[Atlas Sneezed]

The bright flash would simply be the reflected light of the sun off the clould of particles. Kind of like how a cloud is bright white when sunlit. This is especially true for an event at or near the terminator, as above.

[RightWhale]

The kinetic energy of an asteroid stopping suddenly would be enough energy to ionize the body of the asteroid, which would be accompanied by a release of photons, AKA a bright flash. While not in the nuclear range, it would be easily match TNT pound for pound.