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A star shredded by a black hole may have spit out an extremely energetic neutrino
A neutrino that plowed into the
Antarctic ice offers up a cautionary message: Don’t stray too close to the edge
of an abyss.
The subatomic particle may have been blasted
outward when a star was ripped to pieces during a close encounter with a
black hole, physicists report May 11 at arXiv.org. If it holds up, the result
would be the first direct evidence that such star-shredding events can
accelerate subatomic particles to extreme energies. And it would mark only the
second time that a high-energy neutrino has been traced back to its cosmic
origins.
With no electric charge and very
little mass, neutrinos are known to blast across the cosmos at high energies.
But scientists have yet to fully track down how the particles get so juiced up.
One potential source of energetic
neutrinos is what’s called a tidal disruption event. When a star gets too close
to a supermassive black hole, gravitational forces pull the star
apart (SN: 10/11/19). Some of the star’s guts
spiral toward the black hole, forming a hot pancake of gas called an accretion
disk before the black hole gobbles the gas up. Other bits of the doomed star
are spewed outward. Scientists had predicted that such violent events might
beget energetic neutrinos like the one detected.
Spotted on October 1, 2019, the little neutrino packed a punch: an energy of 200 trillion electron volts. That’s about 30 times the energy of the protons in the most powerful human-made particle accelerator, the Large Hadron Collider. The neutrino’s signature was picked up by IceCube, a detector frozen deep in the Antarctic ice. That detector senses light produced when neutrinos interact with the ice.
When IceCube finds a high-energy
neutrino, astronomers scour the sky for anything unusual in the direction from
which the particle came, such as a short-lived flash of light, or transient, in
the sky. This time, astronomers with the Zwicky Transient Facility came up with
a possible match: a tidal disruption event called AT2019dsg.
First observed in April 2019, that event
had been spied emitting light of various wavelengths: visible, ultraviolet,
radio and X-rays. And the maelstrom was still raging when IceCube detected the
neutrino, according to a team of physicists including Marek
Kowalski of the Deutsches Elektronen-Synchrotron, or DESY, in Zeuthen,
Germany.
While intriguing, the association
between the neutrino and the shredded star is not certain, says IceCube physicist
Francis Halzen of the University of Wisconsin–Madison, who was not involved with the new study. “I
don’t know if I have to bet my wallet, but I probably would,” Halzen says. “But
it doesn’t have much money in it.”
The probability that a neutrino and a similar
tidal disruption event would overlap by chance is only 0.2 percent, the
researchers report. But that doesn’t meet physicists’ stringent burden of proof.
“Just one event is difficult to convince [us] this source is really a neutrino
emitter,” says astrophysicist Kohta Murase of Penn
State University. “I am waiting for more data.”
Kowalski declined to comment for this
article, as the paper has not yet been accepted for publication in a scientific
journal.
To have birthed such an energetic
neutrino, the star-shredding event must have first accelerated protons to high
energies. Those protons must then have crashed into other protons or photons (particles
of light). That process produces other particles, called pions, that emit
neutrinos as they decay.
Now, scientists are aiming to pin down
exactly how that acceleration happened. The protons might have been launched
within a wind of debris that flowed outward in all directions. Or they could
have been accelerated in a powerful, geyserlike jet of matter and radiation.
AT2019dsg shows some unusual features
that any explanation should be able to account for. X-rays produced in the event,
for example, appeared to drop off rapidly. So physicists WalterWinter of
DESY and Cecilia Lunardini of Arizona State
University in Tempe suggest May 13 at arXiv.org that the event did produce a
jet, but that a cocoon of material gradually shrouded
the region, hiding the X-rays from view while still allowing the neutrino to escape. Lunardini declined to comment
because the paper is not yet published in a journal.
But Murase argues that for the jet to be
hidden, that means it can’t be that powerful of an outflow, making it hard to
explain the energetic neutrino this way. “If it injects a lot of energy, this
energy gets out,” he says. In a third study posted May 18 at arXiv.org, Murase
and colleagues favor the idea that the protons get accelerated in an outward
flowing wind or in a corona, a superhot
region near the black hole’s accretion disk.
Determining where these particles come
from can help scientists better understand some of the most extreme
environments in the cosmos. Previously, astronomers had matched up a different energetic neutrino with a blazar experiencing a flare-up (SN:7/12/18). A blazar is a bright source of light powered by a
supermassive black hole at the center of a galaxy. Both a blazar flare and a
tidal disruption event “are very special activities, which is when a lot of
energy is released in a small amount of time,” says astrophysicist Ke Fang of Stanford University, who was not involved
with the study.
