Fate of a star destroyed by a black hole

(ORDO NEWS) — In 2019, astronomers observed the closest example to date of a star that was shredded after getting too close to a massive black hole.

This tidal destruction of a sun-like star by a black hole 1 million times more massive than itself occurred 215 million light-years from Earth.

Thankfully, this was the first such event bright enough for UC Berkeley astronomers to study the optical light from the death of a star, in particular the polarization of the light, to learn more about what happened after the star was torn apart.

Their observations, taken on October 8, 2019, show that much of the star’s material was being blown away at high speeds – up to 10,000 kilometers per second – and formed a spherical cloud of gas that blocked much of the high-energy emissions produced by the black hole that was devouring the remnants of the star.

Earlier, other observations of optical light from the explosion, named AT2019qiz, showed that much of the star’s matter was thrown outward by a powerful wind.

But new data on the polarization of light, which was virtually zero at visible or optical wavelengths, at the brightest moment of the explosion, tell astronomers that the cloud was most likely spherically symmetrical.

The results support one answer to the question of why astronomers don’t see high-energy radiation, such as X-rays, from many of the dozens of tidal disruption events observed so far: X-rays, which are produced by material torn from a star and sucked into an accretion disk around a black hole, before falling inward, it is obscured from observation by gas blown outward by powerful winds from the black hole.

The second series of observations on November 6, 29 days after the October observation, showed that the light is very weakly polarized, by about 1%, which indicates that the cloud is thin enough to reveal the asymmetric structure of the gas around the black hole.

UC Berkeley graduate student Kishore Patra noted that this doom scenario could only apply to ordinary tidal disruptions, not strange ones in which relativistic jets of material are ejected from the black hole’s poles. Only additional measurements of the polarization of light from such events will answer this question.

UC Berkeley researchers calculated that 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 the Earth is from the Sun. The optical glow from the hot gas came from a region at a distance of about 30 au.

The 2019 spectropolarimetric observations – a technique for measuring polarization across many wavelengths of light – were carried out on AT2019qiz, a tidal disruption event located in a spiral galaxy in the constellation Eridani.

Zero polarization of the entire spectrum in October indicates a spherically symmetric cloud of gas – all polarized photons balance each other. The slight polarization in the November measurements indicates a slight asymmetry.

Because these tidal disruptions occur so far, at the centers of distant galaxies, they only look like a point of light, and polarization is one of the few clues to the shape of objects.

The study of polarized light actually helps to get information about the distribution of matter in this explosion or, in this case, how the gas and possibly the accretion disk around this black hole is formed.

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