GRB 261011A
GCN Circular 45965
Subject
GRB 261011A: 1.3m DFOT optical upper limit
Date
2026-10-16T16:49:50Z (7 days ago)
From
Yash Sharma at Aryabhatta Research Institute of Observational Sciences <yashandyash30@gmail.com>
Via
Web form
Yash Sharma, Amit K. Ror, Shashi B. Pandey (ARIES) report:
We observed the field of GRB 261011A detected by Swift (Cenko et al., GCN 45896) and SVOM (Huang et al., GCN 45895) with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, India. The observations were started on 2026-10-14 at 18:09:17 UT, i.e., ~ 3.18 days after the Swift/BAT trigger (T0 = 2026-10-11 13:46:41 UT). We obtained multiple frames with an exposure time of 300 s each in the R filter. The images were aligned and stacked.
We do not detect any optical afterglow emission down to the 3-sigma background limit at the optical afterglow position reported by Swift/UVOT (Cenko et al., GCN 45896) and refined by SOAR (Cheng et al., GCN 45922), nor within the 2.9 arcsec error radius of the enhanced Swift-XRT position (Page et al., GCN 45933; Melandri et al., GCN 45934).
We obtain the following 3-sigma upper limit from the stacked image:
Date Start_UT T_start-T0 (days) Filter Exp time (s) Magnitude (3-sigma)
===================================================================================
2026-10-14 18:09:17 ~ 3.18 R 12 x 300 s > 22.0
The non-detection of the optical counterpart is consistent with the rapid fading of the afterglow at z = 2.407 (Pérez-García et al., GCN 45931; Monsalves et al., GCN 45920) and later-epoch optical/NIR upper limits reported by REM (Brivio et al., GCN 45926), SYSU (Wen et al., GCN 45942), and SAO RAS (Moskvitin et al., GCN 45951).
Photometric calibration was performed against secondary standard stars from the Pan-STARRS (PS1) catalog with Zero Point = 4.081 ± 0.015 mag. The reported limiting magnitude is in the AB system and has not been corrected for the Galactic extinction. This circular may be cited.
GCN Circular 45951
Subject
GRB 261011A: SAO RAS and AbAO optical observations
Date
2026-10-14T16:33:53Z (9 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
email
A. Moskvitin (SAO), A. Volnova (IKI), V. Goranskij (SAI MSU, SAO),
V. Vlasyuk (SAO), R. Inasaridze (AbAO), N. Pankov (IKI), A. Pozanenko
(IKI),
on behalf of the SAO GRB follow-up team and the IKI-GRB-FuN report.
We observed the field of GRB 261011A (Huang et al., GCN 45895;
Cenko et al., GCN 45896; Ridnaia et al., GCN 45924; Barthelmy et al.,
GCN 45930; Page et al., GCN 45933; Melandri et al., GCN 45934;
Luo et al., GCN 45939) at redshift z = 2.407 (Pérez-García et al.,
GCN 45931; Monsalves et al., GCN 45920) with the SAO RAS 1-m Zeiss-1000,
0.5-m AS-500(2) and AbAO 0.7-m AS-32 telescopes on October 11.
The OT (Cenko et al., GCN 45896; O'Neill et al., GCN 45897; Fu et al.,
GCN 45898; Izzo, GCNs 45899; 45909; Wu et al., GCN 45900; Shi et al.,
GCN 45903; Aryan et al., GCN 45904; Saikia et al., GCN 45907;
Bochenek and Perley, GCN 45914; Li et al., GCN 45915; Zhao et al.,
GCN 45916; Takahashi et al., GCN 45917; Cheng et al., GCN 45922;
also observed in NIR by Huang et al., GCN 45913) is marginally
detected only in the Zeiss-1000 stacked frame.
Details of the observation are the following.
Date UT start t-T0 Exp, Filter Obj Err UL
Site/Telescope
(mid,days) (n*s) (3sigma)
2026-10-11 20:22:07 0.29806 117*30 R n/d 21.6
SAO/AS-500(2)
2026-10-11 20:56:50 0.30510 3*300 Rc 21.9 0.3 22.0
SAO/Zeiss-1000
2026-10-11 21:01:57 0.32050 49*60 R n/d 20.9
AbAO/AS-32
This preliminary photometry (Vega) is based on nearby SDSS stars
(Lupton 2005 transformation) and not corrected for Galactic extinction.
GCN Circular 45942
Subject
GRB 261011A: Later-epoch J-band upper limit from the SYSU 80 cm infrared telescope
Date
2026-10-13T13:41:51Z (10 days ago)
From
wenly8@mail2.sysu.edu.cn
Via
Web form
Lu-Yao Wen, Jian-Hong Cao, Zhong-Nan Dong, Jin-Ji Li, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Rui-Chen Gao, Jing-Kai Sun, Hao-Ran Zhang, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:
We carried out further J-band observations of GRB 261011A using the Sun Yat-sen University (SYSU) 80 cm infrared telescope. We observed from 2026 October 12 17:37:44.942 to 19:41:05.309 UT, 27.85–29.91 hours after the Swift/BAT trigger. Our earlier J-band detections were reported in Huang et al. (GCN 45913).
We do not detect the source at the position of the counterpart (Cenko et al., GCN 45896; O'Neill et al., GCN 45897; Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901; Shi et al., GCN 45903; Aryan et al., GCN 45904; Arya et al., GCN 45907; Izzo et al., GCN 45909; Huang et al., GCN 45913; Bochenek et al., GCN 45914; Li et al., GCN 45915; Zhao et al., GCN 45916; Takahashi et al., GCN 45917; Monsalves et al., GCN 45920; Cheng et al., GCN 45922), down to a 5-sigma limiting magnitude of J > 19.6 mag (Vega).
