GRB 250818A

Harsha K. H. (IUCAA), Anuraag Arya (IITB), G. Waratkar (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 250818A which was also detected by Fermi/GBM (Fermi GBM Team, GCN Circ. 41402
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 ), and Swift/BAT (Cenko et. al., GCN Circ. 41403
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 ).

The source was clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-08-18 01:32:39.98 UTC. The measured peak count rate associated with the burst is 214 (+77, -0) counts/s above the background in the combined data of all quadrants, with a total of 6518 (+769, -832) counts. The local mean background count rate was 1187 (+3, -3) counts/s. We measure a T90 of 89 (+7, -5) s from the cumulative Veto light curve.

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

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-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 250818A ( S. B. Cenko et al., GCN 41403
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 ) errorbox  30 sec after notice time and 64 sec after trigger time at 2025-08-18 01:32:26 UT, with upper limit up to  18.4 mag. The observations began at zenith distance = 52 deg. The sun  altitude  is -43.8 deg. 

The galactic latitude b =  3 deg., longitude l = 314 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2971328

We obtain a following upper limits.  

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

      69 |         MASTER-OAFA |   C |    10 | 17.6 |        
      90 |         MASTER-OAFA |   C |    20 | 17.4 |        
     116 |         MASTER-OAFA |   C |    20 | 17.6 |        
     148 |         MASTER-OAFA |   C |    30 | 18.0 |        
     184 |         MASTER-OAFA |   C |    30 | 18.1 |        
     225 |         MASTER-OAFA |   C |    40 | 18.3 |        
     276 |         MASTER-OAFA |   C |    50 | 18.1 |        
     337 |         MASTER-OAFA |   C |    60 | 18.4 |        
     408 |         MASTER-OAFA |   C |    70 | 18.1 |        
     494 |         MASTER-OAFA |   C |    90 | 18.1 |        
     601 |         MASTER-OAFA |   C |   110 | 18.2 |        
     722 |         MASTER-OAFA |   C |   120 | 18.1 |        
     848 |         MASTER-OAFA |   C |   120 | 18.0 |        
     974 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1100 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1226 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1352 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1478 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1604 |         MASTER-OAFA |   C |   120 | 18.0 |        
    1730 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1856 |         MASTER-OAFA |   C |   120 | 18.3 |        
    1982 |         MASTER-OAFA |   C |   120 | 18.1 |        
    2108 |         MASTER-OAFA |   C |   120 | 18.1 |        
    2234 |         MASTER-OAFA |   C |   120 | 18.1 |        
    2361 |         MASTER-OAFA |   C |   120 | 18.0 |        
    2487 |         MASTER-OAFA |   C |   120 | 18.0 |        
    2613 |         MASTER-OAFA |   C |   120 | 17.9 |        
    2739 |         MASTER-OAFA |   C |   120 | 17.8 |        
    2865 |         MASTER-OAFA |   C |   120 | 17.9 |        
    2992 |         MASTER-OAFA |   C |   120 | 17.8 |        
    3118 |         MASTER-OAFA |   C |   120 | 17.9 |        
    3244 |         MASTER-OAFA |   C |   120 | 18.1 |        
    3370 |         MASTER-OAFA |   C |   120 | 18.1 |        
    3496 |         MASTER-OAFA |   C |   120 | 18.0 |        
    3622 |         MASTER-OAFA |   C |   120 | 18.1 |        
    3748 |         MASTER-OAFA |   C |   120 | 18.0 |        
    3874 |         MASTER-OAFA |   C |   120 | 18.1 |        
    4000 |         MASTER-OAFA |   C |   120 | 18.0 |        
    4126 |         MASTER-OAFA |   C |   120 | 18.0 |        
    4253 |         MASTER-OAFA |   C |   120 | 18.0 |        
    4379 |         MASTER-OAFA |   C |   120 | 18.1 |        
    4505 |         MASTER-OAFA |   C |   120 | 18.1 |        
    4631 |         MASTER-OAFA |   C |   120 | 17.9 |        
    4757 |         MASTER-OAFA |   C |   120 | 18.0 |        
    4884 |         MASTER-OAFA |   C |   120 | 18.0 |        
    5010 |         MASTER-OAFA |   C |   120 | 18.1 |        
    5135 |         MASTER-OAFA |   C |   120 | 18.0 |        
    5261 |         MASTER-OAFA |   C |   120 | 18.1 |        
    5388 |         MASTER-OAFA |   C |   120 | 18.1 |        
    5515 |         MASTER-OAFA |   C |   120 | 18.0 |        
    5640 |         MASTER-OAFA |   C |   120 | 18.0 |        
Filter C is a clear (unfiltred) band. 


