TITLE: GCN CIRCULAR NUMBER: 40460 SUBJECT: Konus-Wind detection of GRB 250512B / EP250512a DATE: 25/05/15 11:03:14 GMT FROM: Dmitry Svinkin at Ioffe Institute svinkin@mail.ioffe.ru

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

The long-duration GRB 250512B / EP250512a (EP-WXT detection: Zhao et al., GCN 40437; Yang et al., GCN 40448; SVOM-ECLAIRs detection: Maggi et al., GCN 40439) was detected by Konus-Wind (KW) in the waiting mode at about T0 = T0(EP) = 40803 s UT (11:20:03).

A Bayesian block analysis of the KW waiting mode data in the 20-400 keV band reveals a ~8 sigma count-rate increase in the interval from T0+7.4 s to T0+113.3 s.

The KW light curve of this burst is available at http://www.ioffe.ru/LEA/GRBs/GRB250512B/

The total burst fluence is 6.68(-2.23,+1.59)x10^-6 erg/cm^2, and the 2.944 s peak energy flux, measured from T0+10.309 s, is 3.08(-1.14,+0.89)x10^-7 erg/cm^2. (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst, measured from T0+7.4 s to T0+113.3 s, can be described by a power law with exponential cutoff (CPL) model dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha > -1.82 and Ep = 114(-37,+57) keV.

All the quoted errors are estimated at the 68% confidence level. All the presented results are preliminary.

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