arXiv subjects

Torsten A. Ensslin

Publications and source records attributed to Torsten A. Ensslin.

69 records · Page 4Linked to original sources

arXiv2000

Signatures in a Giant Radio Galaxy of a Cosmological Shock Wave at Intersecting Filaments of Galaxies

Sensitive images of low-level, Mpc-sized radio cocoons offer new opportunities to probe large scale intergalactic gas flows outside clusters of galaxies. New radio images of high surface brightness sensitivity at strategically chosen wavelengths of the giant radio galaxy NGC 315 (Mack et al. 1997,1998) reveal significant asymmetries and particularities in the morphology, radio spectrum and polarization of the ejected radio plasma. We argue that the combination of these signatures provides a sensitive probe of an environmental shock wave. Analysis of optical redshifts in NGC 315 vicinity confirms its location to be near, or at a site of large-scale flow collisions in the 100 Mpc sized Pisces-Perseus Supercluster region. NGC 315 resides at the intersection of several galaxy filaments, and its radio plasma serves there as a `weather station' (Burns 1998) probing the flow of the elusive and previously invisible IGM gas. If our interpretation is correct, this is the first indication for a shock wave in flows caused by the cosmological large scale structure formation, which is located in a filament of galaxies. The possibility that the putative shock wave is a source of gamma-rays and ultra high energy cosmic rays is briefly discussed.

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arXiv2000

Isotropization of Ultra-High Energy Cosmic Ray Arrival Directions by Radio Ghosts

The isotropy in the ultra high energy cosmic ray (UHECR) flux observed by Yakutsk and AGASA experiments, is a very strong constraint to production and propagation models alike. Most of the scenarios proposed in the literature should produce a sizable anisotropy as either extragalactic luminous or dark matter is normally associated with the invoked particle sources. We explore the possibility that the magnetic fields in fossil cocoons of former radio galaxies -- so called {\it radio ghosts} -- are able to scatter UHECR in the intergalactic medium giving rise to the observed isotropy. We show, through numerical simulations, under which conditions this process can be operative and the magnitude of the effect. We further demonstrate, that if radio ghosts mix with the ambient medium, they might be able to produce the observed magnetic fields in clusters of galaxies. In the case of mixing, the UHECR isotropization would be even stronger than in our conservative estimates.

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arXiv2000

Reviving Fossil Radio Plasma in Clusters of Galaxies by Adiabatic Compression in Environmental Shock Waves

We give for a plasma with a history of several expansion and contraction phases an analytical model of the evolution of a contained relativistic electron population under synchrotron, inverse Compton and adiabatic energy losses or gains. This is applied to different scenarios for evolution of radio plasma inside the cocoons of radio galaxies, after the activity of the central engine has ceased. It is demonstrated that fossil radio plasma with an age of even up to 2 Gyr can be revived by compression in a shock wave of large-scale structure formation, caused during the merging events of galaxy clusters, or by the accretion onto galaxy clusters. We argue, that this is a highly plausible explanation for the observed cluster radio relics, which are the regions of diffuse radio emission found in clusters of galaxies, without any likely parent radio galaxy seen nearby. An implication of this model is the existence of a population of diffuse, ultra-steep spectrum, very low frequency radio sources located inside and possibly outside of clusters of galaxies, tracing the revival of aged fossil radio plasma by the shock waves associated with large-scale structure formation.

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arXiv2000

Particle Acceleration and Diffusion in Fossil Radio Plasma

The strong activity of radio galaxies should have led to a nearly ubiquitous presence of fossil radio plasma in the denser regions of the inter-galactic medium as clusters, groups and filaments of galaxies. This fossil radio plasma can contain large quantities of relativistic particles (electrons and possibly protons) by magnetic confinement. These particles might be released and/or re-energized under environmental influences as turbulence and shock waves. Possible connections of such processes to the formation of the observed sources of diffuse radio emission in clusters of galaxies (the cluster radio halos and the cluster radio relics) are discussed.

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arXiv2000

Are Cluster Radio Relics Revived Fossil Radio Cocoons?