The limiting magnitude was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction. The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 45939
Subject
GRB 261011A: GECAM-B observation
Date
2026-10-13T12:29:42Z (10 days ago)
From
Jin-Ye Luo <jy_luo2002@163.com>
Via
Web form
Jin-Ye Luo, Chen-Wei Wang, Yue Wang, Shao-Lin Xiong (IHEP) report on behalf of GECAM team:
GECAM-B was triggered on-ground by GRB 261011A at 2026-10-11T13:49:15.250 UTC (denoted as T0), which is also detected by SVOM/ECLAIRs (Huang et al., GCN #45895), Swift/BAT (Cenko et al., GCN #45896; Barthelmy et al., GCN #45930), and Konus-Wind (Ridnaia et al., GCN #45924).
According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of muplti-pulses with a duration (T90) of 40 +21/-9 s.
The GECAM-B light curve can be found here:
https://www.bursthub.cn/gcn/2026/gecambgrb261011A.png
Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).
GCN Circular 45934
Subject
GRB 261011A: Swift-XRT refined Analysis
Date
2026-10-13T07:14:45Z (10 days ago)
Edited On
2026-10-13T12:59:24Z (10 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB),
D.N. Burrows (PSU), J.J. DeLaunay (PSU), S. Lanava (PSU), A.P.
Beardmore (U. Leicester), J.P. Osborne (U. Leicester) and P.A. Evans
report on behalf of the Swift-XRT team:
We have analysed 4.2 ks of XRT data for GRB 261011A, from 301 s to 28.0
ks after the trigger. The data comprise 1.2 ks in Windowed Timing
(WT) mode (the first 10 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. Using 1938 s of PC mode data
and 4 UVOT images, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 21.89722, -21.56052 which is equivalent to:
RA (J2000): 01h 27m 35.33s
Dec(J2000): -21d 33' 37.9"
with an uncertainty of 2.9 arcsec (radius, 90% confidence).
The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=4.1 (+/-0.6). At T+362 s the decay
flattens to an alpha of 1.68 (+0.11, -0.10) before breaking again at
T+792 s to a final decay with index alpha=1.26 (+0.06, -0.05).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.80 (+/-0.05). The
best-fitting absorption column is 7.7 (+1.7, -1.6) x 10^20 cm^-2, in
excess of the Galactic value of 0.0 x 10^00 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 3.6 x 10^-11 (4.1 x 10^-11) erg
cm^-2 count^-1.
A summary of the WT-mode spectrum is thus:
Total column: 7.7 (+1.7, -1.6) x 10^20 cm^-2
Galactic foreground: 1.5 x 10^20 cm^-2
Excess significance: 6.4 sigma
Photon index: 1.80 (+/-0.05)
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01516664.
This circular is an official product of the Swift-XRT team.
GCN Circular 45933
Subject
GRB 261011A: Enhanced Swift-XRT position
Date
2026-10-13T07:11:07Z (10 days ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
K.L. Page, P.A. Evans and M.R. Goad (U. Leicester) report
on behalf of the Swift-XRT team:
Using 1938 s of XRT Photon Counting mode data and 4 UVOT images, we find
an astrometrically corrected X-ray position (using the XRT-UVOT alignment
and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
21.89722, -21.56052 which is equivalent to:
RA (J2000): 01 27 35.33
Dec (J2000): -21 33 37.9
with an uncertainty of 2.9 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed at https://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).
This circular is an official product of the Swift-XRT team.
GCN Circular 45931
Subject
GRB 261011A: 10.4m GTC spectroscopic redshift (z = 2.407)
Date
2026-10-13T05:21:34Z (10 days ago)
From
I. Perez-Garcia at Instituto de Astrofísica de Andalucía <ipg@iaa.es>
Via
Web form
I. Pérez-García (IAA-CSIC), Y.-D. Hu (GXU), A. J. Castro-Tirado (IAA-CSIC), F. J. Aceituno, M.D. Caballero-Garcia, E. Fernandez-Garcia, M. Gritsevich, S. Guziy (IAA-CSIC), D. Garcia-Alvarez, A. Tejero (GRANTECAN, IAC), on behalf of a larger collaboration, report:
We observed the optical afterglow of GRB 261011A detected by SVOM/ECLAIRs (Huang et al. GCN 45895), Swift/BAT (Cenko et al. GCN 45896) and Konus-Wind (Ridnaia et al. GCN 45924), with the 10.4m Gran Telescopio Canarias (equipped with the OSIRIS+ spectrograph) in La Palma (Spain). Long-slit spectra were taken starting on Oct 12, 2026, 00:19 UT (i.e. 10.5 h post burst) with both the R1000R grism (1200 s) and the R1000B grism (600 s), with the slit centred on the optical afterglow position (Cenko et al., GCN 45896; O'Neill et al., GCN 45897; Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901; Shi et al., GCN 45903; Aryan et al., GCN 45904; Arya et al., GCN 45907; Izzo et al., GCN 45909; Huang et al., GCN 45913; Bochenek et al., GCN 45914; Li et al., GCN 45915; Zhao et al., GCN 45916; Takahashi et al., GCN 45917; Cheng et al., GCN 45922).
A faint continuum of the afterglow is detected across the full range. On top of it, we identify low S/N ratio absorption features consistent with Fe II (2344, 2374, 2382 A), Al II (1670 A) and Al III (1854/1862 A), all at a common redshift z = 2.407 +/- 0.001.
A joint analysis of all the expected lines gives the above mentioned, unique solution, at this redshift over the range 1.5 < z < 4.2. Moreover, we notice that the flux drops strongly below ~4270 A, consistent with the damped Lyman-alpha absorption trough at this redshift.
This confirms the tentative redshift z = 2.405 derived by Monsalves et al. (GCN 45920) from the damped Lyman-alpha trough in the VLT/X-shooter spectrum.
