Evidence for energy injection and a fine-tuned central engine at optical
  wavelengths in GRB 070419A
  We present a comprehensive multiwavelength temporal and spectral analysis of
the FRED GRB 070419A. The early-time emission in the $\gamma$-ray and X-ray
bands can be explained by a central engine active for at least 250 s, while at
late times the X-ray light curve displays a simple power-law decay. In
contrast, the observed behaviour in the optical band is complex (from 10$^2$ up
to 10$^6$ s). We investigate the light curve behaviour in the context of the
standard forward/reverse shock model; associating the peak in the optical light
curve at $\sim$450 s with the fireball deceleration time results in a Lorenz
factor $\Gamma \approx 350$ at this time. In contrast, the shallow optical
decay between 450 and 1500 s remains problematic, requiring a reverse shock
component whose typical frequency is above the optical band at the optical peak
time for it to be explained within the standard model. This predicts an
increasing flux density for the forward shock component until t $\sim$ 4
$\times$ 10$^6$ s, inconsistent with the observed decay of the optical emission
from t $\sim$ 10$^4$ s. A highly magnetized fireball is also ruled out due to
unrealistic microphysic parameters and predicted light curve behaviour that is
not observed. We conclude that a long-lived central engine with a finely tuned
energy injection rate and a sudden cessation of the injection is required to
create the observed light curves - consistent with the same conditions that are
invoked to explain the plateau phase of canonical X-ray light curves of GRBs.
