  We investigate real-time tachyon dynamics of unstable D-brane carrying
fundamental string charge. We construct the boundary state relevant for rolling
of modulated tachyon with gauge fields excited on the world-volume, and study
spatial distribution of the fundamental string charge and current as the
D-brane decays. We find that, in contrast to homogeneous tachyon rolling,
spatial modulation of the tachyon field triggers density wave of strings when
electric field is turned on, and of string anti-string pairs when magnetic
field is turned on. We show that the energy density and the fundamental string
charge density are locked together, and evolve into a localized delta-function
array (instead of evolving into a string fluid) until a critical time set by
initial condition of rolling tachyon. When the gauge fields approach the
critical limit, the fundamental strings produced become BPS-like. We also study
the dynamics via effective field theory, and find agreement.