In addition to this, we notice that the Legacy Survey DR10 shows a faint source (r = 25.4 AB), coincident with the afterglow position within 0.1 arcsec, a likely host galaxy candidate.
We thank the staff at GTC for their excellent support.
GCN Circular 45930
Subject
GRB 261011A: Swift-BAT refined analysis
Date
2026-10-13T00:39:46Z (11 days ago)
From
Amy <yarleen@gmail.com>
Via
email
S. D. Barthelmy (GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), S. Laha
(GSFC/UMBC), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), M. J. Moss
(GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC), T.
Sakamoto (AGU) (i.e. the Swift-BAT team):
Using the data set from T-179 to T+723 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 261011A (trigger #1516664)
(Cenko et al., GCN Circ. 45896). The BAT ground-calculated position is
RA, Dec = 21.911, -21.550 deg which is
RA(J2000) = 01h 27m 38.7s
Dec(J2000) = -21d 33' 00.7"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The
partial coding was 29%.
The mask-weighted light curve shows a multi-peaked structure that starts
~T+100 s and ends at ~T+210 s. T90 (15-350 keV) is 78.90 +- 18.49 sec
(estimated error including systematics). Note that the particularly unusual
light curve mentioned in the original circular (Cenko et al., GCN Circ.
45896) is no longer visible in the ground analysis. Further investigation
shows that there were some noisy detectors producing coincidental
pulse-like structures around the GRB time, and thus those pulses seen in
the raw light curve prior to ~T+100 s are not astrophysical.
The time-averaged spectrum from T+103.61 to T+208.74 sec is best fit by a
simple power-law model. The power law index of the time-averaged spectrum
is 1.50 +- 0.07. The fluence in the 15-150 keV band is 6.3 +- 0.3 x 10^-06
erg/cm2. The 1-sec peak photon flux measured from T+171.74 sec in the
15-150 keV band is 3.0 +- 0.4 ph/cm2/sec. All the quoted errors are at the
90% confidence level.
The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1516664
GCN Circular 45926
Subject
GRB 261011A: REM optical/NIR observations
Date
2026-10-12T17:05:52Z (11 days ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio, M. Ferro, P. D'Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:
We observed the field of GRB 261011A, detected by Swift/BAT (Huang et al., GCN 45895; Cenko et al., GCN 45896) and SVOM/ECLAIRs with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile).
The observations were carried out in the g, r, i, z, K bands, started on 2026-10-12 at 01:30:16 UT (i.e. 11.7 hr after the burst) and lasted for about 2.0 hr.
From preliminary photometry, we do not detect the optical/NIR counterpart (Cenko et al., GCN 45896; O'Neill et al., GCN 45897; Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901; Shi et al., GCN 45903; Aryan et al., GCN 45904; Arya et al., GCN 45907; Izzo et al., GCN 45909; Huang et al., GCN 45913; Bochenek et al., GCN 45914; Li et al., GCN 45915; Zhao et al., GCN 45916; Takahashi et al., GCN 45917; Monsalvel et al., GCN 45920; Cheng et al., GCN 45922) down to the following 3sigma limits:
r > 20.6 (AB; calibrated against the PanSTARRS catalogue),
at a mid-time of 12.1 hr after the trigger;
K > 14.2 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 11.8 hr after the trigger.
GCN Circular 45924
Subject
Konus-Wind detection of GRB 261011A
Date
2026-10-12T15:15:46Z (11 days ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A. Ridnaia, D. Frederiks, A. Lysenko, V. Panteleeva,
D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
The long-duration GRB 261011A
(SVOM detection: Huang et al., GCN 45895;
Swift detection: Cenko et al., GCN 45896)
was detected by Konus-Wind (KW) in the waiting mode
at about T0 = T0 (SVOM) = 49747 s UT (13:49:07).
A Bayesian block analysis of the KW waiting mode data
in the 20-400 keV band reveals a ~20 sigma count rate
increase in the interval from T0+1.7 s to T0+66.468 s.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB261011A/
The total burst fluence is 1.30(-0.21,+0.62)x10^-5 erg/cm^2,
and the 2.944 s peak energy flux, measured from T0+10.532 s,
is 4.63(-4.27,+4.74)x10^-7 erg/cm^2/s.
(both in the 20 keV - 10 MeV energy range).
The time-integrated spectrum of the burst,
measured from T0+1.7 s to T0+66.468 s,
can be described by a power law with exponential cutoff (CPL) model
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -1.8(-0.1,+0.2) and Ep = 198(-67,+869) keV.
All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.
GCN Circular 45922
Subject
GRB 261011A: SOAR optical afterglow observations at ~17 hours
Date
2026-10-12T12:56:33Z (11 days ago)
From
Cheng Cheng at CAS South America Center for Astronomy (CASSACA) <chengcheng@nao.cas.cn>
Via
Web form
Cheng Cheng (CASSACA, NAOC), Liping Xin (NAOC), Jia-Sheng Huang (CASSACA, NAOC) report:
We observed the field of GRB 261011A (SVOM/ECLAIRs: Huang et al., GCN 45895; Swift/BAT: Cenko et al., GCN 45896) with the 4.1-m SOAR telescope (GHTS red imager) beginning on 2026-10-12 at 06:45:36 UT, i.e. 17.06 h after the Swift/BAT trigger.
We obtained 3 x 180 s exposures in each of the SDSS g, r and z filters. The frames were trimmed, astrometrically solved, sky-subtracted and median-combined onto a common output grid (0.2894"/pixel), so that the three band images are pixel-aligned. Each stacked image is 1000 x 1000 pixels (4.8' x 4.8'), centred on the target coordinates.
We detect the afterglow at
RA, Dec (J2000) = 01:27:35.40, -21:33:37.7
with an uncertainty of 0.15" (the astrometry is tied to Gaia DR3; a cross-check against LS DR11/DECaLS gives a residual scatter of 0.10"). This position lies 0.52" from the Swift/UVOT position reported by Cenko et al. (GCN 45896), fully consistent with its 1.1" error radius, and refines it.
