Prompt emission of GRB 121217A from gamma-rays to the near-infrared^⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
e-mail: jonnyelliott@mpe.mpg.de
² Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
³ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking Surrey, RH5 6NT, UK
⁴ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁵ Department of Physics and Astronomy, University of Nevada, Las Vegas, NV89154, USA
⁶ University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁷ Center for Research and Exploration in Space Sciences and Technology, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horská 3a/22, 12800 Prague, Czech Republic
⁹ Astrophysics Science Division, NASA, Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁰ Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA
¹¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 København Ø, Denmark
¹² European Southern Observatory, Alonso de Córdova 3107, Vitacura, 19001 Casilla, Santiago 19, Chile

Received: 3 September 2013
Accepted: 13 December 2013

Abstract

The mechanism that causes the prompt-emission episode of gamma-ray bursts (GRBs) is still widely debated despite there being thousands of prompt detections. The favoured internal shock model relates this emission to synchrotron radiation. However, it does not always explain the spectral indices of the shape of the spectrum, which is often fit with empirical functions, such as the Band function. Multi-wavelength observations are therefore required to help investigate the possible underlying mechanisms that causes the prompt emission. We present GRB 121217A, for which we were able to observe its near-infrared (NIR) emission during a secondary prompt-emission episode with the Gamma-Ray burst Optical Near-infrared Detector (GROND) in combination with the Swift and Fermi satellites, which cover an energy range of 5 orders of magnitude (10^-3  keV to 100  keV). We determine a photometric redshift of z = 3.1    ±    0.1 with a line-of-sight with little or no extinction (A_V ~ 0   mag) utilising the optical/NIR SED. From the afterglow, we determine a bulk Lorentz factor of Γ₀ ~ 250 and an emission radius of R < 10¹⁸   cm. The prompt-emission broadband spectral energy distribution is well fit with a broken power law with β₁ = −0.3    ±    0.1 and β₂ = 0.6    ±    0.1 that has a break at E = 6.6    ±    0.9  keV, which can be interpreted as the maximum injection frequency. Self-absorption by the electron population below energies of E_a < 6  keV suggest a magnetic field strength of B ~ 10⁵   G. However, all the best fit models underpredict the flux observed in the NIR wavelengths, which also only rebrightens by a factor of ~2 during the second prompt emission episode, in stark contrast to the X-ray emission, which rebrightens by a factor of ~100. This suggests an afterglow component is dominating the emission. We present GRB 121217A, one of the few GRBs that has multi-wavelength observations of the prompt-emission period and shows that it can be understood with a synchrotron radiation model. However, due to the complexity of the GRB’s emission, other mechanisms that result in Band-like spectra cannot be ruled out.