Gaia24ccy: An outburst followed the footsteps of its predecessor

Abstract

Accretion-driven outbursts in young stellar objects remain poorly understood, largely limited by a statistically small sample of closely followed-up events. This underscores the importance of a thorough exploration of each outbursting object. We studied a peculiar outbursting system, Gaia24ccy, which exhibited two ∆g ∼ 3.8 mag outbursts in 2019 and 2024. The system consists of two unresolved, nearly identical, and rapidly rotating young stars: Gaia24ccy A (1.1419 days) and Gaia24ccy B (1.7898 days). Periodogram analyses just before the onset of the outbursts suggest Gaia24ccy B to be the outbursting component. Unlike any previously known EXor sources, the two outburst profiles show very similar evolution: both rose at the same rate for the first 15 days, followed by multiple "subbursts" on timescales of 10−20 days. The 2019 outburst lasted 145─255 days, while the 2024 outburst persisted for 367 days. We infer the unstable region to lie at rtrigger ≃ 0.019─0.047 au (∼5─12.3R⋆). The accreted mass per event, Macc ∼ 10−5 M⊙, can be provided by a compact inner-disk reservoir. The photometric rise and decay timescales, together with the mid-infrared (MIR) color evolution, favor a thermal─viscous trigger in a hot inner disk, while the presence of rich emission-line spectra indicates concurrent magnetospheric compression—together forming a hybrid picture. Finally, we explain the reddening of the MIR color observed during the outburst as a consequence of the competing emission from the viscous disk and the photosphere.