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UV Spectroscopy of the Superluminous Supernova SN2018bsz

ATel #11790; P. Blanchard, M. Nicholl (Harvard/CfA), R. Chornock (Ohio University), E. Berger (Harvard/CfA), R. Margutti, G. Terreran, D. Coppejans, K. D. Alexander (Northwestern University), D. Milisavljevic (Purdue University)
on 27 Jun 2018; 23:37 UT
Credential Certification: Peter Blanchard (pblanchard@cfa.harvard.edu)

Subjects: Ultra-Violet, Supernovae

We obtained 3 orbits of Hubble Space Telescope observations of the Type I superluminous supernova (SLSN-I) SN2018bsz, the lowest-redshift SLSN-I at z=0.027 (ATels #11659, #11660, #11674; Hiramatsu et al. 2018), on 25 June 2018 UT. The observations consist of 2 orbits of spectroscopy with COS/FUV + G140L and 1 orbit with STIS/NUV-MAMA + G230L (PID: 15488; PI: Blanchard). The resulting UV spectrum covers a rest-frame wavelength range of ~1100 - 3100 angstroms and was obtained at a rest-frame phase of approximately 1 month after peak brightness.

After correcting for Galactic extinction (E(B-V) = 0.207; Schlafly & Finkbeiner 2011) and comparing the spectrum with the near-peak UV spectra of the SLSNe-I Gaia16apd (Yan et al. 2017), SN2017egm (Yan et al. 2018), and iPTF13ajg (Vreeswijk et al. 2014), we find that SN2018bsz is overall similar in spectral shape and shows most of the same strong UV features, namely broad absorptions centered near 2650, 2200, 1750, and 1425 angstroms. However, SN2018bsz does not show obvious distinct absorption features near 2450 and 1950 angstroms (the UV2 and UV4 features in the terminology of Quimby et al. 2018). The UV2 feature has been attributed to Si III or a blend of Si III + Ti III + C II (Quimby et al. 2011; Mazzali et al. 2016) and the UV4 feature has been attributed to Fe III (Howell et al. 2013; Mazzali et al. 2016). In the spectra of Gaia16apd and other SLSNe-I, the UV2 feature appears to weaken with time (Yan et al. 2017; Quimby et al. 2018), and so its absence in SN2018bsz may be attributable to the later phase of the observations. In addition, the features that are present in SN2018bsz appear to be somewhat broader than the other SLSNe-I with UV spectra, which may be causing increased line blending.

The far-UV spectrum from COS shows a similar overall shape to Gaia16apd and SN2017egm, but line features appear relatively weak, which may again be due to increased line blending or the later phase of observations.

Finally, we note that the spectral differences between SN2018bsz and the comparison objects may in part be related to having an unusual abundance pattern, as evidenced by the optical spectra (ATel #11674).

We thank STScI for making these observations possible.