GCN Circular 27596

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

The extremely-bright, short-duration GRB 200415A
(IPN localization: Svinkin et al., GCN Circ. 27585
Not found
 
 
, 27595
Not found
 
 
;
Fermi-GBM observation: Bissaldi et al., GCN Circ. 27587
Not found
 
 
)
triggered Konus-Wind at T0=31681.401 s UT (08:48:01.401).

The burst light curve starts at ~T0-2 ms with the sharp
rise of extremely-bright pulse, with a duration of ~5 ms.
This pulse is followed by a weaker, steadily decaying
emission which is visible up to ~T0+200 ms.
The emission is seen up to ~10 MeV.

As observed by Konus-Wind, the burst had a fluence of
8.1(-0.8, +0.9)x10^-6 erg/cm2,
and a 2-ms peak flux, measured from T0-0.002 s,
of 1.0(-0.08,+0.08)x10^-3 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+0.192 s)
is best fit in the 20 keV - 15 MeV range
by a power law with exponential cutoff (CPL) model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = +0.10(-0.23,+0.27)
and Ep = 818(-112,+136) keV (chi2 = 34/49 dof).
Fitting by a GRB (Band) model yields similar alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.5
(chi2 =33/49 dof).

The spectrum measured near the peak count rate (from T0 to T0+0.064 s)
is best fit in the 20 keV - 15 MeV range by the CPL model
with alpha = +0.12(-0.24,+0.20) and Ep = 1060(-160,+190) keV
(chi2 = 25/36 dof). Fitting by a GRB (Band) model yields similar
alpha and Ep, and an upper limit on the high energy photon index:
beta < -2.6 (chi2 =24/35 dof).

Assuming the likely GRB 200415A association with the nearby Sculptor
galaxy at ~3.5 Mpc (Svinkin et al., GCN 27585
Not found
 
 
, GCN 27595
Not found
 
 
),
we estimate the burst isotropic energy to ~1.3x10^46 erg and
the 2-ms peak luminosity to ~1.6x10^48 erg/s.
These values closely resemble the energetics of the initial pulse of
the SGR 1806-20 giant magnetar flare (GF) measured by KW and HELICON
(Frederiks et al., 2007 AstL 33 1).

GRB 200415A also shows a striking similarity, in light curve, spectrum,
and energy, with GRB 051103 (Frederiks et al., 2007 AstL 33 19),
the extragalactic GF candidate in the M81/M82 group of galaxies
located at nearly the same distance, ~3.6 Mpc.

Even the giant magnetar flare origin of GRB 200415A and GRB 051103
cannot be established unequivocally (the key evidence, pulsating tail,
could hardly be detected by KW or Fermi-GBM from ~3.5 Mpc),
the common nature of the two ultra-bright flares is highly probable.

The Konus-Wind light curve of GRB 200415A is available at
http://www.ioffe.ru/LEA/GRBs/GRB200415_T31681/

A comparison of Konus-Wind light curves of GRB 200415A and GRB 051103
can be found at http://www.ioffe.ru/LEA/GRBs/GRB200415_T31681/051103_200415.pdf


All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.