GRB 250911A
GCN Circular 42376
Subject
GRB 250911A: MAXI/GSC detection
Date
2025-10-22T05:22:50Z (3 hours ago)
From
Hitoshi Negoro at Nihon University/MAXI team <negoro.hitoshi@nihon-u.ac.jp>
Via
Web form
H. Negoro (Nihon U.), T. Mihara (RIKEN), M. Serino (AGU), M. Nakajima, K. Takagi,
H. Takahashi, H. Nishio (Nihon U.), T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN),
T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU),
Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, C. Kang,
T. Nakamoto, M. Uenishi, T. Usuki, S. Yatsuzuka (Ehime U.),
I. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa,
S. Ogawa, M. Kurihara (JAXA), Y. Ueda, K. Fujiwara (Kyoto U.), M. Yamauchi, M. Nishio,
C. Hiraizumi (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.),
W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), and S. Yamada (Tohoku U),
We report on the detection of GRB 250911A by MAXI
(Swift-BAT: GCN 41781, 41812, COLIBRI: GCN 41782, Swift-XRT: GCN 41783, 41784, 41804,
Swift-UVOT: 41800, MASTER: GCN 41806, GECAM-B: GCN 41813, Insight-HXMT: GCN 41827,
Konus-Wind: GCN 41939).
MAXI detected the burst for approximately 40 seconds, centered at around 01:29:46 on
September 11, 2025. The period coincides with the main peak of the burst detected by Swift,
HXMT, and Konus-Wind. The 4-10 keV X-ray flux averaged over the scan was 2041 ± 83 mCrab.
The MAXI light curve is shown at
https://maxi.riken.jp/grbs/250911a/
GRB 250911A was about 0.8 degrees away from the bright NS-LMXB GX 9+9.
Due to the X-ray photon confusion from GX 9+9, we were unable to obtain firm upper
limits or flux in the previous transit at 23:57 UT on September 10 and the next transit
at 03:02 UT.
Some software problem during the burst partly causes the late notice.
GCN Circular 41939
Subject
Konus-Wind detection of GRB 250911A
Date
2025-09-22T16:37:08Z (a month ago)
From
Valentina Panteleeva at Ioffe Institute <panteleevav228@gmail.com>
Via
Web form
V. Panteleeva, 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 250911A
(Swift-BAT detection: Klingler et al., GCN 41781;
Palmer et al., GCN 41812;
GECAM-B detection: Zhang et al., GCN 41813;
Insight-HXMT detection: Wang et al., GCN 41827)
triggered Konus-Wind at T0=05370.240 s UT (01:29:30.240).
The burst light curve shows a multipeaked structure
which starts at ~T0-2.3 s and has a total duration of ~57 s.
The weak precursor that triggered Swift-BAT is seen at ~T0-100 s.
The emission is seen up to ~10 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250911_T05370/
As observed by Konus-Wind, the burst
had the total fluence of 4.54(-0.29,+0.30)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+1.440 s,
of 5.15(-0.88,+0.90)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+41.216 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.85(-0.11,+0.12),
the high energy photon index beta = -2.58(-0.19,+0.14),
the peak energy Ep = 196(-13,+14) keV
(chi2 = 102/97 dof).
The spectrum near the maximum count rate
(measured from T0+0.256 to T0+8.448 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.92(-0.11,+0.12),
the high energy photon index beta = -2.44(-0.19,+0.14),
the peak energy Ep = 257(-25,+26) keV
(chi2 = 88/84 dof).
All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.
GCN Circular 41827
Subject
GRB 250911A: Insight-HXMT detection
Date
2025-09-13T18:17:46Z (a month ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
Chen-Wei Wang, Jin-Peng Zhang, Shao-Lin Xiong, and Chao Zheng report on behalf of the Insight-HXMT team:
At 2025-09-11T01:29:28.250 (T0), Insight-HXMT/HE detected the burst GRB 250911A, which is also detected by Swift/BAT (Palmer et al., GCN 41781) and GECAM-B (Zhang et al., GCN#41813.
