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Spectroscopic classification of SN 2017dfc (= Gaia17baw) with the Lick Shane telescope

ATel #10302; Y.-C. Pan, R. J. Foley (UCSC), G. Canalizo, R. O. Sexton (UCR), A. J. Barth (UCI)
on 21 Apr 2017; 20:10 UT
Credential Certification: Yen-Chen Pan (ypan6@ucsc.edu)

Subjects: Optical, Supernovae

SN 2017dfc (= Gaia17baw) was discovered on 2017 April 9 with magnitude of 18.99 mag. It has since risen to 18.10 mag on 2017 April 19 (http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia17baw/). The SN is located 28".38 north and 63".73 south of the center of M63 (z = 0.001614; mu = 29.8 mag; NED). If the SN is at the distance of M63, it would have an absolute magnitude of -10.9 and -11.8 mag for the two reported photometric epochs, respectively.

We obtained spectroscopic observations of SN 2017dfc with the Kast double spectrograph on the Shane 3-m telescope on UT 2017 April 21. The spectrum indicates that SN 2017dfc is a type-Ia supernova. Using SNID (Blondin & Tonry, 2007, ApJ, 666, 1024), we find the best match is a normal SN Ia about 1-2 days before maximum light. However, all the matches from SNID show that the SN has a redshift of ~0.06. Assuming this redshift, but not accounting for any reddening from M63 and/or its local environment, SN 2017dfc would have an absolute magnitude of -19.3 on 2017 April 19, consistent with the brightness of a near-peak SN Ia.

Examining SDSS and PS1 images of the SN position, there is a faint, extended, relatively red object about 8” to the northwest of SN 2017dfc, which we suggest may be a background host galaxy.

We conclude that SN 2017dfc is not located in the M63, but instead is hosted by a galaxy behind M63 at z ~ 0.06.