  The injection of electrons into a laser wakefield accelerator (LWFA) is
observed to generate an intense coherent ultra-broadband and ultrashort pulse
radiation flash, consistent with the acceleration of electrons from rest to
nearly the speed of light in a distance < ~ 1 $\mu$m. The flash is sufficiently
bright to induce large nonlinear refractive index shifts in optical materials;
we estimate a source brightness temperature of ~$10^{18}$ K. We present
measurements of the flash spectra, coherence, pulse duration, polarization and
angular distribution, providing a detailed picture of electron injection
dynamics in LWFA. These are characteristic of laser-assisted injection of
off-axis electrons, which preserves wake coherence.
