GRB 250529A

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

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
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 )
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
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 ) 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.

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
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 ).

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
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 ), 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).