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.

中文翻译：

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Millinovae：一类新型瞬态超软 X 射线源，没有经典的 Nova 喷发


一些包含白矮星的吸积双星系统（如经典新星或持续超软源）可以发射温度为 ∼ 106 K、光度超过 1035 erg s-1 的低能 X 射线。这些 X 射线被认为起源于白矮星表面的核燃烧，要么是由热核失控（经典新星）引起的，要么是由维持稳定核燃烧的高质量吸积率（持续源）引起的。来自 ASASSN-16oh 的瞬态超软 X 射线的发现挑战了这些想法，因为没有检测到表明经典新星喷发的物质抛射的明确特征，并且这些 X 射线的来源仍然存在争议。目前尚不清楚这颗恒星是同类中的一颗，还是代表了一个更大的、尚未被发现的星群。在这里，我们展示了位于麦哲伦星云方向的 29 颗恒星的发现，它们表现出类似于 ASASSN-16oh 中观察到的长持续时间、对称的光爆发。我们在光学爆发期间观察了其中一个天体，发现它正在发射瞬态超软 X 射线，而没有检测到物质喷射的特征（表明经典新星喷发）。因此，我们提出这些天体形成一组均匀的瞬态超软 X 射线源，我们称之为“毫新星”，因为它们的光度比普通经典新星的光度暗约 1000 倍。