GCN Circular 4989

S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, M.
 Ulanov, and
 T. Cline on behalf of the Konus-Wind team report:

 The long GRB 060418 (Swift-BAT trigger=205851; Falcone et al. GCN 4966
Not found
 
 
;
 Cummings et al. GCN 4975
Not found
 
 
) was observed by Konus-Wind in the waiting
 mode. The emission is clearly seen in the three K-W bands:
G1 (18-70  keV), G2 (70-360 keV) and G3 (360-1360 keV) with high S/N.
 Using an approximation the time-integrated 3-channel K-W spectrum by
 a power law with exponential cutoff model:
 dN/dE ~ E^(-alpha)*exp(-E*(2-alpha)/Ep),
 we have found alpha ~1.5, and Ep ~230 keV.

 As observed by Konus-Wind, the burst had a duration of ~44 s,
 and fluence ~1.6x10^-5 erg/cm2 (in the 20 - 1100 keV energy range).
 Assuming z = 1.49 (Dupree et al. GCN 4969
Not found
 
 
; Vreeswijk and Jaunsen GCN
 4974
Not found
 
 
) and a standard cosmology model with H_0 = 70 km/s/Mpc, Omega_M =
 0.3, Omega_\Lambda =  0.7, the isotropic energy release is E_iso ~9x10^52
erg.

 ~1200 s after the BAT GRB 060418, Konus-Wind detected another burst.
 Whether the second event relates to the first GRB or whether it is a
different
 GRB detected by chance close to first one is not clear at this time.
 The available data cannot confirm or rule out the possibility that the
 second event came from the same location as the Swift GRB:
 a) the Konus-Wind ecliptic latitude response for the second event is
 consistent with the ecliptic latitude response for the Swift GRB.
 b) BAT does not see a rate increase at ~03:26. At this time, GRB 060418
 was out of the BAT FOV (~57 degrees off axis), but a little above the
 Earth horizon (J. Cummings, private communication). So, the BAT data do
 not rule out the possibility the second event came from GRB 060418.
 c) Neither burst was detected by Odyssey, but both bursts are Mars-blocked,
  if the second one comes from the same direction as the first. RHESSI
 observed the first (Swift) burst, but not the second one. The RHESSI
 detection of the first burst was rather weak, and as observed by
Konus, the second burst was weaker than the first one, so it could have
missed.
 the second one (K. Hurley, private communication).
 d) The second event was not detected by Suzaku-WAM. The position of GRB
 060418 was occulted by the Earth from Suzaku at the time of the second
 event (K. Yamaoka, private communication).

 Hence, the situation is ambiguous.

 Possibly, the data from optical telescopes can clarify this issue, if
 there were some optical observations of the GRB 060418 afterglow during
 the time of the second event.

 The K-W light curves of these events are available at
 http://www.ioffe.rssi.ru/LEA/GRBs/GRB060418/