GRB 250821A

M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), M. Pillas (ULiege), S. Antier (OCA/IJCLAB) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 250821A detected by Swift/BAT (Dichiara et al., GCN 41462
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 ) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the iTelescope T32 telescope located at Siding Spring Observatory and operated by M. Freeberg. Our observations started at T0+17 min. 

In our stacked frames, subtracted from the PanSTARRS DR2 template image, we do not detect any optical counterpart inside the Swift/XRT enhanced position (Beardmore et al., GCN 41472
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 ).

We report our follow-up results in the table below:
+---------------+-----------+-----------+----------------------+-------------+
| Tmid-TGRB (hr)| Exp (s)   | Filter    | Magnitude            | Instrument  |
+===============+===========+===========+======================+=============+
| 0.86          | 20 x 180s | Ic (Vega) | 19.4 (U.L., 5 sigma) | T32         | 
+---------------+-----------+-----------+----------------------+-------------+

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot 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/).

A. Y. Lien (U Tampa), S. D. Barthelmy (GSFC),
S. Dichiara (PSU), R. Gupta (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
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-240 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250821A (trigger #1344131)
(Dichiara, et al., GCN Circ. 41462
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 ).  The BAT ground-calculated position is
RA, Dec = 358.427, -28.631 deg which is
   RA(J2000)  =  23h 53m 42.6s
   Dec(J2000) = -28d 37' 53.3"
with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 64%.

The mask-weighted BAT light curve exhibits a short-duration main emission
starting from ~T-0.5 to ~T+1.2 sec, followed by a weaker, extended emission
which last up to ~ T+18 sec.
T90 (15-350 keV) is 1.52 +- 0.37 sec (estimated error including systematics).

The time-averaged spectrum from T-0.46 to T+1.20 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
0.83 +- 0.41.  The fluence in the 15-150 keV band is 1.3 +- 0.3 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T+0.06 sec in the 15-150 keV band
is 0.9 +- 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/1344131

Rosa L. Becerra (U Rome), Niccolò Passaleva (U Rome), Yu-Han Yang (U Rome) and Eleonora Troja (U Rome) report on behalf of the ERC BHianca team:

We observed the field of GRB 250821A (Dichiara et al., GCN 41462
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 ) with the FORS2 imager on the ESO VLT UT1 (Antu). Observations began 18.4 hours after the trigger and were carried out in the R filter with an average airmass of ~1.0.

We do not detect any new source at the enhanced Swift-XRT position (Beardmore et al. GCN 41472
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 ) down to the following 3-sigma limit R>26.1 AB mag calibrated with using nearby stars in the PanSTARRS PS1 DR2 catalogue (Flewelling et al. 2020) and without Galactic extinction correction.

We thank the staff at the VLT for the rapid execution of these observations. 

Chen-Wei Wang, Shao-Lin Xiong and Yue Huang (IHEP) report on behalf of GECAM team:

GECAM-B was triggered on-ground by GRB 250821A at 2025-08-21T10:46:18.000 UTC (denoted as T0), which is also detected by Swift/BAT (S. Dichiara et al., GCN#41462
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 ). According to the GECAM-B light curves in about 40-6000 keV, this burst mainly consists of a single pulse with a duration (T90) of 1.5 +1.5/-0.5 s.

The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecamgrb250821A.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).

M. Beyer Stjerne (KU), K. A. Grimstrup (KU), W. K. Krarup Mirsal (KU), C. Lind-Thomsen (KU), F. Hjorth Gammelgaard (KU), I. L. Róbertsdóttir (KU), J. P. U. Fynbo (DAWN/NBI), K. Valeckas (NOT and KU), R. Holmberg Rasmussen (NOT and Aarhus), D. B. Malesani (DAWN/NBI) and Radboud), report on behalf of a larger collaboration:

We observed the field of the Swift GRB 250821A (Dichiara et al., GCN 41462
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 ) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. A total of 45 min of imaging was secured using the SDSS z filter, with a mean epoch 2025 Aug 22.075 UT (15.0 hr after the Swift/BAT trigger).

No new sources are detected consistent with the Swift/XRT position (Beardmore et al., GCN 41472
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 ), down to a 3-sigma limit of z > 22.8 (AB), calibrated against nearby stars from the Pan-STARRS catalog.

Our result is consistent with, and extends to redder wavelengths, the non-detections reported by Strausbaugh & Cucchiara (GCN 41466
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 ) and Oates & Dichiara (GCN 41471
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 ).

