GCN Circular 41381

A.J. Castro-Tirado, I. Perez-Garcia, F. J. Aceituno, M. D. Caballero-Garcia, S. Guziy, R. Sanchez-Ramirez, E. Fernandez-Garcia, S.-Y. Wu (IAA-CSIC), Y.-D. Hu (GXU), J. Becerra (IAC), M. Gritsevich (Univ. of Helsinki), A. Sintes (Univ. Illes Balears), J. A. Font (Univ. de Valencia), S. B. Pandey (ARIES), B.-B. Zhang (NJU), J. Flores (CAHA), D. Garcia (GTC, IAC) and A. Romero (GTC), on behalf of a larger collaboration, report:

We initially observed the field of the GRB 250814A detected by both Fermi (Fermi GBM team, GCNC 41357) and Swift (Caputo et al. GCNC 41358) using the 1.5m OSN telescope at Observatorio de Sierra Nevada in Granada (clear filter, starting on Aug 14, 20:30 UT). In addition to this facility, we also used both the 2.2m telescope at the Calar Alto Observatory (z-band, starting at 21:55 UT) and the 10.4 GTC telescope (griz-band, starting on Aug 15, 00:00 UT).

This event was proposed to be temporally and spatially coincident with the subthreshold GW trigger S250814bg (the LVK Collaboration, GCNC 41364), which occurred 3.7 s prior to the Fermi GBM trigger (de Barra et al., GCNC 41378).

At the position of the X-ray afterglow detected by Swift/XRT (Salvaggio et al. GCNC 41379), we identify an optical source in our data (in particular on the GTC i-band image) at coordinates (J2000): RA = 20:25:13.87 Dec = +48:23:56.5 (+/- 0.5"), for which we measure i = 23.3 +/- 0.3, which could be the optical counterpart to GRB250814A. This object is consistent with the proposed NIR candidate (Karambelkar et al. GCNC 41377).

Further optical observations would be needed in order to confirm whether this object is the electromagnetic counterpart to S250814bg.