Unruh-deWitt detector in impulsive plane wave spacetimes

We investigate the response function of an inertial Unruh-deWitt detector in an impulsive plane wave spacetime. Through symmetry considerations applied to the Wightman function, we demonstrate that the response function remains invariant for any inertial detector, even for those experiencing a discontinuous lightcone coordinate shift after interacting with the shockwave. This implies that the vacuum state in an impulsive plane wave spacetime is preserved under the associated spacetime symmetries. Additionally, we confirm that the quantum imprint of the shockwave, as discussed in [J. High Energ. Phys. 2021, 54 (2021)], is not an artifact and exhibits a distinct characteristic form. We identify this form by defining a "renormalized" response function for an eternally inertial detector, with Minkowski spacetime as a reference.