Superradiance evolution of black hole shadows revisited

Black hole (BH) shadows can be used to probe new physics in the form of ultralight particles via the phenomenon of superradiant instability. By directly affecting the BH mass and spin, superradiance can lead to a time evolution of the BH shadow, which nonetheless has been argued to be unobservable through very-long-baseline interferometry (VLBI) over realistic observation time scales. We revisit the superradiance-induced BH shadow evolution including the competing effects of gas accretion and gravitational wave (GW) emission and, as a first step towards modeling realistic new physics scenarios which predict the existence of multiple ultralight species, we study the system in the presence of two ultralight bosons, whose combined effect could help reduce the shadow evolution time scale. We find that accretion and GW emission play a negligible role in our results (justifying previous simplified analyses), and that contrary to our intuition the inclusion of an additional ultralight boson does not shorten the BH shadow evolution time scale and hence improve detection prospects. However, we point out an important subtlety concerning the observationally meaningful definition of the superradiance-induced BH shadow evolution time scale, which reduces the latter by about an order of magnitude, opening up the possibility of observing the superradiance-induced BH shadow evolution with upcoming VLBI arrays, provided angular resolutions just below the μas level can be reached. As a concrete example, we show that the angular size of the shadow of SgrA* can change by up to 0.6 μas over a period as short as 16 years, which further strengthens the scientific case for targeting the shadow of SgrA* with next-generation VLBI arrays.