When the upper layer of an accreting neutron star experiences a thermonuclearrunaway of helium and hydrogen, it exhibits an X-ray burst of a few keV with acool-down phase of typically 1~minute. When there is a surplus of hydrogen,hydrogen fusion is expected to simmer during that same minute due to the rpprocess, which consists of rapid proton captures and slow beta-decays ofproton-rich isotopes. We have analyzed the high-quality light curves of 1254X-ray bursts, obtained with the Proportional Counter Array on the Rossi X-rayTiming Explorer between 1996 and 2012, to systematically study the cooling andrp process. This is a follow-up of a study on a selection of 37 bursts fromsystems that lack hydrogen and show only cooling during the bursts. We findthat the bolometric light curves are well described by the combination of apower law and a one-sided Gaussian. The power-law decay index is between 1.3and 2.1 and similar to that for the 37-bursts sample. There are individualbursters with a narrower range. The Gaussian is detected in half of all bursts,with a typical standard deviation of 50~s and a fluence ranging up to 60% ofthe total fluence. The Gaussian appears consistent with being due to the rpprocess. The Gaussian fluence fraction suggests that the layer where the rpprocess is active is underabundant in H by a factor of at least five withrespect to cosmic abundances. Ninety-four percent of all bursts fromultracompact X-ray binaries lack the Gaussian component, and the remaining 6%are marginal detections. This is consistent with a hydrogen deficiency in thesebinaries. We find no clear correlation between the power law and Gaussianlight-curve components.
- X-rays: binaries
- X-rays: Bursts
- Stars: Neutron
- Nuclear reactions
Zand, J. J. M. I. ., Visser, M. E. B., Galloway, D. K., Chenevez, J., Keek, L., Kuulkers, E., Sanchez-Fernandez, C., & Wörpel, H. (2017). Neutron star cooling and the rp process in thermonuclear X-ray bursts. Astronomy & Astrophysics, 606, [A130]. https://doi.org/10.1051/0004-6361/201731281