Simultaneous optical/gamma-ray observations of GRB 121217’s prompt emission

Since the advent of the Swift satellite it has been possible to obtain pre-cise localisations of GRB positions of sub-arcsec accuracy within seconds, facilitating ground-based robotic telescopes to automatically slew to the tar-get within seconds. This has yielded a plethora of observational data for the afterglow phase of the GRB, but the quantity of data (<2KeV) covering the initial prompt emission still remains small. Only in a handful of cases has it been possible obtain simultaneous coverage of the prompt emission in a multi-wavelength regime (gamma-ray to optical), as a result of: observing the field by chance prior to the GRB (e.g. 080319B/naked-eye burst), long-prompt emission (e.g., 080928, 110205A) or triggered on a pre-cursor (e.g., 041219A, 050820A, 061121). This small selection of bursts have shown both correlated and uncorrelated gamma-ray and optical light curve behaviour, and the multi-wavelength emission mechanism remains far from resolved (i.e. single popu-lation synchrotron self-Component, electron distributions, additional neutron components or residual collisions). Such multi-wavelength observations dur-ing the GRB prompt phase are pivotal in providing further insight on the poorly understood prompt emission mechanism. We add to this small sample the Swift burst 121217A that had two distinct periods of prompt emission separ-ated by 700 s, observed by Swift/BAT, Swift/XRT and Fermi/GBM. As a result of the time delay of the second emission, it enabled optical imaging (from 3 to 7 bands) to be taken with the GROND instrument to a resolution as fine as 10s. This multi-wavelength data will hopefully allow us to shed more light on the current picture of prompt emission physics