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GRB 260511B

GCN Circular 44618

Subject
GRB 260511B: NOT optical observations
Date
2026-05-15T16:42:48Z (16 days ago)
From
sqjiang at NAOC <sqjiang@bao.ac.cn>
Via
Web form
S. Q. Jiang, D. Xu (NAOC), D. B. Malesani (DAWN/NBI and Radboud), J. P. U. Fynbo (DAWN/NBI), A. M. Kadela (NOT & NBI) report on behalf of a large collaboration:

We observed the optical counterpart (Wu et al., GCNs 44547, 44552; Sasada et al., GCN 44549; Jiang et al., GCN 44551; Li et al., GCNs 44557, 44593; Vijaykumar et al., GCN 44561; de Ugarte Postigo et al., GCN 44565; Lee et al., GCN 44567; Li et al., GCN 44568; Wang et al., GCN 44571; Belkin et al., GCN 44572; Adami et al., GCN 44573; Saccardi et al., GCN 44574; Rajabov et al., GCN 44576; Pérez-Fournon et al., GCN 44577; Novotný et al., GCN 44580; Moskvitin et al., GCNs 44582, 44600; Calapai et al., GCN 44592; Liu et al., GCN 44610; Li et al., GCN 44615) of GRB 260511B, detected by Fermi/GBM (Fermi GBM Team, GCN 44542), SVOM (Godet et al., GCN 44543), NuSTAR (Waratkar & Grefenstette, GCN 44556), AstroSat CZTI (Arya et al., GCN 44560), and Konus-Wind (Ridnaia et al., GCN 44597) using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera, and obtained 3 x 90 s frames in each of the Sloan g, r, and i bands, and 3x200 s frames in the Sloan z band, starting at 21:12:00.450 UT on 2026-05-12, i.e., 34.192 hr after the Fermi/GBM trigger.

The afterglow is clearly detected in our stacked images, with a magnitude r = 21.33 +/- 0.06 (AB) at a median time of 34.337 hr after the trigger, calibrated against nearby Pan-STARRS stars and not corrected for Galactic extinction.

GCN Circular 44615

Subject
GRB 260511B: GMG Optical Upper Limit
Date
2026-05-15T04:58:07Z (16 days ago)
From
Rui-Zhi Li at Yunnan Observatories, CAS <liruizhi@ynao.ac.cn>
Via
email
R.-Z. Li, B.-T. Wang, F.-F. Song, J. Mao, J.-G. Wang and B. Wang (YNAO, CAS) report:

We observed the field of GRB 260511B (Godet et al., GCN 44543, T0 at 2026-05-11T11:00:32) using the GMG-2.4m telescope at the Lijiang Observatory. 
The observation began at 2026-05-14 12:45:02, about 73.74 hours after the trigger.

The optical counterpart (Wu et al. GCN 44547, Sasada et al. GCN 44549, Jiang et al. GCN 44551, Wu et al. GCN 44552, Vijaykumar et al. GCN 44561, de Ugarte Postigo et al. GCN 44565, Lee et al. GCN 44567, Li et al. GCN 44568, Wang et al. GCN 44571, Belkin et al. GCN 44572, Adami et al. GCN 44573, Saccardi et al. GCN 44574, Rajabov et al. GCN 44576, Perez-Fournon et al. GCN 44577, Novotny et al. GCN 44580, Moskvitin et al. GCN 44582, Calapai et al. GCN 44592, Moskvitin et al. GCN 44600 and Liu et al. GCN 44610) of GRB 260511B was not detected in our r-band image.

The preliminary analysis results are shown as follows:
+-------------------------+----------------+---------------+----------+----------------+
|    Start time [UTC]     |  Exposure [s]  |  Tmid-T0 [h]  |  Filter  |  5-sigma U.L.  |
+=========================+================+===============+==========+================+
| 2026-05-14 12:45:02.730 |      900       |    73.867     |    r     |      21.6      |
+-------------------------+----------------+---------------+----------+----------------+
The given magnitudes are derived based on calibrating against Pan-STARRS1 field stars.

GCN Circular 44610

Subject
GRB 260511B: Mephisto multi-band optical followup
Date
2026-05-14T15:51:44Z (17 days ago)
From
Brajesh Kumar at SWIFAR, YNU <brajesh@ynu.edu.cn>
Via
Web form
Chenxu Liu, Chenxi Shang, Guowang Du, Brajesh Kumar, Xian Liu, Haipeng Lei, Weikang Lin, Yu Pan, Xingzhu Zou, Xinlei Chen, Yuan Fang, Jinghua Zhang, Xufeng Zhu, Tao Wang, Xinzhong Er, Yuanpei Yang (SWIFAR, YNU), Chao Wu (NAOC), Xiangkun Liu, Xiaowei Liu (SWIFAR, YNU) report on behalf of the Mephisto Team:

We performed simultaneous multi-band (uvgriz) photometric observations of GRB 260511B (Godet et al., GCN 44543) at 2026-05-12T16:17:44 (~1.2d after the the SVOM/ECLAIRs trigger) using the 1.6-m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University, located at the Lijiang Observatory. The optical counterpart was clearly detected in our stacked images and preliminary photometry is listed below (without Galactic extinction correction):

-------------------------------------------------------
Start Time (UT)      | Band | Exp(s)  | Mag (AB)
-------------------------------------------------------
2026-05-12T16:17:44  |  u   | 300*2   | 22.45 (+/-0.20)
2026-05-12T16:29:15  |  v   | 300*2   | 21.77 (+/-0.19)
2026-05-12T16:17:47  |  g   | 300*2   | 21.19 (+/-0.08)
2026-05-12T16:29:17  |  r   | 300*2   | 21.03 (+/-0.07)
2026-05-12T16:17:46  |  i   | 300*2   | 20.70 (+/-0.11)
2026-05-12T16:29:17  |  z   | 300*2   | 20.40 (+/-0.16)

These results are consistent with previous optical observations reported by Wu et al. (GCN 44547), Sasada et al. (GCN 44549), Jiang et al. (GCN 44551), Wu et al. (GCN 44552), Vijaykumar et al. (GCN 44561), de Ugarte Postigo et al. (GCN 44565), Lee et al. (GCN 44567), Li et al. (GCN 44568), Wang et al. (GCN 44571), Belkin et al. (GCN 44572), Adami et al. (GCN 44573), Saccardi et al. (GCN 44574), Rajabov et al. (GCN 44576), Perez-Fournon et al. (GCN 44577), Novotny et al. (GCN 44580), Moskvitin et al. (GCN 44582), Calapai et al. (GCN 44592), and Moskvitin et al. (GCN 44600).
----------------------------------------------------------------------------------------------------

Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. The facility is operated by the South-Western Institute for Astronomy Research (SWIFAR), Yunnan University. It provides real-time, high-quality colors of stellar objects. The Mephisto mosaic cameras were installed in October 2025. The first light was achieved in all three channels on 10 October 2025 and presently, these are under the commissioning phase. All the data have been reduced by the Mephisto data processing pipeline. We note that the current data-processing pipeline is still at a preliminary stage, with flux calibration precision of about 2% in the u and v bands, about 1% in the g and r bands, and better than 1% in the i and z bands.

