arXiv subjects

Peter L. Biermann

Publications and source records attributed to Peter L. Biermann.

At least 109 records · Page 6Linked to original sources

arXiv1997

Frontiers of Astrophysics - Workshop Summary

We summarize recent results presented in the astrophysics session during a conference on ``Frontiers of Contemporary Physics''. We will discuss three main fields (High-Energy Astrophysics, Relativistic Astrophysics, and Cosmology), where Astrophysicists are pushing the limits of our knowledge of the physics of the universe to new frontiers. Since the highlights of early 1997 were the first detection of a redshift and the optical and X-ray afterglows of gamma-ray bursts, as well as the first well-documented flares of TeV-Blazars across a large fraction of the electromagnetic spectrum, we will concentrate on these topics. Other topics covered are black holes and relativistic jets, high-energy cosmic rays, Neutrino-Astronomy, extragalactic magnetic fields, and cosmological models.

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arXiv1997

The supergalactic structure and the origin of the highest energy cosmic rays

The recent discoveries of several reliable events of high energy cosmic rays at an energy above 10^20 eV raise questions about their path through the nearby universe. The two analyses of the Haverah Park data set and the Akeno data set appear to have an inconsistent pattern. Both data sets showed some measure of a correlation with the supergalactic plane, the locus of cosmologically nearby galaxies, radio galaxies and clusters of galaxies. We need a reasonable model of the true intergalactic magnetic field. Using recent cosmological simulations of structure formation in the universe, we estimate the magnetic fields which correspond to the upper limits in the Rotation Measure to distant radio sources. We estimate that the magnetic field strength in supergalactic sheets and filaments may be in the range of 0.1 to 1 muG. If such strengths are realized inside our Local Supercluster, this opens up the possibility to focus charged particles in the direction perpendicular to the supergalatic plane, analoguously but in the opposite direction to solar wind modulation. If focusing exists, it means that for all particles captured into the sheets, the dilution with distance d is 1/d instead of 1/d^2. This means in effect, that we may see sources to much larger distances than expected sofar. This effect is relevant only for energies for which the possible distances are smaller than the void scale of the cosmological galaxy distribution, in the range possibly up to 100 Mpc.

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arXiv1997

Cosmic Structure of Magnetic Fields

The simulations of the formation of cosmological structure allows to determine the spatial inhomogeneity of cosmic magnetic fields. Such simulations, however, do not give an absolute number for the strength of the magnetic field due to insufficient spatial resolution. Combining these simulations with observations of the Rotation Measure to distant radio sources allows then to deduce upper limits for the strength of the magnetic field. These upper limits are of order 0.2 - 2 muG along the filaments and sheets of the galaxy distribution. In one case, the sheet outside the Coma cluster, there is a definitive estimate of the strength of the magnetic field consistent with this range. Such estimates are almost three orders of magnitude higher than hitherto assumed usually. High energy cosmic ray particles can be either focussed or strongly scattered in such magnetic filaments and sheets, depending on the initial transverse momentum. The cosmological background in radio and X-ray wavelengths will have contributions from these intergalactic filaments and sheets, should the magnetic fields really be as high as 0.2 - 2 muG.

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arXiv1997

Cosmic Rays VII. Individual element spectra: prediction and data

Based on the earlier papers in this series, we discuss here the errors in the prediction of the model, and compare with the error ranges in the spectra for individual elements. The predictions are spectra of $E^{-2.75 \pm 0.04}$ for hydrogen, and $E^{-2.67 -0.02 \pm 0.02}$ for helium and heavier elements below the knee. For particle energies above $10 Z$ GeV the data give $E^{-2.77 \pm 0.02}$ for hydrogen and $E^{-2.64 \pm 0.02}$ for helium and similar values for all heavier elements combined, where $Z$ is the charge of the nucleus considered. At the higher energy range considered here the secondary elements Li, Be, and B as well as the sub-Fe group have spectra consistent with source-related spallation, such as occurs when a supernova driven explosions runs into a shell around the wind of the predecessor star.

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arXiv1997

Limits on Magnetic Fields and Relativistic Electrons in the Coma Cluster from Multifrequency Observations

Relativistic electrons, seen in the large diffuse radio halo of the Coma cluster of galaxies, should scatter background photons to higher energies. We calculate the inverse Compton contributions from the microwave background, from the local radiation field of elliptical galaxies and from the thermal X-ray emission of the intra cluster medium to the different observed energy bands, and draw restrictions to the shape of the spectrum of the relativistic electron population. The expected electron spectra in different halo formation models are discussed, and signatures for a future distinction of these models are presented, tracing the injection processes and the influence of optical thickness of the radio halo for very low frequency emission due to synchrotron self absorption. Some of the upper limits on the electron spectrum translate into lower limits of the halo magnetic fields, since the observed synchrotron radiation is known. Despite uncertainties in the extrapolation of the radio spectrum to lower frequencies a lower limit to the central field strength of B_o > 0.3 muG seems to be sure. This is comparable to the value resulting from minimal energy arguments. If B_o < 1.2 muG, the recently reported extreme ultraviolet excess of Coma (Lieu et al. 1996) could result from inverse Compton scattered microwave photons. Formulae for transrelativistic Thomson scattering are given in the Appendix.

