Two hundred and fifteen million light-years from Earth, a black hole 1 million times more massive than itself, caused the tidal disruption of a sun-like star.
Fortunately, this was the first stellar death that was bright enough for astronomers at UC Berkeley to study its optical light. Specifically, this is the polarization of light to understand what happened after the star was torn apart.
Scientists found that most of the material blown away by the black hole was blown into a spherical cloud of gas, blocking most of the high-energy emissions produced as it consumed the star’s remains.
An earlier optical light observation from the blast, AT2019qiz, revealed a powerful wind launched much of the star’s matter outward.
In contrast, the data on polarization tells astronomers that the cloud was likely spherically symmetric when the event was most bright.
“This is the first time anyone has deduced the shape of the gas cloud around a tidally spaghettified star,” said Alex Filippenko, UC Berkeley professor of astronomy and a research team member.
Scientists could explain why they haven’t seen high-energy radiation, like X-rays, from many of the up to dozens of tidal disruption events that have been observed: The gas blown outward by powerful winds from the black hole obscures the X-rays produced when matter falls into the black hole from an accretion disk.
Symmetry in space
After disruption, stellar debris has been hypothesized to form an eccentric, asymmetric disk. However, an eccentric disk is expected to exhibit a high degree of polarization, meaning several percent of the total light will be polarized.
For this tidal disruption event, this was not observed.
“One of the craziest things a supermassive black hole can do is to shred a star by its enormous tidal forces,” said team member Wenbin Lu, UC Berkeley assistant professor of astronomy.
The tidal disruption events of stars at the centers of galaxies are among the few ways astronomers can detect and measure supermassive black holes. However, astronomers still do not understand the complex processes after a tidal disruption due to the high computational costs of numerically simulating such events.
Some thirty days after the October observation, the light from the black hole was slightly polarized, about 1%. The asymmetric structure of the gas around the black hole was revealed by the second set of observations on Nov. 6.
Both observations were made with the 3-meter Shane telescope at Lick Observatory near San Jose, California, which is fitted with a Kast spectrograph capable of measuring polarization over the entire spectrum of light.
When light scatters off electrons in a gas cloud, it becomes polarized – its electrical field is predominantly in one direction.
A monstrous cloud
According to calculations by UC Berkeley researchers, polarized light emanates from an elongated, spherical cloud that is one hundred times farther from the star than our planet is from the sun.
Astronomers measured polarization over a wide range of wavelengths of light in 2019 during spectropolarimetric observations of AT2019qiz, a tidal disruption event in the constellation Eridanus.
In October, all the polarized photons were balanced – indicating a spherically symmetric gas cloud. However, a small asymmetry can be seen in the November measurements due to the slight polarization.
As these tidal disruptions occur at such a distance, in distant galaxies, they appear as a point of light, and only their polarization gives information about their shape.
Researchers have noted that these disruptions are so far away that the geometry of the event and structure of the explosion cannot be studied.
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