Ignition column depths of helium-rich thermonuclear bursts from 4U 1728-34

TL;DR

This study analyzes helium-rich thermonuclear bursts from 4U 1728-34, revealing that observed ignition depths are significantly shallower than theoretical predictions, suggesting additional physical processes influence burst ignition.

Contribution

It provides the first detailed comparison of observed burst ignition depths with numerical models, highlighting discrepancies and proposing potential physical explanations.

Findings

Observed ignition depths are below theoretical predictions.

Underestimation of accretion rate is unlikely to explain the discrepancy.

Shear-triggered mixing or fractional fuel coverage may cause earlier ignition.

Abstract

We analysed thermonuclear (type-I) X-ray bursts observed from the low-mass X-ray binary 4U1728-34 by RXTE, Chandra and INTEGRAL. We compared the variation in burst energy and recurrence times as a function of accretion rate with the predictions of a numerical ignition model including a treatment of the heating and cooling in the crust. We found that the measured burst ignition column depths are significantly below the theoretically predicted values, regardless of the assumed thermal structure of the neutron star interior. While it is possible that the accretion rate measured by Chandra is underestimated, due to additional persistent spectral components outside the sensitivity band, the required correction factor is typically 3.6 and as high as 6, which is implausible. Furthermore, such underestimation is even more unlikely for RXTE and INTEGRAL, which have much broader bandpasses. Possible explanations for the observed discrepancy include shear-triggered mixing of the accreted helium to larger column depths, resulting in earlier ignition, or the fractional covering of the accreted fuel on the neutron star surface.