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H. J. Völk

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

7 recordsLinked to original sources
arXiv2009

Theory of cosmic ray and gamma-ray production in the supernova remnant RX J0852.0-4622

Aims. The properties of the Galactic supernova remnant (SNR) RX J0852.0-4622 are theoretically analysed. Methods. An explicitly time-dependent, nonlinear kinetic model of cosmic ray (CR) acceleration in SNRs is used to describe the properties of SNR RX J0852.0-4622, the accelerated CRs and the nonthermal emission. The source is assumed to be at a distance of ~1 kpc in the wind bubble of a massive progenitor star. An estimate of the thermal X-ray flux in such a configuration is given. Results. We find that the overall synchrotron spectrum of RX J0852.0-4622 as well as the filamentary structures in hard X-rays lead to an amplified magnetic field B > 100 muG in the SNR interior. This implies that the leptonic very high energy (VHE) gamma-ray emission is suppressed, and that the VHE gamma-rays are hadronically dominated. The energy spectrum of protons produced over the life-time of the remnant until now may well reach ''knee'' energies. The derived gamma-ray morphology is consistent with the H.E.S.S. measurements. The amount of energy in energetic particles corresponds to about 35% of the hydrodynamic explosion energy. A remaining uncertainty concerns the thermal X-ray flux at 1 keV. A rough estimate, possibly not quite appropriate for the assumed wind bubble configuration, results in it being larger than the nonthermal flux at this energy. Conclusions. It is concluded that this SNR expanding into the wind bubble of a massive star in a dense gas environment can be a hadronic gamma-ray source that is consistent with all existing multi-wavelength constraints, except possibly the thermal X-ray emission.

astro-ph.HE↗
arXiv2008

Hadronic versus leptonic origin of the gamma-ray emission from Supernova Remnant RX J1713.7-3946

Aims. The hadronic vs. leptonic origin of the gamma-ray emission from the Supernova Remnant RX J1713.7-3946 is discussed both in the light of new observations and from a theoretical point of view. Methods. The existing good spatial correlation of the gamma-ray emission and the nonthermal X-ray emission is analyzed theoretically. In addition, the recently published new H.E.S.S. observations define the energy spectrum more precisely, in particular at the high and low energy ends of the instrument's dynamical range. There now exist much more constraining X-ray observations from Suzaku that extend substantially beyond 10 keV. These new data are compared with the authors' previous theoretical predictions, both for dominant hadronic and for simple inverse Compton models. Results. Apart from the well-known MHD correlation between magnetic field strength and plasma density variations, emphasized by the wind-bubble-structure of the remnant, it is argued that the regions of magnetic field amplification also are correlated with enhanced densities of accelerated nuclear particles and the associated streaming instabilities. Therefore a correlation of nonthermal X-ray and \gr emission is not only possible but even to be expected for a hadronic emission scenario. A leptonic origin of the gamma-ray emission would require an implausibly uniform strength of the magnetic field. The observational and theoretical inferences about substantial field amplification in this remnant agree very well with the recent X-ray and \gr observations. Conclusions. All this argues strongly for the dominance of hadronic \grs in the \gr emission spectrum and a fortiori for an overwhelming contribution of nuclear cosmic ray particles to the nonthermal energy in this remnant.

astro-ph↗
arXiv2007

Comparing a model of cosmic ray production in the supernova remnant RX J1713.7-3946 with observations

Explicitly time-dependent, nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) has been employed to investigate the properties of SNR RX J1713.7-3946. Observations of the nonthermal radio and X-ray emission spectra as well as earlier H.E.S.S. measurements of the very high energy $γ$-ray emission were used to constrain the astronomical and the particle acceleration parameters of the system. The model assumes that the object was a core collapse supernova (SN) with a massive progenitor, has an age of $\approx 1600$ yr and is at a distance of $\approx 1$ kpc. It is shown that an efficient production of nuclear CRs, leading to strong shock modification and a large downstream magnetic field strength $B_{\mathrm{d}}\sim100$ $μ$G, can reproduce the observed synchrotron emission from radio to X-ray frequencies together with the \gr spectral characteristics as observed by the H.E.S.S. telescopes. Small-scale filamentary structures observed in nonthermal X-rays provide empirical confirmation for this field amplification scenario which leads to a strong depression of the inverse Compton and Bremsstrahlung fluxes. The results are compared with the latest H.E.S.S. observations.

astro-ph↗
arXiv2007

Theory of cosmic ray and $γ$-ray production in the supernova remnant RX J0852.0-4622 (Vela Jr.)

