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GRB 260226A

GCN Circular 43878

Subject
GRB 260226A: Asiago r-band upper limit
Date
2026-02-28T12:43:06Z (8 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
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).

GCN Circular 43865

Subject
GRB 260226A: Insight-HXMT/HE detection of the very bright burst
Date
2026-02-27T16:11:28Z (8 months ago)
From
yzh807926@163.com
Via
Web form
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 43864

Subject
Konus-Wind detection of GRB 260226A
Date
2026-02-27T13:52:56Z (8 months ago)
From
Dmitry Svinkin at Ioffe Institute <svinkin@mail.ioffe.ru>
Via
Web form
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 43860

Subject
GRB 260226A: CALET Gamma-Ray Burst Monitor detection
Date
2026-02-27T08:55:16Z (8 months ago)
From
Yuta Kawakubo at Aoyama Gakuin University <kawakubo@phys.aoyama.ac.jp>
Via
Web form
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 43859

Subject
GRB 260226A: COLIBRÍ optical upper limits
Date
2026-02-27T06:16:42Z (8 months ago)
From
Camila Angulo Valdez at UNAM <camiangulo@astro.unam.mx>
Via
Web form
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 43858

Subject
GRB 260226A: LCOGT Optical Upper Limit
Date
2026-02-27T03:17:12Z (8 months ago)
From
Robert Strausbaugh at Eastern Illinois University <rstrausbaugh@eiu.edu>
Via
email
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 43855

Subject
GRB 260226A: Glowbug gamma-ray detection
Date
2026-02-26T20:34:03Z (8 months ago)
From
Richard S. Woolf at US Naval Research Laboratory <richard.s.woolf.civ@us.navy.mil>
Via
Web form
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 43854

Subject
GRB 260226A: NuSTAR detection of bright prompt emission
Date
2026-02-26T20:10:43Z (8 months ago)
From
Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>
Via
Web form
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 43853

Subject
GRB 260226A : GROWTH-India Telescope optical observations
Date
2026-02-26T19:55:15Z (8 months ago)
From
V. Swain at IIT Bombay <vishwajeet.s@iitb.ac.in>
Via
Web form

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

Loading...
 
 
) 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 NumberTmid - T0 (hours)RA (deg)Dec (deg)Upper limit (5 sigma)(AB)
13.9542.058.03319.7
24.0341.837.71319.6
34.1241.837.87319.9
44.2141.838.03319.8
54.3041.838.19319.6
64.3941.838.35319.7
74.4741.947.71319.6
84.5641.947.87319.7
94.6541.948.03319.6
104.7541.948.19319.5
114.8441.948.35319.6
124.9342.057.71319.4
135.0242.057.87319.2
145.1042.058.19319.3
155.1942.058.35319.2
165.2842.167.87319.0

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

Loading...
 
 
,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 43852

Subject
GRB 260226A: LAST optical upper limit
Date
2026-02-26T19:42:00Z (8 months ago)
From
Ruslan Konno at Weizmann Institute of Science <ruslankonno@gmail.com>
Via
Web form
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 43851

Subject
GRB 260226A: Fermi GBM observation
Date
2026-02-26T16:21:05Z (8 months ago)
Edited On
2026-03-04T16:10:05Z (8 months ago)
From
Elisabetta Bissaldi at Politecnico and INFN Bari <elisabetta.bissaldi@ba.infn.it>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Elisabetta Bissaldi at Politecnico and INFN Bari <elisabetta.bissaldi@ba.infn.it>
Via
Web form
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 173 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 43850

Subject
GRB 260226A: Fermi-LAT refined analysis
Date
2026-02-26T16:15:30Z (8 months ago)
From
Davide Depalo at Politecnico and INFN Bari <davide.depalo@ba.infn.it>
Via
Web form
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 43847

Subject
Fermi GRB 260226A: Global MASTER-Net observations report
Date
2026-02-26T15:31:16Z (8 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
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 Circular 43846

Subject
GRB 260226A: AstroSat CZTI detection of bright long burst
Date
2026-02-26T15:00:12Z (8 months ago)
From
Anuraag Arya at IIT Bombay <aryaanuraag910@gmail.com>
Via
Web form
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 43844

Subject
GRB 260226A: Fermi-LAT detection of a bright burst
Date
2026-02-26T13:34:45Z (8 months ago)
From
Davide Depalo at Politecnico and INFN Bari <davide.depalo@ba.infn.it>
Via
Web form
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 43843

Subject
GRB 260226A???: BALROG localization (Fermi Trigger 793795080 / GRB 260226443)
Date
2026-02-26T13:23:46Z (8 months ago)
From
Jochen Greiner at MPE <jcgrog@mpe.mpg.de>
Via
email
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 43840

Subject
GRB 260226A: Fermi GBM Final Real-time Localization
Date
2026-02-26T10:48:46Z (8 months ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
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
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