J/A+A/574/A125 Long-term radio variations of QSO J1819+3845 (de Bruyn+, 2015)
The intra-hour variable quasar J1819+3845: 13-year evolution, jet polarization
structure and interstellar scattering screen properties.
de Bruyn A.G., Macquart J.-P.
<Astron. Astrophys. 574, A125 (2015)>
=2015A&A...574A.125D 2015A&A...574A.125D
ADC_Keywords: QSOs ; Radio continuum ; Polarization
Keywords: techniques: high angular resolution -
quasars: individual: J1819+3845 - radiation mechanisms: non-thermal -
scattering - galaxies: active - ISM: clouds
Abstract:
We examine the long-term evolution of the intra-hour variable quasar,
J1819+3845, whose variations have been attributed to interstellar
scintillation by extremely local turbulent plasma, located only 1-3pc
from Earth. The variations in this source ceased some time between
June 2006 and February 2007. The evolution of the source spectrum and
the long-term lightcurve, and the persistent compactness of the source
VLBI structure indicates that the cessation of rapid variability was
associated with the passage of the scattering material out of the line
of sight to the quasar. We present an analysis of the linear
polarization variations and their relation to total intensity
variations. The proper motion of polarized features in the quasar jet
is found to be subluminal. Systematic time delays between Stokes I, Q
and U, in combination with the structure of the source obtained from
8.4GHz VLBI data, confirm the estimate of the screen distance: 1-2pc,
making the screen one of the nearest objects to the Solar System. We
determine the physical properties of this scattering material. We
examine the rotation measures of sources and the diffuse polarized
emission in the surrounding region. We place a limit of 10rad/m2 on
the RM change. The variability of sources near J1819+3845 is used to
deduce that the screen must therefore be either very small (∼100AU) or
patchy.
Description:
Most of the data reduction procedures employed for the data presented
have been described extensively in previous papers (Dennett-Thorpe &
de Bruyn 2003A&A...404..113D 2003A&A...404..113D; Macquart & de Bruyn 2006A&A...446..185M 2006A&A...446..185M;
2007MNRAS.380L..20M 2007MNRAS.380L..20M). We present here a brief summary. All the
observations presented in this paper were obtained with the multi
frequency frontends located at the prime focus of the Westerbork
Synthesis Radio telescope (WSRT). The frontends cover broad frequency
ranges around 350MHz (henceforth 85cm), 1400MHz (21cm), 2300MHz
(13cm), 4900MHz (6cm) and 8500MHz (3.6cm). The data presented in this
paper, and previous papers, now span a period exceeding 13 years.
Objects:
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RA (ICRS) DE Designation(s)
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18 19 26.55 +38 45 01.8 QSO J1819+3845 = MASIV J1819+3845
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
14042002.dat 36 360 *Light curve flux data in Figure 1 for UT 2002-04-14
26022006.dat 44 1446 *Light curve flux data in Figure 7 for UT 2006-02-26
10012003.dat 44 705 *Light curve flux data in Figure 8 for UT 2003-01-10
21022004.dat 44 719 *Light curve flux data in Figure 8 for UT 2004-02-21
12022005.dat 44 720 *Light curve flux data in Figure 8 for UT 2005-02-12
22012006.dat 44 719 *Light curve flux data in Figure 8 for UT 2006-01-22
26012002.dat 27 360 *Light curve flux data in Figure 10 for UT 2002-01-26
14062005.dat 20 721 *Light curve flux data in Figure 11 for UT 2005-06-14
qso21cm.fits 2880 1256 FITS image file of the 21cm Stokes I intensity
distribution of the observation on 2009-05-07
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Note on *.dat: The flux density data are time-ordered with a UNIFORM time step.
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Description of file: qso21cm.fits
The FITS file of QSO J1819+3845 has 950x950 pixels of scale=5.273"/pix.
The resolution (PSF) is 13"x19" in PA=0°.
J1819+3845 was then already in a quiescent state and the image shows
excellent dynamic range with a peak flux density of 127mJy and a noise
level of about 15uJy.
Byte-by-byte Description of file: 14042002.dat
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Bytes Format Units Label Explanations
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4- 9 F6.2 deg HA [-91/91] Hour angle of the target in degrees (G1)
13- 18 F6.2 mJy SI13 Flux density for Stokes I at 13cm (2272MHz)
22- 27 F6.2 mJy SI6 Flux density for Stokes I at 6cm (4900MHz)
31- 36 F6.2 mJy SI3 Flux density for Stokes I at 3cm (8463MHz)
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Byte-by-byte Description of file: 1[02]*.dat 2[12]*.dat 26022006.dat
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Bytes Format Units Label Explanations
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3- 8 F6.2 deg HA [-91/91] Hour angle of the target in degrees (G1)
12- 17 F6.2 mJy SI Flux density for Stokes I at 6cm (4.9GHz)
22- 26 F5.2 mJy SQ Flux density for Stokes Q at 6cm (4.9GHz)
31- 35 F5.2 mJy SU Flux density for Stokes U at 6cm (4.9GHz)
40- 44 F5.2 mJy SV Flux density for Stokes V at 6cm (4.9GHz)
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Byte-by-byte Description of file: 26012002.dat 14062005.dat
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Bytes Format Units Label Explanations
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1- 6 F6.2 deg HA [-91/91] Hour angle of the target in degrees (G1)
8- 13 F6.2 mJy SI3.6 Flux density for Stokes I at 3.6cm (8.5GHz)
15- 20 F6.2 mJy SQ3.6 ? Flux density for Stokes Q at 3.6cm (8.5GHz)
(only for 14062005.dat) (1)
22- 27 F6.2 mJy SI6 ? Flux density for Stokes I at 6cm (4.9GHz)
(only for 26012002.dat)
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Note (1): Note that the Stokes Q data are provided here as a bonus and can be
used to estimate the noise. They are not included in the Figure in the
paper.
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Global notes:
Note (G1): The temporal coordinate is given as hour angle with 2 decimals. An
hour angle step of 0.2506 degrees corresponds to 60 UT seconds.
The hour angle (HA) for the WSRT can be converted to the sideral time (ST) of
the observation via the following relation: HA=ST-RA, where RA is the
Right Ascension of the source. Because the WSRT can only observe a target
between hour angles of -90° and +90° the observation date can then be
used to find the UT range of the 12h run, if needed.
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Acknowledgements:
A.G. de Bruyn, ger(at)astron.nl
(End) A.G. de Bruyn [ASTRON, NFRA], Patricia Vannier [CDS] 13-Sep-2014