Late-Phase Spectropolarimetric Observations of Superluminous Supernova SN 2017egm to Probe the Geometry of the Inner Ejecta. (arXiv:2004.03140v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Saito_S/0/1/0/all/0/1">Sei Saito</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tanaka_M/0/1/0/all/0/1">Masaomi Tanaka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Moriya_T/0/1/0/all/0/1">Takashi J. Moriya</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bulla_M/0/1/0/all/0/1">Mattia Bulla</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Leloudas_G/0/1/0/all/0/1">Giorgos Leloudas</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Inserra_C/0/1/0/all/0/1">Cosimo Inserra</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lee_C/0/1/0/all/0/1">Chien-Hsiu Lee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kawabata_K/0/1/0/all/0/1">Koji S. Kawabata</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mazzali_P/0/1/0/all/0/1">Paolo Mazzali</a>
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 ~ 0.2 %, which indicates an almost
spherical photosphere, with an axial ratio ~ 1.05. The intrinsic polarization
at the late phase increases to ~ 0.8 %, which corresponds to the photosphere
with an axial ratio 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. It
may suggest the presence of a central energy source producing aspherical inner
ejecta.
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 ~ 0.2 %, which indicates an almost
spherical photosphere, with an axial ratio ~ 1.05. The intrinsic polarization
at the late phase increases to ~ 0.8 %, which corresponds to the photosphere
with an axial ratio 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. It
may suggest the presence of a central energy source producing aspherical inner
ejecta.
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