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, observed the field of GRB 250821A (Dichiara

et al., GCN 41462
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 ) starting at 12:02:57, Aug. 21 UT, ~1.28 hours

after the burst. A set of 9x60s clear (roughly R) filter images

were obtained under twilight conditions. We do not detect any new

optical counterpart candidate within the Enhanced XRT error circles

(Beardmore et al., GCN 41472
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 ) in the co-add image. Our upper limit

is about 20.0 (Vega) mag at a mid time of 1.30 hours after the burst.

Our result is consistent with the optical upper limit reported by

other groups (Strausbaugh et al., GCN 41466
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 ; Oates et al., GCN 41471
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 ;

Lipunov et al., GCN 41478
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 ).

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

We have analysed 7.4 ks of XRT data for GRB 250821A, from 150 s to 29.3
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=1.32 (+/-0.20).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.59 (+0.42, -0.28). The
best-fitting absorption column is  3 (+13, -2) x 10^20 cm^-2,
consistent with the Galactic value of 1.6 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 4.2 x 10^-11 (4.4 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3 (+13, -2) x 10^20 cm^-2
Galactic foreground: 1.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.59 (+0.42, -0.28)

If the light curve continues to decay with a power-law decay index of
1.32, the count rate at T+24 hours will be 1.0 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.3 x
10^-15 (4.6 x 10^-15) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01344131.

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

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-Kislovodsk robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the Swift GRB 250821A ( S. Dichiara et al., GCN 41462
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 ) errorbox  41464 sec after notice time and 41495 sec after trigger time at 2025-08-21 22:17:52 UT, with upper limit up to  18.0 mag. The observations began at zenith distance = 72 deg. The sun  altitude  is -33.2 deg. 

The galactic latitude b = -78 deg., longitude l = 24 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2974552

We obtain a following upper limits.  

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

   41585 |   MASTER-Kislovodsk |   C |   180 | 17.4 |        
   41585 |   MASTER-Kislovodsk |   C |   180 | 16.6 |        
   41787 |   MASTER-Kislovodsk |   C |   180 | 18.0 |        
   41787 |   MASTER-Kislovodsk |   C |   180 | 17.2 |        
Filter C is a clear (unfiltred) band. 


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

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

Using 1629 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 250821A, 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 = 358.43974, -28.65327 which is equivalent
to:

RA (J2000): 23h 53m 45.54s
Dec (J2000): -28d 39' 11.8"

with an uncertainty of 2.6 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.

S. R. Oates (Lancaster U.) and S. Dichiara (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250821A
147 s after the BAT trigger (Dichiara et al., GCN Circ. 41462
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 ).
No optical afterglow consistent with the XRT position
(Evans et al., GCN Circ. 41463
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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 first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag/3sigUL
white (FC)        147          297           148           >20.8
   u (FC)        455          475           19            >18.3
white             147          1715          281           >21.1
   v             381          1593          136           >19.1
   b             480          1692          127           >19.9
   u             455          1667          136           >19.6
uvw1             431          1643          58            >18.7
uvm2             406          1618          136           >19.1
uvw2             357          1569          136           >19.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.015 in the direction of the burst
(Schlegel et al. 1998).

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

We observed the Swift GRB 250821A field (Dichiara et al., GCN 41462
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 )  with the LCOGT 1-meter Sinistro instrument at the Siding Spring Observatory, Australia site, on August 21, from 12:00 to 12:32 UT (corresponding to 1.23 to 1.76 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 do not detect an uncatalogued source within the Prompt Swift XRT error region (Evans et al., GCN 41463
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 ) in either band.

The following 5-sigma upper limits are calculated using the PanSTARRS catalog as reference:

r > 23.0
i > 22.3

These magnitudes are not corrected for galactic extinction.

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using 1.6 ks of promptly downlinked XRT event data for GRB 250821A, we
find an enhanced XRT position of the afterglow: RA, Dec: 358.44056,
-28.65369 which is equivalent to:
   RA (J2000)  = 23 53 45.73
   Dec (J2000) = -28 39 13.3
with an uncertainty of 2.9 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/1344131.

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.

S. Dichiara (PSU), R. A. J. Eyles-Ferris (U Leicester),
R. Gupta (NASA GSFC), K. L. Page (U Leicester),
B. Sbarufatti (INAF-OAB) and M. H. Siegel (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 10:46:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250821A (trigger=1344131).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 358.412, -28.618 which is 
   RA(J2000) = 23h 53m 39s
   Dec(J2000) = -28d 37' 05"
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 ~2476 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 10:48:40.1 UT, 142.8 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 358.43969, -28.65380 which is
equivalent to:
   RA(J2000)  = 23h 53m 45.53s
   Dec(J2000) = -28d 39' 13.7"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 155 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.  We cannot determine whether the source
is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.65
x 10^20 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 147 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 none of
the XRT error circle. 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 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.015.

Burst Advocate for this burst is S. Dichiara (sbd5667 AT 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/)