The instrumental polarization of the Nasmyth focus polarimetric differential imager NAOS/CONICA (NACO) at the VLT

Implications for time-resolved polarimetric measurements of Sagittarius A^∗

¹ I. Physikalisches Institut der Universität zu Köln (PH1), Zülpicher Straße 77, 50937 Köln, Germany
e-mail: witzel@ph1.uni-koeln.de
² Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
³ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁴ Instituto de Astrofísica de Andalucía - CSIC, Glorieta de la Astronomía S/N, 18008 Granada, Spain
⁵ Astronomical Institute of the Academy of Sciences, Bocni II 1401/1a, 141 31 Praha 4, Czech Republic
⁶ Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, M5S 3H4 Toronto ON, Canada

Received: 17 May 2010
Accepted: 11 October 2010

Abstract

Context. We report on the results of calibrating and simulating the instrumental polarization properties of the ESO VLT adaptive optics camera system NAOS/CONICA (NACO) in the Ks-band.

Aims. Our goal is to understand the influence of systematic calibration effects on the time-resolved polarimetric observations of the infrared counterpart of the Galactic center super-massive black hole at the position of Sagittarius A* (Sgr A*).

Methods. We use the Stokes/Mueller formalism for metallic reflections to describe the instrumental polarization. The model is compared to standard-star observations and time-resolved observations of bright sources in the Galactic center. The differences between calibration methods are simulated and tested for three polarimetric Ks-band light curves of Sgr A*.

Results. We find the instrumental polarization to be highly dependent on the pointing position of the telescope and about 4% at maximum. Given the statistical uncertainties in the data acquisition, the systematic effects of the employed calibration method are negligible at high-time resolution, as it is necessary and achieved for in the case of Sgr A*. We report a polarization angle offset of 13.2° due to a position angle offset of the λ/2-wave plate with respect to the header value that affects the calibration of NACO data taken before autumn 2009.

Conclusions. With the new model of the instrumental polarization of NACO it is possible to measure the polarization with an accuracy of 1% in polarization degree. The uncertainty of the polarization angle is  ≤ 5° for polarization degrees  ≥ 4%. For highly sampled polarimetric time series we find that the improved understanding of the polarization properties gives results that are fully consistent with the previously used method to derive the polarization. The small difference between the derived and the previously employed polarization calibration is well within the statistical uncertainties of the measurements, and for Sgr A* they do not affect the results from our relativistic modeling of the accretion process.