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

TL;DR

This study analyzes 107 X-ray bursts from the 11 Hz pulsar IGR J17480-2446, revealing short recurrence times, helium ignition in hydrogen-rich layers, and pulsation behaviors that suggest surface-wide ignition regions.

Contribution

It provides the first detailed morphological, spectral, and temporal analysis of bursts from this unique slow-spinning pulsar, highlighting their Type-I nature and pulsation characteristics.

Findings

Shortest burst recurrence time observed from a neutron star

Helium ignition occurs in hydrogen-rich layers

Pulsations are phase-locked and show no frequency drift

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 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 of the bursts observed by the Rossi X-ray Timing Explorer. The recurrence time and the fluence of the bursts clearly anticorrelate with the increase of the persistent X-ray flux. The ratio between the energy generated by the accretion of mass and that liberated during bursts indicate 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 of the rms associated to the pulse frequency is observed during the burst. This behavior would favor a scenario where the flash is ignited within a region which is consistent to be as large as the neutron star surface.