There are celestial sources of RF energy but not generally of narrowband signals. The fact that the signal would be described as "unmodulated" is actually one of its interesting features, in that the narrow bandwidth rules out many celestial sources. It does not rule out all celestial sources, there are certain objects (masers) that could produce such a signal, but they aren't short-term transients and we should have been able to locate it again. So the fact that this was a relatively narrowband signal and had such a short duration makes it pretty consistent with the idea of some sort of artificial communications use.

The instrumentation that detected the signal was very limited in its capabilities, so we have to take the "no detectable modulation" in that context. All we know is that it was largely confined to a 10kHz channel. Many forms of radio communications we use here on earth would not have detectable modulation with this equipment. More recent receivers for SETI purposes collect far smaller bins in both dimensions and would be able to discern the modulation methods we know of, but such are the limitations of data collected in the '70s.

The "Wow signal" being the result of a natural process is certainly a possibility, but its characteristics exclude the natural processes we know about right now. So it's interesting either way: it's either an unknown natural phenomenon, of artificial origin, or (and this possibility cannot be discounted) some sort of measurement error or terrestrial source that we aren't able to figure out now. Even the determination that it was terrestrial noise would be interesting in the context of how to interpret data from radio telescopes.

From the article:

> any modulation with a period of less than 10 seconds or longer than 72 seconds would not have been detectable.

With a carrier of 1420 MHz, I'd expect modulation at a much higher frequency than 0.1 Hz.

The narrow band and power of the signal is what made it stand out.

They are looking for signs of life.

Anything they see that is not certainly a sign of life will be treated as a potential sign of life.

It's special because it's 72 seconds of 0.001% in years of 0.000000000000000000001%