GCN Circular 21594

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
Not found
 
 
; Allam et al., GCN 21530
Not found
 
 
; Yang et al., GCN 21531
Not found
 
 
,
Melandri et al., GCN 21532
Not found
 
 
) possibly associated with the LIGO/Virgo event
G298048 (GCN 21509
Not found
 
 
). 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.