Photometry was carried out with SExtractor in dual mode, using the r-band image as the detection image and measuring MAG_AUTO magnitudes on the g, r and z images with a local background estimate. Magnitudes were calibrated against LS DR11 (DECaLS) stars and are in the AB system:
g = 23.93 +/- 0.14 (phot) +/- 0.08 (zeropoint)
r = 22.75 +/- 0.08 (phot) +/- 0.09 (zeropoint)
z = 22.43 +/- 0.18 (phot) +/- 0.15 (zeropoint)
The first uncertainty is the photometric error and the second is the zeropoint (calibration) uncertainty, which is systematic across the field and therefore should not be treated as an independent error when combining this measurement with others.
The source is unresolved (FWHM = 1.2"), and no host galaxy is detected at this depth; the LS DR11 source at this position (ls_id_dr11 = 48423370429045300) has r = 25.63 +/- 0.30 AB mag.
We thank the SOAR staff for the rapid execution of these observations.
GCN Circular 45920
Subject
GRB 261011A: VLT/X-shooter redshift constraints
Date
2026-10-12T07:37:55Z (11 days ago)
From
Nicolas Monsalves <n.m.monsalvesgonzalez@gmail.com>
Via
Web form
Nicolas Monsalves, Yu-Han Yang and Eleonora Troja (U Rome) report on behalf of the ERC BHianca team:
We observed the field of GRB 261011A (Huang et al., GCN 45895; Cenko et al., GCN 45896) with the X-Shooter spectrograph on the ESO VLT UT3 (Melipal). Observations began at T+13.4 hours and obtained a total of 4 x 600 s spectra at an average airmass of about 1.1.
At the position of the optical counterpart (Cenko et al., GCN 45896; O'Neill et al., GCN 45897; Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901; Shi et al., GCN 45903; Aryan et al., GCN 45904; Arya et al., GCN 45907; Izzo et al., GCN 45909; Huang et al., GCN 45913; Bochenek et al., GCN 45914; Li et al., GCN 45915; Zhao et al., GCN 45916; Takahashi et al., GCN 45917) we detect a faint continuum in all three arms.
In the UVB arm we detect continuum emission down to ~4270 Angstrom, which sets a robust upper limit to the redshift z<3.7. The flux remains consistent with zero between ~4000 and ~4270 Angstrom,with a weak signal reappearing blueward of ~4000 Angstrom. Interpreting this broad feature as a damped Lyman-alpha absorption trough, we infer a tentative redshift of z=2.405.
We thank the staff at the VLT, for the rapid execution of these observations.
GCN Circular 45917
Subject
GRB 261011A : MITSuME Akeno optical afterglow candidate detection
Date
2026-10-12T03:31:32Z (11 days ago)
From
Ichiro Takahashi at Science Tokyo <itakahashi@hp.phys.titech.ac.jp>
Via
Web form
I. Takahashi, M. Sasada, H. Hagio, Y. Kubo, A. Ochi, R. Kato, N. Hosoya, J. Yonamine, Y. Yatsu, N. Kawai (Science Tokyo), and M. Niwano (NAOJ) report on behalf of the MITSuME collaboration:
We observed the field of GRB 261011A detected by Swift (Cenko et al., GCN 45896) and SVOM (Huang et al., GCN 45895) with the optical three-color (g', Rc, and Ic) CCD cameras mounted on the MITSuME 50-cm telescope Akeno.
The observations started at 2026-10-11 13:50:44 UT (243 s after the Swift/BAT trigger). We stacked the images obtained under good observing conditions. We detected an optical afterglow candidate at the position reported by Swift-UVOT (Cenko et al., GCN 45896).
The preliminary photometry of the source is as follows:
T0+[sec] | MID-UT | T-EXP[sec] | Magnitudes
----------------------------------------------------------------------------------------------------
580 | 2026-10-11 13:56:21 | 260 | Rc=16.5+/-0.1, Ic=16.2+/-0.1
----------------------------------------------------------------------------------------------------
T0+ : Elapsed time after the Swift/BAT 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 45916
Subject
GRB 261011A: Optical followup from Sichuan South Observatory
Date
2026-10-12T03:09:29Z (11 days ago)
From
Jiewei Zhao at SWIFAR, YNU <meow.jiewei.zhao@gmail.com>
Via
Web form
Jiewei Zhao, Brajesh Kumar, Yuanpei Yang, Guowang Du (SWIFAR, YNU), Kai Liao, Xinyu Wang, Xing Yao, Yang Zeng and Chunlai Zhang (SSO) report:
We performed observations of the GRB 261011A field (Huang et al., GCN 45895; Cenko et al., GCN 45896; T0 at 2026-10-11T13:46:41) with 60cm telescope located at the Sichuan South Observatory (SSO), China. These observation were acquired from 2026-10-11T14:28:48.22 (~0.70 hours after the trigger) and continued ~2.40 hours. Multiple frames were acquired in L band with different exposure time.
The optical counterpart reported by Swift-UVOT (Cenko et al., GCN 45896) and also confirmed by other facilities (O'Neill et al., GCN 45897; Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901; Shi et al., GCN 45903; Aryan et al., GCN 45904; Arya et al., GCN 45907; Izzo et al., GCN 45909; Huang et al., GCN 45913; Bochenek et al., GCN 45914; Li et al., GCN 45915) was clearly detected in our frames and the preliminary photometry is following:
Start time[MJD], AB Magnitude, Filter
61324.60334, 19.67 +/- 0.19, L
61324.62294, 20.10 +/- 0.14, L
61324.65458, 20.66 +/- 0.20, L
The given magnitudes are not corrected for the expected Galactic extinction.
