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 × 10−13 erg cm−2 s−1 and a spectrum well fitted by a power law or a thermal plasma emission model. This source also shows a UV (2200 Å) excess, which might indicate the presence of mass accretion. For the other targets, we derived X-ray flux upper limits of the order of 10−13 erg cm−2 s−1. These results are consistent with the hypothesis that the collapsed component in these six systems is a NS.