arXiv subjects

H. J. Voelk

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

At least 19 recordsLinked to original sources

arXiv2015

Re-examination of the Expected gamma-ray emission of supernova remnant SN 1987A

A nonlinear kinetic theory, combining cosmic-ray (CR) acceleration in supernova remnants (SNRs) with their gas dynamics, is used to re-examine the nonthermal properties of the remnant of SN 1987A for an extended evolutionary period of 5-50 yr. This spherically symmetric model is approximately applied to the different features of the SNR which consist of (i) a blue supergiant wind and bubble, and (ii) of the swept-up red supergiant (RSG) wind structures in the form of an H II region, an equatorial ring (ER), and an hourglass region. The RSG wind involves a mass loss rate that decreases significantly with elevation above and below the equatorial plane. The model adapts recent three-dimensional hydrodynamical simulations by Potter et al. in 2014 that use a significantly smaller ionized mass of the ER than assumed in the earlier studies by the present authors. The SNR shock recently swept up the ER, which is the densest region in the immediate circumstellar environment. Therefore, the expected gamma-ray energy flux density at TeV energies in the current epoch has already reached its maximal value of $\sim 10^{-13}$ erg cm$^{-2}$ s$^{-1}$. This flux should decrease by a factor of about two over the next 10 years.

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arXiv2012

The nature of gamma-ray emission of Tycho's supernova remnant

The nature of the recently detected HE and VHE gamma-ray emission of Tycho's supernova remnant (SNR) is studied. A nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) is employed to investigate the properties of Tycho's SNR and their correspondence to the existing experimental data, taking into account that the ambient interstellar medium (ISM) is expected to be clumpy. It is demonstrated that the overall steep gamma-ray spectrum observed can be interpreted as the superposition of two spectra produced by the CR proton component in two different ISM phases: The first gamma-ray component, extending up to about $10^{14}$ eV, originates in the diluted warm ISM, whereas the second component, extending up to 100 GeV, comes from numerous dense, small-scale clouds embedded in this warm ISM. Given the consistency between acceleration theory and the observed properties of the nonthermal emission of Tycho's SNR, a very efficient production of nuclear CRs in Tycho's SNR is established. The excess of the GeV gamma-ray emission due to the clouds' contribution above the level expected in the case of a purely homogeneous ISM, is inevitably expected in the case of type Ia SNe.

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arXiv2012

Nonthermal emission of supernova remnant SN 1006 revisited: theoretical model and the H.E.S.S. results

The properties of the Galactic supernova remnant (SNR) SN 1006 are theoretically re-analyzed in the light of the recent H.E.S.S. results. Nonlinear kinetic theory is used to determine the momentum spectrum of cosmic rays (CRs) in space and time in the supernova remnant SN 1006. The physical parameters of the model - proton injection rate, electron-to-proton ratio and downstream magnetic field strength - are determined through a fit of the result to the observed spatially-integrated synchrotron emission properties. The only remaining unknown astronomical parameter, the circumstellar gas number density, is determined by a normalization of the amplitude of the gamma-ray flux to the observed amplitude. The bipolar morphology of both nonthermal X-ray and gamma-ray emissions is explained by the preferential injection of suprathermal nuclei and subsequent magnetic field amplification in the quasi-parallel regions of the outer supernova shock. The above parameters provide an improved fit to all existing nonthermal emission data, including the TeV emission spectrum recently detected by H.E.S.S., with the circumstellar hydrogen gas number density $N_H\approx 0.06 cm^{-3}$ close to values derived from observations of thermal X-rays. The hadronic and leptonic gamma-ray emissions are of comparable strength. The overall energy of accelerated CRs at the present epoch is of the order of 5 % of the total hydrodynamic explosion energy, and is predicted to rise with time by a factor of $\approx 2$. The relevance of CR escape from the SNR for the spectrum of the gamma-ray emission is demonstrated. The sum of the results suggests that SN 1006 is a CR source with a high efficiency of nuclear CR production, as required for the Galactic CR sources, both in flux as well as in cutoff energy.

