Nonthermal Bremsstrahlung and Hard X‐Ray Emission from Clusters of Galaxies (original) (raw)
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On the Nonthermal Emission and Acceleration of Electrons in Coma and Other Clusters of Galaxies
The Astrophysical Journal, 2001
Some clusters of galaxies in addition to thermal bremsstrahlung (TB), emit detectable diffuse radiation from the intercluster medium (ICM) at radio, EUV and hard x-ray (HXR) ranges. The radio radiation must be due to synchrotron by relativistic electrons, and the inverse Compton (IC) scattering by the cosmic microwave background radiation of the same electrons is the most natural source for the HXR and perhaps the EUV emissions. However, simple estimates give a weaker magnetic field than that suggested by Faraday rotation measurements. Consequently, non-thermal bremsstrahlung (NTB) and TB have also been suggested as sources of these emissions. We show that NTB cannot be the source of the HXRs (except for a short period) and that the difficulty with that the low magnetic field in the IC model is alleviated if the effects of observational selection bias, non isotropic pitch angle distribution and spectral breaks in the energy distribution of the relativistic electrons are taken into account. From these consideration and the strength of the EUV emission, we derive a spectrum for the radiating electrons and discuss possible acceleration scenarios for its productions. We show that continuous and in situ acceleration in the ICM of the background thermal electrons is difficult and requires unreasonably high energy input. Similarly acceleration of injected relativistic electrons, say by galaxies, seems unreasonable because it will give rise to a much flatter spectrum of electrons than required, unless a large fraction of energy input is carried away by electrons escaping the ICM, in which case one obtains EUV and HXR emissions extending well beyond the boundaries of the diffuse radio source. A continuous emission by a cooling spectrum resulting from interaction with ICM of electrons accelerated elsewhere also suffers from similar shortcomings. The most likely scenario appears to be an episodic injection-acceleration model, whereby one obtains a time dependent spectrum that for certain phases of its evolution satisfies all the requirements.
Nonthermal Hard X‐Ray Emission in Galaxy Clusters Observed with the B eppo SAX PDS
The Astrophysical Journal, 2004
We study the X-ray emission in a sample of galaxy clusters using the BeppoSAX PDS instrument in the 20 -80 keV energy band. We estimate the non-thermal hard X-ray cluster emission (HXR) by modeling the thermal contribution from the cluster gas and the non-thermal contamination from the unobscured AGN in the clusters. We also evaluate the systematic uncertainties due to the background fluctuations. Assuming negligible contamination from the obscured AGN, the resulting non-thermal component is detected at a 2σ level in ∼50% of the non-significantly AGN-contaminated clusters: A2142, A2199, A2256, A3376, Coma, Ophiuchus and Virgo. The data are consistent with a scenario whereby relaxed clusters have no hard X-ray component of non-thermal origin, whereas merger clusters do, with a 20 -80 keV luminosity of ∼ 10 43−44 h −2 50 erg s −1 . The co-added spectrum of the above clusters indicates a power-law spectrum for the HXR with a photon index of 2.8 +0.3 −0.4 in the 12 -115 keV band, and we find indication that it has extended distribution. These indications argue against significant contamination from obscured AGN, which have harder spectra and centrally concentrated distribution. These results are supportive of the assumption of the merger shock acceleration of electrons in clusters, which has been proposed as a possible origin of the non-thermal hard X-ray emission models. Assuming that the Cosmic Microwave Background photons experience Inverse Compton scattering from the mergeraccelerated relativistic electrons, and thus produce the observed HXR, the measured hard X-ray slope corresponds to a differential momentum spectra of the relativistic electrons with a slope of µ = 3.8 -5.0. In presence of cluster magnetic fields this relativistic electron population produces synchrotron emission with a spectral index of 1.4 -2.1, consistent with radio halo observations of merger clusters. Thus both hard X-ray and radio observations of merger clusters are consistent with the Inverse Compton model. The observed slope of HXR is also consistent with that predicted by the non-thermal bremsstrahlung, which thus cannot be ruled by the fit to the current data, even though this model requires an extreme, untenable cluster energetics. Assuming centrally concentrated distribution of HXR, the data requires a harder slope for the HXR spectrum, which is consistent with secondary electron models, but this model yields a worse fit to the PDS data and thus seems to be disfavored over the primary electron Inverse Compton model.
