SPLOT: a Snapshot survey for Polarised Light in Optical Transients

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 (ESO Faint Object Spectrograph and Camera v2) and SofI (Son of ISAAC) instruments, on the 3.6 m New Technology Telescope 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 polarization 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 Large Synoptic Survey Telescope. The polarimetric measurements have added scientific value to a significant number of the sources and, most importantly, have shown the potential to highlight unclassified transient sources of scientific interest for further study.

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