GCN Circular 41829
Subject
GRB 250912A: VLT observations
Event
Date
2025-09-14T10:06:40Z (10 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
M. Garnichey (LUX-Paris Obs.), G. Corcoran (UCD), B. Schneider (LAM), L. Izzo (INAF/OACN), A. Saccardi (CEA/Irfu), V. D’Elia (ASI/SSDC), N. Habeeb (Leicester), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), A. L. Thakur (INAF/IAPS), N. R. Tanvir (Leicester), S. D. Vergani (LUX-Paris Obs.), report on behalf of the Stargate collaboration:
Following the reports of deep optical upper limits (Wu et al., GCN 41824; Li et al., GCN 41825), we observed the location of the SVOM GRB 250912A (Ducoin et al., GCN 41820; Wang et al., GCN 41826; see also Smith & Meegan, GCN 41828) using the ESO VLT UT4 (Yepun) equipped with the HAWK-I near-infrared imager. A total of 15 min exposure was secured in the J band, at a mean epoch 2025 Sep 13.130 UT (6.02 hr after the trigger).
Image subtraction against archival UKIDSS J-band data reveals no new sources down to the depth of this survey, J ~ 20.6 (AB). As our HAWK-I stack reaches much deeper (limiting magnitude J = 23.9 AB), a more effective search can be carried out by comparison with the Legacy Survey, which is much more sensitive than UKIDSS, but only covers bands different than J.
Within the MXT error circle (Ducoin et al., GCN 41820), many objects are visible in our J-band image that are not present in the Legacy Survey. The brightest one has a magnitude J = 22.15 +/- 0.06 (AB), and is located at coordinates (J2000):
RA = 00:59:44.39
Dec = +11:10:07.0
For this object, we measure an upper limit z > 23.7 AB in the Legacy Survey, indicating either a remarkably red object or a transient/variable source. Based on the available information, we cannot discriminate between these two possibilities at the moment.
Follow-up observations of this source were secured using the ESO VLT UT1 (Antu) equipped with the FORS2 spectrograph. A 3x1000 s spectrum was obtained with the 600z grism (wavelength range 7500 - 10100 AA). No object is detected in the acquisition image (mid time 9.83 hr after trigger) down to a depth of z > 22.7 AB, but a very faint trace is detected in the red part down to ~8550 AA. The very low S/N prevents us from identifying any features, but the continuum detection allows us to set a redshift upper limit z <~ 6.
At the present time, no clear afterglow candidate emerges from our observations. A second, deep J-band image is required in order to conclusively identify any faint, variable sources.
We acknowledge expert support from the ESO observing staff in Paranal, in particular Robert De Rosa, Juan Carlos Olivares, Jonathan Smoker, Hannah Osborne, and Sam Kim.