# Unruh effect and relative acceleration

This is how I understand the Unruh effect: if a person (say, Bob) is moving with acceleration $a$ in his spaceship, then he will see an apparent event horizon around him--that is, the vacuum will appear to have some temperature $T$ from his point of view, where $T$ is given by

$$T=\frac{\hbar a}{2\pi ck_B}$$

Now, let's imagine that Bob is stationary in his spaceship, and Alice is coming towards him at acceleration $a$. Would Bob see the vacuum around Alice have a finite temperature, or does the Unruh effect only occur if the observer themselves are accelerating? I would think that Bob might see space-time "warm" around Alice because she's accelerating towards him from the viewpoint of his reference frame, in the same way that Bob's surroundings are accelerating towards him when he's accelerating. Nevertheless, I am confused on this point.