A cosmic crime scene has revealed the gory details of black holes' eating
habits.
In a new study, astronomers studied a supermassive black hole's destruction
of a star, revealing how these cosmic titans consume the material from
objects that venture too close to them. The research shows that a
significant amount of this material is blown away from, rather than being
consumed by, the black hole.
The grisly act, which happened 215 million light-years from Earth, was first
observed in October 2019 and represents the aftermath of a sunlike star
being destroyed by a black hole over 1 million times its mass. This was the
nearest example of a stellar body being "spaghettified" by the massive tidal
forces generated by a black hole that astronomers have ever spotted.
This so-called tidal disruption event (TDE) occurred in a spiral galaxy in
the constellation Eridanus. It was the first event of this type to be bright
enough in visible light to allow astronomers to study the details of what
happens to material from the star after it is shredded.
By observing the polarization of light from the event, researchers at the
University of California, Berkeley, deduced that much of this stellar
material was blasted away from the black hole at speeds as great as 22
million mph (35 million kph).
This blast , named AT2019qiz, created a cloud of gas that these new
observations of polarized light reveal is spherically symmetrical. This gas
cloud is as wide as 200 times the average distance from Earth to the sun.
This means its radius is 100 times larger than our planet's orbit, and its
outer edge is about 930 million miles (1.5 billion kilometers) from the
central black hole.
"One of the craziest things a supermassive black hole can do is to shred a
star by its enormous tidal forces," Wenbin Lu, an astronomer at UC Berkeley
and co-author of a new paper describing the observations, said in a statement. "These stellar tidal disruption events are one
of the very few ways astronomers know the existence of supermassive black
holes at the centers of galaxies and measure their properties. However, due
to the extreme computational cost in numerically simulating such events,
astronomers still do not understand the complicated processes after a tidal
disruption."
The new findings could explain why astronomers don't see high-energy
emissions, like X-rays, from other TDEs. Such emissions are created as
material from the star is dragged into a thin disk around the black hole.
This material is heated in the disc creating high-energy emissions
which also result when the material falls into the black hole. These
emissions are obscured by these gas clouds blown out by powerful winds.
"This observation rules out a class of solutions that have been proposed
theoretically and gives us a stronger constraint on what happens to gas
around a black hole," Kishore Patra, a graduate student in astronomy at UC
Berkeley and lead author of the paper, said in the statement. "People have been seeing other evidence of wind coming out of these
events, and I think this polarization study definitely makes that evidence
stronger, 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 material in
the star that is spiraling inward doesn't eventually fall into the black
hole ; it's blown away from the black hole," Patra added.
The results seem to contradict a theory proposed by many astronomers: that
when a star is destroyed by a black hole, a highly asymmetrical accretion
disk forms. Such a disk would demonstrate a high degree of polarized light
— something not seen in this TDE.
The second set of observations from this event in November 2019 showed that
the light from it was only slightly polarized. This finding suggested that
the gas cloud of ejected material had thinned enough to reveal the
asymmetrical gas structure around the black hole, the team said.
"The accretion disk itself is hot enough to emit most of its light in
X-rays, but that light has to come through this cloud, and there are many
scatterings, absorptions, and reemissions of light before it can escape out
of this cloud," Patra said. "With each of these processes, the light loses
some of its photon energy, going all the way down to ultraviolet and optical
energies. The final scatter then determines the polarization state of the
photon. So, by measuring polarization, we can deduce the geometry of the
surface where the final scatter happens."
Petra added that the "deathbed scenario" the team observed for this star may
not apply to "oddball" TDEs in which jets of material are ejected at near
light speed only from the poles of the black hole. To answer this question,
further polarization studies of TDEs will be needed.
"Polarization studies are very challenging, and very few people are
well-versed enough in the technique around the world to utilize this," Petra
said. "So, this is uncharted territory for tidal disruption events."
Both sets of observations were made using the 3-meter (10 feet) Shane
telescope at the Lick Observatory near San Jose, California. The telescope
is fitted with the Kast spectrograph, an instrument that can determine the
polarization of light over the full optical spectrum.
The team's paper will be published in September's issue of the journal
Monthly Notices of the Royal Astronomical Society.
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