  In the freeze-tag problem, one active robot must wake up many frozen robots.
The robots are considered as points in a metric space, where active robots move
at a constant rate and activate other robots by visiting them. In the
(time-dependent) online variant of the problem, frozen robots are not revealed
until a specified time. Hammar, Nilsson, and Persson have shown that no online
algorithm can achieve a competitive ratio better than $7/3$ for online
freeze-tag, and asked whether there is any $O(1)$-competitive algorithm. In
this paper, we provide a $(1+\sqrt{2})$-competitive algorithm for online
time-dependent freeze-tag, and show that no algorithm can achieve a lower
competitive ratio on every metric space.