Explicitly time-dependent, nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) has been used to investigate the properties of the very large SNR RX J0852.0-4622. The available observations do not clearly distinguish between a ``nearby'' (at $\sim 200$ pc) and a ``distant'' (at $\sim 1$ kpc) source scenario. Therefore two correspondingly different models were analyzed. While the 200 pc solution can not be a priory excluded, the 1 kpc solution turns out to be clearly preferable for physical reasons. It requires a core collapse supernova (SN) with a massive progenitor in a molecular cloud $\sim 4000$ yrs ago. The overall synchrotron spectrum and the filamentary structures in hard X-rays both consistently lead to an amplified magnetic field $B > 100 μ$G in the SNR interior. This implies a suppression of the leptonic TeV $γ$-ray emission to about 1 percent of the flux measured by the H.E.S.S. telescope system which therefore must be hadronic, consistent with the theoretical solution. Up to the present the 1 kpc solution has already converted $\sim 10$ percent of the explosion energy into nonthermal energy, as expected for a Galactic CR source. Also the derived $γ$-ray morphology is consistent with the H.E.S.S. measurements. For the ``nearby'' solution the leptonic and hadronic $γ$ ray fluxes are in the ratio 1:10 which means that this case is also hadronically dominated. However, the magnetic field strength, consistent with the overall synchrotron spectrum, differs significantly from that derived from the X-ray filaments. Finally, the total mechanical energy released amounts to only $1.8 \times 10^{50}$ erg, uncomfortably low even for a core collapse event.

astro-ph↗
arXiv2004

A comparative study of the spatial distribution of ultraviolet and far-infrared fluxes from M 101

The total ultraviolet (UV) flux (from 1412 to 2718 AA) of M 101 is compared on a pixel-to-pixel basis with the total far-infrared (FIR) flux (from 60 to 170 micron), using the maps of the galaxy taken by GALEX in the near-UV and far-UV and by ISOPHOT at 60, 100 and 170 micron. The main result of this investigation is the discovery of a tight dependence of the FIR/UV ratio on radius, with values monotonically decreasing from around 4 in the nuclear region to nearly zero towards the edge of the optical disk. Although the tightness of this dependence is in part attributable to resolution effects, the result is consistent with the presence of a large-scale distribution of diffuse dust having a face-on optical depth which decreases with radius and which dominates over the more localized variations in opacity between the arm and interarm regions. We also find a trend for the FIR/UV ratio to take on higher values in the regions of diffuse interarm emission than in the spiral-arm regions, at a given radius. This is interpreted quantitatively in terms of the escape probability of UV photons from spiral arms and their subsequent scattering in the interarm regions, and in terms of the larger relative contribution of optical photons to the heating of the dust in the interarm regions.

astro-ph↗
arXiv2004

The theory of synchrotron emission from supernova remnants

The time-dependent nonlinear kinetic theory for cosmic ray (CR) acceleration in supernova remnants (SNRs) is applied studying the properties of the synchrotron emission from SNRs, in particular, the surface brightness-diameter ($Σ-D$) relation. Detailed numerical calculations are performed for the expected range of the relevant physical parameters, namely the ambient density and the supernova explosion energy. The magnetic field in SNRs is assumed to be significantly amplified by the efficiently accelerating nuclear CR component. Due to the growing number of accelerated CRs the expected SNR luminosity increases during the free expansion phase, reaches a peak value at the beginning of the Sedov phase and then decreases again, since in this stage the overall CR number remains nearly constant, whereas the effective magnetic field diminishes with time. The theoretically predicted brightness-diameter relation in the radio range in the Sedov phase is close to $Σ_\mathrm{R}\propto D^{-17/4}$. It fits the observational data in a very satisfactory way. The observed spread of $Σ_\mathrm{R}$ at a given SNR size $D$ is the result of the spread of supernova explosion energies and interstellar medium densities.

astro-ph↗