X-ray detection of a nova in the fireball phase

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 1,2 . 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 3,4,5 . 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 4,6,7,8 , 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. 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.

A Light-curve Analysis of the X-Ray Flash First Observed in Classical Novae

“…Thus, theoretically, no strong emission lines are expected. This is consistent with no prominent emission lines in the observed X-ray spectrum (König et al 2022). Sokolovsky et al (2022) estimated the galactic hydrogen column density to be 1 × 10 19 cm −2  N H  1.86 × 10 20 cm −2 .…”

“…Thanks to the short distance, d = 2.5 kpc, from Earth and very low galactic absorption E(B − V ) ∼ 0.03, the nova was observed in multiple wavelengths, including optical, X-rays, and gamma-rays, from a very early phase of the outburst until a very later phase through the supersoft X-ray source (SSS) phase. X-ray and γ-ray observations are reported by Sokolovsky et al (2022) and König et al (2022).…”

“…The duration of the bright soft X-ray phase is so short that it is called the "X-ray flash" in the rising phase of a nova. Such X-ray flashes have long been expected to be observed, but none were detected until UT 2020 July 7 when the onboard eROSITA instrument Spectrum-Roentgen-Gamma (SRG) scanned the region of YZ Reticuli (König et al 2022).…”

“…This is the first positive detection among any type of nova outbursts. König et al (2022) reported the X-ray flash on UT 2020 July 7 observed with the SRG/eROSITA. This detection was serendipitous during its all-sky survey.…”

Physics of nova outbursts: A theoretical model of classical nova outbursts with self-consistent wind mass loss

“…During a nova outburst the hydrogen-rich envelope expands to a giant size and wind mass loss is accelerated. Such a nova outburst has been calculated by several groups with Henyey-type evolution codes (Prialnik and Kovetz 1995;Kovetz 1998;Denissenkov et al 2013;Idan et al 2013;Wolf et al 2013;Kato et al 2014;Kato et al 2015;Tang et al 2014). These codes, however, encounter numerical difficulties when the nova envelope expands and wind mass loss occurs.…”