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26th of September 2018
ATNF Colloquium
Wide-band, High-resolution JVLA Observations of Cygnus A: Polarimetry and the value of Serendipity
by Rick Perley (NRAO)
Abstract: Cygnus A is the archetypal high luminosity radio galaxy. It is one of the most powerful radio galaxies known, and, at z=0.056, is located extraordinarily close to us. The source is at the center of a dense, highly magnetized X-ray emitting cluster, which has evidently retarded the expansion of its radio lobes, resulting in unusually high surface brightness. The combination of this high brightness and proximity makes this source an ideal candidate for detailed radio imaging studies. Early VLA observations, taken from the late 1970s through the early 1990s, demonstrated the presence of relativistic jets, an extraordinarily high rotation measure (RM) screen with well-defined large-scale structure, and an apparent lack of depolarization on ~0.5 arcsecond (500 pc) scale at 6cm wavelength. The Jansky Very Large Array (JVLA) offers unprecedented and unequalled high-sensitivity, high-resolution, wideband imaging capabilities. An extensive observing program, spanning 2 through 18 GHz, with all four configurations, was executed in 2014-2015, with a key goal of determining the internal thermal gas density within the radio lobes, utilizing the depolarization characteristics of the source emission. The new data show a complex and multi-varied depolarization structure, which has so far defied simple, unique explanations. But perhaps more interesting than the polarimetry and imaging are discoveries resulting from the new data which were not anticipated by the proposers: (i) A weak (~5 mJy) transient offset from the Cygnus A nucleus by 0.4 arcseconds (~400 pc) of extraordinarily small angular scale (~0.1 mas ~ 0.1 pc at the distance of Cygnus A), and (ii) an elliptical, small (100 -- 200 parsec) flat-spectrum, emission region centered on the nucleus, and oriented perpendicularly to the radio jets, with brightness temperature ~700 K. The characteristics are consistent with an ionized gas torus surrounding the nucleus. In this colloquium, I will discuss our interpretations of these new results, and emphasize the importance of careful, human, examination of new and deep observations. If time permits, I will breifly review the NRAO's plans for building a 'Next-Generation Very Large Array'.

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