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arXiv1997

Unstable waves in winds of magnetic massive stars

We use a luminous fast magnetic rotator model to analyze the influence of a magnetic field on the linear waves induced in the wind of a massive star by the radiative instability. We show that a twisted magnetic field can drive a strong wind with a wind efficiency $(\dot{M}v_\infty)/(L/c)>1$ even without multiple scattering. The radiation amplified waves in the wind are modified by the twisted magnetic field so that they can enhance the wind and lead to overestimates for $\dot{M}$ and $v_\infty$. Finally we argue that the spin down time might be consistent with the lifetime derived from the mass loss rate within the uncertainties regarding the stellar structure. Therefore our model may help to explain high, observed values for $\dot{M}$ and $v_\infty$ without being ruled out by the spin down problem.

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arXiv1997

Viscosity in Selfgravitating Accretion Disks

We show that the standard model for geometrically thin accretion disks (alpha-disks) leads to inconsistencies if selfgravity plays a role. This problem arises from the parametrization of viscosity in terms of local sound velocity and vertical disk scale height. A viscosity prescription based on turbulent flows at the critical effective Reynolds number allows for consistent models of thin selfgravitating disks, and recovers the alpha-disk solution as the limiting case of negligible selfgravity. We suggest that such selfgravitating disks may explain the observed spectra of protoplanetary disks and yield a natural explanation for the radial motions inferred from the observed metallicity gradients in disk galaxies.

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arXiv1996

Cosmic Ray Interactions and the Abundances of the Chemical Elements

Our Galaxy is the largest nuclear interaction experiment which we know, because of the interaction between cosmic ray particles and the interstellar material. Cosmic rays are particles, which have been accelerated in the Galaxy or in extragalactic space. Cosmic rays come as protons, electrons, heavier nuclei, and their antiparticles. Up to energies up to some tens of TeV of particle energy it is possible to derive chemical abundances of cosmic rays. It has been proposed that cosmic ray particles can be attributed to three main sites of origin and acceleration, a) supernova shocks in the interstellar medium, b) supernova shocks in a stellar wind of the predecessor star, and c) powerful radio galaxies. This proposal leads to quantitative tests, which are encouraging sofar. Quantitative models for transport and interaction appear to be consistent with the data. Li, Be, B are secondary in cosmic rays, as are many of the odd-Z elements, as well as the sub-Fe elements. At very low energies, cosmic ray particles are subject to ionization losses, which produce a steep low energy cutoff; all particles below the cutoff are moved into the thermal material population, and the particles above it remain as cosmic rays. This then changes the chemical abundances in the interstellar medium, and is a dominant process for many isotopes of Li, Be, B. With a quantitative theory for the origin of cosmic rays proposed, it appears worthwhile to search for yet better spallation cross sections, especially near threshold. With such an improved set of cross sections, the theory of the interstellar medium and its chemical abundances, both in thermal and in energetic particles, could be taken a large step forward.

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arXiv1996

Contributions to the Cosmic Ray Flux above the Ankle: Clusters of Galaxies

Motivated by the suggestion of Kang, Ryu \& Jones (1996) that particles can be accelerated to high energies via diffusive shock acceleration process at the accretion shocks formed by the infalling flow toward the clusters of galaxies, we have calculated the expected particle flux from a cosmological ensemble of clusters. We use the observed temperature distribution of local clusters and assume a simple power-law evolutionary model for the comoving density of the clusters. The shock parameters such as the shock radius and velocity are deduced from the ICM temperature using the self-similar solutions for secondary infall onto the clusters. The magnetic field strength is assume to be in equipartition with the postshock thermal energy behind the accretion shock. We also assume that the injected energy spectrum is a power-law with the exponential cutoff at the maximum energy which is calculated from the condition that the energy gain rate for diffusive shock acceleration is balanced by the loss rate due to the interactions with the cosmic background radiation. In contrast to the earlier paper we have adopted here the description of the cosmic ray diffusion by Jokipii (1987) which leads to considerably higher particle energies. Finally the injected particle spectrum at the clusters is integrated over the cosmological distance to earth by considering the energy loss due to the interactions with the cosmic background radiation. Our calculations show that the expected spectrum of high-energy protons from the cosmological ensemble of the cluster accretion shocks could match well the observed cosmic ray spectrum near $10^{19}$eV with reasonable parameters and models if about $10^{-4}$ of the infalling kinetic energy can be injected into the intergalactic space as the high energy particles.

