GRB 261017A
GCN Circular 45990
Subject
Konus-Wind detection of GRB 261017A
Date
2026-10-19T14:57:57Z (6 hours 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 261017A
(Fermi-GBM detection: Fermi GBM team, GCN 45973;
Mukherjee et al., GCN 45976
Swift-BAT detection: Siegel et al., GCN 45974;
Barthelmy et al., GCN 45985)
triggered Konus-Wind at T0=57906.065 s UT (16:05:06.065).
The burst light curve shows a multipeaked structure
which starts at ~T0-2.4 s and has a total duration of ~10.8 s.
The emission is seen up to ~3 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB261017_T57906/
As observed by Konus-Wind, the burst
had the total fluence of 1.96(-0.14,+0.15)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0-0.064 s,
of 6.37(-1.28,+1.30)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+8.448 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 = -0.06(-0.16,+0.18)
and Ep = 899(-80,+93) keV (chi2 = 90/89 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.0
(chi2 = 90/88 dof).
All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.
GCN Circular 45988
Subject
GRB 261017A: Possible Swift/UVOT Detection
Date
2026-10-19T13:42:13Z (7 hours ago)
From
Paul Kuin at MSSL <npkuin@gmail.com>
Via
email
N.P.M. Kuin, A.A. Breeveld, M.J. Page (MSSL/UCL) and M. H. Siegel (PSU)
report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 261017A
90 s after the BAT trigger (Siegel et al., GCN Circ. 45974).
A source consistent with the XRT position (Beardmore et al.,GCN Circ. 45977)
is detected in the initial UVOT exposures.
Preliminary detections and 3-sigma upper limits using the UVOT photometric
system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures
are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white 90 1377 373 20.41 +/- 0.24
v 631 1427 97 >18.6
b 557 5666 198 >19.2
u 302 5538 501 >19.2
w1 680 1304 78 >18.5
The source varies between exposures, is sometimes absent, and
appears double in two images. It might not be a point source.
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.023 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 45987
Subject
GRB 261017A: Luminance-band upper limit from a 28cm telescope in Chengde
Date
2026-10-19T11:45:33Z (9 hours ago)
From
Chenxi Bao at Peking University <2300011608@stu.pku.edu.cn>
Via
Web form
Chenxi Bao (PKU), Yacheng Kang (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 261017A, detected by Fermi/GBM (GCN 45973; T0 = 2026-10-17T16:05:05) and Swift (Siegel et al., GCN 45974), using a 28cm optical telescope located in Chengde, Hebei, China. Observations began on 2026-10-18 at 19:18:43 UT, approximately 1.1 days after the trigger.
We obtained the Luminance-band image with a total exposure time of 5400 s. No significant source was detected at the position of the reported X-ray counterpart (Siegel et al., GCN 45974; Beardmore et al., GCN 45977; Sbarrato et al., GCN 45979; Barthelmy et al., GCN 45985). We derive a 5-sigma upper limit of L > 21.0 mag at a median time of approximately 1.17 days after the trigger.
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 existing optical/infrared photometric constraints (Fu et al., GCN 45978; Hagio et al., GCN 45980; Tan et al., GCN 45982; Pawar et al., GCN 45986). Given the proposed possibility that GRB 261017A may be a merger-driven long-duration GRB (Maccary et al., GCN 45984), 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 45986
Subject
GRB 261017A: R-band upper limit with the 1.3-m DFOT
Date
2026-10-19T07:20:14Z (13 hours ago)
From
pawarpankaj1492@gmail.com
Via
Web form
Pankaj Pawar, Sarvesh Yadav, Dhruv Jain, Anshika Gupta, Debalina Kar and
Kuntal Misra (ARIES, Nainital, India) report:
We observed the field of GRB 261017A (Siegel et al., GCN 45974) with the
1.3-m Devasthal Fast Optical Telescope (DFOT), India, on 2026 October 17.
Observations started at 22:57:29 UT, approximately 6.87 hours after the
Swift trigger. We obtained six 300-s exposures in the R filter.
