TITLE: GCN CIRCULAR NUMBER: 37938 SUBJECT: EP 241026b: LBT possible optical counterpart (brightening source) DATE: 24/10/29 11:47:25 GMT FROM: Daniele B. Malesani at IMAPP / Radboud University Andrea Rossi (INAF/OAS), Daniele B. Malesani (DAWN/NBI and Radboud Univ.), E. Maiorano (INAF/OAS), report on behalf of a larger collaboration: We performed a second observation of the field of the fast X-ray transient EP241026b (Lian et al., GCN 37902), using the Large Binocular Telescope (LBT), equipped with the LBC instrument. Observations were carried out again in the SDSS r filter, and consisted of 6 exposures by 120 s each. The observation mid time was October 28.244 UT (1.48 days after the EP trigger). Digital image subtraction was carried out with HOTPANTS (Becker, 2015) between this data set and our earlier observation (Malesani et al., GCN 37907). In the difference image, a very obvious transient is detected within the EP/WXT error circle, and is located at coordinates (J2000): RA = 03:45:37.55 Dec = +41:01:51.9 This transient is observed to brighten by about 1.9 mag among the two observations, and has a magnitude r = 21.6 (AB) in our second (brighter) epoch. No source is visible at this position in the Pan-STARRS images of this field. While this behavior is unusual for GRB afterglows, brightening episodes have been noticed recently in a number of EP-detected transients (e.g., EP240414a: Srivastav et al., arXiv:2409.19070, van Dalen et al., arXiv:2409.19056; EP 241021a: Quirola-Vasquez et al., GCN 37930). We also note that this position is only 10 deg off the Galactic plane, so a Galactic origin cannot be excluded. No other transient source is detected in the difference image, down to a limiting magnitude r = 25.1 (AB). We acknowledge excellent support from the LBTO and LBT-INAF staff, particularly O. Kuhn, R. Ansaldi, E. Marini and F. Cusano.