arXiv subjects

Heinrich J. Voelk

Publications and source records attributed to Heinrich J. Voelk.

13 recordsLinked to original sources

arXiv2013

On The {\it Fermi} -Lat Surplus of the Diffuse Galactic Gamma-Ray Emission

Recent observations of the diffuse Galactic \gr emission (DGE) by the {\it Fermi} Large Area Telescope ({\it Fermi}-LAT) have shown significant deviations, above a few GeV until about 100 GeV, from DGE models that use the GALPROP code for the propagation of cosmic ray (CR) particles outside their sources in the Galaxy and their interaction with the target distributions of the interstellar gas and radiation fields. The surplus of radiation observed is most pronounced in the inner Galaxy, where the concentration of CR sources is strongest. The present study investigates this "{\it Fermi}-LAT Galactic Plane Surplus" by estimating the \gr emission from the sources themselves, which is disregarded in the above DGE models. It is shown that indeed the expected hard spectrum of CRs, still confined in their sources (SCRs), can explain this surplus. The method is based on earlier studies regarding the so-called EGRET GeV excess which by now is generally interpreted as an instrumental effect. The contribution from SCRs is predicted to increasingly exceed the DGE models also above 100 GeV, up to \gr energies of about ten TeV, where the corresponding surplus exceeds the hadronic part of the DGE by about one order of magnitude. Above such energies the emission surplus should decrease again with energy due to the finite life-time of the assumed supernova remnant sources. Observations of the DGE in the inner Galaxy at 15 TeV with the Milagro \gr detector and, at TeV energies, with the ARGO-YBJ detector are interpreted to provide confirmation of a significant SCR contribution to the DGE.

astro-ph.HE↗

arXiv2010

Shock acceleration of relativistic particles in galaxy-galaxy collisions

All galaxies without a radio-loud AGN follow a tight correlation between their global FIR and radio synchrotron luminosities, which is believed to be ultimately the result of the formation of massive stars. Two colliding pairs of galaxies, UGC12914/5 and UGC 813/6 deviate from this correlation and show an excess of radio emission which in both cases originates to a large extent in a gas bridge connecting the two galactic disks. We are aiming to clarify the origin of the radio continuum emission from the bridge. The radio synchrotron emission expected from the bridge regions is calculated, assuming that the kinetic energy liberated in the predominantly gas dynamic interaction of the respective interstellar media (ISM) has produced shock waves that efficiently accelerate nuclei and electrons to relativistic energies. We present a model for this acceleration and calculate the resulting radio emission, its spectral index and the expected high-energy gamma-ray emission. It is found that the nonthermal energy produced in the collision is large enough to explain the radio emission from the bridge between the two galaxies. The calculated spectral index at the present time also agrees with the observed value. The deviation of these two interacting galaxy systems from the standard FIR-radio correlation is consistent with the acceleration of an additional population of electrons. This process is not related to star formation and therefore it is expected that the systems do not follow the FIR-radio correlation. The acceleration of relativistic electrons in shocks caused by an ISM collision, in the same way as described here, is likely to take place in other systems as well, as in galaxy clusters and groups or high-redshift systems.

astro-ph.CO↗

arXiv2008

Imaging Very High Energy Gamma-Ray Telescopes

The technique of gamma-ray astronomy at very high energies (VHE: > 100 GeV) with ground-based imaging atmospheric Cherenkov telescopes is described, the H.E.S.S. array in Namibia serving as example. Mainly a discussion of the physical principles of the atmospheric Cherenkov technique is given, emphasizing its rapid development during the last decade. The present status is illustrated by two examples: the spectral and morphological characterization in VHE gamma-rays of a shell-type supernova remnant together with its theoretical interpretation, and the results of a survey of the Galactic Plane that shows a large variety of non-thermal sources. The final part is devoted to an overview of the ongoing and future instrumental developments.

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arXiv2006

Shell-type Supernova Remnants

The role of Supernova Remnants (SNRs) for the production of the Galactic Cosmic Rays is reviewed from the point of view of theory and very high energy gamma-ray experiments. The point is made that theory can describe young SNRs very well, if the evidence from the synchrotron emission is used to empirically determine several parameters of the theory, and thus theory can predict the relative contributions of hadronic and leptonic gamma rays at TeV energies. This is exemplified for several objects that have been observed intensively during the last years. Future key observations are discussed.

astro-ph↗

arXiv2006

New Results from High Energy Gamma-Ray Astronomy

High energy gamma-ray astronomy has recently made significant progresss through ground-based instruments like the {\it H.E.S.S.} array of imaging atmospheric Cherenkov telescopes. The unprecedented angular resolution and the large field of view has allowed to spatially resolve for the first time the morphology of gamma-ray sources in the TeV energy range. The experimental technique is described and the types of sources detected and still expected are discussed. Selected results include objects as different as a Galactic binary Pulsar, the Galactic Center and Supernova Remnants but they also concern the diffuse extragalactic optical/infrared radiation field. Finally, a scan of the Galactic plane in TeV gamma rays is described which has led to a significant number of new TeV sources, many of which are still unidentified in other wavelengths. The field has a close connection with X-ray astronomy which allows the study of the synchrotron emission from these very high energy sources.

