Schönau-Berzdorf, Germany
Schönau-Berzdorf, Germany

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Moss D.,University of Manchester | Stepanov R.,Perm State University | Arshakian T.G.,MPI fur Radioastronomie | Arshakian T.G.,Armenia and Isaac Newton Institute of Chile | And 3 more authors.
Astronomy and Astrophysics | Year: 2012

Context. Magnetic fields in nearby, star-forming galaxies reveal large-scale patterns, as predicted by dynamo models, but also a variety of small-scale structures. In particular, a large-scale field reversal may exist in the Milky Way while no such reversals have been observed so far in external galaxies. Aims. The effects of star-forming regions in galaxies need to be included when modelling the evolution of their magnetic fields, which can then be compared to future radio polarization observations. The conditions leading to large-scale field reversals also need clarification. Methods. Our simplified model of field evolution in isolated disc galaxies includes a standard mean-field dynamo and continuous injection of turbulent fields (the effect of supernova explosions) in discrete star forming regions by implicit small-scale dynamo action. Synthetic maps of radio synchrotron emission and Faraday rotation measures are computed for galaxies at different evolutionary stages. Results.A large-scale dynamo is essential to obtain regular large-scale spiral magnetic fields, as observed in many galaxies. These appear, on kpc scales in near energy equilibrium with the turbulence, after 1-2 Gyr (corresponding to redshift about 4-3). The injection of turbulent fields generates small-scale field structures. Strong injected small-scale fields and a large dynamo number (e.g. rapid rotation) of a galaxy favour the generation of field reversals. Depending on the model parameters, large-scale field reversals may persist over many Gyrs and can survive until the present epoch. Significant polarized radio synchrotron emission from young galaxies is expected at redshift ≤4. Faraday rotation measures (RM) are crucial to detect field reversals. Large-scale RM patterns of rotation measures can be observed at redshift ≤3. Conclusions. Our model can explain the general form of axisymmetric spiral fields with many local distortions, as observed in nearby galaxies. For a slightly different choice of parameters, large-scale field reversals can persist over the lifetime of a galaxy. Comparing our synthetic radio maps with future observations of distant galaxies with the planned Square Kilometre Array (SKA) and its precursors will allow testing and refinement of models of magnetic field evolution. © 2012 ESO.

Beck R.,MPI fur Radioastronomie | Frick P.,Perm State University | Stepanov R.,Perm State University | Sokoloff D.,Moscow State University
Astronomy and Astrophysics | Year: 2012

Context. Modern radio telescopes allow us to record a large number of spectral channels. The application of a Fourier transform to spectropolarimetric data in radio continuum, Faraday rotation measure (RM) synthesis, yields the "Faraday spectrum", which hosts valuable information about the magneto-ionic medium along the line of sight. Aims. We investigate whether the method of wavelet-based RM synthesis can help us to identify structures of regular and turbulent magnetic fields in extended magnetized objects, such as galaxies and galaxy clusters. Methods. The analysis of spectropolarimetric radio observations of multi-scale targets calls for a corresponding mathematical technique. Wavelets allow us to reformulate the RM synthesis method in a scale-dependent way and to visualize the data as a function of Faraday depth and scale. Results. We present observational tests to recognize magnetic field structures. A region with a regular magnetic field generates a broad "disk" in Faraday space, with two "horns" when the distribution of cosmic-ray electrons is broader than that of the thermal electrons. Each field reversal generates one asymmetric "horn" on top of the "disk". A region with a turbulent field can be recognized as a "Faraday forest" of many components. These tests are applied to the spectral ranges of various synthesis radio telescopes. We argue that the ratio of maximum to minimum wavelengths determines the range of scales that can be identified in Faraday space. Conclusions. A reliable recognition of magnetic field structures in spiral galaxies or galaxy clusters requires the analysis of data cubes in position-position-Faraday depth space ("PPF cubes"), observed over a wide and continuous frequency range, allowing the recognition of a wide range of scales as well as high resolution in Faraday space. The planned Square Kilometre Array (SKA) will fulfill this condition and will be close to representing a perfect "Faraday telescope". The combination of data from the Low Frequency Array (LOFAR, at low frequencies) and the Expanded Very Large Array (EVLA, at high frequencies) appears to be a promising approach for the recognition of magnetic structures on all scales. The addition of data at intermediate frequencies from the Westerbork Synthesis Radio Telescope (WSRT) or the Giant Meterwave Radio Telescope (GMRT) would fill the gap between the LOFAR and EVLA frequency ranges. The Global Magneto-Ionic Medium Survey (GMIMS), planned with several single-dish telescopes at low angular resolution, will also provide good scale recognition and high resolution in Faraday space. © 2012 ESO.

