Gamma-ray Burst 260226A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 26 Feb 26 10:38:25 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      44
TRIGGER_NUM:     793795080
GRB_RA:           44.250d {+02h 57m 00s} (J2000),
                  44.610d {+02h 58m 26s} (current),
                  43.562d {+02h 54m 15s} (1950)
GRB_DEC:         +13.817d {+13d 49' 00"} (J2000),
                 +13.921d {+13d 55' 14"} (current),
                 +13.616d {+13d 36' 58"} (1950)
GRB_ERROR:       19.87 [deg radius, statistical plus systematic]
GRB_INTEN:       112 [cnts/sec]
DATA_SIGNIF:     6.60 [sigma]
INTEG_TIME:      1.024 [sec]
GRB_DATE:        21097 TJD;    57 DOY;   26/02/26
GRB_TIME:        38275.96 SOD {10:37:55.96} UT
GRB_PHI:          45.00 [deg]
GRB_THETA:        20.00 [deg]
DATA_TIME_SCALE: 1.0240 [sec]
HARD_RATIO:      0.65
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      91%  GRB
2nd_MOST_LIKELY:   5%  GRO_J0422_32
DETECTORS:       1,0,0, 1,0,0, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:       339.51d {+22h 38m 02s}   -8.63d {-08d 37' 38"}
SUN_DIST:         68.41 [deg]   Sun_angle= -4.3 [hr] (East of Sun)
MOON_POSTN:       93.68d {+06h 14m 42s}  +28.20d {+28d 12' 00"}
MOON_DIST:        47.61 [deg]
MOON_ILLUM:      71 [%]
GAL_COORDS:      163.51,-38.97 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       45.86, -2.89 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260226443/quicklook/glg_lc_medres34_bn260226443.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 88.82,12.58 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 26 Feb 26 10:38:34 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      58
TRIGGER_NUM:     793795080
GRB_RA:           49.383d {+03h 17m 32s} (J2000),
                  49.742d {+03h 18m 58s} (current),
                  48.699d {+03h 14m 48s} (1950)
GRB_DEC:         +11.800d {+11d 48' 00"} (J2000),
                 +11.894d {+11d 53' 40"} (current),
                 +11.618d {+11d 37' 03"} (1950)
GRB_ERROR:       6.63 [deg radius, statistical plus systematic]
GRB_INTEN:       238 [cnts/sec]
DATA_SIGNIF:     19.70 [sigma]
INTEG_TIME:      2.048 [sec]
GRB_DATE:        21097 TJD;    57 DOY;   26/02/26
GRB_TIME:        38275.96 SOD {10:37:55.96} UT
GRB_PHI:          60.00 [deg]
GRB_THETA:        20.00 [deg]
DATA_TIME_SCALE: 2.0480 [sec]
HARD_RATIO:      1.14
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      96%  GRB
2nd_MOST_LIKELY:   1%  Generic SGR
DETECTORS:       1,0,0, 1,0,0, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:       339.51d {+22h 38m 02s}   -8.63d {-08d 37' 38"}
SUN_DIST:         72.76 [deg]   Sun_angle= -4.7 [hr] (East of Sun)
MOON_POSTN:       93.68d {+06h 14m 43s}  +28.20d {+28d 11' 60"}
MOON_DIST:        44.08 [deg]
MOON_ILLUM:      71 [%]
GAL_COORDS:      170.07,-37.36 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       50.13, -6.20 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260226443/quicklook/glg_lc_medres34_bn260226443.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 88.82,12.58 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 26 Feb 26 10:38:44 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      69
TRIGGER_NUM:     793795080
GRB_RA:           49.667d {+03h 18m 40s} (J2000),
                  50.024d {+03h 20m 06s} (current),
                  48.985d {+03h 15m 56s} (1950)
GRB_DEC:         +11.200d {+11d 11' 60"} (J2000),
                 +11.294d {+11d 17' 38"} (current),
                 +11.019d {+11d 01' 07"} (1950)
GRB_ERROR:       3.57 [deg radius, statistical plus systematic]
GRB_INTEN:       3153 [cnts/sec]
DATA_SIGNIF:     92.50 [sigma]
INTEG_TIME:      0.256 [sec]
GRB_DATE:        21097 TJD;    57 DOY;   26/02/26
GRB_TIME:        38275.96 SOD {10:37:55.96} UT
GRB_PHI:          60.00 [deg]
GRB_THETA:        20.00 [deg]
DATA_TIME_SCALE: 0.2560 [sec]
HARD_RATIO:      0.52
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      90%  GRB
2nd_MOST_LIKELY:   5%  GRO_J0422_32
DETECTORS:       1,0,0, 1,0,0, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:       339.51d {+22h 38m 02s}   -8.63d {-08d 37' 38"}
SUN_DIST:         72.90 [deg]   Sun_angle= -4.7 [hr] (East of Sun)
MOON_POSTN:       93.68d {+06h 14m 43s}  +28.20d {+28d 11' 59"}
MOON_DIST:        44.13 [deg]
MOON_ILLUM:      71 [%]
GAL_COORDS:      170.83,-37.62 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       50.25, -6.85 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260226443/quicklook/glg_lc_medres34_bn260226443.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 88.82,12.58 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 26 Feb 26 10:38:43 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      57
TRIGGER_NUM:     793795080
GRB_RA:           40.370d {+02h 41m 29s} (J2000),
                  40.732d {+02h 42m 56s} (current),
                  39.678d {+02h 38m 43s} (1950)
GRB_DEC:         +16.030d {+16d 01' 48"} (J2000),
                 +16.141d {+16d 08' 26"} (current),
                 +15.817d {+15d 49' 00"} (1950)
GRB_ERROR:       7.51 [deg radius, statistical only]
DATA_SIGNIF:     8.60 [sigma]
DATA_INTERVAL:   4.096 [sec]
GRB_DATE:        21097 TJD;    57 DOY;   26/02/26
GRB_TIME:        38275.96 SOD {10:37:55.96} UT
GRB_PHI:          32.00 [deg]
GRB_THETA:        20.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4173 (Gnd S/W Version number)
SUN_POSTN:       339.51d {+22h 38m 02s}   -8.63d {-08d 37' 38"}
SUN_DIST:         65.45 [deg]   Sun_angle= -4.1 [hr] (East of Sun)
MOON_POSTN:       93.68d {+06h 14m 43s}  +28.20d {+28d 11' 59"}
MOON_DIST:        50.10 [deg]
MOON_ILLUM:      71 [%]
GAL_COORDS:      157.91,-39.25 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       42.92,  0.33 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260226443/quicklook/glg_lc_medres34_bn260226443.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_793795080.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        In the LAT Field-of-view.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 26 Feb 26 10:39:09 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      72
TRIGGER_NUM:     793795080
GRB_RA:           45.983d {+03h 03m 56s} (J2000),
                  46.352d {+03h 05m 25s} (current),
                  45.279d {+03h 01m 07s} (1950)
GRB_DEC:         +17.767d {+17d 46' 00"} (J2000),
                 +17.868d {+17d 52' 03"} (current),
                 +17.572d {+17d 34' 19"} (1950)
GRB_ERROR:       3.32 [deg radius, statistical plus systematic]
GRB_INTEN:       10439 [cnts/sec]
DATA_SIGNIF:     844.10 [sigma]
INTEG_TIME:      2.048 [sec]
GRB_DATE:        21097 TJD;    57 DOY;   26/02/26
GRB_TIME:        38275.96 SOD {10:37:55.96} UT
GRB_PHI:          40.00 [deg]
GRB_THETA:        15.00 [deg]
DATA_TIME_SCALE: 2.0480 [sec]
HARD_RATIO:      0.48
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      88%  GRB
2nd_MOST_LIKELY:   8%  GRO_J0422_32
DETECTORS:       1,0,0, 1,0,0, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:       339.51d {+22h 38m 02s}   -8.63d {-08d 37' 37"}
SUN_DIST:         71.09 [deg]   Sun_angle= -4.5 [hr] (East of Sun)
MOON_POSTN:       93.68d {+06h 14m 44s}  +28.20d {+28d 11' 58"}
MOON_DIST:        44.48 [deg]
MOON_ILLUM:      71 [%]
GAL_COORDS:      162.26,-34.80 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       48.57,  0.43 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260226443/quicklook/glg_lc_medres34_bn260226443.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 88.82,12.58 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 26 Feb 26 10:41:20 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      0
TRIGGER_NUM:     793795080
GRB_RA:           41.630d {+02h 46m 31s} (J2000),
                  41.978d {+02h 47m 55s} (current),
                  40.966d {+02h 43m 52s} (1950)
GRB_DEC:          +7.310d {+07d 18' 36"} (J2000),
                  +7.419d {+07d 25' 07"} (current),
                  +7.101d {+07d 06' 03"} (1950)
GRB_ERROR:       4.68 [deg radius, statistical only]
DATA_SIGNIF:     12.70 [sigma]
DATA_INTERVAL:   10.240 [sec]
GRB_DATE:        21097 TJD;    57 DOY;   26/02/26
GRB_TIME:        38275.96 SOD {10:37:55.96} UT
GRB_PHI:          46.00 [deg]
GRB_THETA:        27.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       339.51d {+22h 38m 03s}   -8.63d {-08d 37' 35"}
SUN_DIST:         64.29 [deg]   Sun_angle= -4.2 [hr] (East of Sun)
MOON_POSTN:       93.71d {+06h 14m 50s}  +28.20d {+28d 11' 53"}
MOON_DIST:        52.97 [deg]
MOON_ILLUM:      72 [%]
GAL_COORDS:      166.06,-45.71 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       41.47, -8.36 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260226443/quicklook/glg_lc_medres34_bn260226443.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_793795080.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        In the LAT Field-of-view.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.  
