Swift X-ray non-detection of IRAS 21204+4913 close to its optical-outburst maximum

ATel #17546; Nicolas Grosso (Laboratoire d'Astrophysique de Marseille), Joel Kastner (Rochester Institute of Technology), Kenji Hamaguchi (CRESST NASA/GSFC and UMBC), Michael Richmond (Rochester Institute of Technology).
on 16 Dec 2025; 20:22 UT
Credential Certification: Nicolas Grosso (nicolas.grosso@lam.fr)

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The optical transient TCP J21220926+4926242 was recently announced by Kochanek et al. (ATel #17519) as the counterpart of a star in optical outburst since 2025 Nov 25, and displaying an FUor-like absorption-dominated spectrum. We have identified this object with IRAS 21204+4913, located at the head of a Bok globule (TGU 541 P38; Dobashi et al. 2005, PASJ, 57, S1). We have found that this young stellar object (YSO) is a flat-spectrum source, indicating its possible status as transitioning between an evolved protostar and a classical T Tauri star (CTTS). On the basis of (H-Ks, J-H) colors, we have identified the main component (2MASS J21220568+4926235) of a nearby (30"-West) close (1.1") binary system as a CTTS. From Gaia EDR3, this CTTS has a photogeometric distance of 500 pc (482-522 pc; Bailer-Jones et al. 2021, AJ, 161, 147), and likely resides within or just behind the same Bok globule, suggesting the distance to IRAS 21204+4913 is also ~500 pc.

We report the result of a Target-of-Opportunity observation of IRAS 21204+4913 obtained with the Neil Gehrels Swift Observatory and executed on 2025 Dec 10 (Sequence 03000208002) with an exposure of 4466 s (on-axis). This X-ray observation was obtained close (less than ~4 days) to its optical-outburst maximum, as the ASAS-SN photometry (Shappee et al. 2014, ApJ, 788, 48; Kochanek et al. 2017, PASP, 129, 104502) suggests g~17.0 mag for pre-outburst and a g-band maximum between 2025 Dec 9.08 (Δg~4.9 mag) and Dec 14.15 (Δg~5.3 mag).

From the Swift-XRT LSXPS Upper limit server (Evans et al. 2023, MNRAS, 518, 174), we obtain in the 0.3-10 keV band an upper limit of 2.4E-3 count/s (99.73%, i.e., 3σ, confidence limits; Kraft et al. 1991, ApJ, 374, 344). Combining this Swift visit with the previous one (Sequence 03000208002) executed on 2025 Nov 27 — during the rise phase (ASAS-SN Δg~4.5 mag) — with an exposure of 1793 s (on-axis), leads to a slightly deeper upper-limit of 1.9E-3 count/s (3σ).

We estimate the optical extinction with A_v=12[(H-K)-(H-K)₀] (Wilking et al. 1989), assuming (H-K)₀~0.9 mag for a such flat-spectrum source (Grosso et al. 1997, Nature, 387, 56). From 2MASS H and Ks, we obtain A_v~8 mag.

Using WebPIMMS (v4.15) with a collisionally-ionized equilibrium model (APEC) with N_H=1.3E22 cm^-2 (Vuong et al. 2003, A&A, 408, 581), Z=0.2, kT=1.72 keV — as we inferred from the emission-measure weighted temperatures of the soft and hard plasma components of V1647 Ori's X-ray SED during early outburst (Grosso et al. 2005, A&A, 438, 159) — we estimate that the upper limit of 1.9E-3 count/s corresponds to an absorbed (unabsorbed) flux in the 0.3-10 keV band of 6.5E-14 (2.0E-13) erg/cm²/s (3σ).

These limits suggest that the X-ray flux of IRAS 21204+4913 during its early-eruption stage is somewhat below the absorbed-flux levels observed for V1647 Ori at similar (early) stages of its 2003-2004 eruption (i.e., in the range ~5.0E-14 to ~2.0E-13 erg/cm²/s; Kastner et al. 2004, Nature, 430, 429; Kastner et al. 2006, ApJ, 648, L43). Given the similar pre-outburst mid-IR (IRAS 12 & 25 micron) fluxes and similar optical-outburst amplitudes of both sources, this suggests that the X-ray activity (L_X/L_bol) of IRAS 21204+4913 at early stages of its eruption is weaker than that measured for V1647 Ori early in the latter's eruption. For the distance of 500 pc, the unabsorbed L_X of IRAS 21204+4913 is lower than 6.0E30 erg/s, which is about the typical L_X of a ~1.3 solar-mass YSO as observed in the Taurus molecular cloud (Güdel et al. 2007, A&A, 468, 353).

We thank the Neil Gehrels Swift Observatory PI Brad Cenko, Science Operation and Flight Operations for carrying out this observation.