On the non-thermal high energy radiation of galaxy clusters
Astronomy and Astrophysics, 2004
The origin of the nonthermal EUV and hard X-ray emission "excess" reported from some galaxy clusters has been intensively debated over last several years. The most favored models which refer this excess to relativistic electrons upscattering the 2.7 K CMBR generally requires very low magnetic field, significantly below the estimates derived from the Faraday Rotation Measurements, unless one invokes rather nonstandard assumptions concerning the energy and pitch angle distributions of nonthermal electrons. In this paper we suggest a new model assuming that the "nonthermal" excess is due to synchrotron radiation of ultrarelativistic (multi-TeV) electrons of "photonic" origin. These electrons are continuously introduced throughout the entire intracluster medium by very high energy (hypothetical) γ-rays through interactions with the diffuse extragalactic radiation fields. We present numerical calculations for the Coma cluster, and briefly discuss implications of the model for other galaxy clusters both in the X-and γ-ray energy domains.
On the origin of non-thermal X-radiation from galaxy clusters
2003
The origin of the nonthermal UV and hard Xray emission "excess" reported from some galaxy clusters has been intensively debated since last several years. So far two models have been suggested to explain these radiation components, but both models face significant problems. The most favoured model which refers this excess to relativistic electrons upscattering 2.7 K CMBR requires very low magnetic field (significantly below the estimates derived from the Faraday Rotation Measurements). The second model assumes nonthermal bremsstrahlung, but it requires unacceptably large energy input in sub-relativistic electrons. In this paper we suggest a new model assuming that the "nonthermal" excess is due to synchrotron radiation of ultrarelativistic (multi-TeV) electrons of "photonic" origin. These electrons are continuously implemented throughout the entire intracluster medium by very high energy γ-rays through interactions with the diffuse extragalactic radiation fields. We present numerical calculations for Coma, and briefly discuss implications of the model for other galaxy clusters.
The origin of radio haloes and non-thermal emission in clusters of galaxies
Monthly Notices of the Royal Astronomical Society, 2002
We study the origin of the non-thermal emission from the intracluster medium, including the excess hard X-ray emission and cluster-wide radio haloes, through fitting two representative models to the Coma cluster. If the synchrotron emitting relativistic electrons are accelerated in situ from the vast pool of thermal electrons, then a quasi-stationary solution of the kinetic equation with particle acceleration through turbulence at high energies (> 200 keV) naturally produces a population of supra-thermal electrons responsible for the excess hard X-ray emission through bremsstrahlung. Inverse Compton scattering is negligible at hard X-ray energies in this case. The radio halo flux density constrains the magnetic field strength to a value close to that of equipartition ∼ 1µG. Alternatively, if the relativistic electrons are injected from numerous localised 'external' sources then the hard X-rays are best explained by inverse Compton scattering from GeV electrons, and little of the hard X-radiation has a bremsstrahlung origin. In this case, the magnetic field strength is constrained to ∼ 0.1 − 0.2 µG. Both models assume that the non-thermal emissions are generated by a single electron spectrum, so that only two free parameters, well constrained by the observed hard X-ray and radio halo spectra, are needed in either case. Measurements of the cluster magnetic field will distinguish between the models.
Nonthermal emission from clusters of galaxies
Journal of Cosmology and Astroparticle Physics, 2009
We show that the spectral and radial distribution of the nonthermal emission of massive, M 10 14.5 M ⊙ , galaxy clusters may be approximately described by simple analytic expressions, which depend on the cluster thermal X-ray properties and on two model parameter, β core and η e . β core is the ratio of the cosmic-ray (CR) energy density (within a logarithmic CR energy interval) and the thermal energy density at the cluster core, and η e(p) is the fraction of the thermal energy generated in strong collisionless shocks, which is deposited in CR electrons (protons). Using a simple analytic model for the evolution of intra-cluster medium CRs, which are produced by accretion shocks, we find that β core ≃ η p /200, nearly independent of cluster mass and with a scatter ∆ ln β core ≃ 1 between clusters of given mass. We show that the hard X-ray (HXR) and γ-ray luminosities produced by inverse Compton scattering of CMB photons by electrons accelerated in accretion shocks (primary electrons) exceed the luminosities produced by secondary particles (generated in hadronic interactions within the cluster) by factors ≃ 500(η e /η p )(T /10keV) −1/2 and ≃ 150(η e /η p )(T /10keV) −1/2 respectively, where T is the cluster temperature. Secondary particle emission may dominate at the radio and very high energy ( 1 TeV) γ-ray bands. Our model predicts, in contrast with some earlier work, that the HXR and γ-ray emission from clusters of galaxies are extended, since the emission is dominated at these energies by primary (rather than by secondary) electrons. Our predictions are consistent with the observed nonthermal emission of the Coma cluster for η p ∼ η e ∼ 0.1. The implications of our predictions to future HXR observations (e.g. by NuStar, Simbol-X) and to (space/ground based) γray observations (e.g. by Fermi, HESS, MAGIC, VERITAS) are discussed. In particular, we identify the clusters which are the best candidates for detection in γ-rays. Finally, we show that our model's results agree with results of detailed numerical calculations, and that discrepancies between the results of various numerical simulations (and between such results and our model) are due to inaccuracies in the numerical calculations.