COMMENTS:           
COMMENTS:        NOTE: This GBM event is temporally(9.0<100sec) coincident with the CALET_GBM event (trignum=1456137494).

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 26 Feb 26 10:47:24 UT
NOTICE_TYPE:     Fermi-GBM Final Position
RECORD_NUM:      0
TRIGGER_NUM:     793795080
GRB_RA:           45.270d {+03h 01m 05s} (J2000),
                  45.634d {+03h 02m 32s} (current),
                  44.574d {+02h 58m 18s} (1950)
GRB_DEC:         +15.740d {+15d 44' 24"} (J2000),
                 +15.842d {+15d 50' 32"} (current),
                 +15.543d {+15d 32' 34"} (1950)
GRB_ERROR:       1.00 [deg radius, statistical only]
GRB_DATE:        21097 TJD;    57 DOY;   26/02/26
GRB_TIME:        38275.96 SOD {10:37:55.96} UT
GRB_PHI:          41.00 [deg]
GRB_THETA:        17.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       339.52d {+22h 38m 04s}   -8.63d {-08d 37' 29"}
SUN_DIST:         69.87 [deg]   Sun_angle= -4.4 [hr] (East of Sun)
MOON_POSTN:       93.78d {+06h 15m 06s}  +28.19d {+28d 11' 39"}
MOON_DIST:        45.99 [deg]
MOON_ILLUM:      72 [%]
GAL_COORDS:      163.06,-36.83 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       47.35, -1.32 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260226443/quicklook/glg_lc_medres34_bn260226443.gif
LOC_URL:         http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260226443/quicklook/glg_locplot_all_bn260226443.png
COMMENTS:        Fermi-GBM Final Position.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file should be available by the time this FINAL notice is produced.  
COMMENTS:        This notice was generated completely by automated pipeline processing.  
COMMENTS:        In the LAT Field-of-view.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:           
COMMENTS:        NOTE: This GBM event is temporally(9.0<100sec) coincident with the CALET_GBM event (trignum=1456137494).

GCN Circular #43840
The Fermi GBM team reports the detection of a likely LONG GRB
At 10:37:55 UT on 26 Feb 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260226A (trigger 793795080.95811 / 260226443).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 45.3, Dec = 15.7 (J2000 degrees, equivalent to J2000 03h 01m, 15d 41'), with a statistical uncertainty of 1.0 degrees.
The angle from the Fermi LAT boresight is 17.0 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260226443/quicklook/glg_skymap_all_bn260226443.png
The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260226443/quicklook/glg_healpix_all_bn260226443.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260226443/quicklook/glg_lc_medres34_bn260226443.gif
GCN Circular #43843
T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report:
The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger 793795080 at 10:37:55 on 26 Feb. 2026 were automatically fitted for spectrum and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427; Berlato et al. 2019, ApJ 873, 60).
The best-fit position is:
```
RA(2000.0) = 43.6 deg
Decl.(2000.0) = 9.6 deg
```
The 1 sigma statistical error radius is 0.2 deg. We estimate an additional systematic error of 2 deg.
Further details are available at: https://grb.mpe.mpg.de/grb/GRB260226443/
The Healpix map can be downloaded from: https://grb.mpe.mpg.de/grb/GRB260226443/healpix
The location parameters are available as JSON at: https://grb.mpe.mpg.de/grb/GRB260226443/json
GCN Circular #43844
D. Depalo (Politecnico and INFN Bari), E. Bissaldi (Politecnico and INFN Bari), S. Zhu (DESY), R. Gupta (NASA/GSFC), J. L. Racusin (NASA/GSFC), F. Longo (Univ and INFN Trieste) report on behalf of the Fermi-LAT Collaboration:
At 10:38:18.84 UT on Feb 26, 2026, Fermi-LAT triggered on high-energy emission from GRB 260226A, also detected by GBM (trigger 793795080.95811 / 260226443, GCN Circular 43840) at 10:37:55 UT. Note that this is a rare onboard detection seeded by LAT, independent of the GBM trigger, which is why the trigger times differ. LAT-seeded alerts have a very high threshold and this is only the second GRB to have passed this alert after GRB 090510.
The onboard location is
RA, Dec 42.050, +8.033
with an approximate error radius of 0.5 deg (90% containment, systematic error only). This was 17 deg from the LAT boresight at the time of the trigger. We anticipate providing a refined location within the next 12 hours when the LAT science data for this burst are downlinked and processed.
The Fermi-LAT point of contact for this burst is Niccolò Di Lalla (niccolo.dilalla@stanford.edu).
The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.
GCN Circular #43846
Harsha K. H. (IUCAA), M. Tembhurnikar (IUCAA), S. Salunke (IUCAA), U. Pathak (IITB), A. Arya (IITB), A. Goyal (IITB), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:
Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long, bright GRB 260226A which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 43840), Fermi LAT (Depalo et. al. , GCN Circ. 43844) and Calet (via Trigger Notices, Trig Num. 1456137494).
The source was clearly detected in the CZT detectors in the 20-200 keV energy range. Due to the extremely bright nature of the GRB, all four quadrants of CZT detectors were saturated. The strongest non-saturated peak of the lightcurve occurs at 2026-02-26 10:38:22.65 UTC. This affects the total counts and peak counts reported. The measured peak count rate associated with the burst is 2464 (+267, -180) counts/s above the background in the combined data of three quadrants (out of four), with a total of 6666 (+247, -284) counts. We caution that these numbers are impacted by the data loss from saturation. The local mean background count rate was 258 (+1, -5) counts/s. Using cumulative rates, we measure a T90 of 21 (+1, -1) s. In the preliminary analysis, we find 4947 Compton events associated with this event.