GCN Circular 44600

Subject
GRB 260511B: further SAO RAS optical observations
Date
2026-05-13T22:29:56Z (18 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin, O. Spiridonova (SAO RAS) and A. Pozanenko (IKI),
report on behalf of GRB the follow-up team.

We observed the field of the GRB 260511B discovered by Fermi
(The Fermi GBM team, GCN 44542; Roberts, GCN 44554), SVOM
(Godet et al., GCN 44543), NuSTAR (Waratkar and Grefenstette,
GCN 44556), AstroSat CZTI (Arya et al., GCN 44560),
EP-FXT (Fu et al., GCN 44578), Konus-Wind (Ridnaia et al. GCN 44597)
with the Zeiss-1000, 1m telescope of the SAO RAS
equipped with the CCD photometer. The observations were performed
in Rc band on May 12 and 13 under moderate weather conditions.

The OT (Wu et al., GCN 44547; Sasada et al., GCN 44549;
Jiang et al., GCN 44551; Wu et al., GCN 44552; Vijaykumar et al.,
GCN 44561; de Ugarte Postigo et al., GCN 44565; Lee et al., GCN 44567;
Li et al., GCN 44568; Wang et al., GCN 44571; Belkin et al.,
GCN 44572; Adami et al., GCN 44573; Saccardi et al., GCN 44574;
Rajabov et al., GCN 44576; Pérez-Fournon et al., GCN 44577;
Novotný et al., GCN 44580; Moskvitin and Pozanenko, GCN 44582;
Calapai, GCN 44592; also detected in NIR by Li et al.,
GCN 44557; GCN 44593) is clearly detected in the stacked frames
with the following brightness.

Date       T_start   T_end     t_mid-T0, exp, filter mag   +/- err   UL
           UT        UT        days      s    
2026-05-12T18:57:30--20:17:50  1.35912   2760 Rc     20.93 +/- 0.05  23.1
2026-05-13T19:12:53--21:21:03  2.38641   4800 Rc     21.49 +/- 0.07  23.3

This preliminary photometry is based on nearby SDSS stars
(using Lupton 2005 transformation equations)
and is not corrected for Galactic extinction.

GCN Circular 44597

Subject
Konus-Wind detection of GRB 260511B
Date
2026-05-13T12:12:06Z (18 days ago)
Edited On
2026-05-13T12:50:08Z (18 days ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 260511B
(Fermi-GBM detection: Fermi GBM team, GCN 44542;
Roberts et al., GCN 44554;
SVOM detection: Godet et al., GCN 44543;
NuSTAR detection: Waratkar & Grefenstette, GCN 44556;
AstroSat-CZTI detection: Arya et al., GCN 44560) 
triggered Konus-Wind at T0=39635.267 s UT (11:00:35.267).

The burst light curve shows two separate multipeaked episodes: the first
from ~T0-1.1 s to ~T0+10 s, and the second, which is brighter and spectrally
harder, from ~T0+26 s to ~T0+42 s. The total duration of the burst is ~43.4 s.
The emission is seen up to ~6 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260511_T39635/

As observed by Konus-Wind, the burst
had a fluence of 2.48(-0.11,+0.12)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+30.512 s,
of 6.82(-0.83,+0.84)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+41.216 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.12(-0.09,+0.10)
and Ep = 259(-20,+24) keV (chi2 = 92/99 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.4
(chi2 = 92/98 dof).

The spectrum near the maximum count rate
(measured from T0+24.832 to T0+33.024 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 = -0.87(-0.23,+0.17),
the high energy photon index beta = -2.26(-0.66,+0.13),
the peak energy Ep = 273(-40,+102) keV
(chi2 = 93/81 dof).

Assuming the redshift z=2.006 (de Ugarte Postigo et al., GCN 44565; Wang et al., GCN 44571)
and a standard cosmology model with H_0 = 67.3 km/s/Mpc,
Omega_M = 0.315, and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is 2.53(-0.11,+0.12)x10^53 erg,
the peak luminosity L_iso is 2.09(-0.25,+0.26)x10^53 erg/s,
the rest-frame peak energy of the time-averaged spectrum,
Ep,i,z is 778(-60,+72) keV and the spectrum near the maximum count rate 
Ep,p,z is 821(-120,+307) keV.

With the obtained estimates, GRB 260511B is inside 68% prediction bands for both 
the 'Amati' and the 'Yonetoku' relations derived for the sample of >300 long KW GRBs with 
known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021),
see http://www.ioffe.ru/LEA/GRBs/GRB260511_T39635/GRB260511B_rest_frame.pdf

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

GCN Circular 44593

Subject
GRB 260511B: further J-band observations by the SYSU 80 cm infrared telescope
Date
2026-05-13T02:15:50Z (19 days ago)
From
lijj328@mail2.sysu.edu.cn
Via
Web form
Jin-Ji Li, Chun Chen, Duo-Le Cao, Zhong-Nan Dong, Wei-Sen Huang, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Yan Yu, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80 cm infrared telescope team:

We further observed the field of GRB 260511B detected by SVOM/ECLAIRs (Godet et al., GCN 44543) and Fermi (Fermi GBM team, GCN 44542), using the Sun Yat-sen University (SYSU) 80 cm infrared telescope.

The afterglow (Wu et al., GCN 44547; Sasada et al., GCN 44549; Jiang et al., GCN 44551; Wu et al., GCN 44552; Li et al., GCN 44557; Vijaykumar et al., GCN 44561; Lee et al., GCN 44567; Li et al., GCN 44568; Wang et al., GCN 44571; Belkin et al., GCN 44572; Adami et al., GCN 44573; Saccardi et al., GCN 44574; Rajabov et al., GCN 44576; Pérez-Fournon et al., GCN 44577; Novotný et al., GCN 44580; Moskvitin, GCN 44582; Calapai, GCN 44592) with the redshift of z=2.006 (Postigo et al., GCN 44565) was clearly detected in our stacked J-band images. The photometric results are summarized in the table below. The photometry was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction.

Mid time |Band | Exposure | Mag (Vega)
------------|-------|-------------|------------------
5.362 h  | J   | 700s     | 16.66 +/- 0.14
5.568 h  | J   | 640s     | 16.57 +/- 0.10
5.771 h  | J   | 640s     | 16.85 +/- 0.13
5.985 h  | J   | 560s     | 17.16 +/- 0.18
6.191 h  | J   | 660s     | 17.10 +/- 0.19
7.011 h  | J   | 960s     | 17.11 +/- 0.15
7.367 h  | J   | 1040s    | 17.26 +/- 0.14
7.720 h  | J   | 1100s    | 17.46 +/- 0.18

The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.

