Photometric and Spectroscopic Studies of Type Ic Supernovae SN 2020akf and SN 2021mxx

Technical Summary: Photometric and Spectroscopic Studies of Type Ic Supernovae SN 2020akf and SN 2021mxx

Problem and Context
Stripped-envelope core-collapse supernovae (SE-SNe), specifically Type Ib and Ic, represent the explosive deaths of massive stars that have lost their hydrogen and helium envelopes. While Type Ic supernovae are characterized by the absence of hydrogen and helium lines in their spectra, they exhibit significant diversity in light curve shapes, luminosities, and spectral features. This diversity ranges from "normal" Type Ic events to "broad-lined" Type Ic (Ic-BL) supernovae, which are often associated with gamma-ray bursts and possess extremely high kinetic energies. A subset of events, sometimes termed "transitional," displays properties intermediate between normal and broad-lined classes. Understanding the explosion mechanisms, progenitor systems, and environmental influences of these diverse events requires detailed, object-specific analyses. This paper addresses the need for comprehensive characterization of two such events, SN 2020akf and SN 2021mxx, which were initially misclassified as Type Ia before being re-identified as Type Ic.

Methodology
The authors conducted a multi-wavelength analysis combining optical photometry and medium-resolution spectroscopy for SN 2020akf and SN 2021mxx.

• Observations: Optical imaging in $UBVRI$ bands was performed using the Himalaya Faint Object Spectrograph Camera (HFOSC) on the 2-meter Himalayan Chandra Telescope (HCT). Additional photometric data were incorporated from the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS). Spectroscopic coverage spanned from $\sim-5$ to $+57$ days for SN 2020akf and $\sim-3$ to $+113$ days for SN 2021mxx relative to $B$-band maximum.
• Data Reduction: Standard reduction techniques (bias subtraction, flat-fielding) were applied using IRAF. Host galaxy subtraction was performed using template images to isolate supernova flux. Reddening corrections were applied using Galactic dust maps and comparisons with color templates from the Carnegie Supernova Project.
• Light Curve Analysis: Quasi-bolometric light curves were constructed using the SuperBol code, interpolating multi-band data and correcting for extinction. The evolution of photospheric temperature and radius was derived from these curves.
• Spectral Analysis: Spectral evolution was analyzed by identifying prominent absorption and emission features (e.g., Fe II, Si II, Ca II, C II, O I). Photospheric expansion velocities were estimated by fitting Gaussian profiles to absorption troughs of Fe II $\lambda$5169, Si II $\lambda$6355, and Ca II NIR triplet lines.
• Modeling:
  • Arnett-Valenti Model: A semi-analytical radiation diffusion model was employed to estimate explosion parameters, including synthesized $^{56}$Ni mass ($M_{\text{Ni}}$), ejecta mass ($M_{\text{ej}}$), and kinetic energy ($E_k$). Markov Chain Monte Carlo (MCMC) methods were used to fit the quasi-bolometric light curves.
  • TARDIS: A one-dimensional radiative transfer code was used to model the spectral evolution, deriving density profiles and elemental abundances.
  • MOSFiT: The Modular Open-Source Fitter for Transients was utilized for multi-color light curve modeling to cross-validate explosion parameters.
• Progenitor and Environment: Host galaxy metallicities were estimated using emission line ratios (N2 and O3N2 indices) from SDSS and custom spectroscopy. Progenitor masses were inferred from nebular-phase [O I]/[Ca II] line ratios and comparisons with theoretical nebular models.

Key Results

• Photometric Properties:

  • SN 2020akf: Exhibits a broad light curve with a rise time of $\sim16$ days in the $B$-band. Peak absolute magnitude is $M_V = -17.85 \pm 0.15$ mag, with a peak quasi-bolometric luminosity of $\log L_{\text{bol}} = 42.37 \pm 0.02$ erg s$^{-1}$.
  • SN 2021mxx: Shows a narrower light curve with a faster rise ($\sim10.4$ days) and decline. Peak absolute magnitude is $M_V = -17.60 \pm 0.15$ mag, with a peak quasi-bolometric luminosity of $\log L_{\text{bol}} = 42.21 \pm 0.02$ erg s$^{-1}$.
  • Both events are brighter than typical normal Type Ic SNe (e.g., SN 2007gr) but fainter than Ic-BL events (e.g., SN 1998bw).

