Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: New rotating black holes in non-linear Maxwell $f({\cal R})$ gravity   We investigate static and rotating charged spherically symmetric solutions in
the framework of $f({\cal R})$ gravity, allowing additionally the
electromagnetic sector to depart from linearity. Applying a convenient, dual
description for the electromagnetic Lagrangian, and using as an example the
square-root $f({\cal R})$ correction, we solve analytically the involved field
equations. The obtained solutions belong to two branches, one that contains the
Kerr-Newman solution of general relativity as a particular limit and one that
arises purely from the gravitational modification. The novel black hole
solution has a true central singularity which is hidden behind a horizon,
however for particular parameter regions it becomes a naked one. Furthermore,
we investigate the thermodynamical properties of the solutions, such as the
temperature, energy, entropy, heat capacity and Gibbs free energy. We extract
the entropy and quasilocal energy positivity conditions, we show that
negative-temperature, ultracold, black holes are possible, and we show that the
obtained solutions are thermodynamically stable for suitable model parameter
regions.

35
The categories of this paper are: mtrl-th
3
The categories of this paper are: physics
15
The categories of this paper are: nucl-ex
29
The categories of this paper are: funct-an
37
The categories of this paper are: supr-con
25
The categories of this paper are: patt-sol
27
The categories of this paper are: comp-gas
26
The categories of this paper are: bayes-an
23
The categories of this paper are: solv-int
2
The categories of this paper are: math
24
The categories of this paper are: dg-ga
12
The categories of this paper are: nucl-th
6
The categories of this paper are: astro-ph
33
The categories of this paper are: plasm-ph
31
The categories of this paper are: ao-sci
14
The categories of this paper are: math-ph