GCN Circular 44592

Subject
GRB 260511B: Calapai Observatory, Massa S. Giorgio (Messina), optical observation.
Date
2026-05-13T00:24:14Z (19 days ago)
From
Giovanni Calapai at Calapai Astronomical Observatory, Massa S. Giorgio, Messina, Italy <giovannicalapai@tiscali.it>
Via
Web form
Giovanni Calapai at Calapai Astronomical Observatory, Massa S. Giorgio, (Messina) Italy 
Member of: GRB/UAI Gamma Ray Burst Section of Unione Astrofili Italiani. 

Report:

We observed the field of GRB 260511B detected by Fermi/GBM (Fermi GBM Team, GCN 44542) and SVOM/ECLAIRs (Godet et al., GCN 44543) with the 14 inches Schmidt-Cassegrain (Celestron 14) telescope F/D=7. 
  
The observations were started at 2026-05-11 21:24:23 UT (approximately 10.40 hours after burst) stacking two sets of unfiltered CCD image. The observations were carried out with clear skies and good visibility conditions.

The OT was detected at the following position:

RA    (J2000.0)  12h 50m 17.18s   
Decl. (J2000.0) +04° 03' 10.2" 	 

Photometry was obtained using nearby PanSTARRS stars as follows: 

Observation    Mid-Time    T-T0 (hr)      Exposure        Filter      Mag.     Mag. err.
2026-05-11   21:54:50 UT   10.91           60x60s           CR       18.85     +/-0.04
2026-05-11   23:01:52 UT   12.02           72x60s           CR       18.95     +/-0.04

Magnitude was calibrated with the nearby PanSTARRS stars converted using Lupton (2005) equations. 
No correction for galactic dust extinction was applied.

Our observations are consistent with other already reported Wu et al. (GCN 44547); Sasada et al. (GCN 44549); Jiang et al. (GCN 44551); Wu et al. (GCN 44552); Li et al. (GCN 44557); Vijaykumar et al. (GCN 44561); De Ugarte Postigo et al. (GCN 44565); Lee et al. (GCN 44567); Li et al. (44568); Belkin et al. (GCN 44572); Saccardi et al. (GCN 44574); Rajabov et al. (GCN 44576); Pérez-Fournon et al. (GCN 44577); Novotný et al. (GCN 44580); Moskvitin (GCN 44582).

The message may be cited.

GCN Circular 44582

Subject
GRB 260511B: SAO RAS optical observations
Date
2026-05-12T15:32:19Z (19 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin (SAO RAS) and A. Pozanenko (IKI),
report on behalf of GRB the follow-up team.

We observed the field of the GRB 260511B discovered by Fermi
(The Fermi GBM team, GCN 44542; Roberts, GCN 44554), SVOM
(Godet et al., GCN 44543), NuSTAR (Waratkar and Grefenstette,
GCN 44556), AstroSat CZTI (Arya et al., GCN 44560);
EP-FXT (Fu et al., GCN 44578) with the Zeiss-1000, 1m telescope
of the SAO RAS equipped with the CCD photometer.
Observations were performed in B, V, Rc, Ic filters and started
on May 11, 19:08:40 UT (8.14 hours after the trigger).
   
The OT (Wu et al., GCN 44547; Sasada et al., GCN 44549;
Jiang et al., GCN 44551; Wu et al., GCN 44552; Vijaykumar et al.,
GCN 44561; de Ugarte Postigo et al., GCN 44565; Lee et al., GCN 44567;
Li et al., GCN 44568; Wang et al., GCN 44571; Belkin et al.,
GCN 44572; Adami et al., GCN 44573; Saccardi et al., GCN 44574;
Rajabov et al., GCN 44576; Pérez-Fournon et al., GCN 44577;
Novotný et al., GCN 44580; also detected in NIR by Li et al.,
GCN 44557) is clearly detected in all frames with the following
brightness.

Date       T_start   T_end     t_mid-T0, exp, s  filter mag   +/- err
           UT                  hours      
2026-05-11T19:14:19--19:54:19  8.56306   3*300   B      19.56 +/- 0.03
2026-05-11T19:20:15--19:40:09  8.49444   2*120   V      19.09 +/- 0.04
2026-05-11T19:08:40--19:43:12  8.42333   3*120   Rc     18.75 +/- 0.02
2026-05-11T19:11:24--19:51:31  8.51542   5*120   Ic     18.50 +/- 0.05

This preliminary photometry is based on nearby SDSS stars
(using Lupton 2005 transformation equations)
and is not corrected for Galactic extinction.

GCN Circular 44580

Subject
GRB 260511B: FRAM-ORM optical detection
Date
2026-05-12T13:59:31Z (19 days ago)
From
Filip Novotný at Masaryk University <filip.novotny@talentovani.cz>
Via
Web form
Filip Novotný, Martin Jelínek, Jan Štrobl (ASU CAS Ondrejov, CZ),
Sergey Karpov, Martin Mašek, Petr Janeček, Jakub Juryšek, Jan Ebr, Ronan Cunniffe, Petr Travníček and Michael Prouza (Institute of Physics, Prague, CZ)
report:

We observed the field of GRB 260511B detected by Fermi/GBM (Fermi GBM Team, GCN 44542) and SVOM/ECLAIRs (Godet et al., GCN 44543) with the FRAM robotic telescope at the Observatorio del Roque de los Muchachos (ORM), La Palma, Spain. Observations were carried out in the clear filter from T+9.6 hr to T+17.4 hr with 60-second exposures stacked per epoch.

The optical afterglow(Wu et al., GCN 44547 ; Sasada et al., GCN 44549 ; Jiang et al., GCN 44551 ; Wu et al., GCN 44552 ; Li et al., GCN 44557 ; Vijaykumar et al., GCN 44561 ; de Ugarte Postigo et al., GCN 44565 ; Lee et al., GCN 44567; Li et al., GCN 44568; Wang  et al., GCN 44571; Belkin et al., GCN 44572) of GRB 260511B (FERMI team, GCN 44542 ; Godet et al., GCN 44543; Roberts et al., GCN 44554; Waratkar et al., GCN 44556; Arya et al., GCN 44560) is clearly detected throughout the night, fading from CR = 18.89 +/- 0.10 at T+9.8 hr till about T+16 hr, when the target terminated due to dawn. Magnitudes were reduced using the PyRT pipeline and calibrated to r-band AB magnitudes against the ATLAS catalog. No correction for Galactic extinction has been applied.

