Gamma-ray Burst 250529A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 29 May 25 03:46:45 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      44
TRIGGER_NUM:     770183181
GRB_RA:           35.467d {+02h 21m 52s} (J2000),
                  35.956d {+02h 23m 49s} (current),
                  34.512d {+02h 18m 03s} (1950)
GRB_DEC:         +63.200d {+63d 12' 00"} (J2000),
                 +63.315d {+63d 18' 53"} (current),
                 +62.972d {+62d 58' 19"} (1950)
GRB_ERROR:       35.33 [deg radius, statistical plus systematic]
GRB_INTEN:       64 [cnts/sec]
DATA_SIGNIF:     4.80 [sigma]
INTEG_TIME:      2.048 [sec]
GRB_DATE:        20824 TJD;   149 DOY;   25/05/29
GRB_TIME:        13576.32 SOD {03:46:16.32} UT
GRB_PHI:          42.00 [deg]
GRB_THETA:        75.00 [deg]
DATA_TIME_SCALE: 2.0480 [sec]
HARD_RATIO:      0.00
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:     100%  Unreliable location
2nd_MOST_LIKELY:   0%  n/a
DETECTORS:       0,1,0, 0,0,1, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:        66.29d {+04h 25m 09s}  +21.65d {+21d 38' 47"}
SUN_DIST:         46.38 [deg]   Sun_angle= 2.0 [hr] (West of Sun)
MOON_POSTN:       97.12d {+06h 28m 30s}  +28.19d {+28d 11' 26"}
MOON_DIST:        52.20 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      132.92,  2.10 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       58.32, 45.63 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250529157/quicklook/glg_lc_medres34_bn250529157.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 29.45,-5.70 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 29 May 25 03:46:58 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      67
TRIGGER_NUM:     770183181
GRB_RA:            8.300d {+00h 33m 12s} (J2000),
                   8.679d {+00h 34m 43s} (current),
                   7.562d {+00h 30m 15s} (1950)
GRB_DEC:         +68.733d {+68d 43' 60"} (J2000),
                 +68.873d {+68d 52' 23"} (current),
                 +68.458d {+68d 27' 27"} (1950)
GRB_ERROR:       14.75 [deg radius, statistical plus systematic]
GRB_INTEN:       180 [cnts/sec]
DATA_SIGNIF:     8.10 [sigma]
INTEG_TIME:      1.024 [sec]
GRB_DATE:        20824 TJD;   149 DOY;   25/05/29
GRB_TIME:        13576.32 SOD {03:46:16.32} UT
GRB_PHI:          54.00 [deg]
GRB_THETA:        70.00 [deg]
DATA_TIME_SCALE: 1.0240 [sec]
HARD_RATIO:      0.91
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      96%  GRB
2nd_MOST_LIKELY:   1%  Generic Transient
DETECTORS:       0,1,0, 0,0,1, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:        66.29d {+04h 25m 09s}  +21.65d {+21d 38' 47"}
SUN_DIST:         58.43 [deg]   Sun_angle= 3.8 [hr] (West of Sun)
MOON_POSTN:       97.13d {+06h 28m 30s}  +28.19d {+28d 11' 26"}
MOON_DIST:        63.30 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      121.27,  5.92 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       49.40, 56.53 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250529157/quicklook/glg_lc_medres34_bn250529157.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 29.45,-5.70 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 29 May 25 03:47:04 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      58
TRIGGER_NUM:     770183181
GRB_RA:           50.530d {+03h 22m 07s} (J2000),
                  50.994d {+03h 23m 59s} (current),
                  49.621d {+03h 18m 29s} (1950)
GRB_DEC:         +51.570d {+51d 34' 12"} (J2000),
                 +51.659d {+51d 39' 34"} (current),
                 +51.391d {+51d 23' 29"} (1950)
GRB_ERROR:       14.03 [deg radius, statistical only]
DATA_SIGNIF:     6.30 [sigma]
DATA_INTERVAL:   4.096 [sec]
GRB_DATE:        20824 TJD;   149 DOY;   25/05/29
GRB_TIME:        13576.32 SOD {03:46:16.32} UT
GRB_PHI:          29.00 [deg]
GRB_THETA:        81.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4173 (Gnd S/W Version number)
