If it was an artifact - especially a discarded solar sail, explaining the acceleration - there might be a follow-up.
Imagine you were making a sail-propelled probe, with the intent of slowing the probe so it was captured by the target system. So the probe would have two sails. The first would be deployed after launch, to catch the initial laser beam, then later solar winds, to accelerate to system escape velocity.
You'd then detach that sail, and cruise. Later, you'd deploy a second sail to start braking on your way into the target system, to slow to capture velocity.
The first thing the inhabitants of the target system would see would be the first, discarded, sail as it tumbled through their system and accelerated back out ...
Why? Just accelerate towards the target as long as possible and then flip around. Assuming a columnar source (e.g. from a laser) and non-columnar target (e.g. from our sun), this could be done with minimal (if any) cost in time to arrival.
Carrying two solar sails is expensive. If it’s not, I’m not sure one would be using solar sails at all.
I'm guessing here, as I've never built a sail-propelled interstellar probe :) But I'm assuming that:
1) The characteristics of a good laser-propelled acceleration sail would be different to those of a good solar-powered braking sail.
2) Flipping around while attached to a deployed solar sail would be hard. In my head I'm imagining something like looping a paraglider (https://www.youtube.com/watch?v=-oEeTtQIEjs) but much harder. It'd have to be autonomous (no fly-by-wire at those ranges), and for recovery purposes I'd want to have a spare sail anyway that it could deploy in the case the primary got tangled.
I’m not sure how you’d detach the sail at speed. It’s in front of you with the same velocity. You’ve got to go around it or it around you. And you’ve got to do it at speed without modifying your trajectory.
That’s got to be more difficult than spinning around because any mistakes will be amplified over the vast distances, whereas a slight trajectory problem once you’ve arrived is much less of an issue and is correctable with onboard guidance. If you screw up the detach procedure.... you can’t steer to correct.
Assuming two different methods of acceleration (laser on departure, solar on arrival), why not construct the sail in such a way that the probe goes through the sail? It could pull the sail in, and mechanically route it’s connections to the rear of the craft. If there are material or structural difference needed, it could be incorporated into the design and two opposing surfaces.
Most of the mass is likely to be in the solar sail, so carrying two of them therefore is a major problem. If you need two different surface characteristics in two different directions, which I'm not sure is an issue, just coat the one sail differently on each side.
Sounds a lot like Encounter with Tiber by Buzz Aldrin, only it used a long drogue wire to brake electrostatically. It was only a book, but these details felt realistic enough.
Imagine you were making a sail-propelled probe, with the intent of slowing the probe so it was captured by the target system. So the probe would have two sails. The first would be deployed after launch, to catch the initial laser beam, then later solar winds, to accelerate to system escape velocity.
You'd then detach that sail, and cruise. Later, you'd deploy a second sail to start braking on your way into the target system, to slow to capture velocity.
The first thing the inhabitants of the target system would see would be the first, discarded, sail as it tumbled through their system and accelerated back out ...