GRB 261017A

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
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 ) and Swift (Siegel et al., GCN 45974
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 ), 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
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 ) 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.

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
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 ) and Swift (M. H. Siegel et al., GCN 45974
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 ) 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
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 ). 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).

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.

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
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 ) and Swift (Siegel et al., GCN 45974
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 ), 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
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 ), 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.

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.

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
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 ).
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/"

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/)

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