GCN Circular 37567

D. B. Malesani (DAWN/NBI and Radboud Univ.), A. Martin-Carrillo (UCD), J. An (NAOC/CAS), P. G. Jonker (Radboud Univ.), A. J. Levan (Radboud Univ. and Warwick Univ.), J. Quirola-Vasquez (Radboud Univ.), F. E. Bauer (PUC), D. Xu (NAOC/CAS), H. Sun (NAOC, CAS), report on behalf of the EP collaboration:

We observed the field of EP240919a (Liang et al., GCN 37561
Not found
 
 
; see also Liang et al., GCN 37563
Not found
 
 
), using the Nordic Optical Telescope (NOT) and the ALFOSC camera. The night conditions were not ideal, with clouds and high humidity, but a total of 3x300 and 5x200 s could be secured in the r and z bands, respectively, at mean epochs of 11.2 and 11.5 hr after the EP trigger.

No significant new sources are visible in either band, compared to archival images from Pan-STARRS and the Legacy surveys, down to limiting mangitudes r = 22.5 and z = 22 (AB). A few objects are located within the FXT localization region, but have magnitudes consistent with their archival values.

We note however the presence of a marginally significant source, visible only in the z band, detected at the 1.5-sigma level. It has approximate coordinates (1" error, J2000):

RA = 22:17:06.49
Dec = -09:44:03.1

Follow-up observations were conducted with the Gemini-North telescope equipped with the GMOS-N instrument. Observations began on 2024-09-20 at 08:47 UT (18.0 hr after the EP trigger). In a preliminary analysis, the object is not detected to a limit z > 23.7 (AB). This limit would imply a rather steep decay (power-law index >~ 2.5). Given the low significance and lack of independent confirmation, we do not consider the NOT source as a convincing candidate.