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arXiv1996

The symbiotic system in quasars: black hole, accretion disk and jet

The UV continuum spectrum of quasars and AGN is assumed to originate from an accreting disk surrounding a massive rotating black hole. We discuss the structure and emission spectra of a disk which drives a powerful jet. Due to the large efficiency of extracting energy from the accreting matter in the inner part of the disk close to the massive object, all the energetic conditions for the formation of jets are fulfilled. The total energy going up into the jet depends strongly on the Kerr black hole parameters, on the disk features and on the mass flow and thickness of the jet. The shape of UV spectra of the AGN can be explained by a sub-Eddington accretion disk which drives a jet in the innermost parts.

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arXiv1995

The missing x-rays in Sgr A*: evidence for - not against - a supermassive black hole

We present a simple argument that the missing x-ray flux from the Galactic Center source Sgr~A* ist not evidence {\it against} -- as claimed by Goldwurm et al. 1994 -- but rather indirect evidence {\it for} the presence of a supermassive black hole. The radio spectrum provides a strict {\it lower} limit for the size of Sgr~A* ($R>3\cdot 10^{11}$cm). A more compact source would be completely synchrotron self-absorbed. This size is $10^6$ times larger than a stellar-mass black hole, yet the bolometric radio luminosity is comparable to or even larger than the x-ray luminosity where matter accreting onto a {\it stellar-mass} black hole would inevitably radiate the {\it bulk} of its luminosity. Hence, either the bulk of the accretion power is radiated in the UV (where the limits are higher), or the accretion has to stop at the radio-scale to avoid producing x-rays brighter than the radio emission. Both would be a natural consequence of a supermasive black hole with $\sim10^6M_{\sun}$.

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arXiv1995

ROSAT PSPC observations of six PG quasars and PHL 1657

We report results from the spectral analysis of pointed ROSAT PSPC observations of six PG quasars and the weak-bump quasar PHL 1657. Disregarding other frequency bands, the PSPC data are represented best by simple power-law source spectra ($dN/dE \propto E^Γ$) with a slope $Γ= -2.5\pm 0.4$ and do not show evidence for a more complex structure. However, within the limits given by statistics and systematical errors, a superposition of two power-law spectra, which allows a connection to the observed UV fluxes and to the generally flatter hard-X-ray spectra observed by EXOSAT and GINGA, is found to be indistinguishable from a simple power-law in the PSPC energy band. For the PG quasars, a direct connection to the observed UV flux is obtained with a steep soft-X-ray slope $Γ_{sx}= -3.1\pm 0.3$, fixing the hard-X-ray slope. For PHL 1657, the soft-X-ray spectrum seen by ROSAT is much steeper than the GINGA spectrum, rising towards the EUV in $νF_ν$, and it can not be extrapolated down to meet the exceptionally weak UV flux. A Wien-shaped thermal UV/soft-X-ray bump with a temperature of $\sim $50\,eV connects the UV and soft-X-ray spectra in all cases, but gives only a poor representation of the high signal-to-noise PSPC spectra. We discuss implications of our results for models of the X-ray emission in quasars.

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arXiv1995

Cosmic rays V. The nonthermal radio emission of the old nova GK Per - a signature of hadronic interactions?

Using the model of particle acceleration in shocks in stellar winds developed by Biermann (1993) and Biermann and Cassinelli (1993) we demonstrate that the nonthermal radio emission spectrum from the cataclysmic variable binary star GK Per can be interpreted consistently. We propose that the radio spectrum results from hadronic secondary electrons/positrons, which, in the optically thin case, give a characteristic radio spectrum with a spectral index of +1/3 below a maximum and, for a strong shock in a wind slow compared to the shock speed, of -0.7 above the maximum. We test the proposed concept for consistency with the radio data for OB and Wolf Rayet stars as well as radiosupernovae. There is some evidence that this process is important for the radio emission from active galactic nuclei, both in compact and extended emission. We argue that such a radio spectrum is a unique characteristic for hadronic interactions.

