Astronomy

Gamma rays get an energy boost

Print edition : December 20, 2019

The fading afterglow of GRB 190114C and its galaxy imaged by the Hubble Space Telescope on February 11 and March 12.

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).

Astronomers first recognised the GRB phenomenon 46 years ago. They appeared at random locations in the sky about once a day, on an average. The most common type of GRB occurs when a star runs out of fuel. Its core collapses and forms a black hole, which 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 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 afterglows can be detected up to months, and rarely, even years, 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 a recent issue of “Nature” describe each of the discoveries.

On January 14, just before 4 p.m. EST (January 15, 2.30 a.m IST), 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. Following this, the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) observatory at La Palma in the Canary Islands, Spain, observed the GRB and captured the most energetic gamma rays seen from GRBs just 50 seconds after they were discovered.

The energy of visible light ranges from about 2 to 3 electron volts (eV). With GRB 190114C, MAGIC reported unambiguous Very High Energy (VHE) emission, with energies up to a trillion electron volts (1 TeV). The threshold for being termed VHE is 100 giga or billion eV (GeV). “The discovery of TeV gamma rays from GRB 190114C shows that these explosions are even more powerful than thought before,” said Razmik Mirzoyan of the Max Planck Institute for Physics in Munich. The burst’s optical afterglow captured in Hubble images acquired in February and March shows that the blast originated in a spiral galaxy about 4.5 billion light years away. This means the light from this GRB began travelling 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 (HESS) pointed its 28-metre gamma-ray telescope to the location of the burst, called GRB 180720B. Analysis by HESS revealed that VHE gamma rays with energies up to 440 GeV had been detected. Interestingly, the glow continued for two hours, long after detection, which is both a surprise and an important new discovery.

 

Both the HESS and MAGIC teams interpreted the VHE emission as a distinct afterglow component, which means some additional process must be at work. The best candidate is inverse Compton scattering. High-energy electrons in the jet crash into lower-energy gamma rays and boost them to much higher energies. “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. “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.

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