We do not detect an optical source at the enhanced Swift-XRT position
(Beardmore et al., GCN 45977) in the stacked image. Our preliminary forced
PSF photometry yields the following nominal 5-sigma upper limit:
Mid-time (UT) Tmid-T0 (d) Filter Exposure (s) Mag (Vega)
2026-10-17 23:12:41 0.29695 R 6 x 300 >22.5
T0 is 2026-10-17 16:05:05 UT (Siegel et al., GCN 45974). The mid-time is
the exposure-weighted mean of the individual exposure mid-times.
Photometry was calibrated against 45 nearby Pan-STARRS stars transformed
to approximate Cousins R magnitudes on the Vega system. The limit is based
on the local background noise, including uncertainty in the local sky
estimate, and is not corrected for Galactic extinction.
GCN Circular 45985
Subject
GRB 261017A: Swift-BAT refined analysis
Date
2026-10-18T23:24:53Z (21 hours 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), M. H. Siegel (PSU) (i.e. the Swift-BAT team):
Using the data set from T-179 to T+717 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 261017A (trigger #1519536)
(Siegel et al., GCN Circ. 45974). The BAT ground-calculated position is
RA, Dec = 139.901, 12.644 deg which is
RA(J2000) = 09h 19m 36.2s
Dec(J2000) = +12d 38' 40.0"
with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The
partial coding was 92%.
The mask-weighted light curve shows a main structure with multiple
overlapping pulses that starts and peaks at ~T0, and ends at ~T+10 s. In
addition, there are some weaker pulses that starts from ~T+50 s and ends at
~T+100 s. T90 (15-350 keV) is 89.36 +- 4.78 sec (estimated error including
systematics).
The time-averaged spectrum from T-0.70 to T+93.11 sec is best fit by a
simple power-law model. The power law index of the time-averaged spectrum
is 1.09 +- 0.16. The fluence in the 15-150 keV band is 1.4 +- 0.1 x 10^-06
erg/cm2. The 1-sec peak photon flux measured from T-0.22 sec in the 15-150
keV band is 1.3 +- 0.2 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/1519536
GCN Circular 45984
Subject
GRB 261017A: a short minimum variability timescale compatible with a compact object merger origin
Date
2026-10-18T15:46:59Z (a day ago)
From
ROMAIN LOIC MACCARY at University of Ferrara <romain.maccary@edu.unife.it>
Via
Web form
R. Maccary (University of Ferrara) on behalf of a larger collaboration report:
We determined the minimum variability timescale (MVT) of GRB 261017A (Fermi GBM team, GCN 45973) from the Fermi/GBM light curve (NaI detectors 3, 4, 6, and 7) following the prescriptions of Camisasca et al. (2023), i.e. as the minimum full width half maximum (FWHM_min) of all statistically significant pulses, and found FWHM_min = 31 (-8, +11) ms. Combined with the T90~9.2 s (Fermi GBM team, GCN 45976), in the T90-FWHM_min plot GRB 261017A (green square in the Figure at this link https://drive.google.com/file/d/1KuegJmDUVT0LSCSNSdl47arSIPbaKbdB/view?usp=drive_link) lies in the region populated by other long-lasting merger candidates, such as also GRB 230307A. The MVT corresponds to a feature found at ~2.9 s (see https://drive.google.com/file/d/16Sh5ua-Jc3eqS4GeIlQv2bAZgMFcZ7Cb/view?usp=drive_link).
In addition, with a fluence of 1.22e-5 erg cm-2 and a peak energy Ep~1024 keV (Fermi GBM team, GCN 45976), the positions of GRB 261017A in the Epi-Eiso plane at different redshifts seem to be more compatible with Type-I rather than with Type-II GRBs (see Figure at this link https://drive.google.com/file/d/1txeRPoWaA0bIfNknlq8OOEXjrAcC3Uj1/view?usp=drive_link).
