Sky & Telescope - October 2020 - 27

outbursts from the supermassive black hole in the Milky
Way's core. Although we have seen similar outbursts in other
galaxies, we've seen none large enough in our own galaxy in
the modern era. Without gamma-ray observations, we would
have limited appreciation for just how violent our galaxy's
center can be.

PIE CH A RT: G REGG DINDER M A N / S&T, SOURCE: IRFU / CE A;
FER MI BUBBLES: N ASA GODDA RD SPACE FLIG H T CEN TER

The Future of Gamma-ray Astronomy

The Fourth Fermi
LAT Catalog
Total number of objects: 5,064
Supernova remnants &
pulsar wind nebulae
2.7%
Pulsars
Non-blazar
4.7%
active galaxies
1.5%
Globular clusters,
binaries,
normal galaxies,
and more
1.0%

With the publication of the fourth catalog of sources from
Fermi's Large Area Telescope (LAT), we now know of more
than 5,000 persistent gamma-ray sources spread across the
entire sky. (GRBs generally only last a few seconds, so we
don't include them in this tally.) Based on the first eight years
of Fermi data, this catalog includes more than 3,000 blazars
and nearly 240 pulsars. Along with NASA's Fermi mission,
there are other satellites studying cosmic gamma rays at lower
Blazars
Unknown
energies, including NASA's Neil Gehrels Swift Observatory
61.9%
28.2%
and European missions such as Integral. Each has amassed
its own collection of detections. With so many sources to
study, gamma-ray astronomy has now become a mature field,
contributing to studies of objects visible across the electromagnetic spectrum as well as via particle and gravitationalwave studies.
Approximately one-quarter of the objects in the fourth
Fermi LAT source catalog are unidentified, i.e., not associated with objects visible at other wavelengths of light. Are
these sources just fainter versions of the types of gamma-ray
center of the Milky Way Galaxy, and in dozens of external
emitters with which we are already familiar? Or does some
dwarf spheroidal galaxies. Astronomers have found a mysterifraction of them represent entirely new classes of objects,
ous uptick of gamma rays coming from our galaxy's center, as
going beyond the many new types of sources that Fermi has
well as the Andromeda Galaxy's. But they still debate whether
uncovered? It will take many more years of data and perhaps
the emission comes from dark matter or undiscovered pulsars.
newer, more sensitive detectors to find out.
To date, no unambiguous signs of dark matter annihilation
Possibly the biggest mystery that Fermi continues to study
have been found. Do these heavy dark matter particles exist?
is the nature of dark matter. The Einsteinian heart of Fermi's
Or are the physical theories that invoke these particles incorLAT detector is the conversion of pure energy in the form of
rect? We may have to wait for new generations of gamma-ray
gamma-ray photons into particles with mass: an electronsatellites and other types of instruments to illuminate the
positron pair. If you run this process in reverse and combine
true nature of dark matter.
an electron with a positron, the
But while we are waiting, the
particles annihilate, creating pure
gamma-ray observatories now orbitenergy in the form of two 511 keV
ing Earth will continue to surprise
X-rays. Some scientists suspect a
us with discoveries of new sources,
similar process should happen for
extreme behavior from familiar
dark matter particles, which are
sources, and photons that link to
their own anti-particles in many
other astronomical messengers to
theories. Since these types of theoreveal more secrets from the highrized dark matter particles (often
energy universe.
called "WIMPs" for Weakly Interacting Massive Particles; S&T: Aug.
„ LYNN COMINSKY is a profes2017, p. 28) are much more massor of physics and astronomy at
sive than electrons or positrons, we
Sonoma State University and the
should see gamma-ray signals when
director of EdEon, SSU's STEM
they annihilate.
learning center. When not in front
S FERMI BUBBLES Discovered in 2010 in gamma
To date, Fermi has looked for
of a computer, she is often found
rays, two gargantuan lobes extend above and below our
dark matter signals in many regions
tending to all the critters at the Little
galaxy's disk. This artist's illustration shows just how big
the Fermi bubbles are.
of the sky, including the Sun, the
H-bar Ranch in Petaluma.
sk yandtelescope.org * OCTOBER 2 02 0

27


http://www.skyandtelescope.org

Sky & Telescope - October 2020

Table of Contents for the Digital Edition of Sky & Telescope - October 2020

Contents
Sky & Telescope - October 2020 - Cover1
Sky & Telescope - October 2020 - Cover2
Sky & Telescope - October 2020 - 1
Sky & Telescope - October 2020 - Contents
Sky & Telescope - October 2020 - 3
Sky & Telescope - October 2020 - 4
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