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arXiv2011

Expected gamma-ray emission of supernova remnant SN 1987A

A nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants is employed to re-examine the nonthermal properties of the remnant of SN 1987A for an extended evolutionary period of 5--100 yr. It is shown that an efficient production of nuclear CRs leads to a strong modification of the outer supernova remnant shock and to a large downstream magnetic field $B_\mathrm{d}\approx 20$ mG. The shock modification and the strong field are required to yield the steep radio emission spectrum observed, as well as to considerable synchrotron cooling of high energy electrons which diminishes their X-ray synchrotron flux. These features are also consistent with the existing X-ray observations. The expected \gr energy flux at TeV-energies at the current epoch is nearly $ε_γF_γ\approx 4\times 10^{-13}$ erg cm$^2$s$^{-1}$ under reasonable assumptions about the overall magnetic field topology and the turbulent perturbations of this field. The general nonthermal strength of the source is expected to increase roughly by a factor of two over the next 15 to 20 yrs; thereafter it should decrease with time in a secular form.

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arXiv2010

Nonthermal properties of supernova remnant G1.9+0.3

The properties of the - presumably - youngest Galactic supernova remnant (SNR) G1.9+0.3 are investigated within the framework of nonlinear kinetic theory of cosmic ray acceleration in SNRs. The observed angular size and expansion speed as well as the radio and X-ray emission measurements are used to determine relevant physical parameters of this SNR. Under the assumption that SNR G1.9+0.3 is the result of a Type Ia supernova near the Galactic center (at the distance d=8.5 kpc) the nonthermal properties are calculated. In particular, the expected TeV gamma-ray spectral energy density is predicted to be as low as $ε_γF_γ \approx 5\times 10^{-15}$ erg cm$^{-2}$ s$^{-1}$, strongly dependent ($F_γ\propto d^{-11}$) upon the source distance d.

astro-ph.HE↗

arXiv2009

Nonthermal and thermal emission from the supernova remnant RX J1713.7-3946

A nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) is employed to investigate the properties of SNR RX J1713.7-3946. Observations of the non-thermal radio and X-ray emission spectra as well as the H.E.S.S. measurements of the very high energy gamma-ray emission are used to constrain the astronomical and CR acceleration parameters of the system. It is argued that RX J1713.7-3946 is a core collapse supernova (SN) of type II/Ib with a massive progenitor, has an age of ~1600 yr and is at a distance of ~1 kpc. It is in addition assumed that the CR injection/acceleration takes place uniformly across the shock surface for this kind of core collapse SNR. The theory gives a consistent description for all the existing observational data, including the non-detection of thermal X-rays and the spatial correlation of the X-ray and gamma-ray emission in the remnant. Specifically it is shown that an efficient production of nuclear CRs, leading to strong shock modification and a large downstream magnetic field strength B_d ~140 mkG can reproduce in detail the observed synchrotron emission from radio to X-ray frequencies together with the gamma-ray spectral characteristics as observed by the H.E.S.S. telescopes. The calculations are consistent with RX J1713.7-3946 being an efficient source of nuclear cosmic rays.

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arXiv2009

Cosmic ray acceleration parameters from multi-wavelength observations. The case of SN 1006

The properties of the Galactic supernova remnant SN 1006 are theoretically reanalysed. Nonlinear kinetic theory is used to determine the acceleration efficiency of cosmic rays (CRs) in the supernova remnant SN 1006. The known range of astronomical parameters and the existing measurements of nonthermal emission are examined in order to define the values of the relevant physical parameters which determine the CR acceleration efficiency. It is shown that the parameter values -- proton injection rate, electron to proton ratio and downstream magnetic field strength -- are determined with the appropriate accuracy. In particular also the observed azimuthal variations in the gamma-ray morphology agree with the theoretical expectation. These parameter values, together with the reduction of the gamma-ray flux relative to a spherically symmetric acceleration geometry, allow a good fit to the existing data, including the recently detected TeV emission by H.E.S.S. SN 1006 represents the first example where a high efficiency of nuclear CR production, required for the Galactic CR sources, is consistently established.

astro-ph.HE↗

arXiv2008

Inverse Compton gamma-ray models for remnants of Galactic type Ia supernovae?

