Gamma-ray Burst 250805A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN/BACODINE POSITION NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Tue 05 Aug 25 06:09:29 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     1339848,   Seg_Num: 0
GRB_RA:           24.153d {+01h 36m 37s} (J2000),
                  24.099d {+01h 36m 24s} (current),
                  24.280d {+01h 37m 07s} (1950)
GRB_DEC:         -81.406d {-81d 24' 22"} (J2000),
                 -81.276d {-81d 16' 34"} (current),
                 -81.660d {-81d 39' 37"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       2869 [cnts]    Image_Peak=185 [image_cnts]
TRIGGER_DUR:     1.024 [sec] 
TRIGGER_INDEX:   146     E_range: 25-100 keV
BKG_INTEN:       16074 [cnts]
BKG_TIME:        22131.90 SOD {06:08:51.90} UT
BKG_DUR:         8 [sec]
GRB_DATE:        20892 TJD;   217 DOY;   25/08/05
GRB_TIME:        22148.86 SOD {06:09:08.86} UT
GRB_PHI:           2.98 [deg]
GRB_THETA:        35.21 [deg]
SOLN_STATUS:     0x3
RATE_SIGNIF:     16.82 [sigma]
IMAGE_SIGNIF:    8.91 [sigma]
MERIT_PARAMS:     +1  +0  +0  +0  +2  -1  +0  +0 +33  +0 
SUN_POSTN:       135.56d {+09h 02m 14s}  +16.88d {+16d 53' 02"}
SUN_DIST:        109.89 [deg]   Sun_angle= 7.4 [hr] (West of Sun)
MOON_POSTN:      263.57d {+17h 34m 17s}  -28.48d {-28d 28' 36"}
MOON_DIST:        66.20 [deg]
MOON_ILLUM:      83 [%]
GAL_COORDS:      300.87,-35.54 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      292.01,-68.67 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Coordinates.  
COMMENTS:        This is a rate trigger.  
COMMENTS:        A point_source was found.  
COMMENTS:        This does not match any source in the on-board catalog.  
COMMENTS:        This does not match any source in the ground catalog.  
COMMENTS:        This is a GRB.  
COMMENTS:        This trigger occurred at longitude,latitude = 4.84,1.04 [deg].

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Tue 05 Aug 25 06:13:39 UT
NOTICE_TYPE:     Swift-BAT GRB Lightcurve
TRIGGER_NUM:     1339848,   Seg_Num: 0
GRB_RA:           24.153d {+01h 36m 37s} (J2000),
                  24.099d {+01h 36m 24s} (current),
                  24.280d {+01h 37m 07s} (1950)
GRB_DEC:         -81.406d {-81d 24' 22"} (J2000),
                 -81.276d {-81d 16' 34"} (current),
                 -81.660d {-81d 39' 37"} (1950)
GRB_DATE:        20892 TJD;   217 DOY;   25/08/05
GRB_TIME:        22148.86 SOD {06:09:08.86} UT
TRIGGER_INDEX:   146
GRB_PHI:           2.98 [deg]
GRB_THETA:        35.21 [deg]
DELTA_TIME:      64.00 [sec]
TRIGGER_DUR:     1.024 [sec] 
SOLN_STATUS:     0x3
RATE_SIGNIF:     16.82 [sigma]
IMAGE_SIGNIF:    8.91 [sigma]
LC_URL:          sw01339848000msb.lc
SUN_POSTN:       135.56d {+09h 02m 15s}  +16.88d {+16d 52' 59"}
SUN_DIST:        109.89 [deg]   Sun_angle= 7.4 [hr] (West of Sun)
MOON_POSTN:      263.61d {+17h 34m 27s}  -28.48d {-28d 28' 40"}
MOON_DIST:        66.19 [deg]
MOON_ILLUM:      83 [%]
GAL_COORDS:      300.87,-35.54 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      292.01,-68.67 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Lightcurve.  
COMMENTS:           
COMMENTS:        The next comments were copied from the BAT_POS Notice:  
COMMENTS:        This is a rate trigger.  
COMMENTS:        A point_source was found.  
COMMENTS:        This does not match any source in the on-board catalog.  
COMMENTS:        This does not match any source in the ground catalog.  
COMMENTS:        This is a GRB.  
COMMENTS:        This trigger occurred at longitude,latitude = 4.84,1.04 [deg].

