Nonthermal Bremsstrahlung and Hard X‐Ray Emission from Clusters of Galaxies

Abstract

We have calculated nonthermal bremsstrahlung (NTB) models for the hard X-ray (HXR) tails recently observed by BeppoSAX in clusters of galaxies. In these models, the HXR emission is caused by supra- thermal electrons with energies of D10¨200 keV. We consider models in which these transrelativistic suprathermal particles are the low energy end of a population of electrons being accelerated to high energies by shocks or turbulence (ii accelerating electron ˇˇ models). We also consider a model in which these electrons are the remnant of an older nonthermal population that is losing energy and rejoining the thermal distribution as a result of Coulomb interactions (ii cooling electron ˇˇ models). The supra- thermal populations are assumed to start at an electron kinetic energy 3kT , where T is the temperature of the thermal intracluster medium (ICM). The nonthermal bremsstrahlung spectra —atten at low photon energies because of the lack of low-energy nonthermal particles. The accelerating electron models have HXR spectra that are nearly power laws from D20¨100 keV. However, the spectra are brighter and —atter than given by the nonrelativistic bremsstrahlung cross section because of transrelativistic eUects. The HXR spectrum of the cooling electron model is very —at, and most of the X-ray emission in the HXR energy range (10¨100 keV) actually arises from electrons with much higher energies (D100 MeV). Under the assumption that the suprathermal electrons form part of a continuous spectrum of electrons including highly relativistic particles, we have calculated the inverse Compton (IC) extreme-ultraviolet (EUV), HXR, and radio synchrotron emission by the extensions of the same populations. For acceler- ating electron models with power-law momentum spectra (N(p) P p~k) with which are those k ( 2.7, expected from strong shock acceleration, the IC HXR emission exceeds that caused by NTB. Thus, these models are of interest only if the electron population is cut oU at some upper energy GeV. Similarly, (1 —at-spectrum accelerating electron models produce more radio synchrotron emission than is observed from clusters if the ICM magnetic —eld is kG. The cooling electron model produces vastly too B Z 1 much EUV emission as compared to the observations of clusters. We have compared these NTB models to the observed HXR tails in Coma and Abell 2199. The NTB models require a nonthermal electron population that contains about 3% of the number of electrons in the thermal ICM. If the suprathermal electron population is cut oU at some energy above 100 keV, then the models can —t the observed HXR —uxes and spectral indices in both clusters easily. For accelerating electron models without a cutoU, the electron spectrum must be rather steep, to avoid producing too much IC HXR emission. The Z2.9, model HXR spectra are then rather steep but are marginally consistent with observations of the HXR spectrum in Abell 2199 and Coma or the radio spectrum in Coma. These models can account for the HXR and radio properties of these two clusters but do not produce enough EUV emission. Subject headings: acceleration of particlescosmic raysgalaxies: clusters: general ¨ intergalactic mediumradiation mechanisms: nonthermalX-rays: general