AstroNote 2019-45

We report spectral classifications of 18 low- to intermediate-redshift superluminous supernovae from the Zwicky Transient Facility (Bellm et al. 2018, PASP 131, 995).  Candidate SLSNe were selected from the ZTF public and partnership alert feeds using a custom filter developed by D. Perley and implemented on the GROWTH Marshal (Kasliwal et al. 2019, PASP 131, 997). The filter identifies events of interest using a scoring-based metric that notifies us of sources that show a slow rise time, long duration and/or a high contrast relative to the nearest catalogued galaxy, lack of variability prior to the current observing season, and other metrics.  Our program has classified or confirmed 66 SLSNe since the beginning of full ZTF operations in April 2018.

The table below presents the most recent classifications, associated with observations acquired using Low Resolution Imaging Spectrograph (LRIS) on Keck I on UT 2019-07-02 and using the Double Beam SPectrograph (DBSP) on the Palomar 200 inch telescope on UT 2019-07-03.  Comparisons to SN spectra from the literature were guided by the use of SNID (Blondin & Tonry, 2007, ApJ, 666, 1024) and Superfit (Howell et al. 2005, ApJ, 634, 1190).

Notes:
[A] Peak absolute magnitudes are as measured in the rest-frame wave-band matching the observed r-band.
[R] Still rising, peak magnitude is an upper limit.
[1] Spectrum shows strong intermediate-width H-alpha in emission and no other features. Earlier spectra with DBSP close to peak light showed only a blue continuum, suggesting a possible alternative classification as a SLSN-I with late H-alpha emission (e.g. Yan et al. 2017, ApJ 848, 6)
[2] Previously classified by ePESSTO (Chen et al., ATEL12604). We report a refined redshift of z=0.4103 based on detection of host-galaxy narrow emission lines.
[3] Spectrum shows a good match to 2007bi or PTF09cnd 1 month after peak light, redshifted to z~0.5.
[4] Spectrum shows shows strong intermediate-width Balmer-series emission lines and He I on top of continuum-dominated host galaxy spectrum, with host OII and OIII lines at a consistent redshift.
[5] Supplements a preliminary classification from DBSP observations carried out 2019-04-27 showing a good match to PTF10nmn near peak light. The precise redshift is confirmed from narrow host-galaxy H-alpha in the latest spectrum.
[6] Spectrum resembles late-time Ic SNe redshifted to z~0.26.
[7] Spectrum resembles late-time Ic SNe redshifted to z~0.32.
[8] Redshift from strong host-galaxy emission features at z=0.3562. Spectrum is a good match to late-time spectra of SN2007bi at this redshift.
[9] First reported to TNS by ATLAS (ATLAS19mru). Shows strong narrow host-galaxy emission lines on a largely featureless continuum; the H-alpha line has a weak broad component on which we based the classification. An AGN origin is ruled out by a significant offset (1") from the position of the source in ZTF and PS1 pre-imaging.
[10] First reported to TNS by ATLAS (ATLAS19jlx). Spectrum shows an excellent match to 2007bi or PTF09cnd 1 month after peak light, redshifted to z~0.6.
[11] First reported to TNS by ATLAS (ATLAS19mas). Spectrum shows several broad features which we have not yet successfully matched to any previous transient. However, narrow host-galaxy emission lines are evident at z=0.3642 to fix the redshift and broad hydrogen is clearly absent, so we classify this as a SLSN-I.
[12] Spectrum shows intermediate-width lines of H and He at z=0.3284 as well as strong [OIII] and other emission lines from the host galaxy. Pre-imaging from PS1 and Pan-STARRS rule out an AGN at this location.
[13] Highlighted as an interesting slow transient by Smartt et al. (AstroNote 2019-33). Spectrum shows a blue continuum superimposed with multiple strong intermediate-width Balmer emission lines, narrow host-galaxy emission lines, and no other strong features. The location is consistent with the host nucleus and the spectrum resembles those of AGN, as noted previously by Chornock et al. (2019TNSCR1016....1C). We argue that the UV to optical colors, large contrast versus the host galaxy, lack of prior variability, and lack of spectral lines other than hydrogen favor a SN interpretation.
[14] First reported to TNS by Pan-STARRS (PS19agg). Spectrum shows a good match to PTF09atu redshifted to z=0.67, though it also matches SLSNe-I at slightly lower redshifts. We regard this redshift as tentative.
[15] First reported to TNS by Pan-STARRS (PS19ave). Previously classified as a Ic-BL (Chen et al., TNSCR 938). Spectrum shows similarities to previous SLSNe-I at similar phases, including 2007bi and 09cnd. Its high luminosity, slow evolution, and blue color are also consistent with SLSNe-I.
[16] First reported to TNS by ATLAS (ATLAS19ndu). Redshift from narrow Mg II absorption. The spectrum is very blue with strong rest-frame UV absorption dips consistent with other SLSNe at this redshift closer to peak light.
[17] Spectrum shows strong narrow emission lines associated with the host galaxy, with a broad component from the SN underlying H-alpha. The spectrum is reasonably well-matched to SN 1998s (IIn) shortly after maximum.
[18] Spectrum shows an excellent match to PTF09cnd 16 days before peak, redshifted to z=0.40.

ZTF is a project led by PI S. R. Kulkarni at Caltech (see ATEL #11266), and includes IPAC; WIS, Israel; OKC, Sweden; JSI/UMd, USA; UW,USA; DESY, Germany; NRC, Taiwan; UW Milwaukee, USA and LANL USA. ZTF acknowledges the generous support of the NSF under AST MSIP Grant No 1440341. Alert distribution service provided by DIRAC@UW. Alert filtering is being undertaken by the GROWTH marshal system, supported by NSF PIRE grant 1545949.