TITLE:   GCN CIRCULAR
NUMBER:  35972
SUBJECT: Konus-Wind detection of GRB 240315C (possible counterpart of EP240315a)
DATE:    24/03/21 16:05:26 GMT
FROM:    Dmitry Svinkin at Ioffe Institute <svinkin@mail.ioffe.ru>

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;
Swift-BAT detection: DeLaunay et al., GCN 35971)
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) and during the ~1600 s interval of the transient activity. The positional (GCN 35966) 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; Quirola-Vásquez et al., GCN 35960) 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