A helium-burning white dwarf binary as a supersoft X-ray source.

Type Ia supernovae are cosmic distance indicators1,2, and the main source of iron in the Universe3,4, but their formation paths are still debated. Several dozen supersoft X-ray sources, in which a white dwarf accretes hydrogen-rich matter from a non-degenerate donor star, have been observed5 and suggested as Type Ia supernovae progenitors6-9. However, observational evidence for hydrogen, which is expected to be stripped off the donor star during the supernova explosion10, is lacking. Helium-accreting white dwarfs, which would circumvent this problem, have been predicted for more than 30 years (refs. 7,11,12), including their appearance as supersoft X-ray sources, but have so far escaped detection. Here we report a supersoft X-ray source with an accretion disk whose optical spectrum is completely dominated by helium, suggesting that the donor star is hydrogen-free. We interpret the luminous and supersoft X-rays as resulting from helium burning near the surface of the accreting white dwarf. The properties of our system provide evidence for extended pathways towards Chandrasekhar-mass explosions based on helium accretion, in particular for stable burning in white dwarfs at lower accretion rates than expected so far. This may allow us to recover the population of the sub-energetic so-called Type Iax supernovae, up to 30% of all Type Ia supernovae13, within this scenario.

Nine-hour X-ray quasi-periodic eruptions from a low-mass black hole galactic nucleus.

In the past two decades, high-amplitude electromagnetic outbursts have been detected from dormant galaxies and often attributed to the tidal disruption of a star by the central black hole1,2. X-ray emission from the Seyfert 2 galaxy GSN 069 (2MASX J01190869-3411305) at a redshift of z = 0.018 was first detected in July 2010 and implies an X-ray brightening by a factor of more than 240 over ROSAT observations performed 16 years earlier3,4. The emission has smoothly decayed over time since 2010, possibly indicating a long-lived tidal disruption event5. The X-ray spectrum is ultra-soft and can be described by accretion disk emission with luminosity proportional to the fourth power of the disk temperature during long-term evolution. Here we report observations of quasi-periodic X-ray eruptions from the nucleus of GSN 069 over the course of 54 days, from December 2018 onwards. During these eruptions, the X-ray count rate increases by up to two orders of magnitude with an event duration of just over an hour and a recurrence time of about nine hours. These eruptions are associated with fast spectral transitions between a cold and a warm phase in the accretion flow around a low-mass black hole (of approximately 4 × 105 solar masses) with peak X-ray luminosity of about 5 × 1042 erg per second. The warm phase has kT (where T is the temperature and k is the Boltzmann constant) of about 120 electronvolts, reminiscent of the typical soft-X-ray excess, an almost universal thermal-like feature in the X-ray spectra of luminous active nuclei6-8. If the observed properties are not unique to GSN 069, and assuming standard scaling of timescales with black hole mass and accretion properties, typical active galactic nuclei with higher-mass black holes can be expected to exhibit high-amplitude optical to X-ray variability on timescales as short as months or years9.