X-ray bursts and burst oscillations from the slowly spinning X-ray pulsar IGR J17480-2446 (Terzan 5)

Abstract

The newly discovered 11-Hz accreting pulsar, IGR J17480-2446, located in the globular cluster Terzan 5, has shown several bursts with a recurrence time as short as a few minutes. The source shows the shortest recurrence time ever observed from a neutron star. Here we present a study of the morphological, spectral and temporal properties of 107 bursts observed by the Rossi X-ray Timing Explorer. The recurrence time and the fluence of the bursts clearly anticorrelate with the increase in the persistent X-ray flux. The ratio between the energy generated by the accretion of mass and that liberated during bursts indicates that helium is ignited in a hydrogen-rich layer. Therefore, we conclude that all the bursts shown by IGR J17480-2446 are Type I X-ray bursts. Pulsations could be detected in all the brightest bursts and no drifts of the frequency are observed within 0.25 Hz of the spin frequency of the neutron star. These are also phase-locked with respect to the pulsations observed during the persistent emission and no rise in the rms associated to the pulse frequency is observed during the burst. This behaviour would favour a scenario where the Type I burst, possibly ignited at the polar caps, immediately propagates to the entire neutron star surface.