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

TL;DR

This paper describes the FoReRo2 instrument's polarimetric and spectropolarimetric capabilities, including its design, calibration challenges, and initial observational results on various astronomical objects.

Contribution

It provides the first detailed characterization of FoReRo2's polarimetric modes and demonstrates its scientific potential through multiple observational examples.

Findings

HD 204827 shows short-term PA variability, unsuitable for calibration.

Serkowski's law parameters are statistically analyzed for the first time.

RS Oph exhibits dust formation-related K parameter variability after outburst.

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 the position angle (PA), making it unsuitable for calibration. HD 183143 displays intrinsic polarisation variability, but remains stable in terms of the PA. For the first time, we present a statistical study of the K and lambda_max parameters of Serkowski's law, including the mean value, standard deviation, and distribution. To demonstrate FoReRo2's polarimetric capabilities, we present several examples. The recurrent nova RS Oph shows the variability of Serkowski's K parameter, which is due to dust formation within the first few days after the 2021 outburst. These examples also include the imaging polarimetry of comet C/2019 Y4 (ATLAS) and spectropolarimetric observations of the symbiotic star Z And.