Sky & Telescope - June 2021 - 9

Mars

ASTEROIDS

Second Earth Trojan
Observed

Venus

A RECENTLY DISCOVERED asteroid

appears to be an Earth Trojan, orbiting a
gravitationally stable area with only one
other known occupant.
Amateur astronomer Tony Dunn
reported the potential ﬁnd January 26th
on the Minor Planet Mailing List. Given
the preliminary designation 2020 XL5,
the asteroid is a few hundred meters
across and appears to be orbiting the
L4 Lagrange point.
Trojan asteroids occupy the gravitationally stable Lagrange points about
60° ahead or behind the planets in
their orbits around the Sun. In theory,
Trojan orbits should be stable around
every planet except Saturn, in which

BLACK HOLES

Star-shredding Black
Hole Makes High-energy
Neutrino

ORBIT DIAG R A M: G REGG DINDER M A N / S&T; SOURCE: TON Y DUNN; STA RSHREDDING BL ACK HOLE: DESY / SCIENCE CO M M UNICATION L A B

A SUPERMASSIVE BLACK HOLE some

690 million light-years away tore apart a
star - and may have created neutrinos
in the process.
The IceCube Neutrino Observatory
in Antarctica detected a single highenergy neutrino on October 1, 2019.
The tiny, ghostlike particle came from
a region in Delphinus and packed a
punch of about 200 teraelectron volts
(15 times more energetic than what the
Large Hadron Collider can produce).
Robert Stein (DESY, Germany) and
colleagues observed the area within
hours using the Zwicky Transient Facility. They found the fading glow of a
tidal disruption event (TDE), produced
months earlier when a black hole 30
million times the Sun's mass had shredded a star that ventured too close.
To create a neutrino, a high-energy
proton must slam into another proton
or a photon. This could occur in TDEs,
possibly in the relativistic jets of plasma
that shoot out from near the black hole.

Mercury
Sun

2020 XL5

Earth

S This orbit diagram shows 2020 XL5's trajectory (orange) relative to Earth. View the animation at https://is.gd/2ndearthtrojan.

case Jupiter's gravity pulls them away.
But Earth Trojans have proven difﬁcult
to ﬁnd because they're small and appear
close to the Sun on the sky.

The TDE the team found had ﬂared
up months before, on April 9, 2019, but
while its X-rays had faded rapidly, its
radio emission actually increased. The
radio waves resulted from high-energy
electrons spiraling around magnetic
ﬁeld lines. Whatever accelerated the
electrons would have sped up protons
too, but it's still unclear if that happened within a jet.
Stein's team reports no evidence for
relativistic jets, writing February 22nd
in Nature Astronomy. They posit instead
that the protons accelerate in a broad,
slower-moving outﬂow. The energized
protons then collide with plentiful
ultraviolet photons produced by hot
gas torn from the star
that's farther out.
In the same issue
of Nature Astronomy,
Walter Winter (DESY)
and Cecilia Lunardini
(Arizona State University) suggest that the
TDE did produce jets,
but we can't see them

Earth Trojan orbits can meander -
relative to Earth, 2020 XL5 ranges from
inside Venus's orbit and outward almost
to Mars. The wide-ranging orbit shows
that 2020 XL5 is " almost certainly a
garden-variety bit of rock, " whose orbit
was disturbed during a close pass by
Venus, says Bill Gray, creator of the
Project Pluto astronomical software.
The newest Trojan isn't alone: NASA's
Wide-ﬁeld Infrared Survey Explorer
spacecraft spotted the ﬁrst Earth Trojan,
2010 TK7, in October 2010. But while
that one may orbit stably for about
a quarter million years, calculations
suggest 2020 XL5 will remain a Trojan
only a couple thousand years. Amateur
astronomer Sam Deen says additional
observations to conﬁrm the asteroid's
orbit will be possible late this year.
JEFF HECHT

because they're obscured by outﬂowing
material. But Lunardini acknowledges
that " with only one neutrino observed
from a TDE, you cannot really draw
ﬁrm conclusions, and the data remain
open to interpretation. "
Both teams' models suggest that
TDEs can be efﬁcient particle accelerators, sustaining the process for months
after a stellar snack. However, the
researchers also note there's a 0.5%
chance that the neutrino and TDE are
unrelated. Additional examples of TDEneutrino associations are forthcoming
and will help conﬁrm the link and differentiate between models.
GOVERT SCHILLING

X This artist's impression
shows a tidal disruption
event, which could produce
high-energy neutrinos.
sk yandtelescope.org * JUNE 2 021

https://www.is.gd/2ndearthtrojan http://www.skyandtelescope.org

Sky & Telescope - June 2021

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