SN 2024afav: A Superluminous Supernova with Multiple Light Curve Bumps and Spectroscopic Signatures of Circumstellar Interaction

TL;DR

SN 2024afav is a hydrogen-poor SLSN-I with an unusually multi-peaked light curve. The authors conducted an extensive optical/NIR spectroscopic campaign from $-14$ to $+160$ d, revealing rare features such as narrow blueshifted H$\alpha$ absorption, persistent He I lines, early [O III] emission, and the [O II] + [Ca II] 7300 Å complex, all of which point to post-peak CSM interaction with a hydrogen-rich shell. The timing of these features relative to the light-curve bumps supports a scenario in which ejecta-CSM interaction (forward and reverse shocks) excites and ionizes the outer ejecta, sustaining helium and driving additional luminosity. Comparisons to other bumpy SLSNe-I suggest this SN belongs to a rare sub-group where CSM interaction plays a significant role in shaping both the light curve and the spectrum, with implications for progenitor mass-loss histories and explosion physics in the LSST era.

Abstract

We present a comprehensive optical and near-infrared spectroscopic study of SN 2024afav - a hydrogen-poor superluminous supernova (SLSN-I) that peaks at $\approx$ -20.7 mag and exhibits an unusual multi-bumped light curve. Our spectroscopic observations, spanning phases of -14 to +160 d, reveal several unusual features: (i) a narrow (1,800 km s$^{-1}$) and blueshifted (11,000 km s$^{-1}$) absorption from H$α$ starting at +20 d; (ii) persistent optical and NIR He I lines at all available phases, showing double absorption structure in NIR spectra at +23 d, with a high velocity component at a similar velocity to H$α$; (iii) early appearance of nebular [O III] emission starting at $\approx$ +50 d; and (iv) strong [O II] + [Ca II] 7300 Å emission complex starting at $\approx$ +110 d. These unusual features, and their onset at the time of the light curve bumps, provide compelling evidence of circumstellar interaction between the SN ejecta and a nearby hydrogen-rich shell, as well as the presence of helium in both the outer layers of the progenitor star and in the circumstellar medium. A comparison of SN 2024afav to other SLSNe-I showing bumpy light curves and similar spectral properties (PTF10hgi, SN 2017egm, SN 2019hge), points to a rare sub-group of SLSNe-I in which CSM interaction provides an important modulation to the energy input.

Figures (8)

• Figure 1: Optical/UV light curves of SN 2024afav with magnitudes in the AB system and corrected for Galactic Extinction. The solid vertical ticks on the top axis mark the epochs of optical (black lines) and near-IR (red lines) spectroscopic observations. The black lines represent optical and the red lines represent epochs of Near-IR spectroscopic observations. The light curve rises from discovery to the first peak in $\approx 46$ d and peaks at an absolute magnitude of $M_{c} \approx -20.7$. Subsequently, it exhibits a second peak of comparable brightness $\approx 38$ d later, before decaying sharply and displaying multiple subsequent undulations over the remainder of its evolution in all bands. The $g-r$ (green) and $c-o$ (blue) colors show an overall evolution from blue to red, but with changes to bluer colors around the light curve peaks.
• Figure 3: The pseudo-bolometric light curve (top), photospheric temperature (middle), and photospheric radius (bottom) of SN 2024afav. The grey-shaded regions represent the range of parameters for quantities in the SLSN-I population, as reported by 2024MNRAS.535..471G. The pseudo-bolometric light curve exhibits multiple bumps (marked by vertical lines). The peak luminosity and width of each peak are provided in Table \ref{['tab:peaklist']}. The temperature and radius fluctuations closely follow the peaks.
• Figure 4: Left: Optical spectra of SN 2024afav covering phases of $-14$ d to +160 d. The early-time spectra are blue, with characteristic O2 features. In the post-peak phases we observe other common SLSNe-I features of Fe2, Fe3, Na1D, Si2 and O1, along with unusual narrow features such as [O3] emission and H$\alpha$ absorption. The $\gtrsim +112$ d spectra show nebular NIR Ca2 triplet and [Ca2] + [O2] complex. The colored spectra are smoothed using the Savitzky–Golay filter 2018JPhCS1141a2151S. Right: Comparison of SN 2024afav spectra to SN 2017egm, PTF10hgi, and median SLSNe-I spectra 2025arXiv250321874A at several phases. SN 2024afav shows remarkable similarity to SN 2017egm, PTF10hgi and SN 2019hge including the unusual [O3] and [Ca2] + [O2] complex emission features. The narrow H$\alpha$ feature in the SN 2024afav spectra is at a similar velocity and width to H$\alpha$ in PTF10hgi 2020ApJ...902L...8Y. A detailed identification of all features in the post-peak phase is provided in Figure \ref{['fig:specident']}.
• Figure 5: Detailed identification of spectral features in the optical spectra at $+51$ and $+112$ d. The majority of the absorption features are marked above the spectra using a blueshifted velocity of 7,500 km s$^{-1}$, while the Fe2, Fe3 and Si2 are at 5,500 km s$^{-1}$. The SN emission features are marked below the spectra at their rest frame wavelengths. The host galaxy emission lines are marked with vertical gray dotted lines. The inset shows the photospheric velocity evolution estimated using relatively isolated features of Si2 $\lambda 6355$ Å, O1 $\lambda 6158$ Å, and O1 $\lambda 7774$ Å.
• Figure 6: Top: Evolution of the unusual spectral features [O3] ( left), H$\alpha$ ( middle) and the [O2] + [Ca2] complex ( right). The [O3] feature exhibits contamination from Fe2 in early spectra and displays a shift towards [O3] over time. The H$\alpha$ feature (green dashed line) is narrow ($1,800$ km s$^{-1}$) and remains at a constant blueshift velocity of $\approx 11,000$ km s$^{-1}$. The [O2] + [Ca2] complex appears at $\approx +112$ d and becomes stronger over time. This feature is centered on [O2] implying the stronger presence of [O2] compared to [Ca2]. Telluric absorption is masked in the middle and right panels. Bottom: Line fits to the [O3] $\lambda \lambda 4969, 5007$ Å ( left) and H$\alpha$ ( right) profiles in the MIKE echelle spectrum at $+53$ d. The fitted profile shows a clear presence of [O3] and a contribution from Fe2 $\lambda 4924$ Å. The H$\alpha$ profile exhibits asymmetric absorption, which is due to emission from [O1] $\lambda 6300$ Å.