Effect of accretion on the NS mass and radius measurements: 4U 1608-52

You can find the article from the Arxiv.

Neutron stars are one of the densest stars in our Universe. In some cases these peculiar stars are accompanied by smaller, normal stars forming a so-called low-mass X-ray binary. Due to the strong gravitational pull, the material from the companion star starts to leak towards the more massive neutron star. Rotation of the system, however, complicates the situation and so an accretion disk is formed around the neutron star from where the material will slowly spiral into the surface.

Eventually more and more material will pack to the surface and at some point nuclear burning temperatures and densities are reached. This will start a rapid nuclear fusion reaction on top of the neutron star burning all of the accumulated material in couple of tens of seconds. This will then lead to a bright X-ray flash as massive amounts of energy are released on this nuclear explosion.

The cooling just after these X-ray bursts can be used to constrain the size of the emitting surface of the source as the observed temperatures and fluxes can be compared to theoretical computations. This in turn will result in neutron star mass and radius measurements. On this article, published in MNRAS, we show how the accretion (i.e. the matter falling from the companion) can affect the NS mass and radius measurements done using these X-ray bursts. It turns out that one needs to be extra careful when using the cooling of neutron star atmospheres after the X-ray bursts as the infalling material can induce external heating of the surface. Theoretical models on the other hand assume pure passive cooling so only bursts where the atmosphere shows signs of passive cooling can be used.