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arXiv1995

Galactic jet sources and the AGN connection

In order to further test our hypothesis that jets and disk around compact accreting objects are symbiotic features we investigate the newly discovered superluminal galactic radio jets GRS 1915+105 and GRO J1655-40 and the two famous galactic radio jets 1E1740-2942 and SS 433 within the framework of our couple jet/disk model developed initially for active galactic nuclei (AGN) and the galactic center source Sgr A*. By comparing the ``disk'' and radio core luminosity of those galactic jet sources with our model prediction we can show that they can easily be understood as AGN-like jets where the accretion power onto a central compact object is scaled down by several orders of magnitude. The total power of the jets must be comparable to the disk luminosity - at least for the superluminal sources. To broaden our view we also shortly discuss the situation in other galactic flat spectrum radio source associated with compact objects -- the X-ray binaries Cyg X-1, Cyg X-2, Cyg X-3 and Sco X-1 -- where a jet origin has been proposed earlier on theoretical grounds. In an disk/radio luminosity their radio cores also fall within our model prediction for scaled down radio loud and radio weak AGN-jets. Taking all sources together and comparing their L_disk/radio ratio we find an indication for a similar radio loud/radio weak dichotomy as found earlier for quasar radio cores, however, a larger number of galactic jet sources is needed to confirm this trend.

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arXiv1995

SUPERNOVA BLASTWAVES AND PRE-SUPERNOVA WINDS: THEIR COSMIC RAY CONTRIBUTION

Supernova explosions into predecessor stellar winds can lead to particle acceleration, which we suggest can explain most of the observed cosmic rays of the nuclei of Helium and heavier elements, from GeV in particle energies up to near $3 \, 10^9$ GeV, as well as electrons above about 30 GeV. We go through the following steps: 1) Using a postulated underlying principle that leads to transport coefficients in a turbulent plasma, we derive the properties of energetic particles accelerated in spherical shocks. 2) We suggest that a dynamo working in the inner convection zone of an upper main sequence star can lead to high magnetic field strengths. 3) Such magnetic fields may put additional momentum into stellar winds from the pressure gradient of the toroidal field. 4) We derive a lower limit for the magnetic field strengths. 5) There appears to be a critical Alfvénic Machnumber for electron injection. With this concept we propose an explanation for the observed proton/electron ratio in galactic cosmic rays at GeV energies. 6) We check the model prediction quantitatively on cosmic ray spectrum and chemical composition against airshower data and further cosmic ray data. 7) Finally, we summarize various important caveats, and outline important next steps as well as checks.

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arXiv1995

The origin of cosmic rays

The search for the origin of cosmic rays is a quest of almost a hundred years. A recent theoretical proposal gives quantitative predictions, which can be tested with data. Specifically, it has been suggested, that all cosmic rays can be attributed to just three source sites: i) supernova explosions into the interstellar medium, ii) supernova explosions into a stellar wind, and iii) powerful radiogalaxies. The cosmic rays from any extragalactic source suffer from interaction with the microwave background, leading to the Greisen-Zatsepin-Kuzmin cutoff. While the particle energies, the spectrum and the chemical composition of cosmic rays over the energy range from about GeV to about 100 EeV can be interpreted in the theory, there are exciting measurements now: New measurements show that there are cosmic ray events beyond the Greisen-Zatsepin-Kuzmin cutoff. We discuss here possible sources, and specifically ask whether powerful radiogalaxies are suitable candidates. The basic concepts used here are the minimal hypothesis that the intergalactic magnetic field is given by the galaxy distribution, and the observation that radio galaxies also cluster like galaxies.

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arXiv1995

THE ORIGIN OF COSMIC RAYS

We propose that cosmic rays originate mainly in three sites, a) normal supernova explosions into the interstellar medium, b) supernova explosions into stellar winds, and c) hot spots of powerful radio galaxies. The proposal depends on an assumption about the scaling of the turbulent diffusive transport in cosmic ray mediated shock regions; the proposal also uses a specific model for the interstellar transport of cosmic rays. The model has been investigated in some detail and compared to i) the radio data of OB stars, Wolf Rayet stars, radio supernovae, radio supernova remnants, Gamma-ray line and continuum emission from starforming regions, and the cosmic ray electron spectrum, ii) the Akeno air shower data over the particle energy range from 10 TeV to EeV, and iii) the Akeno and Fly's Eye air shower data from 0.1 EeV to above 100 EeV. Here we specifically discuss in some detail the assumptions inherent in the model proposed, concerning the transport of energetic particles, first in the acceleration region, and second in the Galaxy.

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arXiv1995

The Origin of Galactic Cosmic Rays

Motivated by recent measurements of the major components of the cosmic radiation around 10 TeV/nucleon and above, we discuss the phenomenology of a model in which there are two distinct kinds of cosmic ray accelerators in the galaxy. Comparison of the spectra of hydrogen and helium up to 100 TeV per nucleon suggests that these two elements do not have the same spectrum of magnetic rigidity over this entire region and that these two dominant elements therefore receive contributions from different sources.

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