SN 2024igg: A Super-Chandrasekhar/03fg-like SN exhibiting C II-dominated spectra after explosion

TL;DR

SN 2024igg is a 03fg-like Type Ia supernova exhibiting a C II-dominated spectrum shortly after explosion, challenging standard progenitor scenarios. The authors combine multi-band photometry, spectroscopy, and radiative-transfer modeling (TARDIS) with a pseudo-bolometric light-curve analysis using Transfit to infer $M_{ m ej}=1.54^{+0.22}_{-0.19}\,M_{ m ext{odot}}$ and $M_{ m Ni}=0.547^{+0.082}_{-0.082}\,M_{ m ext{odot}}$, with an explosion time $t_{ m exp}=-18.29^{+0.27}_{-0.14}$ days. The early spectrum is best explained by interaction with a carbon-rich CSM, implying a dense C/O envelope likely produced by a WD-WD merger remnant or an AGB star envelope, which also accounts for the slow line-velocity evolution and UV excess. Nebular-phase spectra showing unshifted forbidden lines support a symmetric, secular-timescale explosion after merger. Collectively, SN 2024igg strengthens the carbon-rich CSM interaction scenario for 03fg-like SNe and constrains progenitor channels and explosion geometry.

Abstract

We present and analyze photometric and spectroscopic observations of the Type Ia supernova (SN Ia) 2024igg, another ``super-Chandrasekhar'' (or 03fg-like) SN whose strong C II $\lambda6580$ feature was initially misidentified as H$α$, thereby constraining its progenitor system, explosion parameters, and physical scenario. SN 2024igg shows many characteristics in common with other 03fg-like objects, such as high ultraviolet flux, slowly declining light curves ($Δm_{15}(B)=0.90\pm0.08$ mag), low expansion velocities, along with strong and persistent C II absorption. Meanwhile, this SN exhibits some remarkable properties within this subgroup, including a moderately low optical luminosity ($M_{\rm max}(B)=-18.99\pm0.15$ mag), a short rise time less than 18.5 days, and strong C II $\lambda6580$. The bolometric analysis yields a $^{56}$Ni mass of $M_{\rm Ni}=0.547\pm0.082$ $M_{\rm \odot}$ and an ejecta mass of $1.54^{+0.22}_{-0.19}$ $M_{\rm \odot}$, marginally exceeding the Chandrasekhar mass. Our TARDIS result indicates that most of the features in the earliest spectrum could be attributed to C II, which is consistent with a model where a supernova explodes within a carbon-rich circumstellar medium (CSM). The CSM interaction would produce a density peak in the ejecta, offering a natural explanation for the slowly evolving line velocities near $-$8000 km s$^{-1}$. The CSM may stem from the debris of a secondary white dwarf in a white-dwarf merger or the envelope of an asymptotic giant branch star. Combined with the unshifted forbidden lines in the spectrum taken at $t\approx\ +$135 days, we suggest that SN 2024igg comes from a symmetric explosion on a secular timescale after the merger.

Figures (8)

• Figure 1: TNT $12' \times 12'$ color-composite ($B/V/r$) image of SN 2024igg and its host galaxy. The reference stars used to calibrate the photometry are marked with circles. The inset shows the zoomed-in region of the SN (indicated with magenta lines) and its host taken on 2024 June 16.
• Figure 2: Left panel: UV and optical light curves of SN 2024igg. The phase is with respect to the $B$-band maximum (MJD = 60452.10; see Section \ref{['subsec:LC_evolution']}). Data in the different filters are shown with different colors, and are shifted vertically for better display. The corresponding instruments are indicated in the legend. Right panel: Power-law fit to the early-time ZTF $r$-band light curve of SN 2024igg. The $r$-band data of SN 2024igg used to fit the power law are shown in red circles. The last nondetections from ZTF $r$ and ATLAS $o$ are plotted as inverted triangles. For comparison, the Clear-band data of SN 2020hvf and the TESS observations of SN 2021zny are shown as salmon diamonds and brown squares, respectively. The flux densities of SN 2024igg are converted from observed magnitudes and are then scaled to the first detection point. For SNe 2020hvf and 2021zny, the phases are shifted to align their explosion time to the last nondetection of SN 2024igg in ZTF $r$, and their flux densities are scaled according to the power-law curve around the epoch of the first detection of SN 2024igg.
• Figure 3: Light-curve and color comparisons of SN 2024igg with a typical normal Type Ia SN 2011fe and four 03fg-like objects: SNe 2009dc, 2020esm, 2020hvf, and 2021zny. All light curves and colors have been corrected for total reddening. The light curves are shown in absolute magnitude. The $I$-band light curves of SN 2020hvf and 2011fe have been transformed to AB magnitude 1983ApJ...266..713O.
• Figure 4: Comparison of the $B$-band light-curve decline rate and the absolute maximum magnitude for a sample of SNe Ia. SN 2024igg is emphasized as a red triangle. Normal SNe Ia from the Pantheon samples 2018ApJ...859..101S are shown as gray dots. The black solid curve represents the best-fit Lira-Phillips relation. The 03fg-like sample, shown as blue triangles, is taken from 2021ApJ...922..205A except for SNe 2009dc 2011MNRAS.412.2735T, 2020esm 2022ApJ...927...78D, 2020hvf 2021ApJ...923L...8J, and 2021zny 2023MNRAS.521.1162D.
• Figure 5: Optical spectral evolution of SN 2024igg and comparison with other SNe Ia. All spectra have been corrected for reddening and host-galaxy redshift. In the left panel, we present the spectral evolution of SN 2024igg from $-13.9$ to $+$134.7 days relative to the $B$-band maximum. Spectra taken with different telescopes are in different colors as indicated in the top legend, and the phase of each spectrum is shown on the right side. Regions of the main telluric absorption are marked by gray vertical bands. The long solid lines indicate Ca II H&K, Fe II$\lambda4924$, S II$\lambda\lambda5454$, 5640, Si II$\lambda6355$, and Ca II$\lambda\lambda\lambda$8498, 8542, 8662 at $-$8000 km s$^{-1}$, as well as C II$\lambda6580$ at $-$10,000 km s$^{-1}$. In the right panel, we show the spectral comparison of SN 2024igg and SNe 2009dc, 2011fe, 2020esm, 2020hvf, and 2021zny at selected epochs ($t \approx\ $$-$15, $-$7, $+$15, and $+$100 days). Spectral features are labeled with short lines in subplots (a), (b), and (c). In subplot (d), rest wavelengths of [Co II] $\lambda5893$, [Ca II] $\lambda7308$, and Ca II$\lambda\lambda\lambda$8498, 8542, 8662 are indicated with gray dashed lines, and those of [Fe II] $\lambda\lambda$7155, 7453 are indicated in lime dotted lines.