Rare ultra-luminous nova spotted in the Small Magellanic Cloud

A rare, extremely luminous X-ray outburst has been observed in the Small Magellanic Cloud, a dwarf galaxy that is a close neighbor of our own Milky Way galaxy. The observations, made by the Neil Gehrels Swift Observatory and other telescopes, were described by an international team of astronomers led Penn State scientists on the Swift team. The researchers attributed the outburst to one of the most luminous nova eruptions ever produced by a white dwarf binary star system.

The observations were described in a paper recently published in the Monthly Notices of the Royal Astronomical Society.

“This is only the second time that we have observed such a bright outburst from this type of white dwarf binary system,” said Thomas Gaudin, a graduate student at Penn State and the first author of the paper. “We hope that this event will provide more insight into how these outbursts are produced and help us to better understand this mysterious class of binary.”

The system that produced this outburst is referred to as CXOU J005245.0–722844. It was recently identified by members of the Einstein Probe team and confirmed by the Swift team as the seventh-known example of a Be/White Dwarf X-ray binary. Be/White Dwarf binaries are binary systems in which a white dwarf star orbits a hot young star surrounded by a disk of stellar material. Astronomers expect these binaries to be commonly observed, Gaudin said, and the lack of known examples is a mystery.

“Novae are explosions that happen when material from a nearby star is deposited onto the surface of a white dwarf,” Gaudin said. “After enough material has been built up, the surface undergoes rapid thermonuclear fusion which creates the outburst. Most novae are events that reach moderate luminosities and decay over the course of several weeks. This nova is strange not just in its extremely luminous behavior but also in its short duration.”

The thermonuclear reaction during the nova is similar to a massive hydrogen bomb exploding—the explosion produces electromagnetic radiation that can be seen by telescopes on Earth and in orbit around Earth. According to the researchers, the nova was visible at optical wavelengths, or visible light, for just under a week and in X-rays for just under two weeks.

“We will need follow-up observations in order to better understand the physics that led to such an outburst, but this is an important first step to getting a better understanding of these systems and potentially why we have not seen as many as we expect,” Gaudin said.