Analyzing data from a lightning mapper and a small portable radiation detector unexpectedly shed light on what a lightning gamma-ray burst might look like – by observing the neutrons generated from the ground by many very heavy showers of cosmic rays. The work took place at the High Altitude Water Cherenkov Cosmic Ray Observatory (HAWC) in Mexico.
“It was an accidental discovery,” said Greg Bowers, a scientist at the Los Alamos National Laboratory and lead author of the study published in Geophysical research letters. “We set up this system to study terrestrial gamma flashes – or lightning gamma-ray bursts – which are usually so bright you can see them from space. The idea was that HAWC would be sensitive to gamma-ray bursts, so we installed a lightning mapper to capture the developing anatomy of lightning and identify the lightning processes that produce them. “
The team, including Xuan-Min Shao and Brenda Dingus also from Los Alamos, used a small, portable particle detector, hoping that a flash of terrestrial gamma rays would generate a clear gamma signal in the small particle detector.
“Our system worked for almost two years and we saw a lot of lightning,” Bowers said. But during these storms, they didn’t observe anything that looked like terrestrial gamma flashes. “However, we saw great count rate gusts on the clear, clear days, which made our heads scratch our heads.”
HAWC data collected at that time showed that, in any case, the large network that includes HAWC had been overwhelmed by extremely large cosmic ray rains – so large that Los Alamos researchers could not estimate their size.
David Smith, a UC Santa Cruz collaborator, discovered that these fine weather bursts had previously been observed by scientists in Russia, who called them “neutron bursts,” and determined that they were the result of the production of neutrons in the ground around the point of impact of the cosmic ray. shower cores.
Previous work that simulated these events had only considered hadrons – a type of subatomic particle – at the heart of downpours. In addition to hadrons and other particles, cosmic ray shower nuclei also contain a lot of gamma rays.
For this work, William Blaine, also from Los Alamos, simulated large showers of cosmic rays and included both hadrons and gamma rays. “We were able to match our observations with the simulations,” Bowers said. “We found that gamma rays produce the same type of neutron burst as hadrons.”
This study suggests that any natural phenomenon that produces a beam of gamma rays pointed at the ground (such as flashes of descending terrestrial gamma rays) could produce a similar signature of “neutron burst”. This is important for future modeling efforts to observe terrestrial gamma flashes.
“This tells us that you can’t just model the gamma rays hitting your detector, you will also have to take into account the neutron burst that occurs nearby,” Bowers said.
The HAWC Observatory includes a collection of water-filled reservoirs on the flanks of the Sierra Negra volcano in Puebla, Mexico, where the thin atmosphere provides better conditions for observing gamma rays. When gamma rays strike molecules in the atmosphere, they produce showers of energetic particles. When some of these particles strike the water inside the HAWC detector tanks, they produce flashes of light called Cherenkov radiation. By studying these Cherenkov lightning, researchers are reconstructing the sources of the downpours to learn more about the particles that caused them.
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