The broad-lined Type Ic supernova 2020lao reveals an energetic explosion with no central-engine signatures

TL;DR

SN 2020lao is an exceptionally well-sampled, early-time broad-lined Type Ic supernova with no detectable central-engine signatures. By combining dense TESS/ZTF photometry, multi-epoch spectroscopy, and spectral synthesis, the study derives $M_{\rm Ni}\approx0.23\ M_\odot$, $M_{\rm ej}\approx3.2\ M_\odot$, and $E_K\approx(23\pm12)\times10^{51}$ erg, yielding $E_K/M_{\rm ej}\approx7\times10^{51}$ erg $M_\odot^{-1}$; a high photospheric velocity of $v_{ph}\approx34{,}800$ km s$^{-1}$ at peak is inferred. The bolometric and spectral analyses, along with a constrained progenitor radius $R_*\lesssim$ a few $R_\odot$, point to a compact Wolf–Rayet-like progenitor and native engine-free, albeit extremely energetic, SN Ic-BL dynamics. Deep radio and X-ray non-detections further limit relativistic ejecta, implying that any jet was either off-axis or choked. Overall, SN 2020lao sits at the energetic extreme of Ic-BL SNe without clear engine-driven afterglow signatures, providing a critical baseline to distinguish viewing-angle effects from intrinsic engine activity in this class.

Abstract

We present early-time observations of the broad-lined Type Ic supernova SN 2020lao, including optical spectroscopy beginning about 48 hr after the inferred explosion and extending to about 100 days. The explosion epoch is constrained with power-law fits to the rising TESS and ZTF light curves, with the first ZTF detection only about 27 hr after explosion. The optical light curves show a rapid 8.8 day rise and a peak luminosity typical of SNe Ic-BL (Mr=-18.5 mag). Unlike some engine-driven events, SN 2020lao shows no early optical excess or afterglow. The lack of a shock-cooling signature in the TESS/ZTF data constrains the progenitor to a compact Wolf Rayet-like star with radius of only a few solar radii. The spectra resemble those of the X-ray flash SN 2006aj but with higher velocities. Arnett modeling of the bolometric light curve, combined with Fe II velocities, yields a nickel mass of about 0.2 solar masses, an ejecta mass of about 3.2 solar masses, and a kinetic energy of about 23x10^51 erg, corresponding to a high specific kinetic energy of about 7x10^51 erg per solar mass. Spectral-synthesis modeling broadly reproduces the photospheric spectra and suggests a somewhat lower but still large specific kinetic energy (about 5 x 10^51 erg per solar mass). Although SN 2020lao and SN 2006aj produced similar 56Ni masses, SN 2020lao exhibits specific kinetic energies larger by a factor of several. Published VLA and Swift/XRT non-detections show no radio or X-ray afterglow, placing strong limits on relativistic ejecta and dense circumstellar material. Given its high specific kinetic energy, the absence of early excess emission and the radio/X-ray non-detections imply that any jet was either far off-axis or choked; otherwise, SN 2020lao represents an extreme non-relativistic SN Ic-BL.

Figures (10)

• Figure 1: An $r$-band image from the Nordic Optical Telescope showing the position of SN 2020lao within its host galaxy, CGCG 169-041.
• Figure 2: The optical $BgVri$-band light curves of SN 2020lao, corrected for reddening and vertically offset by arbitrary constants for clarity. The previous ATLAS $o$-band non-detection limit (orange triangle) obtained $-0.34$ days prior to $t_{exp}$ (vertical dashed line), is plotted alongside the $r$-band light curve. Additionally, ZTF non-detection limits are included. Segments at the top mark the phases when optical (black) and NIR (red) spectroscopy were obtained (see Table \ref{['tab:specjor']}).
• Figure 3: Spectroscopic observations of SN 2020lao. Top: NIR $+21.5$ days spectrum of SN 2020lao. Left: Selected optical spectra of SN 2020lao, corrected for Milky Way reddening and overplotted with a smoothed version. Regions affected by three prevalent telluric features are indicated with vertical gray bands and labeled with Earth symbols. The locations of the absorption minima used to infer the expansion velocities plotted in Fig. \ref{['fig:dopvel']} are marked with filled black dots, with red error bars indicating the wavelength ranges that define the associated uncertainties in the positions of maximum absorption. Right: Comparison between similar epoch spectra of SN 2020lao and SN 2006aj Pian2006Sonbas2008.
• Figure 4: Top panel: Binned TESS light curve adopting a bin size of two hours and plotted vs. phase relative to JD 2458900. Over-plotted is our best-fit power law. Bottom panel: The 2-D probability density of MCMC sample between the fit parameters -- time of explosion, $t_{exp}$, and power-law index, $\alpha$. The contours correspond to 68% and 95% confidence intervals. The solid intersecting black lines indicate the mean of the MCMC sample for parameters $t_{exp}=\mathrm{JD~}2458993.64^{+0.23}_{-0.20}$ (vertical) and $\alpha=1.50^{+0.01}_{-0.01}$ (horizontal).
• Figure 5: Milky Way reddening-corrected apparent colors of SN 2020lao (filled points) are derived from Gaussian Process (GP) interpolations of the observed light curves. Solid lines represent intrinsic SN Ic color-curve templates Stritzinger2018, while open points show the corrected color curves after adjusting for the average offset between the apparent colors and the templates. The colors of SN 2020lao indicate that it experienced minimal host-galaxy reddening.