X-ray detection of a nova in the fireball phase

• Published in: Nature (London) 2022-05, Vol.605 (7909), p.248-250
• Main Authors: König, Ole, Wilms, Jörn, Arcodia, Riccardo, Dauser, Thomas, Dennerl, Konrad, Doroshenko, Victor, Haberl, Frank, Hämmerich, Steven, Kirsch, Christian, Kreykenbohm, Ingo, Lorenz, Maximilian, Malyali, Adam, Merloni, Andrea, Rau, Arne, Rauch, Thomas, Sala, Gloria, Schwope, Axel, Suleimanov, Valery, Weber, Philipp, Werner, Klaus
• Format: Article
• Language: English
• Subjects: 639/33/34/4118; 639/33/34/4127; 639/33/34/864; Atmosphere; Blackbody; Brightening; Burning; Deposition; Dwarf novae; Fireballs; Humanities and Social Sciences; Hydrogen; Luminosity; multidisciplinary; Novae; Photosphere; Science; Science (multidisciplinary); Soft x rays; Stellar atmospheres; White dwarf stars; X ray flashes; X ray sources; X-Rays
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-022-04635-y

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 +0.11 −0.33  × 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. 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 +0.11 −0.33  × 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.