Millinovae: A New Class of Transient Supersoft X-Ray Sources without a Classical Nova Eruption

• Published in: Astrophysical journal. Letters 2024-12, Vol.977 (2), p.L37
• Main Authors: Mróz, Przemek, Król, Krzysztof, Szegedi, Hélène, Charles, Philip, Page, Kim L., Udalski, Andrzej, Buckley, David A. H., Dewangan, Gulab, Meintjes, Pieter, Szymański, Michał K., Soszyński, Igor, Pietrukowicz, Paweł, Kozłowski, Szymon, Poleski, Radosław, Skowron, Jan, Ulaczyk, Krzysztof, Gromadzki, Mariusz, Rybicki, Krzysztof, Iwanek, Patryk, Wrona, Marcin, Mróz, Mateusz J.
• Format: Article
• Language: English
• Subjects: Cataclysmic variable stars; Classical novae; Dwarf novae
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ad969b

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  ∼ 10 ^6 K and luminosities exceeding 10 ^35 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.