Making more observations of high-energy neutrinos
is crucial, Fang says. “This is the only way we can clearly understand how the
universe is operating at this extreme energy.”
Is the coronavirus mutating? Yes. But here’s why you don’t need to panic
In novels and movies,
infectious pathogens mutate and inevitably become more dangerous. In the blockbuster
movie Contagion, for instance, a deadly virus acquires a mutation in
Africa that causes the global death toll to spike in mere days.
Reality, however, is far less
theatrical.
Over the past few months, a
few research groups have claimed to identify new strains of the coronavirus,
called SARS-CoV-2, that’s infecting people around the globe. That sounds scary.
But not only is it sometimes difficult to determine whether a change amounts to
a “new strain,” none of the reported changes to the virus have been shown to
make it more dangerous.
This has led to great
confusion for the general public. Each time such studies surface, fears arise,
and virus experts rush to explain that changes in a virus’s genetic blueprint,
or genome, happen all the time. The coronavirus is no exception.
“In fact, it really just
means that it’s normal,” says Kari Debbink, a virologist at Bowie State
University in Maryland. “We expect viruses to evolve. But not all of those
mutations are meaningful.”
Here’s what it means to find mutations
in the novel coronavirus, and what evidence is needed to actually raise a red
flag.
First, a mutation is just a change
Most of the time, mutations
don’t do anything to a virus at all.
Viruses are simply protein shells that contain either DNA or RNA as their genetic material. In the case of SARS-CoV-2, it’s RNA. The building blocks of RNA, called nucleotides, are arranged in triplets, called codons. These nucleotide trios provide the code for building amino acids, which make up the virus’s proteins. A mutation is a change to one of these nucleotides in the virus’s genetic material — in SARS-CoV-2’s case, one of around 30,000 nucleotides.
Sometimes a mutation in a
triplet is silent, meaning the codon still codes for the same amino acid. But
even when an amino acid does change, the virus might not behave in a way that’s
obviously different. Some mutations could also spawn dysfunctional viruses that
quickly disappear as a result.
And in fact, these changes can
actually be helpful when it comes to tracing the virus’s path around the globe, something researchers have been doing ever
since experts from China released the first coronavirus genetic sequence in
January (SN: 2/13/20). Scientists can
decipher, or sequence, the virus’s RNA to track changes as it infects more people.
They can then track where and how the coronavirus is spreading in a population,
and monitor for further changes in its genetics.
Close relatives
Researchers can use changes in the coronavirus’s genetic blueprint to better understand how it is spreading. By deciphering viruses taken from patients, they can build a tree (pictured, samples are represented by dots and colored by region where they were collected) to show the genetic relationships among those viruses. The pandemic got its start in China at the end of 2019 (blue circles at the bottom) and has since expanded to five other continents. The horizontal lines depict how closely related two viruses are — longer lines mean there are more differences between them.
Tracking SARS-CoV-2’s genetic changes to map its spread, December 2019–May 2020
Epidemiologists are interested
in tracking mutations even if they don’t alter the protein, says Emma Hodcroft,
a molecular epidemiologist at the University of Basel in Switzerland. “But that
doesn’t mean that it’s a new strain or that it’s a virus that behaves
differently.”
A new ‘strain’ of virus doesn’t mean much
The term “strain” is “used
very, very loosely by most scientists,” Hodcroft says. “There isn’t really a
strict definition of the word ‘strain,’” particularly when talking about
viruses. Experts might simply be referring to viruses that aren’t genetically identical
— almost like discussing different people.
Viruses are always changing.
When a virus infects a cell, it begins making copies of its genetic
instructions. Most viruses don’t have the necessary tools to proofread each
string of RNA for mistakes, so the process is error-prone and differences build up over time.
Coronaviruses like
SARS-CoV-2, on the other hand, do have a proofreading enzyme — a rarity for RNA viruses. But that doesn’t mean their genomes don’t have
errors. Changes still accumulate, just more slowly than in other RNA viruses such
as influenza. “Strains,” “variants” or “lineages” are all terms researchers
might use to describe viruses that have identical or closely related strings of
RNA.
But for the general public, a
word like “strain” is often interpreted to mean a whole new scourge. “I think
the use of the term ‘strain’ does little more than cause panic,” says Jeremy
Luban, a virologist at the University of Massachusetts Medical School in
Worcester. “It doesn’t really get at what the important issues are.”
Most mutations aren’t dangerous
A mutation can affect a virus
in a number of ways, but only certain kinds of mutations might make the virus
more dangerous to people. Perhaps the change shields the virus from the immune
system, or makes it
resistant to treatments. Mutations could also alter how easily the virus
spreads among people or cause shifts in disease severity.
