Penn State University astronomers have discovered that the mysterious “cosmic whistles” known as fast radio bursts can pack a serious punch, in some cases releasing a billion times more energy in gamma-rays than they do in radio waves and rivaling supernovae in their explosive power. The discovery, the first-ever finding of non-radio emission from any fast radio burst, drastically raises the stakes for models of fast radio bursts and is expected to further energize efforts to chase down long-lived counterparts to FRBs using X-ray, optical, and radio telescopes.
FRBs, were first discovered in 2007, and in the years since radio astronomers have detected a few dozen of these events. Although they last mere milliseconds at any single frequency, their great distances from Earth—and large quantities of intervening plasma—delay their arrival at lower frequencies, spreading the signal out over a second or more and yielding a distinctive downward-swooping “whistle” across the typical radio receiver band.
“This discovery revolutionizes our picture of FRBs, some of which apparently manifest as both a whistle and a bang,” said Prof Derek Fox, a Penn State. The radio whistle can be detected by ground-based radio telescopes, while the gamma-ray bang can be picked up by high-energy satellites like NASA’s Swift mission. “Rate and distance estimates for FRBs suggest that, whatever they are, they are a relatively common phenomenon, occurring somewhere in the universe more than 2,000 times a day.”
Efforts to identify FRB counterparts began soon after their discovery but have all come up empty until now. In a paper published November 11 in Astrophysical Journal Letters the Penn State team, led by physics graduate student James DeLaunay, reports bright gamma-ray emission from the fast radio burst FRB 131104, named after the date it occurred, Nov 4, 2013. Discovery of the gamma-ray “bang” from FRB 131104, the first non-radio counterpart to any FRB, was made possible by Earth-orbiting Swift satellite, which was observing the exact part of the sky where FRB 131104 occurred as the burst was detected by the Parkes Observatory radio telescope in Parkes, Australia. “Swift is always watching the sky for bursts of X-rays and gamma-rays,” said Neil Gehrels, Goddard Space Flight Center. “What a delight it was to catch this flash from one of the mysterious fast radio bursts.”
“Although theorists had anticipated that FRBs might be accompanied by gamma rays, the gamma-ray emission we see from FRB 131104 is surprisingly long-lasting and bright,” Fox said. The duration of the gamma-ray emission, at 2-6 minutes, is many times the millisecond duration of the radio emission. And the gamma-ray emission from FRB 131104 outshines its radio emissions by more than a billion times, raising estimates of the burst’s energy requirements and suggesting severe consequences for the burst’s surroundings and host galaxy.
Two common models for gamma-ray emission from FRBs exist: one invoking magnetic flare events from magnetars—highly magnetized neutron stars that are the dense remnants of collapsed stars—and another invoking the catastrophic merger of two neutron stars, colliding to form a black hole. According to Prof Kohta Murase, “The energy release we see is challenging for the magnetar model unless the burst is relatively nearby. The long timescale of the gamma-ray emission, while unexpected in both models, might be possible in a merger event if we observe the merger from the side, in an off-axis scenario.”
“In fact, the energy and timescale of the gamma-ray emission is a better match to some types of supernovae, or to some of the supermassive black hole accretion events that Swift has seen,” Fox said. “The problem is that no existing models predict that we would see an FRB in these cases.”
http://phys.org/news/2016-11-cosmic-surprisingly-energetic.htmljCp
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