Max-Planck-Institut für extraterrestrische Physik, Garching, Germany.
Argelander-Institut für Astronomie, Universität Bonn, Bonn, Germany.
Nature. 2023 Mar;615(7953):605-609. doi: 10.1038/s41586-023-05714-4. Epub 2023 Mar 22.
Type Ia supernovae are cosmic distance indicators, and the main source of iron in the Universe, 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 observed and suggested as Type Ia supernovae progenitors. However, observational evidence for hydrogen, which is expected to be stripped off the donor star during the supernova explosion, is lacking. Helium-accreting white dwarfs, which would circumvent this problem, have been predicted for more than 30 years (refs. ), 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 supernovae, within this scenario.
Ia 型超新星是宇宙距离指示器，也是宇宙中铁的主要来源，但它们的形成途径仍存在争议。已经观测到几十颗超软 X 射线源，其中白矮星从非简并供体星吸积富含氢的物质，并被认为是 Ia 型超新星的前身。然而，在超新星爆炸中预计会从供体星上剥离的氢的观测证据却缺乏。三十多年来，人们一直在预测会出现吸积氦的白矮星来解决这个问题（参考文献），包括它们作为超软 X 射线源的出现，但迄今为止尚未被探测到。在这里，我们报告了一个具有吸积盘的超软 X 射线源，其光学光谱完全由氦主导，表明供体星是无氢的。我们将发光和超软 X 射线解释为源自吸积白矮星表面附近的氦燃烧。我们的系统的性质为基于氦吸积的 Chandrasekhar 质量爆炸的扩展途径提供了证据，特别是对于比迄今为止预期的更低吸积率的白矮星中的稳定燃烧。这可能使我们能够在这种情况下恢复到低能量的所谓 Iax 型超新星的种群，占所有 Ia 型超新星的 30%。
