arXiv subjects

Christian R. Kaiser

Publications and source records attributed to Christian R. Kaiser.

35 records · Page 2Linked to original sources

arXiv2003

Hot and cold bubbles in M87

The X-ray data obtained with XMM-Newton is used to investigate the complex structure of the gas in the atmosphere of the Virgo cluster around M87. We construct a simple model for the temperature and density distribution. This model implies that the cumulative mass of the cluster gas is a power-law of its entropy index, $kT n^{-2/3}$, similar to the Hydra cluster. This supports the idea that such power-laws are a direct consequence of gas cooling in a gravitational potential. In the cluster atmosphere hot bubbles of gas injected by the AGN are rising buoyantly. We estimate the age of these structures from the synchrotron radio data and find that this `radiative age' is consistent with the estimated dynamical timescale. However, this requires a spatial separation of the relativistic particles from the magnetic field. The age estimates suggest an activity cycle of the AGN in M87 of roughly $10^8$ years. We show that the largest radio structures are consistent with being the remnants of buoyant bubbles injected by the AGN during an even earlier activity cycle. The wakes behind the currently rising hot bubbles uplift cold gas from the cluster centre. Using a simple model for the trajectory of the cold gas, we demonstrate that the observations by XMM-Newton of a mix of hot and cold gas in the cluster atmosphere in the vicinity of the radio structure can be explained in this scenario. This may also explain the ridges of enhanced X-ray emission from cold gas observed with CHANDRA.

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arXiv2002

AGN as cosmic thermostats

We present a simple analytical model and the results of numerical simulations supporting the idea of periodical heating of cluster gas by AGN outflows. We show why, under this assumption, we are extremely unlikely to observe clusters containing gas with a temperature below about 1 keV. We review the results from numerical hydro-simulations studying the mechanisms by which AGN outflows heat the cluster gas.

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arXiv2002

Simple models of cooling flows

A semi-analytic model of cluster cooling flows is presented. The model assumes that episodic nuclear activity followed by radiative cooling without mass-dropout cycles the cluster gas between a relatively homogeneous, nearly isothermal post-outburst state and a cuspy configuration in which a cooling catastrophe initiates the next nuclear outburst. Fitting the model to Chandra data for the Hydra cluster, a lower limit of $284\Myr$ until the next outburst of Hydra A is derived. Density, temperature and emission-measure profiles at several times prior to the cooling catastrophe are presented. It proves possible to fit the mass $M(σ)$ with entropy index $Pρ^{-γ}$ less than $σ$ to a simple power-law form, which is almost invariant as the cluster cools. We show that radiative cooling automatically establishes this power-law form if the entropy index was constant throughout the cluster gas at some early epoch or after an AGN activity cycle. To high precision, the central value of $σ$ decreases linearly in time. The fraction of clusters in a magnitude-limited sample that have gas cooler than $T$ is calculated, and is shown to be small for $T=2\keV$. Similarly, only 1 percent of clusters in such a sample contain gas with $Pρ^{-γ} < 2\keV\cm^2$. Entropy production in shocks is shown to be small. The entropy that is radiated from the cluster can be replaced if a few percent of the cluster gas passes through bubbles heated during an outburst of the AGN.

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arXiv2002

Modelling Oscillations in the Jet Emission from Microquasar GRS 1915+105

The variability in the infrared to millimetre emission from microquasar GRS 1915+105 is believed to be dominated by the system's relativistic jet. In this paper we develop a time-dependent version of the Blandford & Koenigl (1979) jet emission model and apply it to the oscillations in the infrared and millimetre emission from GRS 1915+105 observed by Fender & Pooley (2000). The resulting model provides a reasonable description of the observed flux oscillations from GRS 1915+105. From a fit of the observed time lag between the flux peaks in the infrared and millimetre emission together with the flux normalisation we were able to determine the model parameters for the GRS 1915+105 jet. We find that to achieve the observed flux levels with the model requires an unphysically large electron density within the jet. We therefore conclude that the Blandford & Koenigl (1979) model cannot explain these observations, either because it does not provide the correct description of the emission from microquasar jets, or because the observed emission variations do not originate in the jet.

