Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/pub/people/atzioumi/iau164/tziou.ps Äàòà èçìåíåíèÿ: Fri Aug 15 11:00:52 1997 Äàòà èíäåêñèðîâàíèÿ: Mon Dec 24 13:35:06 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: molecular cloud

Multi­frequency VLBI observations of the GPS source 1934­638
A. K. Tzioumis, E. A. King, J. E. Reynolds, D. L. Jauncey & R. G. Gough
Australia Telescope National Facility, Australia
R. A. Preston, D. W. Murphy, S. J. Tingay, D. L. Meier & D. L. Jones
Jet Propulsion Laboratory, Pasadena, CA, USA
J. E. J. Lovell & P. G. Edwards
Institute of Space and Astronautical Science, Japan
P. M. McCulloch & M. E. Costa
University of Tasmania, Hobart, Australia
L. Kedziora­Chudczer & D. Campbell­Wilson
University of Sydney, Sydney, Australia
G. D. Nicolson & J. F. H. Quick
Hartebeesthoek Radio Astronomy Observatory, Krugersdorp, South Africa
Abstract. PKS 1934­638 is an archetypal GPS source, peaking at 1.4 GHz and exhibits
almost no flux density variability. VLBI images at frequencies of .843, 2.3, 4.8 & 8.4 were
made with the southern hemisphere VLBI array and they reveal that the source is a 42 mas
compact double. Ther is no detectable change in separation over the last 20 years, yielding
an upper limit of ¸ 0:03c \Sigma 0:2c on any expansion velocity. The spectral shapes of the two
components are remarkably similar, despite indications of finer structure on longer baselines.
Magnetic field calculations indicate fields of a few mGauss and the results are consistent with
equipartition.
PKS 1934­638 has been identified with an 18.9 m galaxy (Kellerman 1966)
at a redshift of 0.183 (Penston & Fosbury 1978). Deep CCD images of this
galaxy show two compact galaxies within 2.9 00 , sharing a common envelope, and
with the radio source located within 0.1 00 of the brighter galaxy (Jauncey et al.,
1986). Optical spectroscopic observations (Fosbury et al., 1987) measure very
high line ratios, indicating larger amounts of gas and dust than found in typical
narrow line radio galaxies.
As is evident in Figure 1 (LHS), PKS 1934­638 shows a very sharply peaked
radio spectrum at 1.4 GHz, first noted by Bolton, Gardner & Mackey (1963).
The radio flux density has been extensively monitored for over 30 years at Parkes,
Hartebeesthoek, Hobart and the Australia Telescope Compact Array (ATCA)
and shows little or no variability. Thus, PKS 1934­638 has become the primary
flux calibrator for the ATCA. There is no detected radio structure at scales –1 00
and no linear polarisation (Ÿ0.1%).
Early VLBI observations (Gubbay et al., 1971; Tzioumis et al., 1989) at
2.3 GHz, indicated that the source is a compact double with a separation of
42.0\Sigma0.2 mas. Recent (1988­92) VLBI observations were made with the South­
ern Hemisphere network (Preston et al., 1989) at 2.3, 4.8 and 8.4 GHz (King
1994) with approximately the same resolution. The source appears remark­
ably similar at these frequencies and the 4.8 GHz image is presented in Fig­
ure 1 (RHS). More than 90% of the total flux density was detected at all fre­
quencies, implying that there is little or no extended structure. About 15%
of the total flux density is detected on intercontinental baselines (Australia to
South Africa) and the visibilities beat rapidly indicating the presence of very
1

PKS1934­638 2
Figure 1. LHS: The power spectra of PKS 1934­638 and its components between
.408 and 8.4 GHz. The total flux density (top set of points) is a compilation from
published results and the component spectra (lower 2 sets of points) are measurements
from VLBI images. RHS: VLBI image of PKS 1934­638 at 4.8 GHz. The peak flux
density is 2:1 Jy:beam \Gamma1 and the contours are at ­0.75, 0.75, 1.5, 3, 6, 12, 24, 48, 96%
(Beam FWHM 10x5.5 mas, PA = \Gamma86 ffi ).
compact components (King 1994). VLBI observations at 843 MHz (Parkes, Ho­
bart, MOST) barely resolve the source (beam 40 x 77 mas), but a consistent
double component model can be fitted.
These multifrequency VLBI observations allow the determination of the
spectral shapes of the compact components, as shown in Figure 1 (LHS). Both
component spectra are very similar and peak near 1.4 GHz. Magnetic field
calculations using equipartition arguments indicate Bme ¸ 5 \Theta 10 \Gamma3 Gauss, and
similarly from synchrotron self­absorption B ssa ¸ 1 \Gamma 3 \Theta 10 \Gamma3 Gauss. Within
the uncertainties inherent in these calculations, these results indicate that the
components of 1934­638 are consistent with equipartition.
No change in component separation has been detected with VLBI observa­
tions spanning over 20 years. Thus there is little or no expansion of this radio
source, with an upper limit on expansion velocity of ¸ 0:03c \Sigma 0:2c.
References
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Jauncey, D. L., et al. 1986. AJ , 92, 1036--1038.
Kellerman, K. I. 1966. Aust. J. Phys. 19, 195--207.
King, E. A. 1994. Ph.D. Thesis, University of Tasmania.
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