• Spectroscopic Evolution:

  • Both SNe display blue continua with prominent Fe II, Ca II, and Mg II features. SN 2020akf shows strong C II lines in the pre-maximum phase, which fade rapidly.
  • Photospheric velocities at maximum light are $\sim15,000$ km s$^{-1}$ for SN 2020akf and $\sim10,000$ km s$^{-1}$ for SN 2021mxx (derived from Fe II $\lambda$5169). These values are consistent with normal to transitional Type Ic SNe and significantly lower than Ic-BL events.
  • Nebular spectra of SN 2021mxx (at 102 and 113 days) are dominated by [O I] $\lambda\lambda$6300, 6363 and [Ca II] $\lambda\lambda$7291, 7324 emission. The [O I]/[Ca II] flux ratio is $\sim0.96$.

• Explosion Parameters:

  • SN 2020akf: Modeled with $M_{\text{Ni}} \approx 0.12 \pm 0.01 M_\odot$, $M_{\text{ej}} \approx 4.71^{+0.50}_{-0.46} M_\odot$, and $E_k \approx 6.33^{+0.68}_{-0.62} \times 10^{51}$ erg. The ratio $E_k/M_{\text{ej}} \approx 0.134$ is comparable to transitional events like SN 2004aw.
  • SN 2021mxx: Modeled with $M_{\text{Ni}} \approx 0.05 \pm 0.01 M_\odot$, $M_{\text{ej}} \approx 0.90^{+0.14}_{-0.18} M_\odot$, and $E_k \approx 0.54^{+0.08}_{-0.12} \times 10^{51}$ erg. This event has an extremely low $M_{\text{ej}}$ and $E_k$, with an $E_k/M_{\text{ej}}$ ratio of $\sim0.06$, typical of normal Type Ic SNe.
  • MOSFiT modeling yields consistent results within $3\sigma$ uncertainties, estimating $M_{\text{Ni}} \approx 0.20 M_\odot$ and $M_{\text{ej}} \approx 3.16 M_\odot$ for SN 2020akf, and $M_{\text{Ni}} \approx 0.09 M_\odot$ and $M_{\text{ej}} \approx 1.53 M_\odot$ for SN 2021mxx.

• Progenitor and Environment:

  • Host galaxy metallicities are near-solar: $\sim0.81 Z_\odot$ for SN 2020akf (KUG 0925+387B) and $\sim0.76 Z_\odot$ for SN 2021mxx (UGC 11380).
  • For SN 2021mxx, the nebular [O I]/[Ca II] ratio and oxygen mass estimate ($\sim0.92 M_\odot$) suggest a progenitor Zero-Age Main Sequence (ZAMS) mass in the range of $13\text{--}18 M_\odot$. The low $[O I]/[Ca II]$ ratio suggests a binary progenitor system.

Significance and Conclusions
The paper classifies SN 2020akf as a transitional Type Ic supernova, possessing spectral and photometric properties intermediate between normal Type Ic and broad-lined Type Ic (Ic-BL) events, similar to SN 2004aw. Its higher ejecta mass and kinetic energy distinguish it from typical normal Type Ic SNe.

Conversely, SN 2021mxx is identified as a normal Type Ic supernova characterized by an extremely low ejecta mass ($\sim0.9 M_\odot$) and kinetic energy. Its parameters place it firmly within the distribution of well-studied normal Type Ic SNe, though its low mass values highlight the diversity within this subclass. The analysis of SN 2021mxx's nebular phase supports a progenitor scenario involving a lower-mass star ($13\text{--}18 M_\odot$) likely stripped via binary interaction.

The study reinforces the utility of combining photometric modeling (Arnett-Valenti, MOSFiT) with spectroscopic analysis (TARDIS, nebular line ratios) to constrain explosion physics and progenitor properties. The results contribute to the growing census of SE-SNe, helping to delineate the boundaries between normal, transitional, and broad-lined Type Ic events and their respective progenitor channels.