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

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 ρ or heartbeat variability and the θ 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 behaviour 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.