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.
Credit: NASA/Fermi and Aurore Simonnet, Sonoma State University
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 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 astronomers have learned about GRBs over the past couple of decades has come from observing their afterglows at lower energies. Now, thanks to these new ground-based detections, they're seeing the gamma rays from GRBs in a whole new way.
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 detected 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 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. That’s 10 times the peak energy Fermi has seen to date.
Data from a different burst, which Fermi and Swift both discovered, confirm afterglows reach these energies. Ten hours after the 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.
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.
Ground-based facilities have detected radiation up to a trillion times the energy of visible light from a cosmic explosion called a gamma-ray burst. This illustration shows the set-up for the most common type. The core of a massive star (left) has collapsed and formed a black hole. This “engine” drives a jet of particles that moves through the collapsing star and out into space at nearly the speed of light. The prompt emission, which typically lasts a minute or less, may arise from the jet’s interaction with gas near the newborn black hole and from collisions between shells of fast-moving gas within the jet (internal shock waves). The afterglow emission occurs as the leading edge of the jet sweeps up its surroundings (creating an external shock wave) and emits radiation across the spectrum for some time — months to years, in the case of radio and visible light, and many hours at the highest gamma-ray energies yet observed. These far exceed 100 billion electron volts (GeV) for two recent GRBs.
The High Energy Stereoscopic System (H.E.S.S.) is an array of five ground-based gamma-ray telescopes located in Namibia in southern Africa. Four 12-meter telescopes — one is visible in the background — surround the larger H.E.S.S. II 28-meter telescope. As gamma rays impact the upper atmosphere, they strike air molecules and break them apart, creating showers of high-energy particles. These particles produce flashes of blue light called Cherenkov radiation, and a camera at the telescope’s focus records them. In July 2018, H.E.S.S. II detected gamma rays with energies up to 440 GeV from GRB 180720B, some of the highest energies ever seen from a gamma-ray burst.
This brief animation illustrates the mirror of one of the MAGIC telescopes at Roque de los Muchachos Observatory on La Palma, Canaray Islands. Two 17-meter telescopes detect flashes of blue light, called Cherenkov radiation, that signal the arrival of gamma rays in the upper atmosphere.