Millihertz quasi-periodic oscillations and thermonuclear bursts from Terzan 5: A showcase of burning regimes

TL;DR

This study analyzes thermonuclear bursts and millihertz QPOs from the neutron star in Terzan 5, revealing new burning regimes, discrepancies with models, and the influence of magnetic fields and spin.

Contribution

It provides the first detailed observational analysis of burst regimes and QPOs from IGR J17480-2446, highlighting discrepancies with existing nuclear burning models.

Findings

Identified four distinct bursting regimes.

Discovered a steep decrease in burst recurrence time with luminosity.

Found evidence for a transition between helium and mixed hydrogen/helium ignition regimes.

Abstract

We present a comprehensive study of the thermonuclear bursts and millihertz quasi-periodic oscillations (mHz QPOs) from the neutron star (NS) transient and 11 Hz X-ray pulsar IGR J17480-2446, located in the globular cluster Terzan 5. The increase in burst rate that we found during its 2010 outburst, when persistent luminosity rose from 0.1 to 0.5 times the Eddington limit, is in qualitative agreement with thermonuclear burning theory yet opposite to all previous observations of thermonuclear bursts. Thermonuclear bursts gradually evolved into a mHz QPO when the accretion rate increased, and vice versa. The mHz QPOs from IGR J17480-2446 resemble those previously observed in other accreting NSs, yet they feature lower frequencies (by a factor ~3) and occur when the persistent luminosity is higher (by a factor 4-25). We find four distinct bursting regimes and a steep (close to inverse cubic) decrease of the burst recurrence time with increasing persistent luminosity. We compare these findings to nuclear burning models and find evidence for a transition between the pure helium and mixed hydrogen/helium ignition regimes when the persistent luminosity was about 0.3 times the Eddington limit. We also point out important discrepancies between the observed bursts and theory, which predicts brighter and less frequent bursts, and suggest that an additional source of heat in the NS envelope is required to reconcile the observed and expected burst properties. We discuss the impact of NS magnetic field and spin on the expected nuclear burning regimes, in the context of this particular pulsar.