Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Redshift of a photon emitted along the black hole horizon   In this work we derive some general features of the redshift measured by
radially moving observers in the black hole background. Let observer 1 cross
the black hole horizon emitting of a photon while observer 2 crossing the same
horizon later receives it. We show that if (i) the horizon is the outer one
(event horizon) and (ii) it is nonextremal, received frequency is redshifted.
This generalizes previous recent results in literature. For the inner horizon
(like in the Reissner-Nordstr\"{o}m metric) the frequency is blueshifted. If
the horizon is extremal, the frequency does not change. We derive explicit
formulas describing the frequency shift in generalized Kruskal- and
Lemaitre-like coordinates.

28
The categories of this paper are: alg-geom
25
The categories of this paper are: patt-sol
36
The categories of this paper are: cmp-lg
21
The categories of this paper are: adap-org
4
The categories of this paper are: cond-mat
13
The categories of this paper are: quant-ph
29
The categories of this paper are: funct-an
23
The categories of this paper are: solv-int
19
The categories of this paper are: eess
12
The categories of this paper are: nucl-th
27
The categories of this paper are: comp-gas
32
The categories of this paper are: atom-ph
10
The categories of this paper are: q-bio
22
The categories of this paper are: chao-dyn
34
The categories of this paper are: chem-ph
14
The categories of this paper are: math-ph