1,720,991 research outputs found
Comptonization of the cosmic microwave background by relativistic plasma
No full textWe 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 × 1066 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 τrel < 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
Internal shock model for microquasars
No full textWe 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 shocks accelerate relativistic particles which emit the observed synchrotron radiation. 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. We investigate the possibility of testing the predictions of this model with resolved radio observations. Finally, we suggest that Doppler-shifted X-ray iron lines, and possibly H-alpha lines, may be emitted by the jet flow of microquasars if thermal instabilities analogous to those in SS433 exist in their jets
The environments and ages of extragalactic radio sources inferred from multi-frequency radio maps
No full textA 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 -models for the gas density distribution from X-ray observations for clusters hosting powerful radio sources
Buoyant radio plasma in clusters of galaxies
No full textRadio galaxies are known to inflate lobes of hot relativistic plasmas into the intergalactic medium. Here we present hydrodynamical and magnetohydrodynamical simulations of these hot plasma bubbles in FR II objects. We focus on the later stages of their evolution after the jet has died down and after the bow shock that surrounded the lobes at earlier stages has vanished. We investigate the evolution of the plasma bubbles as they become subject to Rayleigh–Taylor instabilities. From our simulations we calculate the radio and X-ray emissivities of the bubbles and discuss their appearance in observations. Finally, we investigate the influence of large-scale magnetic fields on the evolution of the bubbles. The issues of re-acceleration and diffusion of relativistic particles are briefly discussed
A self-similar model for extragalactic radio sources
No full textAn analytical model for extragalactic radio sources with pressure-confined jets is presented. We show that the properties of the bow shock and of the gas surrounding these objects force the sources to grow in a self-similar way provided the density in the external atmosphere falls less steeply than 1/d2. Results from observations and numerical simulations are used to develop a self-consistent model for the large-scale structure of FRII objects. The jets in these sources are shown to be stable against turbulence for the observed properties of FRIIs and the divide between FRI and FRII objects in jet power is reproduced. The overall dynamics of a source are derived as a function of time and depend on the external density, the jet power and the scaleheight of the external atmosphere. Derived dynamical ages are consistent with observed spectral ages
Internal shock model for the radio emission of Microquasars
No full textThe superluminal radio components observed in microquasars are usually interpreted as individual ejection events. We discuss some of the shortcomings of this model and propose the internal shock model in quasi-continuous relativistic jets as an alternative. This model can resolve the problems with the single ejection model and is in agreement with radio observations. We outline some further testable model predictions some of which are already confirmed by observations
Modelling oscillations in the jet emission from microquasar GRS 1915+105
No full textThe 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 jet emission model of Blandford & Königl and apply it to the oscillations in the infrared and millimetre emission from GRS 1915+105 observed recently by Fender & Pooley. 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 normalization, 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 & Königl 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
Evolution of buoyant bubbles in M87
No full textThe morphology of the X-ray and radio-emitting features in the central ~50 kpc region around the galaxy M87 strongly suggests that buoyant bubbles of cosmic rays (inflated by an earlier nuclear active phase of the galaxy) rise through the cooling gas at roughly half the sound speed. In the absence of strong surface tension, initially spherical bubbles will transform into tori as they rise through an external medium. Such structures can be identified in the radio images of the halo of M87. During their rise, bubbles will uplift relatively cool X-ray-emitting gas from the central regions of the cooling flow to larger distances. This gas is colder than the ambient gas and has a higher volume emissivity. As a result, rising "radio" bubbles may be trailed by elongated X-ray features, as indeed is observed in M87. We performed simple hydrodynamic simulations to illustrate qualitatively the evolution of buoyant bubbles in the M87 environment
Observations of the unusual counterpart to the X-ray pulsar AX J0051-733
No full textWe report optical and infrared (IR) observations of the ASCA X-ray pulsar system AX J0051-733. The relationship between the X-ray source and possible optical counterparts is discussed. Long-term optical data from over 7 yr are presented which reveal both a 1.4-d modulation and an unusually rapid change in this possible binary period. Various models are discussed
The depolarization properties of powerful radio sources: breaking the radio power versus redshift degeneracy
No full textWe define three samples of extragalactic radio sources of Fanaroff-Riley type II, containing 26 objects in total. The control sample consists of 6C and 7C sources with radio powers of around 1027 W Hz-1 at 151 MHz and redshifts of z~ 1. The other samples contain 3CRR sources either with comparable redshifts but radio powers about a decade larger or with comparable radio powers but redshifts around z~ 0.4. We use these samples to investigate the possible evolution of their depolarization and rotation measure properties with redshift and radio power independently. We used VLA data for all sources at ~4800 MHz and two frequencies within the 1400-MHz band, either from our own observations or from the archive. We present maps of the total intensity flux, polarized flux, depolarization, spectral index, rotation measure and magnetic field direction where not previously published. Radio cores were detected in 12 of the 26 radio sources. Of the sources, 14 show a strong Laing-Garrington effect, but almost all of the sources show some depolarization asymmetry. All sources show evidence for an external Faraday screen being responsible for the observed depolarization. We find that sources at higher redshift are more strongly depolarized. Rotation measure shows no trend with either redshift or radio power. However, variations in the rotation measure across individual sources increase with the redshift of the sources but do not depend on their radio power
- …
