Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Echoes of the past: ultra-high energy cosmic rays accelerated by radio
  galaxies, scattered by starburst galaxies   We explore the possibility that the hotspot of ultrahigh energy cosmic rays
(UHECR) detected by the Telescope Array (TA) from the approximate direction of
M82 and the M81 group of galaxies might be the echo of UHECR emitted by
Centaurus A in an earlier more powerful phase. Echoes from other starburst
galaxies or groups of galaxies may contribute to the UHECR flux at the Earth.
We use an illustrative Monte Carlo model of mono-energetic UHECR transport by
small angle scattering to generate synthetic sky maps. The model informs a
discussion of overall energetics and time and distance scales. We find a viable
echo model for the observed UHECR hotspots if the UHECR luminosity of Centaurus
A 20 Myr ago was 200 times its present luminosity and if the ordered magnetic
field exceeds 10-20nG out to a distance of 400-800kpc in the circumgalactic
medium of M82 and other starburst galaxies.

30
The categories of this paper are: q-alg
2
The categories of this paper are: math
22
The categories of this paper are: chao-dyn
21
The categories of this paper are: adap-org
18
The categories of this paper are: econ
19
The categories of this paper are: eess
7
The categories of this paper are: hep-th
12
The categories of this paper are: nucl-th
17
The categories of this paper are: q-fin
26
The categories of this paper are: bayes-an
0
The categories of this paper are: hep-ph
5
The categories of this paper are: gr-qc
1
The categories of this paper are: cs
29
The categories of this paper are: funct-an
16
The categories of this paper are: stat
37
The categories of this paper are: supr-con