GCN Circular 45932
Si-Yuan Zhu and Pak-Hin Thomas Tam (SYSU) report:
We analyzed GRB 261010A using publicly available data from Fermi/GBM. The burst was detected by Fermi/GBM at 19:17:03 UT on 2026 October 10, as a long-duration GRB with T90 about 54 s (50-300 keV; Fermi GBM Team, GCN 45888).
The 10-1000 keV light curve of GRB 261010A consists of multiple emission episodes. In particular, we identified a precursor emission (PE) episode from T0-0.128 s to T0+0.384 s, with a duration of T100,PE = 0.512 s. The PE is separated from the main emission (ME, from T0 + 3.856 s to T0 + 5.936 s), by a quiescent interval (QE) lasting T100, QE=3.472 s. We adopted the pulse rise time of the PE as its minimum variability timescale (MVT), T_MVT,PE = 0.152 s. The time-integrated spectrum of the ME is best fitted by a power law with an exponential cutoff, yielding a photon index of alpha = -0.94 +/- 0.04 and a peak energy of Ep,ME=970 +/- 198 keV.
To further classify GRB 261010A, we apply the Ep,ME-precursor index (EPI), a diagnostic that uses prompt emission alone (Zhu et al. 2026; [1]). The EPI separates merger-driven (Type I) from collapsar-driven (Type II) GRBs, with Type I GRBs concentrating at EPI > 6.2 and Type II GRBs at EPI < 6.2. Values in the range 6.0 < EPI < 6.4 fall in a transition region where misclassification is possible, whereas EPI > 6.4 and EPI < 6.0 allow safe classification as Type I and Type II, respectively. We calculate EPI = 6.81, suggesting that GRB 261010A is unambiguously classified as a Type I-L (merger-driven) GRB. (see https://tinyurl.com/epi-grb261010a)
The optical afterglow of GRB 261010A (O'Neill et al., GCN 45890; Aceituno et al., GCN 45891) is located at a projected offset of 34.5 kpc from the nearby galaxy LEDA 1781464 at z = 0.070 (Corcoran et al., GCN 45912). GTC/OSIRIS spectroscopy of the red object 2.2 arcsec NE of the afterglow shows a galaxy at z = 0.785 +/- 0.001 (Castro-Tirado et al., GCN 45928), and the afterglow is offset from the centre of that galaxy by 16 kpc in projection.
If LEDA 1781464 at z = 0.070 is the host, the large projected offset of 34.5 kpc supports a merger origin. We obtain Eiso = 5.04E49 erg and Ep,z = 1038 keV of the ME, consistent with the Ep,z-Eiso correlation of Type I (short) bursts. If the galaxy at z = 0.785 is the host. We obtain Eiso = 7.19E51 erg and Ep,z = 1731 keV of the ME, still consistent with the Type I correlation.
Based on our preliminary analysis, GRB 261010A is a Type I-L (merger-driven) GRB candidate. Further analysis of GRB 261010A is ongoing and multi-wavelength follow-up observations are strongly encouraged.
References [1] Zhu et al., ApJ, 1003, 142, 2026