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NUTS update on the spectroscopic follow-up of SN 2018cow

ATel #11836; S. Benetti, A. Pastorello, E. Cappellaro, M. Turatto, L. Tomasella, A. Fiore, A. Reguitti (INAF-OAPd), M. Gromadzki (Warsaw University Astronomical Observatory)
on 10 Jul 2018; 07:59 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Stefano Benetti (stefano.benetti@oapd.inaf.it)

Subjects: Optical, Supernovae

The Nordic Optical Telescope (NOT) Unbiased Transient Survey (NUTS; ATel #8992) reports an update on the spectroscopic follow-up of SN 2018cow (ATel #11727).

The latest observation with the 2.56 m Nordic Optical Telescope equipped with ALFOSC (range 350-950 nm; resolution 1.4 nm FWHM) was performed on UT July 08.03.

The transient spectrum is still dominated by a blue continuum (T_BB~13000 K). A number of broad shallow emission lines are now emerging. After removal of the blue continuum, we tentatively attribute all these emissions to He, namely HeI (447.1, 492.2, 501.5, 587.6, 667.8 and 706.5 nm) and He II (468.6 nm) transitions. The FWHM of the emission identified as He I 5876A is about 7000 km/s. From a narrow, unresolved H-alpha line visible in the spectrum, we estimate a redshift of z = 0.0136 (this value is very close to SDSS redshift, 0.01406, for the parent galaxy). Given this redshift, the peaks of the broad emissions, if correctly identified as He lines, appear significantly redshifted (up to ~70A) with respect to the rest frame position.

Formally, we should define SN 2008cow as a Type Ib supernova, although this event shows an unprecedented spectrum for a supernova of this class.