Many-year measurements of the radio flux of the young supernova remnant Cassiopeia A relative to the radio galaxy Cygnus A were continued at 290 and 151.5 MHz. The new data are used together with previously published observations carried out at decameter, meter, centimeter, and millimeter wavelengths to derive the frequency dependence of the secular variation of the radio flux density of Cas A: $d_\nu [\% year^{ - 1} ] = - (0.63 \pm 0.02) + (0.04 \pm 0.01)\ln \nu [GHz] + (1.51 \pm 0.16) \times 10^{ - 5} (\nu [GHz])^{ - 2.1} $
 . The observed slowing of the secular variations with decreasing frequency at decameter wavelengths can be explained by a decrease in the optical depth of a remnant HII zone around Cas A with time due to recombination of hydrogen atoms. The new derived frequency dependence for the rate of the secular decrease, absolute and relative measurements of the radio flux density of Cas A carried out over the last 25 years, and the absolute spectrum of Cyg A are used to construct the spectrum of Cas A in the range 5–250 000 MHz predicted for epoch 2015.5.

Frequency dependence of the evolution of the radio emission of the supernova remnant Cas A

Radio flux measurements of the Crab nebula have been performed over many years relative to Orion A at 927 MHz and relative to Cygnus A and Virgo A at 151.5 MHz. The inferred average secular rates of decrease in the radio flux of the Crab nebula are d927 MHz = −0.18 ± 0.10% yr−1 over 1977–2000 and d151.5 MHz = −0.3 ± 0.1% yr−1 over 1980–2003. The weighted mean flux-decrease rate averaged over several years of relative measurements at 86, 151.5, 927, and 8000 MHz is dmw = −0.17 ± 0.02% yr−1. The secular flux decrease is frequency independent, with an upper limit of |dα/dt| < 3 × 10−4 yr−1 for the absolute value of the rate of change of the spectral index, and remains constant in time when averaged over long time intervals. The results of our measurements at 151.5 and 927 MHz combined with published absolute measurements at 81.5 and 8250 MHz are used to determine the radio spectrum of the Crab nebula for epoch 2010.0.

Evolution of the Radio Emission of the Crab Nebula from Long-Term Observations at 927 and 151.5 MHz

The crab nebula

RECENTLY Rees and Gunn1 suggested that the 30-Hz wave power probably emerging from the Crab Nebula Pulsar NP0532 does not propagate further than a small region around the pulsar with a radius of about 10% of the nebular radius. Beyond this, in the main body of the nebula, the magnetic field is built up in toroidal form to equipartition strength so that the continuum emission from the bulk of the nebula is entirely synchrotron radiation. The axis of this toroid is aligned with the rotational axis of the pulsar which is assumed to be lying in the plane of the sky along the major axis of the nebula (NW/SE). A test of this proposal is the measurement of a characteristic sign change in the distribution of Faraday rotation or circular polarisation over the nebula. If the nebula is divided into quadrants along its major and minor axes, then adjacent quadrants should have the opposite sign and opposite quadrants the same sign. Since any measured Faraday rotation does not necessarily arise within the body of the nebula, the critical test becomes a detection of the characteristic sign distribution in the small amount of circular polarisation (∼0.1 %) expected at radio frequencies from ordinary synchrotron radiation2. The theoretical frequency dependence of ν−1/2 for the fraction of circular polarisation would then explain the null result of Landstreet and Angel3 (see also ref. 4) at optical frequencies. Here I report a measurement of the distribution of circular polarisation (Stokes parameter V) in the Crab Nebula at 1,415 MHz obtained during a series of high accuracy full polarisation measurements with the Westerbork Synthesis Radio Telescope in November 1971.

Measurement of circular polarisation in the Crab Nebula at 1,415 MHz

While intensity of electromagnetic radiation (radio, infrared, light, or X-ray) gives us primary information about the distribution of the baryonic matter in the Universe, polarisation is a parameter that enables us to investigate many additional details. Polarisation at radio frequencies gives us details of emission processes since the non-thermal synchrotron process dominates at low radio frequencies in emission regions. In addition, polarised radio sources can be used as probes of the intervening interstellar medium through which the radio waves are propagated. Faraday rotation effects are observed and in conjunction with known thermal emission can be used to determine magnetic fields. The Zeeman effect, a direct method of determining magnetic fields, depends on the observation of the circular polarisation components of a spectral line. In this paper I describe the early polarisation observations of radio sources, but in addition I follow the developments through to the present day.

A history of radio astronomy polarisation measurements

: Distributions of the linearly polarized radiation at a frequency of 1418 MHz from the four supernova remnants 3C 10 (Tycho's supernova), Taurus A (Crab Nebula), 3C358 (Kepler's supernova), and Cassiopeia A have been determined by synthesizing a fan beam whose small dimension is along a circle of constant declination. In addition, extremely low upper limits have been placed on the net degree of circular polarization in the integrated radiation from Tau A, 3C358, and Cas A. For the latter two sources, no smaller circularly polarized regions were found within the source dimensions using a resolution of better than 1'. Upper limits to the magnetic field strengths, modified by geometrical factors have also been calculated. (Author)

G. A. Seielstad

Papers

One-dimensional polarization distributions over four supernova remnants at 1418 mhz,