GCN Circular 40961
Subject
GRBs 250702B,D,E / EP250702a: fast fading, extremely red counterpart
Event
Date
2025-07-04T21:57:00Z (3 months ago)
From
Andrew Levan at Radboud University <a.levan@astro.ru.nl>
Via
Web form
A. J. Levan (Radboud & Warwick), A. Martin-Carrillo (UCD), B. Schneider (LAM), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), B. P. Gompertz (Birmingham), G. Corcoran (UCD), M. De Pasquale (Univ. Messina) report on behalf of the Stargate collaboration:
We performed a second observation of the near-infrared source reported by Martin-Carrillo et al. (GCN 40924) likely associated with EP250702a (Cheng et al., GCNs 40906, 40917) and GRB 250702B,D,E detected by Fermi/GBM (Fermi GBM Team, GCNs 40883, 40886, 40890), Swift/BAT (via the GUANO system; DeLaunay et al., GCN 40903) and Konus/Wind (Frederiks et al., GCN 40914). We used the ESO VLT UT4 (Yepun) equipped with the HAWK-I near-infrared camera. We obtained 10-min exposures in each of the H and K bands, starting on 2025 July 4 at 03:16:28 UT (1.56 days after the first Fermi trigger, GRB 250702B).
Compared to our previous measurement (Martin-Carrillo et al., GCN 40924), the counterpart has faded by ~1.3 mag in the K band (and a consistent amount in H). Assuming a power-law decay, this gives a decay index ~1.7 (relative to the first Fermi trigger time).
The source is also extremely red, with an approximate colour of H-K ~ 2.5 (Vega). This is far from what is typically observed in transients and is not straightforward to explain even through dust extinction. The red color also easily explains the lack of optical detections (Siegel, GCN 40952; Busmann et al., GCN 40949; Hua et al., GCN 40943; Pérez-García et al., GCN 40929; Becerra et al., GCN 40918; Kumar et al., GCN 40908).
Our second epoch clearly reveals an extended source under the location of the near-infrared source. The true nature of this extended source is still not confirmed, although it appears morphologically to be a disc-like galaxy. On this assumption, based on the offset and approximate magnitude, we infer a chance probability of ~0.05%. If this is correct, it would suggest an extragalactic origin for GRBs 250702B,D,E / EP250702a.
We acknowledge excellent support from the observing staff in Paranal, in particular Susana Cerda-Hernandez, Thomas Szeifert, Marco Berton and Robert Klement