GCN Circular #41232
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 GRB250805.26 (trigger No 1339848,01h 36m 36.72s , -81d 24m 21.6s, R=0.05) errorbox 18 sec after notice time and 44 sec after trigger time at 2025-08-05 06:09:52 UT, with upper limit up to 17.9 mag. The observations began at zenith distance = 53 deg. The sun altitude is -65.4 deg.
The galactic latitude b = -36 deg., longitude l = 301 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2960226
We obtain a following upper limits.
```
Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

      49 |         MASTER-OAFA |   C |    10 | 17.6 |        
      65 |         MASTER-OAFA |   C |    10 | 17.6 |        
      87 |         MASTER-OAFA |   C |    20 | 17.9 |        
     113 |         MASTER-OAFA |   C |    20 | 17.9 |        
```
Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.
GCN Circular #41233
E. Ambrosi (INAF-IASFPA), S. Dichiara (PSU), R. Gupta (NASA GSFC), K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:
At 06:09:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250805A (trigger=1339848). The BAT on-board calculated location is RA, Dec 24.153, -81.406 which is
```
   RA(J2000) = 01h 36m 37s
   Dec(J2000) = -81d 24' 22"
```
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 ~1800 counts/sec (15-350 keV), at ~0 sec after the trigger.
Due to an observing constraint, Swift will not slew until ~ 57 minutes. There will be no XRT or UVOT data until this time.
Burst Advocate for this burst is E. Ambrosi (elena.ambrosi AT inaf.it). 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/)

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Tue 05 Aug 25 07:08:16 UT
NOTICE_TYPE:     Swift-XRT Nack-Position
TRIGGER_NUM:     1339848,   Seg_Num: 0
POINT_RA:         24.238d {+01h 36m 57s} (J2000)
POINT_DEC:       -81.405d {-81d 24' 16"} (J2000)
IMG_START_DATE:  20892 TJD;   217 DOY;   25/08/05
IMG_START_TIME:  25681.81 SOD {07:08:01.81} UT,   3533.0 [sec] since BAT Trigger Time
COUNTS:          46    Min_needed= 20
STD_DEV:         0.00   Max_StdDev_for_Good=28.44 [arcsec]
PH2_ITER:        4    Max_iter_allowed= 4
ERROR_CODE:      2
COMMENTS:        SWIFT-XRT Nack Position.  
COMMENTS:        Algorithm did not converge; too many iterations.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Tue 05 Aug 25 07:10:54 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1339848,   Seg_Num: 0
POINT_RA:         24.238d {+01h 36m 57s} (J2000)
POINT_DEC:       -81.405d {-81d 24' 17"} (J2000)
POINT_ROLL:      108.675d
IMG_START_DATE:  20892 TJD;   217 DOY;   25/08/05
IMG_START_TIME:  25685.37 SOD {07:08:05.37} UT,   3536.5 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        1.514
N_STARS:         171
X_OFFSET:        424 [pixels]
Y_OFFSET:        619 [pixels]
X_MAX:           1383 [pixels]
Y_MAX:           1578 [pixels]
DET_THRESH:      10
PHOTO_THRESH:    5
SL_URL:          sw01339848000msufc3536.fits
SUN_POSTN:       135.60d {+09h 02m 24s}  +16.87d {+16d 52' 20"}
SUN_DIST:        109.87 [deg]   Sun_angle= 7.4 [hr] (West of Sun)
MOON_POSTN:      264.17d {+17h 36m 40s}  -28.49d {-28d 29' 41"}
MOON_DIST:        66.12 [deg]
MOON_ILLUM:      83 [%]
GAL_COORDS:      300.86,-35.54 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      292.02,-68.68 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Tue 05 Aug 25 07:11:01 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1339848,   Seg_Num: 0
POINT_RA:         24.238d {+01h 36m 57s} (J2000)
POINT_DEC:       -81.405d {-81d 24' 17"} (J2000)
POINT_ROLL:      108.675d
IMG_START_DATE:  20892 TJD;   217 DOY;   25/08/05
IMG_START_TIME:  25685.37 SOD {07:08:05.37} UT,   3536.5 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        1.514
N_STARS:         171
X_OFFSET:        424 [pixels]
Y_OFFSET:        619 [pixels]
X_MAX:           1383 [pixels]
Y_MAX:           1578 [pixels]
DET_THRESH:      10
PHOTO_THRESH:    5
SL_URL:          sw01339848000msufc3536.fits
SUN_POSTN:       135.60d {+09h 02m 24s}  +16.87d {+16d 52' 20"}
SUN_DIST:        109.87 [deg]   Sun_angle= 7.4 [hr] (West of Sun)
MOON_POSTN:      264.17d {+17h 36m 40s}  -28.49d {-28d 29' 41"}
MOON_DIST:        66.12 [deg]