astro-ph↗

arXiv2004

Modelling the spectral energy distribution of galaxies. III. Attenuation of stellar light in spiral galaxies

We present new calculations of the attenuation of stellar light from spiral galaxies using geometries for stars and dust which can reproduce the entire spectral energy distribution from the UV to the FIR/submm and can also account for the surface brightness distribution in both the optical/NIR and FIR/submm. The calculations are based on the model of Popescu et al. (2000), which incorporates a dustless stellar bulge, a disk of old stars with associated diffuse dust, a thin disk of young stars with associated diffuse dust, and a clumpy dust component associated with star-forming regions in the thin disk. The attenuations, which incorporate the effects of multiple anisotropic scattering, are derived separately for each stellar component, and presented in the form of easily accessible polynomial fits as a function of inclination, for a grid in optical depth and wavelength. The wavelength range considered is between 912 AA and 2.2 micron, sampled such that attenuation can be conveniently calculated both for the standard optical bands and for the bands covered by GALEX. The attenuation characteristics of the individual stellar components show marked differences between each other. A general formula is given for the calculation of composite attenuation, valid for any combination of the bulge-to-disk ratio and amount of clumpiness. As an example, we show how the optical depth derived from the variation of attenuation with inclination depends on the bulge-to-disk ratio. Finally, a recipe is given for a self-consistent determination of the optical depth from the Halpha/Hbeta line ratio.

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arXiv2003

Infrared emission towards SN 1987A 11 years after outburst: Measurements with ISOCAM

We present measurements of the mid-infrared (MIR) emission from SN 1987A, made using the Infrared Space Observatory (ISO) 11 years after outburst. They are the only late epoch detections of this source in the thermal IR regime. The position of the source, determined from an offset to an IR-emitting star, suggests that the emission is associated with SN 1987A or its extended supernova remnant (SNR). A predominantly circumstellar origin is however suggested by the size and orientation of the IR-emitting region, which is comparable with the extension of the inner ring seen with the Hubble Space Telescope (HST). The emission is most probably from collisionally-heated circumstellar grains embedded in shocked gas downstream of the blast wave. The MIR extent is consistent with the hypothesis that the blast wave was propagating into material of moderate density interior to the thick inner ring at the epoch of the ISOCAM observations.

astro-ph↗

arXiv2002

Far-Infrared photometry of a statistical sampleof late-type Virgo Cluster galaxies

We present deep diffraction-limited far-infrared (FIR) strip maps of a sample of 63 galaxies later than S0 and brighter than B_T 16.8, selected from the Virgo Cluster Catalogue of Binggeli, Sandage & Tammann. The ISOPHOT instrument on board the Infrared Space Observatory was used to achieve sensitivities typically an order of magnitude deeper than IRAS in the 60 and 100 micron bands and to reach the confusion limit at 170 microns. The averaged 3 sigma upper limits for integrated flux densities of point sources at 60, 100 and 170 microns are 43, 33 and 58 mJy, respectively. 54 galaxies (85.7 percent) are detected at least at one wavelength, and 40 galaxies (63.5 percent) are detected at all three wavelengths. The highest detection rate (85.7 percent) is in the 170 micron band. In many cases the galaxies are resolved, allowing the scale length of the infrared disks to be derived from the oversampled brightness profiles in addition to the spatially integrated emission. The data presented should provide the basis for a variety of statistical investigations of the FIR spectral energy distributions of gas rich galaxies in the local universe spanning a broad range in star-formation activity and morphological types, including dwarf systems and galaxies with rather quiescent star formation activity.

astro-ph↗

arXiv2001

Cold dust in late-type Virgo Cluster galaxies

We have statistically analyzed the spatially integrated far-infrared (FIR) emissions of the complete volume- and luminosity-limited sample of late-type (later than S0) Virgo cluster galaxies measured using the Infrared Space Observatory (ISO) by Tuffs et al. (2002) in bands centered on 60, 100 and 170 microns. 30 out of 38 galaxies detected at all three wavelengths contain a cold dust emission component, present within all morphological types of late-type systems ranging from early giant spirals to Blue Compact Dwarfs (BCDs), and which could not have been recognized by IRAS. The temperatures of the cold dust were found to be broadly distributed, with a median of 18 K, some 8-10 K lower than would have been predicted from IRAS. The derived total dust masses are correspondingly increased by factors of typically 6 - 13. A particularly significant result concerns the low dust temperatures (ranging down to less than 10 K) and large dust masses associated with the Im and BCD galaxies in our sample. We propose two scenarios to account for the FIR characteristics of these systems.

