I'm not sure 'hard-science' is a good tag to apply here. You're looking at something which can transfer information faster than light can.

What do you mean with Ignore the length of time the energy weapon is activated?

@L.Dutch I think that means the victory condition for the defending ship is to avoid all photons of the blow. The question is more about a way to detect incoming attacks than about how the damage would be dealt anyway.

Red shirt: "Commander, we've just been scanned!" Commander: "Is it hostile? shields up and everybody change into yellow shirt please."

Energy beam is photons then it travels at light speed and nothing else can travel faster to arrive before the beam and allow the beam to be known of. However if you have an FTL mechanism in mind then there might be a way within that (but not with "hard science") to use that.

Easy defence: have the defnding ship coated witrha thick layer of material which ablates when struck by high-energy laser. This absorbs some of the energy,more importantly the repulsion effect generated by the jet of ablated material moved the ship out of the beam. Also, that energy is quite low for such a large spot size, it's not going to do that much damage.

@AlexP - The charging of a laser would involve its amplifying medium getting "pumped" until it reaches the "lasing threshold", see here. But if the OP is imagining a gigajoule laser I'd think that would require something like a nuclear pumped laser where the pumping would be done by a nuclear explosion, so I don't think a distant observer could get any more warning than the time between activating a nuclear bomb and the bomb exploding, which is very short.

@Halfthawed see my edit

@L.Dutch-ReinstateMonica what I mean is I don't want people focusing on the set-up, only on the consequence. I don't actually care if the defending ship is getting hit by a lidar speed gun.

@StephenG, see my edit

@AlexP see my edit

@DavidHambling that's not the question. See my response to L.Dutch.

@Hypnosifl see my response to L.Dutch

@Hypnosifl In that scenario, I suppose it is rather the attackers who want to be warned and make evasive manoevers

About the edits: information cannot travel faster than light in a vaccum, no matter how. When somebody turns on a light bulb, there is no way to learn that they turned on the light bulb before the arrival of light from the light bulb. The mechanism of information transfer is irrelevant. (The "in a vacuum" part is very important; stuff can travel faster than light in water etc.)

@HagenvonEitzen - What would the attackers need to be warned of? To be clear, the attackers would be the ones setting off a contained nuclear explosion on their own vessel in order to pump an amplifying medium which would then fire a powerful gigajoule laser at the defending ship.

I guess if we wanted to allow ridiculously advanced technology, we could imagine the ship is surrounded by a swarm of small wormholes which each have one end on board the ship, so observers on the ship can look through them to see distant events prior to the light from those events reaching them through normal (non-wormhole) space. Technically no signal travels faster than light in a local sense here, some light just takes a shortcut. Multiple wormholes in the same area would probably either lead to causality problems or to their being destroyed if the chronology protection conjecture is true.

...but the simpler solution is just to assume gigajoule lasers are ineffective or unfeasible with the technology levels available in the story, so instead they use some other type of weapon like beam of particles with nonzero mass, and even if these particles move at very close to the speed of light, as @AlexP said you could imagine it takes a couple seconds to "charge up" before firing and the charging can be detected from a distance.

@AlexP, so when a traveler from outside our solar system approaches the system, the turmoil of the heliosphere isn't enough to warn of light behind it? That's interesting.