Observations of Classical and Recurrent Novae with X-ray Gratings

TL;DR

This paper reviews X-ray grating observations of 12 novae, revealing white dwarf properties, ejecta behavior, and variability, providing insights into nova physics and hydrogen burning processes.

Contribution

It presents the first comprehensive analysis of nova X-ray spectra from Chandra and XMM-Newton, linking white dwarf temperature, ejecta characteristics, and variability to nova evolution.

Findings

White dwarf temperatures range from 400,000 K to over 1 million K.

Shorter supersoft X-ray phases correlate with lower white dwarf temperatures.

Ejecta exhibit violent phenomena like shell ejection and clumpy emission regions.

Abstract

X-ray grating spectra have opened a new window on the nova physics. High signal-to-noise spectra have been obtained for 12 novae after the outburst in the last 13 years with the Chandra and XMM-Newton gratings. They offer the only way to probe the temperature, effective gravity and chemical composition of the hydrogen burning white dwarf before it turns off. These spectra also allow an analysis of the ejecta, which can be photoionized by the hot white dwarf, but more often seem to undergo collisional ionization. The long observations required for the gratings have revealed semi-regular and irregular variability in X-ray flux and spectra. Large short term variability is especially evident in the first weeks after the ejecta have become transparent to the central supersoft X-ray source. Thanks to Chandra and XMM-Newton, we have discovered violent phenomena in the ejecta, discrete shell ejection, and clumpy emission regions. As expected, we have also unveiled the white dwarf characteristics. The peak white dwarf effective temperature in the targets of our samples varies between ~400,000 K and over a million K, with most cases closer to the upper end, although for two novae only upper limits around 200,000 K were obtained. A combination of results from different X-ray satellites and instruments, including Swift and ROSAT, shows that the shorter is the supersoft X-ray phase, the lower is the white dwarf peak effective temperature, consistently with theoretical predictions. The peak temperature is also inversely correlated with t(2) the time for a decay by 2 mag in optical. I strongly advocate the use of white dwarf atmospheric models to obtain a coherent physical picture of the hydrogen burning process and of the surrounding ejecta.