Research Articles

Gamma-ray bursts (GRBs) are thought to originate from collapsing mas-sive stars or merging compact objects (such as neutron stars or black holes), and are associated with the formation of black holes in distant galaxies. GRB 130427A was detected by both the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) aboard the Fermi Gamma-ray Space Telescope. The LAT is a pair-conversion telescope that ob-serves photons from 20 MeV to> 300 GeV with a 2.4 steradian field of view (1). The GBM consists of 12 sodium iodide (NaI, 8 keV- 1 MeV) and 2 bismuth germanate (BGO, 200 keV-40 MeV) detectors, positioned around the spacecraft to view the entire unoc-culted sky (2). In the standard model of GRBs, the blast wave that produces the initial, bright prompt emission later collides with the external material surrounding the GRB (the circumburst medium) and creates shocks (see, e.g., (3)). These external shocks accelerate charged particles, which produce photons through synchrotron radiation. Until this burst, the high-energy emission from LAT-detected GRBs had been well described by this model, but GRB 130427A challenges this widely ac-cepted model. In particular, the maxi-mum possible photon energy prescribed by this model is surpassed by the the late-time high-energy photons detected by the LAT. The LAT detected high-energy γ-ray emission from this burst for almost a day, including a 95 GeV photon (which was emitted at 128 GeV in the rest frame at redshift z = 0.34 (4)) a few minutes after the burst began and a 32 GeV photon (43 GeV in the rest frame) after more than 9 hours. These are more energetic and detected at con-siderably later times than the previous record holder, an 18 GeV photon de-tected by the Energetic Gamma Ra