  We present a broadband study of gamma-ray burst (GRB) 091024A within the
context of other ultra-long-duration GRBs. An unusually long burst detected by
Konus-Wind, Swift, and Fermi, GRB 091024A has prompt emission episodes covering
~1300 s, accompanied by bright and highly structured optical emission captured
by various rapid-response facilities, including the 2-m autonomous robotic
Faulkes North and Liverpool Telescopes, KAIT, S-LOTIS, and SRO. We also
observed the burst with 8- and 10-m class telescopes and determine the redshift
to be z = 1.0924 \pm 0.0004. We find no correlation between the optical and
gamma-ray peaks and interpret the optical light curve as being of external
origin, caused by the reverse and forward shock of a highly magnetized jet (R_B
~ 100-200). Low-level emission is detected throughout the near-background
quiescent period between the first two emission episodes of the Konus-Wind
data, suggesting continued central-engine activity; we discuss the implications
of this ongoing emission and its impact on the afterglow evolution and
predictions. We summarize the varied sample of historical GRBs with
exceptionally long durations in gamma-rays (>~ 1000 s) and discuss the
likelihood of these events being from a separate population; we suggest
ultra-long GRBs represent the tail of the duration distribution of the long GRB
population.