Nonthermal Radiation and Acceleration of Electrons in Clusters of Galaxies
Cornell University - arXiv, 2002
Recent observations of excess radiation at extreme ultraviolet and hard X-ray energies straddling the well known thermal soft X-ray emission have provided new tools and puzzles for investigation of the acceleration of nonthermal particles in the intercluster medium of clusters of galaxies. It is shown that these radiations can be produced by the inverse Compton upscattering of the cosmic microwave background photons by the same population of relativistic electrons that produce the well known diffuse radio radiation via the synchrotron mechanism. It is shown that the commonly discussed discrepancy between the value of the magnetic field required for the production of these radiation with that obtained from Faraday rotation measures could be resolved by more realistic models and by considerations of observational selection effects. In a brief discussion of the acceleration process it is argued that the most likely scenario is reacceleration of injected relativistic electrons involving shocks and turbulence. The seed electrons cannot be the background thermal electrons because of energetic considerations, and a steady state situation may not agree with the details of the observed spectra. Episodic generation of shocks and turbulence or episodic injection of relativistic electrons is a more likely scenario for acceleration.
High and Low-Energy Nonthermal X-Ray Emission from the Abell 2199 Cluster of Galaxies
Astrophysical Journal, 1999
We report the detection of both soft and hard excess X-ray emission in the cluster of galaxies A 2199, based upon spatially resolved spectroscopy with data from the BeppoSAX, EUVE and ROSAT missions. The excess emission is visible at radii larger than 300 kpc and increases in strength relative to the isothermal component. The total 0.1-100 keV luminosity of this component is 15 % of the cluster luminosity, but it dominates the cluster luminosity at high and low energies. We argue that the most plausible interpretation of the excess emission is an inverse Compton interaction between the cosmic microwave background and relativistic electrons in the cluster. The observed spatial distribution of the non-thermal component implies that there is a large halo of cosmic ray electrons between 0.5-1.5 Mpc surrounding the cluster core. The prominent existence of this component has cosmological implications, as it is significantly changing our picture of a clusters's particle acceleration history, dynamics between the thermal and relativistic media, and total mass budgets.
Astronomy and Astrophysics, 2009
Context. Populations of high energy electrons can produce hard X-ray (HXR) emission in galaxy clusters by up-scattering CMB photons via the inverse Compton scattering (ICS) mechanism. However, this scenario has various astrophysical consequences. Aims. We discuss here the consequences of the presence of a population of high energy particles for the multi-frequency emissivity of the same clusters and the structure of their atmospheres. Methods. We derive predictions for the ICS HXR emission in the specific case of the Ophiuchus cluster (for which an interesting combination of observational limits and theoretical scenarios have been presented) for three main scenarios producing high-E electrons: primary cosmic ray model, secondary cosmic rays model and neutralino DM annihilation scenario. We further discuss the predictions of the Warming Ray model for the cluster atmosphere. Under the assumption to fit the HXR emission observed in Ophiuchus, we explore the consequences that these electron populations induce on the cluster atmosphere. Results. We find that: i) primary electrons can be marginally consistent with the available data provided that the electron spectrum is cutoff at E 30 and E 90 MeV for electron spectral index values of 3.5 and 4.4, respectively; ii) secondary electron models from pp collisions are strongly inconsistent with the viable gamma-ray limits, cosmic ray protons produce too much heating of the intracluster (IC) gas and their pressure at the cluster center largely exceeds the thermal one; iii) secondary electron models from DM annihilation are also strongly inconsistent with the viable gamma-ray and radio limits, and electrons produce too much heating of the IC gas at the cluster center, unless the neutralino annihilation cross-section is much lower than the proposed value. In that case, however, these models no longer reproduce the HXR excess in Ophiuchus. Conclusions. We conclude that ICS by secondary electrons from both neutralino DM annihilation and pp collisions cannot be the mechanism responsible for the HXR excess emission; primary electrons are still a marginally viable solution provided that their spectrum has a low-energy cutoff at E 30−90 MeV. We also find that diffuse radio emission localized at the cluster center is expected in all these models and requires quite low values of the average magnetic field (B ∼ 0.1−0.2 μG in primary and secondary-pp models; B ∼ 0.055−0.39 μG in secondary-DM models) to agree with the available observations. Finally, the WR model (with B ∼ 0.4−2.0 μG) offers, so far, the most accurate description of the cluster in terms of the temperature distribution, heating and pressure and multifrequency spectral energy distribution. Fermi observations of Ophiuchus will provide further constraints to this model.