Gaia24ccy: An outburst followed the footsteps of its predecessor

TL;DR

Gaia24ccy presents a rare, tightly spaced YSO binary where Gaia24ccy B experiences two nearly identical outbursts separated by ~5 years, enabling a direct test of accretion-outburst triggers. The authors combine multi-wavelength photometry and spectroscopy to show rapid initial rises, long-lasting decays, and recurring sub-bursts, with optical blueing and MIR reddening indicating a hybrid disk-driven TI process aided by magnetospheric dynamics (DS). The mass budget ($\sim10^{-5}\,M_\odot$ per event) and the inferred inner-disk trigger radius ($r_{\rm trigger} \sim 0.019$–$0.047$ au) support a scenario where episodic inner-disk mass buildup powers discrete bursts, potentially via clump-mediated delivery. The results suggest a nuanced picture where thermally driven disk instabilities dominate the energy release, with magnetospheric modulation shaping spectral diagnostics, and highlight Gaia24ccy as a key testbed for accretion physics in young binaries with potential periodicity.

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 $Δg \sim$ 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.

Figures (12)

• Figure 1: Upper panel: Multiband light curves of Gaia24ccy, shown with magnitude offsets applied for clarity of multiwavelength evolution. Offsets are: ATLAS $o$ (+4.5), ATLAS $c$ (+4), ASAS-SN $V$ (+0.5), ZTF $zr$ (-1.5), ZTF $zg$ (-3.5), AAVSO $B$ (-4), TMMT $U$ (+2.5), TRT-CTIO $U$ (+2.5), TRT-SBO $B$ (-4). The durations of the 2019 and 2024 outbursts are labeled and shaded with a light orange color. Other band light curves of AAVSO, TMMT, TRT-CTIO, and TRT-SBO are not shown for the clarity of presentation. Lower panel: The evolution of 2019 and 2024 outbursts is shown with ASAS-SN $g$ light curves. The start dates for the 2019 and 2024 outbursts are JD = 2458620, and 2460485, respectively. Grey vertical lines at the bottom mark the epochs of HFOSC observations during the 2024 outburst. The photometry from this work, TMMT, LCOGT, and TRT, is available in machine readable format as the data behind the figure.
• Figure 2: The $K$-band AO image of Gaia24ccy with Near-infrared Camera, Generation II (NIRC2) onboard Keck. The image is reproduced with permission from the authors 2019ApJ...878...45B. The two objects with rotation periods of 1.1419 and 1.7898 days are named Gaia24ccy A and Gaia24ccy B, respectively. Without spatially resolved rotation data, the A/B identification of the upper versus lower component remains undetermined.
• Figure 3: The flux-calibrated optical HFOSC spectra of Gaia24ccy are shown. A few prominent spectral lines are marked for reference.
• Figure 4: The accretion rate profiles during the two outbursts of Gaia24ccy B. Grey diamonds cover the 2019 outburst, while green circles cover the 2024 outburst. The 2019 accretion profile is estimated from the ASAS-SN $g$ light curve, while the 2024 profile is estimated from ASAS-SN $g$, TMMT $U$, LCOGT $u$, and AAVSO $B$ light curves. The two halves of the 2024 outburst, before 118 days and after 205 days, are connected with linear interpolation shown in green dots. Grey and green shaded strips highlight a time interval, and the respective texts are the total mass accreted in that time interval. The colored squares and pentagons represent mass-accretion rates estimated from HFOSC spectral lines. The wavelengths of CaII-IRT-I/II/III are 8498, 8542, and 8662 Å, respectively.
• Figure 5: The linear relations between the $F_{U-excess}$ and $F_{B-excess}, F_{g-excess}$ during the peak of the outburst are shown in dark red and green lines with 1-sigma error in light shades (Equation \ref{['equ:Uexcess_gBexcess']}). $F_{i-excess}$ and intercepts are in units of $10^{-13}\ \mathrm{erg\,s^{-1}\,cm^{-2}}$Å$^{-1}$.