AstroNote 2020-37

We obtained spectroscopy of SN2020bvc (first reported to TNS by ASAS-SN as ASASSN-20bs) with the SPRAT spectrograph on the 2m robotic Liverpool Telescope, acquiring a single 600-second spectrum beginning at 2020-02-08 05:52 (UT), four days after the discovery.  The spectrum was reduced with the standard LT reduction pipeline.  Analysis of the resulting spectrum using Superfit (Howell et al. 2005, ApJ, 634, 1190) shows an excellent match to the prototypical (and GRB-associated) broad-lined Ic SN1998bw at 6 days prior to peak at the redshift of the associated host galaxy (UGC 9379 at z=0.0252; d~110 Mpc assuming h=0.7).

We note that the object displays a peculiar light curve compared to other known Ic-BL SNe, with an extremely fast rise between the most recent non-detection and first detection (Stanek et al., TNS Discovery Report 61070) followed by a steep decay in the subsequent two days (from ZTF, LT, and Swift/UVOT observations).  Spectroscopy during this initial decay (from our own P60/SEDM follow-up and as reported by the Global SN project; TNS Classification Report 6063) shows only hot, featureless spectra except for nebular emission from the underlying star-forming region.  The SN is now rising again.  This event may be related to other broad-lined Ic supernovae with luminous/fast initial rises, such as SN 2018gep (Ho et al. 2019, ApJ 887, 169) or iPTF16asu (Whitesides et al. 2017, ApJ 851, 107) but somewhat less luminous and significantly closer.

We have triggered additional Swift and radio follow-up, and encourage additional monitoring of this interesting transient at all wavelengths.

We also note in passing that this galaxy also hosted SN 2013cu, the prototypical "flash" supernova (Gal-Yam et al. 2014, Nature, 509, 471).