GRB 261010A
GCN Circular 45928
Subject
GRB 261010A: 10.4m GTC spectroscopy of the galaxy 2" from the afterglow
Date
2026-10-12T21:17:07Z (7 hours ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
email
A. J. Castro-Tirado, I. Pérez-García (IAA-CSIC), Y.-D. Hu (GXU), F. J. Aceituno, M.D. Caballero-Garcia, E. Fernandez-Garcia, M. Gritsevich, S. Guziy (IAA-CSIC), on behalf of a larger collaboration, report:
We observed the field of GRB 261010A, detected by Fermi/GBM (the Fermi team, GCN 45888), Swift (Lien et al. GCN 45889), and Konus-Wind (Frederiks et al. GCN 45925), with the OSIRIS+ spectrograph at the 10.4m Gran Telescopio Canarias in La Palma (Spain). Long-slit spectra with the R1000R and R1000B grisms (600 s each) were taken on Oct 11, 2026, starting at 20:28 UT, i.e. 25.2 h post burst. The slit covered both the position of the optical afterglow (O’Neill et al., GCN 45890; Aceituno et al., GCN 45891) and the red object located 2.2 arcsec NE of it (Corcoran et al., GCN 45912).
The spectrum of the redder object confirms the source to be a galaxy, displaying the [O II] 3727 A emission line, the Ca II H&K absorption doublet and the 4000 A break, all consistent at a redshift z = 0.785 +/- 0.001.
At this redshift the projected offset of the afterglow from the galaxy centre is 16 kpc.
If this galaxy at 2” is the host, GRB 261010A would have an isotropic-equivalent energy of E_iso ~ 3-4 x 10^52 erg and a rest-frame peak energy of ~0.8-1.5 MeV (using the Fermi/GBM and Konus-Wind spectral parameters, GCN 45911 and 45925), well above the Amati relation of long GRBs, thus placing this event in the same parameter space as GRB 211211A and GRB 230307A (both long GRBs which originated from a compact binary merger rather than a collapsar).
Further observations are planned.
We thank the GTC staff for the rapid execution of these observations.
GCN Circular 45925
Subject
Konus-Wind detection of GRB 261010A
Date
2026-10-12T16:48:04Z (12 hours ago)
From
Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>
Via
email
D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
The long-duration GRB 261010A (Swift detection:
Lien, et al., GCN 45889; Barthelmy et al., GCN 45918;
Fermi-GBM detection: The Fermi GBM team, GCN 45888;
Palafox & Jameson, GCN 45911)
triggered Konus-Wind (KW) at T0=69424.522 s UT (19:17:04.522).
The KW triggered on a bright, hard-spectrum, multipeaked pulse,
with a duration of ~3 s, which is superimposed over a broad,
less intense emission pulse that started ~10 s before the trigger,
peaked at ~T0+16 s, and lasted for ~70 s.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB261010_T69424/
As observed by Konus-Wind, the burst had
a fluence of (1.72 ± 0.47)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 0.128 s,
of (6.02 ± 1.08)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).
The spectrum of the bright pulse (measured from T0 to T0+8.448 s)
is best fitted in the 20 keV - 15 MeV range by a power law with exponential
cutoff (CPL) model dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.98 (-0.12, + 0.15) and Ep = 837(-194,+299) keV, PGstat = 81/98 dof.
Fitting this spectrum by a Band GRB function yields the same values
of alpha and Ep, and only an upper limit on the high energy photon
index beta of -2.0
The spectrum of the broad pulse (measured from T0+8.448 s to T0+49.408 s)
is best fitted in the 20 keV - 15 MeV range by a simple PL function
with the PL index of 1.92 (-0.07,+0.08),
PGstat = 103/99 dof.
All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.
GCN Circular 45918
Subject
GRB 261010A: Swift-BAT refined analysis
Date
2026-10-12T04:10:48Z (a day 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-120 to T+662 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 261010A (trigger #1516528)
(Lien et al., GCN Circ. 45889). The BAT ground-calculated position is
RA, Dec = 248.767, 26.600 deg which is
RA(J2000) = 16h 35m 04.2s
Dec(J2000) = +26d 36' 01.3"
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The
partial coding was 83%.
