TITLE:   GCN CIRCULAR
NUMBER:  36331
SUBJECT: LIGO/Virgo/KAGRA S240422ed: NOT optical observations of EP240426a
DATE:    24/04/27 08:06:42 GMT
FROM:    Dong Xu at NAOC/CAS <dxu@nao.cas.cn>

X. Liu, Z.P. Zhu, S.Q. Jiang, J. An, S.Y. Fu, T.H. Lu, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), L. Hu (CMU), J. Terwel (NOT) report:

We observed the field of EP240426a, suggested as an X-ray counterpart candidate (Sun et ao., GCN 36313) of LIGO/Virgo/KAGRA S240422ed (LVK, GCN 36236), using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started at 20:40:24 UT on 2024-04-26 and 9x150 s frames were obtained in the Sloan z-band in a seeing ~ 0.8".

Image subtraction of the NOT stacked z-band image to the archival PanSTARRS image shows a residual source, which is positionally consistent with the one found at DECam (Hu et al., GCN 36317), very close to the nucleus of the galaxy 2MASX J08072584-2927344. The source has m(z) = 21.3 +/- 0.2 mag (AB), calibrated with the PanSTAR field. The brightness of the source is largely consistent with that by DECam (Hu et al., GCN 36317). Together with analysis of the archival NIR data (Levan et al., GCN 36325) and the radio data (Anumarlapudi et al., GCN 36329), the source is mostly likely due to AGN activity.

Moreover, we note that there is a relatively bright spot at the north-eastern edge of the galaxy in the NOT image, seemingly at the end of a tail of the galaxy. Its coordinates are R.A. = 08:07:26.13 (J2000) and Dec. = -29:27:30.85 (J2000), with m(z) ~ 21.5 mag (AB). Photometry is significantly affected by the contamination of the galaxy. Given the NOT image is deeper and has better resolution than the PanSTAR image that is used as the template, we're inclined to think the spot is due to the difference of images rather than a physical optical transient.