GRB 250823A

C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC),
R. Gupta (GSFC), H. A. Krimm (NSF),
S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa),
M. J. Moss (GSFC), D. M. Palmer (LANL),
T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC),
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 250823A (trigger #1344586)
(Siegel, et al., GCN Circ. 41511
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 ).  The BAT ground-calculated position is
RA, Dec = 297.465, -24.986 deg which is
   RA(J2000)  =  19h 49m 51.7s
   Dec(J2000) = -24d 59' 09.0"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 75%.

The mask-weighted BAT light curve shows multi-peaked emission 
starting from ~T-25 to ~T+180 sec.
T90 (15-350 keV) is 193.60 +- 10.19 sec (estimated error including systematics).

The time-averaged spectrum from T-23.84 to T+180.82 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.76 +- 0.16.  The fluence in the 15-150 keV band is 1.8 +- 0.2 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+4.24 sec in the 15-150 keV band
is 1.1 +- 0.2 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/1344586
 

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

We have analysed 4.8 ks of XRT data for GRB 250823A, from 111 s to 44.7
ks after the   trigger. The data comprise 313 s in Windowed Timing (WT)
mode (the first 7 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a series of power-law decays. The
light curve initially rises, with an index alpha=-1.5 (+0.0, -0.6). At
T+120 s it breaks to an alpha of 3.33 (+/-0.09). The light curve breaks
again at T+349 s to a decay with alpha=2.31 (+0.23, -0.21),  before a
final break at T+1667 s s after which the decay index is 0.78 (+0.13,
-0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.91 (+/-0.04). The
best-fitting absorption column is  1.99 (+0.16, -0.15) x 10^21 cm^-2,
in excess of the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.92 (+0.28, -0.26)
and a best-fitting absorption column of 1.9 (+1.0, -0.8) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.8 x 10^-11 (5.0 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.9 (+1.0, -0.8) x 10^21 cm^-2
Galactic foreground: 1.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.92 (+0.28, -0.26)

If the light curve continues to decay with a power-law decay index of
0.78, the count rate at T+24 hours will be 9.6 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x
10^-13 (4.8 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/01344586.

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

Alan M. Watson (UNAM), Damien Dornic (CPPM), Rosa L. Becerra (U Roma), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Fredd Sánchez Álvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU), 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), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):

We imaged the field of GRB 250823A, detected by Swift/BAT (Siegel et al., GCN Circ. 41511
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 ) and Fermi/GBM (Fermi GBM team et al., GCN Circ. 41510
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 ) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-08-24 05:54 to 06:33 UTC (from 9.73 to 10.38 hours after the trigger) and obtained 32 minutes of exposure in the i filter.

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 detected the optical counterpart first reported by Gompertz et al. (GCN Circ. 41512
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 ) and subsequently by Fernandez-Garcia et al. (GCN Circ. 41513
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 ), Strausbaugh & Cucchiara (GCN Circ. 41514
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 ), Malesani et al. (GCN Circ. 41520
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 ), Kuin et al. (GCN Circ. 41521
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 ), and Mal et al. (GCN Circ. 41523
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 ), at a preliminary magnitude of:

i = 22.74 +/- 0.21

This measurement is contemporaneous and consistent with the value reported by SVOM/VT (Mal et al., GCN Circ. 41523
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 ).

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.

Y. N. Ma, Z. H. Yao, L. P. Xin, 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) report on behalf of the SVOM/VT team. 

SVOM performed a Target of Opportunity observation of GRB 250823A detected by Swift/BAT (Siegel et al., GCN 41511
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 ) and Fermi/GBM (Fermi GBM Team, GCN 41510
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 ). SVOM/VT began observing the field at 2025-08-23T20:38:57 UTC,  0.48 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 

With X-band data availible, the optical counterpart (Gompertz & Dimple, GCN 41512
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 ; Fernandez-Garcia et al., GCN 41513
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 ; Strausbaugh & Cucchiara, GCN 41514
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 ; Malesani et al., GCN 41520
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 ; Kuin & Siegel, GCN 41521
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 ) at the position consistent with the locations of Swift/XRT (Siegel et al., GCN 41511
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 ; Evans et al., GCN 41515
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 ), was clearly detected in both VT_B and VT_R. It decayed by about 2 mag in 8 hours. The magnitudes are:

mid time (h) | exposure time (s) | band | mag (AB) | mag err
-------------|-------------------|------|----------|--------
    1.934    |      37*70        | VT_B |  21.80   |  0.08 
    1.934    |      37*70        | VT_R |  20.92   |  0.06 
    9.913    |      28*70        | VT_R |  22.8    |  0.3    

Our photometry was not corrected for Galactic extinction.

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. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.

Paul Kuin (UCL/MSSL) and M. H. Siegel (PSU) report on behalf of the
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250823A
126 s after the BAT trigger (Siegel et al., GCN Circ. 41511
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 ). A source
consistent with the XRT position is detected in the initial UVOT exposures.

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              126         1020          334         20.42 +/- 0.17
v                  668         1244           78         18.43 +/- 0.28
b                  594         1169           58         19.25 +/- 0.28
u                  338         1317          304        >19.5
w1                 718         1293           58        >19.1
m2                 692         1268           58        >18.8
w2                 644         1391           97        >19.3

The magnitudes in the table are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 0.111 in the direction of the burst (Schlegel
et al. 1998).

