How often does the Unruh–DeWitt detector click? Regularization by a spatial profile

We analyse within first-order perturbation theory the instantaneous transition rate of an accelerated Unruh–DeWitt particle detector whose coupling to a massless scalar field on four-dimensional Minkowski space is regularized by a spatial profile. For the Lorentzian profile introduced by Schlicht, the zero-size limit is computed explicitly and expressed as a manifestly finite integral formula that no longer involves regulators or limits. The same transition rate is obtained for an arbitrary profile of compact support under a modified definition of spatial smearing. Consequences for the asymptotic behaviour of the transition rate are discussed. A number of stationary and nonstationary trajectories are analysed, recovering in particular the Planckian spectrum for uniform acceleration.