A nearby super-luminous supernova with a long pre-maximum & “plateau” and strong C II features

Abstract

Context. Super-luminous supernovae (SLSNe) are rare events defined as being significantly more luminous than normal terminal stellar explosions. The source of the additional power needed to achieve such luminosities is still unclear. Discoveries in the local Universe (i.e. z < 0.1) are scarce, but afford dense multi-wavelength observations. Additional low-redshift objects are therefore extremely valuable. Aims. We present early-time observations of the type I SLSN ASASSN-18km/SN 2018bsz. These data are used to characterise the event and compare to literature SLSNe and spectral models. Host galaxy properties are also analysed. Methods. Optical and near-IR photometry and spectroscopy were analysed. Early-time ATLAS photometry was used to constrain the rising light curve. We identified a number of spectral features in optical-wavelength spectra and track their time evolution. Finally, we used archival host galaxy photometry together with H ii region spectra to constrain the host environment. Results. ASASSN-18km/SN 2018bsz is found to be a type I SLSN in a galaxy at a redshift of 0.0267 (111 Mpc), making it the lowestredshift event discovered to date. Strong C ii lines are identified in the spectra. Spectral models produced by exploding a Wolf-Rayet progenitor and injecting a magnetar power source are shown to be qualitatively similar to ASASSN-18km/SN 2018bsz, contrary to most SLSNe-I that display weak or non-existent C ii lines. ASASSN-18km/SN 2018bsz displays a long, slowly rising, red 'plateau' of >26 days, before a steeper, faster rise to maximum. The host has an absolute magnitude of -19.8 mag (r), a mass of M * = 1.5 +0.08 −0.33 ×10 9 M , and a star formation rate of = 0.50 +2.22 −0.19 M yr −1 . A nearby H ii region has an oxygen abundance (O3N2) of 8.31±0.01 dex.

SN 2019hcc: a Type II supernova displaying early O ii lines

“…Anderson et al (2014b) performed this analysis on a large set of SNe II, and found a range of 56 Ni masses from 0.007 to 0.079 M , with a mean value of 0.033 M (σ = 0.024). A survey of literature values led to a mean mass 56 Ni = 0.044 M for a sample of 115 SNe II (Anderson 2019). Therefore, we conclude that the value retrieved for SN 2019hcc is within the expected range for a SN II.…”

“…The features usually associated with O II are formed by many tens of overlapping lines (Anderson et al 2018b;Gal-Yam 2019), and can be contaminated by carbon and metal lines, and also by the presence of well-developed Balmer lines, all of which mean the features cannot be uniquely identified as O II. Therefore, whilst SN 2019hcc, SN 2014G, and SN 1998S could be valid candidates to show O II features as the Balmer lines are less prominent, SN 2013ej is less likely as it shows a strong H α profile suggesting the spectrum is dominated by H β at λ4861 and H γ at λ4340.…”

RINGO3 polarimetry of very young ZTF supernovae

“…In general, Type I SLSNe are noted for having low levels of polarization; however, SN 2015bn exhibited a significant rise in polarization after ∼ 20 d post-maximum (Leloudas et al 2017), with the evolution from the pre-maximum to post-maximum state clearly evident in spectropolarimetric observations (Inserra et al 2016). Our detection of significant polarization for SN 2018bsz also occurs ∼ 20 d (rest-frame) after the time of the V-band light-curve maximum (Anderson et al 2018a). Inserra et al (2016) and Leloudas et al (2017) both explained the behaviour of SN 2015bn as being due to a fundamental change in the asymmetry of the ejecta, with early-time emission arising from an almost spherical outer layer, whilst at later times the emission arise from a more aspherical interior (giving rise to the increase in polarization with time).…”

“…Discovered by ASAS-SN on 2018 May 17 (Stanek 2018), it was temporarily classified as a young Type II SN (Hiramatsu et al 2018a). It was later reclassified as a superluminous supernova (for a review see Gal-Yam 2019a), albeit a lower luminosity example (Anderson et al 2018a). Our observations commenced 4 d after the photometric light curve maximum or 69 d after the explosion date proposed by Anderson et al (2018a).…”

“…It was later reclassified as a superluminous supernova (for a review see Gal-Yam 2019a), albeit a lower luminosity example (Anderson et al 2018a). Our observations commenced 4 d after the photometric light curve maximum or 69 d after the explosion date proposed by Anderson et al (2018a). The polarization measurements for SN 2018bsz are presented in Table 6 and the time evolution, with respect to the photometric light curve, is shown on Fig.…”