Thermonuclear X-ray bursts from the 401 Hertz accreting pulsar IGR J17498-2921: indication of burning in confined regions

TL;DR

This study analyzes 12 thermonuclear X-ray bursts from the 401 Hz pulsar IGR J17498-2921 using RXTE data, revealing burst groups, confirming oscillations, and indicating burning in confined surface regions.

Contribution

It provides detailed timing and spectral analysis of bursts from IGR J17498-2921, including the first detection of two PRE bursts with high peak count rate ratios from the same source.

Findings

Confirmed burst oscillations at 8.8σ significance

Identified three burst groups with different luminosities

Detected two PRE bursts with high peak count rate ratio

Abstract

We use the 2011 Rossi X-ray Timing Explorer (RXTE) proportional counter array (PCA) data of the 401 Hz accreting pulsar and burster IGR J17498-2921 to perform timing analysis and time-resolved spectroscopy of 12 thermonuclear X-ray bursts. We confirm previously reported burst oscillations from this source with a much higher significance (8.8\sigma). We notice that the bursts can be divided into three groups: big photospheric radius expansion (PRE) bursts are about ten times more luminous than medium bursts, while the latter are about ten times more luminous than small bursts. The PCA field-of-view of these observations contains several known bursters, and hence some of the observed bursts might not be from IGR J17498-2921. The oscillations during big bursts at the known pulsar frequency show that these bursts were definitely from IGR J17498-2921. We find that at least several of the other bursts were also likely originated from IGR J17498-2921. Spectral analysis reveals that the luminosity differences among various bursts are primarily due to differences in normalizations, and not temperatures, even when we consider the effects of colour factor. This shows burning on a fraction of the stellar surface for those small and medium bursts, which originated from IGR J17498-2921. The low values of the upper limits of burst oscillation amplitude for these bursts suggest a small angle between the spin axis and the magnetic axis. We find indications of the PRE nature of a medium burst, which likely originated from IGR J17498-2921. If true, then, to the best of our knowledge, this is the first time that two PRE bursts with a peak count rate ratio of as high as {\approx} 12 have been detected from the same source.