We theoretically and phenomenologically investigate the question whether the gamma-ray emission from the remnants of the type Ia supernovae SN 1006, Tycho's SN and Kepler's SN can be the result of electron acceleration alone. The observed synchrotron spectra of the three remnants are used to determine the average momentum distribution of nonthermal electrons as a function of the assumed magnetic field strength. Then the inverse Compton emission spectrum in the Cosmic Microwave Background photon field is calculated and compared with the existing upper limits for the very high energy gamma-ray flux from these sources. It is shown that the expected interstellar magnetic fields substantially overpredict even these gamma-ray upper limits. Only rather strongly amplified magnetic fields could be compatible with such low gamma-ray fluxes. However this would require a strong component of accelerated nuclear particles whose energy density substantially exceeds that of the synchrotron electrons, compatible with existing theoretical acceleration models for nuclear particles and electrons. Even though the quantitative arguments are simplistic, they appear to eliminate simplistic phenomenological claims in favor of a inverse Compton gamma-ray scenario for these sources.

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arXiv2008

Internal dynamics and particle acceleration in Tycho's SNR

The consequences of a newly suggested value for the SN explosion energy 1.2x10^{51} erg are explored for the case of Tycho's supernova remnant (SNR). A nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs is employed to investigate the properties of Tycho's SNR and their correspondence to the existing experimental data. It is demonstrated that the large mean ratio between the radii of the contact discontinuity and the forward shock is consistent with the very effective acceleration of nuclear energetic particles at the forward shock. It is also argued that consistency of the value E_{sn} = 1.2x10^{51} erg with the gas dynamics, acceleration theory, and the existing gamma-ray measurements requires the source distance to be greater than 3.3 kpc. The corresponding ambient gas number density is lower than 0.4 cm^{-3}. Since the expected gamma-ray flux strongly depends on the source distance, F_γ\propto d^{-7}, a future experimental determination of the actual gamma-ray flux from Tycho's SNR will make it possible to determine the values of the source distance and of the mean ambient gas density. A simple inverse Compton model without a dominant population of nuclear CRs is not compatible with the present upper limit for the gamma-ray emission for any reasonable ambient interstellar B-field. Given the consistency between acceleration theory and overall, as well as internal, gas dynamics, a future gamma-ray detection would make the case for nuclear particle acceleration in Tycho's SNR incontrovertible in our view.

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arXiv2008

Modeling Bell's Non-resonant Cosmic Ray Instability

We have studied the non-resonant streaming instability of charged energetic particles moving through a background plasma, discovered by Bell (2004). We confirm his numerical results regarding a significant magnetic field amplification in the system. A detailed physical picture of the instability development and of the magnetic field evolution is given.

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arXiv2007

Primary particle acceleration above 100 TeV in the shell-type Supernova Remnant RX J1713.7--3946 with deep H.E.S.S. observations

The shell-type supernova remnant RX J1713.7--3946 was observed during three years with the H.E.S.S. Cherenkov telescope system. The first observation campaign in 2003 yielded the first-ever resolved TeV gamma-ray image. Follow-up observations in 2004 and 2005 revealed the very-high-energy gamma-ray morphology with unprecedented precision and enabled spatially resolved spectral studies. Combining the data of three years, we obtain significantly increased statistics and energy coverage of the gamma-ray spectrum as compared to earlier H.E.S.S. results. We present the analysis of the data of different years separately for comparison and demonstrate that the telescope system operates stably over the course of three years. When combining the data sets, a gamma-ray image is obtained with a superb angular resolution of 0.06 degrees. The combined spectrum extends over three orders of magnitude, with significant gamma-ray emission approaching 100 TeV. For realistic scenarios of very-high-energy gamma-ray production, the measured gamma-ray energies imply efficient particle acceleration of primary particles, electrons or protons, to energies exceeding 100 TeV in the shell of RX J1713.7--3946.

