TITLE: GCN CIRCULAR NUMBER: 38506 SUBJECT: GRB 241209A: VLT/HAWK-I near-IR counterpart candidate DATE: 24/12/09 22:50:27 GMT FROM: Benjamin Schneider at MIT bschn@mit.edu

B. Schneider (LAM), N. Habeeb (U. Leicester), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. Martin-Carrillo (UCD), A. de Ugarte Postigo (OCA and LAM) and N. R. Tanvir (U. Leicester) report on behalf of the Stargate collaboration:

We observed the field of the short GRB 241209A (Parsotan et al., GCN 38476; Fermi GBM team, GCN 38474) with the ESO Very Large Telescope, equipped with the HAWK-I near-infrared camera. We obtained 12 min exposures in both the H and Ks bands, as well as an 11 min exposure in J band starting at 07:51:24 UT on 2024-12-09, i.e. 2.3 hr after the Swift trigger. The observations were obtained under excellent seeing (0.5" in the J band).

The two objects mentioned by de Ugarte Postigo et al. (GCN 38484; see also Saccardi et al., GCN 38487) are both detected in all filters.

Furthermore, in all three bands we detect a faint source overlapping the wings of the brighter galaxy mentioned by de Ugarte Postigo et al. (GCN 38484), approximately 1" SE of the galaxy nucleus. The coordinates of the source are:

RA(J2000) = 10:21:35.14 = 155.39642 DEC(J2000) = +06:19:44.5 = 6.32902 with an uncertainty of 0.5".

At the present time, we cannot state whether this source is transient or pre-existing, as we lack NIR comparison imaging of sufficient quality. Under a worse seeing, this source would be easily blended within the surrounding galaxy light. Further imaging is planned to assess fading.

A comparison of the VLT/FORS2 R-band image (seeing 1.0") reported by de Ugarte Postigo et al. (GCN 38484) with the Legacy survey r-band image reveals only a marginal (~3 sigma, or ~0.1 mag) flux excess at the location of the HAWK-I candidate. Thus, due to systematic errors arising from different PSF and filter shape, we cannot confirm the presence of this source in the FORS2 R-band image at the present time.

We acknowledge the excellent support provided by the Paranal staff, in particular Jonathan Smoker and Juan Carlos Olivares.

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