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An extraordinary gamma-ray burst

A serendipitous close-up look challenges the standard theory of how the most energetic photons are generated.

Gamma-ray bursts (GRBs) signaling the explosive collapse of very distant stars are detected almost daily by a variety of orbiting telescopes. But last April's GRB 130427A, first recognized by the burst monitor aboard NASA's Fermi Gamma-ray Space Telescope, was a record breaker. The energy output of its initial 10-second gamma-ray pulse ranks it among the half dozen most intrinsically luminous GRBs ever recorded. But more important was its proximity, as measured by its modest redshift (z = 0.34). At a distance of "only" 5 billion light-years, it was five times closer than a typical GRB and by far the closest of the hyperluminous GRBs. That felicitous proximity—estimated to occur once or twice a century—has allowed Fermi, shown in the drawing, and an armada of x-ray and optical telescopes to examine the initial burst and its afterglow in unprecedented detail. The observations suggest that GRB 130427A is not an atypical outlier among hyperluminous GRBs. But the serendipitous close-up observations reveal details that challenge the prevailing model of how such GRBs generate their gamma-ray afterglows, with photon energies exceeding 109 eV (1 GeV). Those high-energy photons had been attributed to synchrotron radiation from electrons accelerated to even higher energies by the breakout of the GRB's shock wave from the confines of the collapsed parent star. But that long-accepted scenario cannot account for photon energies as high as 95 GeV recorded by Fermi in the afterglow. It's not yet clear whether the new revelations will require major revisions or just tweaks in the theory of gamma-ray bursts. (M. Ackermann et al., Science, in press.)—Bertram Schwarzschild


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