The backstory is that aliens arrive at the edge of our system, somewhere beyond Pluto. This is because around the sun is a 3d bubble of anti FTL travel, no need to explore this now I just want to add context.
I asked here about what to do to solve the problem of detection. Great ideas and contribution followed. So after we know they are coming in a week we can proceed.
The fleet that arrives is mostly non military ships filled with a splinter group of radical aliens that seceded from a backwater system of theirs under the charismatic leader of this demagogue. This changes everything about the premise but we want to limit the question here.
I want a communication technology that they might be using that we can detect and figure out what it means.
- The aliens don't have full control of their ships. The advanced assisted intelligence that runs it handles most of the operations of that. Those are civilian ships and so while they can travel freely they are hardcoded not to respond to actions that endangers them. However I want feel like cheating and say the AI just threw us a bone or something. The more reasonable the answer is the better.
- The message or communication might be an accident, again they are not an actual military but rather a bunch of normal people, or even the ships automated system trying to establish communication with other ships of their fleet or so.
- Could it be horny teenager looking for human porn? Sure. A curious scientist that wants to learn about us? Maybe.
- The aliens are not stupid and their technology is not stupid. However they can be using all sort of ways to communicate as they not only communicate with themselves but also less advanced species. So their ships should be able to use all sort of communication we can imagine. And again those are not hardened military veterans.
- If you want to invent a whole new method of communication for both us an the aliens it's fine. But at least provide enough details so I can understand it to the extend of utilizing it in a story.
- This is also open to theoretical stuff that should be possible or might be possible within the future. Same rules apply. I can't just write they used X if I know nothing about it.
- Remember they are not actively masking or limiting their communication.
- Also we can bring the entire resources of humanity to bear on this issue. So no limits to how much power or time or people or cooperation we need.
So what sort of technology that they might be using?
Now since I linked the question that limited the original detection and arrival to earth's orbit within a week I think it only makes sense to open the discovery of the "signal" to anything from that week to the moment they are seen with the naked eye from earth.
Bonus points for a faster solution.