The ultimate fate of a star torn apart by a black hole

If a star (red trail) strays too close to a black hole (left), it can be shredded or shredded by the intense gravity. Some of the star’s matter swirls around the black hole, like water in a drain, emitting copious X-rays (blue). Recent studies of these so-called tidal disturbances suggest that a significant portion of the star’s gas is also blown out by intense winds from the black hole, in some cases creating a cloud that obscures the accretion disk and the high-energy events within. † Credit: NASA/CXC/M. Weiss

In 2019, astronomers observed the closest example yet of a star being shredded or “spaghettified” 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 occurred 215 million light-years from Earth. Fortunately, this was the first event so bright that astronomers at the University of California, Berkeley, were able to study the optical light from the star’s death, specifically the polarization of the light, to learn more about what happened after the star was torn apart.

Their October 8, 2019 observations suggest that much of the star’s material was blown away at high speed — up to 10,000 kilometers per second — forming a spherical cloud of gas that blocked most of the high-energy emissions produced as the black hole gobbled up the rest of the star. star up.

Previously, other observations of optical light from the blast, dubbed AT2019qiz, revealed that much of the star’s matter was launched outward in a strong wind. But the new data on the polarization of the light, which was essentially zero at visible or optical wavelengths when the event was brightest, tells astronomers that the cloud was likely spherical symmetric.

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

The results support one answer as to why astronomers don’t see high-energy radiation, such as X-rays, from many of the dozens of tidal disturbances observed so far: the X-rays, which are produced by material ripped from the star and into an accretion disk around dragged into the black hole before falling in are hidden from view by the gas blown out by powerful winds from the black hole.

“This observation excludes a class of solutions that have been theoretically proposed and gives us a stronger constraint on what happens to gas around a black hole,” said UC Berkeley graduate student Kishore Patra, lead author of the study. “People have seen other evidence of wind coming from these events, and I think this polarization study certainly strengthens that evidence in the sense that you wouldn’t get spherical geometry without enough wind. The interesting fact here is that a significant portion of the material in the star that spirals inward doesn’t end up falling into the black hole — it gets blown away from the black hole.”

Polarization reveals symmetry

Many theorists have hypothesized that the stellar debris forms an eccentric, asymmetric disk after perturbation, but an eccentric disk is expected to exhibit a relatively high degree of polarization, which would mean that perhaps a few percent of the total light is polarized. This was not observed for this tidal disturbance.

“One of the craziest things a supermassive black hole can do is tear a star apart by its massive tidal forces,” said team member Wenbin Lu, assistant professor of astronomy at UC Berkeley. “These stellar tidal disturbances are one of the few ways astronomers know about the existence of supermassive black holes at the centers of galaxies and measure their properties. However, due to the extreme computational costs involved in simulating such events numerically, astronomers still do not understand the complicated processes after a tidal disturbance.”

A second set of observations on Nov. 6, 29 days after the October observation, revealed that the light was very slightly polarized, about 1%, suggesting that the cloud had thinned enough to reveal the asymmetric gas structure surrounding 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 full optical spectrum. The light becomes polarized — the 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 get through this cloud, and there are a lot of scatterings, absorptions and re-emissions of light before it can escape this cloud,” Patra said. “In each of these processes, the light loses some of its photon energy, all the way down to ultraviolet and optical energies. The final scattering then determines the polarization state of the photon. So by measuring the polarization, we can infer the geometry of the surface where the final scattering takes place.”

Patra noted that this deathbed scenario may only apply to normal tidal disturbances — not “strange balls,” where relativistic beams of material are expelled from 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 are well versed in the technique around the world to use this,” he said. “So this is uncharted territory for tidal disturbances.”

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

A cloud 100 times larger than 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 came from an area of ​​about 30 au.

The 2019 spectropolarimetric observations — a technique that measures polarization across many wavelengths of light — were from AT2019qiz, a tidal disturbance in a spiral galaxy in the constellation Eridanus. The zero polarization of the entire spectrum in October indicates a spherically symmetrical gas cloud – all polarized photons balance each other. The slight polarization of the November measurements indicates a slight asymmetry. Because these tidal disturbances occur so far away, in the centers of distant galaxies, they seem like only a bright spot, and polarization is one of the few clues to the shape of objects.

“These perturbations are so far away that you can’t really resolve them, so you can’t study the geometry of the event or the structure of these explosions,” 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 — formed around this black hole.”

Death by spaghettification: Scientists record last moments of star devoured by black hole

More information:
Kishore C Patra et al, Spectropolarimetry of the Tidal Disturbance AT 2019qiz: A Quasispheric Reprocessing Layer, Monthly Notices from the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac1727

Provided by University of California – Berkeley

Quote: The ultimate fate of a star torn apart by a black hole (2022, July 11) retrieved July 11, 2022 from

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