A Black Hole has Destroyed a Star, and Used the Wreckage to Pummel Another Star

When a supermassive black hole consumes a star, it doesn’t just swallow it whole. It shreds the star, ripping it apart bit by bit before consuming the remains. It’s a messy process known as a tidal disruption event (TDE). Astronomers occasionally catch a glimpse of TDEs, and one recent one has helped solve a mystery about a type of transient X-ray source.

Known as quasi-periodic eruptions (QPEs), they are soft X-rays that emanate from the centers of galaxies every few hours or a few weeks. QPEs are rare, so they are difficult to study, and we aren’t sure what causes them. One idea is that they are caused by a large star or stellar black hole orbiting the supermassive black hole in such a way that its orbit intersects with the accretion disk of the supermassive black hole. Each time the smaller object passes through the disk, it triggers superheated plasma to release X-rays. We’ve seen a similar effect with blazars, for example.

Given the short periodicity of QPEs the companion object would need to orbit the black hole very closely, just on the edge of a stable orbit distance. And when it starts intersecting with accretion disk material, its orbit will decay on a short cosmic timescale. This would explain why QPEs are so rare. But to prove this model, astronomers would need to observe this happening in real time, which is what a team of astronomers has recently done. The results will be published in Nature later this month.

AT2019qiz seen in X-ray and optical light. Credit: X-ray: NASA/CXC/Queen’s Univ. Belfast/M. Nicholl et al.; Optical/IR: PanSTARRS, NSF/Legacy Survey/SDSS

The story begins with an observation by the Zwicky Transient Facility back in 2019. The ZTF captured an optical flare that had all the markings of a tidal disruption event. It came to be known as TDE AT2019qiz. According to black hole models, when a star is ripped apart, much of the material forms an accretion disk around the black hole within a few years. This would make for perfect QPE conditions if there was a close companion object. So the team aimed the Chandra X-ray Observatory at AT2019qiz occasionally, hoping to capture a quasi-periodic eruption. Sure enough, in 2023, the team started to observe X-ray flashes erupting about every 48 hours. Observations from the Swift and AstroSAT telescopes further confirmed the result.

It isn’t known whether the companion is a star or small black hole, and the team would like to capture more QPEs occurring after known tidal disruption events, but this initial result is pretty clear.

Reference: Nicholl, M., et al. “Quasi-periodic X-ray eruptions years after a nearby tidal disruption event.” arXiv preprint arXiv:2409.02181 (2024).

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