Ignition Column Depths of Helium-rich Thermonuclear Bursts from 4U 1728-34

Abstract

We analyzed thermonuclear (type I) X-ray bursts observed from the low-mass X-ray binary 4U 1728-34 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 (NS) 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 NS surface.