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GRB 250924A

GCN Circular 42018

Subject
GRB 250924A: SVOM/GRM observation
Date
2025-09-28T10:19:57Z (a month ago)
From
tanwj@ihep.ac.cn
Via
Web form
SVOM/GRM team: Wen-Jun Tan, Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Jean-Luc Atteia (IRAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered by GRB 250924A during the routine ground search at 2025-09-24T08:18:46.500 (T0), which is also detected by Swift (Swift team, GCN #41959, GCN #41973，GCN #41974 and GCN #41987) and AstroSat CZTI (M. Tembhurnikar, et al. GCN #41979)

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of two main pulses with a T90 of 20.11 +/-7.67 in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250924A.png

In addition, the position of this burst, as determined by Swift/XRT (RA = 51.87237, Dec = 74.65846, GCN #41973), is located at about 96 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view.

With this localization given by Swift/XRT, the time-averaged spectrum from T0-20 to T0+2 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.52 +0.20/-0.46 and the cutoff energy, parameterized as Epeak, is 265 +165/-101 keV. The event fluence (10-1000 keV) in this time interval is (3.18 +0.47/-0.40)E-06 erg/cm^2. 

The localization of GRB 250922A in the 'Amati' relation diagram is shown at:
https://www.bursthub.cn//admin/static/svgrb250924A_amati.png


The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.

The SVOM point of contact for this burst is: Wen-Jun Tan (IHEP)(tanwj@ihep.ac.cn)

GCN Circular 41987

Subject
GRB 250924A: Swift-BAT refined analysis
Date
2025-09-25T19:39:17Z (a month ago)
From
Hans Krimm at NSF/NASA-GSFC <hkrimm@nsf.gov>
Via
email
D. Sadaula (GSFC/UMBC), 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), T. Sakamoto (AGU), M. H. Siegel (PSU),
(i.e. the Swift-BAT team):

Using the data set from T-240 to T+962 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250924A(trigger #1351790) (Siegel, et al., GCN Circ. 41959).  The BAT ground-calculated position is
RA, Dec = 51.908, 74.665 deg, which is
   RA(J2000)  =  03h 27m 37.8s
   Dec(J2000) = +74d 39' 55.1"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 12%.

The mask-weighted light curve shows a single broad pulse, extending from approximately T-10 to T+20 seconds.  T90 (15-350 keV) is 26.53 +- 3.72 sec (estimated error including systematics).

The time-averaged spectrum from T-9.92 to T+20.00 sec is best fit by a simple power-law model.  The power law index of the time-averaged spectrum is 1.47 +- 0.17.  The fluence in the 15-150 keV band is 2.5 +- 0.3 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T+15.78 sec in the 15-150 keV band is 2.1 +- 0.7 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/1351790

GCN Circular 41982

Subject
GRB 250924A: 1.6m Mephisto optical upper limits
Date
2025-09-25T11:07:51Z (a month ago)
From
liuxk@ynu.edu.cn
Via
Web form
Helong Guo, Yichen Jin, Xingzhu Zou, Guowang Du, Xufeng Zhu, Brajesh Kumar, Xinlei Chen, Yuanpei Yang, Tao Wang, Xiangkun Liu and Xiaowei Liu (all SWIFAR, YNU) report on behalf of Mephisto Team:

Simultaneous multi-band photometric observations of the  GRB 250924A detected by Swift (Siegel et al., GCN 41959) was performed with 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. The observations were started from 16:00:12 2025-09-24 UT (~7.7 hr after the trigger) and six frames of 45 sec each were obtained in uvgr bands. The afterglow candidate (Becerra et. al., GCN 41960; Schneider et al., GCN 41963; Freeberg et. al., GCN 41968; Pankov et al., GCN 41969; Zhang & Filippenko, GCN 41970; Mo et al., GCN 41972, Siegel et al., GCN 41975, Ma et al., GCN 41976) is not detected in our uvgr stacked band images. The 3-sigma upper limit are listed below: 
----------------------------------------------------------------------------
Start_Time(UT)           Filter  Exp (sec)   LimMag(AB)
2025-09-24T16:00:15      u     45*6           >21.34
2025-09-24T16:06:46      v     45*6           >21.66
2025-09-24T16:00:12      g     45*6           >22.22
2025-09-24T16:06:43      r     45*6           >22.46
----------------------------------------------------------------------------
Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21.
----------------------------------------------------------------------------

GCN Circular 41981

Subject
GRB 250924A: SAO RAS optical observations
Date
2025-09-25T10:53:27Z (a month ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin (SAO), V. Goranskij (SAI MSU), N. Pankov (HSE), 
A. Pozanenko (IKI) report on behalf of GRB follow-up collaboration 
and IKI-GRB-FuN.

