Kavli Affiliate: Lijing Shao
| First 5 Authors: Lihang Zhou, Richard Brito, Zhan-Feng Mai, Lijing Shao,
| Summary:
Superradiantly unstable ultralight particles around a classical rotating
black hole (BH) can form an exponentially growing bosonic cloud, which have
been shown to provide an astrophysical probe to detect ultralight particles and
constrain their mass. However, the classical BH picture has been questioned,
and different theoretical alternatives have been proposed. Exotic compact
objects (ECOs) are horizonless alternatives to BHs featuring a reflective
surface (with a reflectivity $mathcal{K}$) in place of the event horizon. In
this work, we study superradiant instabilities around ECOs, particularly
focusing on the influence of the boundary reflection. We calculate the growth
rate of superradiant instabilities around ECOs, and show that the result can be
related to the BH case by a correction factor $g_{mathcal{K}}$, for which we
find an explicit analytical expression and a clear physical interpretation.
Additionally, we consider the time evolution of superradiant instabilities and
find that the boundary reflection can either shorten or prolong the growth
timescale. As a result, the boundary reflection alters the superradiance
exclusion region on the Regge plane, potentially affecting constraints on the
mass of ultralight particles. For a mildly reflective surface
($|mathcal{K}|lesssim 0.5$), the exclusion region is not substantially
changed, while significant effects from the boundary reflection can occur for
an extreme reflectivity ($|mathcal{K}|gtrsim0.9$).
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