GCN Circular 45647
Subject
GRB 260915A: VLT optical and near-infrared upper limits
Event
Date
2026-09-19T16:40:08Z (10 days ago)
From
M. Garnichey at LUX - Paris Observatory <marine.garnichey@obspm.fr>
Via
Web form
J. An (NAOC), R. Brivio (INAF-OAB), G. Corcoran (UCD), V. D’Elia (ASI/SSDC), M. Garnichey (LUX-Paris Obs.), N. Habeeb (U. Leicester), D. B. Malesani (DAWN/NBI and Radboud), A. Saccardi (CEA/Irfu), B. Schneider (LAM), N. R. Tanvir (U. Leicester), S. D. Vergani (LUX-Paris Obs.), X. Liu, D. Xu (NAOC) report on behalf of the Stargate collaboration:
We observed the field of GRB 260915A detected by SVOM/ECLAIRs (Liang et al., GCN 45570; Yang et al., GCN 45612) using the ESO VLT UT1 (Antu) equipped with the FORS2 imager and the ESO VLT UT4 (Yepun) equipped with the HAWK-I near-infrared imager.
We obtained observations in the z-band (on two epochs) and in the YJHK filters (one epoch). A single, faint source, at approximate J2000 coordinates RA = 00:32:37.52, Dec = +08:15:00.1, is detected within the overlap region of the EP/FXT (Shi et al., GCN 45587) and Swift/XRT (Page et al., GCN 45623) error circles. This source is also faintly visible in archival Legacy Survey imaging. A few more, nearby sources are seen consistent with only one of the FXT and XRT error circles. Comparison of the two z-band epochs reveals no variability for any of these sources, so we cannot identify any compelling candidate counterpart of GRB 260915A.
A summary of our observations is provided in the table below.
| t-t0 (hr) | Exposure time (s) | Magnitude (AB) | Instrument | Band |
| ------ | ----------------- | ---------------| ---------- | ---- |
| 5.77 | 10x120 | > 24.5 | FORS2 | z |
| 6.32 | 10x60 | > 24.5 | HAWK-I | Y |
| 6.53 | 8x60 | > 24.0 | HAWK-I | J |
| 6.73 | 8x60 | > 23.8 | HAWK-I | H |
| 6.96 | 10x60 | > 23.4 | HAWK-I | K |
| 29.48 | 10x120 | > 25.0 | FORS2 | z |
Magnitude upper limits are at the 3 sigma level. Optical and NIR photometry was calibrated against the Pan-STARRS DR2 and 2MASS catalogs, respectively, and is not corrected for Galactic extinction.
Our non-detection is consistent with previous reports (Yin et al., GCN 45573; Wu et al., GCN 45575; Zhu et al., GCN 45577; Perez-Garcia et al., GCN 45578; Ducoin et al., GCN 45580; Li et al., GCN 45581; Brivio et al., GCN 45584; Li et al., GCN 45586; Shi et al., GCN 45588; Li et al., GCN 45589; Mohan et al., GCN 45600).
We acknowledge the excellent support from the observing staff in Paranal, in particular Alonso Luna, Camila de Sa Freitas, Gonzalo Burgos, Juan Carlos Olivares, Sol Troncoso, and Thomas Rivinius.