Models of neutron star atmospheres enriched with nuclear burning ashes^⋆,⋆⋆

¹ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
e-mail: joonas.a.nattila@utu.fi
² Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Sand 1, 72076 Tübingen, Germany
³ Kazan (Volga region) Federal University, Kremlevskaya 18, 420008 Kazan, Russia
⁴ European Space Astronomy Centre (ESA/ESAC), Science Operations Department, 28691 Villanueva de la Cañada, Madrid, Spain

Received: 11 May 2015
Accepted: 25 June 2015

Abstract

Context. Low-mass X-ray binaries hosting neutron stars (NS) exhibit thermonuclear (type-I) X-ray bursts, which are powered by unstable nuclear burning of helium and/or hydrogen into heavier elements deep in the NS “ocean”. In some cases the burning ashes may rise from the burning depths up to the NS photosphere by convection, leading to the appearance of the metal absorption edges in the spectra, which then force the emergent X-ray burst spectra to shift toward lower energies.

Aims. These effects may have a substantial impact on the color correction factor f_c and the dilution factor w, the parameters of the diluted blackbody model F_E ≈ wB_E(f_cT_eff) that is commonly used to describe the emergent spectra from NSs. The aim of this paper is to quantify how much the metal enrichment can change these factors.

Methods. We have developed a new NS atmosphere modeling code, which has a few important improvements compared to our previous code required by inclusion of the metals. The opacities and the internal partition functions (used in the ionization fraction calculations) are now taken into account for all atomic species. In addition, the code is now parallelized to counter the increased computational load.

Results. We compute a detailed grid of atmosphere models with different exotic chemical compositions that mimic the presence of the burning ashes. From the emerging model spectra we compute the color correction factors f_c and the dilution factors w that can then be compared to the observations. We find that the metals may change f_c by up to about 40%, which is enough to explain the scatter seen in the blackbody radius measurements.

Conclusions. The presented models open up the possibility of determining NS mass and radii more accurately, and may also act as a tool to probe the nuclear burning mechanisms of X-ray bursts.