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

Outline

1) - The DRAO 26-m Polarization Survey (finished)

- Brief Representation of Data

2) - The DRAO/MPI Rotation Measure Survey (ongoing)

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%

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

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

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

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

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

The l=140° Region

Hα PI Hα PI

Hα PI

VTSS & WHAM

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?

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

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)

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

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!

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

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

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)

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