Sky & Telescope - August 2024 - 18

High-energy Astrophysics
the emission faded quickly, falling under MAGIC's detection
limits within half an hour. Follow-up observations revealed
that the GRB came from a galaxy located several billion lightyears
from Earth.
The high-energy photons MAGIC detected were unexpected.
Astronomers had thought the photons in these bursts
were synchrotron radiation, which is caused by electrons
spiraling along magnetic field lines. But synchrotron radiation
cannot reach such high energies. The finding prompted
researchers to think that a different process could be at work:
inverse Compton scattering, in which photons gain energy after
colliding with speedy electrons. This is the first evidence of
this process occurring in GRBs.
MAGIC has also dipped into the realm of multi-messenger
astronomy by determining the source of a cosmic
neutrino detected by the IceCube experiment in Antarctica.
Neutrinos are extremely hard-to-detect subatomic particles
with no electrical charge and barely any mass (S&T: May
2023, p. 14). Astronomers are actively trying to track cosmic
neutrinos back to their sources in order to understand energetic
physical environments in the universe, but they've had
minimal success.
On September 22, 2017, IceCube detected a very highenergy
neutrino and pinpointed its arrival direction with
some accuracy. The Fermi satellite revealed that the arrival
direction of the neutrino aligned with the location of a flaring
supermassive black hole named TXS 0506+056, whose
light had traveled nearly 4 billion years to reach Earth.
MAGIC then measured the gamma rays the black hole emitted,
which reached 400 GeV, making the TXS 0506+056 black
105
X-rays
GeV gamma rays
TeV gamma rays
104
Einstein & EXOSAT
Against the Elements
The LSTs are designed to withstand winds up to
200 km/h (more than 120 mph) in their parked
positions, lest they become giant windsails and be
blown away. In 2020 the prototype survived winds
above 170 km/h with minimal damage.
hole the most probable neutrino source candidate to date.
" It's very important to try to pinpoint the sources, " Lindfors
says. The neutrino's high energy suggests whatever made
it was an extreme particle accelerator. " Of course you want
to know if it's some violent phenomena in the universe you
already know, or if it's something completely new. "
MAGIC also revealed that pulsars can emit high-energy
gamma rays. There are a few thousand known pulsars, but
only about 400 of them emit gamma rays. In 2008, MAGIC
was the first telescope to measure gamma rays above the
25-GeV range coming from the Crab pulsar, the neutron star
whose supernova in 1054 produced the Crab Nebula. In 2016,
MAGIC scientists announced they'd detected even higher
energies coming from the Crab pulsar, clocking in at a whopping
1.5 TeV.
These findings created a lot of trouble for the models that
Fermi/LAT (3FGL)
Fermi/LAT (1FGL)
103
HEAO A-1
Uhuru (4U)
10
2
Uhuru (2U)
Rockets &
balloons
COS-B (2CG)
10
Sounding
rockets
COS-B (CG)
SAS-2
Whipple (+Mrk 501)
1
1960
Giacconi
et al.
(Sco X-1)
McBreen
et al. (Crab)
1970
Whipple (Crab)
Whipple (+Mrk 421)
1980
18 AUGUST 2024 * SKY & TELESCOPE
1990
Year
Macomb & Gehrels
EGRET (3EG)
EGRET (2EG)
explain how pulsars produce gamma rays, directly ruling
out some of those scenarios. Theoreticians are still trying to
come up with new models that could account for such high
photon energies.
The remnants of smaller stars can also produce gamma
rays, it turns out. Novae are
thermonuclear explosions
that occur when a white dwarf
- the corpse of a star like
our Sun when it has burned
through all the fuel in its core
- steals too much gas from
a companion star (S&T: Apr.
2023, p. 36). The gas builds up
on the white dwarf's surface
until the extreme pressure and
temperature incite a runaway
nuclear reaction. These explosions
are luminous, temporarily
shining as brightly as
10,000 Suns, but not cataclysmic.
Sometimes, astronomers
Fermi/LAT (4FGL)
Fermi/LAT
(4FGL-DR2)
Fermi/LAT
(4FGL-DR4)
Fermi/LAT
(4FGL-DR3)
3rd-generation IACTs:
HESS, MAGIC, VERITAS
+HAWC, LHAASO
& LST
2nd-generation IACTs:
Whipple, HEGRA, CAT,
CANGAROO, TA
2000
2010
2020
GROWING FIELD In the last
60 years, the number of sources
detected at X-ray and gamma-ray
energies has increased substantially.
Astronomers hope that CTAO will increase
the tally of the highest-energy
gamma-ray sources (orange) to more
than 1,000.
Number of sources
STEPHEN FEGAN / CC AT-NC-SA 4.0 LICENSE
Sky & Telescope - August 2024

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