TITLE: GCN CIRCULAR NUMBER: 45228 SUBJECT: GRB 260727A / SVOM sb26072702: RAPAS optical non-detection DATE: 26/07/27 15:20:56 GMT FROM: Thierry Midavaine at SAF - RAPAS <thierrymidavaine@sfr.fr> Pierre-Yves Lechapelain and Thierry Midavaine report on behalf of the RAPAS network: We observed the field of GRB 260727A (SVOM/ECLAIRs trigger sb26072701; Li et al., GCN 45220) in response to the overlapping SVOM broker alert sb26072702 (RA = 331.626 deg, Dec = +74.549 deg, J2000, 2.5 arcmin error radius). Observations were obtained with the RAPAS 0.25 m f/3 robotic telescope and IMX533 sensor at Observatoire de Bretagne Sud (Y80), using RAPAS filters matched to the Gaia G, Gbp, and Grp bands. Data were reduced with the STDPipe/stdweb pipeline; photometry and astrometry are calibrated to Gaia DR3 with spatially varying zero points and color terms. Observations began at 2026-07-27 00:48 UTC (T_GRB + 26 min, with T_GRB = 2026-07-27 00:22:02 UTC from GCN 45220) and continued in a G / Gbp / Grp cycle with 30 s exposures until approximately 02:44 UTC (T_GRB + 2.4 hr). We detect no optical afterglow at the sb26072702 position. Forced aperture photometry at RA = 331.626 deg, Dec = +74.549 deg shows no significant excess on the earliest G exposure (flux S/N = 1.8) or on the deep G co-add (flux S/N = 3.6, below our 5σ detection threshold). No Gaia-uncatalogued source is found within the 2.5 arcmin SVOM error circle. A comparison to Gaia DR3 in a 12 arcmin (0.2 deg) radius around the alert on the first G frame yields five marginal transient candidates at 7–11 arcmin offset; none are confirmed in deeper co-added images. Searches over the full unvignetted RAPAS field (~0.62 deg radius) reveal no plausible afterglow at the SVOM coordinates. We report 5σ limiting magnitudes (Vega, Gaia-calibrated) from the global detection limits of each reduction: Earliest epoch per band (single frame or shortest stack in the sequence): | T_mid - T_GRB | Exp | Band | Limit (5σ) | Note | |---------------|------------|------|------------|-------------| | +25 min | 1 × 30 s | G | >18.3 | | | +31 min | 2 × 30 s | G_BP | >18.6 |2-frame stack| | +36 min | 2 × 30 s | G_RP | >17.4 |2-frame stack| Co-added images from the ToO night: | Band | Co-add | Limit (5σ) | |------|-------------|------------| | G | 17 × 30 s | >20.1 | | G_BP | 18 × 30 s | >19.9 | | G_RP | 14 × 30 s | >18.6 | Mid-exposure times correspond to the first useful epoch in each band and to the weighted mean epoch of each co-added stack, respectively. Acknowledgements : RAPAS ( https://gemini.obspm.fr/20220101-rapas/ ) is a ProAm collaboration created by T. Midavaine (SAF), W. Thuillot (LTE, Obs. de Paris-PSL) and M. Dennefeld (IAP/CNRS and Sorbonne Univ.) and funded by the Paris Observatory under API ProAm of the Scientific Council. It aims at homogenizing observing procedures and filters, and is delivering to a network of french amateur observatories a set of 3 filters which have been chosen to meet the Gaia G, Gbp and Grp spectral bands. This network produces data on astrophysical alerts delivered by various Alert systems (Gaia Alerts, Atlas, ZTF, etc…) and collected by Astro-COLIBRI ( https://astro-colibri.science ). It is similarly progressing towards an homogeneous spectroscopic equipment to deliver SED. Paris Observatory PADC which is on the way to archive all the FITS files with attached processing pipeline. View this GCN Circular online at https://gcn.nasa.gov/circulars/45228. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...