Cosmic Whistle Packs a Surprisingly Energetic Punch

An artist conception of the SGR 1806-20 magnetar including magnetic field lines. After the initial flash, smaller pulsations in the data suggest hot spots on the rotating magnetar’s surface. The data also shows no change in the magentar’s rotation after the initial flash. 

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, November 4, 2013. “I started this search for FRB counterparts without expecting to find anything,” said DeLaunay. “This burst was the first that even had useful data to analyze. When I saw that it showed a possible gamma-ray counterpart, I couldn’t believe my luck!”

Discovery of the gamma-ray “bang” from FRB 131104, the first non-radio counterpart to any FRB, was made possible by NASA’s 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, the mission’s Principal Investigator and chief of the Astroparticle Physics Laboratory at NASA’s 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 two to six 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, dramatically 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 coauthor Kohta Murase, a Penn State professor and theorist, “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.”