  We present a detailed study of the prompt and afterglow emission from Swift
GRB 061126 using BAT, XRT, UVOT data and multi-color optical imaging from ten
ground-based telescopes. GRB 061126 was a long burst (T_90=191 s) with four
overlapping peaks in its gamma-ray light curve. The X-ray afterglow, observed
from 26 min to 20 days after the burst, shows a simple power-law decay with
alpha_X=1.290 \pm 0.008. Optical observations presented here cover the time
range from 258 s (Faulkes Telescope North) to 15 days (Gemini North) after the
burst; the decay rate of the optical afterglow shows a steep-to-shallow
transition (from alpha_1=1.48 \pm 0.06 to alpha_2=0.88 \pm 0.03) approximately
13 min after the burst. We suggest the early, steep component is due to a
reverse shock and show that the magnetic energy density in the ejecta,
expressed as a fraction of the equipartion value, is a few ten times larger
than in the forward shock in the early afterglow phase. The ejecta might be
endowed with primordial magnetic fields at the central engine. The optical
light curve implies a late-time break at about 1.5 days after the burst, while
there is no evidence of the simultaneous break in the X-ray light curve. We
model the broad band emission and show that some afterglow characteristics (the
steeper 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. This
might imply that the X-ray afterglow is due to an additional emission process,
such as late time central engine activity rather than blast-wave shock
emission. The possible chromatic break at 1.5 days after the burst would give
support to the additional emission scenario.
