Magnetic fields in the galactic Universe,as observed in supershells,galaxies, intergalactic and cosmic realms

Here I present a survey of magnetic fields in large objects, from the interstellar supershells (10 pc) up to the edge of the Universe (near a redshift z of 10), with an emphasis on discoveries made in the last decade, be they through particle astronomy or electromagnetic astronomy.For each type of object, the basic observational properties are summarized, and the best theoretical scenario which accounts for the large body of observations is discussed.The strength of these large-scale fields can vary from mGauss to μGauss. Magnetism acts as a tracer of the dynamical histories of cosmological and intracluster events, it guides the motion of the interstellar ionised gas, and it aligns the charged dust particles.     

Magnetic fields of active galactic nuclei and quasars from the SDSS catalog

We analyze the spectropolarimetric observations of 12 candidates for quasars from the spectroscopic database of the SDSS Catalog. The magnetic fields of these objects are estimated in the context of a theory that includes the Faraday rotation of the polarization plane on the mean free path of a photon in the outflow from an accretion disk. As a result, we have determined the column density in the outflow, N _H ∼ 6 × 10²³ cm⁻², and the radial, B ∼ 1 G, and toroidal, B ∼ 600 G, magnetic fields.     

MHD simulations of rayleigh-taylor instability in young supernova remnants

Radio observations shows that young supernova remnants such as Tycho and Cas A generally exhibit a circular clumpy shell. This shell shows a radial magnetic field whose equipartition strength is 2 to 3 orders of magnitude higher than the interstellar field. A simple compression of the ambient field by the shock can explain neither of these observations. We show that the Rayleigh-Taylor instability which occurs at the ejecta/ISM interface can explain these observations. We have done MHD simulations of the instability in the shell of Type-I supernova remnants for the first time by utilizing moving grid technique. Our simulation shows that Rayleigh-Taylor and Kelvin-Helmholtz instabilities amplify ambient magnetic fields locally and produce the clumpy radio shell. Strong magnetic field lines draped around the Rayleigh-Taylor fingers produce the radial B-vector polarization, whereas thermal bremsstrahlung from the dense fingers themselves produce the clumpy X-ray emission.     

Coronal Magnetography of Solar Active Region 8365 with the SSRT and NoRH Radio Heliographs

Microwave maps of solar active region NOAA 8365 are used to derive the coronal magnetograms of this region. The technique is based on the fact that the circular polarization of a radio source is modified when microwaves pass through the coronal magnetic field transverse to the line of sight. The observations were taken with the Siberian Solar Radio Telescope (SSRT) on October 21 – 23 and with the Nobeyama Radio Heliograph (NoRH) on October 22 – 24, 1998. The known theory of wave mode coupling in quasi-transverse (QT) region is employed to evaluate the coronal magnetograms in the range of 10 – 30 G at the wavelength 5.2 cm and 50 – 110 G at 1.76 cm, taking the product of electron density and the scale of coronal field divergence to be constant of 10¹⁸ cm^–2. The height of the QT-region is estimated from the force-free field extrapolations as 6.2 × 10⁹ cm for the 20 G and 2.3 × 10⁹ cm for 85 G levels. We find that on large spatial scale, the coronal magnetograms derived from the radio observations show similarity with the magnetic fields extrapolated from the photosphere.     

Heat blanketing envelopes and thermal radiation of strongly magnetized neutron stars

Strong (B?10⁹ G) and superstrong (B?10¹⁴ G) magnetic fields profoundly affect many thermodynamic and kinetic characteristics of dense plasmas in neutron star envelopes. In particular, they produce strongly anisotropic thermal conductivity in the neutron star crust and modify the equation of state and radiative opacities in the atmosphere, which are major ingredients of the cooling theory and spectral atmosphere models. As a result, both the radiation spectrum and the thermal luminosity of a neutron star can be affected by the magnetic field. We briefly review these effects and demonstrate the influence of magnetic field strength on the thermal structure of an isolated neutron star, putting emphasis on the differences brought about by the superstrong fields and high temperatures of magnetars. For the latter objects, it is important to take proper account of a combined effect of the magnetic field on thermal conduction and neutrino emission at densities ρ?10¹⁰ g?cm^?3. We show that the neutrino emission puts a B-dependent upper limit on the effective surface temperature of a cooling neutron star.     

