TITLE: GCN CIRCULAR NUMBER: 40804 SUBJECT: GRB 250617B: Calapai Observatory, Massa S. Giorgio (Messina), optical observation. DATE: 25/06/21 21:08:57 GMT FROM: Giovanni Calapai at Calapai Astronomical Observatory, Massa S. Giorgio, Messina, Italy giovannicalapai@tiscali.it
Giovanni Calapai at Calapai Astronomical Observatory, Massa S. Giorgio, (Messina) Italy Member of: GRB/UAI Gamma Ray Burst Section of Unione Astrofili Italiani.
Report:
We observed the field of GRB 250617B detected by Swift/BAT (Page et al. GCN 40760) with the 11 inches Schmidt-Cassegrain (Celestron 11) telescope F/D=6,3.
The observations were started at 2025-06-17 23:10:55 UT (approximately 2.15 hours after burst) stacking a two consecutive sets of unfiltered CCD image. The observations were carried out with visibility disturbed by passing clouds.
The OT was detected at the following position:
RA (J2000.0) 22h 11m 35.21s +/- 0.02 Decl. (J2000.0) +32° 43' 57.6" +/- 1.1
The results of our photometry are:
Observation Mid-Time T-T0 (hr) Exposure Filter Mag. S/N Mag. err. 2025-06-18 00:05:21 UT 3.06 90x60s CR 19.24 9.1 +/-0.17 2025-06-18 01:47:28 UT 4.76 88x60s CR 20.75 3.4 +/-0.30
Given the low S/N for the OT of second set, we cannot completely rule out it being due to background fluctuation.
Along the span of our observations and within our uncertainties, the afterglow evolves in agreement with a power-law decay index of ~0.89.
Magnitude was calibrated with the nearby PanSTARRS stars converted using Lupton (2005) equations. No correction for galactic dust extinction was applied.
Our observations are consistent with other already reported Moskvitin et al. (GCN 40761); Jelinek et al. (GCN 40762); Kumar et al. (GCN 40768); Lipunov et al. (GCN 40769); Grossan et al. (GCN 40771); Ma et al. (GCN 40772); Williams et al. (GCN 40773); Schneider et al. (GCN 40774); Antier et al. (GCN 40775); Corcoran et al. (GCN 40776); Turpin et al. (GCN 40777); Siegel et al. (GCN 40778); Pankov et al. (GCN 40783); Gendreau-Distler et al. (GCN 40789).
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