Obviously intense EM rad close by but the intensity decreases as the root of distance from the point source so unless you were close, as others have said, normal space rad would be a greater threat.   No debris either as all matter would be vaporized and ionized.   Would prob be kind of like a ripple in the solar wind.  Really it's far worse exploding one at ground level and creating all those isotopic by products with nowhere to go.

Effects of nuclear explosions in space?
What size supply pipe must be used from a 5000 litre water tank to supply 4 bars of pressure to a house that is 20m below and 300m away from the tank?
