Death to black hole: Astronomers recognize torches due to the star’s...

It is a common misconception that black holes act like cosmic vacuum cleaners, sucking up any matter around them. In reality, only material that goes beyond the event horizon – including light – is swallowed and cannot escape, although black holes are messy eaters too. This means that some of the matter of an object is actually ejected in a powerful jet.

If this object is a star, the process of crushing (or “spaghetting”) by the strong gravitational forces of a black hole takes place outside the event horizon, and part of the star’s original mass is violently ejected outward. This, in turn, can form a rotating ring of matter (also called an accretion disk) around the black hole that emits powerful X-rays and visible light. These jets are one way that astronomers can indirectly infer the presence of a black hole. Now astronomers have recorded the final death throes of a star that was torn apart by a supermassive black hole in such a “tidal disruption event” (TDE). This is described in a new article published in the Royal Astronomical Society’s Monthly Notices.

“The idea that a black hole” sucks in a nearby star “sounds like science fiction. But that’s exactly what happens in a tidal disturbance event, ”said co-author Matt Nicholl of the University of Birmingham. “We were able to examine in detail what happens when a star is eaten by such a monster.”

“A tidal disruption event results from the destruction of a star too close to a supermassive black hole,” said Edo Berger of Harvard University’s Center for Astrophysics, another co-author. “In this case, the star was torn apart, with about half of its mass injected or accumulated in a black hole a million times the mass of the Sun, and the other half ejected outward.”

Death by tidal forces

The idea of ​​being “spaghettified” after falling into a black hole was popularized in Stephen Hawking’s 1988 bestseller. A brief history of time. Hawking imagined an unfortunate astronaut who stepped beyond the event horizon and was exposed to the intense gravitational gradient of the black hole. (The gravitational gradient is the difference in the strength of gravity depending on the orientation of an object.)

For example, if the astronaut fell in the feet first, the pull on the feet would be stronger than on the head. The astronaut was stretched vertically and squeezed horizontally by the tidal forces of the black hole until they resembled a strand of spaghetti. From a physical point of view, it’s the same reason the earth experiences tides: the gravitational pull of the moon pulls the oceans in one direction and smooths them in the other. At least it would be quick; The entire process would take less than a second.

All of this is purely hypothetical and the subject of various thought experiments. On the scale of stars and galaxies, however, some kind of spaghettification is a real phenomenon, but it occurs outside of the black hole’s event horizon rather than inside. These tidal disturbance events are likely to be quite common in our universe, although few have been discovered to date.

For example, in 2018 astronomers announced the first direct image of the consequences of a star being shredded by a black hole 20 million times more massive than our Sun in a pair of colliding galaxies called Arp 299, some 150 million light years from Earth. They used a combination of radio and infrared telescopes, including the Very Long Baseline Array (VLBA), to track a specific formation and expansion of the beam of matter ejected after a star emanated from a supermassive black hole in the center of the One of the colliding galaxies was torn apart.

However, these powerful flashes of light are often hidden behind a curtain of interstellar dust and debris, making them difficult for astronomers to study in more detail. This most recent event (called AT 2019qiz) was discovered shortly after the star was crushed last year, making it easier to study in detail before that curtain of dust and debris was fully formed. Astronomers conducted follow-up observations over the electromagnetic spectrum with several telescopes around the world, including the VLT (Very Large Telescope) array and the NTT (New Technology Telescope), both in Chile, over the next six months.

“Because we recognized it early on, we were actually able to see the curtain of dust and dirt tightening when the black hole triggered a strong flow of material at speeds of up to 10,000 km / s,” said co-author Kate Alexander of der Northwestern University. “This is a unique behind-the-scenes look that provided the first opportunity to determine the origin of the opaque material and watch it in real time as it engulfs the black hole.”

According to Berger, these observations provide the first direct evidence that escaping gas produces the strong optical and radio emissions previously observed during disruption and accretion. “Up until now the nature of these emissions has been hotly debated, but here we see that the two regimes are linked by a single process,” he said.

DOI: Monthly Announcements from the Royal Astronomical Society, 2020. 10.1093 / mnras / staa2824 (Via DOIs).

Listing picture by ESO / M. Kornmesser