I have a Zhonya hidden at a battlefield. I reveal it before my unit dies. Can the other player use a thermo beam?

No, the other player cannot use Thermo Beam to destroy your Zhonya's Hourglass in this situation.

According to RiftJudge FAQ #7377, you cannot react to Zhonya's Hourglass being revealed. When you reveal a hidden card, it finalizes to the chain and resolves immediately as a permanent. Because it resolves immediately upon being finalized, there is no window of opportunity for your opponent to play a spell like Thermo Beam between the moment you reveal it and the moment it is in play.

Furthermore, even if they could play Thermo Beam, it would not destroy the Zhonya's Hourglass because of how it functions:

1. Immediate Recall: As noted in RiftJudge FAQ #2113, Zhonya's Hourglass is gear, and gear cannot remain on battlefields. When it is played from hidden, it is immediately recalled to your base.
2. Timing: By the time the game state would allow for a new chain to be started (after the current chain resolves), the Zhonya's Hourglass is already at your base.
3. Thermo Beam's Target: Thermo Beam kills all gear. While it would affect gear in your base, the specific sequence of revealing Zhonya's to save a unit happens during a chain (as a reaction to the unit dying). Thermo Beam is an [Action] spell, which can only be played during an Open State (when no chain exists). Since you are revealing the Zhonya's in response to a unit dying, the chain is Closed, meaning your opponent cannot play an [Action] spell like Thermo Beam until that chain has fully resolved.

This is my interpretation based on the rules for [Action] timing and the specific mechanics of Zhonya's Hourglass. No official FAQ exists for this specific interaction, but it follows the established rules for chain timing and gear movement.