The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2026-02-26 10:38:15.33 UTC. The veto detector does not show any signs of saturation. The measured peak count rate associated with the burst is 16447 (+216, -227) counts/s above the background in the combined data of all quadrants, with a total of 139460 (+832, -794) counts. The local mean background count rate was 1425 (+4, -6) counts/s. We measure a T90 of 19 (+1, -1) s from the cumulative Veto light curve.
CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.
CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb

GCN Circular #43847
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-Tunka robotic telescope [1] located in Russia (Applied Physics Institute, Irkutsk State University) started inspect of the Fermi GRB 260226A ( Fermi GBM team, GCN 43840) errorbox 11103 sec after notice time and 11134 sec after trigger time at 2026-02-26 13:43:30 UT, with upper limit up to 19.1 mag. The observations began at zenith distance = 59 deg. The sun altitude is -28.9 deg.
The galactic latitude b = -36 deg., longitude l = 164 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3156987
We obtain a following upper limits.

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   11139 | 2026-02-26 13:43:30 |        MASTER-Tunka | (02h 58m 57.34s , +16d 00m 41.9s) |   C |    10 | 18.0 |        
   11139 | 2026-02-26 13:43:30 |        MASTER-Tunka | (02h 59m 23.46s , +15d 56m 15.9s) |   C |    10 | 17.9 |        
   11149 | 2026-02-26 13:43:30 |        MASTER-Tunka | (02h 59m 23.46s , +15d 56m 15.9s) |   C |    30 | 18.8 |  Coadd 
   11163 | 2026-02-26 13:43:54 |        MASTER-Tunka | (02h 58m 52.12s , +15d 59m 40.3s) |   C |    10 | 18.0 |        
   11163 | 2026-02-26 13:43:54 |        MASTER-Tunka | (02h 59m 18.19s , +15d 55m 14.9s) |   C |    10 | 17.9 |        
   11187 | 2026-02-26 13:44:17 |        MASTER-Tunka | (02h 59m 23.99s , +15d 55m 25.8s) |   C |    10 | 17.9 |        
   11187 | 2026-02-26 13:44:17 |        MASTER-Tunka | (02h 58m 57.93s , +15d 59m 51.0s) |   C |    10 | 18.1 |        
   11212 | 2026-02-26 13:44:42 |        MASTER-Tunka | (02h 58m 54.73s , +16d 01m 23.4s) |   C |    10 | 18.0 |        
   11212 | 2026-02-26 13:44:42 |        MASTER-Tunka | (02h 59m 20.78s , +15d 56m 58.6s) |   C |    10 | 17.9 |        
   11222 | 2026-02-26 13:44:42 |        MASTER-Tunka | (02h 59m 20.78s , +15d 56m 58.6s) |   C |    30 | 18.7 |  Coadd 
   11236 | 2026-02-26 13:45:07 |        MASTER-Tunka | (02h 58m 54.78s , +15d 59m 58.2s) |   C |    10 | 17.9 |        
   11236 | 2026-02-26 13:45:07 |        MASTER-Tunka | (02h 59m 20.82s , +15d 55m 33.4s) |   C |    10 | 18.0 |        
   11261 | 2026-02-26 13:45:31 |        MASTER-Tunka | (02h 58m 57.44s , +16d 01m 12.6s) |   C |    10 | 18.0 |        
   11261 | 2026-02-26 13:45:31 |        MASTER-Tunka | (02h 59m 23.48s , +15d 56m 47.9s) |   C |    10 | 17.9 |        
   11284 | 2026-02-26 13:45:55 |        MASTER-Tunka | (02h 58m 52.43s , +16d 00m 36.6s) |   C |    10 | 17.9 |        
   11284 | 2026-02-26 13:45:55 |        MASTER-Tunka | (02h 59m 18.47s , +15d 56m 12.1s) |   C |    10 | 18.0 |        
   11294 | 2026-02-26 13:45:55 |        MASTER-Tunka | (02h 59m 18.47s , +15d 56m 12.1s) |   C |    30 | 18.8 |  Coadd 
   11307 | 2026-02-26 13:46:17 |        MASTER-Tunka | (02h 58m 51.55s , +16d 01m 36.1s) |   C |    10 | 18.0 |        
   11307 | 2026-02-26 13:46:17 |        MASTER-Tunka | (02h 59m 17.60s , +15d 57m 11.6s) |   C |    10 | 17.9 |        
   11329 | 2026-02-26 13:46:40 |        MASTER-Tunka | (02h 58m 58.52s , +16d 00m 35.7s) |   C |    10 | 17.9 |        
   11329 | 2026-02-26 13:46:40 |        MASTER-Tunka | (02h 59m 24.56s , +15d 56m 11.1s) |   C |    10 | 17.9 |        
   11352 | 2026-02-26 13:47:03 |        MASTER-Tunka | (02h 59m 18.76s , +15d 55m 10.5s) |   C |    10 | 17.9 |        
   11362 | 2026-02-26 13:47:03 |        MASTER-Tunka | (02h 59m 18.77s , +15d 55m 10.4s) |   C |    30 | 18.7 |  Coadd 
   11353 | 2026-02-26 13:47:03 |        MASTER-Tunka | (02h 58m 52.73s , +15d 59m 34.6s) |   C |    10 | 18.0 |        
   11374 | 2026-02-26 13:47:25 |        MASTER-Tunka | (02h 58m 57.05s , +15d 59m 57.9s) |   C |    10 | 17.9 |        
   11374 | 2026-02-26 13:47:25 |        MASTER-Tunka | (02h 59m 23.10s , +15d 55m 33.8s) |   C |    10 | 17.9 |        
   11400 | 2026-02-26 13:47:50 |        MASTER-Tunka | (02h 58m 55.01s , +16d 01m 17.3s) |   C |    10 | 17.9 |        
   11400 | 2026-02-26 13:47:50 |        MASTER-Tunka | (02h 59m 21.06s , +15d 56m 53.3s) |   C |    10 | 17.9 |        
   11426 | 2026-02-26 13:48:13 |        MASTER-Tunka | (02h 58m 55.07s , +15d 59m 57.3s) |   C |    15 | 18.2 |        
   11426 | 2026-02-26 13:48:13 |        MASTER-Tunka | (02h 59m 21.10s , +15d 55m 33.2s) |   C |    15 | 18.2 |        
   11441 | 2026-02-26 13:48:13 |        MASTER-Tunka | (02h 59m 21.10s , +15d 55m 33.2s) |   C |    45 | 19.0 |  Coadd 
   11453 | 2026-02-26 13:48:41 |        MASTER-Tunka | (02h 58m 58.09s , +16d 01m 14.6s) |   C |    15 | 18.1 |        
   11453 | 2026-02-26 13:48:41 |        MASTER-Tunka | (02h 59m 24.14s , +15d 56m 50.8s) |   C |    15 | 18.1 |        
   11481 | 2026-02-26 13:49:08 |        MASTER-Tunka | (02h 58m 51.32s , +16d 00m 32.0s) |   C |    15 | 18.1 |        
   11481 | 2026-02-26 13:49:08 |        MASTER-Tunka | (02h 59m 17.36s , +15d 56m 08.1s) |   C |    15 | 18.1 |        
   11514 | 2026-02-26 13:49:41 |        MASTER-Tunka | (02h 58m 51.83s , +16d 01m 31.0s) |   C |    15 | 18.1 |        
   11514 | 2026-02-26 13:49:42 |        MASTER-Tunka | (02h 59m 17.88s , +15d 57m 07.4s) |   C |    15 | 18.1 |        
