TITLE: GCN CIRCULAR NUMBER: 37567 SUBJECT: EP240919a: NOT and Gemini optical observations DATE: 24/09/20 10:07:08 GMT FROM: Daniele B. Malesani at IMAPP / Radboud University d.malesani@astro.ru.nl
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; see also Liang et al., GCN 37563), 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.
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