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H. J. Voelk

Publications and source records attributed to H. J. Voelk.

50 records · Page 3Linked to original sources
arXiv2001

Supernova Remnants: acceleration of particles and gamma-ray emission

Particle acceleration in the dynamically evolving environment of Supernova Remnants is discussed in the framework of a genuinely time-dependent nonlinear theory, assuming spherical symmetry. As a consequence the dependence of injection on the angle between shock normal and external magnetic field direction requires a renormalisation of the calculated particle fluxes. The recent observational results in TeV gamma-rays from such objects are discussed and found to be consistent with theory. We conclude that for the present instrumental sensitivities there are no reasons to draw premature negative conclusions as to the possible origin of the Galactic Cosmic Rays below the "knee" in Supernova Remnants. In addition, theoretical predictions and observations are getting very close. Therefore the coming generation of ground-based and space-borne detectors will decide this basic question of astrophysics.

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arXiv2001

Gas cooling within the diffuse ISM of late-type galaxies

We combine observations of spiral galaxies in the [CII] line at 158 micron, made with the Long Wavelength Spectrometer aboard ISO, with previous data from the Kuiper Airborne Observatory to study the origin of this line, which is the main coolant of the interstellar medium at relatively low temperatures. We also use HI and CO(1-0) observations of these galaxies and estimate the respective line fluxes in the same beam as the [CII] observations. We confirm the existence of a linear relation between the [CII] line intensity and the CO(1-0) line intensity, that we extend to intrinsically fainter galaxies. The dispersion around this relation is significant and due to variations in the far-UV flux, thus in the star formation rate. We find that for the least active galaxies of our sample, in terms of star formation, the rate of [CII] line emission per interstellar hydrogen atom is similar to that in the Solar neighbourhood. For those galaxies, most of the [CII] line emission comes probably from the diffuse cold atomic medium. In more active galaxies, considered globally, the average [CII] line emission is dominated by dense photodissociation regions and to some extent by the warm ionized diffuse medium. This is true in the central region of many spiral galaxies, and probably even in the interarm regions of the most actively star-forming ones.

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arXiv2000

Critical self-organization of astrophysical shocks

There are two distinct regimes of the first order Fermi acceleration at shocks. The first is a linear (test particle) regime in which most of the shock energy goes into thermal and bulk motion of the plasma. The second is an efficient regime when it goes into accelerated particles. Although the transition region between them is narrow, we identify the factors that drive the system to a {\it self-organized critical state} between those two. Using an analytic solution, we determine this critical state and calculate the spectra and maximum energy of accelerated particles.

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arXiv2000

On the gamma-ray fluxes expected from Cassiopeia A

Based on the results of our previous study of the broad band synchrotron emission of Cas A in the framework of a spatial inhomogeneous model, we calculate the fluxes of gamma-radiation that can be expected from this supernova remnant in different energy bands. We show that at energies below 10 GeV gamma-ray fluxes detectable by forthcoming space-borne detectors should be produced due to bremsstrahlung of radio emitting electrons. We predict that the power-law index of the photon flux in the GeV region should be hard, close to the index ~2.2 expected for the acceleration spectrum of the electrons. Photon fluxes accessible to future ground-based gamma-ray detectors could also be expected at very high energies. The fluxes to be expected due to bremsstrahlung and inverse Compton radiation of relativistic electrons at TeV energies should be very steep, and strongly dependent on the characteristic magnetic fields in Cas A. We could expect also significant fluxes of pi^0 -decay gamma-rays produced by relativistic protons. These fluxes may extend with a hard spectrum beyond TeV energies as far as the protons would be accelerated to energies beyond 100 TeV. The hardness of the gamma-ray spectrum at TeV energies could in principle allow one to distinguish between electronic and hadronic origins of those gamma-rays. We discuss also other implications, such as relativistic particle content, or physical parameters in the source, that could be derived from future gamma-ray flux measurements in different energy bands.

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arXiv2000

On energy-dependent propagation effects and acceleration sites of relativistic electrons in Cassiopeia A

We consider the effect of energy dependent propagation of relativistic electrons in a spatially inhomogeneous medium in order to interpret the broad-band nonthermal radiation of the young supernova remnant (SNR) Cassiopeia A. A two-zone model is proposed that distinguishes between compact, bright steep-spectrum radio knots and the bright fragmented radio ring on the one hand, and the remainder of the shell - the diffuse `plateau' - on the other hand. In the framework of this model it is possible to explain the basic features of the spectral and temporal evolution of the synchrotron radiation of Cas A if one assumes that these compact structures correspond to sites of efficient electron acceleration producing hard spectra of accelerated particles with power-law indices ~2.2. The resulting energy distribution of radio electrons in these compact structures becomes significantly steeper than the electron production spectrum on timescales of the energy dependent escape of these electrons into the surrounding diffuse plateau region. We argue that the steepness, rather than the hardness, of the radio spectra of compact bright structures in clumpy sources can in general be considered as a typical signature of sites where strong electron acceleration has built up high gradients in the spatial distribution of radio electrons. Subsequent diffusive escape then modifies their energy distribution, leading to spatial variations of spectral indices within the radio source. Qualitative and quantitative interpretations of a number of observational data of Cas A are given. Predictions following from the model are discussed.

