TITLE: GCN CIRCULAR NUMBER: 37624 SUBJECT: Konus-Wind detection of GRB 240912A DATE: 24/09/29 14:14:30 GMT FROM: Dmitry Svinkin at Ioffe Institute svinkin@mail.ioffe.ru
D. Svinkin, D. Frederiks, M. Ulanov, A. Tsvetkova, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team, report:
The long-duration GRB 240912A (Fermi-GBM detection: The Fermi GBM Team, GCN 37465; Smith and Meegan, GCN 37478; Swift-BAT detection: Evans et al., GCN 37466; Parsotan et al., GCN 37539; INTEGRAL-SPI-ACS detection: Pawar et al., GCN 37473; SVOM-GRM detection: Zheng et al., GCN 37484) triggered Konus-Wind at T0 = 6454.746 s UT (01:47:34.746).
The burst light curve shows a multipeaked structure which starts at ~T0-60 s and has a total duration of ~118 s. The emission is seen up to ~2 MeV.
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB240912_T06454/
As observed by Konus-Wind, the burst had a fluence of 9.57(-0.96,+0.88)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+2.496 s, of 7.32(-1.90,+1.87)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+60.928 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 = -1.36(-0.11,+0.12), the high energy photon index beta = -2.44(-0.54,+0.18), the peak energy Ep = 196(-27,+37) keV (chi2 = 90/96 dof).
The spectrum near the maximum count rate (measured from T0+0.256 s to T0+6.144 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 = -1.12(-0.15,+0.16), the high energy photon index beta = -2.59(-7.41,+0.35), the peak energy Ep = 358(-77,+124) keV (chi2 = 89/72 dof).
Assuming the redshift z=1.234 (de Ugarte Postigo et al., GCN 37469) and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315, and Omega_Lambda = 0.685 (Planck Collaboration, 2014), we estimate the following rest-frame parameters: the isotropic energy release E_iso is 4.0(-0.4,+0.4)x10^53 erg, the peak luminosity L_iso is 6.8(-1.8,+1.7)x10^52 erg/s, the rest-frame peak energy of the time-averaged spectrum Ep,i,z is 437(-60,+82) keV, and the spectrum near the maximum count rate Ep,p,z is 799(-172,+277) keV.
With the obtained estimates, GRB 240912A is inside 68% prediction bands for 'Amati' and 'Yonetoku' relations, for the sample of >300 long KW GRBs with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021), see http://www.ioffe.ru/LEA/GRBs/GRB240912_T06454/GRB240912A_rest_frame.pdf
All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.
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