magnetic field, the cosmic rays stream more
directly with smaller deflections and thus
could potentially reveal the direction of their
sources. At the other extreme, lower-energy
cosmic rays suffer from strong scattering in
intense, small-scale magnetic fields, such as
those associated with the Sun. These mag-
netic fields can imprint preferred directions
in the cosmic rays. Relative motions between
Earth and cosmic-ray sources introduce fur-
ther small asymmetries through the Comp-
ton-Getting effect (3), a version of relativistic
aberration in which intensities are higher
toward the direction of motion.
Until the 1990s, a variety of technical barriers prevented the detection of anisotropies for cosmic rays with teraelectron volts
(1012) and higher energies. Cosmic rays with
these energies originate in our Galaxy and
are relatively unaffected by magnetic fields
in the interior of our solar system. With the
advent of large air-shower arrays, a vast increase occurred in the number of cosmic rays
that can be detected, providing the statistical precision required to map variations in
the cosmic-ray intensities. These detectors
are very accurately calibrated by making
measurements in directions approximately
parallel to the celestial equator. As a result of
Earth’s spin, the detector rotates with respect
to the sky, which averages over variations in
detector response in directions parallel to the
horizon. The resulting data yield information
on the amplitude and locations of variations
in cosmic-ray intensities in right ascension,
a single angular direction across the celestial
sphere. Unfortunately, they are almost blind
to intensity variations that occur in the direction of celestial declination and thus do
not quite offer a full picture of the pattern of
cosmic-ray anisotropies across the sky.
With the excellent statistics on cosmic-
ray events from modern observatories, the
teraelectron volt–to–petaelectron volt (1012
to 1015) energy range reveals a large-scale
pattern in cosmic-ray intensities that can be
represented by a dipole (4–6). A dipole is the
choice description to provide the lowest-or-
der fit that is characterized by an amplitude
between the maximum and minimum inten-
sity and a direction in space that joins these
two regions. The observed anisotropy reveals
a variation between minimum and maxi-
mum cosmic-ray intensities of ~0.2% that is
measured with high statistical significance.
The projected direction of this asymmetry at
teraelectron volt energies appears to line up
with the local direction of the Galactic magnetic field, as expected if cosmic rays stream
along the field lines. However, the direction
of the peak brightness changes at petaelectron volt energies. This might be understood
if there is a local source of high-energy cosmic rays, perhaps associated with the nearby
Vela supernova remnant—an effect similar to
glimpsing the faint glow of a nearby street
lamp through a heavy fog. In addition, a
number of lower-amplitude features remain
after removing the dipole component and
most likely result from interactions between
cosmic rays and the region where the magnetized solar wind terminates at the boundary
of interstellar space (7).
Using the exceptional capabilities of the
Pierre Auger Observatory (see the photo),
the collaboration (2) presents new and
important results on anisotropies seen in
ultrahigh-energy cosmic rays (UHECRs)
with E ≥ 8 EeV that are expected to originate from extragalactic sources. Measurements in an array of 1600 water tanks of
the Cherenkov light produced by air-shower
particles at the Pierre Auger Observatory
map the equatorial component of cosmic-ray arrival directions in the same way as
those of earlier studies. However, the Pierre
Auger Observatory also has telescopes to
detect fluorescence emitted by cosmic-ray
air showers. These data allow access to the
cosmic-ray anisotropies in the direction of
declination. Furthermore, because of the
structure (large zenith angle acceptance) of
the Pierre Auger Observatory, observations
can be taken over an unusually wide part of
the sky, which further improves the results.
The outcome of this study presents a
new perspective on UHECRs with E ≥ 8
EeV. The amplitude of the intensity varia-
tions is about a factor of 10 larger than
the maximum of 0.6% predicted by the
Compton-Getting effect because of our mo-
tion with respect to the cosmic-ray back-
ground (8) and is the highest-amplitude
anisotropy owing to effects beyond the
solar system. Thus, the observed substan-
tial anisotropy is intrinsic to the sources
of UHECRs. After correcting for moderate
bending due to the passage of UHE cosmic
rays through the Galaxy, the authors found
a possible alignment between UHECR ar-
rival directions and the distribution of gal-
axies within ~100 megaparsec (Mpc). The
level of correction for the magnetic deflec-
tion as UHECRs cross the Galaxy depends
on their electric charge, and the adopted
model is consistent with the Pierre Auger
Observatory finding that protons domi-
nate in the relevant energy range.
Although this comparison is tentative
given the uncertainties in both the distribution of the UHECR anisotropy corrected
to outside of the Milky Way and fits to the
projected density distribution of the nearby
galaxy populations (9, 10), it offers important insights into the origins of UHECRs.
The extragalactic origin of the UHECRs with
energies >8 EeV now is strongly supported
by these observations (2). The dipole anisotropy in UHECRs encodes information about
sources of cosmic rays. For example, if the
correlation between UHECR anisotropy and
the projected density distribution of nearby
galaxies holds, then a significant fraction
of the sources of UHECRs with the energies
of ≥8 EeV should lie within a radius of approximately 100 Mpc. j
1. T.K.Gaisser,R.Engel, E.Resconi, Cosmic Rays and Particle
Physics (Cambridge University Press, ed. 2, 2016).
2. The Pierre Auger Collaboration, Science 357, 1266 (2017).
3. A.H.Compton, I.A.Getting, Phys. Rev. 47,817(1935).
4. M.Amenomori etal., Science 314,439(2006).
5. G.Guillian etal. (Super-Kamiokande Collaboration), Phys.
Rev. D 75, 062003 (2007).
6. A.Abassi etal. (IceCube Collaboration), Astrophys.J. 740,
7. M.Ahlers,P.Mertsch, Prog.Nucl.Part.Phys. 94,184(2017).
8. M.Kachelreiß, P.D.Serpico, Phys. Lett. B 640,225(2006).
9. M. Bilicki etal., Astrophys.J. 741, 31 (2011).
10. C.A.P.Bengaly Jr. etal., Month.Not.R.Astron.Soc. 464,768
A panorama is shown of one of the four
fluorescence detector sites, Los Morados,
at the Pierre Auger Observatory, the
largest cosmic-ray detector in the world.