From what do you get numbers?
Not only in this case.
Radiation propagated to all directions (4pi steradian).
Distance to asteroid a few miles.
Bob mentioned asteroid’s mass M = 8.6 x 10^7 tonnes (metric tons).
It corresponds ~1.1 x 10^7 m3 of volume if asteroid is from iron and about 10^8 if from ice.
If asteroid spherical the diameter should has an order of a few hundred meters.
And if even if 100% radiation energy absorption much less than 40%. But some energy will be reflected.