MOON_ILLUM:      83 [%]
GAL_COORDS:      300.86,-35.54 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      292.02,-68.68 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Source List.  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Tue 05 Aug 25 07:12:14 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1339848,   Seg_Num: 0
POINT_RA:         24.238d {+01h 36m 57s} (J2000)
POINT_DEC:       -81.405d {-81d 24' 17"} (J2000)
ROLL:            108.675d
IMG_START_DATE:  20892 TJD;   217 DOY;   25/08/05
IMG_START_TIME:  25685.37 SOD {07:08:05.37} UT,   3536.5 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     776070521
X_OFFSET:        743 [pixels]
Y_OFFSET:        938 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       903
Y_GRB_POS:       1098
BINNING_INDEX:   1
IM_URL:          sw01339848000msuni3573.fits
SUN_POSTN:       135.60d {+09h 02m 24s}  +16.87d {+16d 52' 19"}
SUN_DIST:        109.87 [deg]   Sun_angle= 7.4 [hr] (West of Sun)
MOON_POSTN:      264.18d {+17h 36m 43s}  -28.50d {-28d 29' 42"}
MOON_DIST:        66.11 [deg]
MOON_ILLUM:      83 [%]
GAL_COORDS:      300.86,-35.54 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      292.02,-68.68 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Image.  
COMMENTS:        The GRB Position came from the Window Position in the Mode Command.  
COMMENTS:        The image has 2x2 binning (compression).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Tue 05 Aug 25 07:11:44 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     1339848,   Seg_Num: 0
GRB_RA:           24.2010d {+01h 36m 48.24s} (J2000),
                  24.1460d {+01h 36m 35.04s} (current),
                  24.3284d {+01h 37m 18.81s} (1950)
GRB_DEC:         -81.4013d {-81d 24' 04.6"} (J2000),
                 -81.2713d {-81d 16' 16.7"} (current),
                 -81.6551d {-81d 39' 18.4"} (1950)
GRB_ERROR:       4.7 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  20892 TJD;   217 DOY;   25/08/05
IMG_START_TIME:  25690.00 SOD {07:08:10.00} UT,   3541.1 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:       135.60d {+09h 02m 24s}  +16.87d {+16d 52' 19"}
SUN_DIST:        109.88 [deg]   Sun_angle= 7.4 [hr] (West of Sun)
MOON_POSTN:      264.18d {+17h 36m 42s}  -28.50d {-28d 29' 42"}
MOON_DIST:        66.11 [deg]
MOON_ILLUM:      83 [%]
GAL_COORDS:      300.86,-35.54 [deg] galactic lon,lat of the burst
ECL_COORDS:      292.03,-68.68 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        TAM values, flux and significance fields are not valid.  
COMMENTS:        This position was automatically generated on the ground using  
COMMENTS:        Photon Counting data telemetered via TDRSS (SPER data).  
COMMENTS:        See http://www.swift.ac.uk/sper/docs.php for details.  
COMMENTS:        The probability that this is a serendipitous source in the  
COMMENTS:        SPER window is 0.58% < P(seren) < 1.1%.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Tue 05 Aug 25 07:12:29 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1339848,   Seg_Num: 0
POINT_RA:         24.238d {+01h 36m 57s} (J2000)
POINT_DEC:       -81.405d {-81d 24' 17"} (J2000)
ROLL:            108.675d
IMG_START_DATE:  20892 TJD;   217 DOY;   25/08/05
IMG_START_TIME:  25685.37 SOD {07:08:05.37} UT,   3536.5 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     776070521
X_OFFSET:        743 [pixels]
Y_OFFSET:        938 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       903
Y_GRB_POS:       1098
BINNING_INDEX:   1
IM_URL:          sw01339848000msuni3573.fits
SUN_POSTN:       135.60d {+09h 02m 24s}  +16.87d {+16d 52' 19"}
SUN_DIST:        109.87 [deg]   Sun_angle= 7.4 [hr] (West of Sun)
MOON_POSTN:      264.18d {+17h 36m 44s}  -28.50d {-28d 29' 42"}
MOON_DIST:        66.11 [deg]
MOON_ILLUM:      83 [%]
GAL_COORDS:      300.86,-35.54 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      292.02,-68.68 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Image.  
COMMENTS:        The GRB Position came from the Window Position in the Mode Command.  
COMMENTS:        The image has 2x2 binning (compression).  
COMMENTS:        All 4 attachments are included.