   11529 | 2026-02-26 13:49:42 |        MASTER-Tunka | (02h 59m 17.88s , +15d 57m 07.5s) |   C |    45 | 18.9 |  Coadd 
   11542 | 2026-02-26 13:50:09 |        MASTER-Tunka | (02h 58m 57.84s , +16d 00m 30.9s) |   C |    15 | 18.2 |        
   11542 | 2026-02-26 13:50:09 |        MASTER-Tunka | (02h 59m 23.88s , +15d 56m 07.4s) |   C |    15 | 18.1 |        
   11574 | 2026-02-26 13:50:41 |        MASTER-Tunka | (02h 58m 51.31s , +15d 59m 30.1s) |   C |    15 | 18.2 |        
   11574 | 2026-02-26 13:50:41 |        MASTER-Tunka | (02h 59m 17.33s , +15d 55m 06.6s) |   C |    15 | 18.2 |        
   11604 | 2026-02-26 13:51:11 |        MASTER-Tunka | (02h 58m 58.44s , +15d 59m 43.7s) |   C |    15 | 18.1 |        
   11604 | 2026-02-26 13:51:11 |        MASTER-Tunka | (02h 59m 24.49s , +15d 55m 20.3s) |   C |    15 | 18.1 |        
   11619 | 2026-02-26 13:51:11 |        MASTER-Tunka | (02h 59m 24.49s , +15d 55m 20.2s) |   C |    45 | 19.0 |  Coadd 
   11633 | 2026-02-26 13:51:40 |        MASTER-Tunka | (02h 58m 55.34s , +16d 01m 27.4s) |   C |    15 | 18.1 |        
   11633 | 2026-02-26 13:51:40 |        MASTER-Tunka | (02h 59m 21.38s , +15d 57m 04.2s) |   C |    15 | 18.1 |        
   11662 | 2026-02-26 13:52:09 |        MASTER-Tunka | (02h 58m 55.45s , +15d 59m 35.4s) |   C |    15 | 18.2 |        
   11662 | 2026-02-26 13:52:09 |        MASTER-Tunka | (02h 59m 21.45s , +15d 55m 12.6s) |   C |    15 | 18.1 |        
   11691 | 2026-02-26 13:52:38 |        MASTER-Tunka | (02h 58m 57.68s , +16d 00m 59.7s) |   C |    15 | 18.2 |        
   11691 | 2026-02-26 13:52:38 |        MASTER-Tunka | (02h 59m 23.70s , +15d 56m 36.7s) |   C |    15 | 18.1 |        
   11706 | 2026-02-26 13:52:38 |        MASTER-Tunka | (02h 59m 23.70s , +15d 56m 36.7s) |   C |    45 | 19.1 |  Coadd 
   11721 | 2026-02-26 13:53:08 |        MASTER-Tunka | (02h 58m 52.55s , +16d 00m 26.7s) |   C |    15 | 18.2 |        
   11721 | 2026-02-26 13:53:08 |        MASTER-Tunka | (02h 59m 18.55s , +15d 56m 04.1s) |   C |    15 | 18.1 |        
   11751 | 2026-02-26 13:53:38 |        MASTER-Tunka | (02h 58m 53.33s , +16d 01m 26.5s) |   C |    15 | 18.1 |        
   11751 | 2026-02-26 13:53:38 |        MASTER-Tunka | (02h 59m 19.32s , +15d 57m 03.9s) |   C |    15 | 18.1 |

Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.
[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http : // www.pereplet.ru/lipunov/625.html

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 26 Feb 26 15:42:21 UT
NOTICE_TYPE:     Fermi-LAT Offline Position
TRIGGER_NUM:     793795080
GRB_RA:           41.921d {+02h 47m 41s} (J2000),
                  42.269d {+02h 49m 05s} (current),
                  41.256d {+02h 45m 01s} (1950)
GRB_DEC:          +7.720d {+07d 43' 12"} (J2000),
                  +7.828d {+07d 49' 41"} (current),
                  +7.512d {+07d 30' 42"} (1950)
GRB_ERROR:       6.35 [arcmin radius, 90% containment, statistical only]
GRB_DATE:        21097 TJD;    57 DOY;   26/02/26
GRB_TIME:        38298.84 SOD {10:38:18.84} UT
TRIGGER_ID:      0x20000000
MISC:            0x40000000
SUN_POSTN:       339.71d {+22h 38m 50s}   -8.55d {-08d 32' 53"}
SUN_DIST:         64.46 [deg]   Sun_angle= -4.2 [hr] (East of Sun)
MOON_POSTN:       97.05d {+06h 28m 12s}  +27.96d {+27d 57' 48"}
MOON_DIST:        55.35 [deg]
MOON_ILLUM:      74 [%]
GAL_COORDS:      166.02,-45.21 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       41.87, -8.05 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        Fermi LAT Offline position.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        It is the result of human-in-the-loop processing.  
COMMENTS:        This is a human generated position of a LAT ground detection.  
COMMENTS:        This source corresponds to GBM trigger.

GCN Circular #43848
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-Tunka robotic telescope [1] located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the LAT GRB260226.44 (trigger No 793795080,02h 47m 41.04s , +07d 43m 12.0s, R=0.105833) errorbox 16332 sec after trigger time at 2026-02-26 15:10:31 UT, with upper limit up to 17.0 mag. The observations began at zenith distance = 81 deg. The sun altitude is -40.1 deg.
The galactic latitude b = -45 deg., longitude l = 167 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3157390
We obtain a following upper limits.

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   16337 | 2026-02-26 15:10:31 |        MASTER-Tunka | (02h 46m 41.03s , +08d 13m 10.9s) |   C |    10 | 16.4 |        
   16347 | 2026-02-26 15:10:31 |        MASTER-Tunka | (02h 46m 41.03s , +08d 13m 11.0s) |   C |    30 | 17.0 |  Coadd 
   16338 | 2026-02-26 15:10:31 |        MASTER-Tunka | (02h 46m 22.49s , +08d 16m 39.4s) |   C |    10 | 16.0 |        
   16362 | 2026-02-26 15:10:56 |        MASTER-Tunka | (02h 46m 34.45s , +08d 12m 33.1s) |   C |    10 | 16.5 |        
   16363 | 2026-02-26 15:10:56 |        MASTER-Tunka | (02h 46m 16.08s , +08d 15m 59.0s) |   C |    10 | 15.8 |        
   16384 | 2026-02-26 15:11:18 |        MASTER-Tunka | (02h 46m 33.55s , +08d 13m 32.1s) |   C |    10 | 16.4 |        
   16384 | 2026-02-26 15:11:18 |        MASTER-Tunka | (02h 46m 15.50s , +08d 16m 58.7s) |   C |    10 | 16.0 |        
   16408 | 2026-02-26 15:11:41 |        MASTER-Tunka | (02h 46m 40.83s , +08d 12m 31.4s) |   C |    10 | 16.4 |        
   16408 | 2026-02-26 15:11:41 |        MASTER-Tunka | (02h 46m 22.76s , +08d 15m 57.8s) |   C |    10 | 15.9 |        
   16430 | 2026-02-26 15:12:04 |        MASTER-Tunka | (02h 46m 33.93s , +08d 11m 31.6s) |   C |    10 | 16.3 |        
