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Late-phase spectropolarimetric observations of superluminous supernova SN 2017egm to probe the geometry of the inner ejecta

Saito, Sei, Tanaka, Masaomi, Moriya, Takashi J., Bulla, Mattia, Leloudas, Giorgos, Inserra, Cosimo, Lee, Chien-Hsiu, Kawabata, Koji S. and Mazzali, Paolo 2020. Late-phase spectropolarimetric observations of superluminous supernova SN 2017egm to probe the geometry of the inner ejecta. Astrophysical Journal 894 (2) , 154. 10.3847/1538-4357/ab873b

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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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Publisher: American Astronomical Society
ISSN: 0004-637X
Date of First Compliant Deposit: 16 June 2020
Date of Acceptance: 1 April 2020
Last Modified: 17 Jun 2020 11:16
URI: http://orca.cf.ac.uk/id/eprint/132490

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