When a supermassive black hole devours a star, it doesn't simply swallow it whole. It tears apart the star, tearing it apart piece by piece before consuming the remains. It is a chaotic process called a Tidal Disruption Event (TDE). Occasionally, astronomers catch a glimpse of TDEs, and a recent event has helped solve the mystery of a type of transient X-ray source.
So-called quasi-periodic flares (QPEs) are soft X-rays that emanate from the centers of galaxies every few hours or weeks. QPEs are rare, making them difficult to study, and we are not sure what causes them. One theory is that they are caused by a large star or stellar black hole orbiting the supermassive black hole such that its orbit intersects the supermassive black hole's accretion disk. Each time the smaller object passes the disk, it triggers superheated plasma that releases X-rays. We have observed a similar effect in blazars, for example.
Given the short periodicity of the QPEs, the companion object would have to orbit the black hole very closely, just on the edge of a stable orbital distance. And as it begins to intersect with accretion disk material, its orbit will decay on short cosmic timescales. This would explain why QPEs are so rare. But to prove this model, astronomers would have to observe this happening in real time, which is what a team of astronomers recently did. The results will be published in Nature later this month.
AT2019qiz seen in X-ray and optical light. Photo 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 from the Zwicky Transient Facility in 2019. The ZTF detected an optical flare that showed all the signs of a tidal disruption. It became known as TDE AT2019qiz. According to black hole models, when a star is torn apart, much of the matter forms an accretion disk around the black hole within a few years. This would give perfect QPE conditions if there was a nearby companion object. So the team occasionally pointed the Chandra X-ray observatory at AT2019qiz, hoping to capture a quasi-periodic burst. In fact, the team began observing X-ray flashes approximately every 48 hours in 2023. Observations from the Swift and AstroSAT telescopes further confirmed the result.
It's not known whether the companion is a star or a small black hole, and the team would like to capture more QPEs that occur after known tidal disruption events, but this initial result is pretty clear.
Reference: Nicholl, M., et al. “Quasiperiodic X-ray bursts years after a nearby tidal event.” arXiv preprint arXiv:2409.02181 (2024).
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