Fitting a power law F(t) ~ t^(-alpha) to our data yields alpha = 1.39 +/- 0.29, consistent with the power-law decay reported by earlier observers (GCN 44547, 44549, 44551, 44561).

GCN Circular 44578

Subject
GRB 260511B: EP-FXT follow-up observation
Date
2026-05-12T12:55:37Z (19 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
S.-Y. Fu (HUST), X. Tian (GXU), H.-N. Yang, Z.-X. Ling (NAOC, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of GRB 260511B detected by Fermi/GBM (the Fermi GBM team, GCN 44542) and SVOM/ECLAIRs (SVOM/sb26051102, Godet et al., GCN 44543). The follow-up obervation started at 2026-05-11T11:41:41 UTC, approximately 40.7 minutes after the SVOM trigger, with a total exposure time of 5.9 ks.

On-ground analysis of the FXT data found an uncatalogued fading source at R.A., Dec. = 192.574, 4.0549 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic) near the ECLAIRs error circle. This position is consistent with the optical counterpart reported by SVOM/C-GFT (Wu et al., GCN 44547) and MITSuME (Sasada et al. GCN 44549). The average FXT 0.3-10 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density fixed at the Galactic value, intrinsic hydrogen column density of 3.8 (+-0.10) ×10^21 cm^-2, and a photon index of 2.10 +- 0.04. The derived average unabsorbed flux in the 0.3-10 keV band is 2.07 (+-0.05)×10^-11 erg s^-1 cm^-2.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

GCN Circular 44577

Subject
GRB 260511B: LCO optical counterpart detection
Date
2026-05-12T12:22:26Z (19 days ago)
From
Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>
Via
Web form
I. Pérez-Fournon (IAC and ULL), F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, 
I. Correa-Plasencia, E. Lekaroz-Urriza, M. Quintana-Ansaldo (all ULL), A. López-Oramas (IAC and ULL), and D. Aguado (IAC and ULL)

We report on Las Cumbres Observatory (LCO) observations of the long GRB 260511B, detected by Fermi GBM (Fermi GBM team, GCN circ. 44542; Roberts et al., GCN circ. 44554), SVOM ECLAIRs and MXT (Godet et al., GCN circ. 44543), NuSTAR (Waratkar and Grefenstette, GCN circ. 44556), and AstroSat (Arya et al., GCN circ. 44560).

We observed the field of GRB 260511B with one of the two LCO 1-m telescopes equipped with Sinistro cameras located at the LCO node at McDonald Observatory, Texas, in the SDSS r´ (300 sec), g' (200 sec), and i' (200 sec) filters. The first observation, in the SDSS r' filter, started on 2026-05-12 at 03:00:47 UT, 16.00 hours after the Fermi and SVOM trigger. The optical conterpart, first reported by Wu et al. (GCN circ. 44547), at a redshift of z = 2.006 (de Ugarte Postigo et al., GCN circ. 44565; Wang et al., GCN circ. 44571; and Adami et al., GCN circ. 44573), is clearly detected in our LCO images. The AB magnitudes, calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction are:

r' = 19.81 +/- 0.06 (mid-time 16.05 hours after the trigger)

g' = 20.21 +/- 0.06 (mid-time 16.67 hours after the trigger)

i' = 19.71 +/- 0.07 (mid-time 16.73 hours after the trigger)

These results are consistent with other optical and near-IR detections of the afterglow (Wu et al., GCN circ. 44547;  Sasada et al., GCN circ. 44549; Jiang et al., GCN circ. 44551; Wu et al., GCN circ. 44552; Li et al., GCN circ. 44557; Vijaykumar et al., GCN circ.  44561; de Ugarte Postigo et al., GCN circ. 44565; Lee et al., GCN circ. 44567; Li et al., GCN circ. 44568; Wang et al., GCN circ.  44571; 
Belkin et al., GCN circ. 44572; Adami et al., GCN circ. 44573; Saccardi et al., GCN circ. 44574); 
and Rajabov et al., GCN circ. 44576).

This work makes use of observations from the Las Cumbres Observatory global telescope network (LCO program IAC2026A-011, SGLF and Superluminous Supernovae surveys).

This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).

GCN Circular 44576

Subject
GRB260511B: MAO/AZT-22 optical observations
Date
2026-05-12T11:51:54Z (19 days ago)
From
Yodgor Rajabov at UBAI <rajabov@astrin.uz>
Via
Web form
Y. Rajabov, B.Abidkhanov, O. Burkhonov, S. Ehgamberdiev, Y. Tillayev (UBAI), T. Boyqobilov, A. Shaymanov (Maidanak Observatory/UBAI) report on behalf of UBAI team.

We observed the field of GRB 260511B detected by SVOM (Godet et al., GCN 44543) and Fermi (Fermi GBM team, GCN 44542) with the AZT-22 1.5m telescope of the Maidanak Observatory (MAO) starting on 2026-05-11 at 16:49:08 UT, i.e, ~6.81 hours after the FERMI GBM trigger. In total we obtained 6x300 s exposures in the R-band using 4kx4k CCD SNUCAM camera (Im et al., 2010). 
We detect the optical counterpart reported by many teams (Wu et al., GCN 44547; Sasada et al., GCN 44549; Jiang et al., GCN 44551; Wu et al., GCN 44552; Vijaykumar et al., GCN 44561; de Ugarte Postigo et al., GCN 44565; Lee et al., GCN 44567; Li et al., GCN 44568; Belkin et al., GCN 44572; Saccardi et al., GCN 44574).
Preliminary photometry is as follows:
Date       UTstart  Exptime     t-T0       Filter OT    Err.    UL    SNR	Site/Telescope
                    (nxs)     (hours)                 
2026-05-11 16:49:08 6x300      6.81       R   18.32   0.01    23.0	142	MAO/AZT-22
The red shift (z = 2.006) measured by de Ugarte Postigo et al., GCN 44565. Then confirmed by Wang et al., GCN 44571 and Adami et al., GCN 44573.
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained in Johnson Cousins filters were calibrated using the Gaia DR3 Synphot catalog. We used PanSTARRS DR2 catalog for template subtraction.  The data has not corrected for the Galactic extinction.
Maidanak astronomical observatory (MAO) is an observational facility of the Ulugh Beg Astronomical Institute (UBAI), Uzbekistan Academy of Sciences (http://maidanak.uz/).

GCN Circular 44574

Subject
GRB 260511B: LCO optical observations
Date
2026-05-12T09:48:43Z (19 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
A. Saccardi (CEA/Irfu), C. Wu (NAOC), J. X. Cao (GXU),  L. Zhang (IHEP), D. Kong (GXU), report on behalf of the SVOM mission team:

We observed the field of the SVOM/ECLAIRs GRB 260511B (Godet et al., GCN 44543), also detected by Fermi/GBM (Fermi GBM team, GCN 44542; Roberts et al., GCN 44554), NuSTAR (Waratkar et al., GCN 44556), and AstroSat (Arya et al., GCN 44560), with the LCO 1m telescope at Cerro Tololo Observatory equipped with the Sinistro instrument.

