Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Penrose process, superradiance and ergoregion instabilities   Superradiant scattering is a radiation enhancement process that takes place
in many contexts, and which has recently found exciting applications in astro
and particle physics. In the framework of curved spacetime physics, it has been
associated with the classical Penrose process for particles. Superradiance is
usually also associated with bosonic fields around geometries with ergoregions
and horizons. These notions are in clear tension however: the Penrose process
occurs for horizonless geometries, and particles are composed of fermions.
  Here, we resolve the tension in its different aspects, by showing that (i)
superradiance occurs for self-interacting fermions on flat spacetime; (ii)
superradiance occurs also for horizonless geometries, where it leads to an
ergoregion instability. Ultracompact, horizonless geometries will usually
respond with echoes of growing amplitude, until rotational (or electrostatic)
energy is extracted from the object; (iii) the Fourier-domain analysis leads to
absence of superradiance when horizons are not present. We elucidate why this
analysis fails to give meaningful results; (iv) finally, we show that
superradiant, ergoregion instabilities have a particle analog of similar growth
timescales and which can power the formation of a structure outside a compact,
rotating star.

29
The categories of this paper are: funct-an
26
The categories of this paper are: bayes-an
32
The categories of this paper are: atom-ph
9
The categories of this paper are: nlin
18
The categories of this paper are: econ
25
The categories of this paper are: patt-sol
4
The categories of this paper are: cond-mat
36
The categories of this paper are: cmp-lg
22
The categories of this paper are: chao-dyn
21
The categories of this paper are: adap-org
13
The categories of this paper are: quant-ph
33
The categories of this paper are: plasm-ph
35
The categories of this paper are: mtrl-th
27
The categories of this paper are: comp-gas
34
The categories of this paper are: chem-ph
14
The categories of this paper are: math-ph