BACKGROUND:
IRAS 21204+4913 is a flat spectrum SED (Class I/II) YSO with historical variability at the ~2 mag (peak-to-peak) level in the optical and ~1 mag (peak-to-peak) level in the mid-infrared.
In late 2025, the source began a systematic brightening of 4-5 mag in the optical, as reported and documented in the above ATels.
NEW INFORMATION:
We obtained a near-infrared spectrum on 2025-05-08 UT with Keck/NIRES, covering 0.98-2.45 μm.
The observed and de-reddened spectrum are shown below.
The outburst spectrum of IRAS 21204+4913, shown in black below, compares well with that of the median spectrum of 28 bona fide FU Ori sources (Portnoi et al. 2026), shown in blue below.
In combination with the evidence presented in
ATel 17519
and in
Burlak et al. (2026)
for a hot FG-type optical spectrum,
the evidence here for both cool molecules and a pattern of weak NaI, CaI plus strong SiI, MgI in the infrared spectrum,
seems to confirm that the 2025 outburst of IRAS 21204+4913 was indeed a transition to FU Ori type accretion.