We present our spectropolarimetric observations of SN 2017egm , a Type I superluminous supernova ( SLSN-I ) in a nearby galaxy NGC 3191 , with Subaru telescope at 185.0 days after the g -band maximum light .
This is the first spectropolarimetric observation for SLSNe at late phases .
We find that the degree of the polarization in the late phase significantly changes from that measured at the earlier phase .
The spectrum at the late phase shows a strong Ca emission line and therefore we reliably estimate the interstellar polarization component assuming that the emission line is intrinsically unpolarized .
By subtracting the estimated interstellar polarization , we find that the intrinsic polarization at the early phase is only \sim 0.2 % , which indicates an almost spherical photosphere , with an axial ratio \sim 1.05 .
The intrinsic polarization at the late phase increases to \sim 0.8 % , which corresponds to the photosphere with an axial ratio \sim 1.2 .
A nearly constant position angle of the polarization suggests the inner ejecta are almost axisymmetric .
By these observations , we conclude that the inner ejecta are more aspherical than the outer ejecta .
This may suggest the presence of a central energy source producing aspherical inner ejecta .