Sky & Telescope - May 2023 - 18

Particle Astrophysics
nos in the universe may well originate in rather inconspicuous
active galactic nuclei (AGN) like M77, which are powered
by supermassive black holes.
Such studies might help solve the riddle of the origin of the
most energetic cosmic rays. Astronomers suspect that these
" Mike Tyson particles, " which carry as much energy as a powerfully
pitched baseball, are produced in the cores of remote
AGN. The idea is that strong magnetic fields or shock waves
in the vicinity of a central supermassive black hole accelerate
the particles. But since cosmic rays cannot be traced back
to their source, observers need to find neutrinos produced
in the same environments to confirm their suspicions. The
TXS 0506+056 and M77 results suggest that blazars and AGN
do indeed produce most ultra-high-energy cosmic rays.
Friendly Competition
But if there's a large, successful neutrino observatory operational
in Antarctica, why bother to build another one on the
bottom of the sea? While enjoying the Mediterranean sun
and keeping an eye on the slowly subsiding waves, Paschal
Coyle, who is also the spokesperson of the KM3NET collaboration,
explains the need for both cooperation and " friendly
competition " between different groups: Findings by one
facility need to be corroborated - or falsified! - by other
instruments, using different equipment and applying different
analysis techniques. (A third cubic-kilometer-size detector
known as Baikal-GVD, for Gigaton Volume Detector, is under
construction at the 1,240-meter-deep bottom of Lake Baikal
in eastern Russia.)
KM3NET also has its own unique selling points. In particular,
the European observatory will be able to pinpoint
the arrival direction of neutrinos much more precisely. " We
expect to achieve a directional accuracy of less than a tenth
of a degree, " says Coyle. " That's about 10 times better than
IceCube, which is more prone to light scattering due to tiny
bubbles in the ice. "
In addition, because of its location in the Northern
Hemisphere, KM3NET will be able to study a different part
of the universe than IceCube. Surprisingly, the section of sky
each detector specializes in lies below, not above, the sensors.
Here's why. Not all muons observed by IceCube or KM3NET
result from neutrinos smashing into atoms. As mentioned
before, muons are also produced when cosmic rays hit our
planet's atmosphere. These atmospheric muons can traverse
at most a few kilometers of rock, ice, or ocean before they
decay. But neutrinos fly right through Earth. Only for a muon
traveling " upward " through the detector can you be sure it
was created by a neutrino, not by a cosmic ray hitting the
planet's atmosphere. As a result, IceCube and KM3NET are
most efficient in detecting rare astrophysical neutrinos when
they look " downward. "
Action Time
Around four o'clock in the afternoon, there's a sudden burst
of activity aboard ship. The waves that prevented the deployment
of new detector lines earlier in the day have finally
subsided, and it's time for action. But first, an old line has
to be hoisted up to solve an issue with the power supply.
From the deck of Janus II (a nearby research vessel that is
part of the KM3NET sea operation), an underwater robot is
lowered down in a special cage and disappears beneath the
surface. Meanwhile, on the Castor 02, crew members wearing
safety helmets and life jackets are preparing a heavy lifting
hook, which they attach to a kilometers-long steel cable on a
giant reel. Soon enough, and with a lot of noise, the cable is
unwinding over large pulleys, and the hook starts its 90-minute
descent to the seafloor, some 2,450 meters below.
At the end of the afternoon, people gather to follow the
activities of the underwater robot live on a TV screen in the
small mess room of the Castor 02. Coyle and his team members
can't take their eyes off the screen. It feels like watching
a science-fiction movie. The propellers of the remotely conDEPLOYMENT
Left: The landing craft Castor 02, with the huge portal crane used to deploy the detector modules. Center: The DOMs are carefully
coiled up inside a spherical cage, shown here on deck. Right: An underwater robot is lowered from the neighboring research vessel Janus II.
18 MAY 2023 * SKY & TELESCOPE
GOVERT SCHILLING (2)

Sky & Telescope - May 2023

Table of Contents for the Digital Edition of Sky & Telescope - May 2023