An Ultra Low Mass and Small Radius Compact Object in 4U 1746-37?

TL;DR

This study uses X-ray burst data and Monte Carlo simulations to suggest that the neutron star in 4U 1746-37 may be an ultra low mass, small radius compact object, possibly a quark star or quark-cluster star.

Contribution

It introduces a novel analysis considering geometric effects and reflection in constraining the mass and radius of the neutron star in 4U 1746-37, proposing the existence of an ultra low mass compact object.

Findings

Possible low mass neutron star with ~0.41 solar masses.

Alternative scenario suggests an even lower mass (~0.21 solar masses).

Implication of a self-bound quark star or quark-cluster star.

Abstract

Photospheric radius expansion (PRE) bursts have already been used to constrain the masses and radii of neutron stars. RXTE observed three PRE bursts in 4U 1746-37, all with low touchdown fluxes. We discuss here the possibility of low mass neutron star in 4U 1746-37 because the Eddington luminosity depends on stellar mass. With typical values of hydrogen mass fraction and color correction factor, a Monte-Carlo simulation was applied to constrain the mass and radius of neutron star in 4U 1746-37. 4U 1746-37 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 in the mass and radius constraints of 4U 1746-37. If the reflection of the far side accretion disc is accounted, a low mass compact object (mass of $0.41\pm0.14~M_{\odot}$ and radius of $8.73\pm1.54~\rm km$ at 68% confidence) exists in 4U 1746-37. If another effect operated, 4U 1746-37 may contain an ultra low mass and small radius object ($M=0.21\pm0.06~M_{\odot},~R=6.26\pm0.99~\rm km$ at 68% confidence). Combined all possibilities, the mass of 4U 1746-37 is $0.41^{+0.70}_{-0.30}~M_\odot$ at 99.7% confidence. For such low mass NS, it could be reproduced by a self-bound compact star, i.e., quark star or quark-cluster star.