Meli A.,Ghent University | Biermann P.L.,MPI fur Radioastronomie
Astronomy and Astrophysics | Year: 2013

Context. Shocks in jets and hot spots of active galactic nuclei (AGN) are one prominent class of possible sources of very high-energy cosmic-ray particles (above 1018 eV). Extrapolating their spectrum to their plausible injection energy from some shock implies an enormous hidden energy for a spectrum of index ~-2. Some analyses suggest the particles' injection spectrum at source to be as steep as-2.4 to-2.7, which exacerbates the problem, by a factor of 106. Nevertheless, it seems implausible that more than at the very best 1/3 of the jet energy goes into the required flux of energetic particles, thus one would need to allow for the possibility that there is an energy problem, which we would like to address in this work. Aims. Sequences of consecutive oblique shock features, or conical shocks, have been theoretically predicted and eventually observed in many AGN jets. Based on that, we use by analogy the Comptonization effect and propose a scenario of a single injection of particles consecutively accelerated by several oblique shocks along the axis of an AGN jet. Methods. We developed a test-particle approximation Monte Carlo simulations to calculate particle spectra by acceleration at such a shock pattern while monitoring the efficiency of acceleration by calculating differential spectra. Results. We find that the first shock of a sequence of oblique shocks establishes a low-energy power-law spectrum with ~E -2.7. The following consecutive shocks push the spectrum up in energy, rendering flatter distributions with steep cut-offs, and characteristic depletion at low energies, which could explain the puzzling apparent extra source power. Conclusions. Our numerical calculations show a variation of spectral indices, a general spectral flattening, and starved spectra, which connect to the relativistic nature of the shocks, the multiple shock acceleration conditions, and the steepness of the magnetic field to the shock normal. This helps in understanding the jet-magnetic field geometry and the irregular or flat spectra observed in many AGN jets (e.g., CenA, 3C 279, PKS 1510-089). Furthermore, the E-2.4-E-2.7 ultra-high-energy cosmic-ray injected source spectra claimed by many authors might be explained by the superposition of several, perhaps many, emission sources, all of which end their particle shock-acceleration sequence with flatter, starved spectra produced by two or more consecutive oblique shocks along their jets. It might also imply a mixed component of the accelerated particles above 1019 eV. Moreover, the present acceleration model can explain the variability of inverted gamma-ray spectra observed in high redshifted flaring extragalactic sources. © 2013 ESO.

Moss D.,University of Manchester | Sokoloff D.,Moscow State Pedagogical University | Beck R.,MPI fur Radioastronomie | Krause M.,MPI fur Radioastronomie
Astronomy and Astrophysics | Year: 2014

Context. Galactic encounters are usually marked by a substantial increase in synchrotron emission of the interacting galaxies when compared with the typical emission from similar non-interacting galaxies. This increase is believed to be associated with an increase in the star formation rate and the turbulent magnetic fields resulting from the encounter, while the regular magnetic field is usually believed to decrease as a result of the encounter. Aims. We attempt to verify these expectations. Methods. We consider a simple, however rather realistic, mean-field galactic dynamo model where the effects of small-scale generation are represented by random injections of magnetic field resulting from star forming regions. We represent an encounter by the introduction of large-scale streaming velocities and by an increase in small-scale magnetic field injections. The latter describes the effect of an increase in the star formation rate caused by the encounter. Results. We demonstrate that large-scale streaming, with associated deviations in the rotation curve, can result in an enhancement of the anisotropic turbulent (ordered) magnetic field strength, mainly along the azimuthal direction. This leads to a significant temporary increase of the total magnetic energy during the encounter; the representation of an increase in star formation rate has an additional strong effect. In contrast to expectations, the large-scale (regular) magnetic field structure is not significantly destroyed by the encounter. It may be somewhat weakened for a relatively short period, and its direction after the encounter may be reversed. Conclusions. The encounter causes enhanced total and polarized emission without increase in the regular magnetic field strength. The increase in synchrotron emission caused by the large-scale streaming can be comparable to the effect of the increase in the star formation rate, depending on the choice of parameters. The effects of the encounter on the total magnetic field energy last only slightly longer than the duration of the encounter (ca. 1 Gyr). However, a long-lasting field reversal of the regular magnetic field may result. © ESO, 2014.

