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Probing the cosmic-ray content of galaxy clusters by stacking Fermi-LAT count maps


Huber, B; Tchernin, C; Eckert, D; Farnier, C; Manalaysay, A; Straumann, U; Walter, R (2013). Probing the cosmic-ray content of galaxy clusters by stacking Fermi-LAT count maps. Astronomy and Astrophysics, 560(A&A):A64.

Abstract

Aims. Radio observations have shown that galaxy clusters are giant reservoirs of cosmic rays (CR). Although a gamma-ray signal from the cluster volume is expected to arise through interactions of CR protons with the ambient plasma, a confirming observation is still missing.
Methods. We searched for a cumulative gamma-ray emission in the direction of galaxy clusters by analysing a collection of stacked Fermi-LAT count maps. Additionally, we investigated possible systematic differences in the emission between cool-core and non-cool-core cluster populations.
Results. Making use of a sample of 53 clusters selected from the HIFLUGCS catalog, we do not detect a significant signal from the stacked sample. The upper limit on the average flux per cluster derived for the total stacked sample is at the level of a few 10-11   ph   cm-2   s-1 at a 95% confidence level in the 1–300 GeV band, assuming power-law spectra with photon indices 2.0, 2.4, 2.8, and 3.2. Separate stacking of the cool-core and non-cool-core clusters in the sample lead to similar values of around 5 × 10-11   ph   cm-2   s-1 and 2 × 10-11   ph   cm-2   s-1, respectively.
Conclusions. Under the assumption that decaying π0, produced in collisions between CRs and the ambient thermal gas, are responsible for gamma-ray emission, we set upper limits on the average CR content in galaxy clusters. For the entire cluster population, our upper limit on the gamma-ray flux translates into an upper limit on the average CR-to-thermal energy ratio of 4.6% for a photon index of 2.4, although it is possible for individual systems to exceed this limit. Our 95% upper limits are at the level expected from numerical simulations, which most likely suggests that the injection of CR at cosmological shocks is less efficient than previously assumed.

Abstract

Aims. Radio observations have shown that galaxy clusters are giant reservoirs of cosmic rays (CR). Although a gamma-ray signal from the cluster volume is expected to arise through interactions of CR protons with the ambient plasma, a confirming observation is still missing.
Methods. We searched for a cumulative gamma-ray emission in the direction of galaxy clusters by analysing a collection of stacked Fermi-LAT count maps. Additionally, we investigated possible systematic differences in the emission between cool-core and non-cool-core cluster populations.
Results. Making use of a sample of 53 clusters selected from the HIFLUGCS catalog, we do not detect a significant signal from the stacked sample. The upper limit on the average flux per cluster derived for the total stacked sample is at the level of a few 10-11   ph   cm-2   s-1 at a 95% confidence level in the 1–300 GeV band, assuming power-law spectra with photon indices 2.0, 2.4, 2.8, and 3.2. Separate stacking of the cool-core and non-cool-core clusters in the sample lead to similar values of around 5 × 10-11   ph   cm-2   s-1 and 2 × 10-11   ph   cm-2   s-1, respectively.
Conclusions. Under the assumption that decaying π0, produced in collisions between CRs and the ambient thermal gas, are responsible for gamma-ray emission, we set upper limits on the average CR content in galaxy clusters. For the entire cluster population, our upper limit on the gamma-ray flux translates into an upper limit on the average CR-to-thermal energy ratio of 4.6% for a photon index of 2.4, although it is possible for individual systems to exceed this limit. Our 95% upper limits are at the level expected from numerical simulations, which most likely suggests that the injection of CR at cosmological shocks is less efficient than previously assumed.

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15 citations in Web of Science®
19 citations in Scopus®
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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Physics Institute
Dewey Decimal Classification:530 Physics
Language:English
Date:2013
Deposited On:05 Feb 2014 15:31
Last Modified:05 Apr 2016 17:30
Publisher:EDP Sciences
ISSN:0004-6361
Free access at:Publisher DOI. An embargo period may apply.
Publisher DOI:https://doi.org/10.1051/0004-6361/201321947

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