How far away are nuclear explosions detectable?

[james_davis_nicoll]

Thomas Womack's answer is rather startling:

"Ten megatons worth of soft X-rays, which is about 4*10^16 joules in
photons of about 10^-16 joules apiece; so about 4*10^32 photons, which
is about 0.3 per square metre on the surface of a one-light-year
sphere. So a large-area X-ray detector might be able to notice it,
particularly if it's detecting when the photons arrive and so can see
that it's a single sharp flash.

The Sun's energy output is 4*10^26 watts, whilst a big bomb might be
10^24 watts at the final stages of the doubling -- which makes it
absolute magnitude 11, and detectable with a photon-timing detector
watching through a ten-metre telescope anywhere in the Milky Way."