Sky & Telescope - May 2021 - 16

High-energy Particles

Austrian physicist Victor Hess discovered cosmic rays
back in 1912, but studying them in detail has been frustratingly difﬁcult. As soon as high-energy particles enter Earth's
atmosphere, they collide with nitrogen and oxygen nuclei,
producing an avalanche of rapidly interacting and decaying
secondary particles. Only by capturing and measuring these
ﬂeeting air showers can scientists hope to learn more about
the primary culprits. " From our data, we can derive the number of secondary particles at ground level, the arrival direction of the air shower, and the total energy, " says Antonella
Castellina (INFN Torino, Italy), co-spokesperson of the
international Pierre Auger Observatory, currently the largest
cosmic-ray observatory in the world.
For ultra-high-energy cosmic rays (UHECRs), an additional complicating factor is their sheer rarity. Particles with
energies above 8×1018 electron volts (8 EeV, about 1% of the
punch of a high-velocity baseball) only arrive at Earth at a
rate of one per square kilometer per year. Little wonder that
the origin of UHECRs is still pretty mysterious, even though
the ﬁrst one was detected almost 60 years ago, in 1962.
" Finding the sources is by far the biggest challenge, " says
theorist Luis Anchordoqui (City University of New York).
Not that scientists are completely in the dark about the
origin of UHECRs. In 2017, the Pierre Auger collaboration
presented convincing evidence that they originate from
beyond the Milky Way, as most researchers already suspected on theoretical grounds. Because of their high energy,
UHECRs experience a smaller deﬂection by magnetic ﬁelds
than lower-energy cosmic rays - in general, less than 25°.
Looking at the arrival directions of more than 30,000 of
these " baseball particles " over a period of 12 years, scientists
found that about 6.5% more UHECRs are coming from one
half of the sky than from the opposite half. This anisotropy

Energetic air-shower particles
pass through the tanks at
velocities that are higher than
the speed of light in water.
is not aligned with the galactic center, as you might expect
in the case of a galactic origin, but with the slightly lopsided
distribution of external galaxies within 150 million lightyears or so.
One year later, the team announced a possible association of even more energetic UHECRs (above 4×1019 eV) with
relatively nearby starburst galaxies (galaxies with an exceptionally high star-formation rate) - in particular, NGC 4945
in the constellation Centaurus, NGC 253 (Sculptor), M83
(Hydra), and NGC 1068 (Cetus). " At present, it's just a strong
hint, " says Castellina - the statistical signiﬁcance of the
correlation isn't yet large enough to claim a discovery. More
data are crucial to obtain a higher degree of conﬁdence, she
explains. " We keep observing. "

A Sea of Water Tanks
Located in the Pampa Amarilla northeast of the Argentinian
town of Malargüe, the Pierre Auger Observatory (named after
the French physicist who pioneered air-shower observations)
covers an area of 3,000 square kilometers - about the size of
Rhode Island. Its some 1,660 surface detectors, spaced about a
mile apart, consist of tanks that look like giant Jacuzzis, each
holding 12,000 liters of puriﬁed water. Sensitive silicon photomultipliers register the faint Cherenkov radiation produced
when energetic air-shower particles pass through the tanks at
velocities that are higher than the speed of light in water - a

X TEAM EFFORT Outreach coordinator Greg Snow (1954-2019) stands
with the final tank of the 1,600 originally deployed, signed by collaboration members. Later, the array team installed additional tanks, part of a
more densely spaced section to enable detections at lower energies.

PIERRE AUG ER OBSERVATORY: LUK AS NELLEN / PIERRE AUG ER
OBSERVATORY / CC BY-SA 2.0; SIG NED TA NK: PIERRE AUG ER
COLL A BOR ATION / CC BY-SA 2.0

T PIERRE AUGER Long-distance view of a line of the observatory's
Cherenkov water tanks (small white structures).

M AY 2 0 2 1 * S K Y & T E L E S C O P E

Sky & Telescope - May 2021

Table of Contents for the Digital Edition of Sky & Telescope - May 2021