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arXiv2002

Modelling Oscillations in GRS 1915+105 Jet Emission

We have studied time variability in the flux from the flat spectrum synchrotron radiation of the Blandford & Konigl (1979) model for relativistic, conical jets. The resulting model has been applied to the flux variation of the flat spectrum of GRS 1915+105 observed by Fender & Pooley (2000). This comparison has highlighted a fundamental problem of the Blandford & Konigl (1979) model in that it requires unphysically large electron densities to explain the flux levels observed from the flat spectrum of GRS 1915+105.

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arXiv2002

Hot Bubbles from Active Galactic Nuclei as a Heat Source in Cooling Flow Clusters

The hot plasma permeating clusters of galaxies often shows a central peak in the X-ray surface brightness that is coincident with a drop in entropy. This is taken as evidence for a cooling flow where the radiative cooling in the central regions of a cluster causes a slow subsonic inflow. Searches in all wavebands have revealed significantly less cool gas than predicted indicating that the mass deposition rate of cooling flows is much lower than expected. However, most cooling flow clusters host an Active Galactic Nucleus (AGN) at their centres. AGN can inflate large bubbles of hot plasma that subsequently rise through the cluster atmosphere, thus stirring this gas. Here we report on the results from highly resolved hydrodynamic simulations which for the first time show that buoyant bubbles increase the cooling time in the inner cluster regions and thereby significantly reduce the deposition of cold gas. This work demonstrates that the action of AGN in the centres of cooling flow clusters can explain the observed absence of large quantities of cooled gas.

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arXiv2002

The death of FRII radio sources and their connection with radio relics

Radio relic sources in galaxy clusters are often described as the remnants of powerful radio galaxies. Here we develop a model for the evolution of such relics after the jets cease to supply energy to the lobes. This includes the treatment of a relic overpressured with respect to its gaseous surroundings even after the jets switch off. We also determine the radio emission of relics for a large variety of assumptions. We take into account the evolution of the strength of the magnetic field during the phase of relativistic particle injection into the lobes. The resulting spectra show mild steepening at around 1 GHz but avoid any exponential spectral cut-offs. The model calculations are used to fit the observed spectra of six radio relics. The quality of the fits is excellent for {\it all} models discussed. Unfortunately, this implies that it is virtually impossible to determine any of the important source parameters from the observed radio emission alone.

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arXiv2001

Pair annihilation and radio emission from galactic jet sources: The case of Nova Muscae

In the hard X-ray spectra of some X-ray binaries line features at around 500 keV are detected. We interpret these as arising from pair annihilation in relativistic outflows leading to a significant Doppler shift of the lines' frequencies. We show how this can be used to accurately determine the bulk velocity and orientation to the line of sight of the outflows. Constraints on the energy requirements of such outflows are also derived. Furthermore, we show that a small fraction of pairs escaping the annihilation region may give rise to the radio synchrotron emission observed in some of these objects. We apply these ideas to the hard X-ray and radio observations of Nova Muscae 1991. In this object, the energy requirements seem to rule out a large proton fraction in the outflows.

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arXiv2000

The environments and ages of extragalactic radio sources inferred from multi-frequency radio maps

