TITLE: GCN CIRCULAR NUMBER: 21594 SUBJECT: LIGO/Virgo G298048: VLT linear polarimetry DATE: 17/08/19 14:13:15 GMT FROM: Klaas Wiersema at U of Leceister K. Wiersema (University of Leicester), S. Covino (INAF-OAB), A. Melandri (INAF-OAB) and E. Palazzi (INAF-IAFS Bo) report on behalf of a large collaboration: We obtained optical linear polarimetry measurements of object SSS17a (Coulter et al., GCN 21529; Allam et al., GCN 21530; Yang et al., GCN 21531, Melandri et al., GCN 21532) possibly associated with the LIGO/Virgo event G298048 (GCN 21509). We used the ESO Very Large Telescope UT1 (Antu) equipped with the FORS2 instrument. Observations started on 23:09 UT on 18 August 2017, i.e. during twilight, and continued until the source reached airmass 2. FORS2 uses a Wollaston element and a rotating halfwave plate to perform imaging polarimetry; additionally we used the FORS2 R_special filter (similar to R) . We obtained a large series of imaging polarimetry datasets, each consisting of 4 angles of the wave plate, using exposure times of 60 seconds for each exposure; the transient is bright in our data. In a preliminary analysis of a subset of our data, we find no evidence of optical linear polarisation, with a preliminary upper limit on the linear polarisation of P_linear < 1%. We expect this value to change somewhat through a more thorough analysis in the future, e.g. using more accurate models to correct for the host galaxy emission and rapidly varying twilight contribution. Our polarisation limit is a first for a kilonova candidate, and indicates a relatively smooth emission region, with no evidence of significant asymmetry at the time of observations. Similarly, this limit may indicate that there is no large contribution of non-thermal emission in the observed wavelength at the time of observations. We acknowledge excellent support from the ESO staff, J. M. Corral-Santana and S. Brilliant, and in particular the visiting observer, M. Lendl, who so kindly gave up some of their observing time.