NASA catches a black hole gobbling up a star in an unusual way

The tidal disruption is marked by the absence of jets of gases.
Jijo Malayil
A disk of hot gas swirls around a black hole
A disk of hot gas swirls around a black hole

NASA/JPL-Caltech 

A wandering star has yet again come too close to a black hole whose gravitational force is tearing it apart. The star is located around 250 million light-years away, making it the fifth-closest star observed to have gone through a similar process. The event, termed AT2021ehb by Nasa, "took place in a galaxy with a central black hole about 10 million times the mass of our Sun," according to a release

Researchers at Nasa hope to identify more complex black hole-feeding behaviors by observing such events. A new study published in the Astrophysical Journal mentions their work using Nasa's NuSTAR (Nuclear Spectroscopic Telescope Array) to observe the rise in high-energy X-ray light around the black hole after the star was completely ruptured due to the gravitational force of the black hole. The activity is followed by the formation of a hot structure above the black hole called a corona. 

NuSTAR, which is sensitive to observing such wavelengths of light and the distance, enabled researchers to capture the process of the corona's formation and its evolution. 

"The work demonstrates how the destruction of a star by a black hole – a process formally known as a tidal disruption event – could be used to understand better what happens to material that’s captured by one of these behemoths before it’s fully devoured," said a release. 

The importance of AT2021ehb

AT2021ehb is of particular interest to researchers as its short duration of tidal disruption helps them to analyze how the gravitational force of a black hole influences the material around it, the pattern of lights formed, and its physical transformation. 

"Tidal disruption events are a sort of cosmic laboratory. They’re our window into the real-time feeding of a massive black hole lurking in the center of a galaxy," said study co-author Suvi Gezari, an astronomer at the Space Telescope Science Institute in Baltimore. 

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An unusual phenomenon 

In such tidal disruptions, jets of gas that flow in opposite directions are seen with the formation of a corona near the black hole. "Corona emits higher-energy X-rays than any other part of a black hole, but scientists don’t know where the plasma comes from or exactly how it gets so hot". However, with AT2021ehb, the corona formation was unexpected as there were no jets. 

"We’ve never seen a tidal disruption event with X-ray emission like this without a jet present, and that’s really spectacular because it means we can potentially disentangle what causes jets and what causes coronae,” said Yuhan Yao, lead author of the study and a graduate student at Caltech in Pasadena, California. 

The team is looking to analyze more tidal disruptions to better understand and confirm the events seen with AT2021ehb and other unusual activities.

Abstract

We present X-ray, UV, optical, and radio observations of the nearby (≈78 Mpc) tidal disruption event AT2021ehb/ZTF21aanxhjv during its first 430 days of evolution. AT2021ehb occurs in the nucleus of a galaxy hosting a≈107 M⊙ black hole (MBH inferred from host galaxy scaling relations). High-cadence Swift and Neutron Star Interior Composition Explorer (NICER) monitoring reveals a delayed X-ray brightening. The spectrum first undergoes a gradual soft → hard transition and then suddenly turns soft again within 3 days at δt≈272 days during which the X-ray flux drops by a factor of 10. In the joint NICER+NuSTAR observation (δt = 264 days, harder state), we observe a prominent nonthermal component up to 30 keV and an extremely broad emission line in the iron K band. The bolometric luminosity of AT2021ehb reaches a maximum of when the X-ray spectrum is the hardest. During the dramatic X-ray evolution, no radio emission is detected, the UV/optical luminosity stays relatively constant, and the optical spectra are featureless. We propose the following interpretations: (i) the soft → hard transition may be caused by the gradual formation of a magnetically dominated corona; (ii) hard X-ray photons escape from the system along solid angles with low scattering optical depth (∼a few) whereas the UV/optical emission is likely generated by reprocessing materials with much larger column density—the system is highly aspherical; and (iii) the abrupt X-ray flux drop may be triggered by the thermal–viscous instability in the inner accretion flow, leading to a much thinner disk.