The RoboPol sample of optical polarimetric standards^★

¹ Institute of Astrophysics, Foundation for Research and Technology-Hellas, N. Plastira 100, Vassilika Vouton, 71110 Heraklion, Greece
e-mail: blinov@ia.forth.gr
² Department of Physics, and Institute for Theoretical and Computational Physics, University of Crete, Voutes University campus, 70013 Heraklion, Greece
³ South African Astronomical Observatory, PO Box 9, Observatory, 7935 Cape Town, South Africa
⁴ Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune-411007, India
⁵ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
⁶ Finnish Centre for Astronomy with ESO, FINCA, University of Turku, Quantum, Vesilinnantie 5, 20014 Turku, Finland
⁷ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
⁸ Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden
⁹ University of California, Los Angeles, Geophysics and Space Physics Department of Earth, Planetary, and Space Sciences, Los Angeles, USA
¹⁰ Owens Valley Radio Observatory, California Institute of Technology, MC 249-17, Pasadena, CA 91125, USA
¹¹ Department of Astronomy/Steward Observatory, Tucson, AZ 85721-0065, USA
¹² Section of Astrophysics, Astronomy & Mechanics, Department of Physics, National and Kapodistrian University of Athens, Panepistimiopolis Zografos 15784, Greece
¹³ Department of Physics, Gauhati University, Guwahati - 781014, Assam, India
¹⁴ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University in Toruń, Grudziadzka 5, 87-100 Toruń, Poland
¹⁵ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, 1200 E California Blvd, MC 249-17, Pasadena, CA 91125, USA
¹⁶ Institut de Radioastronomie Millimétrique, Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
¹⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁸ Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa
¹⁹ Joint Institute for VLBI ERIC, Oude Hoogevceensedijk 4, 7991 PD Dwingeloo, The Netherlands

Received: 29 April 2023
Accepted: 19 July 2023

Abstract

Context. Optical polarimeters are typically calibrated using measurements of stars with known and stable polarization parameters. However, there is a lack of such stars available across the sky. Many of the currently available standards are not suitable for medium and large telescopes due to their high brightness. Moreover, as we find, some of the polarimetric standards used are in fact variable or have polarization parameters that differ from their cataloged values.

Aims. Our goal is to establish a sample of stable standards suitable for calibrating linear optical polarimeters with an accuracy down to 10⁻³ in fractional polarization.

Methods. For 4 yr, we have been running a monitoring campaign of a sample of standard candidates comprised of 107 stars distributed across the northern sky. We analyzed the variability of the linear polarization of these stars, taking into account the non-Gaussian nature of fractional polarization measurements. For a subsample of nine stars, we also performed multiband polarization measurements.

Results. We created a new catalog of 65 stars (see Table 2) that are stable, have small uncertainties of measured polarimetric parameters, and can be used as calibrators of polarimeters at medium and large telescopes.