Superradiance in deformed Kerr black holes

Recent strong-field regime tests of gravity are so far in agreement with general relativity. In particular, astrophysical black holes appear all to be consistent with the Kerr spacetime, but the statistical error on current observations allows for small yet detectable deviations from this description. Here we study superradiance of scalar and electromagnetic test fields in deformed Kerr spacetimes and we observe that for large deformations superradiance is highly suppressed with respect to the Kerr case. Surprisingly, for small deformations there exists a range of values for the deformation parameter for which the maximum amplification factor is larger than the Kerr one. We also provide a first result about the superradiant instability of these deformed spacetimes against massive scalar fields.