Fermi/GBM observations of the ultra-long GRB 091024 A burst with an optical flash

Aims. In this paper we examine gamma-ray and optical data of GRB 091024, a gamma-ray burst (GRB) with an extremely long duration of T-90 approximate to 1020 s, as observed with the Fermi Gamma-ray Burst Monitor (GBM). Methods. We present spectral analysis of all three distinct emission episodes using data from Fermi/GBM. Because of the long nature of this event, many ground-based optical telescopes slewed to its location within a few minutes and thus were able to observe the GRB during its active period. We compare the optical and gamma-ray light curves. Furthermore, we estimate a lower limit on the bulk Lorentz factor from the variability and spectrum of the GBM light curve and compare it with that obtained from the peak time of the forward shock of the optical afterglow. Results. From the spectral analysis we note that, despite its unusually long duration, this burst is similar to other long GRBs, i.e. there is spectral evolution (both the peak energy and the spectral index vary with time) and spectral lags are measured. We find that the optical light curve is highly anti-correlated to the prompt gamma-ray emission, with the optical emission reaching the maximum during an epoch of quiescence in the prompt emission. We interpret this behavior as the reverse shock (optical flash), expected in the internal-external shock model of GRB emission but observed only in a handful of GRBs so far. The lower limit on the initial Lorentz factor deduced from the variability time scale (Gamma(min) = 195(-110)(+ 90)) is consistent within the error to the one obtained using the peak time of the forward shock (Gamma(0) = 120) and is also consistent with Lorentz factors of other long GRBs.