GCN Circular 45915
Subject
GRB 261011A: GMG Optical Observation
Date
2026-10-12T01:58:20Z (11 days ago)
Edited On
2026-10-12T12:51:27Z (11 days ago)
From
Rui-Zhi Li at Yunnan Observatories, CAS <liruizhi@ynao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Rui-Zhi Li at Yunnan Observatories, CAS <liruizhi@ynao.ac.cn>
Via
Web form
R.-Z. Li, B.-T. Wang, F.-F. Song, J. Mao, K.-X. Lu, J.-G. Wang and B. Wang (YNAO, CAS) report:
We observed the field of GRB 261011A (Huang et al., GCN 45895; Cenko et al., GCN 45896; T0 at 2026-10-11T13:46:41) using the GMG-2.4m telescope at the Lijiang Observatory. The observation began at 2026-10-11T16:41:50, about 2.92 hours after the trigger.
The optical counterpart reported by Swift-UVOT (Cenko et al., GCN 45896) and by other facilities (O'Neill et al., GCN 45897; Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901; Shi et al., GCN 45903; Aryan et al., GCN 45904; Arya et al., GCN 45907; Izzo et al., GCN 45909; Huang et al., GCN 45913; Bochenek et al., GCN 45914) was detected in an r-band image.
The preliminary analysis results are shown as follows:
| Start time [UTC] | Exposure [s] | Tmid-T0 [h] | Filter | AB Magnitude | 5-sigma U.L. |
|---|---|---|---|---|---|
| 2026-10-11 16:41:50.018 | 300 | 2.961 | r | 20.46 ± 0.04 | 22.2 |
The given magnitudes are derived based on calibration against Legacy Survey DR10 field stars, and are not corrected for the expected Galactic foreground extinction, corresponding to a reddening of E(B-V) = 0.019 mag in the direction of the optical counterpart (Schlafly & Finkbeiner 2011).
GCN Circular 45914
Subject
GRB 261011A: Liverpool Telescope optical observations
Date
2026-10-12T00:58:10Z (12 days ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
Web form
A. Bochenek, D. A. Perley (LJMU), report:
We observed the field of GRB 261011A (Huang et al., GCN 45895; Cenko et al., GCN 45896) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 8x100s exposures in SDSS gr filters, starting at 2026-10-11 23:08:04 UT, approximately 9.36 hours after Swift trigger.
We detect a counterpart in the stacked images, at a position consistent with that reported by Cenko et al. (GCN 45896). Photometry is given below; the g-band detection is tentative.
| MJD (mid) | T_mid-T_0 | Filter | Mag. (AB) |
| ------------ | ------------ | --------- | ------------ |
| 61324.97054 | 9.51 h | g | 22.46 ± 0.29 |
| 61324.98405 | 9.84 h | r | 21.98 ± 0.16 |
The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.
GCN Circular 45913
Subject
GRB 261011A: J-band observations by the SYSU 80cm infrared telescope
Date
2026-10-12T00:22:24Z (12 days ago)
Edited On
2026-10-12T01:56:22Z (11 days ago)
From
Jianhong Cao at Sun Yat-sen University <caojh35@mail2.sysu.edu.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Jianhong Cao at Sun Yat-sen University <caojh35@mail2.sysu.edu.cn>
Via
Web form
Si-Rui Huang, Jin-Ji Li, Zhong-Nan Dong, Jia-Qi Lin, Pu Lin, Yun Shi, Jian-Hong Cao, Hao-Nan Yang, Rui-Chen Gao, Jing-Kai Sun, Hao-Ran Zhang, 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 261011A, detected by SVOM/ECLAIRs (Huang et al., GCN 45895) and Swift/BAT (Cenko et al., GCN 45896), using the Sun Yat-sen University (SYSU) 80cm infrared telescope. Our observations were carried out on 2026 Oct 11 from 16:27:30 to 17:07:28 UT, approximately 2.68–3.35 hours after the Swift/BAT trigger, using stacked images in the J band.
We clearly detect the near-infrared afterglow at the position of the previously reported optical counterpart (Cenko et al., GCN 45896; O'Neill et al., GCN 45897). The photometric results are listed below (times refer to the midpoints of the observations):
* **16:37:14 UT (T0 + 2.84 hr):** 58 x 20 s (1160 s total exposure), J = 18.07 +/- 0.07 mag (Vega).
* **16:57:34 UT (T0 + 3.18 hr):** 59 x 20 s (1180 s total exposure), J = 18.65 +/- 0.14 mag (Vega).
The source shows a decline of approximately 0.58 mag between the two epochs.
The photometry was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction.
An additional approximately one hour of J-band observations was obtained, and the data are currently being processed.
The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 45909
Subject
GRB 261011A: continued LCO optical observations
Date
2026-10-11T20:09:17Z (12 days ago)
From
luca.izzo@inaf.it
Via
Web form
L. Izzo (INAF-OACn and DARK/NBI) reports on behalf of a larger collaboration:
We continued to observe the afterglow of GRB261011A (Huang et al., GCN #45895; Cenko et al., GCN #45896) with the Sinistro instrument mounted on the 1-m telescope of the LCO network, located at the Sutherland Observatory, South Africa. Observations started on 2026 October 11 at 18:56 UT (5.17 hr after the GRB trigger). We obtained a single image of 300s in the SDSS-g and SDSS-r filters.
The optical counterpart reported by Swift-UVOT (Cenko et al., GCN #45896) and by other ground-based facilities (O'Neill et al., GCN #45897, Fu et al., GCN #45898, Izzo et al., GCN #45899, Wu et al., GCN #45900, Yin et al., GCN #45901, Shi et al., GCN #45903, Aryan et al., GCN #45904, Arya et al., GCN #45907) is faintly detected in both images. We measure preliminary magnitudes of g = 21.91 +/- 0.17 mag and r = 21.09 +/- 0.14 (AB), calibrated against nearby stars from the Pan-STARRS catalog.
