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We present a detailed study of the prompt and afterglow emission from SwiftGRB 061126 using BAT, XRT, UVOT data and multi-color optical imaging from tenground-based telescopes. GRB 061126 was a long burst (T_90=191 s) with fouroverlapping peaks in its gamma-ray light curve. The X-ray afterglow, observedfrom 26 min to 20 days after the burst, shows a simple power-law decay withalpha_X=1.290 \pm 0.008. Optical observations presented here cover the timerange from 258 s (Faulkes Telescope North) to 15 days (Gemini North) after theburst; the decay rate of the optical afterglow shows a steep-to-shallowtransition (from alpha_1=1.48 \pm 0.06 to alpha_2=0.88 \pm 0.03) approximately13 min after the burst. We suggest the early, steep component is due to areverse shock and show that the magnetic energy density in the ejecta,expressed as a fraction of the equipartion value, is a few ten times largerthan in the forward shock in the early afterglow phase. The ejecta might beendowed with primordial magnetic fields at the central engine. The opticallight curve implies a late-time break at about 1.5 days after the burst, whilethere is no evidence of the simultaneous break in the X-ray light curve. Wemodel the broad band emission and show that some afterglow characteristics (thesteeper decay in X-ray and the shallow spectral index from optical to X-ray)are difficult to explain in the framework of the standard fireball model. Thismight imply that the X-ray afterglow is due to an additional emission process,such as late time central engine activity rather than blast-wave shockemission. The possible chromatic break at 1.5 days after the burst would givesupport to the additional emission scenario.

Multiwavelength analysis of the intriguing GRB 061126: the reverse shock scenario and magnetization