Clusters of galaxies are revealing themselves as powerful sources of nonthermal radiation in a wide range of wavelengths. In order to account for these multifrequency observations, equipartition of cosmic rays (CRs) with the thermal gas in clusters of galaxies is often invoked. This condition might suggest a dynamical role played by cosmic rays in the virialization of these large-scale structures and is now testable through gamma-ray observations. We show here, in the specific case of the Coma and Virgo clusters, for which upper limits on the gamma-ray emission exist, that equipartition implies gamma-ray fluxes that are close to or even in excess of the EGRET limit, depending on the adopted model of CR injection. We use this limit to constrain the validity of the equipartition condition. We also show that, contrary to what was claimed in previous calculations, the equipartition assumption implies gamma-ray fluxes in the TeV range that can be detected even by currently operating gamma-ray observatories if the injection cosmic-ray spectrum is flatter than E-2.4.
On the equipartition of thermal and nonthermal energy in clusters of galaxies
Blasi P
1999-01-01
Abstract
Clusters of galaxies are revealing themselves as powerful sources of nonthermal radiation in a wide range of wavelengths. In order to account for these multifrequency observations, equipartition of cosmic rays (CRs) with the thermal gas in clusters of galaxies is often invoked. This condition might suggest a dynamical role played by cosmic rays in the virialization of these large-scale structures and is now testable through gamma-ray observations. We show here, in the specific case of the Coma and Virgo clusters, for which upper limits on the gamma-ray emission exist, that equipartition implies gamma-ray fluxes that are close to or even in excess of the EGRET limit, depending on the adopted model of CR injection. We use this limit to constrain the validity of the equipartition condition. We also show that, contrary to what was claimed in previous calculations, the equipartition assumption implies gamma-ray fluxes in the TeV range that can be detected even by currently operating gamma-ray observatories if the injection cosmic-ray spectrum is flatter than E-2.4.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
