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REPORT OF THE WESTERN EXTENSION PUBLIC AFFAIRS COMMITTEE
Teaching/Communication/Extension/Profession,
Radio Linear and Circular Polarization from M81*
We present results from archival Very Large Array (VLA) data and new VLA
observations to investigate the long term behavior of the circular polarization
of M81*, the nuclear radio source in the nearby galaxy M81. We also used the
Berkeley-Illinois-Maryland Association (BIMA) array to observe M81* at 86 and
230 GHz. M81* is unpolarized in the linear sense at a frequency as high as 86
GHz and shows variable circular polarization at a frequency as high as 15 GHz.
The spectrum of the fractional circular polarization is inverted in most of our
observations. The sign of circular polarization is constant over frequency and
time. The absence of linear polarization sets a lower limit to the accretion
rate of . The polarization properties are strikingly
similar to the properties of Sgr A*, the central radio source in the Milky Way.
This supports the hypothesis that M81* is a scaled up version of Sgr A*. On the
other hand, the broad band total intensity spectrum declines towards milimeter
wavelengths which differs from previous observations of M81* and also from Sgr
A*.Comment: Astronomy & Astrophysics, minor changes, matching the published
version, also available at
http://www.mpifr-bonn.mpg.de/staff/abrunthaler/pub.shtm
The Allen Telescope Array Twenty-centimeter Survey -- A 700-Square-Degree, Multi-Epoch Radio Dataset -- II: Individual Epoch Transient Statistics
We present our second paper on the Allen Telescope Array Twenty-centimeter
Survey (ATATS), a multi-epoch, ~700 sq. deg. radio image and catalog at 1.4
GHz. The survey is designed to detect rare, bright transients as well as to
commission the ATA's wide-field survey capabilities. ATATS explores the
challenges of multi-epoch transient and variable source surveys in the domain
of dynamic range limits and changing (u,v) coverage.
Here we present images made using data from the individual epochs, as well as
a revised image combining data from all ATATS epochs. The combined image has
RMS noise 3.96 mJy / beam, with a circular beam of 150 arcsec FWHM. The
catalog, generated using a false detection rate algorithm, contains 4984
sources, and is >90% complete to 37.9 mJy. The catalogs generated from snapshot
images of the individual epochs contain between 1170 and 2019 sources over the
564 sq. deg. area in common to all epochs. The 90% completeness limits of the
single epoch catalogs range from 98.6 to 232 mJy.
We compare the catalog generated from the combined image to those from
individual epochs, and from the NRAO VLA Sky Survey (NVSS), a legacy survey at
the same frequency. We are able to place new constraints on the transient
population: fewer than 6e-4 transients / sq. deg., for transients brighter than
350 mJy with characteristic timescales of minutes to days. This strongly rules
out an astronomical origin for the ~1 Jy sources reported by Matsumura et al.
(2009), based on their stated rate of 3.1e-3 / sq. deg.Comment: 28 pages, 12 figures, ApJ accepte
Space VLBI Observations Show in the Quasar NRAO 530
We present here space-based VLBI observations with VSOP and a southern
hemisphere ground array of the gamma-ray blazar NRAO 530 at 1.6 GHz and 5 GHz.
The brightness temperature of the core at 1.6 GHz is K. The
size is near the minimum observable value in the direction of NRAO~530 due to
interstellar scattering. The 5 GHz data show a single component with a
brightness temperature of K, significantly in excess of
the inverse Compton limit and of the equipartition brightness temperature limit
(Readhead 1994). This is strong evidence for relativistic motion in a jet
requiring model-dependent Doppler boosting factors in the range 6 to 60. We
show that a simple homogeneous sphere probably does not model the emission
region accurately. We favor instead an inhomogeneous jet model with a Doppler
boosting factor of 15.Comment: 12 pages, 2 figures. Accepted for publication in ApJ Letter
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