Our observation started on 2026-05-12 at 00:14:08 UT (about 13.23 hr after the trigger) and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters.

The optical counterpart (Wu et al., GCN 44547; Sasada et al., GCN 44549; Jiang et al., GCN 44551; Wu et al., GCN 44552; Li et al., GCN 44557; Vijaykumar et al., GCN 44561; de Ugarte Postigo et al., GCN 44565; Lee et al. GCN 44567; Li et al., GCN 44568; Wang et al., GCN 44571; Belkin et al., GCN 44572; Adami et al., GCN 44573) is clearly detected in our images.

We measure the following magnitudes calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:

r = 19.68 +/- 0.02 AB (mid-time 13.29 hr after the trigger);
z = 19.71 +/- 0.09 AB (mid-time 18.14 hr after the trigger).

This project is funded by the SVOM collaboration.

GCN Circular 44573

Subject
GRB 260511B: OHP/T193 spectroscopic observations
Date
2026-05-12T09:05:55Z (19 days ago)
From
Christophe Adami at LAM <christophe.adami@lam.fr>
Via
Web form
C. Adami (LAM/Pytheas/AMU), D. Russeil (LAM/Pytheas/AMU), E. Le Floc'h (CEA-Saclay) report on behalf of a larger collaboration and on behalf of the transient team of the OHP 2026 special call:

We observed the optical counterpart (Wu et al., GCN 44547 ; Sasada et al., GCN 44549 ; Jiang et al., GCN 44551 ; Wu et al., GCN 44552 ; Li et al., GCN 44557 ; Vijaykumar et al., GCN 44561 ; de Ugarte Postigo et al., GCN 44565 ; Lee et al., GCN 44567; Li et al., GCN 44568; Wang  et al., GCN 44571; Belkin et al., GCN 44572) of GRB 260511B (FERMI team, GCN 44542 ; Godet et al., GCN 44543; Roberts et al., GCN 44554; Waratkar et al., GCN 44556; Arya et al., GCN 44560) in spectroscopic mode using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained a total of 145 min of exposures (10min + 15min + 4 x 30min) with the MISTRAL spectroscopic blue setting from 4200 to 8000 AA, starting at 2026-05-11 22:04:34 UT under modest weather conditions.

The host environment at z=2.006 detected by the GTC and reported by de Ugarte Postigo et al. (GCN 44565) and Wang et al. (GCN 44571) is visible in our data, in particular with the 2344 AA and 2600 AA (rest-frame) FeII lines observed at a consistent redshift of z=2.007 +/- 0.001. 

We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen.

GCN Circular 44572

Subject
GRB 260511B: GOTO optical counterpart detection
Date
2026-05-12T08:15:30Z (19 days ago)
From
Sergey Belkin at Monash University <sergey.belkin@monash.edu>
Via
Web form
S. Belkin, G. Ramsay, D. O'Neill, B. P. Gompertz, M. Wortley, R. Starling, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, A. Kumar, M. Pursiainen, on behalf of GOTO collaboration

We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the Fermi/GBM (Fermi GBM team, GCN 44542) and SVOM/ECLAIRs (Godet et al. GCN 44543) triggers for GRB 260511B. Observations started at 2026-05-11 21:09:05 UT, (+10.13h post trigger) in the GOTO L-band (400-700 nm).

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations (Lyman et al. 2026).

We detect the previously reported optical counterpart of GRB 260511B at a position consistent with the source reported by Wu et al. (GCN 44547), Sasada et al. (GCN 44549), Jiang et al. (GCN 44551), Wu et al. (GCN 44552), Vijaykumar et al. (GCN 44561), de Ugarte Postigo et al. (GCN 44565), Lee et al. (GCN 44567), and  Li et al. (GCN 44568). The source is detected in the GOTO L-band (400-700 nm) with L=19.45 ± 0.09 AB mag at a mid-time of 2026-05-11 21:44:06 UT (+10.73 h).

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).

GCN Circular 44571

Subject
GRB 260511B: Xinglong 2.16m/BEFOSC redshift confirmation
Date
2026-05-12T07:55:07Z (19 days ago)
Edited On
2026-05-12T13:42:29Z (19 days ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
J. Wang, L. P. Xin, J. Zheng,  H. L. Li, C. Wu, Y. L. Qiu, X. H. Han, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, J. Y. Wei (NAOC) reports on behalf of SVOM follow-up team

The follow-up for the optical counterpart (Wu et al. GCN 44547; Sasada et al. GCN 44549; Jiang et al. GCN 44551; Wu et al. GCN 44552; Li et al. GCN 44557; Vijaykumar et al. GCN 44561) of GRB 260511B (Fermi GBM team GCN 44542; Godet et al. GCN 44543; Roberts et al. GCN 44544; Waratkar et al. GCN 44556; Arya et al. GCN 44560; de Ugarte Postigo et al., GCN 44565; Lee et al, GCN 44567; Li et al., GCN 44568) within the errorbox of X-ray counterpart (Godet et al. GCN 44543; Li et al., GCN 44566;) was performed at 2026-05-11T12:46:21 UTC, about 1.76 hours after the trigger, using the NAOC 2.16m telescope at Xinglong observatory, China. 

A total of 2*1800s exposure was performed by BEFOSC spectrograph in our spectroscopic observation. The wavelength ranges from 4000-8800A in the obsever frame. After combining the two frames, absorption features of multiple ions: CIV, FeII and MgII can be identified at a common redshift of ~2.006. In addition, there is a feature possibly due to MgII double let resulted from an intervening system at redshift of 1.435. Our results are consistent with the report of redshift (de Ugarte Postigo et al., GCN 44565). The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).

GCN Circular 44568

Subject
GRB 260511B: SVOM/VT optical observations
Date
2026-05-12T04:28:34Z (20 days ago)
Edited On
2026-05-12T13:42:03Z (19 days ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, C. Wu, Y. L. Qiu, L. P. Xin, Y. N. Ma, Z. H. Yao, J. R. Xu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu), L. Zhang (IHEP) and D. Kong (GXU) report on behalf of the SVOM/VT team.

SVOM/VT performed observations of GRB260511B triggered by SVOM/ECLAIRs (sb26051102, Godet et al., GCN 44543). The burst was also detected by Fermi-GBM (Fermi GBM team, GCN 44542; Roberts et al., GCN 44554)，AstroSat (Arya et al., GCN 44560) and NuSTAR (Waratkar et al. GCN 44556). Our available data via automatic slew started at 2026-05-11T14:26:43 UTC, approximately 3.45 hours post trigger, in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. The ToO observations started at 2026-05-11T20:37:09 UTC, about 9.62 hours post trigger.

