  An intriguing hypothesis, that there exists a decaying X-ray emission
component before the GRB trigger, has been suggested in order to explain the
shallow decay phase of the X-ray afterglow detected in many Swift GRBs. If this
"prior emission" is from an external shock, one would expect a corresponding
optical emission component during the GRB prompt emission phase. In this paper
we apply the available prompt optical emission data (both detections and upper
limits) to constrain such a scenario. We fit the shallow and normal decay
segments of the XRT light curves in our sample with a T_\Delta-shifted single
power law, and extrapolate the X-ray flux back to the time of the early optical
observations. We then use the synchrotron spectrum predicted by the standard
external shock model to extrapolate from the X-ray flux to the optical band,
and obtain the possible range of the predicted optical flux. Finally, we
compare the predictions with the observations. In the cases where later optical
data are available, we also compare the shapes of the optical light curves to
the predicted optical light curves from the external shock prior emission
model. We find that for a good fraction of GRBs (4 out of 8; up to December
2006), the available data already impose severe constraints on the hypothesis.
In particular, the expected optical flux from the prior external shock model is
higher than what the data allow. We conclude that if the shallow-decay X-ray
component were from a prior emission component, it would have to be of an
internal origin with optical flux suppressed.
