4U 1746–37: An ultra‐low‐mass compact star candidate

Abstract

AbstractThe Rossi X‐ray Timing Explorer observed three photospheric radius expansion bursts in 4U 1746‐37, all with low touchdown fluxes. We discuss the possibiity of a low‐mass neutron star in 4U 1746‐37 if the touchdown flux corresponds to its Eddington limit. 4U 1746‐37 is a dipping binary system which has a high inclination angle. Two geometric effects, the reflection of the far side accretion disc and the obscuration of the near side accretion disc have also been included. We apply a Monte‐Carlo simulation, with typical values of hydrogen mass fraction and color correction factor, to constrain the mass and radius of the neutron star in 4U 1746‐37. We found that the mass of 4U 1746‐37 is 0.41+0.70–0.30 M® at 99.7 % confidence which is an ultra‐low‐mass compact star candidate. It could be reproduced by a self‐bound compact star, i.e., quark star or quark‐cluster star, from an ccretion‐induced collapse process. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)