TITLE:   GCN CIRCULAR
NUMBER:  45105
SUBJECT: GRB 260705A: Candidate Optical Counterpart from SOAR and Gemini-South and Possible Association with GRB 260705B
DATE:    26/07/07 15:52:15 GMT
FROM:    James Freeburn at University of North Carolina at Chapel Hill <jamesfreeburn54@gmail.com>

James Freeburn (UNC), Tomás Ahumada (NOIRLab), Jillian Rastinejad (UMD), Gokul Srinivasaragavan (UMD/Northwestern), Huei Sears (Rutgers), Anirudh Salgundi (UNC), report on behalf of a larger collaboration:

We observed the field of GRB 260705A (SVOM burst-id sb26070502) detected by SVOM (Saccardi et al., GCN 45079) and Fermi (Hamburg et al., GCN 45080) using the SOAR and Gemini-South telescopes. We obtained 8 x 300 s of i-band imaging with SOAR using the Goodman High Throughput Spectrograph (SOAR2026A-018; PI: Andreoni) in imaging mode with the Red camera at a mid-time of 2026-07-07 10:19 UT, corresponding to ~45 hrs after the trigger.  We performed SOAR data reduction with a custom pipeline using pre-reduced images from the Goodman pipeline (Torres et al. 2018). We obtained  4 x 120 s of r-band imaging at three different pointings using GMOS on Gemini-South (PI: Srinivasaragavan) at a mid-time of 2026-07-07 09:35 UT and performed image reduction with DRAGONS (Labrie et al. 2019). For both images, we performed image subtraction using SFFT (Hu et al. 2022) against DECam Legacy Survey images. The GMOS image covers ~77% of the SVOM localization (Saccardi et al., GCN 45079), while the resampled Goodman image covers ~65% of the localization. 

We detect a candidate optical counterpart at:

R.A. (J2000) = 03:53:12.49 (58.30204 d) +/- 0.5’’
Dec. (J2000) =  -31:39:08.61 (-31.65239 d) +/- 0.5’’

with AB magnitudes, calibrated with SkyMapper DR4 (Onken et al., 2024), of:

r = 21.48 +/- 0.08
i = 21.44 +/- 0.04

These magnitudes have not been corrected for Galactic extinction of A_V = 0.028 mag. The brightness of this source is consistent with the upper limits found by LCO (Shi et al., GCN 45084), though these observations were taken ~30 hours prior.

This candidate optical counterpart is embedded in a catalogued source, WISEA J035312.50-313908. WISEA J035312.50-313908.7 has no reported proper motion in Gaia DR3 (Gaia Collaboration 2022) and is classified as a ‘high-confidence galaxy’ in DES DR2 (Abbott et al. 2021), therefore we conclude the source is a galaxy. WISEA J035312.50-313908.7 has a photometric redshift of z =  0.178 +/- 0.009 from Legacy Survey DR10 (Duncan et al. 2022). The candidate optical counterpart is consistent with a nuclear origin with an offset of ~0.4” from the host center. We disfavor this counterpart as an AGN given the WISE color of W1-W2  = -0.05 (Cutri et al. 2013) and the absence of an AGN match in the Milliquas catalog (Flesch et al. 2023). 

We note that GRB 260705B, detected by Fermi (GCN 45081), has a localization consistent with GRB 260705A and was detected ~ 11 hr (~ 40,000 s) after.  If these bursts are connected to the same source, this suggests a possible identification as an ultra-long GRB.  The extended gamma-ray duration and nuclear location are also reminiscent of jetted TDEs (e.g., Swift J1644+57, Bloom et al. 2011; Swift J2058+05, Cenko et al. 2012; Swift J1112-82, Brown et al. 2015).

Further observations are planned to assess the variability of this source. We thank the staff at Gemini-South and SOAR for their assistance with the observations.