The Enigmatic Type Icn Supernova 2024abvb Located ~22 kpc from Its Host Galaxy

Abstract

We report multiwavelength observations of the highly offset (~22.4 kpc) SN 2024abvb, the sixth Type Icn supernova to date. With a peak magnitude of Mr = -19.55 +/- 0.11 mag, it is among the most luminous in the existing sample and shows similar colours and decline rates to other SNe Icn. The early optical spectra show a blue continuum with narrow C II features (vFWHM ~ 2000 km s^-1), consistent with a typical wind velocity of a Wolf-Rayet star. The absence of C III lambda 5696 emission at the time of explosion is consistent with a Type Ibn supernova; however, the lack of narrow He lines in both the optical and near-infrared spectra supports a SNe Icn classification. Unlike the majority of SNe Icn, we do not detect broad features in the late-time (7-21 days relative to o-band peak) spectral phase of SN 2024abvb. Semi-analytical modelling of the light curves shows that it can be reproduced by ~2.6 Msun of SN ejecta interacting with ~0.3 Msun of circumstellar material (CSM), both larger than other SNe Icn but consistent with rapidly evolving SNe Ibn. The metallicity at the SN location is significantly lower than the global metallicity of its host galaxy, suggesting that line-driven mass loss required to strip the progenitor of its H and He envelopes was likely inefficient. We estimate the star-formation-rate history at the location of SN 2024abvb and find that it lies at the bottom ~5th percentile among SESNe hosts, inconsistent with a Wolf-Rayet progenitor. Based on its spectral features, local and host environment properties, and host-galaxy offset, we favour an 8-10 Msun star stripped by a compact companion as the progenitor, with a sufficient runaway velocity to reach the observed offset.

Figures (18)

• Figure 1: The left panel is a $15\hbox{$^{\prime}$}\times 15\hbox{$^{\prime}$}$ image of SN 2024abvb constructed using multi $Bgr$-band Swope images that are mapped to the blue, green, and red channels of the image, respectively. The right panel is a $2\hbox{$^{\prime}$}\times 2\hbox{$^{\prime}$}$ cutout with the position of SN 2024abvb marked. The arrow indicates the host galaxy PSO J011055.760-054416.73 with an apparent magnitude of $m_{AB}=18.27$ mag Flewelling2020.
• Figure 2: The UV, optical, and NIR light curves of SN 2024abvb. Upper limits are denoted with hollow triangles. The grey solid line indicates the time of discovery (MJD $60636.3$), which is $~3$ days after the estimated time of explosion (MJD $60633.4$). The spectral epochs with optical (NIR) spectra are highlighted with solid blue (dashed green) lines. The phase is given relative to the $o$-band maximum (MJD $60645.3$). To guide the eye, shaded interpolation curves are overlaid for each filter band, indicating the Gaussian process fits within $1\sigma$.
• Figure 3: Left: The host-galaxy stellar mass versus star formation rate (SFR) for SESNe and SNe IIn. SN 2024abvb ($\log_{10}(\text{M}_{*}/\text{M}_{\odot}) = 9.66^{+0.12}_{-0.12}$) is denoted with the pink star. The SNe IIn sample (taken from Schulze2021) is shown as grey pentagons, the SNe Ibn (taken from Farias et al. in prep.) are shown as green squares, and the SNe Icn are shown as orange circles. Using BlastJones2024_Blast, we also compare SN 2024abvb to SN 2023emq (purple triangle) and SN 2023xgo (blue triangle), the two transitional SNe Ibn/Icn candidates found in the literature. Right: We compare the host-galaxy stellar mass and the Specific Star-Formation Rate (sSFR) for SESNe and SNe IIn.
• Figure 4: Histograms and scatter plots of the host galaxy properties of interacting SESNe against projected distance. SNe Ibn (green squares, 44) are collected from Farias et al. (in prep.), Sanders2013 and Dong2025, while the SNe Icn (orange circles, 5), transitional SNe Ibn/Icn candidates (purple and blue triangles), and SN 2024abvb (pink star) data are from BlastJones2024_Blast. The markers are the same across all three plots. The vertical uncertainty for SN 2024abvb is smaller than the plotted star, as the projected distance is derived from a distance modulus with a 0.1 mag uncertainty. The top histograms across the three plots show the number (N) of SNe per stellar mass, SFR, or stellar metallicity, respectively. Left: host-galaxy stellar mass vs. projected distance. Middle: SFR vs. projected distance. Right: stellar metallicity vs. projected distance. SN 2024abvb has the largest projected offset amongst the SNe Icn and transitional SNe Ibn/Icn candidates, while its host-galaxy properties are broadly consistent with the SESNe population.
• Figure 5: $r$-band (left) and $g$-band (right) light-curve comparison of SN 2024abvb with a similar supernovae. Note that since the explosion time for SN 2006jc is not well-constrained, we align the estimated maximum-light epoch from Pastorello2007 to the maximum of the shaded SNe Ibn template from Hosseinzadeh2017. SN 2024abvb exhibits a rapid rise in its early evolution and is among the brightest sources.