TITLE: GCN CIRCULAR NUMBER: 41939 SUBJECT: Konus-Wind detection of GRB 250911A DATE: 25/09/22 16:37:08 GMT FROM: Valentina Panteleeva at Ioffe Institute panteleevav228@gmail.com

V. Panteleeva, D. Frederiks, A. Lysenko, A. Ridnaia, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report:

The long-duration GRB 250911A (Swift-BAT detection: Klingler et al., GCN 41781; Palmer et al., GCN 41812; GECAM-B detection: Zhang et al., GCN 41813; Insight-HXMT detection: Wang et al., GCN 41827) triggered Konus-Wind at T0=05370.240 s UT (01:29:30.240).

The burst light curve shows a multipeaked structure which starts at ~T0-2.3 s and has a total duration of ~57 s. The weak precursor that triggered Swift-BAT is seen at ~T0-100 s. The emission is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB250911_T05370/

As observed by Konus-Wind, the burst had the total fluence of 4.54(-0.29,+0.30)x10^-5 erg/cm2, and the 64-ms peak flux, measured from T0+1.440 s, of 5.15(-0.88,+0.90)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst (measured from T0 to T0+41.216 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.85(-0.11,+0.12), the high energy photon index beta = -2.58(-0.19,+0.14), the peak energy Ep = 196(-13,+14) keV (chi2 = 102/97 dof).

The spectrum near the maximum count rate (measured from T0+0.256 to T0+8.448 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.92(-0.11,+0.12), the high energy photon index beta = -2.44(-0.19,+0.14), the peak energy Ep = 257(-25,+26) keV (chi2 = 88/84 dof).

All the quoted errors are at the 68% confidence level. All the quoted values are preliminary.

View this GCN Circular online at https://gcn.nasa.gov/circulars/41939. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...