NASA telescopes have observed a massive black hole unusually close to Earth snacking on an ‘unlucky’ star. The black hole, which is about 10 million times the mass of the Sun, is located 250 million light-years from Earth in the centre of another galaxy. This event was the fifth-closest example of a black hole destroying a star ever observed.
The event is called AT2021ehb, according to NASA. The ratio of the sizes of the black hole and the Sun is almost equal to that between the Titanic and a bowling ball.
A corona was formed as the star reached its doom
After the star was thoroughly ruptured by the black hole’s gravity, astronomers observed a dramatic increase in high-energy X-ray light around the black hole, indicating that an extremely hot structure was formed above the black hole, as the stellar material was pulled towards its doom. This hot structure is called a corona.
NASA’s NuSTAR and its role in observing the event
The study describing the findings was recently published in the Astrophysical Journal. According to the study, NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array) satellite is the most sensitive space telescope capable of observing these wavelengths of light, and the proximity of the event provided an unprecedented view of the corona’s formation and evolution.
All about a tidal disruption event
The destruction of a star by a black hole is known as a tidal disruption event. The study describes how a tidal disruption event could be used to better understand what happens to material that is captured by such a massive celestial body before it is fully devoured.
Hot gas that has accumulated over many years, sometimes millennia, surrounds most black holes. The gas forms disks billions of kilometres wide around the behemoths. Sometimes, these disks shine brighter than entire galaxies.
However, a single star being torn apart and consumed stands out even around these bright sources, especially around much less active black holes.
The process often takes only a few weeks or months, from start to finish. Tidal disruption events are of interest to astronomers due to their observability and short duration. Astronomers often focus on finding out how the black hole’s gravity manipulates the material around it, and creates incredible light shows and new physical features.
Tidal disruption events are a “cosmic laboratory”
In a statement released by NASA, Suvi Gezari, an astronomer at the Space Telescope Science Institute (STScI) in Baltimore, and one of the co-authors on the paper, said tidal disruption events are a sort of “cosmic laboratory”, and astronomers’ “window” into the real-time feeding of a massive black hole lurking in the centre of a galaxy.
Star was stretched like a “long noodle” of hot gas
The side of the star nearest to the black hole was pulled harder than the far side of the star, during the tidal disruption event. This stretched the entire thing apart and left nothing but a “long noodle” of hot gas.
During such events, the stream of gas gets whipped around a black hole, colliding with itself. This results in shock waves and outward flows of gas that generate visible light, along with wavelengths that are not visible to the human eye, such as ultraviolet light and X-rays.
Following the generation of outward flows of gas, the material starts to settle into a disk rotating around the black hole, similar to water circling a drain, with friction resulting in low-energy X-rays. The series of events for AT2021ehb took place over just 100 days.
How the event was observed
According to NASA, the event was first spotted on March 1, 2021, by the Zwicky Transient Facility (ZTF) located at the Palomar Observatory in Southern California, and was subsequently studied by NASA’s Neil Gehrels Swift Observatory and Neutron star Interior Composition Explorer (NICER) telescope, which observers longer X-ray wavelengths than Swift.
NASA’s NuSTAR started observing the system around 300 days after the event was first spotted.
What was surprising about the event?
When NuSTAR detected a corona, which is a cloud of hot plasma or gas atoms with their electrons stripped away, scientists were surprised. This is because coronae usually appear with jets of gas that flow in opposite directions from a black hole.
However, in the case of the tidal disruption event analysed in the new study, there were no jets. This made the corona observation unexpected.
Scientists do not exactly know where the plasma of coronae comes from, or how it gets so hot.
Yuhan Yao, the lead author on the paper, said astronomers have never seen a tidal disruption event with X-ray emission like this without a jet present, and this is really spectacular because it means scientists can potentially disentangle what causes jets and what causes coronae. She added that the observations of the new event are in agreement with the idea that magnetic fields have something to do with how the corona forms, and the astronomers want to know what is causing the magnetic field to get so strong.