Stokes Diagnostics of 2D MHD-Simulated Solar Magnetogranulation

The properties of solar magnetic fields on scales less than the spatial resolution of solar telescopes are studied. A synthetic infrared spectropolarimetric diagnostic based on a 2D MHD simulation of magnetoconvection is used for this. Analyzed are two time sequences of snapshots that likely represent two regions of the network fields with their immediate surroundings on the solar surface with unsigned magnetic flux densities of 300 and 140 G. In the first region from the probability density functions of the magnetic field strength it is found that the most probable field strength at log τ ₅=0 is equal to 250 G. Weak fields (B<500 G) occupy about 70% of the surface, whereas stronger fields (B>1000 G) occupy only 9.7% of the surface. The magnetic flux is −28 G and its imbalance is −0.04. In the second region, these parameters are correspondingly equal to 150 G, 93.3%, 0.3%, −40 G, and −0.10. The distribution of line-of-sight velocities on the surface of log τ ₅=−1 is estimated. The mean velocity is equal to 0.4 km s⁻¹ in the first simulated region. The average velocity in the granules is −1.2 km s⁻¹ and in the intergranules it is 2.5 km s⁻¹. In the second region, the corresponding values of the mean velocities are equal to 0, −1.8, and 1.5 km s⁻¹. In addition the asymmetry of synthetic Stokes V profiles of the Fe i 1564.8 nm line is analyzed. The mean values of the amplitude and area asymmetry do not exceed 1%. The spatially smoothed amplitude asymmetry is increased to 10% whereas the area asymmetry is only slightly varied.     

Magnetic fields at the surfaces of stars

Summary Magnetic fields have now been detected in stars in several parts of the Hertzsprung-Russell diagram. Roughly dipolar fields ranging in strength between 3× 10² and 3×10⁴ G are found in many chemically peculiar A and B main sequence stars. Dipolar fields are also found in some 2–3% of white dwarfs, but with strengths between 1×10⁶ and 5×10⁸ G. In both these types of stars, the observed fields vary as the underlying star turns, but do not change in a secular manner. In solartype stars, structurally complex fields of a few kG are found with filling factors of the order of 0.1 to 0.8. Further indirect evidence of fields in cool main sequence stars is provided by detection of visible and ultraviolet line emission (chromospheric activity), x radiation (coronal matter), and giant starspots. In this review, we survey the observations of stellar magnetism in all these types of stars, as well as efforts to model the observed magnetic fields and associated photospheric peculiarities and activity.This article was processed by the author using the Springer-Verlag T_EX AAR macro package 1991.     

Estimation of spicule magnetic field using observed kink waves

We apply the method of MHD seismology to estimate the magnetic field in spicules using observed kink waves. We include the effects of gravitational stratification, the neglect of which leads to an error of around 30% in the estimation of the magnetic field. With stratification included, we find the magnetic field in spicules in the range of 8–16 G. We also estimate a density of 7.4 × 10⁻¹⁰ kg m⁻³ in spicules. The estimated values of magnetic field and density are in agreement with the available observations. Improved measurement of height, oscillation period, and plasma density in spicules will further enhance the precision of this method.     

APEX and ATCA observations of the southern hot core G327.3-0.6 and its environs

There is no generally accepted evolutionary scheme for high mass star formation yet. A simple approach to address this problem is to cover several of the known stages during the formation of massive stars in the same cloud and then investigate their properties trying to construct an evolutionary sequence. Here we present such a project conducted with complementary APEX and ATCA observations. These observations show a compact and bright single hot core in the G327.3-0.6 region on a 0.03 pc scale with a mass of 500 M_⊙ and 0.5–1.5 10⁵ L_⊙. Additionally a clumpy filament is seen in N₂H⁺. Together with cm continuum observations, the data reveal like pearls on a string several stages of massive star formation, with likely the youngest stages hiding in the cold N₂H⁺ cores analysed with a multilevel study of the APEX and ATCA observations.     

Interstellar neutral hydrogen filaments at high galactic latitudes and the bennett pinch

Observed properties of interstellar neutral hydrogen filaments suggest the presence of the Bennett pinch as described by the Carlqvist relationship with rotation around the filament axes included. A brief summary is first given of three ways in which a filament model for interstellar cloud structure was tested. Preliminary results from highresolution HI mapping of gas and dust in an apparent HI cloud indicate that the neutral gas and dust within and around its boundary is itself highly filamentary. An attempt to detect magnetic fields in this and similar features using the Zeeman effect technique at the 21-cm wavelength of interstellar neutral hydrogen set upper limits of a fewµG. In contrast, the strength of the toroidal magnetic field expected from the examination of the Carlqvist relationship is of order 5µG, which would be produced by a current of 1.4 · 10¹³ A. Zeeman effect technology is at present not able to detect toroidal magnetic fields of this order at the edge of barely resolved HI filaments. Nevertheless, currently available high-resolution HI data suggest that interstellar filament physics should take into account the role of currents and pinches for creating and stabilizing the structures.     

