When astronomers first observed a jagged or ‘spaghettified’ star, after getting too close to a massive black hole in 2019, they determined that much of the star’s material was being thrown around outward in a strong wind from the optical light emitted by the explosion. . Now, astronomers at the University of California, Berkeley (UCB) have analyzed the polarization of that light to determine that the cloud was likely spherically symmetrical, adding further evidence for the presence of that powerful wind.
“This is the first time anyone has inferred the shape of the gas cloud around a tidal spaghettied star,” said co-author Alex Filippenko, UCB astronomer. The latest findings appeared in a recent article published in the Monthly Notices of the Royal Astronomical Society.
As we reported earlier, an object that passes beyond a black hole’s event horizon, including light, is engulfed and cannot escape, although black holes are also disorderly eaters. This means that part of an object’s material is actually ejected in a powerful jet. If this object is a star, the process of being shredded (or “spaghettified”) by the powerful gravitational forces of a black hole occurs outside the event horizon, and some of the original mass of the star is violently ejected outwards. This can form a spinning ring of matter (i.e. an accretion disk) around the black hole that emits powerful X-rays and visible light. Jets are a way for astronomers to indirectly infer the presence of a black hole.
In 2018, astronomers announced the first direct image of the aftermath 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 about 150 million light-years away. of the earth. A year later, astronomers recorded the final agony of a star being shredded by a supermassive black hole in such a “tidal disturbance event” (TDE), dubbed AT 2019qiz. The star split apart, about half of its mass feeding – or accreting – into a black hole a million times the mass of the Sun, and the other half was ejected outward.
These powerful bursts of light are often shrouded in a curtain of interstellar dust and debris, making it difficult for astronomers to study them in more detail. But AT 2019qiz was discovered shortly after the star was shredded last year, making it easier to study in detail, before that curtain of dust and debris had fully formed. Astronomers made follow-up observations across the entire electromagnetic spectrum over the next six months, using multiple telescopes around the world. These observations provided the first direct evidence that gas exiting during disturbance and accretion produces the powerful optical and radio emissions previously observed.
Astronomers knew that the emitted optical light had a slight 1% polarization based on observations from the 3-meter Shane Telescope at Lick Observatory near San Jose, California; the observatory includes a spectrograph to determine the polarization of optical light. The light would have polarized after scattering electrons into the gas cloud. Given how far away these TDEs tend to be, they usually appear as a simple bright spot, and polarization is one of the few properties hinting at the shape of objects.
According to co-author Kishore Patra, much of the light emitted by the accretion disk would have started out in the X-ray regime, but as it passed through the gas cloud, that light continued to lose energy through various scatterings, absorptions and re-emissions, eventually emerging in the optical regime. “The final scattering then determines the polarization state of the photon,” Patra said. “So by measuring the polarization we can infer the geometry of the surface where the final scattering occurs.”
Based on polarization measurements from October 2019 showing zero polarization, 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 larger than Earth’s orbit. However, measurements taken a month later revealed a 1% polarization of the light, suggesting that the cloud had thinned and taken on a slight asymmetry.
“This observation rules out a 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 of wind coming out of these events, and I think this polarization study definitely reinforces that evidence, in the sense 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 star’s inward-rotating matter doesn’t end up falling into the black hole – it gets blown out of the black hole.
DOI: Royal Astronomical Society Monthly Notices, 2022. 10.1093/mnras/stac1727 (About DOIs).