SUN_POSTN:        66.29d {+04h 25m 09s}  +21.65d {+21d 38' 47"}
SUN_DIST:         32.27 [deg]   Sun_angle= 1.0 [hr] (West of Sun)
MOON_POSTN:       97.13d {+06h 28m 30s}  +28.19d {+28d 11' 25"}
MOON_DIST:        41.46 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      145.33, -4.63 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       62.28, 31.86 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250529157/quicklook/glg_lc_medres34_bn250529157.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_770183181.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 29 May 25 03:47:16 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      92
TRIGGER_NUM:     770183181
GRB_RA:            7.083d {+00h 28m 20s} (J2000),
                   7.440d {+00h 29m 46s} (current),
                   6.386d {+00h 25m 33s} (1950)
GRB_DEC:         +60.300d {+60d 17' 60"} (J2000),
                 +60.440d {+60d 26' 25"} (current),
                 +60.024d {+60d 01' 25"} (1950)
GRB_ERROR:       5.82 [deg radius, statistical plus systematic]
GRB_INTEN:       724 [cnts/sec]
DATA_SIGNIF:     36.20 [sigma]
INTEG_TIME:      1.024 [sec]
GRB_DATE:        20824 TJD;   149 DOY;   25/05/29
GRB_TIME:        13576.32 SOD {03:46:16.32} UT
GRB_PHI:          50.00 [deg]
GRB_THETA:        65.00 [deg]
DATA_TIME_SCALE: 1.0240 [sec]
HARD_RATIO:      0.77
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      97%  GRB
2nd_MOST_LIKELY:   2%  Generic Transient
DETECTORS:       0,1,0, 0,0,1, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:        66.29d {+04h 25m 09s}  +21.65d {+21d 38' 48"}
SUN_DIST:         56.08 [deg]   Sun_angle= 3.9 [hr] (West of Sun)
MOON_POSTN:       97.13d {+06h 28m 31s}  +28.19d {+28d 11' 25"}
MOON_DIST:        65.59 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      120.07, -2.44 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       39.24, 50.59 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250529157/quicklook/glg_lc_medres34_bn250529157.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 29.45,-5.70 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 29 May 25 03:47:07 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      71
TRIGGER_NUM:     770183181
GRB_RA:            7.000d {+00h 28m 00s} (J2000),
                   7.359d {+00h 29m 26s} (current),
                   6.298d {+00h 25m 12s} (1950)
GRB_DEC:         +62.400d {+62d 23' 60"} (J2000),
                 +62.540d {+62d 32' 25"} (current),
                 +62.123d {+62d 07' 24"} (1950)
GRB_ERROR:       9.75 [deg radius, statistical plus systematic]
GRB_INTEN:       196 [cnts/sec]
DATA_SIGNIF:     13.80 [sigma]
INTEG_TIME:      2.048 [sec]
GRB_DATE:        20824 TJD;   149 DOY;   25/05/29
GRB_TIME:        13576.32 SOD {03:46:16.32} UT
GRB_PHI:          50.00 [deg]
GRB_THETA:        65.00 [deg]
DATA_TIME_SCALE: 2.0480 [sec]
HARD_RATIO:      0.85
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      96%  GRB
2nd_MOST_LIKELY:   1%  Generic Transient
DETECTORS:       0,1,0, 0,0,1, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:        66.29d {+04h 25m 09s}  +21.65d {+21d 38' 47"}
SUN_DIST:         56.73 [deg]   Sun_angle= 3.9 [hr] (West of Sun)
MOON_POSTN:       97.13d {+06h 28m 31s}  +28.19d {+28d 11' 25"}
MOON_DIST:        65.11 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      120.22, -0.35 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       41.32, 52.24 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250529157/quicklook/glg_lc_medres34_bn250529157.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 29.45,-5.70 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 29 May 25 03:48:27 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      2
TRIGGER_NUM:     770183181
GRB_RA:           27.930d {+01h 51m 43s} (J2000),
                  28.349d {+01h 53m 24s} (current),