Moss D.,University of Manchester | Sokoloff D.,Moscow State Pedagogical University | Beck R.,MPI fur Radioastronomie
Astronomy and Astrophysics | Year: 2012

Context. The magnetic field of galaxies is believed to be produced by internal dynamo action, but can be affected by motion of the galaxy through the surrounding medium. Observations of polarized radio emission of galaxies located in galaxy clusters have revealed noticeable features of large-scale magnetic configurations, including displacements of the magnetic structures from the optical images and tails, which are possible imprints of ram pressure effects arising from motion of the galaxies through the intracluster medium. Aims. We present a quantitative dynamo model, which attempts to describe the above effects. In contrast, to the traditional problem of a wind affecting a body with a prescribed magnetic field, we investigate how a non-magnetized wind flow affects a magnetic field that is being self-excited by galactic dynamo action. Methods. To isolate the leading physical effects, we exploit a simple dynamo model that can describe relevant effects. In particular, we use what is known as the "no-z" approximation for the mean-field dynamo equations. Results. In a suitable parametric range we obtain displacements of the large-scale magnetic field, as well as magnetic tails. However, the specific details of their locations are quite counterintuitive. The direction of displacement is perpendicular to, rather than parallel to, the wind direction. The point at which the tail emerges from the galaxy depends on details of the model. The tail is eventually directed downstream. In the simplest case the magnetic tail begins in the region where the wind decreases the total gas velocity. Any wind that penetrates the galaxy modifies the intrinsic dynamo action. These features are different from those found in ram-pressure models. Conclusions. Any determination of galactic motion through the cluster medium from observational data needs to take the effects of dynamo action into account. © 2012 ESO.

Indriolo N.,Johns Hopkins University | Neufeld D.A.,Johns Hopkins University | Gerin M.,French National Center for Scientific Research | Geballe T.R.,Gemini Observatory | And 3 more authors.
Astrophysical Journal | Year: 2012

Absorption lines from the molecules OH+, H2O +, and H+ 3 have been observed in a diffuse molecular cloud along a line of sight near W51 IRS2. We present the first chemical analysis that combines the information provided by all three of these species. Together, OH+ and H2O+ are used to determine the molecular hydrogen fraction in the outskirts of the observed cloud, as well as the cosmic-ray ionization rate of atomic hydrogen. H + 3 is used to infer the cosmic-ray ionization rate of H2 in the molecular interior of the cloud, which we find to be ζ2 = (4.8 ± 3.4) × 10-16s-1. Combining the results from all three species we find an efficiency factor - defined as the ratio of the formation rate of OH+ to the cosmic-ray ionization rate of H - of ε = 0.07 ± 0.04, much lower than predicted by chemical models. This is an important step in the future use of OH + and H2O+ on their own as tracers of the cosmic-ray ionization rate. © 2012. The American Astronomical Society. All rights reserved.

Espinoza C.M.,University of Manchester | Lyne A.G.,University of Manchester | Kramer M.,MPI fur Radioastronomie | Manchester R.N.,CSIRO | Kaspi V.M.,McGill University
Astrophysical Journal Letters | Year: 2011

PSR J1734-3333 is a radio pulsar rotating with a period P = 1.17 s and slowing down with a period derivative , the third largest among rotation-powered pulsars. These properties are midway between those of normal rotation-powered pulsars and magnetars, two populations of neutron stars that are notably different in their emission properties. Here we report on the measurement of the second period derivative of the rotation of PSR J1734-3333 and calculate a braking index n = 0.9 0.2. This value is well below 3, the value expected for an electromagnetic braking due to a constant magnetic dipole, and indicates that this pulsar may soon have the rotational properties of a magnetar. While there are several mechanisms that could lead to such a low braking index, we discuss this observation, together with the properties exhibited by some other high- rotation-powered pulsars, and interpret it as evidence of a possible evolutionary route for magnetars through a radio-pulsar phase, supporting a unified description of the two classes of the object. © 2011. The American Astronomical Society. All rights reserved.

