X-ray detection of a nova in the fireball phase

TL;DR

This paper reports the first confirmed detection of a short, soft X-ray flash from a nova during its fireball phase, matching theoretical predictions and providing new insights into nova energetics.

Contribution

It provides the first observational evidence of the predicted X-ray fireball phase in a classical nova, confirming key aspects of nova theory.

Findings

Detected a bright, soft X-ray flash 11 hours before optical brightening

Constrained the flash duration to less than 8 hours

Spectral analysis consistent with a hot white dwarf photosphere

Abstract

Novae are caused by runaway thermonuclear burning in the hydrogen-rich envelopes of accreting white dwarfs, which results in the envelope to expand rapidly and to eject most of its mass. For more than 30 years, nova theory has predicted the existence of a "fireball" phase following directly the runaway fusion, which should be observable as a short, bright, and soft X-ray flash before the nova becomes visible in the optical. Here we present the unequivocal detection of an extremely bright and very soft X-ray flash of the classical Galactic nova YZ Reticuli 11 hours prior to its 9 mag optical brightening. No X-ray source was detected 4 hours before and after the event, constraining the duration of the flash to shorter than 8 hours. In agreement with theoretical predictions, the source's spectral shape is consistent with a black body of $3.27^{+0.11}_{-0.33}\times 10^5$ K ($28.2^{+0.9}_{-2.8}$ eV), or a white dwarf atmosphere, radiating at the Eddington luminosity, with a photosphere that is only slightly larger than a typical white dwarf. This detection of the expanding white dwarf photosphere before the ejection of the envelope provides the last link of the predicted photospheric lightcurve evolution and opens a new window to measure the total nova energetics.