We observed the field of the GRB 250924A (Siegel et. al, GCN 41897;
Osborne et. al, GCN 41973; Campana et. al, GCN 41974) with two SAO RAS
telescopes, 1-m Zeiss-1000 equipped with the CCD-photometer on
September 24/25, 23:41:32--00:27:16, and 6-m BTA equipped 
with the focal reducer SCORPIO-2 (Afanasiev & Moiseev,  2011, BaltA, 
20, 363) on September 25, 01:42:55--01:58:24 UT.

The optical transient (Siegel et. al, GCN 41959; Becerra et. al, 
GCN 41960; Schneider et. al, GCN 41963; Freeberg et. al, GCN 41968;
Pankov et. al, GCN 41969; Zheng and Filippenko, GCN 41970; 
Siegel, GCN 41975; Ma et al., GCN 41976; Cotter et al., GCN 41980; 
and also observed in NIR, Mo et. al, GCN 41972) is clearly detected 
in our stacked frames. Preliminary results are as following.

Date       UTstart   Exptime   t-T0       Filter OT    Err.    UL  Tel.
                     s         (mid, days)                (3sigma)
2025.09.24 23:41:32  2520   0.65689       Rc  23.18 +/- 0.24  23.5 1-m
2025.09.25 01:42:55  600    0.73068       Rc  23.07 +/- 0.10  24.3 6-m

The photometry calibrated against nearby USNO-B1.0 stars (R2 magnitudes)
and not corrected for the Galactic extinction.

GCN Circular 41980

Subject
GRB 250924A: NOT optical observations
Date
2025-09-25T07:54:34Z (2 months ago)
From
Laura Cotter at University College Dublin <laura.cotter@ucdconnect.ie>
Via
Web form

L. Cotter (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), A. de Ugarte Postigo (LAM), B. Schneider (LAM), G. Corcoran (UCD), K. Valeckas (NOT and NBI), report on behalf of a larger collaboration:

We observed the optical afterglow of GRB 250924A (Siegel et al., GCN 41959

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) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager. A total of 3x300 s of imaging in the r band was acquired.

At the mid epoch of 2025 Sep 25.117 UT (18.49 hr after the Swift/BAT trigger), we measure a magnitude of r = 23.55 +/- 0.15 (AB), calibrated against nearby objects from the Pan-STARRS catalog, and not corrected for Galactic extinction.

Comparing our result to previous measurements (e.g. Pankov et al., GCN 41969

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), the afterglow is significantly fainter than the extrapolation using the decay rate observed at early times (Schneider et al., GCN 41963; Zhang & Filippenko, GCN 41970), indicating a steepening of the light curve.

GCN Circular 41979

Subject
GRB 250924A: AstroSat CZTI detection
Date
2025-09-25T06:29:43Z (2 months ago)
From
Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>
Via
Web form
M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), S. Salunke (IUCAA), A. Arya (IITB), A. Goyal (IITB), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration GRB 250924A which was also detected by Swift/BAT (Siegel et al., GCN Circ. 41959).

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-09-24 08:18:32.00 UTC. The measured peak count rate associated with the burst is 48 (+17, -5) counts/s above the background in the combined data of three quadrants (out of four), with a total of 967 (+370, -133) counts. The local mean background count rate was 197 (+1, -3) counts/s. Using cumulative rates, we measure a T90 of 27 (+9, -5) s. 

The source was also faintly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range.

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb

GCN Circular 41976

Subject
GRB 250924A: SVOM/VT optical observation
Date
2025-09-25T03:51:53Z (2 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y. N. Ma, L. P. Xin, Z. H. Yao, Y. L. Qiu, C. Wu, H. L. Li, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), R. Z. Li (YNAO), D. F. Kong (GXU), report on behalf of the SVOM/VT team:

SVOM/VT performed a Target of Opportunity observation of GRB 250924A detected by Swift (Siegel et al., GCN 41959; Osborne et al., GCN 41973; Campana et al., GCN 41974; Siegel et al., GCN 41975). SVOM/VT began observing the field at 2025-09-24T11:07:35 UTC, 2.818 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 

The optical counterpart is detected in both VT_B and VT_R, the magnitudes are as follows:

mid time (h) | exposure time (s) | band | mag (AB) | mag err
-------------|-------------------|------|----------|--------
    3.103    |      38*50        | VT_B |  21.15   |  0.06 
    3.089    |      40*50        | VT_R |  20.25   |  0.05 
Our photometry was not corrected for Galactic extinction.