   16431 | 2026-02-26 15:12:04 |        MASTER-Tunka | (02h 46m 15.91s , +08d 14m 57.9s) |   C |    10 | 15.8 |        
   16454 | 2026-02-26 15:12:27 |        MASTER-Tunka | (02h 46m 40.26s , +08d 11m 42.7s) |   C |    10 | 16.1 |        
   16454 | 2026-02-26 15:12:27 |        MASTER-Tunka | (02h 46m 22.16s , +08d 15m 07.0s) |   C |    10 | 15.3 |        
   16478 | 2026-02-26 15:12:52 |        MASTER-Tunka | (02h 46m 37.35s , +08d 12m 56.4s) |   C |    10 | 15.0 |        
   16479 | 2026-02-26 15:12:52 |        MASTER-Tunka | (02h 46m 19.39s , +08d 16m 23.6s) |   C |    10 | 14.8 |        
   16503 | 2026-02-26 15:13:17 |        MASTER-Tunka | (02h 46m 37.23s , +08d 11m 26.2s) |   C |    10 | 14.7 |        
   16503 | 2026-02-26 15:13:17 |        MASTER-Tunka | (02h 46m 19.51s , +08d 14m 56.8s) |   C |    10 | 14.6 |        
   16526 | 2026-02-26 15:13:40 |        MASTER-Tunka | (02h 46m 39.73s , +08d 13m 11.3s) |   C |    10 | 15.8 |        
   16527 | 2026-02-26 15:13:40 |        MASTER-Tunka | (02h 46m 21.80s , +08d 16m 35.6s) |   C |    10 | 15.2 |        
   16550 | 2026-02-26 15:14:03 |        MASTER-Tunka | (02h 46m 33.44s , +08d 12m 29.2s) |   C |    10 | 15.3 |        
   16550 | 2026-02-26 15:14:03 |        MASTER-Tunka | (02h 46m 15.66s , +08d 15m 55.3s) |   C |    10 | 14.9 |        
   16576 | 2026-02-26 15:14:30 |        MASTER-Tunka | (02h 46m 34.78s , +08d 13m 28.4s) |   C |    10 | 15.3 |        
   16577 | 2026-02-26 15:14:30 |        MASTER-Tunka | (02h 46m 17.03s , +08d 16m 54.9s) |   C |    10 | 15.1 |        
   16607 | 2026-02-26 15:15:01 |        MASTER-Tunka | (02h 46m 40.60s , +08d 12m 25.3s) |   C |    10 | 15.4 |        
   16607 | 2026-02-26 15:15:01 |        MASTER-Tunka | (02h 46m 22.93s , +08d 15m 51.3s) |   C |    10 | 14.9 |        
   16630 | 2026-02-26 15:15:23 |        MASTER-Tunka | (02h 46m 33.66s , +08d 11m 26.0s) |   C |    10 | 15.4 |        
   16630 | 2026-02-26 15:15:23 |        MASTER-Tunka | (02h 46m 16.01s , +08d 14m 52.2s) |   C |    10 | 15.1 |        
   16653 | 2026-02-26 15:15:46 |        MASTER-Tunka | (02h 46m 41.11s , +08d 11m 47.9s) |   C |    10 | 16.1 |        
   16653 | 2026-02-26 15:15:46 |        MASTER-Tunka | (02h 46m 24.19s , +08d 15m 25.7s) |   C |    10 | 14.8 |        
   16676 | 2026-02-26 15:16:09 |        MASTER-Tunka | (02h 46m 37.35s , +08d 13m 13.3s) |   C |    10 | 15.6 |        
   16676 | 2026-02-26 15:16:09 |        MASTER-Tunka | (02h 46m 19.66s , +08d 16m 38.3s) |   C |    10 | 15.2 |        
   16832 | 2026-02-26 15:18:43 |        MASTER-Tunka | (02h 46m 34.69s , +08d 13m 32.9s) |   C |    15 | 16.1 |        
   16832 | 2026-02-26 15:18:43 |        MASTER-Tunka | (02h 46m 16.54s , +08d 16m 52.9s) |   C |    15 | 15.5 |        
   16859 | 2026-02-26 15:19:09 |        MASTER-Tunka | (02h 46m 40.75s , +08d 12m 32.7s) |   C |    15 | 16.0 |        
   16859 | 2026-02-26 15:19:09 |        MASTER-Tunka | (02h 46m 22.88s , +08d 15m 54.8s) |   C |    15 | 15.4 |        
   16884 | 2026-02-26 15:19:37 |        MASTER-Tunka | (02h 46m 35.06s , +08d 11m 32.5s) |   C |    10 | 15.8 |        
   16884 | 2026-02-26 15:19:38 |        MASTER-Tunka | (02h 46m 17.19s , +08d 14m 52.2s) |   C |    10 | 15.2 |        
   16908 | 2026-02-26 15:20:02 |        MASTER-Tunka | (02h 46m 41.38s , +08d 11m 36.4s) |   C |    10 | 16.1 |        
   16908 | 2026-02-26 15:20:02 |        MASTER-Tunka | (02h 46m 23.62s , +08d 14m 57.3s) |   C |    10 | 15.4 |        
   16947 | 2026-02-26 15:20:41 |        MASTER-Tunka | (02h 46m 38.12s , +08d 13m 07.4s) |   C |    10 | 16.1 |        
   16947 | 2026-02-26 15:20:41 |        MASTER-Tunka | (02h 46m 20.25s , +08d 16m 28.2s) |   C |    10 | 15.4 |        
   16969 | 2026-02-26 15:21:02 |        MASTER-Tunka | (02h 46m 38.11s , +08d 11m 37.5s) |   C |    10 | 16.0 |        
   16969 | 2026-02-26 15:21:03 |        MASTER-Tunka | (02h 46m 19.74s , +08d 14m 48.1s) |   C |    10 | 15.5 |        
   16993 | 2026-02-26 15:21:26 |        MASTER-Tunka | (02h 46m 40.15s , +08d 13m 08.3s) |   C |    10 | 15.8 |        
   16993 | 2026-02-26 15:21:26 |        MASTER-Tunka | (02h 46m 22.32s , +08d 16m 29.7s) |   C |    10 | 15.3 |        
   17017 | 2026-02-26 15:21:51 |        MASTER-Tunka | (02h 46m 35.71s , +08d 12m 33.0s) |   C |    10 | 16.1 |        
   17017 | 2026-02-26 15:21:51 |        MASTER-Tunka | (02h 46m 17.88s , +08d 15m 54.5s) |   C |    10 | 15.4 |        
   17042 | 2026-02-26 15:22:16 |        MASTER-Tunka | (02h 46m 35.36s , +08d 13m 30.6s) |   C |    10 | 15.9 |        
   17042 | 2026-02-26 15:22:16 |        MASTER-Tunka | (02h 46m 17.46s , +08d 16m 51.1s) |   C |    10 | 15.2 |        
   17068 | 2026-02-26 15:22:41 |        MASTER-Tunka | (02h 46m 40.47s , +08d 12m 31.9s) |   C |    10 | 15.9 |        
   17068 | 2026-02-26 15:22:41 |        MASTER-Tunka | (02h 46m 22.01s , +08d 15m 41.5s) |   C |    10 | 15.5 |        
   17092 | 2026-02-26 15:23:05 |        MASTER-Tunka | (02h 46m 35.02s , +08d 11m 33.0s) |   C |    10 | 15.7 |        
   17092 | 2026-02-26 15:23:05 |        MASTER-Tunka | (02h 46m 17.26s , +08d 14m 55.9s) |   C |    10 | 15.2 |        
   17114 | 2026-02-26 15:23:27 |        MASTER-Tunka | (02h 46m 41.21s , +08d 11m 47.1s) |   C |    10 | 15.8 |        
   17114 | 2026-02-26 15:23:27 |        MASTER-Tunka | (02h 46m 23.36s , +08d 15m 07.6s) |   C |    10 | 15.5 |        
   17139 | 2026-02-26 15:23:52 |        MASTER-Tunka | (02h 46m 38.16s , +08d 13m 22.7s) |   C |    10 | 15.8 |        
   17139 | 2026-02-26 15:23:52 |        MASTER-Tunka | (02h 46m 19.90s , +08d 16m 35.1s) |   C |    10 | 15.3 |        