Jacob Smith (UAH), B. Mailyan (Florida Tech) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

At 20:10:10.67 UT on 23 August 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250823A (trigger 777672615/250823840),
which was also detected by Swift BAT (M. H. Siegel et al. 2025, GCN 41511
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 ), 
and optical follow-up was first reported by Liverpool Telescope (B. P. Gompertz et al. 2025, GCN 41512
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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 72 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 13 s (50-300 keV). The time-averaged spectrum
from T0-6.1 to T0+10.2 s is best fit by
a simple power law function with index -1.67 +/- 0.05. 
A power law function with an exponential high-energy cutoff fits equally well with power law index = -1.44 +/- 0.17 and Epeak = 206.5 +/- 95.8.

The event fluence (10-1000 keV) in this time interval is
(1.9 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+1.1 s in the 10-1000 keV band is 1.8 +/- 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/

P.A. Evans, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 950 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 250823A, 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 = 297.47814, -24.99557 which is equivalent
to:

RA (J2000): 19h 49m 54.75s
Dec (J2000): -24d 59' 44.1"

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.

R. Strausbaugh (Eastern Illinois University), A. Cucchiara (NASA) report on behalf of a larger collaboration:

We observed the Swift GRB 250823A field (Siegel et al., GCN 41511
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 )  with the LCOGT 1-meter Sinistro instrument at the Cerro Tololo Interamerican Observatory, Chile site, on August 23, from 23:17 to 23:49 UT (corresponding to 3.12 to 3.65 hours after the GRB trigger time) with the SDSS r and i filters.

We performed a series of 3x300s exposures in i-band and r-band.  We detect an uncatalogued source coincident with the XRT error region, and other optical detections (Gompertz et al., GCN 41512
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 ; Fernandez-Garcia et al., GCN 41513
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 ) in both bands.

The following magnitudes are calculated using the PanSTARRS catalog as reference:

r = 21.2 +/- 0.15
i = 22.5 +/- 0.55

These magnitudes are not corrected for galactic extinction.

E. Fernandez-Garcia, I. Perez-Garcia, M.D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, S.-Y. Wu and A. J. Castro-Tirado (IAA-CSIC, Granada), P. J. Meintjes and H. J. van Heerden (UFS, Bloemfontein), A. Martin-Carrillo and L. Hanlon (UCD, Dublin), C. J. Perez del Pulgar (UMA, Malaga), Y.-D. Hu (GXI, Guangxi) and M. Gritsevich (Univ. of Helsinki), on behalf of a larger collaboration, report:

Following the detection of GRB250823A by Fermi (Fermi GBM Team, GCN Circ. 41510
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 ) and Swift/BAT (Siegel et al., GCN Circ. 41511
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 ), the BOOTES-6/DPRT 0.6m robotic telescope at Boyden Observatory in Maselspoort (South Africa) automatically observed the Swift/XRT X-ray afterglow location starting on Aug. 23, 20:12:44 UT (i.e. 157 s after trigger). A series of images in clear filter were gathered and the optical counterpart reported by the LT (Gompertz et al., GCNC 41512) was detected at early times in our dataset. We measure a preliminary magnitude of 19.30 +- 0.15 on the co-added 10 s x 45 image (at 20:28:26 UT mid time). Further data analysis is ongoing.

We thank the staff at Boyden Observatory for their excellent support.

B. P. Gompertz (U. Birmingham) and Dimple (U. Birmingham) report on behalf of a larger collaboration:

We conducted follow-up observations of GRB 250823A (Fermi GBM Team, GCN Circ. 41510
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 , Siegel et al., GCN Circ. 41511
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 ) with the IO:O camera on the 2m robotic Liverpool Telescope (LT). Observations began at 21:26:55 UT on 2025-08-23, 1.28 hr after the Swift trigger, and consisted of 5 x 120 s exposures in each of the SDSS g, r, i, and z filters.

We detect a new optical source with an AB magnitude of r = 21.25 +/- 0.09 (mid-time t0+1.45 hr) at a position of RA(J2000) 19:49:54.7, Dec -24:59:44.9 (0.2” uncertainty). The source is not present in archival Pan-STARRs imaging and is spatially consistent with the localisation by Swift/XRT (Siegel et al., GCN Circ. 41511
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 ). We therefore conclude it is the optical counterpart of GRB 250823A.

Magnitudes are calibrated against nearby Pan-STARRS stars and are not corrected for Galactic extinction.

M. H. Siegel (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC),
N. P. M. Kuin (UCL-MSSL), S. Lanava (PSU), K. L. Page (U Leicester),
D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 20:10:07 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250823A (trigger=1344586).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 297.521, -24.976 which is 
   RA(J2000) = 19h 50m 05s
   Dec(J2000) = -24d 58' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). The BAT light curve shows a complex structure 
with a duration of 200 sec.  The peak count rate
was ~1600 counts/sec (15-350 keV), at ~4 sec after the trigger.

The XRT began observing the field at 20:17:28.0 UT, 440.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 297.47788,
-24.99582 which is equivalent to:
   RA(J2000)  = 19h 49m 54.69s
   Dec(J2000) = -24d 59' 45.0"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 157 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 (1.12 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.9
(+3.01/-2.59) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
338 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 19.2 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 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.111.

Some data, such as earlier XRT observations, are not available due to
a telemetry gap, and will be produced on the full ground download.

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 20:10:10 UT on 23 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250823A (trigger 777672615.66839 / 250823840).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 315.5, Dec = -23.6 (J2000 degrees, equivalent to J2000 21h 02m, -23d 36'), with a statistical uncertainty of 8.7 degrees.

The angle from the Fermi LAT boresight is 70.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250823840/quicklook/glg_skymap_all_bn250823840.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250823840/quicklook/glg_healpix_all_bn250823840.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250823840/quicklook/glg_lc_medres34_bn250823840.gif