Characterization of High-polarization Stars and Blazars with DIPOL-1 at Sierra Nevada Observatory

Abstract

We report here the performance and first results of the new multiband optical polarimeter DIPOL-1, installed at the Sierra Nevada Observatory 90 cm T90 telescope (SNO, Granada, Spain). DIPOL-1 is equipped with a plane parallel calcite plate and λ/2 retarder for modulating the intensity of two perpendicularly polarized beams, and a high readout speed CMOS camera that allows for fast, time-dense coverage. We characterize the performance of this instrument through a series of tests on zero- and high-polarization standard stars. The instrumental polarization in the Nasmyth focus was well determined, with a very stable contribution of 4.0806% ± 0.0014% in the optical R band. For bright high-polarization standards (m R < 8) we reach precisions <0.02% in polarization degree and 0.°1 in polarization angle for exposures of 2–4 min. The polarization properties of these stars have been constrained, providing more recent results also about possible variability for future studies of some of the most used calibrators. Moreover, we have tested the capability of observing much fainter objects, in particular through blazar observations, where we reach a precision of <0.5%−0.6% and <0.°5 for faint targets (m R ∼ 16.5) with exposures of ∼1 hr. For brighter targets (m R ∼ 14.5−15), we can aim for time-dense observations with errors <0.2%−0.4% and <1°−1.°5 in 5–20 min. We have successfully performed a first campaign with DIPOL-1, detecting significant polarized emission of several blazars, with special attention to the highest ever polarization degree measured from blazar 3C 345 at ∼32%.

IOP4, the Interactive Optical Photo-Polarimetric Python Pipeline

“…IOP4 is an open-source, interactive photo-polarimetric pipeline written in Python. Its results have already featured in a number of refereed and high impact publications (e.g., Middei et al 2023aMiddei et al , 2023bDi Gesu et al 2023;Ehlert et al 2023;Marshall et al 2023;Peirson et al 2023;Kim et al 2024;Otero-Santos et al 2024; among others), following the footsteps of its predecessor IOP3 19 (e.g., Di Gesu et al 2022;Liodakis et al 2022, etc), and has provided data to many ongoing studies. Most of this data comes from the MAPCAT (Agudo et al 2012) and TOP-MAPCAT programs, having reduced more than 600 GB of data from the these programs as of 2024 June.…”

“…The DIPOL-1 polarimeter is thoroughly described in Piirola et al (2020) and Otero-Santos et al (2024). It is based on a λ/2 retarder plate attached to a rotator and a high readout speed CMOS camera.…”