Siebenmorgen R.,ESO | Heymann F.,Ruhr University Bochum | Krugel E.,MPI fur Radioastronomie
EAS Publications Series | Year: 2011

We present a Monte Carlo (MC) radiative transfer code for complex three dimensional dust distributions and include transiently heated PAH. The correctness of the code is confirmed by comparison with benchmark results. The method makes use of the parallelization capabilities of modern vectorized computing units like graphic cards. The computational speed grows linearly with the number of graphical processing units (GPU). On a conventional desktop PC, our code is up to a factor 100 faster when compared to other MC algorithms. As an example, we compute the dust emission of proto-planetary disks. We simulate how a mid-IR instrument mounted at a future 42 m ELT will detect such disks. Two cases are distinguished: a homogeneous disk and a disk with an outward migrating planet, producing a gap and a spiral density wave. We find that the resulting mid-IR spectra of both disks are almost identical. However, they can be distinguished at those wavelengths by coronographic, dual-band imaging. Finally, the emission of PAHs exposed to different radiation fields is computed. We demonstrate that PAH emission depends not only on the strength but also strongly on the hardness of the radiation, a fact which has often been neglected in previous models. We find that hard photons (>20 eV) easily dissociate all PAHs in the disks of T Tauri stars. To explain the low, but not negligible detection rate (<10%) of PAHs in T Tau disks, we suggest that turbulent motions act as a possible path for PAH survival. © EAS, EDP Sciences 2011.

Kramer M.,MPI fur Radioastronomie
AIP Conference Proceedings | Year: 2011

In 1992 the first extrasolar planets were discovered. But unlike expected, these planets were not located around normal stars. The central object of the system that is now known to consist of (at least) three planets, is a radio pulsar. This discovery was possible due to the unique precision of pulsar timing which allows us to detect even asteroid-sized bodies around pulsars. If sensitivity is not a problem, where are the other systems? Only one further pulsar planetary system is known, raising the question about the formation processes of pulsar planets and their conditions. In the future, we can expect to perform a 'complete' census of pulsar planets in the Galaxy, promising to answer the open questions once and for all. © 2011 American Institute of Physics.

Berkhuijsen E.M.,MPI fur Radioastronomie | Beck R.,MPI fur Radioastronomie | Tabatabaei F.S.,MPI fur Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2013

We investigate the effect of propagation of cosmic ray electrons (CRE) on the non-thermal (NTH; synchrotron)-far-infrared correlations in M 31 and M 33. The thermal (TH) and NTH emission components of the radio continuum emission at 1.4 GHz and one higher frequency are compared with dust emission from M 31 and M 33 using Spitzer data. In both galaxies the TH emission is linearly correlated with the emission from warm dust (24 μm, 70 μm), but the power laws of the NTH-FIR correlations have exponents b < 1 that increase with increasing frequency. Furthermore, the values of b forM33 are significantly smaller (b {minus tilde} 0.4) than those for M 31 (b {minus tilde} 0.6). We interpret the differences in b as differences in the diffusion length of the CRE. We estimate the diffusion length in two ways: (1) by smoothing the NTH emission at the higher frequency until the correlation with NTH emission at 1.4 GHz has b = 1, and (2) by smoothing the TH emission until the correlation with the NTH emission at the same frequency has b = 1, assuming that the TH emission represents the source distribution of the CRE. Our smoothing experiments show thatM31 only has a thin NTH disc with a scale height of h = 0.3-0.4 kpc at 1.4 GHz, whereas M 33 has a similar thin disc as well as a thick disc with scale height hthick {minus tilde} 2 kpc. In the thin discs, the (deprojected) diffusion length at 1.4 GHz is {minus tilde}1.5 kpc, yielding a diffusion coefficient of 2 × 1028 cm2 s-1. The structure, strength and regularity of the magnetic field in a galaxy as well as the existence of a thick disc determine the diffusion of the CRE, and hence, the power-law exponent of the NTH-FIR correlations. © 2013 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society.

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