   17163 | 2026-02-26 15:24:16 |        MASTER-Tunka | (02h 46m 38.13s , +08d 11m 34.8s) |   C |    10 | 15.8 |        
   17163 | 2026-02-26 15:24:16 |        MASTER-Tunka | (02h 46m 20.56s , +08d 15m 02.0s) |   C |    10 | 15.3 |        
   17187 | 2026-02-26 15:24:40 |        MASTER-Tunka | (02h 46m 41.99s , +08d 13m 24.9s) |   C |    10 | 16.0 |        
   17187 | 2026-02-26 15:24:40 |        MASTER-Tunka | (02h 46m 24.35s , +08d 16m 51.1s) |   C |    10 | 15.3 |        
   17209 | 2026-02-26 15:25:03 |        MASTER-Tunka | (02h 46m 35.05s , +08d 12m 30.7s) |   C |    10 | 15.9 |        
   17209 | 2026-02-26 15:25:03 |        MASTER-Tunka | (02h 46m 17.41s , +08d 15m 58.0s) |   C |    10 | 15.2 |        
   17231 | 2026-02-26 15:25:25 |        MASTER-Tunka | (02h 46m 34.52s , +08d 13m 31.1s) |   C |    10 | 15.8 |        
   17231 | 2026-02-26 15:25:25 |        MASTER-Tunka | (02h 46m 16.43s , +08d 16m 48.8s) |   C |    10 | 15.2 |        
   17258 | 2026-02-26 15:25:51 |        MASTER-Tunka | (02h 46m 41.61s , +08d 12m 31.5s) |   C |    10 | 15.8 |        
   17258 | 2026-02-26 15:25:51 |        MASTER-Tunka | (02h 46m 24.00s , +08d 15m 56.3s) |   C |    10 | 15.3 |        
   17281 | 2026-02-26 15:26:14 |        MASTER-Tunka | (02h 46m 34.33s , +08d 11m 30.5s) |   C |    10 | 15.8 |        
   17281 | 2026-02-26 15:26:14 |        MASTER-Tunka | (02h 46m 16.53s , +08d 14m 52.9s) |   C |    10 | 15.2 |        
   17305 | 2026-02-26 15:26:38 |        MASTER-Tunka | (02h 46m 40.80s , +08d 11m 40.7s) |   C |    10 | 16.0 |        
   17305 | 2026-02-26 15:26:38 |        MASTER-Tunka | (02h 46m 22.93s , +08d 14m 59.1s) |   C |    10 | 15.2 |        
   17330 | 2026-02-26 15:27:03 |        MASTER-Tunka | (02h 46m 37.97s , +08d 13m 22.4s) |   C |    10 | 15.7 |        
   17330 | 2026-02-26 15:27:03 |        MASTER-Tunka | (02h 46m 20.40s , +08d 16m 48.5s) |   C |    10 | 15.2 |        
   17355 | 2026-02-26 15:27:29 |        MASTER-Tunka | (02h 46m 38.21s , +08d 11m 42.8s) |   C |    10 | 15.8 |        
   17356 | 2026-02-26 15:27:29 |        MASTER-Tunka | (02h 46m 20.53s , +08d 15m 06.2s) |   C |    10 | 15.2 |        
   17379 | 2026-02-26 15:27:52 |        MASTER-Tunka | (02h 46m 40.47s , +08d 13m 29.7s) |   C |    10 | 15.8 |        
   17379 | 2026-02-26 15:27:52 |        MASTER-Tunka | (02h 46m 22.85s , +08d 16m 53.5s) |   C |    10 | 15.2 |        
   17404 | 2026-02-26 15:28:18 |        MASTER-Tunka | (02h 46m 34.36s , +08d 12m 28.4s) |   C |    10 | 15.8 |        
   17404 | 2026-02-26 15:28:18 |        MASTER-Tunka | (02h 46m 16.62s , +08d 15m 50.9s) |   C |    10 | 15.2 |        
   17428 | 2026-02-26 15:28:41 |        MASTER-Tunka | (02h 46m 35.07s , +08d 13m 27.8s) |   C |    10 | 15.7 |        
   17428 | 2026-02-26 15:28:42 |        MASTER-Tunka | (02h 46m 17.38s , +08d 16m 50.8s) |   C |    10 | 15.1 |        
   17457 | 2026-02-26 15:29:10 |        MASTER-Tunka | (02h 46m 39.98s , +08d 12m 26.4s) |   C |    10 | 15.7 |        
   17457 | 2026-02-26 15:29:10 |        MASTER-Tunka | (02h 46m 22.36s , +08d 15m 50.5s) |   C |    10 | 15.1 |        
   17480 | 2026-02-26 15:29:33 |        MASTER-Tunka | (02h 46m 34.99s , +08d 11m 25.9s) |   C |    10 | 15.7 |        
   17480 | 2026-02-26 15:29:33 |        MASTER-Tunka | (02h 46m 17.41s , +08d 14m 49.8s) |   C |    10 | 15.2 |        
   17507 | 2026-02-26 15:30:01 |        MASTER-Tunka | (02h 46m 40.94s , +08d 11m 33.4s) |   C |    10 | 15.7 |        
   17507 | 2026-02-26 15:30:01 |        MASTER-Tunka | (02h 46m 23.14s , +08d 14m 53.9s) |   C |    10 | 15.1 |        
   17531 | 2026-02-26 15:30:24 |        MASTER-Tunka | (02h 46m 37.85s , +08d 13m 16.0s) |   C |    10 | 15.7 |        
   17531 | 2026-02-26 15:30:24 |        MASTER-Tunka | (02h 46m 20.32s , +08d 16m 41.5s) |   C |    10 | 15.0 |        
   17553 | 2026-02-26 15:30:46 |        MASTER-Tunka | (02h 46m 37.93s , +08d 11m 54.6s) |   C |    10 | 15.7 |        
   17553 | 2026-02-26 15:30:46 |        MASTER-Tunka | (02h 46m 20.09s , +08d 15m 13.6s) |   C |    10 | 15.0 |        
   17576 | 2026-02-26 15:31:10 |        MASTER-Tunka | (02h 46m 40.42s , +08d 12m 54.8s) |   C |    10 | 15.6 |        
   17576 | 2026-02-26 15:31:10 |        MASTER-Tunka | (02h 46m 22.84s , +08d 16m 17.8s) |   C |    10 | 14.9 |        
   17600 | 2026-02-26 15:31:33 |        MASTER-Tunka | (02h 46m 34.70s , +08d 12m 22.5s) |   C |    10 | 15.6 |        
   17600 | 2026-02-26 15:31:33 |        MASTER-Tunka | (02h 46m 16.99s , +08d 15m 44.7s) |   C |    10 | 15.1 |        
   17626 | 2026-02-26 15:32:00 |        MASTER-Tunka | (02h 46m 35.77s , +08d 13m 23.5s) |   C |    10 | 15.6 |        
   17626 | 2026-02-26 15:32:00 |        MASTER-Tunka | (02h 46m 18.12s , +08d 16m 47.2s) |   C |    10 | 15.1 |        
   17650 | 2026-02-26 15:32:23 |        MASTER-Tunka | (02h 46m 40.81s , +08d 12m 21.6s) |   C |    10 | 15.7 |

GCN Circular #43850
D. Depalo (Politecnico and INFN Bari), E. Bissaldi (Politecnico and INFN Bari), A. Holzmann Airasca (UniTrento and INFN Bari), R. Gupta (NASA/GSFC), F. Longo (University and INFN, Trieste), S. Lopez (CNRS / IN2P3), R. Martinelli (University and INFN, Trieste), and J. L. Racusin (NASA/GSFC) report on behalf of the Fermi-LAT team:
At 10:38:18.84 UT on Feb 26th, 2026 Fermi-LAT triggered and detected high-energy emission from GRB 260226A (Depalo et al., GCN Circular 43844), which was also detected by Fermi-GBM (trigger 793795080 / 260226443, GCN Circular 43840) and Astrosat (Harsha et al., GCN Circular 43846).
The best LAT on-ground location is found to be
RA, Dec = 41.93, 7.73 (J2000)
with an error radius of 0.11 deg (90 % containment, statistical error only).
