A spectacularly radiant ray of light from over 700 million light years away proves to be a gift that continues to give.
Released from the heart of a galaxy, it was a star’s dying electromagnetic scream as it was torn apart and partially consumed by a black hole about 5 million times the mass of the sun – and a new analysis has shown that it underwent the rather appropriately named process with ‘spaghettification’ when it died.
Star death is usually violent affairs, but death by spaghetti identification is perhaps the most violent of them all. It happens when a star wanders a little too close to a black hole, and the tidal force in its gravitational field pulls and pulls the star so hard that it ends up being torn apart ̵
When the star is torn apart, some of the resulting waste spaghetti – in a long, thin thread of material a bit like a spaghetti noodle, which is wound into a thin filament that wraps around and feeds into the black hole.
We can detect this process because it releases a radiant light burst produced by gravity and frictional effects in the accession disk around the black hole. This light gradually fades as the material is emptied.
Just such an event was discovered in April 2019, picked up by the Zwicky Transient Facility, which monitors the sky looking for transient events. The event in question, later named AT2019dsg, was unusually bright, even for a tidal disturbance event that blazed over X-ray, ultraviolet, optical, and radio wavelengths.
The inner edge of the accession disk closest to the black hole is the hottest part of the disk and therefore emits the most energetic radiation, X-rays. That we can detect X-rays at all means that we stare down the pole in the supermassive black hole – otherwise it would be obscured by the outer areas of the accession disk.
Think about looking at a donut side by side. You can not see the hole, can you?
Of course, we have seen X-rays in tidal disturbances, but there was something very different about AT2019dsg. An international team of astronomers led by Giacomo Cannizzaro and Peter Jonker of the Netherlands Institute for Space Research studied electromagnetic output and discovered absorption lines.
Absorption lines are good by the way. When astronomers take pictures of light if the light has traveled through something that slows down or blocks some wavelengths, such as gas or dust, it will appear as dark lines on the spectrum.
They appear in all sorts of places, but the poles of supermassive black holes are usually not among them. Accretion disks usually revolve around the equatorial region – you know like Saturn’s rings – and right down on the rod there is usually quite clear.
The variety and breadth of these absorption lines was also strange. The observed configuration seemed to suggest several strands of material, like a ball of string.
This again suggested that something was wrapped around the black hole at an unusual angle, such as the spaghetti filaments from a temporarily disturbed star, perhaps thrown away from the bulk of the star debris.
“When a star is disturbed, the debris can form highly self-engraving currents,” the researchers wrote in their paper.
“What we could see are absorption lines caused by such currents, where the different orbital motions and projected velocities of these different currents cause the variation in the width of the lines.
“To have this, we would need some of the self-engraving currents to be deflected by large angles while the bulk of the disturbed material circulates to an access disk.”
If the team’s interpretation of spaghettification proves to be correct, it would be the first time we actually had direct evidence that this process took place.
AT2019dsg has come to our attention before, when scientists discovered what appeared to be a high-energy neutrino spit out of the event. One has to wonder what other surprises this unusually bright star death has in store.
The research has been published in Monthly announcements from the Royal Astronomical Society.