The position and the brightness are consistent with the previous reports (Siegel et al., GCN 41959; Becerra et al., GCN 41960; Lipunov et al., GCN 41962; Schneider et al., GCN 41963; Freeberg et al., GCN 41968; Pankov et al., GCN 41969; Zheng et al., GCN 41970; Saikia et al., GCN 41971; Mo et al., GCN 41972; Siegel et al., GCN 41975).

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Centre for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.

GCN Circular 41975

Subject
GRB 250924A: Swift/UVOT Detection
Date
2025-09-24T23:25:48Z (2 months ago)
From
Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>
Via
Web form
M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250924A
193 s after the BAT trigger (Siegel et al., GCN Circ. 41959).
A source consistent with the XRT position is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  03:27:29.66 =  51.87358 (deg.)
    Dec (J2000) = +74:39:34.0  =  74.65945 (deg.)
with an estimated uncertainty of 0.43 arc sec. (radius, 90% confidence).

Preliminary detections and 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: 

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

white              193          342          147         17.80 +/- 0.04
v                  681         1234           58        >18.4
b                  606          626           19         18.75 +/- 0.30
u                  351          601          246         18.03 +/- 0.08
w1                 730         1110           39        >19.0
m2                1065         1085           19        >17.6
w2                 656         1210           78        >19.6

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.315 in the direction of the burst
(Schlegel et al. 1998).

GCN Circular 41974

Subject
GRB 250924A: Swift-XRT refined Analysis
Date
2025-09-24T21:44:35Z (2 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
S. Campana (INAF-OAB), A. Melandri (INAF-OAR), D.N. Burrows (PSU), S.
Lanava (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page
(U. Leicester), T. Sbarrato (INAF-OAB) and P.A. Evans report on behalf
of the Swift-XRT team:

We have analysed 8.3 ks of XRT data for GRB 250924A, from 181 s to 34.9
ks after the   trigger. The data comprise 80 s in Windowed Timing (WT)
mode (the first 1 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=5.2 (+/-0.5), followed by a break at T+301 s to an alpha
of 1.05 (+/-0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 4.0 (+0.4, -0.3). The
best-fitting absorption column is  5.1 (+/-0.8) x 10^21 cm^-2, in
excess of the Galactic value of 2.8 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.06 (+0.20, -0.13)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.6 x 10^-11 (5.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.8 (+0.7, -0.0) x 10^21 cm^-2
Galactic foreground: 2.8 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.06 (+0.20, -0.13)

If the light curve continues to decay with a power-law decay index of
1.05, the count rate at T+24 hours will be 8.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.9 x
10^-13 (4.4 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/01351790.

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

GCN Circular 41973

Subject
GRB 250924A: Enhanced Swift-XRT position
Date
2025-09-24T21:34:07Z (2 months 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 985 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 250924A, 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 = 51.87237, +74.65846 which is equivalent
to:

RA (J2000): 03h 27m 29.37s
Dec (J2000): +74d 39' 30.5"

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 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 41972

Subject
GRB 250924A: J-band detection with WINTER
Date
2025-09-24T19:38:12Z (2 months ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
Via
Web form
Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (Caltech), Viraj Karambelkar (Columbia/CCA), Benjamin Schneider (LAM), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:

We observed the field of GRB 250924A (Siegel et al., GCN 41959) 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 at 2025-09-24T08:26:28 UTC in the J band (~8 minutes after the GRB trigger), consisting of 15 x 120 s 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). 

We detect a clear source at the optical counterpart location (Siegel et al., GCN 41959; Becerra et al., GCN 41960; Lipunov et al., GCN 41962; Schneider et al., GCN 41963; Freeberg et al., GCN 41968; Pankov et al., GCN 41969; Zheng et al., GCN 41970; Saikia et al., GCN 41971), with magnitude J ~ 18.6 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.

