GCN Circular 36144

Subject
EP 240416a: Xinglong and NOT decaying optical counterpart
Event
EP240416a
Date
2024-04-17T07:45:53Z (2 years ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
J. An, X. Liu, S.Q. Jiang, Z.P. Zhu, J. Zheng, S. Liu, J.J. Jin, S.Y. Fu, T.H. Lu, Z. Fan, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI) report on behalf of a large collaboration:

We observed the field of the X-ray transient, EP 240416a, detected by EP/WXT (Cheng et al., GCN 36138Loading...
 
 ) using the 2.16-m telescope located at Xinglong, Hebei, China. We obtained 5 x 360 s R-band frames with a median time of 2024-04-16T17:18:06, i.e., 14.6 hr after the EP trigger.

The stacked R-band image has a limiting magnitude of R ~ 22 mag, calibrated with the PanSTAR field. Inspection of the whole EP/WXT 3 arcmin error circle produces a few low S/N optical counterpart candidates.

We subsequently performed r-band photometry at the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera, and obtained 3 x 300 s frames in the Sloan r-band with a median time of 2024-04-17T01:00:00, i.e., 22.3 hr after the EP trigger.

The stacked r-band image has a limiting magnitude of r ~ 23.5 mag, calibrated with the PanSTAR field. Inspection of the whole EP/WXT 3 arcmin error circle also produces a few optical counterpart candidates.

The candidates from the two epochs are corssmatched in position and only one single optical transient (OT) is left. It is localized at

R.A. = 13:32:34.51 (J2000)
Dec. = -13:37:48.88 (J2000)

with an uncertainty of radius ~ 0.3 arcsec, being consistent with the candidate reported by Kinder (Chen et al., GCN 36139Loading...
 
 ). This OT has m(R) = 21.8 +/- 0.4 (Vega) in the Xinglong image and decayed to m(r) = 22.66 +/- 0.18 (AB) in the NOT image, calibrated with the PanSTARRS field. The decay rate is consistent with that for typical GRB optical afterglows.

Therefore, we suggest that the OT is very likely the optical counterpart of EP 240416a, which is likely a GRB event.