Gaia24ccy: An outburst followed the footsteps of its predecessor

TL;DR

Gaia24ccy experienced two similar outbursts in 2019 and 2024, revealing insights into accretion processes in young stellar objects through detailed analysis of its outburst profiles, disk dynamics, and emission features.

Contribution

This study provides the first detailed analysis of a double-outburst in Gaia24ccy, highlighting a hybrid accretion mechanism involving thermal-viscous triggers and magnetospheric effects.

Findings

Outbursts lasted 145-255 and 367 days respectively.

Unstable region located at 0.019-0.047 au from the star.

Accreted mass per event approximately 10^-5 solar masses.

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 $\Delta g \sim$ 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 a very similar evolution: both rose at the same rate for the first 15 days, followed by many 'sub-bursts' on the timescale 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 $r_{\rm trigger} \simeq 0.019-0.047$ au ($\sim5-12.3 R_\star$). The accreted mass per event $M_{\rm acc}\sim10^{-5} M_\odot$ can be provided by a compact inner-disk reservoir. The photometric rise/decay timescales and the mid-infrared color evolution favor a thermal-viscous trigger in a hot inner disk, while the appearance of rich emission-line spectra indicates concurrent magnetospheric compression - together best described by a hybrid picture. Finally, we explain the reddening of the mid-infrared color observed during the outburst as a consequence of the competing emission from the viscous disk and the photosphere.