GCN Circular 35972

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

The long-duration GRB 240315C
(IPN triangulation: Svinkin et al., GCN 35966
Not found
 
 
;
Swift-BAT detection: DeLaunay et al., GCN 35971
Not found
 
 
)
was detected by Konus-Wind (KW) in the waiting mode
at T0=73018 s UT (20:16:58).

The burst was observed over the high and variable background
due to a solar particle event. A Bayesian block analysis of the KW waiting mode data in the 20-1600 keV band reveals a >20 sigma count rate increase in the interval from T0 to T0+47 s.
The burst light curve shows a multi-peaked structure with the brightest peak around ~T0+24 s (we note a 6 s gap in the data starting from ~T0+47 s).

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB240315_T73000/

The spectrum of the burst, measured from T0 to T0+47 s,
can be described by a power law with exponential cutoff (CPL) model
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -1.09(-0.25,+0.36) and Ep = 410(-159,+388) keV.

The spectrum near the peak count rate, measured from T0+15 s to T0+29 s,
can be described by a CPL model with
alpha = -0.80(-0.30,+0.38) and Ep = 459(-155,+304) keV.

The total burst fluence is 1.63(-0.40,+0.64)x10^-5 erg/cm^2,
and the 2.944 s peak energy flux, measured from T0+23 s,
is 9.13(-2.84,+3.65)x10^-7 erg/cm^2.
(both in the 20 keV - 10 MeV energy range).

The burst was detected ~374 s after the start time of the fast X-ray transient EP240315a (T0(EP)=20:10:44 UT; Zhang et al., GCN 35931
Not found
 
 
) and during the ~1600 s interval of the transient activity. The positional (GCN 35966
Not found
 
 
) and temporal coincidence of this burst with the transient
supports the conclusion that the EP240315a is the GRB counterpart.
Dimmer emission episodes of GRB 240315C may be missed by Konus-Wind due to the high and variable background.

Assuming GRB 240315C is indeed connected with EP240315a at
redshift z=4.859 (Saccardi et al., GCN 35936
Not found
 
 
; Quirola-Vásquez et al., GCN 35960
Not found
 
 
) and a standard cosmology model with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315, and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is 7.1(-1.7,+2.8)x10^53 erg,
the peak luminosity L_iso is 2.3(-0.7,+0.9)x10^53 erg/s,
and the rest-frame peak energies of the time-integrated and
peak spectra, Ep,i,z = 2402(-932,+2273) keV and Ep,p,z = 2689(-902,+1781) keV, respectively.

With the obtained estimates, GRB 240315C is inside 90% prediction band for the 'Amati' relation and inside 90% prediction band for the 'Yonetoku' relation derived for the sample of >300 long KW GRBs with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021),
see http://www.ioffe.ru/LEA/GRBs/GRB240315_T73000/GRB240315C_rest_frame.pdf