This was 26 deg from the LAT boresight at the time of the trigger.
The data from the Fermi-LAT show a significant increase in the event rate that is spatially and temporally correlated with the trigger with high significance. The photon flux above 100 MeV in the time interval 0 - 1.5 ks after the GBM trigger is (1.50 ± 0.06)E-4 ph/cm2/s. The estimated photon index above 100 MeV is -3.25 ± 0.08. The highest energy photon has an energy of 1.1 GeV and occurs at 420 s after trigger time.
The Fermi-LAT point of contact for this burst is Niccolò Di Lalla (niccolo.dilalla@stanford.edu).
The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.
GCN Circular #43851
E. Bissaldi (Politecnico and INFN Bari), O.J. Roberts (Uni. of Galway, Ireland), P. Veres (UAH) and A. von Kienlin (MPE) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 10:39:05 UT on 26 February 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260226A (trigger 793795080/260226443), which was also detected by Fermi-LAT (Depalo et al. 2026, GCN 43844 and 43850) and AstroSat CZTI (Harsha et al. 2026, GCN 43846).
The Fermi GBM on-ground location (GCN 43840) is consistent with the Fermi-LAT position.
The angle from the Fermi LAT boresight is 26 degrees.
The GBM light curve consists of a very bright and structured emission episode with a duration (T90) of about 82 s (50-300 keV).
The time-averaged spectrum from T0 to T0+81 s is best fit by a Band function with Epeak = 723 +/- 8 keV, alpha = -0.95 +/- 0.01, and beta = -2.47 +/- 0.02.
The event fluence (10-1000 keV) in this time interval is (4.54 +/- 0.01)E-04 erg/cm^2. The 1-sec peak photon flux measured starting from T0+19.3 s in the 10-1000 keV band is 166 +/- 2 ph/s/cm^2.
The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html
For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"
GCN Circular #43852
R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), S. Fainer (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), and S. Spitzer (WIS) report on behalf of the LAST Collaboration.
We report observations of GRB 260226A, reported by Fermi-GBM (Fermi GBM Team, GCN 43840), as well as by Fermi-LAT (Depalo et al, GCN 43844) and AstroSat CZTI (Harsha et al, GCN 43846). Observations were conducted with the Large Array Survey Telescope (LAST; Ofek et al. 2023, PASP 135, 5001; Ben-Ami et al. 2023, PASP 135, 5002).
Observations of GRB 260226A were taken over 10 sequential epochs in clear band (similar to the Gaia Bp band) with 4 telescopes simultaneously. The first epoch began at 2026-02-26 16:30:51UTC (T-T0 = 5.88 h). Each epoch consists of 20x20s exposures per telescope.
We coadd a total of 4x120x20s (mid. T-T0=6.67 h) exposure images and perform image subtraction using a reference image of the field. We do not detect any clear new optical source up to a 5-sigma limiting magnitude of 20.15 (AB) within the error region reported in Bissaldi et al., GCN 43851. Performing image subtraction between the latest (mid. T-T0 = 7.31 h) and earliest (mid. T-T0 = 5.94 h) 20x20s exposure images also shows no source brighter than 19.4 mag decaying by more than 0.5 mag. Our results are consistent with other optical non-detections (Lipunov et al., GCN 43847,43848).
LAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/).

GCN Circular #43853
A.P. Saikia, V. Vijaykumar, T. Mohan, A. Devaraj, S. Patil, V. Swain, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:
We observed the field of Fermi GRB 260226A (Fermi GBM team, GCN 43840) also detected by Fermi LAT (Depalo et al., GCN 43844, 43850), with 0.7m GROWTH-India Telescope (GIT). The observations started at 2026-02-26 14:33:34 UT, i.e 3.9 hours after the GBM trigger. We tiled the LAT localization region, observed the field at a position angle of approximately 45 degrees, and covered about 74% of the localization area. No new transients were detected within the observed region. Each exposure was taken in the r′ filter with an exposure time of 300 seconds. The photometric results are summarized below:

Tile Number | Tmid - T0 (hours)    | RA (deg) | Dec (deg) |Upper limit (5 sigma)(AB) |
| ------------ | ------ | --------------- | ------------------------- | -------------- |
| 1            | 3.95   | 42.05           | 8.033                     | 19.7           |
| 2            | 4.03   | 41.83           | 7.713                     | 19.6           |
| 3            | 4.12   | 41.83           | 7.873                     | 19.9           |
| 4            | 4.21   | 41.83           | 8.033                     | 19.8           |
| 5            | 4.30   | 41.83           | 8.193                     | 19.6           |
| 6            | 4.39   | 41.83           | 8.353                     | 19.7           |
| 7            | 4.47   | 41.94           | 7.713                     | 19.6           |
| 8            | 4.56   | 41.94           | 7.873                     | 19.7           |
| 9            | 4.65   | 41.94           | 8.033                     | 19.6           |
| 10           | 4.75   | 41.94           | 8.193                     | 19.5           |
| 11           | 4.84   | 41.94           | 8.353                     | 19.6           |
| 12           | 4.93   | 42.05           | 7.713                     | 19.4           |
| 13           | 5.02   | 42.05           | 7.873                     | 19.2           |
| 14           | 5.10   | 42.05           | 8.193                     | 19.3           |
| 15           | 5.19   | 42.05           | 8.353                     | 19.2           |
| 16           | 5.28   | 42.16           | 7.873                     | 19.0           |

The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.
Our results are consistent with other optical non-detections (Lipunov et al., GCN 43847,43848, Konno et al., 43852)
The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 8' x 11' field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.

GCN Circular #43854
G. Waratkar (Caltech) and B. Grefenstette (Caltech) report on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group:
The NuSTAR SINGS working group reports the detection of bright prompt emission from the long-duration GRB 260226A in both the NuSTAR CsI anti-coincidence shields. Details of the search algorithm will be described in a future paper.
The NuSTAR SINGS algorithm, triggered at 2026-02-26 10:38:05.0, shows a detection of GRB 260226A consistent with the detections by Fermi/GBM (Fermi GBM Team, GCN Circ. 43840) and AstroSat/CZTI (Harsha et al., GCN Circ. 43846).
The NuSTAR CsI shield data are recorded at 1 Hz. We detect a single bright peak lasting for ~20-s, consistent with the bright episode from the lightcurve of Fermi/GBM. The peak count rate is ~14000-cps with a baseline rate of ~1000-cps during this time period. We also see clear evidence above 100 keV in the CdZnTe detectors, lasting for ~20-s.
The Fermi/LAT localization (Depalo et al., GCN Circ. 43844) at RA = 42.05, Dec = 8.033 implies an offset from the NuSTAR boresight of 91-deg (i.e. from the side of the instrument) and an offset from the geocenter of 68-deg.
Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260226A
Information on NuSTAR SINGS can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/
NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington.
GCN Circular #43855
R. Woolf, C.C. Cheung, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:
The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 260226A, which was also detected by Fermi GBM (GCN 43840), Fermi LAT (GCN 43844), AstroSat CZTI (GCN 43846), NuSTAR (GCN 43854), and CALET (Trigger 1456137494).
Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2026-02-26 10:38:07.336 with a duration of 18.6s and a total significance of about 61.8 sigma. The light curve comprises three primary peaks at ~T0+6s, T0+7.5s and T0+10.5s, followed by three lesser peaks at ~T0+14s, T0+16s and T0+18s. Note that data from ~T0+9.5s to T0+11s suffered from deadtime in various detectors. Additionally, CLLB inorganic scintillation detectors [2], internal to the Glowbug shielded chassis, registered counts above background that are coincidence in time with the main Glowbug detectors.
The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS.
Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12.
```
[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108
```
Distribution Statement A: Approved for public release. Distribution is unlimited.