Luckily,
such mutations are rare. Unfortunately, they can be hard to identify.
A preliminary study published
May 5 at bioRxiv.org, for instance, found a mutation in the SARS-CoV-2 spike, a
protein on the outside of the coronavirus that allows it to break into cells. This
new variant is now found more often in places like Europe and the United States
than the original form of the coronavirus. That may mean the change makes the virus more transmissible, the authors concluded. But the study lacked
laboratory experiments to support the claim.
Other explanations could also
explain the pattern. The SARS-CoV-2 variant with the mutation could have ended
up in certain regions thanks to random chance — a person infected with a virus that had the new
mutation just happened to hop on a plane — and might have nothing to do with the
virus itself. The study didn’t provide enough evidence to distinguish among the
possibilities.
“What I think has been
potentially confusing to people is that we’re watching this very normal process
of [viral] transmission and mutation happen in real time,” says Louise Moncla,
an evolutionary epidemiologist at the Fred Hutchinson Cancer Research Center in
Seattle. “And there’s this real desire to understand whether these mutations
have any functional difference.”
‘Take a deep breath,’ experts say, and expect mutations
To understand whether a
single mutation changes how the virus works, “it’s not just going to be one
experiment,” Bowie State virologist Debbink says. “It takes a lot of research.”
In addition to examining
genetic sequences of viruses from coronavirus patients all over the world,
researchers will also rely on studies in lab-grown cells or animals. Such studies
could help pinpoint whether viruses with particular mutations behave
differently. Competition experiments —
where two different viruses are mixed in cells in a dish or used to infect an
animal — can help scientists figure out which variant is more successful at
making copies of itself, that is, which one “wins.”
Other types of tests could
reveal if mutations in the coronavirus’ spike protein alter how strongly it
attaches to the protein on human cells that allows it to get inside the cells, virologist
Luban says (SN: 2/3/20), or whether changes
modify how easily the virus gets into a cell after binding.
But lab results might not provide
the full picture either. “Just because something’s different in a cell doesn’t
necessarily mean that it’s different when you scale that up to the whole human
body,” Hodcroft says. “At the end of the day, you’re going to need some animal
studies or some really good human data.”
These studies take time. Meanwhile,
more coronavirus mutations are guaranteed to pop up over the coming months — and experts will continue to track
them.
“The data will tell us whether we need to worry, and in what way we need to worry,” Moncla says. “Everyone should take a deep breath and realize that this is exactly what we’ve always expected to happen, and we don’t necessarily need to be concerned.”
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New species of scaly, deep-sea worms named after Elvis have been found
A new
look at the critters known as “Elvis worms” has the scale worm family all shook
up.
These
deep-sea dwellers flaunt glittery, iridescent scales reminiscent of the sequins
on Elvis’ iconic jumpsuits (SN: 1/23/20).
“For a while, we thought there was just one kind of Elvis worm,” says Greg
Rouse, a marine biologist at the Scripps Institution of Oceanography in La
Jolla, Calif. But analysis of the creatures’ genetic makeup shows that Elvis worms comprise four species
of scale worm,
Rouse and colleagues report May 12 in ZooKeys.
Rouse’s
team compared the genetic material of different Elvis worms with each other,
and with DNA from other scale worm species. This analysis places Elvis worms in
the Peinaleopolynoe genus of scale worms, which includes two other known
species — one found off the coast of Spain, the other off California.
The four
newly identified Elvis worm species are scattered across the Pacific, from P.
elvisi and P. goffrediae in Monterey Canyon off California to P.
orphanae in the Gulf of California by Mexico and P. mineoi near
Costa Rica.
These
deep-sea Elvis impersonators share some common traits, such as nine pairs of scales.
But each species has its own distinct flare. P. elvisi’s gold and pink iridescent
color scheme earned it the honor of keeping the worms’ namesake in its official
title. P. orphanae, on the other hand, mostly sports rainbow-sparkled
scales of a bluish hue.
The researchers don’t know why Elvis worms have evolved such eye-catching scales, since the animals live in the dark, deep sea. It could just be a side effect of developing thicker scales over time, which happen to refract more light, Rouse says. Thicker scales could come in handy in a fight, since Elvis worms are apparently biters, a behavior discovered while watching a worm skirmish. “Suddenly, they started doing this amazing jitterbugging — wiggling, and then fighting and biting each other” on their scales, Rouse says. “No one’s ever seen any behavior like this in scale worms.”