Equatorial scattering and the structure of the broad-line region in Seyfert nuclei: evidence for a rotating disc

We present detailed scattering models confirming that distinctive variations in polarization across the broad Hα line, which are observed in a significant fraction of type 1 Seyfert galaxies, can be understood in terms of a rotating line-emitting disc surrounded by a coplanar scattering region (the equatorial scattering region). The predicted polarization properties are: (i) averaged over wavelength, the position angle (PA) of polarization is aligned with the projected disc rotation axis and hence also with the radio source axis; (ii) the polarization PA rotates across the line profile, reaching equal but opposite (relative to the continuum PA) rotations in the blue and red wings; and (iii) the degree of polarization peaks in the line wings and passes through a minimum in the line core. We identify 11 objects that exhibit these features to different degrees. In order to reproduce the large-amplitude PA rotations observed in some cases, the scattering region must closely surround the emission disc and the latter must itself be a relatively narrow annulus – presumably the Hα-emitting zone of a larger accretion disc. Asymmetries in the polarization spectra may be attributable to several possible causes, including bulk radial infall in the equatorial scattering region, or contamination by polar scattered light. The broad Hα lines do not, in general, exhibit double-peaked profiles, suggesting that a second Hα-emitting component of the broad-line region is present, in addition to the disc.     

Magnetic field limits and spectral variability in the Circinus galaxy H2O megamasers

We have used the Australia Telescope Compact Array (ATCA) to search for polarized emission from the Circinus galaxy H₂O megamasers. No linear or circular polarized emission was detected with 3σ upper limits of 24 and 39 mJy, respectively, corresponding to fractional polarizations of 0.6 and 0.9 per cent for the strongest line. These results allow us to place upper limits on the strength of the nuclear magnetic field of ≤150 mG near the outer edge of the masing disc and ≤360 mG at the inner edge.
Rapid variability, thought to be due to interstellar scintillation, is a well-known feature of the Circinus H₂O megamasers. Our Stokes-I data show evidence for changes in the shape of the maser line spectral profiles, as well as their intensity. In addition to the rapid variations, the maser light curves also exhibit variations with a time-scale of the order of days (although our observations are too short to characterize this well). These behaviours support the hypothesis that the Circinus H₂O megamasers undergo diffractive interstellar scintillation, which has previously only been observed in pulsars and in one active galactic nucleus.     

A change detection algorithm for terrain surface moisture mapping based on multi‐year passive microwave remote sensing (Examples of SSM/I and TMI Channels)

As an indication of the surface polarized emission, a polarization index (PI) of microwave radiance from the terrain surface (half‐space of canopy‐soil land) is derived from the radiative transfer model. This PI separates the radiance effects of the canopy‐soil moisture and interference from surface roughness and atmosphere, and is suitable to describe the change of terrain surface moisture, especially for extreme drought or flood conditions. As an example, the statistics of the monthly average < PI > from 6 years' data of the Defense Meteorological Satellite Program (DMSP) SSM/I observations at the lowest frequency 19·35 GHz channel as available are applied for the demonstration of the surface moisture status over a large and heterogeneous territory such as China. The deviation of the PI data at the same month from the average < PI > , i.e. Δ_nPI(≡(PI? < PI>)/ < PI>), gives prominence to focusing moisture variation of terrain surface, and its anomaly shows possible drought or flood occurrence in extreme conditions. The Δ_nPI mapping is validated by the typical examples of the drought in China's Shanxi area in May 2001 and the flood around China's Yangtze River in August 1998, respectively. Our approach is recommended for lower frequency channels to minimize the influence from vegetation canopy for future applications (such as the channels of the Advanced Microwave Scanning Radiometer [AMSR‐E] launched in May 2002 and microwave imaging radiometer of China's Fengyun satellite series). When the monthly < PI > and the ground truth of average volumetric moisture < m_v > of the region are correctly evaluated, it is tractable to retrieve the soil land surface moisture by using the PI data at the same month and the same region without much knowledge of surface roughness, vegetation canopy and other factors. As an example, the retrieval of m_v is favourably tested by using the Tropical Rainfall Measuring Mission (TRMM) Tropical Microwave Imager (TMI) data. Copyright © 2007 John Wiley & Sons, Ltd.     

VSOP polarization observations of the BL Lacertae object OJ 287

VLBI total intensity and linear polarization images of the BL Lacertae object OJ 287 have been obtained at     using a global ground array and the HALCA orbiting antenna, and at     two weeks earlier using the VLBA. In the ground-based 6-cm images, the source is dominated by a core–jet double structure the components of which are essentially unresolved. The baselines to the orbiting antenna resolve both of these compact components. In the VSOP images, the ground-based 'core' breaks up into several distinct components, demonstrating that this region is dominated by the contribution of bright, optically thin knots of jet emission. A very similar structure is observed in the 1.3-cm image. The magnetic field in the core is transverse, becomes longitudinal in the inner jet, then makes a sharp transition to a region of transverse field further from the core. This suggests that the field in the outer jet has become highly ordered in the transverse direction owing to the action of a shock; the physical nature of the extended region of longitudinal field closer to the core is not clear. The availability of nearly simultaneous observations with comparable resolution at widely spaced frequencies enabled detection of a ≃90° rotation in polarization position angle for the core, owing to the transition from the optically thick (6 cm) to the optically thin (1.3 cm) regime.     

G332.5−5.6, a new Galactic supernova remnant

We present radio observations of the source G332.5−5.6, a candidate supernova remnant (SNR). Observations have been performed with the Australia Telescope Compact Array (ATCA) at two frequencies, 1.4 and 2.4 GHz. Our results confirm that G332.5−5.6 is an SNR, with a spectral index  α=−0.7 ± 0.2  for the whole source and an average fractional polarization of ∼35 per cent at 2.4 GHz. The central component is coincident with extended X-ray emission, and the distance to the SNR is estimated to be ∼3.4 kpc. Based on its radio and X-ray morphology, this SNR should be classified as a composite, and we suggest that it belongs to a trident-shaped subclass like G291.0−0.1.