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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 mass1,2. 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 optical3,4,5. 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 predictions4,6,7,8, the source’s spectral shape is consistent with a black-body of 3.27+0.11−0.33 × 105 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.

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Fig. 1: Sky images of all seven eROSITA cameras combined (0.2–0.6 keV).
Fig. 2: Multi-wavelength light curve of YZ Ret.
Fig. 3: Comparison of measured and simulated spectra of the X-ray flash and the ratios between the best-fit models and the data.

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Data availability

SIMPUT files for the best-fit model and calibrated eROSITA products of the observation are available from data are provided with this paper.

Code availability

The SIXTE code and eROSITA instrument files are publicly available at The eROSITA analysis software, eSASS, is available at


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This work is based on data from eROSITA, the soft X-ray instrument aboard SRG, a joint Russian–German science mission supported by the Russian Space Agency (Roskosmos), in the interests of the Russian Academy of Sciences represented by its Space Research Institute (IKI), and the Deutsches Zentrum für Luft- und Raumfahrt (DLR). This work was supported by the Bundesministerium für Forschung und Technologie under DLR grants 50 QR 1603, 50 QR 2103 and 50 QR 2104. G.S. acknowledges support from the Spanish MINECO grant PID2020-117252GB-I00. V.S. thanks the Deutsche Forschungsgemeinschaft (DFG) for financial support (WE1312/53-1). This research has made use of ISIS functions (ISISscripts) provided by the ECAP/Remeis observatory and MIT ( We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research.

Author information

Authors and Affiliations



R.A. identified the original event. The eROSITA near-real-time analysis was developed by I.K., A.R. and P.W. The final data extraction and reduction was performed by O.K., J.W., R.A., S.H. and A. Malyali, including calibration information from K.D. The pattern fraction analysis for SIXTE was performed by O.K., S.H., T.D. and K.D. SIXTE was being developed by T.D., C.K., M.L., O.K. and J.W. The interpretation of the results was done by O.K., J.W., G.S., A. Merloni, A. Malyali, T.D., K.W., V.D., A.S., F.H. and R.A. The WD atmosphere models were provided by V.S., T.R. and K.W. The manuscript was written by O.K. and J.W. J.W. and A.S. acquired funding for this work.

Corresponding authors

Correspondence to Ole König or Jörn Wilms.

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The authors declare no competing interests.

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Nature thanks Frederick Walter and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 eROSITA spectrum of YZ Reticuli taken during the supersoft state between t0+176.3 d and t0+186.3 d.

The total exposure time is 640 s, the spectrum is background subtracted. The spectrum can be described by a 20.7+0.7−0.4 eV black-body under an equivalent hydrogen column density of 7.1+0.3−0.9 × 1020 cm−2 and a 0.3–2 keV absorbed flux of 1.35(9) × 10−12 erg cm−2 s−1. The reconstructed source position is 1.2’’ from the optical position, which makes source confusion very unlikely. The fit is based on Cash statistics47, error bars are given at the 1σ confidence level. Panel b shows the residuals using only an absorbed black-body model. While the statistics are formally better when including an additional APEC model, as shown in c, the data are consistent with the background level at energies > 0.6 keV

Source data

Extended Data Fig. 2 Comparison of the observed eROSITA slew lightcurve of the X-ray flash and the (averaged) simulation of a constant source with the best-fit black-body parameters.

The trough shape is due to pattern pile-up when the source passes the center of the FoV and vignetting. The last few seconds of the lightcurve show a possible decline in brightness. Error bars are at the 1σ level, the blue shaded region indicates the 3σ flux uncertainty. The inset shows the observed source and (averaged) simulated image

Source data

Extended Data Fig. 3 Comparison of simulated and measured pattern fractions in order to verify the pile-up model of SIXTE.

The pre-flight pattern fractions from the TRoPIC prototype camera are shown for comparison48. Error bars are at the 1σ level

Source data

Extended Data Fig. 4 Parameter contours of the black-body and atmosphere model fits. The contours show the Δχ2 between the averaged simulated spectra with respect to the observed spectrum.

A systematic uncertainty of 10% is assigned to each simulated spectrum. Contour lines give the Δχ2 values for 2 degrees of freedom. The crosses denote the best-fit values, which are used in Fig. 3

Source data

Extended Data Table 1 Best-fit models of the X-ray flash of YZ Ret.

Supplementary information

Source data

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König, O., Wilms, J., Arcodia, R. et al. X-ray detection of a nova in the fireball phase. Nature 605, 248–250 (2022).

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