A 3-dimensional model of the synchrotron emissivity of the cocoons of powerful extragalactic radio sources of type FRII is constructed. Bulk backflow and energy losses of the relativistic electrons, radiative and adiabatic, are self-consistently taken into account. Thus the model is an extension of spectral aging methods including the underlying source dynamics into the age estimates. The discrepancies between spectral ages and dynamical ages arising from earlier methods are resolved. It is also shown that diffusion of relativistic particles within the cocoon is unlikely to significantly change the particle spectrum and thus the emitted radio spectrum. Projection along the line of sight yields 1 or 2-dimensional surface brightness distributions which can be compared with observations. From the model parameters constraints on the source age, the density of the source environment and the angle to the line of sight can be derived. Application of the method to Cygnus A, 3C 219 and 3C 215 show that the method provides robust estimates for the model parameters for sources with comparatively regularly shaped radio lobes. The resolution of the radio maps required is only moderate. Within the large uncertainties for the orientation angle, the three example sources are found to be consistent with orientation-based unification schemes of radio-loud AGN. In the case of Cygnus A the gas density of the environment is found to agree with independent X-ray measurements. For 3C 219 and 3C 215 the densities derived from the model are apparently too low. It is suggested that these discrepancies are caused by overestimates of slope and core radius of $β$-models for the gas density distribution from X-ray observations for clusters hosting powerful radio sources.

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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

Internal shock model for Microquasars

We present a model for the radio outbursts of microquasars based on the assumption of quasi-continuous jet ejection. The jets are `lit up' by shock fronts traveling along the jets during outbursts. The observed comparatively flat decay light curves combined with gradually steepening spectral slopes are explained by a superposition of the radiation of the aging relativistic particle population left behind by the shocks. This scenario is the low energy, time-resolved equivalent to the internal shock model for GRBs. We show that this model predicts energy contents of the radiating plasma similar to the plasmon model. At the same time, the jet model relaxes the severe requirements on the central source in terms of the rate at which this energy must be supplied to the jet. Observations of `mini-bursts' with flat spectral slopes and of infrared emission far from the source centre suggest two different states of jet ejections: (i) A `mini-burst' mode with relatively stable jet production and weak radio emission with flat spectra and (ii) an outburst mode with strong variations in the jet bulk velocities coupled with strong radio emission with steeper spectra. We also show that the continuous jets in microquasars should terminate in strong shocks and possibly inflate radio lobes similar to extragalactic jet sources. Finally, we investigate the possibility of testing the predictions of this model with resolved radio observations.

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arXiv1999

Radio galaxies with a `double-double' morphology: II - The evolution of double-double radio galaxies and implications for the alignment effect in FRII sources

A Double-Double Radio Galaxy (DDRG) is defined as consisting of a pair of double radio sources with a common centre. In this paper we present an analytical model in which the peculiar radio structure of DDRGs is caused by an interruption of the jet flow in the central AGN. The new jets emerging from the restarted AGN give rise to an inner source structure within the region of the old, outer cocoon. Standard models of the evolution of FRII sources predict gas densities within the region of the old cocoon that are insufficient to explain the observed properties of the inner source structure. Therefore, additional material must have passed from the environment of the source through the bow shock surrounding the outer source structure into the cocoon. We propose that this material is warm clouds ($\sim\!10^4$ K) of gas embedded in the hot IGM which are eventually dispersed over the cocoon volume by surface instabilities induced by the passage of cocoon material. The derived lower limits for the volume filling factors of these clouds are in good agreement with results obtained from optical observations. The long time scales for the dispersion of the clouds ($\sim\!10^7$ yr) are consistent with the apparently exclusive occurrence of the DDRG phenomenon in large ($\ga 700$ kpc) radio sources and with the observed correlation of the strength of the optical/UV alignment effect in $z\!\sim\!1$ FRII sources with their linear size.

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arXiv1999

Multi-frequency observations as a key to source and environment parameters of FRII objects

Our knowledge of the environments of radio-loud AGN is still sketchy. However, to understand the jet phenomenon it is important to know about the properties of the surroundings in which jets are formed and evolve. Here I present an analytical model of the radio surface brightness distribution of the large scale structure of FRII-type radio sources. The `virtual maps' resulting from this model can be compared with observed maps to obtain estimates for a range of source properties from the model. These properties include parameters describing the gas density distribution of the source environment, the energy transport rate of the jets and the orientation angle of the source jet axis with respect to the line of sight. The model is tested using radio maps of Cygnus A for which there are independent measurements of some of these parameters available in the literature. The model estimates agree well with these observations. Varying the resolution of the radio maps used in thecomparison does not change the results significantly.