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arXiv1999

Implications of a Nonthermal Origin of the Excess EUV Emission from the Coma Cluster of Galaxies

The inverse Compton (IC) interpretation of the excess EUV emission, that was recently reported from several clusters of galaxies, suggests that the amount of relativistic electrons in the intracluster medium is highly significant, W_e>10^{61} erg. Considering Coma as the prototype galaxy cluster of nonthermal radiation, we discuss implications of the inverse Compton origin of the excess EUV fluxes in the case of low intracluster magnetic fields of order 0.1 muG, as required for the IC interpretation of the observed excess hard X-ray flux, and in the case of high fields of order 1 muG as suggested by Faraday rotation measurements. Although for such high intracluster fields the excess hard X-rays will require an explanation other than by the IC effect, we show that the excess EUV flux can be explained by the IC emission of a `relic' population of electrons driven into the incipient intracluster medium at the epoch of starburst activity by galactic winds, and later on reenergized by adiabatic compression and/or large-scale shocks transmitted through the cluster as the consequence of more recent merger events. For high magnetic fields B > 1 muG the interpretation of the radio fluxes of Coma requires a second population of electrons injected recently. They can be explained as secondaries produced by a population of relativistic protons. We calculate the fluxes of gamma-rays to be expected in both the low and high magnetic field scenarios, and discuss possibilities to distinguish between these two principal options by future gamma-ray observations.

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arXiv1999

On the radio spectral index of galaxies

The radio emission of a galaxy consists of thermal bremsstrahlung, synchrotron emission from discrete supernova remnants, and diffuse synchrotron emission from cosmic ray electrons spread over the galactic disk and halo. Each of these components has a different spectral index so that the total radio spectral index of a galaxy depends sensitively on their relative contribution and on the processes shaping the diffuse synchrotron emission. In the present paper we calculate the contribution of supernova remnants to the total synchrotron emission and conclude that it is about 10%. This moderate contribution has a noticeable effect on the nonthermal spectral index, lowering it by $\simeq 0.1$ for steep spectra. We then calculate the diffuse synchrotron emission in two simple models, a diffusion and a convection model. We find that the spatially integrated nonthermal spectral index is in general a poor diagnostic for the type of propagation or the importance of energy losses. Spatially resolved radio data for the halo of galaxies are necessary in order to draw firm conclusions. The steepening of the spectrum away from the disk is a clear indication that synchrotron and inverse Compton losses are taking place during the propagation of cosmic ray electrons in the halo. In those galaxies for which spatially resolved data for the halos exist such a steepening has been found. We conclude therefore that energy losses are generally important and that cosmic ray electrons cannot freely escape from galaxies.

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arXiv1999

Effectiveness of TeV Gamma-Ray Observations at Large Zenith Angles with a Stereoscopic System of Imaging Atmospheric Cherenkov Telescopes

The sensitivity of imaging atmospheric Cherenkov telescopes (IACTs) in TeV gamma-ray observations reachs its maximum at small zenith angles (< 30 degree) which provide the minimum attainable energy threshold of an instrument. However, for a specific telescope site a number of gamma-ray sources, or source candidates, can only be observed at much larger zenith angles (< 60 degree). Moreover the observations at large zenith angles allow to extend the observation time window for any object seen at small zenith angles, as well as to enlarge the dynamic energy range of an instrument towards the highest observable energies of gamma-rays. Based on Monte Carlo simulations we present here the results on the sensitivity of a stereoscopic system of 5 IACTs in observations at large zenith angles. We point out some important parameters of the telescope design which could substantially improve the efficiency of such observations with forthcoming IACT arrays like CANGAROO III, HESS and VERITAS.

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arXiv1998

Clusters of Galaxies: magnetic fields and nonthermal emission

The nonthermal particle content of galaxy clusters should in part have a cosmological component generated during the early starburst phase of the member galaxies. This is reviewed in the framework of a simple cluster formation model suggested previously. It implies a nonthermal energy fraction of about 10 percent for the Intracluster gas. We also propose a mechanism for the early generation of Intracluster magnetic fields in terms of Galactic Winds. It results in typical field strengths of about 0.1 microGauss. Such comparatively weak fields are consistent with an inverse Compton origin of the excess EUV and hard X-ray emission of the Coma cluster, given the radio synchrotron emission. The required relativistic electrons must have been accelerated rather recently, less than a few billion years ago, presumably in cluster accretion shocks. This is in contrast to the hadronic nonthermal component which accumulates on cosmological time scales, and whose pion-decay TeV gamma-ray emission is expected to be larger, or of the same order as the inverse Compton TeV emission. This gamma-radiation characterizes the energetic history of cluster formation and should be observable with future arrays of imaging atmospheric Cherenkov telescopes.

