Broad-band polarimetric investigation of the Type II-plateau supernova 2013ej

We present results based on follow-up observations of the Type II-plateau supernova (II-P SN) 2013ej at six epochs spanning a total duration of similar to 37 d. The RC-band linear polarimetric observations were carried out between the end of the plateau and the beginning of the nebular phase, as demarcated in the photometric light-curve. The contribution arising from interstellar polarization (ISP) was constrained using two approaches, namely based on observations of foreground stars lying within a 5-arcmin and a 10 degrees radius of the SN location, and by investigating the extinction arising from the Milky Way and host galaxy in the direction of the SN. Our analysis revealed that in general the intrinsic polarization of the SN was higher than the polarization values for the foreground stars and exhibited an increasing trend during our observations. After correcting for the ISP of similar to 0.6 per cent, the maximum intrinsic polarization of SN 2013ej was found to be 2.14 +/- 0.57 per cent. Such a strong polarization has rarely been seen in II-P SNe. If this procedure is justified (i.e. the 'polarization bias' effect is indeed negligible) the polarization could be caused by the asymmetry of the inner ejecta of the SN, because the ISP towards the SN location is estimated to be, at most, 0.6 per cent.