The afterglow (Wu et al. GCN 44547; Sasada et al. GCN 44549; Jiang et al. GCN 44551; Wu et al. GCN 44552; Li et al. GCN 44557; Vijaykumar et al. GCN 44561; Lee et al., GCN 44567) with the redshift of z=2.006 (Postigo et al., GCN 44565) was clearly detected in both channels. The following measurements are in the AB magnitude and are not corrected for Galactic extinction:

Mid time | Band | Exposure Time | Brightness
 3.450 h   VT_B     100 s       18.00 +/- 0.03 mag
 3.450 h   VT_R     100 s       17.61 +/- 0.03 mag    
10.159 h   VT_B     100 s       19.52 +/- 0.05 mag
10.159 h   VT_R     100 s	19.05 +/- 0.05 mag

The source showed a fading slope of about -1.3 during the observations above.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.

GCN Circular 44567

Subject
GRB 260511B: SVOM/COLIBRÍ (FM-GFT) optical observations
Date
2026-05-12T04:22:36Z (20 days ago)
From
William H. Lee at UNAM <wlee@astro.unam.mx>
Via
Web form
William H. Lee (UNAM), Dalya Akl (NYUAD), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), 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), L. Zhang (IHEP), and D. Kong (GXU) report:

We imaged the field of the SVOM/ECLAIRs and Fermi/GBM GRB 260511B (Fermi GBM Team, GCN Circ. 44529; Godet et al., GCN Circ. 44532) and also detected by AstroSat (Arya et al. GCN Circ. 44560) and NuSTAR (Waratkar et al. GCN Circ. 44556) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-12 03:24 to 03:43 UTC (from 16.35 to 16.67 hours after the trigger) and obtained 16 minutes of simultaneous exposure in r and z filters.

The data were reduced, coadded, calibrated, and analysed with the COLIBRÍ ASU pipeline.
The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

At the position of the optical counterpart reported by SVOM/C-GFT (Wu et al. GCN Circ. 44547), MITSuME (Sasada et al. GCN Circ. 44549), TRT (Jiang et al. GCN Circ. 44551), SVOM/VT (Wu et al. GCN Circ. 44552), we detect a source with preliminary magnitudes of: 

r = 19.99 +/- 0.01,

z = 19.67 +/- 0.02.

Compared with the optical observations reported by de Ugarte Postigo (GCN Circ. 44565), we estimate an optical decay temporal index of alpha ≈ -1.3 for the source.

We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
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 44565

Subject
GRB 260511B: OSIRIS+/GTC redshift z=2.006
Date
2026-05-11T23:06:28Z (20 days ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
A. de Ugarte Postigo (LAM), C. C. Thoene (AbAO), A. Martin-Carrillo (UCD), S. Geier (GTC), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), L. Izzo (INAF/OACN and DARK/NBI), G. Lombardi (GTC), D. B. Malesani (DAWN/NBI and Radboud), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), A. M. García Rodríguez (GTC) and D. González González (GTC) report:

We observed the optical counterpart (Wu et al. GCN 44547; Sasada et al. GCN 44549; Jiang et al. GCN 4455; Wu et al. GCN 44552; Waratkar et al. GCN 44556; Li et al. GCN 44557; Vijaykumar et al. GCN 44561) of GRB 260511B (Fermi GBM team GCN 44542; Godet et al. GCN 44543; Roberts et al. GCN 44554; Arya et al. GCN 44560) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument.

In the 30-s acquisition image (beginning on 2026-05-11 at 21:05:13.010 UT, that is 10.08 hr after trigger), the optical afterglow is well detected with a preliminary magnitude of r = 19.30 ± 0.05 (AB), calibrated against nearby SDSS objects, and not corrected for Galactic extinction.

A total of 3 spectra of 900 s were secured, starting on 2026-05-11 at 21:11:43.011
 UT (10.19 hr after trigger), using grism R1000B. Continuum is visible over the complete wavelength range 3620-7800 AA. A trough is detected at the blue edge of the spectrum at ~3650 AA, which we identify as a DLA. A number of strong metal absorption features are detected, which we interpret as due to SII, SiII, SiII*, OI, CII, SiIV, CIV, FeII, FeII* AlII, NiII, NiII*, AlIII, CrII, ZnII, and MnII all at a common redshift of 2.006 +/- 0.001, which we suggest to be the redshift of GRB 260511B. Multiple intervening systems are also detected in the spectrum, including a strong system at z = 1.437 with lines due to SiII, CIV, FeII, AlII, AlIII, MgII.

This work has used the GRBspec database at http://grbspec.eu (de Ugarte Postigo et al. 2014, doi:10.1117/12.2055774).

GCN Circular 44561

Subject
GRB 260511B: GROWTH-India Telescope optical observations
Date
2026-05-11T17:39:26Z (20 days ago)
From
V. Swain at IIT Bombay <vishwajeet.s@iitb.ac.in>
Via
Web form

V. Vijaykumar (IITB), A.P. Saikia (IITB), V. Swain (IITB), T. Mohan (IITB), S. Patil (IITB), V. Bhalerao (IITB), G.C. Anupama (IIA), S. Barway (IIA) and R. Norbu (IAO) report on behalf of the GIT team:

We observed the field of GRB 260511B detected by Fermi (Fermi GBM Team, GCN 44542

Loading...
 
 
), with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2026-05-11 16:19:32 (UTC), i.e., 5.32 hours after the trigger, and obtained multiple exposures in the SDSS r', g', and i' filters. We detect the optical counterpart in our images at the position reported by Wu et al., GCN 44547. The photometric upper limit is as follows:

MJD (mid)Filtertmid-t0 (hours)Exposure Time (sec)Magnitude (AB)
61171.68255r'5.371x40018.27 +/- 0.05
61171.68758g'5.491x40018.54 +/- 0.06
61171.69242i'5.611x36018.10 +/- 0.10

The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.

Our results are consistent with other optical observations (Wu et al., GCN 44547

Loading...
 
 
and GCN 44552; Sasada et al. GCN 44549; Jiang et al., GCN 44551.)

The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree 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 44560

Subject
GRB 260511B: AstroSat CZTI detection
Date
2026-05-11T17:32:28Z (20 days ago)
Edited On
2026-05-11T17:39:40Z (20 days ago)
From
Anuraag Arya at IIT Bombay <aryaanuraag910@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Anuraag Arya at IIT Bombay <aryaanuraag910@gmail.com>
Via
Web form
A. Arya (IITB), A. Goyal (IITB), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), S. Salunke (IUCAA), 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 GRB 260511B which is also detected by Fermi/GBM (Fermi GBM Team, GCN Circ. 44542), SVOM/ECLAIRs (Godet et al., GCN Circ. 44543), and NuSTAR (Waratkar et. al. , GCN Circ. 44556).

