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Polarized light reveals the ultimate fate of a star spaghettided by a black hole

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Increase / If a star (red trail) gets too close to the black hole (left), it can be torn apart or spaghetized by the strong gravity. Some of the star’s material swirls around the black hole like water in a sewer, emitting abundant X-rays (blue).

NASA/CXC/M. Weiss

When astronomers first observed a star in 2019 that was torn apart or “spaghetti” after approaching a massive black hole too close, they determined that much of the star’s material had been blasted outward by a powerful wind due to the optical light emitted by the explosion. . Now, astronomers at the University of California, Berkeley (UCB) have analyzed the polarization of this light to determine that the cloud was likely spherically symmetrical, adding further evidence for the presence of this powerful wind.

“This is the first time anyone has determined the shape of a gas cloud around a star that has undergone tidal spaghettization,” said co-author Alex Filippenko, a UCB astronomer. The latest results appeared in a recent article published in the Monthly Notices of the Royal Astronomical Society.

As we have previously reported, an object that passes beyond the event horizon of a black hole, including light, is absorbed and cannot escape, although black holes are also dirty eaters. This means that part of the object’s matter is actually ejected by a powerful jet. If this object is a star, the process of being shredded (or “spaghettized”) by the black hole’s powerful gravitational forces occurs outside the event horizon, and some of the star’s original mass is forcefully thrown out. This can form a rotating ring of matter (also known as an accretion disk) around the black hole, which emits powerful X-rays and visible light. Jets are one way astronomers can indirectly infer the presence of a black hole.

In 2018, astronomers announced the first direct image of the aftermath of a star exploding from a black hole 20 million times more massive than our Sun in a pair of colliding galaxies called Arp 299 about 150 million light-years from Earth. A year later, astronomers recorded the final agony of a star torn apart by a supermassive black hole in just such a “tidal disruption” (TDE), dubbed AT 2019qiz. The star was torn apart, with about half of its mass falling into or accreting into a black hole 1 million times the mass of the Sun, while the other half was thrown out.

These powerful flashes of light are often shrouded in a veil of interstellar dust and debris, making them difficult for astronomers to study in detail. But AT 2019qiz was detected shortly after the star was destroyed last year, making it easier to study in detail before the dust and debris curtain was fully formed. The astronomers made follow-up observations in the electromagnetic spectrum over the next six months using several telescopes around the world. These observations provided the first direct evidence that the outflowing gas during destruction and accretion produces the powerful optical and radio emission observed previously.

An artist's conception of a star being tidally torn apart by the powerful gravity of a supermassive black hole.
Increase / An artist’s conception of a star being tidally torn apart by the powerful gravity of a supermassive black hole.

Astronomers knew that the emitted optical light had a slight polarization of 1 percent based on observations from the 3-meter Shane telescope at the Lick Observatory near San Jose, California; the observatory has a spectrograph to determine the polarization of optical light. The light must have become polarized after the scattering of electrons in the gas cloud. Given how far away such TDEs are, they usually look like points of light, and polarization is one of the few properties that hints at the shape of objects.

According to co-author Kishore Patra, most of the light emitted by the accretion disk should have started in the X-ray mode, but as it passed through the gas cloud, this light continued to lose energy through various scatterings, absorptions and re-emissions, eventually occurring in the optical mode. “The final scattering determines the polarization state of the photon,” Patra said. “So by measuring the polarization, we can determine the geometry of the surface where the final scattering occurs.”

Based on October 2019 polarization measurements showing zero polarization, the Berkeley scientists calculated that the light came from a spherical cloud with a surface radius of about 100 astronomical units (AU), or about 100 times the Earth’s orbit. However, measurements taken a month later showed a 1 percent polarization of the light, suggesting that the cloud had become thinner and slightly asymmetric.

“This observation rules out the class of solutions that have been proposed theoretically and gives us a stronger constraint on what happens to the gas around a black hole,” Patra said. “People have seen other evidence that these events cause wind, and I think this polarization study definitely makes that evidence more convincing in the sense that you can’t get a spherical geometry without enough wind. The interesting fact here is that much of the matter in a star that spirals inwards doesn’t end up in the black hole—it gets blown out of the black hole.”

DOI: Monthly Notices of the Royal Astronomical Society 2022 10.1093/mnras/stac1727 (About DOI).

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