                  27.110d {+01h 48m 26s} (1950)
GRB_DEC:         +54.490d {+54d 29' 24"} (J2000),
                 +54.615d {+54d 36' 53"} (current),
                 +54.243d {+54d 14' 35"} (1950)
GRB_ERROR:       3.99 [deg radius, statistical only]
DATA_SIGNIF:     18.90 [sigma]
DATA_INTERVAL:   11.264 [sec]
GRB_DATE:        20824 TJD;   149 DOY;   25/05/29
GRB_TIME:        13576.32 SOD {03:46:16.32} UT
GRB_PHI:          36.00 [deg]
GRB_THETA:        69.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:        66.29d {+04h 25m 09s}  +21.65d {+21d 38' 48"}
SUN_DIST:         43.51 [deg]   Sun_angle= 2.5 [hr] (West of Sun)
MOON_POSTN:       97.14d {+06h 28m 34s}  +28.19d {+28d 11' 22"}
MOON_DIST:        55.27 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      131.69, -7.35 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       48.17, 39.69 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250529157/quicklook/glg_lc_medres34_bn250529157.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_770183181.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 29 May 25 03:48:17 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      0
TRIGGER_NUM:     770183181
GRB_RA:           29.060d {+01h 56m 14s} (J2000),
                  29.490d {+01h 57m 58s} (current),
                  28.220d {+01h 52m 53s} (1950)
GRB_DEC:         +56.380d {+56d 22' 48"} (J2000),
                 +56.503d {+56d 30' 12"} (current),
                 +56.136d {+56d 08' 08"} (1950)
GRB_ERROR:       6.09 [deg radius, statistical only]
DATA_SIGNIF:     15.10 [sigma]
DATA_INTERVAL:   8.192 [sec]
GRB_DATE:        20824 TJD;   149 DOY;   25/05/29
GRB_TIME:        13576.32 SOD {03:46:16.32} UT
GRB_PHI:          37.00 [deg]
GRB_THETA:        70.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:        66.29d {+04h 25m 09s}  +21.65d {+21d 38' 48"}
SUN_DIST:         44.08 [deg]   Sun_angle= 2.5 [hr] (West of Sun)
MOON_POSTN:       97.14d {+06h 28m 34s}  +28.19d {+28d 11' 22"}
MOON_DIST:        54.63 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      131.86, -5.36 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       50.06, 41.07 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250529157/quicklook/glg_lc_medres34_bn250529157.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_770183181.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 29 May 25 03:55:34 UT
NOTICE_TYPE:     Fermi-GBM Final Position
RECORD_NUM:      0
TRIGGER_NUM:     770183181
GRB_RA:            7.060d {+00h 28m 14s} (J2000),
                   7.411d {+00h 29m 39s} (current),
                   6.374d {+00h 25m 30s} (1950)
GRB_DEC:         +54.740d {+54d 44' 24"} (J2000),
                 +54.880d {+54d 52' 49"} (current),
                 +54.464d {+54d 27' 49"} (1950)
GRB_ERROR:       4.23 [deg radius, statistical only]
GRB_DATE:        20824 TJD;   149 DOY;   25/05/29
GRB_TIME:        13576.32 SOD {03:46:16.32} UT
GRB_PHI:          45.00 [deg]
GRB_THETA:        60.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:        66.29d {+04h 25m 10s}  +21.65d {+21d 38' 51"}
SUN_DIST:         54.68 [deg]   Sun_angle= 3.9 [hr] (West of Sun)
MOON_POSTN:       97.22d {+06h 28m 54s}  +28.18d {+28d 11' 05"}
MOON_DIST:        67.17 [deg]
MOON_ILLUM:      6 [%]
GAL_COORDS:      119.56, -7.98 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       34.24, 46.13 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250529157/quicklook/glg_lc_medres34_bn250529157.gif
LOC_URL:         http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250529157/quicklook/glg_locplot_all_bn250529157.png
COMMENTS:        Fermi-GBM Final Position.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file should be available by the time this FINAL notice is produced.  
COMMENTS:        This notice was generated completely by automated pipeline processing.  
COMMENTS:        This is likely a Long GRB.