The Insight-HXMT/HE light curve mainly consists of multi-pulses with a T90 of 24.5 +2.5/-1.5 s. The 1s peak rate, measured from T0+5.250 s, is 1857 cnts/sec. The total counts from this burst is 11720 counts.
The HXMT/HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb250911A.png
All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope.
Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org.
GCN Circular 41813
Subject
GRB 250911A: GECAM-B detection
Date
2025-09-12T13:00:39Z (a month ago)
From
zhangjinpeng@ihep.ac.cn
Via
Web form
Jin-Peng Zhang, Yang-Zhao Ren, Chen-Wei Wang, Shao-Lin Xiong and Yue Huang (IHEP) report on behalf of GECAM team:
GECAM-B was triggered on-ground by GRB 250911A at 2025-09-11T01:29:28.250 UTC (denoted as T0), which was also observed by Swift (Palmer et al., GCN 41781). According to the GECAM-B light curves in about 40-6000 keV, this burst mainly consists of a weak precursor and a bright pulse with multiple spikes, with a duration (T90) of 35.4 +1.1/-1.7 s.
The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecambgrb250911A.png
GECAM-B localized this burst to the following position (J2000):
Ra: 262.0 deg
Dec: -20.2 deg
Err: 4.1 deg (1-sigma, statistical only)
This localization is consistent with Swift observation (Palmer et al., GCN 41781; Beardmore et al., GCN 41784).
The time-averaged spectrum from T0 - 5 s to T0 + 40 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.52 +0.10/-0.12 and the cutoff energy, parameterized as Epeak, is 330 +180/-80 keV. The event fluence (10-1000 keV) in this time interval is (3.52 +0.25/-0.32)E-05 erg/cm^2.
The 'Amati' relation diagram of GRB 250911A is shown at:
https://www.bursthub.cn//admin/static/gecambgrb250911A_amati.png
Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).
GCN Circular 41812
Subject
GRB 250911A: Swift-BAT refined analysis
Date
2025-09-12T10:16:27Z (a month ago)
From
Rahul Gupta at NASA GSFC <rahul.gupta@nasa.gov>
Via
Web form
D. M. Palmer (LANL), S. D. Barthelmy (GSFC),
R. Gupta (GSFC), N. J. Klingler (GSFC/UMBC/CRESSTII),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),
M. J. Moss (GSFC), T. Parsotan (GSFC),
D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU)
(i.e. the Swift-BAT team):
Using the data set from T-239 to T+200 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250911 A (trigger #1348587)
(Klingler, et al., GCN Circ. 41781). The BAT ground-calculated position is
RA, Dec = 263.752, -16.841 deg which is
RA(J2000) = 17h 35m 00.6s
Dec(J2000) = -16d 50' 28.0"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 29%.
The mask-weighted BAT light curve shows two emission episodes
(a precursor followed by main emission) separated by quiescent
gap of ~ 80 sec. T90 (15-350 keV) is 128.42 +- 87.23 sec
(estimated error including systematics).
