Did a black hole eating a star generate a neutrino? Unlikely, new study
shows
New calculations show that a black hole slurping down a star may not have generated enough energy to launch a neutrino

Date:
October 13, 2021
Source:
Harvard-Smithsonian Center for Astrophysics
Summary:
New calculations show that a black hole slurping down a star may
not have generated enough energy to launch a neutrino.



FULL STORY ==========================================================================
In October 2019, a high-energy neutrino slammed into Antarctica. The
neutrino, which was remarkably hard to detect, peaked astronomers'
interest: what could generate such a powerful particle?

========================================================================== Researchers traced the neutrino back to a supermassive black hole that
had just ripped apart and swallowed a star. Known as a tidal disruption
event (TDE), AT2019dsg occurred just months earlier -- in April 2019 --
in the same region of the sky where the neutrino had come from. The
monstrously violent event must have been the source of the powerful
particle, astronomers said.

But new research casts doubt on that claim.

In a study published this month in the Astrophysical Journal,
researchers at the Center for Astrophysics | Harvard & Smithsonian and Northwestern University, present extensive new radio observations and
data on AT2019dsg, allowing the team to calculate the energy emitted
by the event. The findings show AT2019dsg generated nowhere near the
energy needed for the neutrino; in fact, what it spewed out was quite "ordinary," the team concludes.

Black Holes are Messy Eaters While it may seem counterintuitive, black
holes do not always swallow everything in reach.



========================================================================== "Black holes are not like vacuum cleaners," says Yvette Cendes, a
postdoctoral fellow at the Center for Astrophysics who led the study.

When a star wanders too close to a black hole, gravitational forces
begin to stretch, or spaghettify, the star, Cendes explains. Eventually,
the elongated material spirals around the black hole and heats up,
creating a flash in the sky that astronomers can spot from millions of
light years away.

"But when there's too much material, black holes can't eat it all smoothly
at once," says Kate Alexander, a study co-author and postdoctoral fellow
at Northwestern University who calls black holes 'messy eaters.' "Some of
the gas gets spewed back out during this process -- like when babies eat,
some of the food ends up on the floor or the walls." These leftovers
get flung back into space in the form of an outflow, or jet - - which,
if powerful enough, could theoretically generate a subatomic particle
known as a neutrino.

An Unlikely Source for Neutrinos Using the Very Large Array in New Mexico
and Atacama Large Millimeter/ submillimeter Array (ALMA) in Chile, the
team was able to observe AT2019dsg, some 750 million light years away,
for more than 500 days after the black hole had started consuming
the star. The extensive radio observations make AT2019dsg the most
well-studied TDE to date and revealed that the radio brightness peaked
around 200 days after the event began.



========================================================================== According to the data, the total amount of energy in the outflow was
equivalent to the energy radiated by the Sun over the course of 30
million years. While that may sound impressive, the powerful neutrino
spotted on Oct. 1, 2019 would require a source 1,000 times more energetic.

"Instead of seeing the bright jet of material needed for this, we see
a fainter radio outflow of material," Alexander explains. "Instead of a powerful firehose, we see a soft wind." Cendes adds, "If this neutrino
somehow came from AT2019dsg, it begs the question: Why haven't we spotted neutrinos associated with supernovae at this distance or closer? They
are much more common and have the same energy velocities." The team
concludes it's unlikely that the neutrino came from this particular
TDE. If it did, however, astronomers are far from understanding TDEs
and how they launch neutrinos.

"We're probably going to check-in on this one again," says Cendes,
who believes there's still much to learn. "This particular black hole
is still feeding." TDE AT2019dsg was first discovered on April 9, 2019
by the Zwicky Transient Facility in Southern California. The neutrino,
known as IceCube-191001A, was detected by the IceCube Neutrino Observatory
in the South Pole six months later.

========================================================================== Story Source: Materials provided by Harvard-Smithsonian_Center_for_Astrophysics. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Y. Cendes, K. D. Alexander, E. Berger, T. Eftekhari,
P. K. G. Williams,
R. Chornock. Radio Observations of an Ordinary Outflow from the
Tidal Disruption Event AT2019dsg. The Astrophysical Journal, 2021;
919 (2): 127 DOI: 10.3847/1538-4357/ac110a ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211013081601.htm

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