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arXiv1999

Heating of the IGM by FRII radio sources

We present results of a numerical integration of the hydrodynamical equations governing the self-similar, two-dimensional gas flow behind the bow shock of an FRII radio source embedded in an IGM with a power law density profile. The model predicts pressure gradients within the cocoons consistent with modest backflow. For very steep external density profiles sources may well not expand in a self-similar fashion and in this case the model is not self-consistent. The assumption of ram pressure confinement of the cocoons perpendicular to the jet axis is found to overestimate the ratio of the pressure in front of the radio hot spots and that in the cocoons. Based on the properties of the gas between bow shock and cocoon we calculate the X-ray surface brightness of the flow. This emission is found to be a good tracer of the density distribution within the flow and varies significantly with the properties of the unshocked IGM. The cooling-time of the shocked IGM is found to be comparable to, or greater than, the Hubble time. The influence of a radio source on the evolution of its gaseous surroundings therefore extends well beyond the limited life time of the source itself. We compare our results with the X-ray map of Cygnus A and find some evidence for cold, dense gas clumps in the surroundings of this object. The extended X-ray emission observed around 3C 356 may also be caused by the bow shock of this radio source. We also present an empirical model for the X-ray emission of the shocked IGM due to thermal bremsstrahlung.

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arXiv1998

On the cosmological evolution of the FRII radio source population

We present an analytical model for the cosmological evolution of the FRII source population. Based on an earlier model for the intrinsic radio luminosity - linear size evolution of these objects, we construct theoretical source samples. The source distributions in the radio power - linear size plane of these samples are then compared with that of an observed flux-limited sample. We find that the source parameters determining the radio luminosity of FRII objects can not be independent of each other. The best-fitting models predict the jet power to be correlated either with the life time of the source or with the shape of the density distribution of the source environment. The latter case is consistent with the observed tendency of the most luminous radio sources at high redshift to be located in richer and more extended environments than their low redshift counterparts. We also find evidence for a class of FRII sources distinctly different from the main population. These sources are extremely old and/or are located in very underdense environments. The luminosity function of FRII sources resulting from the model is in good agreement with previous results for high luminosity sources. The apparent luminosity evolution of the radio luminosity function is not reproduced because of the high flux limit of the used comparison sample. The cosmological evolution of the median linear size of FRII sources is found to be mild.

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arXiv1998

The mutual interaction of powerful radio galaxies and their environments

Using a self-similar model for the expansion of cocoons surrounding the jets in powerful extragalactic radio sources (type FRII), we investigate the influence of the properties of the gas surrounding these objects on their evolution. The variation of their radio luminosity as a function of linear size for individual sources is determined in dependence of the density distribution of the IGM the source is embedded in. Based on these results, the cosmological evolution of the FRII population and hence that of the environments of these sources can be constrained. The environments are found to be denser and/or more extended at high redshifts. The bow shock surrounding the cocoon of FRII sources compresses and heats the IGM. A numerical integration of the hydrodynamic equations governing the gas flow between the bow shock and the cocoon boundary is presented. From this we determine the appearance of the large scale structure of radio galaxies in X-rays and the cooling times for the IGM heated by the passage of the bow shock. This has important implications for the cosmological evolution of the IGM.

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arXiv1997

Evolutionary tracks of FRII sources through the P-D diagram

This is the second in a series of papers presenting an analytical model for the evolution of FRII radio sources. In this paper we evaluate the expected radio emission from a radio source incorporating energy loss processes for the relativistic electrons. By combining these results with our earlier dynamical model we calculate evolutionary tracks through the Power-Linear size diagram. These tracks are in good agreement with the observed distribution of sources in this diagram. The effects of different forms for the evolution of the magnetic field in the cocoon, the redshift of the source, the environment of the source and the defining parameters of the jet are investigated. The evolutionary tracks are found to be insensitive to the assumed form of the magnetic field evolution. Some evidence against protons as a major constituent of the jet material is also found.

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