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arXiv2007

Spectrum of cosmic rays, produced in supernova remnants

Nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants is employed to calculate CR spectra. The magnetic field in SNRs is assumed to be significantly amplified by the efficiently accelerating nuclear CR component. It is shown that the calculated CR spectra agree in a satisfactory way with the existing measurements up to the energy $10^{17}$ eV. The power law spectrum of protons extends up to the energy $3\times 10^{15}$ eV with a subsequent exponential cutoff. It gives a natural explanation for the observed knee in the Galactic CR spectrum. The maximum energy of the accelerated nuclei is proportional to their charge number $Z$. Therefore the break in the Galactic CR spectrum is the result of the contribution of progressively heavier species in the overall CR spectrum so that at $10^{17}$ eV the CR spectrum is dominated by iron group nuclei. It is shown that this component plus a suitably chosen extragalactic CR component can give a consistent description for the entire Galactic CR spectrum.

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arXiv2006

New evidence for strong nonthermal effects in Tycho's supernova remnant

For the case of Tycho's supernova remnant (SNR) we present the relation between the blast wave and contact discontinuity radii calculated within the nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. It is demonstrated that these radii are confirmed by recently published Chandra measurements which show that the observed contact discontinuity radius is so close to the shock radius that it can only be explained by efficient CR acceleration which in turn makes the medium more compressible. Together with the recently determined new value $E_{sn}=1.2\times 10^{51}$ erg of the SN explosion energy this also confirms our previous conclusion that a TeV gamma-ray flux of $(2-5)\times 10^{-13}$ erg/(cm$^2$s) is to be expected from Tycho's SNR. Chandra measurements and the HEGRA upper limit of the TeV gamma-ray flux together limit the source distance $d$ to $3.3\leq d\leq 4$ kpc.

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arXiv2006

Gamma-ray emission expected from Kepler's SNR

Nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) is used to investigate the properties of Kepler's SNR and, in particular, to predict the gamma-ray spectrum expected from this SNR. Observations of the nonthermal radio and X-ray emission spectra as well as theoretical constraints for the total supernova (SN) explosion energy E_sn are used to constrain the astronomical and particle acceleration parameters of the system. Under the assumption that Kepler's SN is a type Ia SN we determine for any given explosion energy E_sn and source distance d the mass density of the ambient interstellar medium (ISM) from a fit to the observed SNR size and expansion speed. This makes it possible to make predictions for the expected gamma-ray flux. Exploring the expected distance range we find that for a typical explosion energy E_sn=10^51 erg the expected energy flux of TeV gamma-rays varies from 2x10^{-11} to 10^{-13} erg/(cm^2 s) when the distance changes from d=3.4 kpc to 7 kpc. In all cases the gamma-ray emission is dominated by π^0-decay gamma-rays due to nuclear CRs. Therefore Kepler's SNR represents a very promising target for instruments like H.E.S.S., CANGAROO and GLAST. A non-detection of gamma-rays would mean that the actual source distance is larger than 7 kpc.

