Evidence for an EXor - FUor accretion state transition in TCP J21220926+4926242

ATel #17556; Bringfried Stecklum (TLS Tautenburg)
on 23 Dec 2025; 10:45 UT
Credential Certification: Bringfried Stecklum (stecklum@tls-tautenburg.de)

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Kochkina et al. (ATel #17519) reported the outburst of TCP J21220926+4926242, during which a FUor-like, absorption-dominated optical spectrum was observed. Grosso et al. (ATel #17546) identified the source as IRAS 21204+4913, a flat-spectrum young stellar object (YSO), and obtained upper limits on its X-ray emission during the brightening. The source belongs to the Class I/II YSO candidates of Marton et al. (2016) and exhibits long-term optical and infrared variability reported by Gaia (Mowlavi et al. 2021) and NEOWISE (Mei et al. 2023). Strong Hα emission was detected by IPHAS (Barentsen et al. 2014), indicating ongoing accretion activity.

The 0.75-5 µm spectrophotometry obtained by SPHEREx (Bock et al. 2025) does not show ice absorption features, arguing against deep embeddedness or strong foreground obscuration. This is consistent with the moderate extinction of A_V ~ 8 mag derived by Grosso et al. (ATel #17546).

The pre-burst temporal behavior of the source can be characterized using optical and infrared survey data. Light curves retrieved from ATLAS (Tonry et al. 2018) and (NEO)WISE (Wright et al. 2010) were used for this purpose. The W1 and W2 light curves were supplemented by magnitudes (shown as bold symbols) derived from the recent SPHEREx spectrophotometry using the corresponding WISE filter transmission profiles.

The link below provides the corresponding plot. The ATLAS orange-band light curve (420-650 nm; shown in black), spanning 2016-2025, exhibits pronounced variability with an amplitude approaching three magnitudes on different timescales. Hints of quasi-periodic variability may be present. The most recent rapid rise, which began in mid-November 2025, is clearly visible and still ongoing as of MJD 61031.23932 (2025 Dec 22 UTC).

The (NEO)WISE W1 (blue) and W2 (red) light curves show single-exposure photometry to illustrate the variability during the bi-annual visits, each lasting about one day. These data indicate an elevated flux level during the initial WISE mission (2010-2011). Subsequently, the source faded between 2018 and 2020 before entering a renewed brightening phase. The slopes of the W1 and W2 light curves indicate that the source became bluer when getting brighter. (NEO)WISE photometry of a nearby comparison object (2MASS 21222348+4931194) confirms that the early bright state is intrinsic to the target and not an instrumental artifact.

Interestingly, the URAT R-band magnitude of 17.1 mag (epoch 2013.3) is nearly three magnitudes fainter than the photographic magnitudes reported in USNO-A2.0 (14.4 mag; epoch 1952.7) and USNO-B1.0 (14.5 mag; epoch 1975.8). Such a large difference is difficult to attribute solely to photometric systematics, suggesting that the source may have experienced a previous high accretion state.

The short-term photometric variability observed prior to the outburst, together with the blueing during brightening, is characteristic of EXor-like accretion behavior (e.g., Herbig 2007), commonly attributed to magnetospheric accretion. In contrast, the current spectral appearance is FUor-like, and the amplitude of the ongoing rise is likewise typical of FUor outbursts. The recent transition of Hα from emission to pure absorption is remarkable and indicates that the accretion flow has entered a regime in which a dense, optically thick, hot inner disk dominates the optical emission. This provides observational evidence that a YSO displaying EXor-like variability in a low accretion state can transition into a FUor-type outburst state.

Barentsen, G. et al. 2014, MNRAS 444,3230
Bock, J. J. et al. 2025, arXiv:2511.02985
Herbig, G. H. 2007, AJ 133, 2679
Marton, G. et al. 2016, MNRAS 458, 3479
Mei, Y. et al. 2023, ApJS 264, 38
Mowlavi, N. et al. 2021,A&A 648, 44
Tonry, J. L. et al. 2018, PASP 130, 064505
Wright, E. L. et al. 2010, AJ 140, 1868

Optical and IR light curves