Statistical and Physical Study of One-Sided Planetary Nebulae

We have investigated the spatial orientation of one-sided Planetary Nebulae. Most of them if not all are interacting with the interstellar medium. Seventy percent of the nebulae in our sample have inclination angles larger than 45^° to the galactic plane and 30% of the inclination angles are less than 45^°. Most of the selected objects are old, evolved planetary nebulae with large dimensions, and not far away from the galactic plane. Seventy-five percent of the objects are within 160 pc from the galactic plane. The enhanced concavity arc can be explained physically as a result of the `planetary nebulae – interstellar matter' interaction. We have discussed the possible effect of the interstellar magnetic field in the concavity regions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Solar Magnetic Carpet I: Simulation of Synthetic Magnetograms

This paper describes a new 2D model for the photospheric evolution of the magnetic carpet. It is the first in a series of papers working towards constructing a realistic 3D non-potential model for the interaction of small-scale solar magnetic fields. In the model, the basic evolution of the magnetic elements is governed by a supergranular flow profile. In addition, magnetic elements may evolve through the processes of emergence, cancellation, coalescence and fragmentation. Model parameters for the emergence of bipoles are based upon the results of observational studies. Using this model, several simulations are considered, where the range of flux with which bipoles may emerge is varied. In all cases the model quickly reaches a steady state where the rates of emergence and cancellation balance. Analysis of the resulting magnetic field shows that we reproduce observed quantities such as the flux distribution, mean field, cancellation rates, photospheric recycle time and a magnetic network. As expected, the simulation matches observations more closely when a larger, and consequently more realistic, range of emerging flux values is allowed (4×10¹⁶ – 10¹⁹ Mx). The model best reproduces the current observed properties of the magnetic carpet when we take the minimum absolute flux for emerging bipoles to be 4×10¹⁶ Mx. In future, this 2D model will be used as an evolving photospheric boundary condition for 3D non-potential modeling.     

Are the magnetic fields of millisecond pulsars ∼108 G?

It is generally assumed that the magnetic fields of millisecond pulsars (MSPs) are ～10⁸ G. We argue that this may not be true and the fields may be appreciably greater. We present six evidences for this: (1) The ～10⁸G field estimate is based on magnetic dipole emission losses which is shown to be questionable; (2) The MSPs in low mass X-ray binaries (LMXBs) are claimed to have <10¹¹ G on the basis of a Rayleygh-Taylor instability accretion argument. We show that the accretion argument is questionable and the upper limit 10¹¹ G may be much higher; (3) Low magnetic field neutron stars have difficulty being produced in LMXBs; (4) MSPs may still be accreting indicating a much higher magnetic field; (5) The data that predict ～10⁸ G for MSPs also predict ages on the order of, and greater than, ten billion years, which is much greater than normal pulsars. If the predicted ages are wrong, most likely the predicted ～10⁸ G fields of MSPs are wrong; (6) When magnetic fields are measured directly with cyclotron lines in X-ray binaries, fields ?10⁸ G are indicated. Other scenarios should be investigated. One such scenario is the following. Over 85% of MSPs are confirmed members of a binary. It is possible that all MSPs are in large separation binaries having magnetic fields >10⁸ G with their magnetic dipole emission being balanced by low level accretion from their companions.     

The ordinary URCA process in the presence of positrons and large magnetic fields

The effect of positron capture on the ordinary URCA neutrino luminosity in a zero magnetic field is investigated for several values of the degeneracy parameter and the range of temperatures 5×10⁸K–5×10¹⁰K. The rate for this process is then compared with those in large magnetic fields (on the order ofH _c =m ² c ³/eh=4.414×10¹⁰ G). The results indicate that positron capture reduces the effect of large magnetic fields on this process at high temperatures.     

