A nearby He-rich superluminous supernova at photospheric phases

TL;DR

SN 2021bnw is a nearby He-rich SLSN Ib whose extensive optical/NIR photometry and spectroscopy reveal He features in the NIR at late times and a wedge-shaped light-curve with two post-maximum bumps. A TARDIS spectral synthesis test supports He identification in the NIR, while STELLA hydrodynamics–radiation transport modeling favors a two-component powering scenario in which ejecta-CSM interaction with a He-rich CSM couples with a central power source. The inferred parameters point to a large Ni supply ($\sim$1.7 $M_\odot$), a substantial ejecta mass ($\sim$15 $M_\odot$), and a He-rich CSM mass ($\sim$6.5 $M_\odot$), though early-time data remain challenging to reproduce. Collectively, the work expands the sample of He-rich SLSNe Ib and provides stringent constraints on progenitor masses and explosion physics, informing models of massive-star death and circumstellar environments.

Abstract

Aim. We present and interpret the data of the nearby hydrogen-deficient but helium-rich superluminous supernova SN~2021bnw which reached a magnitude of -20.7 at maximum luminosity in g band. Methods. We discuss the light curves and spectra of SN 2021bnw based on its spectro-photometric follow up exploiting different observational facilities. We reproduce the NIR spectrum of SN 2021bnw with TARDIS to inspect the chemical composition at late photospheric phases and identify helium features. We also use a STELLA model coupling hydrodynamics and radiation transport to constrain the physical parameters of the explosion assmunig a 56Ni+CSM scenario. Results. We suggest that SN 2021bnw was mainly powered by the interaction of the ejecta with a previously lost He-rich circumstellar material, coupled with a central power source. Conclusions. This work expands the data sample of He-rich superluminous supernovae rich (SLSNe Ib) and, assuming a single progenitor scenario, can constrain the masses and the physics of their progenitors.

Figures (11)

• Figure 1: Left panel: $u$-, $B$-, $g$-, $V$-, $r$-, $i$-, ATLAS $cyan$-, ATLAS $orange$-, $z$-, $J$-, $H$-, $K$-filter LCs (green, dark blue, red, purple, blue, magenta, orange, light green, violet, dark gray, light gray, yellow symbols) of SN 2021bnw. LCs are displayed in apparent (left axis) and absolute magnitude (right axis) and are shifted by a constant (shown in the label). The dashed line shows the polynomial fit used to estimate the maximum luminosity epoch. LCs in different filters are colored with different colors as labelled to their right side. Different symbols correspond to different instruments, as labelled in gray on the top right corner. Magnitudes are reported in the AB magnitude system. Right panel: residuals after the subtraction of the best-fit one-dimensional polynomial between 2 and 80 days after maximum luminosity in $B,g,V,r$ bands. The dashed black line marks the zero-residual level for reference. In this panel, the dots are colour-coded as in the left panel and the detection limits were removed for visualization purpose.
• Figure 2: Pseudo-bolometric LC of SN 2021bnw. The $^{56}$Co slope is shown for comparison purpose.
• Figure 3: Spectral evolution of SN 2021bnw. The dotted lines mark the line identifications labelled on their top and refer to the emission component at rest frame. On the right side of each spectrum the rest-frame phase with respect to the $g$-band maximum luminosity is reported (in days). The spectra at phases $-12$, 25, 35, 62, 66, 72, 86 and 96 have been re-binned (with a bin size of 5 Å) and smoothed with a Savitzky-Golay filter for visualization purpose.
• Figure 4: Velocities of SN 2021bnw measured fitting a gaussian to the absorption minima of the O i$\lambda\,7774$ (red dots) and from the blackbody radius $R_{\rm BB}$(blue triangles, see Fig. \ref{['fig:photosphere']}). For comparison, we also plot a $\propto t^{-2/3}$ curve (dashed black line).
• Figure 5: Left panel: NIR spectrum of SN 2021bnw at 87 days after maximum. The grey-shaded areas mark the regions contaminated by telluric absorptions. Right panel: same as the left panel but zooming the region close to 1.1 $\mu$m. In both of them, the wavelengths corresponding to the emission components of the NIR features from Helium, Magnesium and Oxygen are marked with different colors (see the legend on the top-right corner), solid and dashed lines correspond to neutral and singly-ionized ones, respectively.