References:
GCN Circular 45982
Subject
GRB 261017A: J band upper limit by the SYSU 80cm infrared telescope
Date
2026-10-18T11:14:16Z (a day ago)
From
Rui-Chen Gao <ayxzxn0100@gmail.com>
Via
Web form
Zhi-Xiang Tan, Zhong-Nan Dong, Rui-Chen Gao, Jin-Ji Li, Jia-Qi Lin, Pu Lin, Yun Shi, Jing-Kai Sun, Hao-Nan Yang, Hao-Ran Zhang, P. H. Thomas Tam, Rong-Feng Shen, and Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:
We observed the field of GRB 261017A, detected by Fermi (Fermi GBM team, GCN 45973) and Swift (Siegel et al., GCN 45974), with the Sun Yat-sen University (SYSU) 80cm infrared telescope in the J band. The observations were carried out on 2026-10-17, from 20:13:49 to 20:43:33 UT, beginning 4.15 hr after the BAT trigger. A total of 86 frames of 20 s each were stacked, giving 1720 s on source.
We do not detect any new source within the enhanced Swift-XRT error circle (Beardmore et al., GCN 45977) in our stacked image. The 5-sigma limiting magnitude of the stack is J = 18.6 mag (Vega)
The SYSU 80cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 45980
Subject
GRB 261017A : MITSuME Akeno optical upper limits
Date
2026-10-18T08:27:49Z (2 days ago)
From
hagio.h.ffca@m.isct.ac.jp
Via
Web form
H. Hagio, I. Takahashi, M. Sasada, A. Ochi, Y. Kubo, 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 261017A detected by Fermi (Fermi GBM team, GCN 45973) and Swift (M. H. Siegel et al., GCN 45974) with the optical three-color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50-cm telescope Akeno.
The observation started at 2026-10-17 17:05:50 UT (1.01hr after the trigger). We stacked the images taken under good conditions. We did not detect any obvious point sources within the Swift/XRT error region (Beardmore et al., GCN 45977). We obtained the 5-sigma limits of the stacked images as follows:
T0+[sec] | MID-UT | T-EXP[sec] | 5-sigma limits
------------------------------------------------------------------------------------------------
4434 | 2026-10-17 17:18:59 | 720 (g'), 780 (Rc, Ic) | g'>18.2, Rc>18.6, Ic>18.2
9242 | 2026-10-17 18:39:07 | 6840 (g'), 6900 (Rc, Ic) | g'>20.1, Rc>20.3, Ic>19.7
------------------------------------------------------------------------------------------------
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 45979
Subject
GRB 261017A: Swift-XRT refined Analysis
Date
2026-10-18T07:57:11Z (2 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.J. DeLaunay (PSU), S.
Lanava (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page
(U. Leicester), M.G. Bernardini (INAF-OAB) and P.A. Evans report on
behalf of the Swift-XRT team:
We have analysed 6.3 ks of XRT data for GRB 261017A, from 70 s to 43.9
ks after the trigger. The data comprise 98 s in Windowed Timing (WT)
mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode.
The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.7 (+0.5, -0.4). At T+100 s the decay
steepens to an alpha of 5.0 (+0.7, -0.3) before breaking again at T+260
s to a final decay with index alpha=1.12 (+0.30, -0.26).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.16 (+0.08, -0.07). The
best-fitting absorption column is 4.7 (+3.5, -1.5) x 10^20 cm^-2,
consistent with the Galactic value of 3.2 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 5.4 x 10^-11 (5.5 x
10^-11) erg cm^-2 count^-1.
A summary of the WT-mode spectrum is thus:
Total column: 4.7 (+3.5, -1.5) x 10^20 cm^-2
Galactic foreground: 3.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.16 (+0.08, -0.07)
If the light curve continues to decay with a power-law decay index of
1.12, the count rate at T+24 hours will be 8.1 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.4 x
10^-14 (4.5 x 10^-14) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01519536.
This circular is an official product of the Swift-XRT team.
