TITLE: GCN CIRCULAR NUMBER: 30540 SUBJECT: Fermi GBM Sub-Threshold Detection of GRB 210726A DATE: 21/07/28 15:21:41 GMT FROM: Peter Veres at UAH P. Veres (UAH) reports on behalf of the Fermi-GBM Team: "Swift-BAT detected GRB 210726A at 19:19:03 UT (GCN 30523). There was no Fermi-GBM onboard trigger around the event. The GBM targeted search [1], the most sensitive, coherent search for GRB-like signals was run from +/-30 s around the BAT trigger time. A transient source was identified whose most significant timescale according to the search is 512 ms, and a location consistent with the Swift-BAT event. The GBM targeted search event was found with the highest significance with a "soft" spectral template (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7). The GBM light curve consists of a single peak. The time-averaged spectrum from T0+0.6 s to T0+1.4 s is best fit by a power law function with index -2.03 +/- 0.3 (T0 is 2021-07-26 19:19:03 UT). A power law function with an exponential high-energy cutoff fits equally well, but it is not statistically preferred over the power law; the fit measures the power law index 0.23 +/- 1.67 and the cutoff energy, parameterized as Epeak, is 41 +/- 10 keV. The event fluence (10-1000 keV) in this time interval for the power law with the exponential cutoff is (6.4 +/- 1.7)E-08 erg/cm^2. Using the photometric redshift z=0.35 reported by Watson et al. (GCN 30534), we derive an isotropic equivalent energy in the 1-10,000 keV range of (2.1 +/- 0.5)E+49 erg. This analysis is preliminary. [1] Goldstein et al. 2019 arXiv:1903.12597 "