Discovery of GRS 1915+105 variability patterns in the Rapid Burster

TL;DR

This paper reports the discovery of two new variability patterns in the neutron star binary MXB 1730-335, resembling black hole variability types, suggesting these behaviors are not exclusive to black holes and can occur at various accretion rates.

Contribution

The study identifies and characterizes two new variability patterns in a neutron star system, linking them to known black hole behaviors and challenging previous assumptions about their origins.

Findings

Discovered quasi-periodic oscillations with a 7-minute period.

Observed deep dips 44 minutes apart, similar to black hole variability.

Variability occurs at sub-Eddington accretion rates.

Abstract

We report the discovery of two new types of variability in the neutron star low-mass X-ray binary MXB 1730-335 (the 'Rapid Burster'). In one observation in 1999, it exhibits a large-amplitude quasi-periodic oscillation with a period of about 7 min. In another observation in 2008, it exhibits two 4-min long 75 per cent deep dips 44 min apart. These two kinds of variability are very similar to the so-called $\rho$ or 'heartbeat' variability and the $\theta$ variability, respectively, seen in the black hole low-mass X-ray binaries GRS 1915+105 and IGR J17091-3624. This shows that these types of behavior are unrelated to a black hole nature of the accretor. Our findings also show that these kinds of behaviour need not take place at near-Eddington accretion rates. We speculate that they may rather be related to the presence of a relatively wide orbit with an orbital period in excess of a few days and about the relation between these instabilities and the type II bursts.