Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Particle acceleration in relativistic outflows   In this review we confront the current theoretical understanding of particle
acceleration at relativistic outflows with recent observational results on
various source classes thought to involve such outflows, e.g. gamma-ray bursts,
active galactic nuclei, and pulsar wind nebulae. We highlight the possible
contributions of these sources to ultra-high-energy cosmic rays.

23
The categories of this paper are: solv-int
37
The categories of this paper are: supr-con
1
The categories of this paper are: cs
3
The categories of this paper are: physics
12
The categories of this paper are: nucl-th
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
24
The categories of this paper are: dg-ga
14
The categories of this paper are: math-ph
35
The categories of this paper are: mtrl-th
34
The categories of this paper are: chem-ph
20
The categories of this paper are: acc-phys
0
The categories of this paper are: hep-ph
17
The categories of this paper are: q-fin
26
The categories of this paper are: bayes-an