The nature of ASASSN-24fw’s occultation: modelling the event as dimming by optically thick rings around a substellar companion

ABSTRACT ASASSN-24fw, monitored by the All-Sky Automated Survey for SuperNovae, underwent a rapid and deep dimming event beginning in late 2024 and lasting until 2025 June. The pre-dimming spectral energy distribution indicates that ASASSN-24fw is an F-type main-sequence star with a persistent infrared excess, corresponding to a fractional luminosity of about 10 per cent. Public survey light curves show that the star is otherwise photometrically stable, implying that the dimming was caused by an external occulter. While long-duration stellar dimming events have become increasingly common in recent years, ASASSN-24fw is distinguished by a pronounced flat-bottomed light curve lasting nearly 200 d. We analyse this event using available photometric and spectroscopic data obtained during the dimming phase. Two independent light-curve models are applied. The first characterizes the multiple phases of the ingress structure, while the second model suggests that the occulter is a gas-giant companion, likely a brown dwarf, with a minimum mass of 3.42 M$_{\rm {J}}$ and an extended ring system with a radial extent of approximately 0.17 au. Near-infrared spectra obtained during dimming show enhanced infrared excess and are consistent with an M8-type brown dwarf companion. Optical spectra reveal variable H$_{\alpha }$ emission during the event. Post-dimming spectra show no evidence for ongoing accretion, suggesting that the H$\alpha$ variability is associated with the occulter rather than the host star. We conclude that ASASSN-24fw represents a rare and unusual system, and that further observations are required to better constrain its stellar properties, circumstellar environment, and evolutionary history.