GRB 261017A

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