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Article overview
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Constraining the electron and proton acceleration efficiencies in merger shocks in galaxy clusters | | F. Vazza
; D. Eckert
; M. Brueggen
; B. Huber
; | | Date: |
11 May 2015 | | Abstract: | Radio relics in galaxy clusters are associated with powerful shocks that
(re)accelerate relativistic electrons. It is widely believed that the
acceleration proceeds via diffusive shock acceleration. In the framework of
thermal leakage, the ratio of the energy in relativistic electrons to the
energy in relativistic protons should should be smaller than $K_{
m e/p} sim
10^{-2}$. The relativistic protons interact with the thermal gas to produce
$gamma$-rays in hadronic interactions. Combining observations of radio relics
with upper limits from $gamma$-ray observatories can constrain the ratio
$K_{
m e/p}$. In this work we selected 10 galaxy clusters that contain double
radio relics, and derive new upper limits from the stacking of $gamma$-ray
observations by FERMI. We modelled the propagation of shocks using a
semi-analytical model, where we assumed a simple geometry for shocks and that
cosmic ray protons are trapped in the intracluster medium. Our analysis shows
that diffusive shock acceleration has difficulties in matching simultaneously
the observed radio emission and the constraints imposed by FERMI, unless the
magnetic field in relics is unrealistically large ($gg 10 ~
m mu G$). In all
investigated cases (also including realistic variations of our basic model and
the effect of re-acceleration) the mean emission of the sample is of the order
of the stacking limit by FERMI, or larger. These findings put tension on the
commonly adopted model for the powering of radio relics, and imply that the
relative acceleration efficiency of electrons and protons is at odds with
predictions of diffusive shock acceleration, requiring $K_{
m e/p} geq
10-10^{-2}$. | | Source: | arXiv, 1505.2782 | | Services: | Forum | Review | PDF | Favorites | |
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