GCN Circular 46170
Subject
EP261105a: VLT/FORS2 optical detection
Event
Date
2026-11-06T11:52:47Z (10 hours ago)
From
Roberto Ricci at INAF-IRA <ricci@ira.inaf.it>
Via
Web form
Hira Waseem, Muskan Yadav, Nicolas Monsalves, Narges Shahamat, Roberto Ricci, Niccolò Passaleva, Yuhan Yang, and Eleonora Troja (U Rome) report on behalf of the ERC BHianca team:
We observed the field of EP261105a detected by Einstein Probe (Guo et al., GCN 46155; Guo et al. 46168) with the FORS2 imager on the ESO VLT UT1 (Antu). FORS2 observations began on 2026-11-06 at 04:10 UT, 18.0 hours after the trigger and were carried out in the R-band at an average airmass ~1.5 and with an average seeing ~0.6 arcsec.
At the position of the candidate optical afterglow reported by SVOM/VT (Ma et al., GCN 46160) we detect a source with R ~ 24.3 AB mag, consistent with the non-detection reported by LCO at 2.6 hours (van Hoof et al., GCN 46162). Compared with the SVOM/VT detection (VT_R = 22.3 at 0.85 hours; Ma et al., GCN 46160), our measurement implies a power-law decay with temporal index alpha ~ -0.5. Magnitudes are calibrated using nearby stars in the Legacy Survey DR10 catalog (Dey et al. 2019), and are not corrected for Galactic extinction.
No catalogued source is visible at the position of the optical counterpart, implying that any underlying galaxy would be faint. We note that the counterpart is located ~4 arcsec from the centre of a bright (r~18 AB mag) galaxy with photometric redshift of z = 0.055 +/- 0.014 in the Legacy Survey DR10. The chance coincidence probability for this galaxy is P_cc ~ 0.01 (Bloom et al. 2002) . If the association is confirmed, the projected offset corresponds to ~4.5 kpc at z = 0.055.
Further observations are planned.
We thank the staff at the VLT for the rapid execution of these observations.