Constraints on the mass and radius of the accreting neutron star in the Rapid Burster

TL;DR

This study uses a photospheric radius expansion burst observed by Swift/XRT to estimate the mass and radius of the neutron star in the Rapid Burster, constraining its equation of state.

Contribution

It provides the first mass and radius measurements of the Rapid Burster's neutron star using a Bayesian analysis of a photospheric radius expansion burst.

Findings

Mass = 1.1 +/- 0.3 solar masses

Radius = 9.6 +/- 1.5 km

Rules out stiff equations of state for the neutron star

Abstract

The Rapid Burster (MXB 1730-335) is a unique object, showing both type I and type II X-ray bursts. A type I burst of the Rapid Burster was observed with Swift/XRT on 2009 March 5, showing photospheric radius expansion for the first time in this source. We report here on the mass and radius determination from this photospheric radius expansion burst using a Bayesian approach. After marginalization over the likely distance of the system (5.8-10 kpc) we obtain M=1.1+/-0.3 M_sun and R=9.6+/-1.5 km (1-sigma uncertainties) for the compact object, ruling out the stiffest equations of state for the neutron star. We study the sensitivity of the results to the distance, the color correction factor, and the hydrogen mass fraction in the envelope. We find that only the distance plays a crucial role.