TITLE: GCN CIRCULAR NUMBER: 40095 SUBJECT: GRB 250404A: Calapai Observatory, Massa S. Giorgio (Messina), optical observations. DATE: 25/04/06 17:44:24 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 250404A detected by Fermi Gamma-ray Burst Monitor (GBM team, GCN 40050) with the 11 inches Schmidt-Cassegrain (Celestron 11) telescope F/D=6,3.

The observations were started at 2025-04-04 22:45 UT (approximately 8.42 hours after burst) stacking a set of unfiltered CCD image. The observations were made with modest visibility conditions: passing clouds and Moon (illumin. 48%, dist. 19°)

The OT was detected at the following position (+/- 0.3 arcsec):

RA (J2000.0) 08h 20m 14.59s Decl. (J2000.0) +35° 31' 41.5"

Photometry was obtained using nearby PanSTARRS stars as follows:

Observation Mid-Time T-T0 (hr) Exposure Filter Mag. Err. 2025-04-04 23:27:00 UT 9.12 82x60s CR 20.16 +/-0.2

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 Jiang et al. (GCN 40052), Perez-Garcia et al. (GCN 40053), Du et al. (GCN 40058), Zhu et al. (GCN 40061), Konno et al. (GCN 40063), Odeh et al. (GCN 40064), Lipunov et al. (GCN 40066), Min-He et al. (GCN 40069), Lipunov et al. (GCN 40070), Perez-Fournon et al. (GCN 40071), Jelinek et al. (GCN 40072), Rakotondrainibe et al. (GCN 40073), Taguchi et al. (GCN 40074), Brivio et al. (GCN 40075), Ruocco (GCN 40076), Martin-Carrillo et al. (GCN 40080), Pavoni et al. (GCN 40083), Li et al. (GCN 40084), Yin et al. (GCN 40085), Pankov et al. (GCN 40086), Chen et al. (GCN 40088), Chen et al. (GCN 40089), Pankov et al. (GCN 40090), Leonini et al. (GCN 40093).

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