This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 101004719.
GCN Circular 45907
Subject
GRB 261011A: GROWTH-India Telescope optical observations
Date
2026-10-11T19:15:24Z (12 days ago)
Edited On
2026-10-11T19:32:31Z (12 days ago)
From
Aditya Pawan Saikia at IIT Bombay <adityaps@iitb.ac.in>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Aditya Pawan Saikia at IIT Bombay <adityaps@iitb.ac.in>
Via
Web form
R. Arya (IITB), Harshita (IITB), V. Vijaykumar (IITB), S. Patil (IITB), A.P. Saikia (IITB), V. Swain (IITB), V. Bhalerao (IITB), G.C. Anupama (IIA), S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:
We observed the field of GRB 261011A detected by SWIFT (Cenko et al., GCN 45896), with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2026-10-11 17:11:43 UT, i.e., 3.329 hours after the trigger, and obtained multiple exposures in the SDSS r' filter. We detect the optical counterpart in our images at the position reported by SWIFT-UVOT. The photometric results are as follows:
| MJD (mid) | Filter | tmid-t0 (hours) | Exposure Time (sec) | Magnitude (AB) | Limiting Magnitude (5-sigma) |
|---|---|---|---|---|---|
| 61324.720 | r' | 3.565 | 3x300 | 20.93 +/- 0.10 | 21.15 |
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 (O'Neill et al., GCN 45897, Fu et al., GCN 45898, Izzo et al., GCN 45899, Wu et al., GCN 45900, Yin et al., GCN 45901, Shi et al., GCN 45903, and Aryan et al., GCN 45904)
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 45906
Subject
GRB 261011A: Luminance-band upper limit from a 28cm telescope in Chengde
Date
2026-10-11T18:53:04Z (12 days ago)
Edited On
2026-10-16T12:34:21Z (7 days ago)
From
Y.C. Kang at Peking University <yckang@stu.pku.edu.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Y.C. Kang at Peking University <yckang@stu.pku.edu.cn>
Via
Web form
Chenxi Bao (PKU), Yacheng Kang (PKU), Xinmiao Zhao (PKU), and Zhuokai Liu (PKU) report on behalf of the Interestar Collaboration and the PKU HiTF (High-energy Transients Follow-up) group:
We observed the field of GRB 261011A, detected by Swift/BAT (Cenko et al., GCN 45896; T0 = 2026-10-11T13:46:41) and SVOM/ECLAIRs (Huang et al, GCN 45895), using a 28cm optical telescope located in Chengde, Hebei, China. The observation began on 2026-10-11 at 15:46:33 UT, approximately 1.99 hr after the Swift/BAT trigger.
We obtained the Luminance-band image with a total exposure time of 1800 s. No significant source is detected at the position of the reported optical counterpart (Cenko et al., GCN 45896; O'Neill et al., GCN 45897; Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901; Shi et al., GCN 45903; Aryan et al., GCN 45904; Buckley et al., GCN 45905). We derive a 5-sigma upper limit of L > 19.65 mag.
Preliminary photometric calibration was performed using nearby catalogued field stars. The reported limit is an approximate AB-equivalent magnitude and has not been corrected for Galactic extinction. Our non-detection is consistent with the reported fading behavior of the optical counterpart. Further optical follow-up observations are encouraged.
The 28cm optical telescope is operated by the Interestar Collaboration, which contributes to the rapid follow-up of high-energy transients as part of the PKU HiTF group.
GCN Circular 45905
Subject
Swift GRB 261011A: Global MASTER-Net observations report
Date
2026-10-11T17:46:58Z (12 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
D.Buckley (SAAO),
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 (Lomonosov MSU)
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
A.Sosnovskij (Lomonosov MSU)
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory)
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
R.Rebolo (The Instituto de Astrofisica de Canarias)
O.Gress, N.Budnev(ISU)
M. Segura, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA)
MASTER-SAAO robotic telescope [1] located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB 261011A ( S. B. Cenko et al., GCN 45896) errorbox 13536 sec after notice time and 13752 sec after trigger time at 2026-10-11 17:35:53 UT, with upper limit up to 18.0 mag. Observations started at twilight. The observations began at zenith distance = 67 deg. The sun altitude is -11.5 deg.
The galactic latitude b = -79 deg., longitude l = 181 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3524824
We obtain a following upper limits.
Tmid-T0 | Site |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________
13842 | MASTER-SAAO | C | 180 | 18.0 |
13842 | MASTER-SAAO | C | 180 | 17.8 |
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 45904
Subject
GRB 261011A: Optical follow-up observations with Kinder
Date
2026-10-11T16:42:15Z (12 days ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
A. Aryan, Z.-Y. Chen, T.-W. Chen, W.-J. Hou, Y.-H. Lee (all NCU), A. K. H. Kong (NTHU), S. Yang (HNAS), S. J. Smartt, J. H. Gillanders (both Oxford), A. Sankar.K, K. N.-T. Ho, A. Dutta, M.-H. Lee, C.-S. Lin, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), K. W. Smith, H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:
As part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86,doi:10.3847/1538-4357/adb428), we conducted multi-band optical follow-up observations of GRB 261011A, detected by SVOM (Huang et al., GCN 45895) and Swift (Cenko et al., GCN 45896).
Imaging was acquired in the g-band using the 1m Lulin One-meter Telescope (LOT), and in the r- and i-bands using the 40cm Seisi Lulin Telescope (SLT). Our observations started at MJD 61324.6155, approximately 0.99 hr after the Swift/BAT trigger.
The optical counterpart reported by Swift/UVOT (Cenko et al., GCN 45896), GOTO (O'Neill et al., GCN 45897), and more instruments (Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901; Shi et al., GCN 45902) was clearly detected in all three bands.
