SN 2024abvb: a Type Ibn/Icn supernova with evidence of helium and an extreme offset from its host galaxy

Abstract

We present spectroscopic and photometric observations and analysis of SN 2024abvb, a peculiar transitional Type Ibn/Icn supernova located at an unusually large projected distance from its host galaxy (21.5 kpc). SN 2024abvb displays an extended rise time in the $g$- and $o$-bands (10.1 and 10.6 days respectively), followed by a linear decline in all photometric bands. Comparisons with other supernova subclasses show that the photometric and spectroscopic evolution of SN 2024abvb are distinct from Type~Ibn and Type Icn events, with a higher peak $r$-band luminosity and lower blackbody temperatures. Spectra reveal an initial blue continuum and narrow P-Cygni profiles, with C {\sc ii} $λ$5890 dominating in emission, persisting at late phases, and showing a rapid decline in the expansion velocity. Weak He {\sc i} $λ$5876 features are tentatively detected at early times. Analysis of progenitor scenarios rules out thermonuclear origins based on incompatible light curve shapes and spectral signatures. A rare massive star progenitor appears unlikely given the low local star formation rate. The most plausible origin is an ultra-stripped supernova scenario involving a binary system; this best explains the observed separation from the host, the low circumstellar material mass, the fast photometric evolution and the low nickel production, although a discrepancy in model versus observed ejecta mass remains. These results reinforce the classification of SN 2024abvb as a distinctive Type Ibn/Icn event and highlight the diversity of progenitor channels for interacting supernovae.

Figures (14)

• Figure 1: Left Panel:$gri$ image stack from Las Cumbres Observatory (LCO) data (taken on 2024/11/28) with SN and host labelled. A 1 arcmin scale bar has been plotted at the top left of the image, with north oriented up and east to the left. Right Panel:$gri$ image stack from Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys (LIS, Dey_2019) data (taken on 2019/08/08), all markers are the same across both images. Although we see clear spiral arms in the LIS image, the SN position is offset from the host.
• Figure 2: The SciPy and emcee optimised fit for the $c$-band data
• Figure 3: Light curves of SN 2024abvb from AFOSC, ATLAS, Pan-STARRS, LCO, NTT, Swift, NOT, ZTF, REM, TTT, Moravian, BlackGem and GOTO for all available bands from the UV-NIR. The phase is given relative to the $V$-band maximum. The legend indicates which shape and colour is associated with specific bands. Non-detections are shown with downward arrows. The rise times and decline rates for each band can be seen in Table \ref{['tab:rise_times']}. The vertical black lines indicate the epochs when spectra were taken. The $L$- and $q$-bands refer to GOTO and BlackGem bands, respectively. The figure is not extinction corrected. $griz$ are AB magnitudes, as are those of the surveys ATLAS, PS1 and GOTO. $UBVJHK$ are Vega magnitudes.
• Figure 4: Comparison photometry in the $r$-band (absolute magnitude) for SN 2024abvb, the Type Ibn (SN 2010al; 2015MNRAS.449.1921P) and two other Type Icn (SNe 2022ann and 2019hgp; Davis_2023).
• Figure 5: The top panel shows the bolometric light curve. The second panel shows the estimated temperature derived from the black-body fits. The third panel shows the photospheric radius. The bottom panel shows the contribution of each part of the electromagnetic spectrum over time (UV: $uvw2$, $uvm2$, $uvw1$. Optical: $u$, $B$, $g$, $V$, $c$, $r$, $o$, $i$, $z$. NIR: $J$, $H$, $K$). Each panel, besides the last, shows a comparison between SNe 2024abvb and 2019hgp 2022Natur.601..201G. SN 2019hgp is the closest photometric analogue, therefore we use this for comparison.