X-ray detection of a nova in the fireball phase

Novae are caused by runaway thermonuclear burning in the hydrogen-rich envelopes of accreting white dwarfs, which leads to a rapid expansion of the envelope and the ejection of most of its mass(1,2). Theory has predicted the existence of a 'fireball' phase following directly on from the runaway fusion, which should be observable as a short, bright and soft X-ray flash before the nova becomes visible in the optical(3-5). Here we report observations of a bright and soft X-ray flash associated with the classical Galactic nova YZ Reticuli 11 h before its 9 mag optical brightening. No X-ray source was detected 4 h before and after the event, constraining the duration of the flash to shorter than 8 h. In agreement with theoretical predictions(4,6-8), the source's spectral shape is consistent with a black-body of 3.27(-0.33)(+0.11) x 10(5) K (28.2(-2.8)(+0.9 )eV), or a white dwarf atmosphere, radiating at the Eddington luminosity, with a photosphere that is only slightly larger than a typical white dwarf.

Automated summary

Novae are caused by runaway thermonuclear burning in the hydrogen-rich envelopes of accreting white dwarfs. This study reports observations of a bright and soft X-ray flash associated with the classical Galactic nova YZ Reticuli, which appeared 11 hours before its optical brightening. The flash lasted less than 8 hours and its spectral shape is consistent with a black-body or a white dwarf atmosphere radiating at the Eddington luminosity.