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arXiv1996

The FIR/radio correlation in starburst galaxies - constraints on starburst models

This paper presents an analysis of the correlation between the far-infrared (FIR) and the radio emission of starburst galaxies. Data for interacting galaxies, many of which are undergoing a starburst, and for normal galaxies have been analysed and compared in order to test for any influence of the star-formation activity on the ratio between the FIR and the radio emission at 2.4 GHz, $\q24$. No statistically significant indication for such an influence has been found: There is neither a significant difference between $\q24$ of the two samples nor a dependence of this ratio on the starburst strength. This observational fact is unexpected because of the different physical conditions and the short time-scale of the star-formation activity in a starburst. In order to interprete the observations the FIR and the radio emission during a starburst have been modeled. The following conclusions could be drawn from the observed constancy of $\q24$: A strong and fast ($\approx 10^7$ yr) increase of the magnetic field at the beginning of the starburst is required in order to maintain a constant $\q24$. Otherwise the strong Inverse Compton losses that are due to the intense radiation field in a starburst would lower the synchrotron emission drastically resulting in a value of $\q24$ significantly higher than the observed one. Furthermore, the time-scale of the variation of the star-formation rate has to be longer than some $10^7$ yr. For lower values the different time-scales of the FIR and the radio emission produce large fluctuations of $\q24$.

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arXiv1996

A quantitative model of the FIR/radio correlation for normal late-type galaxies

This paper investigates the physical reasons for the existence, the tightness and the near universality of the FIR/radio correlation for late-type field galaxies. We develop theoretical models for the radio and far-infrared (FIR) emission of normal galaxies and study the influence of their main parameters on the ratio of the two emissions. In addition, data are used from a sample of 114 late-type galaxies which allow an estimate of the mean energy densities of the radiation field and the magnetic field, and of the dust opacity. These data reveal, for the first time, a reasonably good, linear correlation between the energy density of the radiation field and the energy density of the magnetic field. Furthermore we find that on average the galaxies are marginally optically thick for the non-ionizing UV light. Including their extended magnetic halos, galaxies are also found to be on average marginally optically thick regarding the radiative energy losses of the Cosmic Ray electrons. Combining these semi-empirical results with the theoretical emission models we show that in the optically thick case the linear correlation between the energy densities of the radiation field and the magnetic field is a necessary condition for the existence of the observed FIR/radio correlation. Furthermore we show that the individual dispersion of the FIR to radio continuum ratio caused by uncertainties of any single one of the parameters considered is significantly less than the empirical dispersion of the FIR/radio correlation. The radio and the FIR emission are mainly determined by their sources, i.e. massive supernova precursor stars, because galaxies act in a reasonable approximation as calorimeters for the stellar UV radiation and for the energy flux of the Cosmic Ray electrons.

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arXiv1994

Cosmic Ray Hydrodynamics at Shock Fronts

When a magnetised, thermal plasma containing a shock wave efficiently accelerates Cosmic Rays (CR) a reaction is exerted on the fluid structure. The simplest description of this process involves three conservation equations for the background fluid, with the CR pressure gradient included as an extra force, plus a hydrodynamic equation for the CR energy density. Since the CR spectrum is not determined in this approach two quantities are introduced, the ratio of CR pressure to energy density and a mean diffusion coefficient, that must be specified to close the system of equations. Physical arguments are presented for how these quantities should evolve and then compared with time dependent numerical solutions of acceleration at a plane parallel, piston driven shock.

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arXiv1993

Very High Energy Gamma-Rays from AGN: Cascading on the Cosmic Background Radiation Fields and the Formation of Pair Halos

Recent high energy gamma-ray observations (E>100 MeV) of blazar AGN show emission spectra with no clear upper energy cutoff. AGN, considered to be possible sources for the highest energy cosmic rays, may have emission extending well into the VHE (very high energy, E>100 GeV) domain. Because VHE gamma-rays are absorbed by pair production on the intergalactic background radiation, much of this emission is not visible. Electromagnetic cascades initiated by absorbed VHE gamma-rays, however, may be observable. Since the velocities of e+e- pairs produced in a cascade are likely to be isotropized by an ambient random magnetic field, extended ``halos'' (R>1 Mpc) of pairs will be formed around AGN with VHE emission. Cascade radiation from these pair halos is emitted isotropically and is observable below a few TeV. The halo radiation can be distinguished by its characteristic variation in spectrum and intensity with angular distance from the central source. This variation depends weakly on the details of the central source model, e.g., the orientation and beaming/opening angle of an emitting jet. Limiting or determining the intensity of the pair halo can thus serve as a model-independent bound on or measure of the VHE power of AGN. Next-generation Cherenkov telescopes may be able to image a pair halo.

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