GCN Circular #43858
R. Strausbaugh (Eastern Illinois University), A. Cucchiara (NASA) report on behalf of a larger collaboration:
We observed the Fermi GRB 260226A field (Fermi GBM Team, GCN 43840) with the LCOGT 1-meter Sinistro instrument at the McDonald Observatory, USA site, on February 27, from 01:46 to 02:18 UT (corresponding to 15.15 to 15.68 hours after the GRB trigger time) with the SDSS r and i filters.
We performed a series of 3x300s exposures in i-band and r-band. We do not detect any uncatalogued sources within the Fermi LAT error region (Depalo et al., GCN 43850) in either band.
The following 5-sigma upper limits are calculated using the PanSTARRS catalog as reference:
```
r > 21.6
i > 21.1
```
These magnitudes are not corrected for galactic extinction.
GCN Circular #43859
Camila Angulo (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of the Fermi GRB 260226A (Fermi GBM Team, GCN Circ. 43840, Fermi LAT, GCN Circ. 43844) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-02-27 02:54:20 to 03:52:25 UTC (from 16.27 to 17.25 hours after the trigger) and obtained 45 minutes of simultaneous exposures in the r and z filters.
The data were reduced and coadded with the ASU pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked images, comparing to LS (Dey et al. 2019), we do not detect any new source at the LAT source position (Depalo et al., GCN Circ. 43850) down to the following 5-sigma limits:
```
r > 22.86
z > 21.99
```
These upper limits are consistent with the ones reported by Lipunov et al. (GCN Circ. 43847, 43848), Konno et al. (GCN Circ. 43852), Saikia et al. (GCN Circ. 43853), and Strausbaugh et al. (GCN Circ. 43858).
We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular #43860
K. Kobayashi (Waseda U), A. Yoshida, T. Sakamoto, S. Sugita, Y. Kawakubo (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR), S. Torii, Y. Akaike, Y. Shimizu, T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC), M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena), and the CALET collaboration:
The long GRB 260226A (Fermi GBM Final Real-time Localization: Fermi GBM team, GCN Circ. 43840; BALROG localization: Preis et al., GCN Circ 43843; Fermi-LAT detection: Depalo et al., GCN Circ 43844; AstroSat CZTI detection: Harsha et al., GCN Circ 43846; Fermi-LAT refined analysis: Depalo et al., GCN Circ 43850; Fermi GBM observation: Bissaldi et al., GCN Circ 43851; NuSTAR detection: Waratkar et al., GCN Circ 43854; Glowbug gamma-ray detection: Woolf et al., GCN Circ 43855) triggered the CALET Gamma-ray Burst Monitor (CGBM) at 10:38:04.46 UTC on 26 February 2026 (https://cgbm.calet.jp/cgbm_trigger/flight/1456137494/index.html). The burst signal was seen by all CGBM detectors.
The burst light curve shows a multi-peaked structure that starts at T-3.9 sec, peaks at T+11.1 sec, and ends at T+37.0 sec. The T90 and T50 durations measured by the SGM data are 16.0 +/- 0.3 sec and 8.2 +/- 0.1 sec (40-1000 keV), respectively.
The ground-processed light curve is available at
https://cgbm.calet.jp/cgbm_trigger/ground/1456137494/
The CALET data used in this analysis are provided by the Waseda CALET Operation Center located at Waseda University.
GCN Circular #43864
D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, A. Tsvetkova, M. Ulanov, and T. Cline on behalf of the Konus-Wind team, report:
The bright, long-duration GRB 260226A (Fermi-GBM detection: The Fermi GBM team, GCN 43840; Bissaldi eat al., GCN 43851; BALROG localization: Preis and Greiner, GCN 43843; Fermi-LAT detection: Depalo et al., GCN 43844, 43850; AstroSat-CZTI detection: Harsha et al., GCN 43846; NuSTAR-ACS detection: Waratkar et al., GCN 43854; Glowbug detection: Woolf et al., GCN 43855; CALET-GBM detection: Kobayashi et al., GCN 43860) triggered Konus-Wind at T0=38287.440 s UT (10:38:07.440).
The burst light curve shows a very bright, multi-peaked emission pulse which starts at ~T0-9 s and has a duration of ~30 s. This pulse is followed by a weaker, smoothly decaying emission tail, visible up to the end of the KW triggered data record (~T0+250 s). The emission is seen up to ~20 MeV.
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB260226_T38287/
As observed by Konus-Wind, the burst had the total fluence of 7.27(-0.17,+0.17)x10^-4 erg/cm2, and the 64-ms peak flux, measured from T0+10.688 s, of 1.33(-0.06,+0.06)x10^-4 erg/cm2/s (both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst (measured from T0 to T0+142.080 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 = -0.93(-0.02,+0.03), the high energy photon index beta = -2.33(-0.07,+0.06), the peak energy Ep = 613(-25,+26) keV (chi2 = 118/97 dof).
The spectrum near the maximum count rate (measured from T0+10.240 to T0+10.752 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 = -0.64(-0.06,+0.07), the high energy photon index beta = -2.11(-0.08,+0.07), the peak energy Ep = 772(-77,+80) keV (chi2 = 48/57 dof).
All the quoted errors are at the 68% confidence level. All the quoted values are preliminary.
GCN Circular #43865
Zheng-Hang Yu, Chen-Wei Wang and Shao-Lin Xiong (IHEP) report on behalf of the Insight-HXMT team:
At 2026-02-26T10:38:11.100 (T0), Insight-HXMT/HE detected the long bright burst GRB 260226A, which is also detected by Fermi/GBM (Fermi GBM Team, GCN #43840), Fermi/LAT(D. Depalo et al., GCN #43844) , AstroSat-CZTI (Harsha et al., GCN #43846), Glowbug (Woolf et al., GCN #43855), CALET-GBM (Kobayashi et al., GCN #43860), and Konus-Wind(D. Svinkin et al., GCN #43864)
The Insight-HXMT/HE light curve mainly consists of two eposides with a T90 of about 76 s. The 1s peak rate, measured from T0+1.5 s, is 15858 cnts/sec. Insight-HXMT/HE detected a total of 264251 counts form this burst. However, the HE detector was suffered from saturation during the brightest peak due to the high-brightness.
The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb260226A.png
All measurements above are made with the CsI detectors of Insight-HXMT/HE operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope.
Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://hxmten.ihep.ac.cn/
GCN Circular #43878
A. Reguitti (INAF/OAPd), P. D'Avanzo (INAF/OAB), D. B. Malesani (DAWN/NBI and Radboud), L. Tomasella (INAF/OAPd) report on behalf of the CIBO collaboration:
We observed the field of the bright GRB 260226A (Depalo et al., GCN 43850; Bissaldi et al., GCN 43851; Harsha et al., GCN 43846; Waratkar & Grefenstette, GCN 43854; Woolf et al.. GCN 43855; Kobayashi et al., GCN 43860; Svinkin et al., GCN 43864; Yu et al., GCN 43865) using the 67/92 Schmidt Telescope located at the Asiago observatory (Italy). Observations consisted of five r-band exposures of 240 s each, with a mid time Feb 26.766 UT (7.77 hr after the Fermi/GBM trigger).
Image subtraction was carried out using the DESI Legacy survey (Dey et al. 2019, doi:10.3847/1538-3881/ab089d) as template. No new source is detected within the revised Fermi/LAT error circle (Depalo et al., GCN 43850) down to a limiting magnitude r > 20.4 AB (calibrated against nearby objects from the Pan-STARRS catalog).
Our non detection is consistent with other reports from the literature (Konno et al., GCN 43852; Saikia et al., GCN 43853; Strausbaugh & Cucchiara, GCN 43858; Angulo et al., GCN 43859).

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Jochen Greiner, last update: 28-Feb-2026 [Disclaimer]