TITLE: GCN CIRCULAR NUMBER: 45944 SUBJECT: GRB 261010A: a short minimum variability timescale and a distinct pulse evolution compatible with a merger origin DATE: 26/10/13 15:27:50 GMT FROM: ROMAIN LOIC MACCARY at University of Ferrara R. Maccary, C. Guidorzi, M. Bulla (University of Ferrara) A. Rossi, G. Stratta, (INAF-OAS),S. Klose (TLS Tautenburg) on behalf of a larger collaboration report: We determined the minimum variability timescale (MVT) of GRB 261010A (Fermi GBM team, GCN 45888) from the Fermi/GBM light curve (NaI detectors 0, 1 and 3) following the prescriptions of Camisasca et al. (2023), i.e. as the minimum full width half maximum (FWHM_min) of all statistically significant pulses, and found FWHM_min = 82 (-21, +28) ms. Combined with the T90~54 s (Fermi GBM team, GCN 45911), in the T90-FWHM_min plot GRB 261010A (green square in the Figure at this link https://drive.google.com/file/d/1xVNgXHV105Zqyeva3hRCc_OcizuGEC-0/view?usp=drive_link) lies in the region populated by other long-lasting merger candidates, such as also GRB 191019A. In addition, the waiting times show a distinct temporal evolution (see https://drive.google.com/file/d/1udGHzr9tzvQvIQNng_tFnlRK30SuZp2v/view?usp=drive_link), which has been recently identified as an indicator of merger origin (Maccary et al. 2026a,b) This is in agreement with the prompt-based merger classification made by Zhu & Tam, GCN 45932. In addition, assuming either a redshift of z= 0.070 (Malesani et al., GCN 45912) or z=0.785 (Castro-Tirado et al., GCN 45928), a fluence of 1.72e-5 erg cm-2 would correspond to either Eiso~2.0e+50 erg or Eiso~2.9e+52 erg. Combined with Ep~837 keV (Frederiks et al., GCN 45925), the two possible positions of GRB261010A in the Epi-Eiso plane seem to be more compatible with Type-I rather than Type-II GRBs (see Figure at this link https://drive.google.com/file/d/1_NaXnsKbUKEF0SW9yx8TgjdR4Rb7QgY9/view?usp=drive_link). We caution that this Figure was obtained using the peak energy computed using the main pulse, the time-integrated peak energy computed on the whole burst could be lower (e.g., Fermi/GBM team, GCN 45911) which would put the point at z=0.785 slightly closer to Type-II GRBs (see https://drive.google.com/file/d/1z5MI3L9aGI8hhrtL5kHurP6my6yEtAPU/view?usp=drive_link) . References: - Minimum variability timescale (MVT): Camisasca et al., 2023,https://doi.org/10.1051/0004-6361/202245657 - Maccary et al., 2025,A&A,https://doi.org/10.1051/0004-6361/202555418 - Distinctive pulse evolution: Maccary et al., 2026, A&A, https://doi.org/10.1051/0004-6361/202659083