GCN Circular 43391

Subject

EP260110a: NOT and GTC Optical and Near-Infrared Observations

Event

EP260110a

Date

2026-01-13T16:52:27Z (a day ago)

From

Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>

Via

Web form

G. Corcoran (UCD), P. G. Jonker (Radboud), J. Sanchez-Sierras (Radboud), A. Martin-Carrillo (UCD), J. A. Quirola-Vasquez (Radboud), A. van Hoof (Radboud), D. Mata Sanchez (IAC), D. Slumstrup (GTC), N. Castro Rodríguez (GTC), V. Vuolteenaho (NOT and Oulu Univ.) report on behalf of a larger collaboration:

We observed the field of the fast X-ray transient EP260110a (Zhang et al., GCNs 43363

, ) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera as well as in the near infrared with the Gran Telescopio Canarias (GTC) equipped with the EMIR instrument. Our observations consisted of two epochs of 3 x 300 s in SDSS-r band and 5 x 200 s in SDSS-z, taken from the NOT, and a single epoch in H with a total exposure time of 1280 s taken from the GTC.

Performing image subtraction with PyZOGY on our r-band stacked images and visually comparing the other bands to a stack of archival r-band CFHT images we do not detect any new sources within the EP/FXT uncertainty region in any of our images. We obtain the following upper limits:

Mid time (UT)	Time since trigger (hr)	Band	3 sigma UL (AB)
2026-01-11 01:46:32	13.80	r	23.5
2026-01-11 02:04:27	14.10	z	22.3
2026-01-11 06:35:44	19.02	H	23.3
2026-01-12 03:03:04	39.08	r	24.2
2026-01-12 03:20:59	39.38	z	22.7

The r and z band upper limits were calibrated using nearby stars from the Pan-STARRS catalog while the 2MASS catalog was used for the H-band observations and the photometry is not corrected for Galactic extinction. Our upper limits are consistent with Perez-Garcia et al. (GCN 43364

), Kang et al. (GCN ), Antier et al. (GCN ), Zheng et al. (GCN ), Lipunov et al. (GCN ), Li et al. (GCN ), and Zhu et al. (GCN ).

We note the presence of two faint sources within the EP/FXT uncertainty region in both our r- and H-band images, which are also seen in the archival CFHT imagery. While no residual is visible at their position in the r-band subtraction, due to the lack of sufficiently deep H-band templates we cannot test for NIR variability at this time.