GCN Circular #40567
The Fermi GBM team reports the detection of a likely LONG GRB
At 03:46:16 UT on 29 May 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250529A (trigger 770183181.316048 / 250529157).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 7.1, Dec = 54.7 (J2000 degrees, equivalent to J2000 00h 28m, 54d 42'), with a statistical uncertainty of 4.2 degrees.
The angle from the Fermi LAT boresight is 60.0 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250529157/quicklook/glg_skymap_all_bn250529157.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/bn250529157/quicklook/glg_healpix_all_bn250529157.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250529157/quicklook/glg_lc_medres34_bn250529157.gif
GCN Circular #40629
A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin on behalf of the HEND/Mars Odyssey team,
D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team,
A. Goldstein, M. S. Briggs, C. Wilson-Hodge, and E. Burns on behalf of the Fermi GBM team,
S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, A. Tohuvavohu, and J. DeLaunay on behalf of the Swift-BAT team,
and
W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr, and A.S. Gardner on on behalf of the GRS-Odyssey GRB team, report:
The long-duration GRB 250529A (Fermi-GBM detection: Fermi GBM team, GCN 40567) was detected by Fermi (GBM), Konus-Wind, Swift (BAT), and Mars-Odyssey (HEND) at about 13576 s UT (03:46:16). The burst was outside the coded field of view of the BAT.
We have triangulated it to a preliminary, 3 sigma error box whose coordinates are:
```
 ---------------------------------------------
  RA(2000), deg                 Dec(2000), deg
 ---------------------------------------------
 Center:
   18.193 (01h 12m 46s) +58.274 (+58d 16' 25")
 Corners:
   21.244 (01h 24m 58s) +56.039 (+56d 02' 19")
   14.840 (00h 59m 22s) +60.456 (+60d 27' 22")
   14.829 (00h 59m 19s) +60.366 (+60d 21' 57")
   21.225 (01h 24m 54s) +55.944 (+55d 56' 37")
 ---------------------------------------------
```
The error box area is 1252 sq. arcmin, and its maximum dimension is 5.6 deg (the minimum one is 4 arcmin). The Sun distance was 48 deg.
This localization may be improved.
The IPN localization is consistent with, but reduces the area of, the Fermi-GBM (GCN 40567) localization.
A triangulation map and HEALPix FITS file are posted at http://www.ioffe.ru/LEA/GRBs/GRB250529_T13600/IPN/ The HEALPix triangulation map is the multi-order HEALPix in units of probability density.
The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular.
GCN Circular #40654
A. Kumar, D. O’Neill, B. P. Gompertz, G. Ramsay, R. Starling, S. Belkin, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration:
We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022; Dyer et al. 2024) in response to the Fermi/GBM detected GRB 250529A (GBM770183181; Fermi GBM Team, GCN 40567).
Targeted observations were performed with GOTO-North beginning at 2025-05-29 03:59:57 UT (+0.23h post trigger) and continued through to 2025-05-29 04:49:24 UT (+1.05h post trigger). Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm).
Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks.
Following the refinement of the GRB localisation (Kozyrev et al., GCN 40629), we reviewed our sources consistent with the new IPN annulus. We identify a new transient at RA, Dec = 18.071949, 58.347004 (equivalent to RA = 01:12:17.27 and Dec = +58:20:49.21), contained within the annulus. The source was initially detected with a magnitude of L = 18.76 ± 0.28 mag (+0.90h after the GBM trigger). The second epoch, taken at +1.05h post trigger, shows marginal evidence for fading, with a measured magnitude of L = 19.14 ± 0.12 mag. Observations taken on the following night (t0+25h) yield only a 3-sigma upper limit of L > 19.05 mag.
We find no evidence of the source prior to the GRB trigger time in previous GOTO observations, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021).
While the candidate lies close to the Galactic plane (latitude -4.4 degrees) and has only weak evidence for fading, the significant reduction of the localisation area now makes it a much stronger (but still uncertain) afterglow candidate for GRB 250529A.
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction.
GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).

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Jochen Greiner, last update: 09-Jun-2025 [Disclaimer]