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Polarimetric and spectropolarimetric observations with FoReRo2: Instrument overview and standard stars monitoring

Yanko Nikolov, Galin Borisov, Stefano Bagnulo, Plamen Nikolov, Rumen Bogdanovski, Tanyu Bonev

TL;DR

This paper characterizes FoReRo2, a dual-channel polarimetric/spectropolarimetric instrument on the Rozhen 2-m telescope, detailing its upgrades, calibration procedures, and data-reduction pipeline. It assesses instrumental polarization, chromatism, and position-angle corrections, and demonstrates polarimetric precision better than 0.1% in the synthetic $V$ band through long-term monitoring of standard stars. The study presents a statistical Serkowski-law analysis of $K$ and $\\lambda_{max}$ across a large star sample, and illustrates FoReRo2’s capabilities with RS Oph, Z And, and comet C/2019 Y4 (ATLAS). The results provide a dependable calibration baseline and showcase FoReRo2’s applicability to both stellar and Solar System targets, including transient dust phenomena and Raman-scattering diagnostics in symbiotic stars.

Abstract

In this paper, we present a description and characterisation of the 2-Channel Focal Reducer Rozhen (FoReRo2), with an emphasis on its polarimetric and spectropolarimetric observation modes. FoReRo2 is a multimode instrument mounted at the Ritchey-Chretien(RC) focus of the 2-meter Ritchey-Chretien-Coude (RCC) telescope at the Bulgarian National Astronomical Observatory (BNAO) - Rozhen. The primary targets of this instrument include novae, Be/X-ray binaries, symbiotic stars, asteroids, and comets. Standard stars with a high degree of polarisation are an essential part of the polarimetric observations. In our sample, HD 204827 shows short-term variability in position angle, making it unsuitable for calibration. HD 183143 displays intrinsic polarisation variability but remains stable in position angle. For the first time, we present a statistical study of K and λ_max of the Serkowski law, including their mean value, standard deviation, and distribution. In order to show the FoReRo2 polarimetric capabilities, we also present several examples. The recurrent nova RS Oph shows variability of Serkowski's K parameter due to dust formation within the first few days after the 2021 outburst. The examples also include imaging polarimetry of comet C/2019 Y4 (ATLAS) and spectropolarimetric observations of the symbiotic star Z And.

Polarimetric and spectropolarimetric observations with FoReRo2: Instrument overview and standard stars monitoring

TL;DR

This paper characterizes FoReRo2, a dual-channel polarimetric/spectropolarimetric instrument on the Rozhen 2-m telescope, detailing its upgrades, calibration procedures, and data-reduction pipeline. It assesses instrumental polarization, chromatism, and position-angle corrections, and demonstrates polarimetric precision better than 0.1% in the synthetic band through long-term monitoring of standard stars. The study presents a statistical Serkowski-law analysis of and across a large star sample, and illustrates FoReRo2’s capabilities with RS Oph, Z And, and comet C/2019 Y4 (ATLAS). The results provide a dependable calibration baseline and showcase FoReRo2’s applicability to both stellar and Solar System targets, including transient dust phenomena and Raman-scattering diagnostics in symbiotic stars.

Abstract

In this paper, we present a description and characterisation of the 2-Channel Focal Reducer Rozhen (FoReRo2), with an emphasis on its polarimetric and spectropolarimetric observation modes. FoReRo2 is a multimode instrument mounted at the Ritchey-Chretien(RC) focus of the 2-meter Ritchey-Chretien-Coude (RCC) telescope at the Bulgarian National Astronomical Observatory (BNAO) - Rozhen. The primary targets of this instrument include novae, Be/X-ray binaries, symbiotic stars, asteroids, and comets. Standard stars with a high degree of polarisation are an essential part of the polarimetric observations. In our sample, HD 204827 shows short-term variability in position angle, making it unsuitable for calibration. HD 183143 displays intrinsic polarisation variability but remains stable in position angle. For the first time, we present a statistical study of K and λ_max of the Serkowski law, including their mean value, standard deviation, and distribution. In order to show the FoReRo2 polarimetric capabilities, we also present several examples. The recurrent nova RS Oph shows variability of Serkowski's K parameter due to dust formation within the first few days after the 2021 outburst. The examples also include imaging polarimetry of comet C/2019 Y4 (ATLAS) and spectropolarimetric observations of the symbiotic star Z And.

Paper Structure

This paper contains 23 sections, 12 equations, 19 figures, 6 tables.

Table of Contents

  1. Introduction
  2. The Two-Channel-Focal-Reducer Rozhen
  3. Observing strategy and data reduction
  4. Observations and basic reduction steps
  5. Beam swapping technique
  6. Instrumental polarisation
  7. Slit vs. slitless spectropolarimetry
  8. Correction for the chromatism (wavelength dependence) of the half-wave plate
  9. Correction for position angle.
  10. Selection of aperture for imaging polarimetry.
  11. Spectropolarimetric observations of high polarisation standards
  12. Searching for variability.
  13. FoReRo2 observational polarimetric examples
  14. Serkowski law: relationship between $K$ and $\lambda_{\rm max}$
  15. Fitting the polarisation spectra with the Serkowski law
  16. ...and 8 more sections

Figures (19)

  • Figure 1: FoReRo2 attached at the RC focus of the 2m telescope.
  • Figure 2: Drawing of the FoReRo2. The positions of the 'HWP' and 'Wollaston prism' are marked. At that position, the Wollaston is outside of the light beam.
  • Figure 3: Half-wave plate and its control unit.
  • Figure 4: Fluxes measured over a full rotation cycle of the HWP, Top panel: Fluxes in the o- ($\square$) and e- ($\bigtriangleup$) beams. Mid panel: the measured quantity ($F_{\rm o} - F_{\rm e})/(F_{\rm o} + F_{\rm e})$ and its best-fit. Bottom panel: residuals. More details are given in the text.
  • Figure 5: Raw frames obtained with FoReRo2 in polarimetric mode. Top panel: Imaging polarimetry of the comet C/2019 Y4 (ATLAS). Bottom panel: Spectropolarimetry of the Be/X-ray binary star X Per. The $H_{\alpha}$ emission line is visible. Both frames show the o- and e-beams split by the Wollaston prism
  • ...and 14 more figures