The time-averaged spectrum from T-1.35 to T+147.24 sec is best fit by a power law
with an exponential cutoff. This fit gives a photon index 1.00 +- 0.20,
and Epeak of 170.5 +- 84.5 keV (chi squared 56.83 for 56 d.o.f.). For this
model the total fluence in the 15-150 keV band is 1.5 +- 0.0 x 10^-05 erg/cm2
and the 1-sec peak flux measured from T+103.94 sec in the 15-150 keV band is
8.6 +- 0.6 ph/cm2/sec. A fit to a simple power law gives a photon index
of 1.35 +- 0.05 (chi squared 66.73 for 57 d.o.f.). 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/1348587
GCN Circular 41806
Subject
GRB 250911A: MASTER optical counterpart observations
Date
2025-09-12T00:53:31Z (a month ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.M.Lipunov (Lomonosov MSU, Moscow),
C.Francile, F. Podesta, R.Podesta, E. Gonzalez (OAFA),
A.Kuznetsov, A.Sankovich, G.Antipov, P.Balanutsa, E.Gorbovskoy, N.Tiurina, M.Shilova, A.Chasovnikov, V.Topolev, K.Zhirkov, Ya.Kechin, V.Senik, D.Vlasenko (Lomonosov MSU),
D.Buckley, (SAAO, South Africa)
O.Gress, N.Budnev (ISU, Irkutsk),
A.Sosnovskij (Crao RAS),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
V.M.Pillet, R.Rebolo Lopez (The Instituto de Astrofisica de Canarias, Spain),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez,J.Martinez,A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysic Observatory, Mexico)
MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru,Argentina started Swift GRB 250911A (Klingler et al. GCN 41781) observations automatically at 2025-09-11 02:25:44UT and in own planner mode befor it.
There is GRB optical counterpart MASTER OT J173500.02-165114.40 with mOT~18.81
at Swift position (XRT Perri et al. GCN 41783, Beardmore et al. GCN 41784, Dichiara et al. GCN 41804;
UVOT Siegel et al. GCN 41800), first reported by COLIBRI (Angula et al. GCN 41782).
All images reduction will be continued.
The message may be cited.
References
1) Lipunov et al. 2010, MASTER Robotic Net, Advances in Astronomy, vol. 2010, p.30L
2) Lipunov,V.G. Kornilov, E.S. Gorbovskoy, N.V. Tyurina, A.S. Kuznetsov,
"ASTRONOMICAL ROBOTIC NETWORKS and OPERATIVE MULTICHANNEL ASTROPHYSICS. Based on
the example of the global MASTER network.", The Moscow State University
Publishing House. In the series "Works of Outstanding Scientists of Moscow
State University", http://www.pereplet.ru/lipunov/625.html#625)
GCN Circular 41804
Subject
GRB 250911A: Swift-XRT refined Analysis
Date
2025-09-11T20:18:36Z (a month ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
S. Dichiara (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti
(INAF-OAB), C. Salvaggio (INAF-OAB), D.N. Burrows (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:
We have analysed 6.6 ks of XRT data for GRB 250911A, from 3.4 ks to
48.4 ks after the trigger. The data are entirely in Photon Counting
(PC) mode.
The light curve can be modelled with a power-law decay with a decay
index of alpha=1.52 (+/-0.08).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.80 (+0.16, -0.15). The
best-fitting absorption column is 3.9 (+1.0, -0.5) x 10^21 cm^-2,
consistent with the Galactic value of 3.4 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 4.5 x 10^-11 (6.3 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 3.9 (+1.0, -0.5) x 10^21 cm^-2
Galactic foreground: 3.4 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.80 (+0.16, -0.15)
If the light curve continues to decay with a power-law decay index of
1.52, the count rate at T+24 hours will be 0.015 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.8 x
10^-13 (9.5 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01348587.
This circular is an official product of the Swift-XRT team.
GCN Circular 41800
Subject
GRB 250911A: Swift/UVOT Detection
Date
2025-09-11T16:49:55Z (a month ago)
From
Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>
Via
Web form
M. H. Siegel (PSU) and N. J. Klingler (GSFC/UMBC/CRESSTII) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 250911A 3.5 ks after the BAT trigger (Klingler et al., GCN Circ. 41781). A fading afterglow consistent the XRT position (Beardmore et al., GCN Circ. 41784) and COLIBRI afterglow (Angulo et al., GCN Circ. 41782) is detected in the initial UVOT exposures.
The preliminary UVOT position is:
RA (J2000) = 17:35:00.03 = 263.75011 (deg.)
Dec (J2000) = -16:51:14.9 = -16.85413 (deg.)
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).
Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag/3sigUL
white 3483 4252 344 19.95+/-0.16
v 4462 4662 196 >19.20
v 26629 26655 25 >17.87
b 3847 4047 196 19.40+/-0.22
b 10440 10955 503 20.58+/-0.33
u 3642 5214 330 19.18+/-0.19
u 21026 21075 48 >18.94
uvw1 4872 5072 196 >19.53
uvw1 20120 21019 885 >20.01
uvm2 4667 4867 196 >19.38
uvm2 16021 16815 782 >20.49
uvw2 4258 4458 196 >19.50
uvw2 25723 26622 885 >20.09
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.4185 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 41784
Subject
GRB 250911A: Enhanced Swift-XRT position
Date
2025-09-11T06:48:39Z (a month 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 2239 s of XRT Photon Counting mode data and 6 UVOT
images for GRB 250911A, 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 = 263.75012, -16.85418 which is equivalent
to:
RA (J2000): 17h 35m 00.03s
Dec (J2000): -16d 51' 15.0"
with an uncertainty of 2.0 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed at http://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 41783
Subject
GRB 250911A: Swift-XRT observations
Date
2025-09-11T05:16:58Z (a month ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
M. Perri (SSDC & INAF-OAR), P.A. Evans (U. Leicester) and J.A. Kennea
(PSU) report on behalf of the Swift-XRT team:
The XRT began observing the field of GRB 250911A at 02:25:07.6 UT,
3441.5 seconds after the BAT trigger. Using promptly downlinked data we
find a fading, uncatalogued X-ray source located at RA, Dec 263.75102,
-16.85448 which is equivalent to:
RA(J2000) = 17h 35m 00.24s
Dec(J2000) = -16d 51' 16.1"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 53 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (3.44 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.3
(+3.44/-2.88) x 10^21 cm^-2 (90% confidence).
GCN Circular 41782
Subject
GRB 250911A: COLIBRÍ optical counterpart candidate
Date
2025-09-11T03:50:26Z (a month ago)
From
Camila Angulo Valdez at UNAM <camiangulo@astro.unam.mx>
Via
Web form
Camila Angulo (UNAM), Sarah Antier (OCA), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM):
We imaged the field of the Swift GRB 250911A (Palmer et al., GCN Circ. 41781) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed at 2025-09-11 02:41 UTC (1.2 hours after the trigger) and obtained 60 second exposures in the r and z filters.
The data were analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
We detect an uncatalogued source consistent with the BAT 3.0 arcmin error circle (Palmer et al., GCN Circ. 41781) at:
RA(J2000) = 17:35:00.05 = 263.75021 degrees
Dec(J2000) = -16:51:14.40 = -16.8540 degrees
with an uncertainty of 0.5 arcsec.
The preliminary magnitudes derived for that source are:
r = 19.19 +/- 0.08
z = 18.46 +/- 0.10
We suggest that this source is the optical counterpart of the GRB.
Further observations are ongoing.
We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular 41781
Subject
GRB 250911A: Swift detection of a burst
Date
2025-09-11T01:43:39Z (a month ago)
From
David Palmer at LANL <palmer@lanl.gov>
Via
email
N. J. Klingler (GSFC/UMBC/CRESSTII), S. Dichiara (PSU),
R. Gupta (NASA GSFC), A. Y. Lien (U Tampa) and D. M. Palmer (LANL)
report on behalf of the Neil Gehrels Swift Observatory Team:
At 01:27:46 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250911A (trigger=1348587). Due to an observing constraint
Swift did not immediately slew to the burst.
The BAT on-board calculated location is
RA, Dec 263.762, -16.844 which is
RA(J2000) = 17h 35m 03s
Dec(J2000) = -16d 50' 36"
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 20 sec. The peak count rate
was ~1200 counts/sec (15-350 keV), at ~3 sec after the trigger.
Due to an observing constraint, Swift will not slew until T0+50
minutes. There will be no XRT or UVOT data until this time.
Burst Advocate for this burst is N. J. Klingler (noelklin AT umbc.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/)