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

¹ INAF–Osservatorio Astronomico di Brera, Via E. Bianchi 46, Merate (LC) 23807, Italy
e-mail: boris.sbarufatti@inaf.it
² Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
⁴ INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, via A. Corti 12, 20133 Milano, Italy
⁵ Università dell’Insubria, Dipartimento di Scienza e Alta Tecnologia, Via Valleggio 11, 22100 Como, Italy

Received: 18 January 2024
Accepted: 20 April 2024

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.