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

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

We have analysed 5.5 ks of XRT data for GRB 250818A, from 86 s to 83.2
ks after the   trigger. The data comprise 247 s in Windowed Timing (WT)
mode (the first 10 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+4.4 ks) can be modelled with a
power-law decay with a decay index of alpha=1.15 (+/-0.04).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.18 (+/-0.06). The
best-fitting absorption column is  1.63 (+/-0.08) x 10^22 cm^-2, in
excess of the Galactic value of 6.8 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.76 (+0.17, -0.16)
and a best-fitting absorption column of 1.69 (+0.25, -0.23) x 10^22
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 6.6 x 10^-11 (1.2 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.69 (+0.25, -0.23) x 10^22 cm^-2
Galactic foreground: 6.8 x 10^21 cm^-2
Excess significance: 7.2 sigma
Photon index:	     1.76 (+0.17, -0.16)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01343270.

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

S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), R. Gupta
(GSFC), H. A. Krimm (NSF), 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-721 to T+482 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 250818A (trigger #1343270)
(Cenko et al., GCN Circ. 41403
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 ).  The BAT ground-calculated position is
RA, Dec = 213.816, -58.010 deg which is
   RA(J2000)  =  14h 15m 15.8s
   Dec(J2000) = -58d 00' 36.5"
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 44%.

The mask-weighted light curve shows a multi-peaked structure that starts at
~T-15 s and ends at ~T+140 s. The main peak occurs at ~T+78 s. T90 (15-350
keV) is 106.86 +- 3.79 sec (estimated error including systematics).

The time-averaged spectrum from T-15.92 to T+141.43 sec is best fit by a
simple power-law model.  The power law index of the time-averaged spectrum
is 1.85 +- 0.04.  The fluence in the 15-150 keV band is 1.2 +- 0.03 x
10^-05 erg/cm2. The 1-sec peak photon flux measured from T+77.63 sec in the
15-150 keV band is 5.5 +- 0.3 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/1343270/

M. H. Siegel (PSU) and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250818A
162 s after the BAT trigger (Cenko et al., GCN Circ. 41403
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 ).
No optical afterglow consistent with the XRT position (Osborn et al., GCN
Circ. 41403
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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

u (fc)             162          412          246         >20.3
b                  418          438           19         >18.6
v                32212        32641          429.        >20.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 1.602 in the direction of the burst
(Schlegel et al. 1998).

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

Using 431 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 250818A, 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 = 213.80012, -58.01403 which is equivalent
to:

RA (J2000): 14h 15m 12.03s
Dec (J2000): -58d 00' 50.5"

with an uncertainty of 2.4 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.

C. de Barra (University College Dublin) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 01:31:23.00 UT on 18 August 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250818A (trigger 777173487/250818063)
which was also detected by Swift BAT (S. B. Cenko et al. 2025, GCN 41403
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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 146 degrees.

The GBM light curve consists of multiple emission episodes with a duration (T90)
of about 101 s (50-300 keV). The time-averaged spectrum
from T0-9.2 to T0+94.2 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.83 +/- 0.14 and the cutoff energy,
parameterized as Epeak, is 83 +/- 5 keV.

The event fluence (10-1000 keV) in this time interval is
(1.62 +/- 0.06)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+78 s in the 10-1000 keV band is 8.8 +/- 0.7 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/"

S. B. Cenko (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),
H. A. Krimm (NSF), A. Y. Lien (U Tampa), 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 01:31:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250818A (trigger=1343270).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 213.833, -58.007 which is 
   RA(J2000) = 14h 15m 20s
   Dec(J2000) = -58d 00' 26"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Due to a communications gap, only the
BAT light curve after T+8s is available.  This later lightcurve
shows a complex structure out to at least T+150s.  The peak count
rate in the available lightcurve was ~5500 counts/sec (15-350 keV), 
at ~80 sec after the trigger.

The XRT began observing the field at 01:33:00.6 UT, 97.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 213.8002, -58.0134 which is equivalent to:
   RA(J2000)  = 14h 15m 12.05s
   Dec(J2000) = -58d 00' 48.2"
with an uncertainty of 4.9 arcseconds (radius, 90% containment). This
location is 66 arcseconds from the BAT onboard position, within the BAT
error circle. 

The initial flux in the 0.1 s image was 1.09e-07 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
162 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. Because of the density of catalogued
stars, further analysis is required to report an upper limit for any afterglow
in the region. No correction has been made for the large, but uncertain,
extinction expected.

Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov).
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 01:31:22 UT on 18 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250818A (trigger 777173487.99771 / 250818063).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 232.7, Dec = -62.9 (J2000 degrees, equivalent to J2000 15h 30m, -62d 53'), with a statistical uncertainty of 7.8 degrees.

The angle from the Fermi LAT boresight is 140.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818063/quicklook/glg_skymap_all_bn250818063.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/bn250818063/quicklook/glg_healpix_all_bn250818063.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818063/quicklook/glg_lc_medres34_bn250818063.gif