What can NuSTAR do for thermonuclear X-ray bursts?

Abstract

Unstable thermonuclear burning on the surface of accreting neutron stars is commonly observed as type I X-ray bursts. The flux released during some strong bu rsts can temporarily exceed the Eddington limit, driving the neutron star photosphere to such large radii that heavy-element ashes of nuclear burning are ejected in the burst expansion wind. We have investigated the possibility of observing with NuSTAR some X-ray bursters selected for their high burst rate and trend to exhibit so-called superexpansion bursts. Our main ambition is to detect the photoionization edges associated with the ejected nuclear ashes, and identify the corresponding heavy elements. A positive identification of such edges would probe the nuclear burning processes, and provide a measure of the expans ion wind velocity as well as the gravitational redshift from the neutron star. Moreover, we expect that the high sensitivity of NuSTAR in hard X-rays will make it possible to study the behavi our of the accretion emission during the bursts, which is an important parameter to constrain the properties of the X-ray burst emission and thermonuclear burning.