In late 2019, light reached the Earth from the destruction of a star, the light journeying for 215 million years from a spiral galaxy in Eridanus. The star's end came about not from an internal explosion, but from a close encounter with a supermassive black hole. It's the first time astronomers have been able to witness an event like this so clearly
Anything that gets too deep in a black hole's energy well becomes spaghettified, stretched out into long thin strips by the intense gravitational force closer to the hole. More prosaically known as tidal disruption, we've seen signs of this happening to stars several times, but our capacity to observe them has been limited by distance and intervening material.
Astronomers have been puzzled by the lack of X-Ray and higher energy radiation from the tidal disruption events we have seen. A paper in Monthly Notices of the Royal Astronomical Society on the 2019 event offers a possible explanation, while eliminating others.
As Dr Wenbin Lu of the University of California, Berkeley commented in a statement; “One of the craziest things a supermassive black hole can do is shred a star by its enormous tidal forces. These stellar tidal disruption events are one of very few ways astronomers know the existence of supermassive black holes at the center of galaxies and measure their properties.”
The distances at which they have occurred, however, has left us with only simplified understanding of what is going on.
The story began with the observation of a surge in optical light, named AT2019qiz, coming from the area around a million solar mass black hole feeding on a star of approximately the Sun's mass, and peaking on October 8.
Astronomers concluded that while most of the star was getting spaghettified, a powerful wind spread some of its material into a cloud. The new paper reports negligible levels of polarization in light at the event's brightest point. From this, the authors inferred the cloud is close to spherically symmetric – the first time we've seen a tidal disruption event closely enough to draw conclusions about the shape. They estimate its radius as 100 times that of the Earth's orbit.
By November 6, 2019, a small amount of polarization was detected, indicating the cloud had thinned sufficiently so we were seeing light from the asymmetric structure closer to the black hole itself.
The near-spherical shape means there is plenty of gas blocking our view of the black hole, and therefore sapping the energy of the X-rays that should be being produced from black hole's accretion disk. By the time they are repeatedly scattered, the X-Rays have stretched to ultraviolet or even optical wavelengths.
"This observation rules out a class of solutions that have been proposed theoretically and gives us a stronger constraint on what happens to gas around a black hole," said Berkeley graduate student Kishore Patra in a statement.
In particular, the findings support previous suggestions spaghettification events are associated with powerful winds. “The interesting fact here is that a significant fraction of the material in the star that is spiraling inward doesn't eventually fall into the black hole – it's blown away from the black hole." Patra added.
However, whether AT2019qiz is typical of tidal disruptions remains to be seen.
The observations were only possible because the Kast spectropolarimeter at the Lick Observatory can measure the polarization proportion of light as faint as 17th magnitude, which AT2019qiz exceeded at its brightest.