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The mechanism that causes the prompt-emission episode of gamma-ray bursts(GRBs) is still widely debated despite there being thousands of promptdetections. The favoured internal shock model relates this emission tosynchrotron radiation. However, it does not always explain the spectral indicesof the shape of the spectrum, often fit with empirical functions.Multi-wavelength observations are therefore required to help investigate thepossible underlying mechanisms that causes the prompt emission. We present GRB121217A, for which we were able to observe its near-infrared (NIR) emissionduring a secondary prompt-emission episode with the Gamma-Ray Burst OpticalNear-infrared Detector (GROND) in combination with the Swift and Fermisatellites, covering an energy range of 0.001 keV to 100 keV. We determine aphotometric redshift of z=3.1+/-0.1 with a line-of-sight extinction of A_V~0mag, utilising the optical/NIR SED. From the afterglow, we determine a bulkLorentz factor of Gamma~250 and an emission radius of R<10^18 cm. Theprompt-emission broadband spectral energy distribution is well fit with abroken power law with b1=-0.3+/-0.1, b2=0.6+/-0.1 that has a break atE=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 suggesta magnetic field strength of B~10^5 G. However, all the best fit modelsunderpredict the flux observed in the NIR wavelengths, which also onlyrebrightens by a factor of ~2 during the second prompt emission episode, instark contrast to the X-ray emission, which rebrightens by a factor of ~100,suggesting an afterglow component is dominating the emission. We present GRB121217A one of the few GRBs for which there are multi-wavelength observationsof the prompt-emission period and show that it can be understood with asynchrotron radiation model.

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