Radio Asymmetry Research Articles

Asymmetric Expansion of the Youngest Galactic Supernova Remnant G1.9+0.3

Abstract The youngest Galactic supernova remnant (SNR) G1.9+0.3, produced by a (probable) SN Ia that exploded ∼1900 CE, is strongly asymmetric at radio wavelengths, much brighter in the north, but bilaterally symmetric in X-rays. We present the results of X-ray expansion measurements that illuminate the origin of the radio asymmetry. We confirm the mean expansion rate (2011–2015) of 0.58% yr−1, but large spatial variations are present. Using the nonparametric “Demons” method, we measure the velocity field throughout the entire SNR, finding that motions vary by a factor of 5, from to yr−1. The slowest shocks are at the outer boundary of the bright northern radio rim, with velocities v s as low as 3600 km s−1 (for an assumed distance of 8.5 kpc), much less than v s = 12,000–13,000 km s−1 along the X-ray-bright major axis. Such strong deceleration of the northern blast wave most likely arises from the collision of SN ejecta with a much denser than average ambient medium there. This asymmetric ambient medium naturally explains the radio asymmetry. In several locations, significant morphological changes and strongly nonradial motions are apparent. The spatially integrated X-ray flux continues to increase with time. Based on Chandra observations spanning 8.3 yr, we measure its increase at yr−1. The SN ejecta are likely colliding with the asymmetric circumstellar medium ejected by the SN progenitor prior to its explosion.

A New Mechanism Explaining the Asymmetric X‐Ray Emission from Supernova Remnant PKS 1209−52

The interaction of a supernova remnant with its surrounding environment is a powerful source of radiation in various spectral regions and at different stages in its development. The study of these emissions can therefore help to investigate the interstellar medium surrounding a supernova. For example, the highly asymmetric X-ray emission from PKS 1209-52 has previously been described as resulting from the supernova shock front on the eastern side of the remnant interacting with a small, dense interstellar cloud (or cloud core). In this paper we describe a possible mechanism through which the kinetic energy of the expanding shock is converted to energetic electrons. The particular interaction is a plasma streaming instability known as the modified two-stream instability that creates plasma waves in the lower hybrid frequency range that can then go on to accelerate electrons to high energy. A Fokker-Planck equation describing the diffusion of the electrons to high energies by lower hybrid waves is solved for both Gaussian and Lorentzian wave spectral distributions. We also simulate the interaction between the supernova remnant with the dense interstellar cloud using a two-dimensional fluid code. The nonthermal electrons can then generate X-ray emission via bremsstrahlung and also line radiation through interactions with the ambient medium. The maximum energy of these electrons is limited to the few keV energy range and therefore does not lead to any synchrotron radio emission that would otherwise lead to a radio asymmetry in the remnant that is not seen.

Radio asymmetry in 3C 99

The N-galaxy 3C 99, a Compact Steep-spectrum Source showing a triple asymmetric structure, has been observed with several arrays of radio telescopes at sub-arcsecond resolution. New images from MERLIN, European VLBI Network and the Very Long Baseline Array show that the source components detected in the central region of 3C 99 have steep spectral indices, making it difficult to determine which of them is the core. The asymmetric radio structure of 3C 99 is explained using the Scheuer-Baldwin continuous streaming model with a rather large difference in the interstellar medium density on the two sides of the central region. The age of 3C 99 has been estimated; the source is found younger than 106 years.

Optical polarization in distant radio galaxies

We review the optical polarization properties of radio galaxies with redshifts greater than 0.1. We find a strong tendency for the E-vector to be either perpendicular or parallel to the radio axis, and perpendicular to the axis in all the high-redshift galaxies. The observed degree of linear polarization is correlated with redshift and radio power, and is anticorrelated with the rest-frame wavelength of the measurement and the radio spectral index. No correlation is found with absolute magnitude. For the high-polarization objects (P ≥ 3 per cent), the polarization also increases with the radio asymmetry parameter Q, which is thought to be related to the interstellar-medium density ratio. After correcting the polarization for stellar dilution, only weak correlations with redshift and rest-frame wavelength remain, suggesting that the redshift dependence could be entirely explained as a wavelength-dependent contrast effect. It is possible to divide the radio galaxies into two groups: the high-redshift objects (z ≥ 0.6) with high perpendicular polarization and strong radio-optical alignments, and the low-redshift sources with low polarization having either parallel or perpendicular E-vectors. The dominant polarization mechanism for the first group must be the scattering of nuclear radiation. We present a dust-scattering model that explains the structure, the polarization properties and the spectral energy distribution of the ultraviolet aligned light with optically thin Mie scattering of quasar radiation emitted in a cone of 47° half-opening angle. The required amount of spherically distributed dust is |$(1-3) \times {10}^{8} {\text M}_{\odot}$| (consistent with the IRAS data), which produces only a small dust extinction (Av ~ 0.1) along a line of sight to a quasar. The low-redshift radio galaxies do not have a single dominant polarization mechanism. In addition to scattering, polarization by transmission through aligned dust grains and synchrotron radiation can play a role in some of these objects.

VLBI observations of a complete sample of radio galaxies. II - The parsec-scale structure of NGC 315

We present multifrequency radio observations of the low-luminosity (Fanaroff-Riley type 1) radio galaxy NGC 315 at parsec resolution. The observations were obtained at 1.6,5, and 8.4 GHz with the European and US VLBI networks. In addition VLA observations were used to obtain arcsec resolution maps at the same frequencies. The radio morphology of NGC 315 is asymmetric at milliarcsec resolution. It shows nuclear emission with a core size of 0.05 pc, and a one-sided jet. We place a limit on the milliarcsec jet to counterjet brightness ratio of B_jet_/B_cjet_ >= 50 at 1.6 GHz. At arcsec resolution we see a counterjet which is shorter and much more diffuse than the main jet. We derive a spectral index profile at parsec resolution, and estimate the physical conditions in the parsec-scale jet. If we assume that the parsec-scale asymmetry is caused by Doppler boosting of an intrinsically identical jet-counterjet pair, the derived brightness ratio gives β cos θ >~ 0.54, which gives an angle a between the source axis and the line of sight <~ 58^deg^. However, Doppler boosting cannot account for the asymmetry observed on the arcsec scale, given the much lower velocity flow estimated for the arcsec jet. This, together with the lack of any visible motion of the features in the maps at different epochs, suggests that an intrinsic asymmetry in the nuclear engine of NGC 315 is likely to be responsible for the observed radio asymmetry.

Very Large Array observations of Uranus at 2.0 CM

Radio observations of Uranus obtained at 2.0 cm with the B configuration of the VLA during April 1985 are reported. The calibration and data-reduction procedures are described in detail, and the results are presented in tables, maps, and graphs and compared with IRIS 44-micron observations (Hanel et al., 1986). Features discussed include highest brightness centered on the pole rather than on the subearth point, a decrease in brightness temperature (by up to 9 K) at latitudes between -20 and -50 deg (well correlated with the IRIS data), and disk-center position (corrected for the observed radio asymmetry) in good agreement with that found on the basis of the outer contours of the image. 15 references.

Solar Radio Asymmetry at 4-Metres Wave-length

IN 1948, Hey, Parsons and Phillips1 reported that at 4.1-metres wave-length bursts of radio emission, usually of a few minutes duration, occurred more often in association with flares on the eastern half of the sun's disk than on the western half. Subsequent observations have confirmed this conclusion. In a recent analysis of radio emissions at 1.5-metres wavelength, Dodson, Hedeman and Owren2 could find no evidence of this asymmetry. The following reasons may explain why they failed to observe the effect.