Electron and proton acceleration efficiency by merger shocks in galaxy clusters
2015 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 451, no 2, 2198-2211 p.Article in journal (Refereed) Published
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 be smaller than K-e/p similar to 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-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 (>> 10 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-e/p >= 10 - 10(-2).
Place, publisher, year, edition, pages
[Vazza, F.; Bruggen, M.] Hamburger Sternwarte, D-21029 Hamburg, Germany. [Eckert, D.] Univ Geneva, Dept Astron, CH-1290 Versoix, Switzerland. [Huber, B.] Royal Inst Technol KTH, SE-10691 Stockholm, Sweden., 2015. Vol. 451, no 2, 2198-2211 p.
acceleration of particles, radiation mechanisms: non-thermal, shock waves, galaxies: clusters: intracluster medium, gamma-rays: galaxies: clusters
Astronomy, Astrophysics and Cosmology
IdentifiersURN: urn:nbn:se:kth:diva-174245DOI: 10.1093/mnras/stv1072ISI: 000360830000080ScopusID: 2-s2.0-84938151651OAI: oai:DiVA.org:kth-174245DiVA: diva2:861465
QC 201510162015-10-162015-10-022015-11-19Bibliographically approved