Emission of cyclotron radiation by interstellar planets

In the solar system all planets that have significant magnetic fields also emit electron cyclotron radiation, usually near the auroral regions around the magnetic poles. In this study we use scaling laws based on solar system data to estimate the power and frequency of the auroral cyclotron emissions from interstellar planets (or sub-brown dwarfs). The emission can be powered either by motion of the planet through the interstellar plasma or by unipolar induction due to a moon. According to our results, in interstellar space the unipolar induction mechanism is potentially more effective than the motional emission mechanism. Typical emission power is around 10¹⁰-10¹² W, but significantly stronger emissions are obtained in the most optimistic estimates. We have to conclude that detection of a rogue Jupiter would be very difficult, if not impossible with the radio telescopes available now or in the near future, but in very favorable conditions a much more massive and rapidly rotating (or otherwise strongly magnetized) gas giant with a large nearby moon could be detected up to ∼57 pc distance with the square kilometer array. There may be a few thousand large enough interstellar planets this close to the solar system. For reference, we point out that according to previous studies some known hot Jupiters are expected to emit up to 10¹⁴-10¹⁶ W of cyclotron radiation, orders of magnitude more than the typical interstellar planets discussed here. However, these emissions have not yet been detected.     

Complex of Meteoroid Orbits with Eccentricities Near 1 and Higher

In our work, the method that can help to predict the existence of distant objects in the Solar system is demonstrated. This method is connected with statistical properties of a heliocentric orbital complex of meteoroids with high eccentricities. Heliocentric meteoroid orbits with high eccentricities are escape routes for dust material from distant parental objects with near-circular orbits to Earth-crossing orbits. Ground-based meteor observations yield trajectory information from which we can derive their place of possible origin: comets, asteroids, and other objects (e.g. Kuiper Objects) in the Solar system or even interstellar space. Statistical distributions of radius vectors of nodes, and other parameters of orbits of meteoroids contain key information about position of greater bodies. We analyze meteor orbits with high eccentricities that were registered in 1975–1976 in Kharkiv (Ukraine). The orbital data of the Kharkiv electronic catalogue are received from observations of radiometeors with masses 10⁻⁶−10⁻³ g.     

A multiwavelength study of the supernova remnant G296.8-0.3

We report XMM-Newton observations of the Galactic supernova remnant G296.8-0.3, together with complementary radio and infrared data. The spatial and spectral properties of the X-ray emission, detected towards G296.8-0.3, was investigated in order to explore the possible evolutionary scenarios and the physical connexion with its unusual morphology detected at radio frequencies. G296.8-0.3 displays diffuse X-ray emission correlated with the peculiar radio morphology detected in the interior of the remnant and with the shell-like radio structure observed to the northwest side of the object. The X-ray emission peaks in the soft/medium energy range (0.5–3.0 keV). The X-ray spectral analysis confirms that the column density is high (N _H∼0.64×10²² cm⁻²) which supports a distant location (d>9 kpc) for the SNR. Its X-ray spectrum can be well represented by a thermal (PSHOCK) model, with kT∼0.86 keV, an ionization timescale of 6.1×10¹⁰ cm⁻³ s, and low abundance (∼0.12 Z _⊙). The 24 μm observations show shell-like emission correlated with part of the northwest and southeast boundaries of the SNR. In addition a point-like X-ray source is also detected close to the geometrical center of the radio SNR. The object presents some characteristics of the so-called compact central objects (CCO). Its X-ray spectrum is consistent with those found at other CCOs and the value of N _H is consistent with that of G296.8-0.3, which suggests a physical connexion with the SNR.     

Spectral Line and Continuum Radiation Propagation in a Clumpy Medium

We discuss the propagation of spectral line and continuum radiation in a clumpy medium and give general expressions for the observed absorption or emission from a cloud population. We show that the affect of the medium clumpiness can usually be characterised by a single number multiplying the mean column opacity. Our result provides a simpler proof and generalization of the result of Martin et al. (1984). The formalism provides a simple way to understand the effects of clumping on molecular line profiles and ratios; for example, how clumping effects the interpretation of ¹³CO(1–0) to ¹²CO(1–0) line ratios. It also can be used as a propagation operator in physical models of clumpy media where the incident radiation effects the spectral line emissivity. We are working to extend the formalism to the propagation of masers in a clumpy medium, but in this case, there are special difficulties because formal expectation values are not characteristic of observations because they are biased by rare events.     

Magnetorotational instabilities and pulsar kick velocities

At the end of their birth process, neutron stars can be subject to a magnetorotational instability in which a conversion of kinetic energy of differential rotation into radiation and kinetic energies is expected to occur at the Alfvén timescale of a few ms. This birth energy conversion predicts the observed large velocity of neutron stars if during the evolving of this instability the periods are of a few ms and the magnetic fields reach values of 10¹⁶ G.