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arXiv2006

Theory of cosmic ray production in the supernova remnant RX J1713.7-3946

A nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) is employed to investigate the properties of SNR RX J1713.7-3946. Observations of the nonthermal radio and X-ray emission spectra as well as the H.E.S.S. measurements of the very high energy gamma-ray emission are used to constrain the astronomical and the particle acceleration parameters of the system. Under the assumptions that RX J1713.7-3946 was a core collapse supernova (SN) of type II/Ib with a massive progenitor, has an age of \approx 1600 yr and is at a distance of \approx 1 kpc, the theory gives indeed a consistent description for all the existing observational data. Specifically it is shown that an efficient production of nuclear CRs, leading to strong shock modification, and a large downstream magnetic field strength B_d ~ 100 mkG can reproduce in detail the observed synchrotron emission from radio to X-ray frequencies together with the gamma-ray spectral characteristics as observed by the H.E.S.S. telescopes. Small-scale filamentary structures observed in nonthermal X-rays provide empirical confirmation for the field amplification scenario which leads to a strong depression of the inverse Compton and Bremsstrahlung fluxes. Going beyond that and using a semi-empirical relation for young SNRs between the resulting CR pressure and the amplified magnetic field energy upstream of the outer SN shock as well as a moderate upper bound for the mechanical explosion energy, it is possible to also demonstrate the actual need for a considerable shock modification in RX J1713.7-3946. It is consistent with RX J1713.7-3946 being an efficient source of nuclear cosmic rays.

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arXiv2005

Simple Model of the Outflow from Starburst Galaxies: Application to Radio Observations

We present an analytical model for the hydrodynamic outflow from the disk of a starburst galaxy. The model is used to calculate the cosmic ray propagation and the radio intensity distribution in the nuclear starbust region of NGC 253. We find that the cosmic ray energy production rate of the central 600 pc of NGC 253 is about $3\cdot 10^{41}$ erg s$^{-1}$, that is about 15 percent of the total mechanical supernova power. For this inner region we estimate a terminal outflow velocity of 900 km s$^{-1}$ and a mass loss rate of $\dot{M}=2÷4M_\odot$ per year.

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arXiv2005

Dependence of the gamma-ray emission from SN 1006 on the astronomical parameters

We use nonlinear kinetic theory to study the remnant dynamics and the particle acceleration as well as the properties of the nonthermal emission from the supernova remnant SN 1006. The known range of astronomical parameters is examined to determine whether it encompasses the existing synchrotron emission data. Given the present-day spatial extent and expansion rate of the object, it is shown that the hadronic gamma-ray flux is very sensitive to the ambient gas density N_H and that the existing H.E.S.S. upper limit requires N_H < 0.1 cm^{-3}. The strength of the amplified magnetic field downstream of the shock is about 150 μG.

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arXiv2004

Magnetic Field Amplification in Tycho and other Shell-type Supernova Remnants

It is shown that amplification of the magnetic field in supernova remnants (SNRs) occurs in all six objects where morphological measurements are presently available in the hard X-ray continuum at several keV. For the three archetypical objects (SN 1006, Cas A and Tycho's SNR) to which nonlinear time-dependent acceleration theory has been successfully applied up to now, the global theoretical and the local observational field strengths agree very well, suggesting in addition that all young SNRs exhibit the amplification effect as a result of very efficient acceleration of nuclear cosmic rays (CRs) at the outer shock. Since this appears to be empirically the case, we may reverse the argument and consider field amplification as a measure of nuclear CR acceleration and it has indeed been argued that acceleration in the amplified fields allows the CR spectrum from SNRs to reach the knee in the spectrum or, in special objects, even beyond. The above results are furthermore used to investigate the time evolution of field amplification in young SNRs. Although the uncertainties in the data do not allow precise conclusions regarding this point, they rather clearly show that the ratio of the magnetic field energy density and the kinetic energy density of gas flow into the shock is of the order of a few percent if the shock speed is high enough V_s > 10^3 km/s, and this ratio remains nearly constant during the SNR evolution. The escape of the highest energy nuclear particles from their sources becomes progressively important with age, reducing also the cutoff in the π^0 -decay gamma-ray emission spectrum with time after the end of the sweep-up phase. Simultaneously the leptonic gamma-ray channels will gain in relative importance with increasing age of the sources.

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