| Abstract: We present our study of a long-period stellar dimming event, ASASSN-24fw, observed by the All-Sky Automated Survey for SuperNovae. ASASSN-24fw is a main-sequence F-type star that underwent a rapid fading event beginning in late 2024 and lasting until June 2025, with a total duration of approximately 275 days. Prior to dimming, its spectral energy distribution indicates a persistent infrared excess with a fractional luminosity of about 10%. We modeled this excess using a two-component blackbody model, finding effective temperatures of ~1100 K for the warm component and ~400 K for the cold component. Publicly available survey light curves show that ASASSN-24fw is otherwise photometrically stable, indicating that the dimming is caused by an external occulter. While long-period dimming events have become more common in recent years, ASASSN-24fw is distinguished by a unique flat-bottomed light curve lasting nearly 200 days. To constrain the properties of the occulter and the host system, we analyzed multi-band photometric data and spectroscopic observations obtained during and after the dimming event. We applied two independent light-curve modeling approaches. The first parametrizes the light curve morphology and reveals multiple ingress phases, suggesting variations in the density of the occulting material. The second model favors an interpretation in which the occulter is a gas giant, likely a brown dwarf, with a minimum mass of 3.42 M_Jup surrounded by an extended ring system with a radius of approximately 0.17 au. Near-infrared spectra obtained during dimming show enhanced infrared excess and are consistent with a late-type object, likely an M8 brown dwarf. Optical spectra taken during dimming exhibit variable Hα emission, while post-dimming spectra show no evidence of accretion, implying that the variability is associated with the occulter rather than the host star. Systems like ASASSN-24fw are rare and provide valuable insight into circumstellar environments and the evolution of complex occulting systems. |
