Non-LTE Luminosity and Abundance Diagnostics of Classical Novae in X-rays

TL;DR

This study models the X-ray spectra of classical novae using non-LTE atmospheric calculations to diagnose their luminosity and elemental abundances, highlighting the importance of atomic data and line opacities.

Contribution

It introduces a detailed non-LTE modeling approach for nova X-ray spectra incorporating high-ionization atomic data, improving diagnostics of nova properties.

Findings

N VI and N VII lines are effective temperature indicators

Model fitting suggests some novae are near or above the Eddington limit

Line opacities significantly influence atmospheric parameters and spectra

Abstract

Classical novae are significant sources of interstellar material, especially carbon, nitrogen and oxygen. These standard candles are only behind supernovae and $\gamma$-ray bursts as the third brightest objects in the sky, and the most probable progenitors of type Ia supernovae. After a nova outburst the system enters into the constant bolometric luminosity phase and the nova maintains a stable hydrogen burning in the surface layers of the white dwarf. As the expanding shell around the nova attenuates, progressively deeper and hotter layers become visible. At the end of the constant bolometric luminosity phase, the hottest layers are exposed and novae radiate X-rays. This work uses the static, plane-parallel model atmosphere code TLUSTY to calculate atmospheric structure and SYNSPEC to calculate synthetic X-ray spectra. It was necessary to incorporate atomic data for the highest ionization stages of elements ranging from hydrogen to iron in both programs. Atomic data on energy levels, bound-free, bound-bound transitions and natural broadening were taken from NIST and TOPbase. Extensive tests revealed the importance of line opacities on atmospheric parameters and on the final spectra. A correlation can be defined between effective temperature and surface gravity. The spectral appearance is not very sensitive to the joint changes of both. Due to this effect both parameters might be over-estimated with static models. These tests also showed that N VI and N VII lines are good indicators of the effective temperature. Model fitting of V4743 Sgr and V2491 Cyg confirmed the anticipated impact of modeling geometry and stellar wind. The ionization balance and line profiles indicate that both novae are close to or over the Eddington limit. These results are consistent with previous studies; further and unambiguous details require a comprehensive update of TLUSTY, what is under way.