GCN Circular #41234
J.A. Kennea (PSU), G. Cusumano (INAF-IASF PA) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team:
The XRT began observing the field of GRB 250805A at 07:08:01.8 UT, 3532.9 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 24.20214, -81.40161 which is equivalent to:
```
   RA(J2000)  = 01h 36m 48.51s
   Dec(J2000) = -81d 24' 05.8"
```
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 30 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 (8.44 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.6 (+3.34/-2.82) x 10^21 cm^-2 (90% confidence).
GCN Circular #41236
X. Liu (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), L.B. He, Z.P. Zhu, S.Q. Jiang, J. An, D. Xu (NAOC), S.Y. Fu (HUST) report on behalf of a large collaboration:
We observed the field of GRB 250805A (Ambrosi et al, GCN 41233), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. Observations started at 06:27:50.5 UTC on 2025-08-05, i.e., ~ 0.3 hr after the Swift/BAT trigger, and 10 x 200 s frames were obtained in the Sloan r- and i- bands.
No new optical source is detected in the co-added r- and i-band images within the Swift/XRT error circle (Kennea et al, GCN 41234). The limiting magnitudes are as follow:
```
| Tmid(s) | band | upper limit(3sigma) |
| :-----: | :--: | :-----------------: |
| 2746.99 |  i   |        20.7         |
| 3545.62 |  r   |        20.8         |
```
The magnitudes are calibrated against Legacy Survey DR10 and are not corrected for Galactic extinction.
GCN Circular #41238
A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) report on behalf of the Swift-XRT team.
Using 912 s of XRT Photon Counting mode data and 3 UVOT images for GRB 250805A, 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 = 24.21207, -81.40174 which is equivalent to:
```
RA (J2000): 01h 36m 50.90s
Dec (J2000): -81d 24' 06.3"
```
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.
GCN Circular #41240
K.L. Page (U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-OAR), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 6.1 ks of XRT data for GRB 250805A, from 3.5 ks to 27.4 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=0.89 (+/-0.08).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.63 (+/-0.14). The best-fitting absorption column is 1.4 (+/-0.5) x 10^21 cm^-2, in excess of the Galactic value of 8.4 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 4.4 x 10^-11 (5.1 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     1.4 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 8.4 x 10^20 cm^-2
Excess significance: 2.0 sigma
Photon index:	     1.63 (+/-0.14)
```
If the light curve continues to decay with a power-law decay index of 0.89, the count rate at T+24 hours will be 0.042 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x 10^-12 (2.1 x 10^-12) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01339848.
This circular is an official product of the Swift-XRT team.
GCN Circular #41241
L. Izzo (INAF/OACn & DARK/NBI), I. F. Giudice (INAF/OACn), N. Tanvir (U. Leicester), A. Levan (Radboud U.), A. Martin-Carrillo (UCD), M. De Pasquale (U. Messina), A. Takur (INAF-IAPS), V. D’Elia (ASI/SSDC) report on behalf of the Stargate collaboration:
We observed the region of the X-ray counterpart (Kennea et al., GCN #42134) of GRB 250805A (Ambrosi et al., GCN #41233) using the ESO/VLT UT3 (Melipal) equipped with the X-Shooter spectrograph.
A 30-second acquisition image in the r-band, initiated on August 05, 2025, at approximately 09:30 UT (3.3 hours post-burst), revealed no optical counterpart within the initial Swift-XRT error region (Kennea et al., GCN #42134). Due to the absence of an optical source within this region, spectroscopic observations were not executed.
However, we have identified the presence of an uncatalogued optical source outside the initial XRT error region, situated approximately 6 arcsec away from the XRT centroid. This source aligns with the enhanced Swift-XRT position reported later by Beardmore et al. (GCN #41238), indicating that it is likely the optical counterpart of GRB 250805A.
We acknowledge the excellent support from the ESO observing staff in Paranal, in particular Robert Klement and Luca Sbordone.
GCN Circular #41242
H. L. Li, Y. N. Ma, L. P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, 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), A. Li (BNU), Y. D. Hu (GXU) report on behalf of the SVOM/VT team.
SVOM performed a Target of Opportunity observation of GRB 250805A detected by Swift/BAT (Ambrosi et al., GCN 41233; Evans et al., GCN 41234; Page et al., GCN 41240). SVOM/VT began observing the field at 2025-08-05 07:00:42 UTC, 50.4 min after the Swift trigger time, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
An uncatalogued source, compared to Lagacy survey, is found using VT X-band data, with a distance of 2.7 arcsec to Swift/XRT enhanced position (Beardmore et al., GCN 41238). The optical source is at R.A., Dec 24.216108, -81.402177 degrees:
```
  RA  (J2000) =  01:36:51.86
  Dec (J2000) = -81:24:07.84
```
with an uncertainty of 0.5 arcsec.
