Fermi/GBM observations of the ultra-long GRB 091024: A burst with an
  optical flash
  In this paper we examine gamma-ray and optical data of GRB 091024, a
gamma-ray burst (GRB) with an extremely long duration of T90~1020 s, as
observed with the Fermi Gamma-Ray Burst Monitor (GBM). 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. 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.