GCN Circular 41971

Subject
GRB 250924A: GROWTH-India Telescope Upper Limits
Date
2025-09-24T18:08:50Z (2 months ago)
From
Varun Bhalerao at IIT Bombay <varunb@iitb.ac.in>
Via
Web form

A.P. Saikia, T. Mohan, V. Swain, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and R. Norbu (IAO) report on behalf of the GIT team:

We observed the field of Swift GRB 250924A (Siegel et al., GCN 41959

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), with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2025-09-24 14:01:24 UT, i.e., 5.7 hours after the GRB trigger. We obtained multiple exposures in r'. We did not detect the source in our stacked image within the Swift UVOT localization region (Siegel et al., GCN 41959). The photometric upper limits are as follows:

MJD (mid)Filtertmid-t0 (hr)Exposure Time (sec)Upper limit (AB)
60942.618958r'6.5511 x 36021.5

The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction. Our observations span a similar time frame as Pankov et al. (GCN 41969

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), and are broadly consistent with their measurements.

The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.

GCN Circular 41970

Subject
GRB 250924A: KAIT optical observations
Date
2025-09-24T17:33:43Z (2 months ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
WeiKang Zheng (UCB) and Alexei V. Filippenko (UCB) report on

behalf of the KAIT GRB team:


The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at

Lick Observatory, automatically responded to the Swift GRB

250924A (Siegel et al., GCN 41959) starting at 08:21:50 UT, 201s

after the burst. Observations lasted for about 1 hour with degraded

sky condition (cloudy), and a set of clear (roughly R) filter images

were obtained. We clearly detected the reported fading afterglow

(Siegel et. al, GCN 41959; Becerra et. al, GCN 41960; Schneider

et. al, GCN 41963; Freeberg et. al, GCN 41968; Pankov et al.,

GCN 41969). We measure its brightness decayed from 16.41 +/- 0.03

mag (Vega) at 3.52 minutes to 19.7 +/- 0.3 mag at 59.33 minutes after

the burst. The data after 300s can be fit with a single power law

with a decay index of -0.96 +/- 0.02.

GCN Circular 41969

Subject
GRB 250924A: Mondy optical observations, afterglow fading
Date
2025-09-24T16:19:46Z (2 months ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), E. Klunko (ISTP) report on behalf of GRB-IKI-FuN:

We observed the field of GRB 250924A detected by Swift (Siegel et. al, GCN 41897) with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy). We are obtaining the images of the GRB field in the R-filter. The optical afterglow initially detected by Swift/UVOT (Siegel et. al, GCN 41897) and further observed by (Becerra et. al, GCN 41960; Schneider et. al, GCN 41963; Freeberg et. al, GCN 41968) is clearly visible in the stacked image of first 10 obtained images. Preliminary photometry is as follows:

Date       UTstart  Exptime    t-T0       Filter OT    Err.    UL
                     (nxs)     (mid, days)                  (3sigma)
2025-09-24 14:37:48  10x120    0.27036      R    21.47 0.09   22.5

The photometry was calibrated against nearby USNO-B1.0 stars (R2-mags) and not corrected for the Galactic extinction. We note that the afterglow has faded at ~3.7 mag in ~6.5 hr since detection. Further observations are ongoing.

GCN Circular 41968

Subject
GRB 250924A: Kilonova-Catcher optical afterglow detection
Date
2025-09-24T14:39:52Z (2 months ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
M. Freeberg (KNC), C. Andrade(UMN), D. Turpin (CEA-Saclay/Irfu), S. Antier (OCA/IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 250924A detected by Swift/BAT (Siegel et al., GCN 41959) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the TEC160FL telescope operated by M. Freeberg. Our observations started at TGRB+1.0hr. 

In our stacked frames, subtracted from the PanSTARRS DR2 template image, we detect the optical counterpart reported by Swift/UVOT (Siegel et al., GCN 41959), DDOTI (Becerra et al., GCN 41960) and COLIBRI (Schneider et al., GCN 41963).

We report our follow-up results in the table below 

+---------------+-----------+--------+----------------+-------------+
| Tmid-TGRB (hr)| Exp (s)   | Filter | Magnitude      | Instrument  |
+===============+===========+========+================+=============+
| 1.63          | 8 x 300s  | r (AB) | 20.03 +/- 0.10 | TEC160FL    | 
| 2.64          | 18 x 180s | i (AB) | 19.76 +/- 0.19 | TEC160FL    | 
+---------------+-----------+--------+----------------+-------------+

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 catalog. 

We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). 

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).

GCN Circular 41963

Subject
GRB 250924A: COLIBRÍ optical observations
Date
2025-09-24T12:35:04Z (2 months ago)
From
Benjamin Schneider at MIT <bschn@mit.edu>
Via
Web form
Benjamin Schneider (LAM), Camila Angulo (UNAM), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Rosa L. Becerra (U Roma), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU), 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) and Ny Avo Rakotondrainibe (LAM):

We imaged the field of the Swift GRB 250924A (Siegel et al., GCN Circ. 41959) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-09-24 08:33:51 to 09:46:24 UTC (from 15 to 88 minutes after the trigger) and obtained images in the g, r, i, z and y filters.

