SPLOT: a snapshot survey for polarized light in optical transients

Abstract

We present SPLOT, a small scale pilot survey to test the potential of snapshot (single epoch) linear imaging polarimetry as a supplementary tool to traditional transient follow-up. Transients exist in a vast volume of observational parameter space and polarimetry has the potential to highlight sources of scientific interest and add value to near real-time transient survey streams. We observed a sample of ∼ 50 randomly selected optical transients with the EFOSC2 and SofI instruments, on the 3.6m New Technology Telescope (NTT) to test the feasibility of the survey. Our sample contained a number of interesting individual sources: a variety of supernovae, X-ray binaries, a tidal disruption event, blazar outbursts, and, by design, numerous transients of unknown nature. We discuss the results, both for the individual sources and the survey in detail. We provide an overview on the success and limitations of SPLOT and also describe a novel calibration method for removing instrumental polarisation effects from Nasymth-mounted telescopes. We find that a SPLOT-like survey would be a benefit to the large scale future transient survey streams such as LSST. The polarimetric measurements have added scientific value to a significant number of the sources and, most importantly, has shown the potential to highlight unclassified transient sources of scientific interest for further study.

Polarimetry of the superluminous transient ASASSN-15lh

“…This high level of polarization was interpreted as arising in a relativistic jet-like outflow. Higgins et al (2019) reported a single epoch of V-band polarimetry of OGLE16aaa, measuring 1.81 ± 0.42 per cent, which is of similar magnitude to the polarization measured for ASASSN-15lh by us. Lee et al (2020) observed an evolution in the polarization of AT 2019dsg, with the degree of polarization decreasing from 9.6 ± 2.6percent (at 4 d after light-curve maximum) to 2.0 ± 1.0 per cent (34 d later).…”

RINGO3 polarimetry of very young ZTF supernovae

“…The degree of polarization was further corrected for bias using the Modified ASymptotic (MAS) estimator of true polarization p MAS (Plaszczynski et al 2014). We follow Higgins et al (2019) by characterizing polarization measurements with p MAS /σ p < 3 as nondetections, and quote the 95 per cent upper limit.…”

Linear and Circular Polarimetry of the Optically Bright Relativistic Tidal Disruption Event AT 2022cmc

“…Polarimetric observations of "ordinary" optical TDEs (van Velzen et al 2020) were scarce until recently, including few measurements of individual events (Higgins et al 2019;Lee et al 2020;Holoien et al 2020). Leloudas et al (2022) gathered spectral polarimetry for a sample of three optical TDEs and demonstrated that the continuum polarization is wavelength independent, while emission lines depolarize the spectrum.…”