) that show strong signs of containing a NS. The source 2MASS J15274848$+$3536572 was detected in the X-ray band with a flux of $5 $ erg cm$^ $ s$^ $ and a spectrum well fitted by a power law or a thermal plasma emission model. This source also shows a UV (2200 AA ) excess, which might indicate the presence of mass accretion. For the other targets, we derived X-ray flux upper limits of the order of $10^ $ erg cm$^ $ s$^ $ . These results are consistent with the hypothesis that the collapsed component in these six systems is a NS.