GRB 260121A

D. Svinkin (Ioffe), D. Frederiks (Ioffe), D. Kapshtan (Geoscan), M. Orlov (Geoscan), 
V. Bazunova (Geoscan), E. Shchepin (Geoscan), E. Meltsov (Geoscan), 
K. Starikov (Geoscan, SPbSU), A. Khogoev (Geoscan), 
A. Razdobarin (SPbAU), and D. Dolmatov (SPbSU),
on behalf of the 239Alferov team, report:

The long-duration GRB 260121A 
(IPN triangulation: Kozyrev et al., GCN 43564
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 ;
Konus-Wind detection: Ridnaia et al., GCN 43611
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 )
was detected by 239Alferov 3U CubeSat at about 8552 s (02:22:32) UT.

The burst light curve, as measured by 239Alferov detector, shows 
a single pulse with a duration of ~13 s. The emission is seen up to above ~1 MeV.

The light curve recorded by 239Alferov is available at:
http://www.ioffe.ru/LEA/GRBs/GRB260121_T08549/239Alferov/

239Alferov is an educational 3U CubeSat mission at SSO combining amateur radio 
and space research objectives. The gamma-ray detector onboard 239Alferov contains 
a 64x72x15 mm NaI(Tl) scintillator read out by a SiPM array and covers the energy range 
from ~20 keV to ~1000 keV. The spacecraft is now at a commissioning stage, 
with scientific observations started in December 2025. 
Currently, the instrument records count rates in five energy bands:  
~20-50 keV, ~50-100 keV, ~100-500 keV, 500-1000 keV, and > 1000 keV, with 1 s temporal resolution.
The surveys span a couple of days with about a day-long analysis and maintenance intervals 
between the observations.
The ground segment of the mission utilizes the SONIKS open ground station network.

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

The long-duration GRB 260121A
(IPN triangulation: Kozyrev et al., GCN 43564
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triggered Konus-Wind at T0=8549.319 s UT (02:22:29.319).

The burst light curve shows a single pulse
which starts at ~T0-0.5 s and has a total duration of ~21.6 s.
The emission is seen up to ~4 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260121_T08549/

As observed by Konus-Wind, the burst
had a fluence of 2.79(-0.46,+0.47)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+0.512 s,
of 2.09(-0.31,+0.32)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+10.240 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.82(-0.11,+0.13),
the high energy photon index beta = -2.07(-0.33,+0.18),
the peak energy Ep = 366(-60,+75) keV
(chi2 = 97/97 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+2.304 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.75(-0.12,+0.15),
the high energy photon index beta = -1.78(-0.11,+0.09),
the peak energy Ep = 367(-73,+90) keV
(chi2 = 123/97 dof).

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.

A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin
on behalf of the MGNS/BepiColombo team,

J. Benkhoff on behalf of the BepiColombo team,

D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia,
and T. Cline on behalf of the Konus-Wind team,

A. Goldstein, M. S. Briggs, C. Wilson-Hodge, M. Hui,
and E. Burns on behalf of the Fermi GBM team,

G. Waratkar, J.Joshi, V. Bhalerao, D. Bhattacharya, A. Arya,
and S. Vadawale, on behalf of the Astrosat-CZTI team,

E. Bozzo and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team,

S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, A. Tohuvavohu,
and J. DeLaunay on behalf of the Swift-BAT team,

and

D. Kapshtan, M. Orlov, V. Bazunova, E. Shchepin, E. Meltsov,
K. Starikov, A. Khogoev, on behalf of the 239Alferov team,
report:

The long-duration GRB 260121A
was detected by Fermi (GBM; in continuous TTE data),
Konus-Wind, Astrosat (CZTI), INTEGRAL (SPI-ACS),
239Alferov CubeSat, and BepiColombo-MGNS
at about 8552 s UT (02:22:32).

We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:
---------------------------------------------
  RA(2000), deg                 Dec(2000), deg
 ---------------------------------------------
 Center:
  213.591 (14h 14m 22s) -62.629 (-62d 37' 45")
 Corners:
  212.178 (14h 08m 43s) -65.088 (-65d 05' 17")
  212.129 (14h 08m 31s) -64.984 (-64d 59' 01")
  214.879 (14h 19m 31s) -60.111 (-60d 06' 38")
  214.922 (14h 19m 41s) -60.220 (-60d 13' 11")
 ---------------------------------------------
The error box area is 855 sq. arcmin, and its maximum
dimension is 5.13 deg (the minimum one is 3 arcmin).
The Sun distance was 72 deg.

The localization may be improved.

A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB260121_T08549/IPN/
The HEALPix triangulation map is the multi-order HEALPix in units of
probability density.

The Konus-Wind time history and spectrum will be given
in a forthcoming GCN Circular.