The mask-weighted light curve shows a main pulse that starts and peaks at
~T0 and ends at ~T+3 s. In addition, there are some weak emission that
starts prior to the main pulse at ~T-10 s and lasts till ~T+70 s. T90
(15-350 keV) is 71.50 +- 14.07 sec (estimated error including systematics).
The time-averaged spectrum from T-9.28 to T+76.45 sec is best fit by a
simple power-law model. The power law index of the time-averaged spectrum
is 1.43 +- 0.17. The fluence in the 15-150 keV band is 3.1 +- 0.4 x 10^-6
erg/cm2. The 1-sec peak photon flux measured from T+0.26 sec in the 15-150
keV band is 3.6 +- 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/1516528
(In memory of Scott Barthelmy, the BAT team dedicates the rest of our
circulars to Scott.)
GCN Circular 45912
Subject
GRB 261010A: NOT confirmation of the optical afterglow
Date
2026-10-11T22:16:41Z (a day ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), D. O’Neill (Birmingham), L. Izzo (INAF/OAC and DARK/NBI), B. P. Gompertz (Birmingham), A. Martin-Carrillo (UCD), A. Schenone-Zanuzzi (NOT), E. Georgiou (NOT and Turku Univ.) report on behalf of a larger collaboration:
We observed again the position of the Swift and Fermi GRB 261010A (Lien et al., GCN 45889; Palafox & Jameson, GCN 45911) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager. A total of 6x300 s were secured in the SDSS r band, with a mean epoch 2026 Oct 11.85154 UT (25.152 hr after trigger).
The candidate optical counterpart (O’Neill et al., GCN 45890; Aceituno et al., GCN 45891) is no longer detected in our images. Performing image subtraction using the Legacy survey as template, we measure a limiting magnitude r > 23.9 (AB; calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction).
We also note that the refined Swift/XRT position (Osborne et al., GCN 45894) has slightly shifted compared to the promptly-released coordinates (Lien et al., GCN 45889), and is now fully consistent with the location of the optical counterpart candidate. We thus conclude that this object is indeed the afterglow of GRB 261010A.
The position of the afterglow is about 25" (34.5 kpc in projection) away from the nearby galaxy LEDA 1781464 at redshift z = 0.070. The chance association probability is of order 1% (comparable to that for the fainter, redder object 2" away from the afterglow; O’Neill et al., GCN 45890). We note that, if placed at z = 0.070, our limit implies a luminosity > 1.5 mag fainter than AT 2017gfo at a comparable phase (e.g. Villar et al. 2017, doi:10.3847/2041-8213/aa9c84).
GCN Circular 45911
Subject
GRB 261010A: Fermi GBM Observation
Date
2026-10-11T21:47:48Z (a day ago)
Edited On
2026-10-12T01:56:32Z (a day ago)
From
Eva MP at INAOE <eva.palafox@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Eva MP at INAOE <eva.palafox@gmail.com>
Via
Web form
E. Palafox (INAOE) and A. Jameson (USRA) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 19:17:03.16 UT on 10 October 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 261010A (trigger 813352628/261010804).
which was also detected by Swift BAT (A. Y. Lien et al. 2026, GCN 45889).
The Fermi GBM on-ground location is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 22 degrees.
The GBM light curve consists of a bright emission episode follwed by several lower peaks with a duration (T90)
of about 54 s (50-300 keV). The time-averaged spectrum
from T0-7.4 to T0+54.0 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.30 +/- 0.05 and the cutoff energy,
parameterized as Epeak, is 450 +/- 90 keV.
The event fluence (10-1000 keV) in this time interval is
(1.70 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.2 s in the 10-1000 keV band is 8.2 +/- 0.3 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 45894
Subject
GRB 261010A: Enhanced Swift-XRT position
Date
2026-10-11T10:24:10Z (2 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester)
report on behalf of the Swift-XRT team.