The source is detected in both VT_B and VT_R images. It decayed by 0.7 mag within 1.89 hours of VT observation. The magnitudes are derived as below:
```
Mid time            | exposure time (s) | band | mag (AB) | mag err
--------------------|-------------------|------|----------|--------
1.24 hours          | 37*70             | VT_B | 22.79    | 0.10   
1.24 hours          | 37*70             | VT_R | 21.16    | 0.04    
```
Our photometry was not corrected for Galactic extinction.
Given the fading of its brightness, the uncatalogued source is proposed to be the optical afterglow of the burst. Our result is consistent with the report by ESO/VLT (Izzo et al., GCN 41241).
The color of VT_B-VT_R~1.5 mag suggests that it shall be a low or intermediate redshift GRB.
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.
GCN Circular #41252
T. Sakamoto (AGU), E. Ambrosi (INAF-IASFPA), 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), D. Sadaula (GSFC/UMBC) (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 250805A (trigger #1339848) (Ambrosi, et al., GCN Circ. 41233). The BAT ground-calculated position is RA, Dec = 24.124, -81.400 deg which is
```
   RA(J2000)  =  01h 36m 29.8s 
   Dec(J2000) = -81d 23' 59.0" 
```
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 65%.
The mask-weighted light curve displays a main emission period between T0-25 sec and T0+30 sec composed of multiple pulses with the strongest pulse beginning at T0. There is dim but significant emission beginning at T0-60 sec. The T90 (15-350 keV) is 79.37 +- 11.98 sec (estimated error including systematics).
The time-averaged spectrum from T-60.66 to T+36.25 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.58 +- 0.53, and Epeak of 68.0 +- 15.8 keV (chi squared 54.47 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 2.2 +- 0.2 x 10^-06 erg/cm2 and the 1-sec peak flux measured from T+0.46 sec in the 15-150 keV band is 1.7 +- 0.2 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.59 +- 0.11 (chi squared 67.84 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/1339848
GCN Circular #41265
M. H. Siegel (PSU) and Ambrosi (INAF-IASFPA) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 250805A 3536 s after the BAT trigger (Ambrosi et al., GCN Circ. 41233). No optical afterglow consistent with the XRT position (Beardmore et al., GCN Circ. 41238) or optical afterglow (Izzo et al., GCN Circ. 41241; Li et al., GCN Circ. 41241) 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

white_FC          3536         3686          147         >21.3
white             3536         4306          344         >21.7
b                 3902         4101          197         >20.9
u                 3696         3895          197         >20.6
w2                4312         4460          146         >20.7
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.092 in the direction of the burst (Schlegel et al. 1998).
GCN Circular #41267
P. Jaquiery, R. Hellot (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 250805A (Ambrosi et al., GCN 41233) detected by Swift/BAT with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a Celestron C14 telescope located at Beverly-Begg Observatory and operated by P. Jaquiery and the CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot. Our observations started at T0+1.2hr.
In our stacked frames, subtracted from the Legacy Survey DR10 template image, we do not detect any optical counterpart inside the Swift/XRT position (Kennea et al., GCN 41234; Beardmore et al., GCN 41238) and at the position of the optical afterglow candidate reported by the VLT/X-shooter (Izzo et al., GCN 41241) and SVOM/VT (Li et al., GCN 41242) teams.
We report our follow-up results in the table below:
```
+---------------+-----------+------------+----------------------+-------------+
| Tmid-TGRB (hr)| Exp (s)   | Filter     | Magnitude            | Instrument  |
+===============+===========+============+======================+=============+
| 1.9           | 28 x 120s | sdssr (AB) | 18.3 (U.L., 5 sigma) | C14-BBO     | 
| 26.1          |  5 x 600s | sdssr (AB) | 19.5 (U.L., 5 sigma) | CDK17-AITP  | 
+---------------+-----------+------------+----------------------+-------------+
```
These upper limits are compatible with the magnitudes and upper limits reported by (Liu et al., GCN 41236; Izzo et al., GCN 41241; Li et al., GCN 41242; Siegel et al., GCN 41265)
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 SkyMapper DR4 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/).

Back to JG's homepage

Jochen Greiner, last update: 07-Aug-2025 [Disclaimer]