AstroNote 2025-284

We report the identification and spectroscopic observations of a bright near-IR (NIR) transient found in the regular survey operations of the Wide-field Infrared Transient Explorer (WINTER, Lourie et al. 2020, Frostig et al. 2024). WINTER is a 1 sq. deg. field-of-view camera on the Palomar 1-m telescope and is surveying part of the Galactic plane in the NIR Y, J, and Hs bands at a cadence of two weeks (Frostig et al. 2025). 

WNTR25fhdoi / AT205sqe was first identified as a candidate transient in WINTER data by the WINTER real-time data processing pipeline implemented using mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565) on UT 2025-07-30 with a J-band magnitude of 15.8 +/- 0.1 (AB) at coordinates of RA = 19:14:51.63 Dec = +08:00:06.40

The source has also been independently discovered and reported to the Transient Name Server by the XOSS (http://xjltp.china-vo.org/kats25o021.html) and ATLAS (Tonry et al. 2018) surveys. 

Combining the WINTER lightcurve with the publicly available ATLAS and Zwicky Transient Facility (ZTF, Bellm et al. 2018) lightcurves, we find that this source has been gradually brightening in the optical and NIR bands for at least the last three months. In this timeframe, the source has brightened by 4.5 magnitudes in the r-band from r=21.5 mag to r=17 mag (AB).

We obtained a NIR spectrum of this source with the Triplespec spectrograph (Herter et al. 2008) on the 200-inch Hale telescope on UT 2025-08-08. Our spectrum shows strong, narrow (<500 km/s), emission lines of hydrogen and helium centered at z=0, with the helium lines exhibiting P-cygni profiles. We do not detect any clear, strong features of carbon, nitrogen, or oxygen commonly seen in classical novae in this spectrum.

The slow photometric evolution and the spectroscopic features of AT2025sqe are reminiscent of the slow Galactic transient Gaia 22alz (Nova Velorum 2022, Aydi et al. 2023) - whose slow brightening has been suggested to be a rare observational signature of the reprocessing of X-ray or ultraviolet emission during the “X-ray / UV-flash” phase of a nova outburst (Aydi et al. 2023). 

Further observations to understand the nature of this transient are underway, and multiwavelength observations are encouraged.