  Previous studies have shown that an Unruh-DeWitt (UDW) detector, when coupled
linearly to a massless scalar field and permitted to fall radially into certain
black holes, will exhibit non-monotonicity in its transition properties near
the horizon. Specifically, the transition probability of a detector freely
falling into a (3+1)-dimensional Schawrzschild black hole, when considering the
Unruh and Hartle-Hawking vacuum states, was shown to possess a local extremum
at horizon crossing [K.K. Ng et al., New J. Phys. 24 (2022) 103018]. The
transition rate of a detector falling into a static (2+1)-dimensional
Ba\~nados-Teitelboim-Zanelli (BTZ) black hole, for the Hartle-Hawking state,
was also found to have multiple local extrema near the horizon under certain
parameter settings [M.R. Preciado-Rivas et al., arXiv:2402.14908v1]. These
discoveries are of interest, as they suggest that the event horizon of a black
hole may be distinguishable to a local probe when QFT effects are included. In
this paper, we explore the problem of a UDW detector falling freely into a
rotating BTZ black hole. We numerically compute the detector's transition rate
for different values of black hole mass, black hole angular momentum, detector
energy gap, and field boundary conditions at infinity. Our results lead to a
more generalized description of the behaviour of particle detectors in BTZ
black hole spacetime, from which the previous non-rotating BTZ case can be
retrieved in the limit as angular momentum vanishes.
