The "26-m Polarization Survey" (2002-2005)

MAIK WOLLEBEN

Some Introduction

At 408 MHz emission is mostly synchrotron emission and some thermal emission (at 1.4 GHz too!).

Synchrotron emission is polarized, and in 1962 polarized radio emission from the sky was observed.

Haslam 408 MHzTotal IntensityWielebinski et al.

1962

Some Introduction

Before the 26-m Polarization Survey, the surveys by Brouw & Spoelstra (1976) were the best all-sky polarization data available.

About 1800 pointings, rotating receiver, carefully calibrated.

A very good data set but: highly undersampled and covering northern hemisphere only.

A better data set was needed to help calibrating Effelsberg data.

Primary Motivation: "Absolute Calibration of Effelsberg Survey"

The Effelsberg Medium Latitude Survey (Reich et al. 2004): 1.4 GHz, 20 MHz bandwidth, +/- 20 deg lat.

Loss of largest scale structure due to the way data were observed.

Large-scale structures needed to come from other surveys.

Dwingeloo data insuffciently sampled, new data were required.

A survey with the Galt Telescope was proposed to supplement the Dwingeloo data single: same frequency, analog polarimeter, and base the whole calibration on Dwingeloo...

My first "encounter" with the Galt Telescope

2001: got a picture sent from a travelling companion of "some radio observatory" in Penticton.

2002 - 2003: my first year at DRAO as a PhD student.

2005 - 2011: then two post-docs at DRAO.

Preparing the MPIfR Analog Polarimeter

Analog polarimeter, used on the Effelsberg telescope (until recently).

Incoming hands of polarization are split, one into the total power detector, other one into the multiplier.

Total power and cross-correlations need to be calibrated separately!

The one we shipped to DRAO was carefully overhauled in Effelsberg.

The Receiver

The plan was to use the 1.4 GHz receiver of the Galt Telescope, which was used for HI observations.

Receiver produced linear polarization: X and Y.

Long discussions about whether the hybrid should be placed ahead or behind the LNAs... Carl Heiles convinced us that it should go in ahead of the LNA!

Also, whether the calibration signal should be injected into the X and Y components or after the hybrid into the R and L components.

Wielebinski insisted on using circulators.

At DRAO: Preparation for the Survey

Junkyard Wars

The Receiver

There was an envelope in the receiver!

A note written by John...

...there were many more notes in different places...

120 V hurts!

A bird nest in the feed horn doesn't help!

No training in operating the lift, no saftey gear either...

The Software

Initial plan was to use the Effelsberg data acquisiton software (NOD).

but....

...then decided to use aips++ and glish.

The Rotating Dipole

We needed to test the receiver and polarimeter.

Rotating dipole was found.

But difficult to align with boresight.

The Rotating Sky

North Celestial Pole worked better...

Convinced the thesis committee that project should be continued.

The Survey

TELESCOPE PARAMETER

System Temperature: 125 K

SURVEY PARAMETER

Frequency: 1410 MHz

Bandwidth: 12 MHz

Observing mode: drift scanning

Pixel-size: 15 arcmin

Integration time: 60 s / pixel

Drift Scans

about 350 Meridian drift scans

all scans carried out by night (to avoid solar interference and ionospheric FR)

fully sampled along right ascension

incomplete sampling along declination

41% of the sky fully sampled

Accuracy

Correlation of Stokes U (left panel) and Q (right panel) values from the DRAO survey with the Dwingeloo survey before adjusting the temperatures to the Effelsberg scale.

Temperature scale was checked against EMLS.

Result

We got more than we expected:

More data... (two years of data instead of one).

First year of observations at DRAO.

Second year remotely from Bonn.

Calibration based on Dwingeloo and Effelsberg.

The 26-m Polarization Survey at 1.4 GHz

The Fan Region

From the survey paper (Wolleben et al. 2006):

The Fan Region has no obvious counterpart in total intensity.

Earlier investigators considered this polarized emission to originate at distances under 500 pc.

We detected definite depolarization by a number of HII regions seen against the Fan Region.

On the basis of this new evidence we conclude that the polarized emission from the Fan Region originates over a range of distances, extending from 500 pc to a few kpc, the latter distance corresponding to the distance of the Perseus arm.

The North Polar Spur

The North Polar Spur

Wolleben (2007)

Was it Worth it?

Of course!

The survey helped to emphasize the importance of absolute calibration of polarimetric data of diffuse emission.

And provided the data needed...

About 8-10 citations per year...

Was it Worth it?

ISM and Galactic Magnetic Fields

Radio observational constraints on Galactic 3D-emission models, Sun et al. (2008), 181 citations

Science with ASKAP. The Australian square-kilometre-array pathfinder, Johnston et al. (2008), 135 citations

A Survey of Extragalactic Faraday Rotation at High Galactic Latitude: The Vertical Magnetic Field of the Milky Way Toward the Galactic Poles, Mao et al. (2010), 69 citations

The Outer Scale of Turbulence in the Magnetoionized Galactic Interstellar Medium, Haverkorn et al (2008), 68 citations

The Southern Galactic Plane Survey: Polarized Radio Continuum Observations and Analysis, Haverkorn et al. (2006), 65 citations

A New Model for the Loop I (North Polar Spur) Region, Wolleben (2007), 61 citations

Constraining models of the large scale Galactic magnetic field with WMAP5 polarization data and extragalactic rotation measure sources, Jansson et al. (2009), 49 citations

Was it Worth it?

CMB Foreground

Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Polarization Analysis, Page et al. (2007), 807 citations

COrE (Cosmic Origins Explorer) A White Paper, The COrE Collaboration (2010), 97 citations

Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Bayesian Estimation of Cosmic Microwave Background Polarization Maps, Dunkley et al. (2009), 70 citations

Prospects for polarized foreground removal, Dunkley et at. (2009), 65 citations

Foregrounds for observations of the cosmological 21 cm line, Bernardi et al. (2009), 62 citations

Was it Worth it?

For soil moisture mapping the sky is the unwanted contribution...

Conclusion: to a lot of people the 26-m survey helped remove the unwanted signal from their data.

Led to GMIMS...

Earth-Viewing L-Band Radiometer Sensing of Sea Surface Scattered Celestial Sky Radiation—Part II: Application to SMOS

Reul et al. (2008)

Geoscience and Remote Sensing. Vol 46, Issue 3

Abstract: We examine how the rough sea surface scattering of L-band celestial sky radiation might affect the measurements of the future European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission. For this purpose, we combined data from several surveys to build a comprehensive all-sky L-band celestial sky brightness temperature map for the SMOS mission that includes the continuum radiation and the hydrogen line emission rescaled for the SMOS bandwidth.