A black hole gnaws away at a star every 22 days, then slowly devours it

Astronomers working with NASA’s Neil Girls Swift Observatory have discovered something unusual. The observatory’s X-ray Telescope (XRT) has captured emissions from a supermassive black hole (SMBH) in a galaxy about 500 million light-years away. The black hole repeatedly feeds on an unfortunate star that got too close.

When SMBHs feed on a star, it is called a tidal disturbance event (TDE). These events occur when a star gets too close to a massive hole, and strong gravity pulls material away from the star. Matter is sucked into the accretion disk outside the black hole, heated, and emits radiation, especially X-rays. Sometimes, the star is destroyed. The black hole’s overwhelming gravity literally pulls the star into a stream of gas. Sometimes, it survives, but not for long.

Sometimes, the black hole feeds on the star repeatedly. This can happen when a star follows a highly eccentric orbit. As it passes around its center, it gets very close to the black hole. The star bulges toward the hole and sheds some of its gas. When this happens, astronomers call it recurrent tidal disturbance, or partial tidal disturbance. Eventually, the star will lose enough mass and will no longer be able to maintain its cohesion. It’s mass against mass, and the supermassive black hole always wins. Astronomers are interested in and closely studying the recurring TDE phenomenon.

The radiation from all this activity can tell astronomers about the properties of the black hole and the star. Every time the black hole adds more stellar material to its accretion disk, it releases energy. The type and amount of energy across different wavelengths reveals a lot about the star and the black hole.

This newly discovered recurring TDE is called Swift J0230 (Swift J023017.0+283603.) and it is located half a billion light-years away in the constellation Triangle. It appears to nibble on the stricken star every 25 days, although this may vary by a few days. Each brightening event lasts about 10 to 15 days, but the longest event was about 20 days, and the shortest event was less than one day.

A new paper presents this discovery. It is titled “Quasi-periodic monthly outbursts caused by recurrent stellar disturbance from a massive black hole,” and is published in the journal Nature Astronomy. The lead author is Phil Evans, an astrophysicist at the University of Leicester and a long-time member of the SWIFT team.

The Swift Observatory was built to detect and study gamma ray bursts (GRBs). These are very energetic explosions that are observed in other galaxies. Swift was very efficient and detected the 1,000th GRB in 2015. It also detected four GRBs in one day. The observatory has an X-ray telescope that can help locate the GRB more precisely as well as monitor the afterglow produced by the explosion. But we are also supposed to quickly notice any transient X-ray phenomena, including TDEs.

In June 2022, Swift J0230 was spotted for the first time. According to Evans and his colleagues, a star similar to our Sun orbits a small star with a mass of more than 200,000 solar masses. Every time a black hole collides with a star, it absorbs about three times the Earth’s mass of gas.

Swift J0230 takes a bite out of the star it orbits about every 25 days. But other known recurring TDE waves have different frequencies. One well-known example has a frequency of 114 days, and another has a short nine-hour gap between events.

There are two known objects that are closely related to Swift J0230 emissions. One is a quasi-periodic eruption (QPE), and the other is a cyclic nuclear explosion (PNT).

Astronomers believe that QPEs are white dwarfs interacting with central low-mass black holes. PNTs are explosions much longer in both duration and repetition period, and astronomers believe they are main sequence stars interacting with a black hole and emitting significant optical light. But the Swift J0230 bridges the gap between the two. It is likely a main sequence star orbiting a modest-mass black hole. This discovery gives astronomers an opportunity to study how different types of stars interact with black holes of different masses.

J0230 was not easy to spot. It was discovered by astronomers using a new data processing technique involving an automated search developed by Evans. It is called the Swift Transient X-ray Detector.

Every time Swift observes a portion of the sky, it downloads data. The Swift X-ray Transient Detector then processes the data and compares it to previous Swift X-ray observations of the same area looking for changes. If a transient detector finds a change in X-rays from the same region of the sky, it sounds an alert, and astronomers can quickly coordinate follow-up observations.

Evans’ new data processing system paid off almost immediately.

S said “SWIFT J0230 was discovered only about two months after Phil launched its program,” said Bradley Sinko, SWIFT’s principal investigator at NASA’s Goddard Space Flight Center. “It bodes well for the detector’s ability to identify other transient events and for SWIFT’s future exploration of new areas of science.”

Data analysis is a vital part of modern astronomy. It plays a major role in upcoming observatories such as the Vera Rubin Observatory. It could also give new life to old telescopes like the Swift Observatory. Swift was launched in 2004, and there is no way to update its hardware. But by taking advantage of advances in data analysis, astronomers are extracting new observations from its data. Without Evans’ new data technology, Swift would never have discovered Swift J0230.

“SWIFT’s hardware, software, and skills of its international team have enabled it to adapt to new areas of astrophysics over its lifetime,” said lead author Evans.

Systems like the Swift J0230 are difficult to detect. They have modest fluxes and no emissions outside the X-ray range. They require repeated observations to discover, unlike QPEs, which can be found in archival data. Real-time detectors, such as the Evans system, are essential because they can lead to rapid follow-up observations.

Hopefully the system will find more objects like Swift J0230.

“Finding this event within three months of enabling this real-time search suggests that it is reasonably common, and we can expect to discover more objects of this class using sensitive, wide-field X-ray instruments such as eRosita and, in the near future, Einstein Probe.