Indications for a slow rotator in the Rapid Burster from its thermonuclear bursting behaviour

TL;DR

This study analyzes thermonuclear bursts from the Rapid Burster, revealing evidence of a slow neutron star spin through spectral and bursting behavior, which differs from typical low-mass X-ray binaries.

Contribution

It provides the first evidence suggesting a slow spin neutron star in the Rapid Burster based on burst spectral properties and bursting rate trends.

Findings

Bursts lacking canonical cooling are still thermonuclear.

Bursting rate increases with luminosity beyond typical thresholds.

Similar behavior to the slow-spinning 11 Hz pulsar IGR J17480-2446.

Abstract

We perform time-resolved spectroscopy of all the type I bursts from the Rapid Burster (MXB 1730-335) detected with the Rossi X-ray Timing Explorer. Type I bursts are detected at high accretion rates, up to \sim 45% of the Eddington luminosity. We find evidence that bursts lacking the canonical cooling in their time-resolved spectra are, none the less, thermonuclear in nature. The type I bursting rate keeps increasing with the persistent luminosity, well above the threshold at which it is known to abruptly drop in other bursting low-mass X-ray binaries. The only other known source in which the bursting rate keeps increasing over such a large range of mass accretion rates is the 11 Hz pulsar IGR J17480$-$2446. This may indicate a similarly slow spin for the neutron star in the Rapid Burster.