GCN Circular 45922
Cheng Cheng (CASSACA, NAOC), Liping Xin (NAOC), Jia-Sheng Huang (CASSACA, NAOC) report:
We observed the field of GRB 261011A (SVOM/ECLAIRs: Huang et al., GCN 45895; Swift/BAT: Cenko et al., GCN 45896) with the 4.1-m SOAR telescope (GHTS red imager) beginning on 2026-10-12 at 06:45:36 UT, i.e. 17.06 h after the Swift/BAT trigger.
We obtained 3 x 180 s exposures in each of the SDSS g, r and z filters. The frames were trimmed, astrometrically solved, sky-subtracted and median-combined onto a common output grid (0.2894"/pixel), so that the three band images are pixel-aligned. Each stacked image is 1000 x 1000 pixels (4.8' x 4.8'), centred on the target coordinates.
We detect the afterglow at
RA, Dec (J2000) = 01:27:35.40, -21:33:37.7
with an uncertainty of 0.15" (the astrometry is tied to Gaia DR3; a cross-check against LS DR11/DECaLS gives a residual scatter of 0.10"). This position lies 0.52" from the Swift/UVOT position reported by Cenko et al. (GCN 45896), fully consistent with its 1.1" error radius, and refines it.
Photometry was carried out with SExtractor in dual mode, using the r-band image as the detection image and measuring MAG_AUTO magnitudes on the g, r and z images with a local background estimate. Magnitudes were calibrated against LS DR11 (DECaLS) stars and are in the AB system:
g = 23.93 +/- 0.14 (phot) +/- 0.08 (zeropoint)
r = 22.75 +/- 0.08 (phot) +/- 0.09 (zeropoint)
z = 22.43 +/- 0.18 (phot) +/- 0.15 (zeropoint)
The first uncertainty is the photometric error and the second is the zeropoint (calibration) uncertainty, which is systematic across the field and therefore should not be treated as an independent error when combining this measurement with others.
The source is unresolved (FWHM = 1.2"), and no host galaxy is detected at this depth; the LS DR11 source at this position (ls_id_dr11 = 48423370429045300) has r = 25.63 +/- 0.30 AB mag.
We thank the SOAR staff for the rapid execution of these observations.