GCN Circular 45978
Subject
GRB 261017A: Nanshan/NEXT optical upper limit
Date
2026-10-18T03:53:39Z (2 days ago)
From
syfu@nao.cas.cn
Via
Web form
S.Y. Fu, L. Lei (HUST), Z.P. Zhu, J. An, X. Liu, S.Q. Jiang, L.B. He, D. Xu (NAOC), X. Gao (Urumqi No.1 Senior High School), J.Z. Liu (XAO) report:
We observed the field of GRB 261017A detected by Fermi (Fermi GBM team, GCN 45973) and Swift (Siegel et al., GCN 45974), using the NEXT telescope located at Nanshan, Xinjiang, China. Observation started at 2026-10-17T21:45:23, i.e., 5.67 hours post-burst, and a series of r-band frames were obtained.
No uncatalogued optical transient is detected in the stacked image within the enhanced Swift-XRT error circle (Beardmore et al., GCN 45977), down to a 5-sigma limiting magnitude of r ~ 21.0, calibrated with Pan-STARRS DR2 stars in the field.
We acknowledge the excellent support from X. Gao for enabling these observations.
GCN Circular 45977
Subject
GRB 261017A: Enhanced Swift-XRT position
Date
2026-10-18T01:19:55Z (2 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester)
report on behalf of the Swift-XRT team.
Using 967 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 261017A, 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 = 139.90347, +12.63779 which is equivalent
to:
RA (J2000): 09h 19m 36.83s
Dec (J2000): +12d 38' 16.0"
with an uncertainty of 2.5 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 was automatically generated, and is an official product of the
Swift-XRT team.
GCN Circular 45976
Subject
GRB 261017A: Fermi GBM Observation
Date
2026-10-18T00:51:46Z (2 days ago)
From
oindabimukherjee@gmail.com
Via
Web form
O. Mukherjee (USRA), L. Scotton (UAH) and P. Veres (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 16:05:05.10 UT on 17 October 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 261017A (trigger 813945910/261017670),
which was also detected by Swift BAT (M. H. Siegel et al. 2026, GCN 45974).
The Fermi GBM on-ground location is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 67 degrees.
The GBM light curve consists of a single emission episode with a duration (T90)
of about 9.2 s (50-300 keV). The time-averaged spectrum
from T0-0.6 to T0+10 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.44 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 1024 +/- 22 keV.
The event fluence (10-1000 keV) in this time interval is
(1.22 +/- 0.03)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.26 s in the 10-1000 keV band is 4.3 +/- 0.2 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html
For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"
GCN Circular 45974
Subject
GRB 261017A: Swift detection of a burst
Date
2026-10-17T16:18:01Z (2 days ago)
From
Jimmy DeLaunay at PSU <jjd330@psu.edu>
Via
email
M. H. Siegel (PSU), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), 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) and D. M. Palmer (LANL)
report on behalf of the Neil Gehrels Swift Observatory Team:
At 16:05:05 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 261017A (trigger=1519536). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 139.900, +12.644 which is
RA(J2000) = 09h 19m 36s
Dec(J2000) = +12d 38' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a complex
structure with a duration of about 11 sec. The peak count rate
was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 16:06:26.4 UT, 81.0 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 139.9021, 12.6394 which is equivalent to:
RA(J2000) = 09h 19m 36.50s
Dec(J2000) = +12d 38' 21.8"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 18 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy.
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 89 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag.
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for extinction.
Burst Advocate for this burst is M. H. Siegel (siegel AT swift.psu.edu).
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 45973
Subject
GRB 261017A: Fermi GBM Final Real-time Localization
Date
2026-10-17T16:15:32Z (2 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 16:05:05 UT on 17 Oct 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 261017A (trigger 813945910.098688 / 261017670).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 141.9, Dec = 14.8 (J2000 degrees, equivalent to J2000 09h 27m, 14d 48'), with a statistical uncertainty of 2.4 degrees.
The angle from the Fermi LAT boresight is 64.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261017670/quicklook/glg_skymap_all_bn261017670.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/bn261017670/quicklook/glg_healpix_all_bn261017670.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261017670/quicklook/glg_lc_medres34_bn261017670.gif