Prompt emission of GRB 121217A from gamma-rays to the NIR

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, often fit with empirical functions. 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, covering an energy range of 0.001 keV to 100 keV. We determine a photometric redshift of z=3.1+/-0.1 with a line-of-sight extinction of A_V~0 mag, utilising the optical/NIR SED. From the afterglow, we determine a bulk Lorentz factor of Gamma~250 and an emission radius of R<10^18 cm. The prompt-emission broadband spectral energy distribution is well fit with a broken power law with b1=-0.3+/-0.1, b2=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~10^5 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, suggesting an afterglow component is dominating the emission. We present GRB 121217A one of the few GRBs for which there are multi-wavelength observations of the prompt-emission period and show that it can be understood with a synchrotron radiation model.

13 pages, 8 figures, 6 tables, accepted for publication in Astronomy & Astrophysics

References in corpus (5)

• An online repository of Swift/XRT light curves of GRBs
• The Internal-Collision-Induced Magnetic Reconnection and Turbulence (ICMART) Model of Gamma-Ray Bursts
• GROND - a 7-channel imager
• Observations of the Naked-Eye GRB 080319B: Implications of Nature's Brightest Explosion
• Energy input and response from prompt and early optical afterglow emission in gamma-ray bursts

Cited by in corpus (16)

• Physics of Gamma-Ray Bursts Prompt Emission
• Prompt optical emission as a signature of synchrotron radiation in gamma-ray bursts
• Highly Luminous Supernovae associated with Gamma-Ray Bursts I.: GRB 111209A/SN 2011kl in the Context of Stripped-Envelope and Superluminous Supernovae
• A morphological analysis of gamma-ray burst early optical afterglows
• Capturing the electromagnetic counterparts of binary neutron star mergers through low latency gravitational wave triggers
• The sharpness of gamma-ray burst prompt emission spectra
• A Correlated Study of Optical and X-ray Afterglows of GRBs
• The Optical/NIR afterglow of GRB 111209A: Complex yet not Unprecedented
• GROND coverage of the main peak of Gamma-Ray Burst 130925A
• A Large Catalogue of Multi-wavelength GRB Afterglows I: Color Evolution And Its Physical Implication
• Fallback accretion on to a newborn magnetar: long GRBs with giant X-ray flares
• Limits on optical polarization during the prompt phase of GRB 140430A
• Hunting Gravitational Waves with Multi-Messenger Counterparts: Australia's Role
• The Spectral Sharpness Angle of Gamma-ray Bursts
• The Double-Burst Nature and Early Afterglow Evolution of Long GRB 110801A
• Simultaneous optical/gamma-ray observations of GRB 121217's prompt emission