Virgili, F. J.Mundell, C. G.Pal, V.Guidorzi, C.Margutti, R.Melandri, A.Harrison, R.Kobayashi, S.Chornock, R.Henden, A.Updike, A. C.Cenko, S. B.Tanvir, N. R.Steele, I. A.Cucchiara, A.Gomboc, A.Levan, A.Cano, Z.Mottram, C. J.Clay, N. R.Bersier, D.Kopač, D.Japelj, J.Filippenko, A. V.Li, W.Svinkin, D.Golenetskii, S.Hartmann, D. H.Milne, P. A.Williams, G.O, P. T.Fox, D. B.Berger, Edo2017-03-022013Virgili, F. J., C. G. Mundell, V. Pal’shin, C. Guidorzi, R. Margutti, A. Melandri, R. Harrison, et al. 2013. “Grb 091024a and the Nature of Ultra-Long Gamma-Ray Bursts.” The Astrophysical Journal 778 (1) (November 1): 54. doi:10.1088/0004-637x/778/1/54. http://dx.doi.org/10.1088/0004-637x/778/1/54.0004-637Xhttp://nrs.harvard.edu/urn-3:HUL.InstRepos:30496613We 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 ± 0.0004. We find no correlation between the optical and γ-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 (RB ≈ 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.en-USgamma-ray burst: generalgamma-ray burst: individualJournal Article2017-03-0210.1088/0004-637x/778/1/54