The source was clearly detected in the CZT detectors in the 20-200 keV energy range.The light curve showed multiple peaks of emission with the strongest peak at  2026-05-11 11:01:02.99 UTC. The measured peak count rate associated with the burst is 398 (+41, -36) counts/s above the background in the combined data of all quadrants, with a total of 3131 (+342, -481) counts.The local mean background count rate was 282 (+2, -4) counts/s. We measure a T90 of 39 (+3, -2) s from the cumulative CZT light curve.

The source was 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 2026-05-11 11:01:02.74. The measured peak count rate associated with the burst is 1362 (+85, -90) counts/s above the background in the combined data of all quadrants, with a total of 7777 (+766, -805) counts.The local mean background count rate was 1287 (+4, -5) counts/s. We measure a T90 of 37 (+16, -3) 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 data products like interactive and downloadable light curves for this GRB can be found at:
https://astrosat.iucaa.in/cift/cift_products/516193231.75/S516193231.75_details.html

CZTI GRB detections are reported regularly on the payload site at:
 http://astrosat.iucaa.in/czti/?q=grb

GCN Circular 44557

Subject
GRB 260511B: J-band observations by the SYSU 80 cm infrared telescope
Date
2026-05-11T16:10:48Z (20 days ago)
From
lijj328@mail2.sysu.edu.cn
Via
Web form
Jin-Ji Li, Chun Chen, Duo-Le Cao, Zhong-Nan Dong, Wei-Sen Huang, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Yan Yu, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:

We observed the field of GRB 260511B (SVOM burst-id sb26051102) detected by SVOM/ECLAIRs (Godet et al., GCN 44543) and Fermi (Fermi GBM team, GCN 44542), using the Sun Yat-sen University (SYSU) 80 cm infrared telescope. Our observations started on 2026 May 11 at 13:51 UT, 2.9 hours after the GRB trigger, with a total exposure time of 1800 s in the J-band images.

We clearly detect the source at the position of the optical counterpart (Wu et al., GCN 44547; Sasada et al., GCN 44549; Jiang et al., GCN  44551; Wu et al., GCN 44552), and we measure J = 16.02 +/- 0.07 Vega mag. The photometry was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction. 

Further observations are planned.

The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.

GCN Circular 44556

Subject
GRB 260511B: NuSTAR detection of prompt emission
Date
2026-05-11T16:08:21Z (20 days 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 prompt emission from the long-duration GRB 260511B 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 2026-05-11T11:00:31.7 UTC, shows a detection of GRB 260511B, also detected by Fermi/GBM (Fermi GBM Team, GCN 44542) and SVOM (Godet et al., GCN 44532).

The NuSTAR CsI shield data are recorded at 1 Hz. We detect an initial faint burst followed by a 20-s long brighter burst consistent with the Fermi/GBM lightcurve. The peak count rate is ~2500-cps with a baseline rate of ~1000-cps during this time period. We also see marginal evidence in the signal above 100 keV in the CZT detectors.

The optical counterpart candidate (Wu et al., GCN 44547) at RA = 192.57345, Dec = 4.05512 implies an offset from the NuSTAR boresight of 93-deg (i.e. from the side of the instrument) and an offset from the geocenter of 164-deg.

Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260511B 

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 44554

Subject
GRB 260511B: Fermi GBM Detection
Date
2026-05-11T15:37:39Z (20 days ago)
From
oliver.roberts@universityofgalway.ie
Via
Web form
O.J. Roberts (Uni. Of Galway, Ireland) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 11:00:31.70 UT on 11 May 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260511B (trigger 800190036 / 260511459), which 
was also detected by SVOM /ECLAIRs and MXT (Godet et al. 2026, GCN 44543), 
with an optical counterpart initially detected by C-GFT (Wu et al. 2026, GCN 44547). 
The GBM localization is consistent with the SVOM position.
The angle from the Fermi LAT boresight is 100 degrees.

The GBM light curve consists of a two bright emission episodes with a duration (T90)
of about 41 s (50-300 keV). The time-averaged spectrum from T0-0.5 to T0+47.6 s is 
best fit by a Band function, with Epeak = 209 +/- 16 keV, alpha = -1.01 +/- 0.04, 
and beta = -2.14 +/- 0.08.

The event fluence (10-1000 keV) in this time interval is
(2.62 +/- 0.04) E-05 erg/cm^2. The 1-s peak photon flux measured
starting from T0+33.5 s in the 10-1000 keV band is 18.3 +/- 0.4 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 44552

Subject
GRB 260511B: SVOM/VT optical observation with VHF data
Date
2026-05-11T15:08:48Z (20 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form

C. Wu, L. P. Xin, Y. L. Qiu, H. L. Li (NAOC), J. T. Palmerio (CEA/Irfu), L. Zhang (IHEP), D. Kong (GXU) on behalf of the SVOM/VT team.

After the trigger by SVOM/ECLAIRs at 2026-05-11T11:00:32 UTC (T0), SVOM performed an automatic slew on the burst of GRB 260511B location (Godet et al.,GCN 44543; The Fermi GBM team, GCN 44542

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;). SVOM/VT took images of the field in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.

From a preliminary analysis of the 1-bit subimage and source list downloaded via VHF network, the candidate (Wu et al., GCN 44547

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; Sasada et al., GCN 44549; Jiang et al., GCN 44551) was clearly detected in VT-VHF data. The candidate's magnitude is :

date-obs (UTC)mid-time (T-T0)exposureVT_B mag(AB)VT_R mag(AB)
2026-05-11T13:14:192.23 hr5*50 sec17.83 +/- 0.01--

Magnitude was not corrected for Galactic reddening. (Note:The missing magnitude is likely caused by either onboard processing interference or saturation of the bright source at early epoch.）

The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.

GCN Circular 44551

Subject
GRB 260511B: TRT optical observation
Date
2026-05-11T14:21:35Z (20 days ago)
From
sqjiang at NAOC <sqjiang@bao.ac.cn>
Via
Web form
S.Q. Jiang (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), L.B. He, X. Liu, Z.P. Zhu, J. An, D. Xu (NAOC), S.Y. Fu (HUST) report:

We observed the field of GRB 260511B detected by SVOM (Godet et al., GCN 44543) and Fermi (Fermi GBM team, GCN 44542), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). Observation started at 2026-05-11T12:52:30.855 UT, i.e., 1.867 hrs post-burst, and a series of R-band frames were obtained.

The optical counterpart (Wu et al., GCN 44547; Sasada et al., GCN 44549) is clearly detected in our images with a brightness of R = 16.82 +/- 0.01 at a median time of 1.875 hrs since trigger, calibrated with Pan-STARRS DR2 stars in the field and not corrected for Galactic extinction.

