TITLE:   GCN CIRCULAR
NUMBER:  40132
SUBJECT: EP250227a: Further NOT optical observations
DATE:    25/04/10 09:17:11 GMT
FROM:    Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>

D. B. Malesani (DAWN/NBI and Radboud), J. Sanchez-Sierras (Radboud), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD), A. J. Levan (Radboud and Warwick), M. A. Diaz Teodori (NOT and Turku Univ.), report on behalf of a larger collaboration:

We obtained ten 300-s observations in the r band of the field of the fast X-ray transient EP250227a (Wen et al., GCN 39532) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager. The mid time was 2025 Apr 5.09 UT (36.9 days after the EP trigger).

In Malesani et al. (GCN 39533), based on observations taken 22.4 hr after the EP trigger, we noted a possible optical transient (not consistent with the EP/FXT localisation) within the EP/WXT error circle, revealed by image subtraction using the Legacy survey as template. The new observation was obtained with the same instrument, filter, and pixel scale as our early one, in order to provide a better reference. We then carried out again image subtraction using both ISIS (Alard 2000, A&AS, 144, 363) and PyZOGY (Zackay et al. 2016, doi:10.3847/0004-637X/830/1/27).

With the new template, no credible residual is detected at the position of the early optical candidate (Malesani et al., GCN 39533), which we thus consider to be spurious. We set an upper limit r > 23.8 AB to any transient within the EP/WXT error circle, at a time of 22.4 hr after the trigger.

In our April 5 observation, one faint object is detected inside the EP/FXT error circle (at coordinates RA = 13:36:34.67, Dec = -13:49:23.18, magnitude r = 24.52 +- 0.23 AB). Its probability of a chance association with the X-ray localization is large. As a result, no connection to EP250227a can be established.