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

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 ≃45 per cent of the Eddington luminosity. We find evidence that bursts lacking the canonical cooling in their time-resolved spectra are, nonetheless, 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.