We used AutoPhOT (Brennan & Fraser 2022, A&A, 667, A62) to perform PSF photometry. Preliminary measurements are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude
LOT | g | 61324.6155 | 0.99 | 60 * 1 | 20.47 +/- 0.10
SLT | r | 61324.6157 | 1.04 | 300 * 1 | 19.91 +/- 0.07
SLT | i | 61324.6194 | 1.09 | 300 * 1 | 19.69 +/- 0.07
Using the nearly simultaneous g- and r-band measurements at approximately 1.04 hr after the trigger, and correcting for Galactic foreground extinction, we obtain (g-r) = 0.64 +/- 0.11 mag. Using the earliest r- and i-band measurements, we obtain (r-i) = 0.20 +/- 0.10 mag. Thus, the source shows a relatively red (g-r) colour while remaining considerably flatter between r and i.
The presented magnitudes are calibrated using field stars from the ATLAS-RefCat2 catalog from MAST (Tonry et al. 2018, ApJ, 867, 105) and are not corrected for Galactic foreground extinction of A_g = 0.07, A_r = 0.05 and A_i = 0.04 mag in the direction of the transient (Schlafly & Finkbeiner 2011).
The methodology, details on the Lulin Observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.
GCN Circular 45903
Subject
GRB 261011A: SVOM/C-GFT optical observations
Date
2026-10-11T16:07:29Z (12 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y.Y. Shi(GXU), Z. Kang (CHO),S.J. Yin (GXU), C. Wu (NAOC), 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), Massimiliano Lincetto (CPPM), Damien Dornic (CPPM) report on behalf of the SVOM/C-GFT team:
We observed the field of GRB 261011A detected by SVOM/ECLAIRs (Huang et al., GCN 45895) with LATIOS on SVOM/C-GFT. Observations started at 2026-10-11T13:52:56 UTC, ~3.87 min after the trigger.
The optical candidate (Cenko et al., GCN 45896; O'Neill et al., GCN 45897; Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901) was detected in our images of band i. The magnitude is:
| [date-obs(mid-time)] | Mid_t-T0(min) | exposure time (s) | band | mag (AB) | mag_err |
|---|---|---|---|---|---|
| 2026-10-11T13:53:01 | 3.95 | 10s | i | 15.25 | +/- 0.06 |
The photometry was calibrated against DESI Legacy Imaging Surveys DR10 catalogue and no correction for Galactic dust extinction was applied.
We thank the observation assistants B.-W. Li and H.-X. 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 arcmin x 21 arcmin 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 45901
Subject
GRB 261011A: LCO optical observations
Date
2026-10-11T15:53:31Z (12 days ago)
Edited On
2026-10-11T18:44:33Z (12 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
S.J. Yin(GXU),J.G. Ducoin(CPPM),D. B. Malesani (DAWN/NBI and Radboud), C. Wu (NAOC), A. Saccardi (CEA/Irfu), L.P. Xin, X.H. Han, P.P. Zhang (NAOC),Y.Y .Shi, Y.X. Guan(GXU), Massimiliano Lincetto (CPPM), Damien Dornic (CPPM) report on behalf of the SVOM mission team:
We observed the field of the SVOM/ECLAIRs GRB 261011A (Huang et al., GCN 45895) with the LCO 1m telescope at Siding Spring Observatory equipped with the Sinistro instrument.
Our observation started on 2026-10-11 at 14:04:15.8 UT and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters.
The optical counterpart (Cenko et al., GCN 45896, Neill et al., CGN 45897, Fu et al., GCN 45898, Izzo et al., GCN 45899, Wuet al., GCN 45900) is detected in our images. We measure the following magnitudes calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:
r = 19.87 +/- 0.02 AB (mid-time 0.96 hr after the trigger);
z = 17.52 +/- 0.02 AB (mid-time 0.34 hr after the trigger).
This project is funded by the SVOM collaboration.
GCN Circular 45900
Subject
GRB 261011A: SVOM/VT optical observation from VHF data
Date
2026-10-11T15:46:41Z (12 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
C. Wu (NAOC), L. P. Xin, Y. L. Qiu (NAOC), J. T. Palmerio (CEA/Irfu), Massimiliano Lincetto (CPPM), Damien Dornic (CPPM), on behalf of the SVOM/VT team.
After the trigger by SVOM/ECLAIRs (sb26101102) at 2026-10-11T13:49:04 UTC(T0), SVOM performed an automatic slew on the burst location ( Huang et al., GCN 45895).
From a preliminary analysis of the 1-bit subimage and source list downloaded via the VHF network, the candidate reported by Swift/UVOT (Cenko et al., GCN 45896), Neill et al. (GCN 45897), Fu et al. (GCN 45898), Izzo et al. (GCN 45899) was clearly detected in VT VHF data. Details are presented below.
This candidate was detected in both VT_R and VT_B. Its magnitudes are:
| date-obs (UTC) | mid-time (t-T0) | exposure | VT_B mag(AB) | VT_R mag(AB) |
|---|---|---|---|---|
| 2026-10-11T13:56:30 | 7.4 min | 6x50 s | 17.27 ± 0.10 | 16.260 ± 0.008 |
| 2026-10-11T14:02:20 | 13.3 min | 6x50 s | 18.13 ± 0.10 | 17.17 ± 0.10 |
Magnitudes were not corrected for Galactic reddening.
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 45899
Subject
GRB 261011A: LCO optical observations
Date
2026-10-11T15:39:30Z (12 days ago)
From
luca.izzo@inaf.it
Via
Web form
L. Izzo (INAF-OACn and DARK/NBI) reports on behalf of a larger collaboration:
We observed the field of GRB261011A (Cenko et al., GCN #45896) with the Sinistro instrument mounted on the 1-m telescope of the LCO network, located at Siding Spring, Australia. Observations started on 2026 October 11 at 14:25 UT (0.65 hr after the GRB trigger). We obtained a single image of 300s in the SDSS-g and SDSS-r filters.
