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GCN Circular 35062

Konus-Wind detection of GRB 231115A (a probable Magnetar Giant Flare from M82)
2023-11-16T12:43:51Z (6 months ago)
Dmitry Frederiks at Ioffe Institute <>
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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, Dalessi et al. GCN 35044;
INTEGRAL (IBAS) detection: Mereghetti et al., GCN 35037;
Glowbug detection: Cheung et al., GCN 35045)
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

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;
Mereghetti et al., GCN 35037; Burns GCN 35038),
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.

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