astro-ph↗

arXiv2000

Early Starbursts and Magnetic Field Generation in Galaxy Clusters

We propose a mechanism for the early generation of the mean Intracluster magnetic field in terms of magnetized galactic winds. These winds are the result of starburst phases of the cluster galaxies, assumed to produce the predominant population of early type galaxies in mergers of gas-rich progenitors. After further cluster contraction typical field strengths are $10^{-7}$ Gauss. This estimate may increase to the level of $10^{-6}$ Gauss if more extreme galactic parameters, and subsequent shear amplification of the field are considered. The topology of the field is one of almost unconnected Wind Bubbles with Parker-type spiral field configurations over scales of the distance between galaxies. Further cluster accretion, that continues chaotically in space and time up to the present, will perturb these "large-scale" mean fields on smaller or at best comparable spatial scales. The small scale fields in the resulting turbulent fluctuation spectrum should be able to confine relativistic particles over times longer than the age of the Universe. The nonthermal particle content of galaxy clusters should therefore also have a "cosmological" hadronic component generated during the early starburst phase of the member galaxies. Already by itself it implies a nonthermal energy fraction of about 10 percent for the Intracluster gas which should then be detectable by future gamma-ray telescopes.

astro-ph↗

arXiv2000

TeV Gamma Rays Expected from Supernova Remnants in Different Uniform Interstellar Media

Calculations of the expected TeV $γ$-ray emission, produced by accelerated cosmic rays (CRs) in nuclear collisions, from supernova remnants evolving in a uniform interstellar medium (ISM) are presented. The aim is to study the sensitivity of $γ$-ray production to a physical parameter set. Apart from its general proportionality to N_H, it is shown that the $γ$-ray production essentially depends upon the ratio of the CR diffusion coefficient $κ$ to a critical value $κ_{crit}=10(B_0/5 μ{G})(N_H/0.3 {cm}^{-3})^{-1/3}κ_B$, where B_0 and N_H are the magnetic field and hydrogen number density of the ISM, and $κ_B$ denotes the Bohm diffusion coefficient. If $κ$ is of the same order or lower than $κ_{crit}$, then the peak TeV $γ$-ray flux in the Sedov evolutionary phase, normalized to a distance of 1 kpc, is about 10^{-10}(N_H/0.3 {cm}^{-3}) photons cm^{-2} s^{-1}. For a CR diffusion coefficient that is significantly larger than $κ_{crit}$, the CR cutoff energy is less than 10 TeV and the expected $γ$-ray flux at 1 TeV is considerably below the presently detectable level of 10^{-12} photons cm^{-2} s^{-1}. The same is of course true for a SNR in the rarified, so-called hot ISM.

astro-ph↗

arXiv2000

Galactic Gamma-Ray Background Radiation from Supernova Remnants

The contribution of the Source Cosmic Rays (SCRs), confined in Supernova Remnants, to the diffuse high energy \gr emission above 1 GeV from the Galactic disk is studied. \grs produced by the SCRs have a much harder spectrum compared with those generated by the Galactic Cosmic Rays which occupy a much larger residence volume uniformly. SCRs contribute less than 10% at GeV energies and become dominant at \gr energies above 100 GeV. The contributions from $π^0$-decay and Inverse Compton \grs have comparable magnitude and spectral shape, whereas the Bremsstrahlung component is negligible. At TeV energies the contribution from SCRs increases the expected diffuse \gr flux almost by an order of magnitude. It is shown that for the inner Galaxy the discrepancy between the observed diffuse intensity and previous model predictions at energies above a few GeV can be attributed to the SCR contribution.

astro-ph↗

arXiv2000

Kinetic theory of cosmic ray and gamma-ray production in supernova remnants expanding into wind bubbles

Abbreviated Abstract: A kinetic model of particle acceleration in supernova remnants (SNRs) is extended to study the cosmic ray (CR) and associated high energy gamma-ray production during SN shock propagation through the inhomogeneous circumstellar medium of a progenitor star that emits a strong wind. The wind forms a low-density bubble surrounded by a swept-up shell of interstellar matter. Gamma rays are produced as a result of decay of pions which in turn are the result of collisions of CRs with nuclei of the thermal plasma. The time evolution of the SNRs is followed numerically, taking into account the nonlinear backreaction of the accelerated CRs. Examples typical for SN type Ib and SN type II explosions are considered. Apart from the confirmation of the known fact that acceleration is extremely rapid and that the upper momentum cutoff is reached almost immediately after the explosion due to the high wind magnetic field, it is also shown that the CRs are accelerated with high efficiency. The CR energy spectrum, ultimately produced in the SNRs, has a power law form with index = 2.0 to 2.1 in a wide energy range up to a maximum energy which is about 10^14 eV if the CR diffusion coefficient is as small as the Bohm limiting value. However, the expected pi^0-decay gamma-ray flux is considerably lower than in the case of a uniform interstellar medium; a relatively high gamma-ray luminosity in the TeV band, detectable at distances of several kpc, is only expected in the case of a relatively dense ISM with a number density above 10/cc.

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