On Jan. 14, 2019, the Major Atmospheric Gamma
Imaging Cherenkov (MAGIC) observatory in the Canary Islands captured the
highest-energy light every recorded from a gamma-ray burst. MAGIC began
observing the fading burst just 50 seconds after it was detected thanks
to positions provided by NASA's Fermi and Swift spacecraft (top left
and right, respectively, in this illustration). The gamma rays packed
energy up to 10 times greater than previously seen. Credits: NASA/Fermi and Aurore Simonnet, Sonoma State University. Download high-resolution images from NASA’s Scientific Visualization Studio
The fading afterglow of GRB 190114C and its home
galaxy were imaged by the Hubble Space Telescope on Feb. 11 and March
12, 2019. The difference between these images reveals a faint,
short-lived glow (center of the green circle) located about 800
light-years from the galaxy’s core. Blue colors beyond the core signal
the presence of hot, young stars, indicating that this is a spiral
galaxy somewhat similar to our own. It is located about 4.5 billion
light-years away in the constellation Fornax. Credits: NASA, ESA, and V. Acciari et al. 2019. Download high-resolution images from NASA’s Scientific Visualization Studio
A pair of distant explosions discovered by NASA’s Fermi Gamma-ray Space Telescope and Neil Gehrels Swift Observatory
have produced the highest-energy light yet seen from these events,
called gamma-ray bursts (GRBs). The record-setting detections, made by
two different ground-based observatories, provide new insights into the
mechanisms driving gamma-ray bursts.
Astronomers first recognized the GRB phenomenon 46 years ago. The blasts appear at random locations in the sky about once a day, on average.
The most common type of GRB occurs when a star much more massive than
the Sun runs out of fuel. Its core collapses and forms a black hole,
which then blasts jets of particles outward at nearly the speed of
light. These jets pierce the star and continue into space. They produce
an initial pulse of gamma rays — the most energetic form of light — that
typically lasts about a minute.
As the jets race outward, they interact with surrounding gas and emit
light across the spectrum, from radio to gamma rays. These so-called
afterglows can be detected up to months — and rarely, even years — after
the burst at longer wavelengths.
“Much of what we’ve learned about GRBs over the past couple of
decades has come from observing their afterglows at lower energies,”
said Elizabeth Hays, the Fermi project scientist at NASA’s Goddard Space
Flight Center in Greenbelt, Maryland. “Now, thanks to these new
ground-based detections, we’re seeing the gamma rays from gamma-ray
bursts in a whole new way.”
Two papers published in the journal Nature describe each of the discoveries. A third paper analyzes one of the bursts using a rich set of multiwavelength data from observatories in space and on the ground. A fourth paper, accepted by The Astrophysical Journal, explores the Fermi and Swift data in greater detail.
On Jan. 14, 2019, just before 4 p.m. EST, both the Fermi and Swift
satellites detected a spike of gamma rays from the constellation Fornax.
The missions alerted the astronomical community to the location of the
burst, dubbed GRB 190114C.
One facility receiving the alerts was the Major Atmospheric Gamma Imaging Cherenkov (MAGIC)
observatory, located on La Palma in the Canary Islands, Spain. Both of
its 17-meter telescopes automatically turned to the site of the fading
burst. They began observing the GRB just 50 seconds after it was
discovered and captured the most energetic gamma rays yet seen from
these events.
The energy of visible light ranges from about 2 to 3 electron volts.
In 2013, Fermi’s Large Area Telescope (LAT) detected light reaching an
energy of 95 billion electron volts (GeV), then the highest seen from a
burst. This falls just shy of 100 GeV, the threshold for so-called very
high-energy (VHE) gamma rays. With GRB 190114C, MAGIC became the first facility to report unambiguous VHE emission, with energies up to a trillion electron volts (1 TeV). That’s 10 times the peak energy Fermi has seen to date.
“Twenty years ago, we designed MAGIC specifically to search for VHE
emission from GRBs, so this is a tremendous success for our team,” said
co-author Razmik Mirzoyan, a scientist at the Max Planck Institute for
Physics in Munich and the spokesperson for the MAGIC collaboration. “The
discovery of TeV gamma rays from GRB 190114C shows that these
explosions are even more powerful than thought before. More importantly,
our detection facilitated an extensive follow-up campaign involving
more than two dozen observatories, offering important clues to the
physical processes at work in GRBs.”
These included NASA’s NuSTAR mission, the European Space Agency’s XMM-Newton X-ray satellite, the NASA/ESA Hubble Space Telescope,
in addition to Fermi and Swift, along with many ground-based
observatories. Hubble images acquired in February and March captured the
burst’s optical afterglow. They show that the blast originated in a
spiral galaxy about 4.5 billion light-years away. This means the light
from this GRB began traveling to us when the universe was two-thirds of
its current age.
Another paper presents observations of a different burst, which Fermi
and Swift both discovered on July 20, 2018. Ten hours after their
alerts, the High Energy Stereoscopic System (H.E.S.S.)
pointed its large, 28-meter gamma-ray telescope to the location of the
burst, called GRB 180720B. A careful analysis carried out during the
weeks following the event revealed that H.E.S.S. clearly detected VHE gamma rays with energies up to 440 GeV.
Even more remarkable, the glow continued for two hours following the
start of the observation. Catching this emission so long after the GRB’s
detection is both a surprise and an important new discovery.
Scientists suspect that most of the gamma rays from GRB afterglows
originate in magnetic fields at the jet’s leading edge. High-energy
electrons spiraling in the fields directly emit gamma rays through a
mechanism called synchrotron emission.
But both the H.E.S.S. and MAGIC teams interpret the VHE emission as a
distinct afterglow component, which means some additional process must
be at work. The best candidate, they say, is inverse Compton scattering.
High-energy electrons in the jet crash into lower-energy gamma rays and
boost them to much higher energies.
In the paper detailing the Fermi and Swift observations, the
researchers conclude that an additional physical mechanism may indeed be
needed to produce the VHE emission. Within the lower energies observed
by these missions, however, the flood of synchrotron gamma rays makes
uncovering a second process much more difficult.
“With Fermi and Swift, we don’t see direct evidence of a second
emission component,” said Goddard’s S. Bradley Cenko, the principal
investigator for Swift and a co-author of the Fermi-Swift and
multiwavelength papers. “However, if the VHE emission arises from the
synchrotron process alone, then fundamental assumptions used in
estimating the peak energy produced by this mechanism will need to be
revised.”
Future burst observations will be needed to clarify the physical
picture. The new VHE data open a new pathway for understanding GRBs, one
that will be further extended by MAGIC, H.E.S.S. and a new generation of ground-based gamma-ray telescopes now being planned.
The Fermi Gamma-ray Space Telescope is an astrophysics and particle
physics partnership managed by NASA's Goddard Space Flight Center in
Greenbelt, Maryland. Fermi was developed in collaboration with the U.S.
Department of Energy, with important contributions from academic
institutions and partners in France, Germany, Italy, Japan, Sweden and
the United States.
Goddard manages the Swift mission in collaboration with Penn State in
University Park, the Los Alamos National Laboratory in New Mexico and
Northrop Grumman Innovation Systems in Dulles, Virginia. Other partners
include the University of Leicester and Mullard Space Science Laboratory
in the United Kingdom, Brera Observatory and the Italian Space Agency
in Italy.
Editor: Rob Garner
Source: NASA/Fermi Space Telescope