Using 821 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 261010A, 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 = 248.74803, +26.58216 which is equivalent
to:
RA (J2000): 16h 34m 59.53s
Dec (J2000): +26d 34' 55.8"
with an uncertainty of 3.0 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 45891
Subject
GRB 261010A: possible optical afterglow detected with the 1.5m OSN
Date
2026-10-10T21:42:39Z (2 days ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
email
F.J. Aceituno, E. J. Fernandez-Garcia, A. J. Castro-Tirado, I. Perez-Garcia, M.D. Caballero-Garcia, M. Gritsevich, S. Guziy (IAA-CSIC) and Y.-D. Hu (GXI), on behalf of a larger collaboration, report:
Following the detection of GRB 261010A by both Fermi (Fermi team, GCN 45888) and Swift/BAT (Lien et al. GCN 45889), we triggered the 1.5m telescope at Sierra Nevada Observatory (OSN) in Granada (Spain), starting on Oct 10, 19:49 UT (~32 min post trigger) in the VRI filters. On the coadded image (7x300s, I-band), an uncatalogued object is detected slightly outside the Swift/XRT error circle, on the outskirts of a galaxy visible in the DSS2 archival plate. For the transient we measure a preliminary magnitude of I = 21.0 mag, at coordinates (J2000) = 16:34:59.45 +26:34:57.5, in agreement with the NOT findings (O’Neill et al. GCN 45890). Further observations are needed to confirm its relation to GRB 261010A.
We thank the staff at OSN for their excellent support.
GCN Circular 45890
Subject
GRB 261010A: NOT possible optical counterpart
Date
2026-10-10T21:39:17Z (2 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. O’Neill (Birmingham), B. Schneider (LAM), D. B. Malesani (DAWN/NBI and Radboud), B. Gompertz (Birmingham), L. Izzo (INAF/OACN and DARK/NBI), A. Martin-Carrillo (UCD), N. R. Tanvir (Leicester), A. Schenone-Zanuzzi (NOT), report on behalf of a larger collaboration:
We observed the position of the Swift and Fermi GRB 261010A (Lien et al., GCN 45889; Fermi GBM team, GCN 45888) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started as soon as weather conditions cleared, on 2026 Oct 10.874 UT (1.7 hr after the GRB trigger).
Both by visual inspection and by digital image subtraction (versus Pan-STARRS and Legacy), we note the presence of a source not seen in archival imaging, with the following coordinates (J2000):
RA = 16:34:59.46
Dec = +26:34:57.4
This position is slightly (~2") offset from a red source visible in the Legacy Survey images; the relation between these two objects is unclear. After template subtraction, we measure r ~ 21.36 +- 0.06 (AB) for the new source (not corrected for Galactic extinction).
The above coordinates are ~7" off the centre of the currently best available Swift/XRT position, which has a quoted uncertainty of 3.6". The association of this object with GRB 261010A is therefore tentative at the moment, and awaits confirmation. Further observations are planned and encouraged.
GCN Circular 45889
Subject
GRB 261010A: Swift detection of a burst
Date
2026-10-10T19:34:53Z (2 days ago)
From
Jimmy DeLaunay at PSU <jjd330@psu.edu>
Via
email
A. Y. Lien (U Tampa), R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL),
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 19:17:07 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 261010A (trigger=1516528). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec = 248.752, 26.609 deg, which is
RA(J2000) = 16h 35m 00s
Dec(J2000) = 26d 36' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a single-peaked
structure with a duration of at least 5 sec. The peak count rate
was ~7000 counts/sec (15-350 keV), at ~0 sec after the trigger.
There may be additional pulse structures that are caused either
by the burst or by entry into the SAA. Additional ground data are
required to distinguish the burst emission from background noise.
The XRT began observing the field at 19:18:41.4 UT, 94.2 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 248.7491, 26.5802 which is equivalent to:
RA(J2000) = 16h 34m 59.78s
Dec(J2000) = +26d 34' 48.7"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
location is 104 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 data are unavailable at this time
Burst Advocate for this burst is A. Y. Lien (yarleen AT gmail.com).
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 45888
Subject
GRB 261010A: Fermi GBM Final Real-time Localization
Date
2026-10-10T19:27:41Z (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 19:17:03 UT on 10 Oct 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 261010A (trigger 813352628.162212 / 261010804).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 244.7, Dec = 27.6 (J2000 degrees, equivalent to J2000 16h 18m, 27d 36'), with a statistical uncertainty of 3.3 degrees.
The angle from the Fermi LAT boresight is 25.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261010804/quicklook/glg_skymap_all_bn261010804.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/bn261010804/quicklook/glg_healpix_all_bn261010804.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261010804/quicklook/glg_lc_medres34_bn261010804.gif