A new model for the, so called, `cluster radio relics' is presented (Ensslin & Gopal-Krishna 2000). Fossil radio cocoons, resulting from the former activity of radio galaxies, should contain a low energy relativistic electron population and magnetic fields. Electrons with an age of even up to 2 Gyr can be re-accelerated adiabatically to radio emitting energies, if the fossil radio plasma gets compressed in an environmental shock wave. Such a wave can be caused by a merging event of galaxy clusters, or by the accretion onto galaxy clusters. An implication of this model is the existence of a population of diffuse, ultra-steep spectrum, very low-frequency radio sources located inside and possibly outside of clusters of galaxies, tracing the revival of aged fossil radio plasma by the shock waves associated with large-scale structure formation.

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arXiv2000

Radio Halos of Galaxy Clusters from Hadronic Secondary Electron Injection in Realistic Magnetic Field Configurations

We investigate the possibility that radio halos of clusters of galaxies are caused by synchrotron emission of cosmic ray electrons (CRe), which were produced by cosmic ray protons (CRp) interacting hadronically with the intra-cluster medium (ICM) protons. We perform cosmological magneto-hydrodynamics (MHD) simulations to obtain a sample of ten magnetized galaxy clusters. They provide realistic models of the gas and magnetic field distribution, needed to predict the CRe production rates, their cooling, and their synchrotron emissivity. We assume a CRp population within the ICM with an energy density which has a constant ratio to thermal energy density. This ratio is adjusted in such a way that one of the simulated clusters reproduces the radio luminosity of the radio halo of the Coma cluster of galaxies. Our model exhibits the observed low degree of radio polarization and has a similar radial emission profile as the Coma cluster. We provide estimates for the expected gamma ray and neutrino flux. The necessary CRp/thermal energy ratio is 4 ... 14% ${(E_{p,min}/GeV)}^{-0.375}$ (for the range of magnetic field strengths suggested by Faraday measurements), where $E_{p,min}$ is the lower kinetic energy cutoff of the CRp with spectral index $α_p \approx 2.375$. Assuming this ratio to be the same in the whole set of simulated clusters a $T_x-L_ν$ relation is predicted which follows the observed relation well.

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arXiv2000

Comptonization of the Cosmic Microwave Background by Relativistic Plasma

(Abridged) We investigate the spectral distortion of the cosmic microwave background (CMB) caused by relativistic plasma. Within the Thomson regime, an exact analytic expression for the photon scattering kernel of a momentum power-law electron distribution is given. The ultra-relativistic Sunyaev-Zeldovich (SZ) decrement measures the electron number and not the energy content. On the other hand, the relativistic SZ increment at higher frequencies depends strongly on the spectral shape of the electrons. We calculate the expected Comptonization due to the energy release of radio galaxies, which we estimate to be 3 10^66 erg Gpc^-3. We investigate Comptonization from (a) the part of the released energy which is thermalized and (b) the relativistic, remnant radio plasma, which may form a second, relativistic phase in the inter galactic medium, nearly unobservable for present day instruments (presence of so called 'radio ghosts'). We find a thermal Comptonization parameter due to (a) of y = 10^-6 and (b) an optical depth of relativistic electrons in old radio plasma of tau_rel < 10^-7. If a substantial fraction of the volume of clusters of galaxies is filled with such old radio plasma the SZ effect based determination of the Hubble constant is biased to lower values. Finally, it is shown that a supra-thermal population of electrons in the Coma cluster would produce a signature in the Wien-tail of the CMB, which is marginally detectable with a multifrequency measurement by the Planck satellite. Such a population is expected to exist, since its bremsstrahlung would explain Coma's recently reported high energy X-ray excess.

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arXiv2000

Theoretical Implications of Diffuse Non-Thermal Emission from Clusters of Galaxies

A short review on theoretical implications of non-thermal emission (radio, extreme ultraviolet, high energy X-ray) from the intra-cluster medium is given. The origin of cluster radio halos and cluster radio relics is discussed within the framework of a network of processes producing a non-thermal electron population. Emphasis is given to the role of old, remnant, presently invisible relativistic plasma released by former radio galaxies.

