X-Ray Flashes on Helium Novae

A helium nova occurs on a white dwarf (WD) accreting hydrogen-deficient matter from a helium star companion. When the mass of a helium envelope on the WD reaches a critical value, unstable helium burning ignites to trigger a nova outburst. A bright soft X-ray phase appears in an early outbursting phase of a helium nova before it optically rises toward maximum. Such an X-ray bright phase is called the X-ray flash. We present theoretical light curves of X-ray flashes for 1.0, 1.2, and 1.35 M _☉ helium novae with mass accretion rates of (1.6–7.5) × 10^‑7 M _☉ yr^‑1. Long durations of the X-ray flashes (100 days–10 yr) and high X-ray luminosities (∼10³⁸ erg s^‑1) indicate that X-ray flashes are detectable as a new type of X-ray transient or persistent X-ray sources. An X-ray flash is a precursor of optical brightening, so that the detection of X-ray flashes on helium novae enables us to plan arranged observation for optical premaximum phases that have been one of the frontiers of the study of novae. We found a candidate object of helium-burning X-ray flash from the literature on extragalactic X-ray surveys. This X-ray transient source is consistent with our X-ray flash model of a 1.35 M _⊙ WD.