The final fate of a star destroyed by a black hole

In 2019, astronomers observed the closest example to date of a star being shredded or “spaghetti” after getting too close to a massive black hole.

That tidal disruption of a Sun-like star by a black hole 1 million times more massive than itself took place 215 million light-years from Earth. Fortunately, this was the first such event bright enough for astronomers at the University of California, Berkeley to study the optical light of stellar death, specifically the polarization of light, to learn more about what happened. after the star was torn apart.

His observations on October 8, 2019 suggest that much of the star’s material was ejected at high speeds (up to 10,000 kilometers per second) and formed a spherical cloud of gas that blocked most of the high-energy emissions produced as star. black hole engulfed the rest of the star.

Previously, other observations of the optical light from the explosion, called AT2019qiz, revealed that much of the star’s matter was blown outward in a powerful wind. But new data about the polarization of the light, which was essentially zero at visible or optical wavelengths when the event was at its brightest, tells astronomers that the cloud was likely spherically symmetric.

“This is the first time anyone has deduced the shape of the gas cloud around a tidally spaghettined star,” said Alex Filippenko, a UC Berkeley professor of astronomy and a member of the research team.

The results support an answer to why astronomers don’t see high-energy radiation, like X-rays, from many of the dozens of tidal disruption events observed to date: X-rays, which are produced by material ripped from the star. and drawn into an accretion disk around the black hole before falling inward, hidden from view by gas blown out by the black hole’s powerful winds.

“This observation rules out a class of solutions that have been proposed theoretically and gives us a stronger constraint on what ha

ppens to the gas around a black hole,” said UC Berkeley graduate student Kishore Patra, lead author of the study. “People have been seeing other evidence of wind coming from these events, and I think this polarization study definitely strengthens that evidence, in that you wouldn’t get a spherical geometry without having a sufficient amount of wind. The interesting fact here is that a significant fraction of the material in the star that is spiraling inward does not eventually fall into the black hole, it is ejected from the black hole.”

Polarization reveals symmetry

Many theorists have hypothesized that stellar debris forms an eccentric lopsided disk after breakup, but an eccentric disk is expected to show a relatively high degree of polarization, which would mean that perhaps a significant percentage of the total light is polarized This was not observed for this tidal disruption event.

“One of the craziest things a supermassive black hole can do is rip a star apart with its huge tidal forces,” said team member Wenbin Lu, an assistant professor of astronomy at UC Berkeley. “These stellar tidal disruption events are one of the few ways that astronomers learn about the existence of supermassive black holes at the centers of galaxies and measure their properties.

However, due to the extreme computational cost in numerically simulating such events, astronomers still do not understand the complicated processes after a tidal disruption.”

If a star (red trail) gets too close to a black hole (left), it can be shredded or spaghetti-shaped by intense gravity. Some of the star’s matter swirls around the black hole, like water down a drain, emitting a large amount of X-rays (blue). Recent studies of these so-called tidal disruption events suggest that a significant fraction of the star’s gas is also blown away by the black hole’s intense winds, in some cases creating a cloud that obscures the accretion disk and high-energy events. that happen inside. . Credit: NASA/CXC/M. Weiss

A second set of observations on November 6, 29 days after the October observation, revealed that the light was slightly polarized, about 1%, suggesting that the cloud had thinned enough to reveal the gas structure. asymmetric around the black hole. Both observations came from the 3-meter Shane telescope at the Lick Observatory near San Jose, California, which is equipped with the Kast spectrograph, an instrument that can determine the polarization of light across the entire optical spectrum. Light becomes polarized (its electric field vibrates mainly in one direction) when it scatters electrons in the gas cloud.

“The accretion disk itself is hot enough to emit most of its light in X-rays, but that light has to pass through this cloud, and there’s a lot of scattering, absorption, and re-emission of light before it can.” escape from this cloud. Patra said. “With each of these processes, the light loses some of its photon energy, down to ultraviolet and optical energies. The final scattering determines the polarization state of the photon. So by measuring the polarization, we can deduce the geometry of the surface where the final scattering occurs.”

Patra noted that this deathbed scenario can apply only to normal tidal disruptions, not “weird balls,” in which relativistic jets of material are ejected by the black hole’s poles. Only more measurements of the polarization of light from these events will answer that question.

“Polarization studies are very challenging, and very few people around the world are versed enough in the art to use this,” he said. “So this is uncharted territory for tidal disruption events.”

Patra, Filippenko, Lu, and UC Berkeley researcher Thomas Brink, graduate student Sergiy Vasylyev, and postdoctoral fellow Yi Yang reported their observations in a paper that has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

A cloud 100 times larger than the Earth’s orbit

The UC Berkeley researchers calculated that the polarized light was emitted from the surface of a spherical cloud with a radius of about 100 astronomical units (au), 100 times farther from the star than Earth is from the sun. An optical glow of hot gas emanated from a region at around 30 au.

The 2019 spectropolarimetric observations, a technique that measures polarization across many wavelengths of light, were of AT2019qiz, a tidal disruption event located in a spiral galaxy in the constellation Eridanus. Zero polarization of the entire spectrum in October indicates a spherically symmetric gas cloud: all polarized photons balance each other. The slight polarization of the November measurements indicates a small asymmetry. Because these tidal disruptions occur so far away, at the centers of distant galaxies, they appear only as a point of light, and polarization is one of the few indications of the objects’ shapes.

“These disruption events are so far away that they can’t really be resolved, so you can’t study the geometry of the event or the structure of these bursts,” Filippenko said. “But studying polarized light actually helps us deduce some information about the distribution of matter in that explosion or, in this case, how the gas, and possibly the accretion disk, forms around this black hole.”