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arXiv1999

Radio Ghosts

We investigate the possibility that patches of old radio plasma (`radio ghosts') of former radio galaxies form a second distinct phase of the inter-galactic medium (IGM), not mixed with the thermal gas. The separation of this phase from the ambient gas and its resistance against eroding turbulent forces is given by magnetic fields, which are expected to be roughly in pressure equilibrium with the surrounding medium. Since patches of this plasma are largely invisible in the radio we use the term `radio ghost' to characterize their nature. Possibilities and difficulties of different detection strategies of ghosts are discussed. These involve radio emission, cosmic microwave background (CMB) and starlight Comptonization, and Faraday rotation. Re-activation of the electron population in shock waves of cosmological structure formation, which seems to lead to the cluster radio relic phenomena. We discuss the role radio ghosts can have: They are able to store relativistic particles for cosmological times, but are also able to release them under the influence of very strong turbulence. This might happen during a major merger event of clusters of galaxies. The released relativistic proton population could produce the observed radio halos of some cluster of galaxies via hadronic reactions with the background gas leading to the production of secondary electrons and positrons. Destroyed ghosts, mixed with the IGM can help to magnetize it. Finally, the strong field strength within ghosts should have a significant impact on the propagation of extragalactic high energy cosmic rays.

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arXiv1999

Non-thermal Origin of the EUV and HEX Excess Emission of the Coma Cluster - the Nature of the Energetic Electrons

The excess in extreme-ultraviolet (EUV) radiation and high energy X-rays (HEX) of Coma may be modeled by power law spectra. Their origin could be inverse Compton (IC) emission by relativistic electrons. The scattered photon fields can either be the CMB or the starlight of the galaxies within the cluster. If the EUV excess is due to CMB-IC, a strongly inhomogeneous magnetized intra- cluster medium (ICM) is required, in which the density of the relativistic electrons is anticorrelated with the magnetic fields. This could be understood if the electrons were accelerated during a major merger event within the last 2 Gyr and cooled afterwards in the inhomogeneous fields. If the EUV excess is due to starlight-IC, a population of relativistic, very low energy electrons has to be present, with a high energy density. In order to survive Coulomb losses, these electrons have to be separated from the cluster gas by confining magnetic fields. This could be remnant radio plasma from the epoch of violent quasar activity. The observed narrow radial profile of the EUV excess emission is due to the narrow photon distribution for this model. Both models favor very inhomogeneous magnetic field and relativistic electron distributions. IC models for the HEX excess seem to be implausible: CMB-IC leads to a mainly unmagnetized ICM, in contradiction to Faraday rotation measurements. Starlight scattering electrons would overproduce EUV photons due to simultaneously CMB-IC. We propose that the observed HEX excess is due to bremsstrahlung of a small high energy power-law tail of the mainly thermal ICM electron distribution. Such a tail is expected due to in-situ acceleration by turbulence.

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arXiv1998

Cosmic Rays in the Galactic Center Region

EGRET data on the Gamma ray emission from the inner Galaxy have shown a rather flat spectrum, extending to about 50 GeV. It is usually assumed that these gamma-rays arise from the interactions of cosmic ray nuclei with ambient matter. Cosmic Ray particles have been observed up to 3 10^20 eV, with many arguments suggesting, that up to about 3 10^18 eV they are of Galactic origin. Cosmic ray particles get injected by supernova explosions. Their injected spectrum is steepened by diffusive losses from the Galaxy to yield the observed spectrum. As cosmic ray particles roam around in the Galactic disk, they encounter molecular clouds and through p-p collisions produce gamma rays from pion decay. The flux and spectrum of these gamma rays is then a clear signature of cosmic rays throughout the Galaxy. Star formation activity peaks in the central region of the Galaxy, around the Galactic Center. Looking then at the gamma ray spectrum of the central region of our Galaxy yields clues as to where the cosmic ray particles interact, and with what spectrum. Using the FLUKA Monte-Carlo, we have modelled this spectrum, and find a best fit for a powerlaw spectral index of 2.34, rather close to the suggested injection spectrum for supernovae which explode into their own winds. This suggests that most cosmic ray interaction happens near the sources of injection; this is consistent with the spectrum of cosmic ray nuclei derived from spallation. One consequence is that cosmic ray heating and ionization should be strong in the Galactic Center region.

