SN 2021lwz: Another Exotic Luminous and Fast Evolving Optical Type Ic Broad-Lined Supernova ?

TL;DR

SN 2021lwz is a rapid, luminous transient at $z=0.065$ discovered in a faint dwarf host. Comprehensive spectroscopy, multi-band photometry, and polarimetry reveal an Ic-Ic-BL-like photosphere pre-peak, evolving into broader post-peak features, with a low ejecta mass $M_{ej}\sim0.24\,M_\odot$ and a magnetar-driven power source (spin-down luminosity $L_0\sim3\times10^{45}$ erg s$^{-1}$, $B_{14}\sim1.8$, $P_{ms}\sim5.8$ ms, $t_{SD}\sim1$ day). Arnett Ni powering fails to explain the light curve, while magnetar modeling provides a physically plausible fit, implying a compact progenitor in a high specific star-formation-rate dwarf ($M_\ast\sim10^{6.7}\,M_\odot$, SFR $\sim0.04\,M_\odot$ yr$^{-1}$). The event shares similarities with rare transients like SN 2014ft, iPTF 16asu and SN 2018gep, yet remains distinct from typical H-poor SLSNe and LFBOTs, highlighting a continuum between stripped-envelope SNe and magnetar-powered luminous transients. The host environment and polarization analysis further constrain the geometry and progenitor channel, suggesting an ultra-stripped-like engine with limited ejecta mass and a spherical photosphere, with implications for the diversity of rapidly evolving, luminous transients.

Abstract

Context. Current large-scale, high-cadence surveys, such as the Zwicky Transient Facility (ZTF), provide detections of new and rare types of transients and supernovae whose physical origins are not well understood. Aims. We investigate the nature of SN 2021lwz at a redshift z=0.065, an overluminous supernova (SN) of absolute magnitude, $M_{g} \sim -20.1$ AB, falling in the lower range of superluminous supernovae (SLSNe) luminosities, and discovered in a faint dwarf galaxy with an absolute magnitude of $M_{g} \simeq -14.5$ AB. Methods. SN 2021lwz is studied using optical spectroscopy, photometry and imaging linear polarimetry obtained during several follow-up campaigns. All the data are used to analyse and model the evolution of the explosion. Comparisons with other SNe of well known or rarer types are investigated. Results. SN 2021lwz belongs to the rare class of rapidly evolving transients. The bolometric light curve rises in about 7 days to a peak luminosity of about 5 x10^{43} erg/s, at a rate of 0.2 mag/day close to the peak. Spectroscopy modeling reveals more similarities with a normal Type Ic-like SN than with a SLSN before peak, showing broadened lines after peak. Light curve modeling shows that the Arnett model of the bolometric light curve using a radioactive source ($^{56}$ Ni) is not able to reasonably explain the light curve evolution. A magnetar model seems more appropriate, suggesting that the explosion of low ejecta mass ($M_{\rm ej} \sim 0.24 ~M_\odot$) took place in a low mass ($M \sim 10^{6.66}~M_\odot$) dwarf galaxy of specific star-formation rate about ten times larger than typical star-forming galaxies. Conclusions. Given its spectroscopic properties and the low ejecta mass needed to model its light-curve, SN 2021lwz does not match with many core-collapse H-poor SNe Types. It shares similarities with rarer transients like SN 2014ft, iPTF 16asu and SN 2018gep.

Figures (21)

• Figure 1: Rise times $t_{rise,10\%}$ versus $g$-band absolute magnitudes $M_{g}$ for SLSNe-I 2023ApJ...943...41C, normal SNe Ic 2021AA...651A..81B and SNe Ic-BL 2019AA...621A..71T. As in 2023ApJ...943...41C the $M_{g}$ of the Sn Ic and SN Ic-BL is computed from the r-band magnitudes using a color correction of $\sim 0.36$ mag. The rise times of these two populations are measured from the explosion dates in the r-band, which are slightly longer than $t_{rise,10\%}$. The data used for the labelled individual sources are from our compilation discussed in Section \ref{['discussion']}.
• Figure 2: LS DR 10 image of SN 2021lwz's host, as shown by the faint feature centred in the yellow circle, and its environment. The field-of-view is of about 120$\arcsec$ by 90$\arcsec$ in size. North is up and East is left in the Equatorial frame.
• Figure 3: Spectral sequence of SN 2021lwz. Spectra taken with the P60 are shown in blue; with the NOT in orange; and with the Keck I telescope in green. All the spectra were smoothed using a Savitzky-Golay low pass filter. Un-smoothed data and measurement errors are shown with a lighter shade of the same color. Each spectrum is marked with its phase in rest frame days relative to maximum g-band flux as well as the telescope used for measurement. The Fe II P-Cygni profile minima used for velocity estimation are marked in dark red. Figure \ref{['fig:PCygni']} zooms in onto the Fe II lines.
• Figure 4: NIR spectrum of SN 2021lwz obtained with NIRES on Keck II. Also shown are the SESNe templates spectra of Helium-Rich (green) and Helium-Poor (red) at phase $+13$ days produced by 2022ApJ...925..175S. The NIR spectra of the He-Rich SLSN-I SN 2019hge at phase $+34$ days 2020ApJ...902L...8Y, and of the peculiar and bright Type Ic SN 1998bw at phase d$+8$2001ApJ...555..900P, are also shown for comparisons. The observed spectra smoothed using Savitzky-Golay low pass filters are shown in gray. The vertical gray bands show velocity line ranges in bins of 5000 km$/$s.
• Figure 5: Absolute magnitude (rest frame) light curves (see Sec. \ref{['Light_Curves_analysis']} for details) from the ATLAS and Liverpool Telescope SDSS filters as triangles, the Swift UVOT filters as squares and the ZTF filters as circles. The phases of available spectra are shown as black triangles at the top along with the phases of the polarimetry shown as red triangles. Phase $=0$ is at peak ZTF $g$-band flux. The light curves were shifted by the indicated amounts for illustration purposes.