GCN Circular 35062

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

The short duration, hard spectrum GRB 231115A
(Fermi GBM detection: Fermi GBM team GCN 35035
Not found
 
 
, Dalessi et al. GCN 35044
Not found
 
 
;
INTEGRAL (IBAS) detection: Mereghetti et al., GCN 35037
Not found
 
 
;
Glowbug detection: Cheung et al., GCN 35045
Not found
 
 
)
triggered Konus-Wind (KW) at T0= 56183.509 s UT (15:36:23.509).

The burst light curve shows a single FRED-like pulse,
which starts, at ~T0-0.028 s with the fast (<4 ms) rise of
the emission intensity, which peaks at ~T0-0.016 s
and has a total duration of ~66 ms.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB231115_T56183/

As observed by Konus-Wind, the burst had
a fluence of (7.3 ± 0.1)x10^-7 erg/cm^2 and
a 16-ms peak energy flux, measured from T0 - 0.018 s,
of (2.2 ± 0.45)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0-0.028 s to T0+0.036 s)
is best fit in the 20 keV - 1.5 MeV range by a power law with exponential
cutoff (CPL) model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = 0.69(-0.54,+0.96) and Ep = 495(-77,+118) keV.
A blackbody (BB) spectral model fits the spectrum equally well,
with the BB temperature kT = 114(-10,+9) keV.

Assuming the likely GRB 231115A association with the nearby
M82 galaxy at ~3.5 Mpc (D’Avanzo et al., GCN 35036
Not found
 
 
;
Mereghetti et al., GCN 35037
Not found
 
 
; Burns GCN 35038
Not found
 
 
),
we estimate the burst isotropic energy Eiso to ~1.1x10^45 erg and
the 16-ms peak luminosity Liso to ~3.2x10^46 erg/s.
These values are in the range the of the energetics of initial pulses
of magnetar giant flares (MGFs), which, assuming the short rise time
and the hard energy spectrum of the burst, supports its MGF origin.

At 3.5 Mpc, the characteristic radius of the emission region,
estimated from the KW blackbody spectral fit, is R~30 km,
the same order of magnitude as the radius of a neutron star or its magnetosphere.

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