polarization surveys with the drao 26-m telescope at 1.4 ghz maik wolleben, t. landecker, o. davison...
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Polarization Surveys with thePolarization Surveys with the
DRAO 26-m Telescope at 1.4 GHzDRAO 26-m Telescope at 1.4 GHz
Maik Wolleben, T. Landecker, O. DavisonMaik Wolleben, T. Landecker, O. Davison
Dominion Radio Astrophysical ObservatoryDominion Radio Astrophysical Observatory
W. Reich, R. WielebinskiW. Reich, R. Wielebinski
Max-Planck-Institut für RadioastronomieMax-Planck-Institut für Radioastronomie
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Outline
1) - The DRAO 26-m Polarization Survey (finished)
- Brief Representation of Data
2) - The DRAO/MPI Rotation Measure Survey (ongoing)
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Specifications
TELESCOPE PARAMETER
diameter: 25.6mpolar mountingresolution: 37 arcminaperture efficiency: 55 %hard limits: -34 to 90 degsystem temperature: 125 K
SURVEY PARAMETER
frequency: 1410 MHzbandwidth: 12 MHzobserving mode: drift scanningpixel-size: 15 arcminintegration time: 60 s / pixel
First observing period: Nov 2002 – May 2003, coverage 17%
Second observing period: Jun 2004 – Mar 2005, coverage 42%
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Coverage
- about 350 Meridian drift scans- about 350 Meridian drift scans
- carried out by night (to avoid solar interference and ionospheric FR) - carried out by night (to avoid solar interference and ionospheric FR)
- fully sampled along right ascension- fully sampled along right ascension
- incomplete sampling along declination- incomplete sampling along declination
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Calibration
After correction:After correction:● Effelsberg-MB scaleEffelsberg-MB scale● Agrees with Agrees with southern sky survey southern sky survey (Parkes-MB)(Parkes-MB)● Pole temperature Pole temperature 80mK80mK
Stokes U' Stokes Q'
this response pattern (above) is not calibrated (max %pol roughly 6%)
1. ¼ of the observed signal is coming through the side lobes2. instrumental polarization: side lobes are polarized → ground radiation correction
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Calibration
Observing & Calibration Strategy
about 1000 pointings congruent with the Leiden/Dwingeloo1 polarization survey: → provides zero-levels in Stokes U and Q (absolute levels) → used for the gain calibration of the receiving system (Müller matrix)
ground radiation profiles: → required to extrapolate absolute zero-levels below 0° declination
comparison with Effelsberg Medium Latitude Survey: → refines the (relative) temperature scale
- includes correction for main-beam instrumental polarization
- residual side lobe polarization visible along the Galactic plane (l ≈0° - 60°)
- systematic errors due to scanning effects (system temperature variations)
1 A survey of linear polarization at 1415 MHz: Spoelstra, T. A. Th., A&AS, 1972
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Errors
correlation coefficients rU=0.89, rQ=0.86:
→ rms-noise: 12 mK (U), 33 mK (Q)
NCP-measurement gives:→ rms-noise of 12 mK in U and Q
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Map of Polarized Intensity
combined with southern sky polarization survey at 1.4 GHz(Testori, J. C.; Reich, P.; Reich, W., in The Magnetized Interstellar Medium, 2004
the ultimate goal: all-sky polarization map
preliminary version
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The l=140° Region
Hα PI Hα PI
Hα PI
VTSS & WHAM
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The l=140° Region
Hα PI
HII regions can be used to:
- derive the distance to the origin of polarized emission
- derive local synchrotron emissivity towards HII regions
> more complicated if there is depolarization and Faraday rotation <
apparently, some HII regions do not cause depolarization:
- the role of magnetic fields in HII regions?
- HII regions with/without B-field?
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The l=140° Region
B-star
dist: ≈100 - 400 pc
S 203
dist: ≈3.8 kpc
S 185
dist: ≈210 pc
O-star
dist: ≈1.2 kpc
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The “Depolarization Patch“
observation
- low PI and %pol towards inner Galaxy
- small scale structure
- striking sharp upper and lower boundaries
intuitive statements
Either caused by: - depolarization along the line-of-sight (depth depolarization) pro: symmetry about Galactic plane
total power
or- depolarization by a local Faraday screen pro: explains lack of polarized emission from the North-Polar Spur (dist. ≈150 pc)
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Data Availability
data available within the next two months!data available within the next two months!
then, interpolated data can be downloaded as:
J2000
fits format
- interpolated U, Q
- coverage mask
(to retrieve original coverage)
- anything else (if requested)
Galactic
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Future: The DRAO/MPI Rotation Measure Survey
frequency range: 1300 MHz – 1800 MHz
# channels: about 1,000 – 10,000
fully Nyquist sampled between declination -30° and +90°
scans along the Meridian (not drift scanning)
rms per 500 MHz band: 0.3 mK
observing time: 1.5 years
What's next? Digital Polarimetry!
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Future: The DRAO/MPI Rotation Measure Survey
RMRM RMRM RM RM ..... RM
25xne = 0.05 cm-3
L = 150 pcB║ = -4...4 μG
RM = -25...25 rad/m2
Example 1
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Future: The DRAO/MPI Rotation Measure Survey
ne = 0.04 cm-3
B║ = 3 μGd = 0...4 kpc
RM = 0.81·ne·B║·d
Example 2
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Future: The DRAO/MPI Rotation Measure Survey
the observed polarization angle will probably not depend linearly on λ2
→ observed RM depends on frequency!
RM-survey products: Stokes U & Qdata cubes
Stokes Idata cube
Rotation MeasureSynthesis
RM-cube(pol. int. vs. Faraday depth)
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Summary
http://mpifr-bonn.mpg.de/div/konti/26msurvey
http://www.drao.nrc.ca/26msurvey
1) DRAO 26-m Polarization Survey
2) DRAO/MPI Rotation Measure Survey
(data will be released soon)
- receiving system currently under development
- survey start scheduled for May 2006
- total observing time required: 1.5 years