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GCN Circular 36207

EP240420a: Nanshan optical counterpart observations
2024-04-21T09:48:59Z (3 months ago)
Edited On
2024-04-21T14:47:07Z (3 months ago)
Dong Xu at NAOC/CAS <>
Edited By
Leo P. Singer at NASA/GSFC <> on behalf of Dong Xu at NAOC/CAS <>
U. Mahmut, A. Iskandar, J.Z. Liu, Y. Zhang (XAO), J. An, Z.P. Zhu, X. Liu, S.Q. Jiang, S.Y. Fu, D. Xu (NAOC) report on behalf of a large collaboration:

We observed the field of the X-ray transient, EP 240420a, detected by EP/WXT (Zhang et al., GCN 36194) using the 1-m Nanshan Optical Wide-field Telescope (NOWT) and the Half-Meter-Telescope (HMT) located at Nanshan, Xinjiang, China.

At series of 300 s and 200 s NOWT frames in the V-band were obtained starting at 16:00:05 UT on 2024-04-20 and the median time is 4.58 hr after the EP trigger. An optical transient (OT) is marginally detected at the counterpart candidate position reported by NOT (An et al., GCN 36202) with V ~ 21.0 mag, calibrated with the PanSTAR field.

At HMT 40 x 60 frames without any filter were obtained and the median time is 4.32 hr after the EP trigger. No credible source is detected at the NOT position, down to a 3-sigma limiting magnitude of r ~ 20.0, calibrated with the PanSTAR field.

The NOT candidate is within the EP/FXT error circle (Han et al., GCN 36203). Comparison of the Nanshan observation with the NOT observation shows that the OT is decaying, being consistent with the behavior of GRB optical afterglows.

We therefore think the OT is the optical counterpart of EP 240420a, which could be a GRB.

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