The data were reduced and coadded with the COLIBRÍ pipeline and 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 the optical counterpart reported by source Siegel et al., GCN Circ. 41959 and Becerra et al., GCN Circ. 41960 at: 

RA(J2000) = 3:27:29.71 = 51.8738 degrees
Dec(J2000) = +74:39:33.7 = +74.6594 degrees

with an uncertainty of 0.5 arcsec.

The preliminary magnitudes derived for that source are:

Start (UT)          | mid T0+T (s)  | Band | Exp  | Mag  | Error
--------------------|---------------|------|------|------|------
2025-09-24T09:01:29 | 2853  | g | 3x60  | 20.12 | 0.05
2025-09-24T08:33:51 | 1054  | r | 3x60  | 18.90 | 0.04
2025-09-24T09:03:50 | 2994  | i | 3x60  | 19.38 | 0.05
2025-09-24T08:33:51 | 1054  | z | 3x60  | 18.39 | 0.05
2025-09-24T09:02:40 | 3963  | y | 16x60 | 19.26 | 0.06 

From subsequent r-band images obtained between 0.26 and 2.21 hr after the burst, we measure a temporal decay slope of alpha = 0.6 +/- 0.1.

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 41962

Subject
Swift GRB 250924A: Global MASTER-Net observations report
Date
2025-09-24T12:04:41Z (2 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, 
G.Antipov,  A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-Tunka robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the Swift GRB 250924A ( M. H. Siegel et al., GCN 41959) errorbox  13196 sec after notice time and 13280 sec after trigger time at 2025-09-24 11:59:49 UT, with upper limit up to  16.2 mag. Observations started at twilight.  The observations began at zenith distance = 49 deg. The sun  altitude  is -10.0 deg. 

The galactic latitude b = 15 deg., longitude l = 133 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2998975

We obtain a following upper limits.  

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   13310 |        MASTER-Tunka |   C |    60 | 16.2 |        
   13310 |             MASTER- |   C |    60 | 16.2 |  Coadd 
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

GCN Circular 41960

Subject
GRB 250924A: DDOTI Detection of the Optical Counterpart
Date
2025-09-24T09:00:57Z (2 months ago)
Edited On
2025-09-24T13:58:04Z (2 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Roma), Sahil Atri (U Roma), Alan M. Watson (UNAM), Camila Angulo Valdez (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM), Margarita Pereyra (UNAM) and Eleonora Troja (U Roma) report:

We observed the field of GRB 250924A detected by Swift (Siegel et al., GCN Circ. 41959) with the DDOTI/OAN wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir (http://ddoti.astroscu.unam.mx) on the night of 2025-09-24 UTC.

DDOTI observed the Swift/UVOT position (Siegel et al., GCN Circ. 41959) from 08:35 UTC to 08:40 UTC (from T+17.3 to T+ 22.1 min after the trigger), obtaining a total exposure of 4 minutes.

By comparing our observations with the USNO-B1 and Pan-STARRS PS1 DR2 catalogs, we detect an uncatalogued source with a preliminary AB magnitude of:

w = 19.47 +/- 0.19.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra of San Pedro Mártir.

GCN Circular 41959

Subject
GRB 250924A: Swift detection of a burst with an optical counterpart
Date
2025-09-24T08:35:33Z (2 months ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
M. H. Siegel (PSU), J. J. DeLaunay (PSU), R. Gupta (NASA GSFC),
S. Lanava (PSU), K. L. Page (U Leicester) and D. M. Palmer (LANL)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 08:18:29 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250924A (trigger=1351790).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 51.865, +74.652 which is 
   RA(J2000) = 03h 27m 28s
   Dec(J2000) = +74d 39' 07"
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 30 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~17 sec after the trigger.

The XRT began observing the field at 08:21:32.8 UT, 183.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
51.87141, 74.65894 which is equivalent to:
   RA(J2000)  = 03h 27m 29.14s
   Dec(J2000) = +74d 39' 32.2"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 25 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 consistent with the Galactic value of 2.77
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.03e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 192 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	03:27:29.62 =  51.87340
  DEC(J2000) = +74:39:34.0  =  74.65945
with a 90%-confidence error radius of about 0.62 arc sec. This position is 1.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.74 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.316.

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/)
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