Using new data analysis tool on Swift, scientists discover black hole snacking on a star

by Haygen Warren

Scientists, using NASA’s Neil Gehrels Swift Observatory, have discovered a black hole that is continuously snacking on a star that is similar to our Sun. Located in a galaxy around 500 million light-years away from Earth, the black hole’s constant nibbling on the star, a phenomenon known as a tidal disruption event, creates a bright burst of light. This burst of light subsequently lit up the galaxy, which allowed for its discovery.

The black hole and tidal disruption event — referred to as Swift J023017.0+283603, or Swift J0230 for short — is only one part of this story, though, as the discovery was made using a new groundbreaking data analysis method from Swift’s X-ray telescope (XRT) instrument. The technique will allow for a new era of science from Swift, which is a three-telescope space observatory designed to observe highly energetic cosmic phenomena at multiple wavelengths.

“Swift’s hardware, software, and the skills of its international team have enabled it to adapt to new areas of astrophysics over its lifetime. Neil Gehrels, the mission’s namesake, oversaw and encouraged many of those transitions. Now, with this new ability, it’s doing even more cool science,” said Swift team member and lead author Phil Evans of the University of Leicester.

So, what exactly are tidal disruption events?

As mentioned, tidal disruption events are cosmic phenomena that occur when a star orbits a bit too close to a black hole. When the star approaches the black hole, the immense gravitational forces from the black hole create extreme tides that tear the star apart into a long, thin stream of gas and other cosmic material. One edge of this stream of gas swings around into the black hole while the other end gets thrown out of the system as a whole.

Rendering of a tidal disruption event. Note one end of the long stream of gas swinging into the black hole with the other being thrown away from the black hole. (Credit: NASA/CXC/U. Michigan/J. Miller et al./M.Weiss)

When a tidal disruption event occurs, bright flares of multi-wavelength light are created when the stream of gas interacts with a disk of material that is already orbiting the black hole. Scientists will then look for these flares to study the system’s characteristics, size, and more. Furthermore, scientists use the characteristics of the star and black hole to determine the emission they’re seeing from the event, allowing them to create a wide range of different scenarios and behaviors that can be used to categorize tidal disruption events.

However, not all tidal disruption events result in the immediate destruction of the star. Sometimes, a star will orbit a black hole at a distance where the tidal forces aren’t strong enough to completely destroy the star but still pull gas and material from the star. The star will continue to orbit the black hole until the star loses too much gas and material and finally disintegrates. These events are referred to as partial tidal disruptions or repeating tidal disruptions, and Swift J0230 is one of these events.

Other examples of repeating tidal disruptions include an event wherein an outburst of energy occurred once every 114 days, which could have been caused by a star orbiting a black hole with 78 million times the mass of our Sun. Another repeating disruption event saw a burst of energy once every nine hours from a black hole with a mass equal to that of 400,000 Suns.

Swift’s XRT first observed Swift J0230 on June 22, 2022. The instrument noticed a bright flash of light from within a galaxy located 500 million light-years away in the Triangulum constellation. After its initial sighting of the flare, XRT continued observing the galaxy and noted nine additional flares once every few weeks.

Evan et al. believe Swift J0230 is a good candidate for a repeating tidal disruption event, wherein a star similar to our Sun is being repeatedly tugged on by a black hole with a mass nearly 200,000 times that of our Sun. The team estimates that the star loses around three Earth masses of gas and material every time it makes its closest approach to the black hole.

When Swift J0230 was first observed by the XRT, scientists weren’t immediately sold on the idea of a repeating tidal disruption event. To be sure, the teams checked Swift’s Ultraviolet/Optical Telescope instrument but found nothing, which meant that the event was only emitting X-rays.

“We searched and searched for the event brightening in the data collected by Swift’s Ultraviolet/Optical Telescope. But there wasn’t any sign of it. The galaxy’s variability was entirely in X-rays. That helped rule out some other potential causes,” said Swift team member Alice Breeveld of the University College London’s Mullard Space Science Laboratory.

While several other repeating tidal disruption events have been observed, the contents of Swift J0230 are allowing scientists to model and compare how different types of stars interact with differently-sized black holes in repeating tidal disruptions.

Swift J0230 occured within this galaxy, named 2MASX J02301709+2836050, as seen by the Pan-STARRS telescope. (Credit: Neils Bohr Institute/Daniele Malesani)

The discovery of Swift J0230 was only possible because of a new XRT data analysis program called the Swift X-ray Transient Detector, which was developed by Evans and provides scientists with an automated catalog of all XRT observations.

When XRT observes a portion of the sky, the data the instrument collected is immediately sent back to Earth. When the data reaches the ground, the program automatically compares it to previous XRT observations of the same portion of the sky. If the program finds that X-rays have changed, it immediately alerts scientists, allowing them to quickly coordinate additional observations of that region of the sky. This is what happened with Evans et al. during the discovery of Swift J0230, and proves that the program is a useful tool for scientists interested in studying highly energetic cosmic phenomena like tidal disruption events.

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

When Swift launched in 2004, it was originally designed to only study gamma-ray bursts, which are the most powerful and energetic explosions in the universe. However, in the years since its launch, scientists have noted Swift’s capability to observe a wide range of cosmic events and objects, and have begun using the spacecraft to observe comets and tidal disruption events.

Evans et al.’s results were recently published in the journal Nature Astronomy on Sept. 7. 

(Lead image: artist’s illustration of Swift with a gamma-ray burst in the background. Credit: Spectrum Astro)

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