GRB 261005B

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A. D'Ai
(INAF-IASFPA), S. Campana (INAF-OAB), A. Melandri (INAF-OAR), S. Lanava
(PSU), S. Dichiara (PSU), D.N. Burrows (PSU), A.P. Beardmore (U.
Leicester) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 5.1 ks of XRT data for GRB 261005B, from 4.4 ks to
81.3 ks after the   trigger. The data are entirely in Photon Counting
(PC) mode. Using 3116 s of PC mode data and 6 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 = 255.33918, +1.58871
which is equivalent to:

RA (J2000): 17h 01m 21.40s
Dec(J2000): +01d 35' 19.4"

with an uncertainty of 2.1 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial rise, with a power-law
index of alpha=-1.5 (+0.0, -1.0), followed by a break at T+8550 s  to
an alpha of 2.7 (+0.6, -0.5).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.15 (+/-0.13). The
best-fitting absorption column is  1.5 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 9.0 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 6.1 x 10^-11 (6.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.5 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 9.0 x 10^20 cm^-2
Excess significance: 1.7 sigma
Photon index:	     1.15 (+/-0.13)

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01515414.

This circular is an official product of the Swift-XRT team.

A. A. Breeveld (UCL-MSSL), N. Klingler (NASA-GSFC/UMBC/CRESST II) and T.M. Parsotan (GSFC) report on behalf of the Swift/UVOT team:

The Swift/UVOT started observing the field of GRB 261005B 4424 s after the BAT trigger (Parsotan et al., GCN Circ. 45812
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 ).
No optical afterglow consistent with the enhanced XRT position (Page et al., GCN Circ. 45815
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 ) is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the data available are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white             5451         5651          197         >20.4
b                 5247         5446          197         >19.8
u                 5040         5239          197         >19.3
v                 4424         4624          197         >18.3
uvw1              4835         5035          197         >18.7
uvw2              5658         5836          175         >19.1
uvm2              4629         4829          197         >18.1


The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.12 in the direction of the burst (Schlegel et al. 1998).

Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:

We observed the field of GRB 261005B (Parsotan et al., GCN 45812
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 ; Godwin et al., GCN 45816
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 ) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).

Observations began on 2026-10-06 at 02:30:42.092 UTC in the J band (4.86 hr after the BAT trigger), consisting of 15*120 sec exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565
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 ).

We do not detect any new source at the enhanced Swift-XRT position (Page at al., GCN 45815
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 ).
We obtain the following 5-sigma upper limit: J = 19.17 mag (AB).

WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.

Matt Godwin (UAH) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 21:39:13.77 UT on 05 October 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 261005B (trigger 812929158/261005902),
which was also detected by Swift BAT (T.M. Parsotan et al. 2026, GCN 45812
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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 consist of multiple peaks with a duration (T90)
of about 22 s (50-300 keV). The time-averaged spectrum
from T0-6.4 to T0+20.6 s is best fit by a power law function with an exponential high-energy cutoff.
The power law index is -0.56 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 110 +/- 8 keV.

The event fluence (10-1000 keV) in this time interval is
(8 +/- 2)E-07 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+13 s in the 10-1000 keV band is 1.56 +/- 0.19 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/"

K.L. Page, P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the
Swift-XRT team:

Using 1707 s of XRT Photon Counting mode data and 3 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 =
255.33924, 1.58872 which is equivalent to:

RA (J2000):  17 01 21.42
Dec (J2000): +01 35 19.4

with an uncertainty of 2.2 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.

T. M. Parsotan (GSFC), R. Brivio (INAF-OAB), J. J. DeLaunay (PSU),
R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (U Tampa),
A. Mei (INAF-OAB), K. L. Page (U Leicester), D. M. Palmer (LANL) and
C. Salvaggio (INAF-OAB) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 21:39:06 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 261005B (trigger=1515414).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec = 255.327, +1.590 deg  which is 
   RA(J2000) = 17h 01m 19s
   Dec(J2000) = 01d 35' 25"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 40 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~4 sec after the trigger.

The XRT began observing the field at 21:41:10.7 UT, 124.1 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.

There are no UVOT data available at this time.

Burst Advocate for this burst is T. M. Parsotan (tyler.parsotan 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/)