We detect a bright source consistent with the position of the optical afterglow reported by Swift-UVOT (Cenko et al., GCN #45896) and by other ground-based facilities (O'Neill et al., GCN #45897; Fu et al., GCN #45898). We measure preliminary magnitudes of g = 20.01 +/- 0.02 mag and r = 19.47 +/- 0.02 (AB), calibrated against nearby stars from the Pan-STARRS catalog.
This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 101004719. We also acknowledge the use of the ECSnoopy package by E. Cappellaro.
GCN Circular 45898
Subject
GRB 261011A: TRT optical afterglow detection
Date
2026-10-11T14:55:20Z (12 days ago)
From
syfu@nao.cas.cn
Via
Web form
S.Y. Fu (HUST), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), J. An, L.B. He, S.Q. Jiang, X. Liu, D. Xu, Z.P. Zhu (NAOC), L. Lei (HUST) report:
We observed the field of GRB 261011A detected by SVOM (Huang et al., GCN 45895) and Swift (Cenko et al., GCN 45896) using the 0.7-m telescope of the Thai Robotic Telescope network (TRT). A series of Sloan r-band frames were obtained.
The optical afterglow is detected at the Swift/UVOT position (Cenko et al., GCN 45896) with r ~ 19.1 at a median time of 37.6 mins after the Swift trigger.
Observations are ongoing.
GCN Circular 45897
Subject
GRB 261011A: GOTO optical counterpart detection
Date
2026-10-11T14:26:21Z (12 days ago)
From
d.s.oneill@bham.ac.uk
Via
Web form
D. O'Neill,B. P. Gompertz, M. Shrestha, R. Starling, A. Kumar, G. Ramsay, 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 and M. Pursiainen report 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 Swift/BAT (Cenko et al. GCN 45896 and SVOM ECLIARS (Huang et al, GCN 45895) alerts for GRB 261011A.
Observations covering the localisation area began at 2026-10-11 13:53:16 UT, (+0.11h post trigger) with an average 5-sigma depth of 21.2 mag.
Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks (Lyman et al. 2026).
We detect the counterpart reported in Cenko et al. GCN 45896.The source was detected with L = 16.32 ± 0.01 AB mag (+0.11h). We find no evidence of the source prior to the GRB trigger time in previous GOTO observations taken at 2026-10-11 13:40:24 (-0.1h) down to a depth of >20.57 AB mag, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021).
GCN Circular 45896
Subject
GRB 261011A: Swift detection of an unusual burst
Date
2026-10-11T14:20:50Z (12 days ago)
Edited On
2026-10-11T15:17:11Z (12 days ago)
From
Jimmy DeLaunay at PSU <jjd330@psu.edu>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Amy Lien at GSFC <amy.y.lien@nasa.gov>
Via
email
S. B. Cenko (GSFC), S. D. Barthelmy (GSFC), S. Dichiara (PSU),
R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (U Tampa),
M. J. Moss (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL),
and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:
At 13:46:41 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 261011A (trigger=1516664). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 21.925, -21.529 which is
RA(J2000) = 01h 27m 42s
Dec(J2000) = -21d 31' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a broad soft pulse
from T0 to T0+130 sec and a shoulder extending out to T0+260 sec for a total
duration of about 260 sec. The peak count rate was ~8000 counts/sec
(15-350 keV), at ~95 sec after the trigger.
The XRT began observing the field at 13:51:57.1 UT, 315.8 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. We are waiting for the full dataset to detect and localise the
XRT counterpart.
UVOT took a finding chart exposure of nominal 150.000 seconds with the White
filter starting 321 seconds after the BAT trigger. There is a candidate
afterglow in the list of sources generated on-board at
RA(J2000) = 01:27:35.39 = 21.89746
DEC(J2000) = -21:33:37.2 = -21.56032
with a 90%-confidence error radius of about 1.10 arc sec. This position is
145.7 arc sec. from the center of the BAT error circle. The estimated magnitude
is 17.78. No correction has been made for extinction.
The BAT lightcurve is very unusual in that it combines a 260 second long
low energy emission mostly below 50 keV, with a ~5 second short peak
structure around T+95 seconds visible at all energies up the to 100-350 keV band.
Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)
GCN Circular 45895
Subject
GRB 261011A: SVOM detection of a long bright burst
Date
2026-10-11T14:20:12Z (12 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Yiming Huang (APC), Massimiliano Lincetto (CPPM), Damien Dornic (CPPM), Hui Yang (IRAP), Pierre Maggi (ObAS) report on behalf of the SVOM mission team:
At 2026-10-11T13:49:04 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 261011A (SVOM burst-id sb26101102).
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 22 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 90.76 in the [8-120] keV energy band over a time window of 40.96 seconds starting at 2026-10-11T13:49:04.
The localization of the best alert is R.A., Dec. 21.9017, -21.5422 degrees (J2000) with a 90% confidence level (C.L.) radius of 2.17 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 about 66.77 (-1.48 +1.35) s.
This burst also triggered SVOM/GRM at 2026-10-11T13:49:07 on a timescale of 1 seconds with an SNR of 7. The SVOM/GRM light curve showed a multiple peak structure with a T90 duration of about 63.09 (-0.74 +0.67) s.
SVOM slewed to the burst.
SVOM/MXT began observing the field at 2026-10-11T13:56:38 UTC, 454 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 21.9074, -21.5577 degrees:
R.A. (J2000) = 1h27m37.79s
Dec. (J2000) = -21d33m27.83s
with a 90% C.L. radius of 60.42 arcseconds.
This location is 0.99 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 Yiming HUANG: huangym@ihep.ac.cn.
Please contact the BA by email if you require additional information.