Helium Ignition on Accreting Neutron Stars with a New Triple-alpha Reaction Rate

TL;DR

This study examines how a new triple-alpha reaction rate affects helium ignition and X-ray burst properties on accreting neutron stars, revealing discrepancies with observations that challenge the new rate's validity.

Contribution

It provides the first detailed analysis of the impact of Ogata et al.'s (2009) reaction rate on neutron star ignition conditions and compares predictions with observational data.

Findings

Lower ignition column densities at low accretion rates.

Predicted burst energies are significantly lower than observed.

The results question the validity of the new reaction rate.

Abstract

We investigate the effect of a new triple-alpha reaction rate from Ogata et al. (2009) on helium ignition conditions on accreting neutron stars and on the properties of the subsequent type I X-ray burst. We find that the new rate leads to significantly lower ignition column density for accreting neutron stars at low accretion rates. We compare the results of our ignition models for a pure helium accretor to observations of bursts in ultra-compact X-ray binary (UCXBs), which are believed to have nearly pure helium donors. For mdot > 0.001 mdot_Edd, the new triple-alpha reaction rate from Ogata et al. (2009) predicts a maximum helium ignition column of ~ 3 x 10^9 g cm^{-2}, corresponding to a burst energy of ~ 4 x 10^{40} ergs. For mdot ~ 0.01 mdot_Edd at which intermediate long bursts occur, the predicted burst energies are at least a factor of 10 too low to explain the observed energies of such bursts in UCXBs. This finding adds to the doubts cast on the triple-alpha reaction rate of Ogata et al. (2009) by the low-mass stellar evolution results of Dotter & Paxton (2009).