4U 1746–37: An ultra-low-mass compact star candidate
The 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.