Issue |
A&A
Volume 446, Number 1, January IV 2006
|
|
---|---|---|
Page(s) | 185 - 200 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20053293 | |
Published online | 09 January 2006 |
Diffractive interstellar scintillation of the quasar J1819+3845 at
21 cm
1
Kapteyn Astronomical Institute, University of Groningen, PO Box 800, Groningen 9700 AV, The Netherlands
2
National Radio Astronomy Observatory, PO Box 0, Socorro NM 87801, USA e-mail: jmacquar@nrao.edu
3
Netherlands Foundation for Research in Astronomy, Dwingeloo, The Netherlands e-mail: ger@astron.nl
Received:
25
April
2005
Accepted:
12
September
2005
We report the discovery of fast, frequency-dependent
intensity variations from the scintillating intra-day variable quasar
J1819+3845 at cm which resemble diffractive
interstellar scintillations observed in pulsars. The observations were
taken with the Westerbork Synthesis Radio Telescope on a dozen
occasions in the period between Aug. 2002 and Jan. 2005.
The data were sampled at both high temporal and
high frequency resolution and have an
overall simultaneous frequency span of up to 600 MHz. In constructing
the light curves and dynamic spectra the confusion from background
sources has been eliminated. The timescale (down to 20 min) and
the bandwidth (frequency decorrelation bandwidth of 160 MHz)
of the observed variations jointly imply that the component of the
source exhibiting this scintillation must possess a brightness
temperature well in excess of the inverse Compton limit. A specific
model in which both the source and scintillation pattern are isotropic
implies a brightness temperature
K,
where previous estimates place the distance to the scattering medium
in the range
pc, yielding a minimum brightness
temperature >20 times the inverse Compton limit. An independent
estimate of the screen distance using the 21 cm scintillation
properties alone indicates a minimum screen distance of
pc and a brightness temperature above
K.
There is no evidence for anisotropy in the scattering medium or source
from the scintillation characteristics, but these estimates may be
reduced by a factor comparable to the axial ratio if the source is
indeed elongated. The observed scintillation
properties of J1819+3845 at 21 cm are compared with those at 6 cm,
where a significantly larger source size has been deduced for the bulk
of the emission by Dennett-Thorpe & de Bruyn (2003).
However, opacity effects within the source and
the different angular scales probed in the regimes of weak and strong
scattering complicate this comparison.
Key words: galaxies: quasars: individual: J1819+3845 / galaxies: active / scattering / radiation mechanisms: non-thermal / techniques: high angular resolution
© ESO, 2006
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