GCN Circular 44549

Subject
GRB 260511B : MITSuME Akeno optical afterglow candidate detection
Date
2026-05-11T13:40:52Z (20 days ago)
From
Mahito Sasada at Institute of Science Tokyo <sasada@hp.phys.titech.ac.jp>
Via
Web form
M. Sasada, I. Takahashi, H. Hagio, Y. Kubo, A. Ochi, R. Kato, N. Hosoya, J. Yonamine, Y. Yatsu and N. Kawai (Science Tokyo) report on behalf of the MITSuME collaboration:

We observed the field of GRB 260511B detected by SVOM (Godet et al., GCN 44543) with the optical three-color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50-cm telescope Akeno. 

The observation started at 2026-05-11 11:11:58 UT (11.4 min after the trigger). We stacked the images taken under good conditions. We detected a candidate of the optical afterglow in all-band images at R.A. = 192.5739 deg and Dec. = 4.0554 deg, which is near the candidate position of SVOM/ECLAIRs reported by Wu et al., GCN 44547. Here we report the preliminary magnitudes of the source as follows. 

T0+[sec] | MID-UT | T-EXP[sec] | magnitudes
---------------------------------------------------------------------------------------------------------------
716 | 2026-05-11 11:12:28 | 60 | Rc=13.1+/-0.1, Ic=13.1+/-0.1
1337 | 2026-05-11 11:22:49 | 240 | g'=14.9+/-0.1, Rc=14.9+/-0.1, Ic=14.5+/-0.1
---------------------------------------------------------------------------------------------------------------
T0+ : Elapsed time after the trigger
T-EXP: Total exposure time

We used the PS1 catalog for flux calibration. The catalog magnitudes in PS1 g, r and i bands were converted to our g', Rc and Ic band magnitudes following Tonry et al. (2012), Table 6. The magnitudes are expressed in the AB system. The images were processed in real-time through the MITSuME GPU reduction pipeline (Niwano et al. 2021, PASJ; https://github.com/MNiwano/Eclaire).

GCN Circular 44547

Subject
GRB 260511B SVOM/C-GFT optical counterpart detection
Date
2026-05-11T13:03:12Z (20 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form

C. Wu (NAOC), Z. Kang (CHO), L.P. Xin, X.H. Han, P.P. Zhang, X.M. Lu (NAOC), Z.W. Li, Y. Lv (CHO), R.S. Zhang, Y.J. Xiao, Y.L. Qiu, J. Wang, J.S. Deng, L. Huang, J.Y. Wei (NAOC) ，O. Go-det, M. Brunet (IRAP), L. Zhang (IHEP), D. Kong (GXU) report on behalf of the SVOM/C-GFT team:

We observed the field of GRB 260511B (SVOM burst-id sb26051102) detected by SVOM/ECLAIRs (Godet et al., GCN 44543

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) with LATIOS on SVOM/C-GFT. Observations start-ed at 2026-05-11T11:29:37 UTC, ~0.48 hr after the trigger.

An uncatalogued optical source compared to Pan-STARRS1 is detected in our images within the ECLAIRs localization error circle. Its position is R.A., Dec. 192.57345, 4.05512 degrees:

R.A. (J2000) = 12h50m17.63s
Dec. (J2000) = +04d03m18.4s

with an uncertainty of ~0.5 arcsec. The magnitudes is mag_r = 15.56 +/- 0.04 at 45.32 min post-trigger.

The photometry was calibrated against nearby Pan-STARRS1 stars and no correction for Galactic dust extinction was applied.

We propose that this source is the optical counterpart of GRB 260511B. Further analyses are ongoing.

We thank the observation assistants Bo-Wen Li and Hong-Xv Xue at Jilin observatory for their excellent support.

The Chinese Ground Follow-up Telescope (C-GFT) for the SVOM mission is located at Jilin Station, Changchun Observatory, National Astronomical Observatories, CAS. It features two instruments: (1) CATCH at the Cassegrain focus with a 21 arcsec x 21 arcsec FOV for simultaneous g/r/i-band imaging, and (2) LATIOS, a 4k x 4k CMOS camera at the prime focus with a 1.28 deg x 1.28 deg FOV that images in g, r, and i bands via filter switching.

GCN Circular 44543

Subject
GRB 260511B: SVOM detection of a burst
Date
2026-05-11T11:25:44Z (20 days ago)
Edited On
2026-05-12T13:41:53Z (19 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
O. Godet, M. Brunet (IRAP), L. Zhang (IHEP), D. Kong (GXU) report on behalf of the SVOM mission team:

At 2026-05-11T11:00:32 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260511B (SVOM burst-id sb26051102).


This transient was also detected by Fermi/GM (GCN Circular 44542).


The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.


The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 15 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 45.21 in the [8-120] keV energy band over a time window of 5.10 seconds starting at 2026-05-11T11:01:00.


The localization of the best alert is R.A., Dec. 192.606, 4.094 degrees:

R.A. (J2000) = 12h50m25.47s

Dec. (J2000) = 4d05m40.00s

with a 90% confidence level (C.L.) radius of 2.63 arcmin (including systematic error of 2 arcmin added in quadrature).


The SVOM/ECLAIRs light curve showed a multiple peak structure with a T90 duration of 41.2 -0.6/+1.0 s (5-120 keV).


This burst also triggered SVOM/GRM at 2026-05-11T11:00:31 on a timescale of 0.10 seconds with an SNR of 7.

The SVOM/GRM light curve showed a multiple peak structure with a T90 duration of 54.2 -5.2/+5.1 s (8-1100 keV).


SVOM slewed to the burst.


SVOM/MXT began observing the field at 2026-05-11T11:03:32 UTC, 180 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 192.5735, 4.0405 degrees:

R.A. (J2000) = 12h50m17.65s

Dec. (J2000) = 4d02m25.68s

with a 90% C.L. radius of 32.02 arcseconds.

This location is 3.78 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received.

VT began observing the field after the slew. The analysis of the data will be published in a future circular.


The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.


The Burst Advocate (BA) on shift for this alert is zhangli: zhangli at ihep.ac.cn

Please contact the BA by email if you require additional information.

GCN Circular 44542

Subject
GRB 260511B: Fermi GBM Final Real-time Localization
Date
2026-05-11T11:10:56Z (20 days 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 11:00:31 UT on 11 May 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260511B (trigger 800190036.70394 / 260511459).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 192.2, Dec = 5.0 (J2000 degrees, equivalent to J2000 12h 48m, 5d 00'), with a statistical uncertainty of 1.4 degrees.

The angle from the Fermi LAT boresight is 101.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260511459/quicklook/glg_skymap_all_bn260511459.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/bn260511459/quicklook/glg_healpix_all_bn260511459.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260511459/quicklook/glg_lc_medres34_bn260511459.gif
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