Polarimetry at the 30m telescope

This site collects information relevant to polarimetric observations at the IRAM 30m telescope. The observing method XPOL makes use of the dual-polarization feature of EMIR and the cross-correlation capability of the VESPA. The method is described in the Technical Document No.2 (see below). The calibration of the instrumental phase is explained in detail in this presentation.

XPOL observations are possible in any of the 4 EMIR bands. Instrumental effects have been best characterised in the 3mm band (see Technical document No.3). The least experience exists at 0.9mm, but a preliminary Technical document (No. 4) is available upon request. Polarimetric observations of extended sources present the complex problem of contamination by the polarized sidelobes. Our current knowledge of these sidelobes is collected in the section 'Beam Maps' below. A strategy to estimate the influence of the polarized sidelobes on slightly extended linearly polarized sources is described in the scientific document No.3. Their influence on the circular polarization of an extended source is demonstrated in Scientific document No.4.

News

• On 15-Jun-2016, G.Paubert reported on tests. He finds a "frequency dependent leakage of I into both U and V in the receiver cabin" at 2mm with E150. This is from looking at a single onoff with the wobbler on Mars.

• On 9-May 2016, F.Herpin sent an observers report, saying that the strong leakage of I into V at 3mm with E090 severely hinders Zeeman observations to determine the magnetic field along the line of sight. The level of the instrumental circular polarisation (Stokes V) reaches 1.4%. This is based on observations of CN lines in IRC+10216 in March 2016. Comparison IRC+10216 spectra of 2006 do not show this leakage into V.

• The 3mm band setup was upgraded in November 2015: the mixers were exchanged against NOEMA type mixers, one orthomode transducer was installed to split polarizations. The performance of XPOL was evaluated as reported by A.Sievers on 1-April 2016 here. The instrumental circular polarisation was found to be rather large, at a level of 2%. The V map of Uranus now shows a central maximum, while previously it had looked very different, exhibiting two lobes (cf.

Mars map at 3mm of 2009 below).

• The external mirrors of EMIR, including the polarizer grid and the external cold load, had to be adapted to the overall new receiver cabin optics in April 2015. After re-installation, polarimetry was commissioned. See a preliminary report by A.Sievers of 19-Oct 2015 here.

Observing

An XPOL observation is made in the wobbler-switching observing mode. This PaKo script sets up a simple XPOL observation at 3mm. More complex setups where two EMIR bands or two VESPA subbands are recorded are possible. The VESPA polarimetric capabilities are concisely described in Table 4 of the VESPA user guide. Observers are advised to check the good performance of XPOL by observing a polarization angle calibrator, the Crab nebula or the steep spectrum quasar 3C286 (see Scientific publications No. 1 and 5 below).

A separate note contains information on how to estimate the statistical error of an polarimetric observation. In addition, the note describes Zeeman observations and gives an estimate of the sensitivity for magnetic fields.

Beam maps

(under construction)

The following table lists the beam maps obtained with EMIR as of September 2012. The source name provides a link to the Stokes IQUV maps. The (x.y) cross on these maps indicates the horizontal and vertical directions in the Nasmyth cabin. Note that not all maps have been obtained under adequate weather conditions which can be judged from the appearance of non-gaussian features on the Stokes I maps. Copies of these maps are available in GILDAS format.

Technical documents

1. Versatile IF polarimeter at the IRAM 30m telescope

   • Thum et al. 2003 including the first mm observation of the polarization of the moon's limb

2. XPOL—the Correlation Polarimeter at the IRAM 30-m Telescope

   • Thum et al. 2008, PASP, 120, 777

3. Polarimetry with EMIR/XPOL - commisioning report (August 2010)

   • Wiesemeyer & Thum 2010, IRAM Report

4. Polarimetry with EMIR/XPOL and the new optics - commissioning report (October 2015)

   • Sievers et al. 2015, IRAM Report

5. Polarimetry with EMIR/XPOL: E090 with OMT - commissioning report (April 2016)

   • Sievers, 2016, IRAM Report

Scientific publications (selected for illustrating specific technical aspects)

1. Measurement of the Crab nebula polarization at 90 GHz as a calibrator for CMB experiments

   • Aumont et al. 2010

2. A sensitive upper limit to the circular polarization of the Crab nebula at λ3 mm

   • Wiesemeyer et al. 2011

3. Observations of the Goldreich-Kylafis effect in star-forming regions with XPOL at the IRAM 30 m telescope

   • Forbrich et al. 2008 This publication includes examples for assessing the magnitude of instrumental polarization due to sidelobes

4. A 3.5 mm Polarimetric Survey of Radio-loud Active Galactic Nuclei

   • Agudo et al. 2010

5. 3C 286: a bright, compact, stable, and highly polarized calibrator for millimeter-wavelength observations

   • Agudo et al. 2012

Background material

1. IAU convention concerning Stokes Parameters

   • Radioastronomy, including the 30m telescope, uses these IAU conventions (see last page). Note however that according to this convention the sign of Stokes V is opposite to the usage in traditional physics as described by Born & Wolf in "Classical Optics"

2. The sign of Stokes V in 30m observations

   • with XPOL has been determined through several methods. They are described in a short note. It was verified that Stokes V spectra generated by the 30m data acquisition program have the correct sign according to the IAU convention.