GCN Circular 36714

N. Pankov (HSE, IKI), A. Pozanenko (IKI), R. Ya. Inasaridze (AbAO) report on behalf of GRB-IKI-FuN collaboration:

We observed the refined error circle of EP240618a (Sun et al., GCN 36690
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) with AS-32 telescope of Abastumani Observatory (AbAO) equipped with CCD photometer and the R-filter to search for an optical counterpart. The observations  were taken on dates 2024-06-18 and 2024-06-19. The first series is consist of 66x60 sec images, while latter one is 120x60 sec. Using image differencing, we find no new source within the localization circle or any brightness excess of catalog sources. The obtained results are consistent with observations by other telescopes (Wu et al., GCN 36701
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; Adami et al., GCN 36702
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; Tinyanont et al., GCN 36710
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; Akl et al., GCN 36712
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). Preliminary upper limits are following:

Date       UT start  t-T0     Exp.   Filter   OT     Err.  UL(3 sigma)
                   (mid, days)  (s)
2024-06-18 18:18:51  0.524398 64x60  R        n/d    n/d   19.6
2024-06-19 21:45:02  1.667581 120x60 R        n/d    n/d   20.1

The difference image has an upper limit of 19.9 mag. The magnitudes were calibrated using nearby stars from USNO-B1.0 (R2 magnitude) catalog. The magnitudes are not corrected for the Galactic extinction towards EP240618a, which is A(V) = 0.4294 mag (S&F 2011).