A strongly truncated inner accretion disk in the Rapid Burster

TL;DR

This study reveals a strongly truncated inner accretion disk in the Rapid Burster, supporting magnetospheric models for Type-II bursts and providing detailed spectral analysis with relativistic reflection modeling.

Contribution

First to model the Fe-K line with relativistic reflection in the Rapid Burster, revealing a strongly truncated disk and estimating the neutron star's magnetic field strength.

Findings

Disk truncated at ~87 km from the neutron star

Magnetic field strength estimated at ~6.2 x 10^8 G

Supports magnetospheric gating models for Type-II bursts

Abstract

The neutron star (NS) low-mass X-ray binary (LMXB) the Rapid Burster (RB; MXB 1730-335) uniquely shows both Type-I and Type-II X-ray bursts. The origin of the latter is ill-understood but has been linked to magnetospheric gating of the accretion flow. We present a spectral analysis of simultaneous Swift, NuSTAR and XMM-Newton observations of the RB during its 2015 outburst. Although a broad Fe-K line has been observed before, the high quality of our observations allows us to model this line using relativistic reflection models for the first time. We find that the disk is strongly truncated at $41.8^{+6.7}_{-5.3}$ gravitational radii ($\sim 87$ km), which supports magnetospheric Type-II burst models and strongly disfavors models involving instabilities at the innermost stable circular orbit. Assuming that the RB magnetic field indeed truncates the disk, we find $B = (6.2 \pm 1.5) \times 10^8$ G, larger than typically inferred for NS LMXBs. In addition, we find a low inclination ($i = 29\pm2^{\rm o}$). Finally, we comment on the origin of the Comptonized and thermal components in the RB spectrum.