ePESSTO reclassification of SN2018bsz as the lowest redshift SLSN to date

ATel #11674; J. P. Anderson (ESO), Luc Dessart (CNRS UMI), P. Pessi (IALP, ESO), S. J. Smartt (QUB), C. Inserra (Southampton), G. Leloudas (DARK), R. Roy (IUCAA), A. Gal-Yam (Wizemann), J. Tonry, L. Denneau, A. Heinze, H. Weiland (IfA, Univ. of Hawaii), B. Stalder (LSST) A. Rest (STScI), K. W. Smith, O. McBrien, D. R. Young (QUB), D. E. Wright (Univ. of Minnesota)
on 30 May 2018; 19:51 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Joseph Anderson (janderso@eso.org)

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ePESSTO, the extended Public ESO Spectroscopic Survey for Transient Objects (see Smartt et al. 2015, A&A, 579, 40 http://www.pessto.org ), reports a reclassification of SN2018bsz (RA=16:09:39.19, DEC=-32:03:45.2).

SN2018bsz was discovered (ATEL#11660) by the All Sky Automated Survey for SuperNovae ASAS-SN (see Shappee et al. 2014, ApJ, 788, 48 and http://www.astronomy.ohio-state.edu/~assassin/index.shtml ) and also recovered by the ATLAS survey, see Tonry et al. (2011, PASP, 123, 58) and Tonry et al. (ATel #8680).

Initial spectral classifications of SN2018bsz were reported by both Hiramatsu et al. (TNS Classification Report: 2018TNSCR.679....1H) and ePESSTO (ATEL#11659) (both spectra can be found on wiserep: https://wiserep.weizmann.ac.il). Both classified the transient as a young type II supernova (SNII) due to the presence of a strong P-Cygni profile consistent in wavelength with H-alpha. However, later inspection of the spectra have revealed features that cast doubt on a SNII classification: no other Balmer lines are observed; a broad emission feature is seen redwards of H-alpha; there are a number of P-Cygni profiles observed between 4000 and 5000 Angstroms that are not usually observed in spectra of young SNeII. We tentatively identify CII lines (4745, 5890, 6580 and 7234 Angstroms) with blue-shifted absorption troughs at around 13,000 kms-1. Such lines have been predicted previously by spectral models of Super-Luminous SNe (SLSNe, see Dessart et al. 2012, MNRAS, 426 26), but are rarely detected at such strength. A number of spectral features are observed between 4000 and 5000 Angstroms, however the emission peaks appear offset in wavelength from the lines usually identified as OII in SLSNe. By comparing with spectra of other SLSNe I (using SNID: Blondin & Tonry, 2007, ApJ, 666, 1024), we obtain good matches with several objects including PTF12dam at -15 days and PTF09atu at -20 days.

Inspecting the ATLAS photometry reveal a current o-band magnitude of 15.4 that translates to an absolute magnitude of almost -20 at the distance of the host galaxy. The ATLAS photometry also suggests that the transient is still rising (last photometry obtained four days ago). SN2018bsz was first detected by ATLAS at an o-band magnitude of 18.2+/-0.2 on MJD 58219, and there is a non-detection on MJD 58207 that constrains the explosion to have occurred around 50 days ago.

We conclude that SN2018bsz is a type I SLSN with strong CII spectral features. Given a host galaxy redshift of 0.0267, this makes SN2018bsz the lowest-redshift SLSN to date. Given this reclassification, we may expect SN2018bsz to rise to be as bright as 14th apparent magnitude.

Further multi-wavelength observations are strongly encouraged.