{
  "bibcode": "2021GCN.30657....1F",
  "body": "D. Frederiks, S. Golenetskii, A.Lysenko, A. Ridnaia,\nD. Svinkin, A. Tsvetkova,  M. Ulanov, and T. Cline,\non behalf of the Konus-Wind team, report:\n\nThe long GRB 210818A (Swift detection: Beardmore et al., GCN 30652;\nAGILE detection: Ursi et al., GCN 30654)\ntriggered Konus-Wind (KW) at T0=3729.710 s UT (01:02:09.710).\n\nThe burst light curve started at ~T0 with a bright, short (<0.5s),\nhard-spectrum initial pulse, which was followed, after a ~0.5 s interval,\nby a similarly bright and hard, multi-peaked emission, which gradually decays until ~T0+90 s.\nWe also note a QPO-like behavior of the light curve in the interval\nfrom ~T0+1.6 s to ~T0+3.2 s, with the quasi period of ~0.21 s.\n\nThe emission is seen up to ~7 MeV.\n\nThe Konus-Wind light curve of this GRB is available at\nhttp://www.ioffe.ru/LEA/GRBs/GRB210818_T03729/\n\nAs observed by Konus-Wind, the burst had\na fluence of (2.67 �� 0.12)x10^-4 erg/cm^2 and\na 64-ms peak energy flux, measured from T0 + 0.128 s,\nof (5.12 �� 0.54)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).\n\nThe time-averaged spectrum of the burst (measured from T0 to T0+80.640 s)\nis best fit in the 20 keV - 20 MeV range\nby a power law with exponential cutoff (CPL) model:\ndN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)\nwith  alpha = -0.99(-0.03,+0.03) and Ep = 1107(-71,+78) keV (chi2 = 117/98 dof).\nFitting this spectrum by a Band function yields the same alpha and Ep,\nand an upper limit on the high energy photon index: beta < -3.2 (chi2 = 117/97 dof).\n\nThe spectrum near the peak count rate (measured from T0 to T0+0.256 s,\nthat corresponds to the short initial pulse)\nis best fit in the 20 keV - 20 MeV range by the CPL model\nwith alpha = -0.28(-0.13,+0.14) and Ep = 1247(-133,+47) keV (chi2 = 38/58 dof).\nFitting this spectrum by a Band function yields the same alpha and Ep,\nand an upper limit on the high energy photon index: beta < -3.4 (chi2 = 38/57 dof).\n\nAll the quoted errors are estimated at the 90% confidence level.\nAll the presented results are preliminary.",
  "circularId": 30657,
  "createdOn": 1629310986000,
  "email": "fred@mail.ioffe.ru",
  "subject": "Konus-Wind detection of GRB 210818A",
  "submitter": "Dmitry Frederiks at Ioffe Institute  <fred@mail.ioffe.ru>",
  "eventId": "GRB 210818A"
}