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arXiv1998

Shock Waves of the Large-Scale Structure Formation in the Universe

Simulations of structure formation in the Universe predict accretion shock waves at the boundaries of the large-scale structures as sheets, filaments, and clusters of galaxies. If magnetic fields are present at these shocks, particle acceleration should take place, and could contribute to the observed cosmic rays of high energies. When the radio plasma of an old invisible radio lobe is dragged into such a shock wave, the relativistic electron population will be reaccelerated and the plasma becomes radio-luminous again. Such tracers of the accretion shock waves are observed at the boundaries of some clusters of galaxies: the so-called cluster radio relics, which are large regions of diffuse radio emission, without any parent galaxy nearby. The observed properties of the cluster radio relics are naturally explained by accretion shock waves. Radio relics therefore give the first evidence for the existence of accretion shocks of the large-scale structure formation and they allow investigations of the shock properties.

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arXiv1998

Black Hole Energy Release to the Gaseous Universe

We estimate the energy release of black hole formation to the intra-cluster medium of the Coma cluster and find that this is comparable to the present day energy content. Therefore an energetic and maybe hydrostatic influence is possible. Our calculations rely on the assumption of an universal black hole to galaxy mass ratio (more exactly: spheroidal mass component of the stellar population), for which there is growing evidence. On a cosmological scale, there is also an energy release of black hole formation comparable to what is expected to be present within the thermal gas, caused by the process of structure formation. This indicates an important dynamical influence, neglected by present day structure formation simulations. This estimate of cosmological black hole energy release is independent of the black hole to galaxy mass ratio, but consistent with its value.

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arXiv1997

Cluster Radio Relics as a Tracer of Shock Waves of the Large-Scale Structure Formation

We present evidence for the existence of shock waves caused by the formation of the large-scale structure. In some clusters of galaxies peripherally located sources of extended diffuse radio emission exist, the so-called cluster radio relics. They have steep radio spectra but no apparent cutoff, as old remnants of radio galaxies usually have. Therefore particle acceleration has to take place within them. We propose that shock structures of the cosmological large-scale matter flows are responsible for the acceleration of relativistic electrons: cluster accretion shocks and bow shocks of merger events. We develop a theory of radio plasma having traversed these shocks and compare it to observational data of nine radio relics (0038-096, 0917+75, 1140+203, 1253+275, 1712+64, 1706+78, 2006-56, 2010-57, 1401-33) and their host clusters (A85, A786, A1367, Coma, A2255, A2256, A3667, S753). The necessary accretion power, the spectral index of the radio spectrum, the acceleration efficiency of the shock, the diffusion coefficient in the post-shock region, and the predicted radio polarization in all of our examples fit into a coherent interpretation of the observational data. Since polarization measurements are available only for four sources, the predictions of our theory can be independently checked using other examples. The predicted values of the shock compression ratio, density and temperature of the infalling gas, magnetic field strength of the shocked and unshocked radio plasma are discussed within the frame of structure formation theory.

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arXiv1997

Limits on Magnetic Fields and Relativistic Electrons in the Coma Cluster from Multifrequency Observations

Relativistic electrons, seen in the large diffuse radio halo of the Coma cluster of galaxies, should scatter background photons to higher energies. We calculate the inverse Compton contributions from the microwave background, from the local radiation field of elliptical galaxies and from the thermal X-ray emission of the intra cluster medium to the different observed energy bands, and draw restrictions to the shape of the spectrum of the relativistic electron population. The expected electron spectra in different halo formation models are discussed, and signatures for a future distinction of these models are presented, tracing the injection processes and the influence of optical thickness of the radio halo for very low frequency emission due to synchrotron self absorption. Some of the upper limits on the electron spectrum translate into lower limits of the halo magnetic fields, since the observed synchrotron radiation is known. Despite uncertainties in the extrapolation of the radio spectrum to lower frequencies a lower limit to the central field strength of B_o > 0.3 muG seems to be sure. This is comparable to the value resulting from minimal energy arguments. If B_o < 1.2 muG, the recently reported extreme ultraviolet excess of Coma (Lieu et al. 1996) could result from inverse Compton scattered microwave photons. Formulae for transrelativistic Thomson scattering are given in the Appendix.

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