Time and energy dependent characteristics of Thermonuclear burst oscillations

TL;DR

This study analyzes the temporal and spectral properties of thermonuclear burst oscillations across multiple sources, revealing insights into frequency drifts and phase lags related to physical processes during bursts.

Contribution

It provides a comprehensive analysis of energy-dependent amplitude and phase lag behaviors, and their relation to burst oscillation mechanisms across different neutron star sources.

Findings

Frequency drifts are more common at lower spin frequencies.

No systematic energy evolution of phase lag observed across bursts.

Both soft and hard energy lags are present in different cases.

Abstract

We have investigated temporal and spectral properties of a large sample of thermonuclear bursts with oscillations from eight different sources with spin frequencies varying from 270 to 620 Hz. For our sample we chose those bursts, for which the oscillation is sufficiently strong and of relatively long duration. The emission from the hot-spot that is formed during a thermonuclear burst is modulated by several physical processes and the burst oscillation profiles unavoidably carry signatures of these. In order to probe these mechanisms, we examined the amplitude and phase lags of the burst oscillations with energy. We also studied the frequency variation of oscillations during these thermonuclear bursts. We observed that the frequency drifts are more frequent in the cases where the spin frequency is lower. We found that the phase lag of the burst oscillations shows no systematic evolution with energy between the bursts, and also in between different sources. In 7 cases, we do indeed observe lag of soft energy photons, while there are a significant number of cases for which hard lag or no lag is observed.