Late-phase Spectropolarimetric Observations of Superluminous Supernova SN 2017egm to Probe the Geometry of the Inner Ejecta

Sei Saito*, Masaomi Tanaka, Takashi J. Moriya, Mattia Bulla, Giorgos Leloudas, Cosimo Inserra, Chien Hsiu Lee, Koji S. Kawabata, Paolo Mazzali

*Corresponding author for this work

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Abstract

We present our spectropolarimetric observations of SN 2017egm, a Type I superluminous supernova (SLSN-I) in a nearby galaxy NGC 3191, with the 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 (ISP) component assuming that the emission line is intrinsically unpolarized. By subtracting the estimated ISP, 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. This may suggest the presence of a central energy source producing aspherical inner ejecta.

Original languageEnglish
Article number154
JournalAstrophysical Journal
Volume894
Issue number2
Number of pages7
ISSN0004-637X
DOIs
Publication statusPublished - 2020

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