Sky & Telescope - May 2021 - 19

likely to interact with normal matter than their lower-energy
siblings - because of their higher energy, they have a larger
cross section, as physicists say. While low-energy neutrinos
easily pass through a planet like Earth, the particles that
GRAND is after have trouble penetrating a few tens of
kilometers of solid rock. So if their source is low above the
horizon - hidden from view by the mountains, as seen from
GRAND - they will enter the mountains from behind and
most likely interact with a nucleus somewhere in the rock.
These interactions produce short-lived particles that may
emerge from the near side of the mountain range, in full view
of the GRAND radio antennas.
E > 57 EeV
60°

30°

-30°

Unseen region
(gray area)
pe

rga

lact

ic p l a n e

t
lac

-60°

0
2
4
6
8 10 12 14 16 18
Events observed, summed over 20°-radius circle

E > 8 EeV
90°

Unseen

24h

Galactic
center

By then, however, the UHECR riddle may have been solved by
GRAND, the prototype of which is now taking shape in western China. Just like the new radio antennas of Pierre Auger,
GRAND will also detect air showers produced by cosmic-ray
particles, but its ultimate goal is to hunt for extremely energetic neutrinos. " These high-energy neutrinos have never been
observed before, " says Timmermans, " but we're very conﬁdent
they must exist, although they're apparently quite rare. "
No matter how cosmic-ray particles are accelerated to
UHECR levels, explains Timmermans, you expect them to
incidentally crash into atomic nuclei in the ambient interstellar medium - probably close to their source, where densities
are much higher than in intergalactic space. Through E =
mc2, the sheer energy of these collisions is converted into a
wide variety of new particles, including uncharged neutrinos.
According to theoretical calculations, these neutrinos carry
some 5% of the energy of the original cosmic-ray particle. In
other words, a burst of UHECRs of 1020 electron volts should
be accompanied by neutrinos of a few times 1018 eV.
Because magnetic ﬁelds don't deﬂect neutrinos, any highenergy neutrino arriving here on Earth points directly back
to its origin. Unfortunately, not all of the particles will come
from pinpoint sources: UHECRs can also create high-energy
neutrinos by interacting with the cosmic background radiation, which is all over the sky. But any neutrinos produced
together with the UHECRs will betray where the most energetic particles in the universe originate, with a precision of
better than half a degree. " It would be great to really discover
one or more discrete sources on the sky, " says astroparticle
physicist Sijbrand de Jong (Radboud University, The Netherlands), who pioneered the Auger radio observations and is
also a member of the GRAND collaboration.
But how do you detect these high-energy neutrinos? That's
where the Tibetan Plateau mountains come in. Somewhat
counterintuitively, the most energetic neutrinos are more

TELESCOPE A RR AY HOTSPOT: R. U. A BBASI E T A L. / ASTR OPHYSICAL JOUR NAL LE T TERS
2014 (79 0:L 21); COSMIC DIPOLE: PIERRE AUG ER COLL A BOR ATION

Geography to the Rescue

space mission for the study of ultra-high-energy cosmic
rays to NASA in 2016. In 2019, the researchers submitted a
detailed design study for review by the 2020 Decadal Survey
on Astronomy and Astrophysics of the National Academies,
which is expected to report its recommendations this spring.
If the proposal receives a high ranking and NASA gives the
green light, POEMMA could launch by the end of the decade,
according to Olinto.
POEMMA consists of two identical spacecraft circling
Earth in the same orbit, some 300 kilometers behind each
other. The stereo vision of its two eyes - 4-meter wide-angle
telescopes equipped with sensitive ultraviolet cameras -
would enable a full 3D reconstruction of any ﬂuorescence
trail in the atmosphere. " What we need is a lot of events
with directions, " says Olinto, " and over time, POEMMA
will observe large swaths of the Earth's nightside. Within
ﬁve years or so, it should be possible to map the sources of
UHECRs. "

Su
per
gala
ctic

plane

lac

ti c

-90°

0.46

0.42
events/km2 per steradian per year

0.38

S ORIGIN HINTS In the Northern Hemisphere, the Telescope Array has
detected more extremely energetic cosmic rays coming from a hotspot
centered in Ursa Major (top). Operating in the Southern Hemisphere, the
Pierre Auger Observatory has seen more cosmic rays (including those
of slightly lesser energy) coming from one half of the sky than the other
(bottom). Unfortunately, the two experiments see different parts of the
sky and have different energy cutoffs, so it's difficult to compare their
data. (The supergalactic plane is the plane of nearby galaxy clusters.)
s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 1

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Sky & Telescope - May 2021

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