Millinovae: A New Class of Transient Supersoft X-Ray Sources without a Classical Nova Eruption
Udalski, Andrzej; Mróz, Przemek; Szymański, Michał K.; Skowron, Jan; Poleski, Radosław; Soszyński, Igor; Pietrukowicz, Paweł; Kozłowski, Szymon; Iwanek, Patryk; Ulaczyk, Krzysztof; Wrona, Marcin; Gromadzki, Mariusz; Buckley, David A. H.; Rybicki, Krzysztof; Page, Kim L.; Dewangan, Gulab; Charles, Philip; Mróz, Mateusz J.; Król, Krzysztof; Szegedi, Hélène; Meintjes, Pieter
Poland, South Africa, United Kingdom, India, Israel, United States
Abstract
Some accreting binary systems containing a white dwarf (such as classical novae or persistent supersoft sources) are seen to emit low-energy X-rays with temperatures of ∼ 106 K and luminosities exceeding 1035 erg s‑1. These X-rays are thought to originate from nuclear burning on the white dwarf surface, either caused by a thermonuclear runaway (classical novae) or a high mass-accretion rate that sustains steady nuclear burning (persistent sources). The discovery of transient supersoft X-rays from ASASSN-16oh challenged these ideas, as no clear signatures of mass ejection indicative of a classical nova eruption were detected, and the origin of these X-rays remains controversial. It was unclear whether this star was one of a kind or representative of a larger, as yet undiscovered, group. Here, we present the discovery of 29 stars located in the direction of the Magellanic Clouds exhibiting long-duration, symmetrical optical outbursts similar to that seen in ASASSN-16oh. We observed one of these objects during an optical outburst and found it to be emitting transient supersoft X-rays, while no signatures of mass ejection (indicative of a classical nova eruption) were detected. We therefore propose that these objects form a homogeneous group of transient supersoft X-ray sources, which we dub "millinovae" because their optical luminosities are approximately a 1000 times fainter than those of ordinary classical novae.