I just played two games in a row on vassal where this came up.

In both cases, my opponent's first ship to shoot only had my ship with rebel captive as an option. Both times they had stress already, and really didn't want to shoot and get a second stress.

Do you have to attack every round if you can? Or can you choose to not attack?

You are under no obligation to actually make an attack.

But note that you have obligation to defend, though

Page 10:

During this phase, each ship may perform one attack against one enemy ship that is inside its firing arc and within range.

Emphasis mine. May means optional.

Hmm interesting...I guess that makes rebel captive even more powerful if it can scare a player into not even attacking you!

Hmm interesting...I guess that makes rebel captive even more powerful if it can scare a player into not even attacking you!

I've not shot at Tarn with a Z-95 because it was a range three shot and Tarn wasn't a priority. However my opponent would have enjoyed the free target lock so that he could down my Z-95 before it fired off its assault missile.