Gamma-ray Burst 250601A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sun 01 Jun 25 17:29:54 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      45
TRIGGER_NUM:     770491780
GRB_RA:           77.550d {+05h 10m 12s} (J2000),
                  77.550d {+05h 10m 12s} (current),
                  77.552d {+05h 10m 12s} (1950)
GRB_DEC:         -67.033d {-67d 01' 59"} (J2000),
                 -67.003d {-67d 00' 09"} (current),
                 -67.093d {-67d 05' 35"} (1950)
GRB_ERROR:       7.40 [deg radius, statistical plus systematic]
GRB_INTEN:       410 [cnts/sec]
DATA_SIGNIF:     22.50 [sigma]
INTEG_TIME:      1.024 [sec]
GRB_DATE:        20827 TJD;   152 DOY;   25/06/01
GRB_TIME:        62975.54 SOD {17:29:35.54} UT
GRB_PHI:         149.00 [deg]
GRB_THETA:        35.00 [deg]
DATA_TIME_SCALE: 1.0240 [sec]
HARD_RATIO:      0.30
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      97%  GRB
2nd_MOST_LIKELY:   1%  Generic Transient
DETECTORS:       0,0,0, 0,0,0, 0,1,0, 1,0,1, 0,0,
SUN_POSTN:        69.94d {+04h 39m 45s}  +22.15d {+22d 09' 15"}
SUN_DIST:         89.34 [deg]   Sun_angle= -0.5 [hr] (East of Sun)
MOON_POSTN:      148.28d {+09h 53m 08s}  +15.51d {+15d 30' 31"}
MOON_DIST:        97.00 [deg]
MOON_ILLUM:      36 [%]
GAL_COORDS:      277.55,-34.63 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      348.79,-85.08 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250601729/quicklook/glg_lc_medres34_bn250601729.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 317.07,13.53 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sun 01 Jun 25 17:30:12 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      58
TRIGGER_NUM:     770491780
GRB_RA:           92.580d {+06h 10m 19s} (J2000),
                  92.705d {+06h 10m 49s} (current),
                  92.334d {+06h 09m 20s} (1950)
GRB_DEC:         -54.830d {-54d 49' 47"} (J2000),
                 -54.837d {-54d 50' 10"} (current),
                 -54.818d {-54d 49' 04"} (1950)
GRB_ERROR:       4.64 [deg radius, statistical only]
DATA_SIGNIF:     24.00 [sigma]
DATA_INTERVAL:   1.024 [sec]
GRB_DATE:        20827 TJD;   152 DOY;   25/06/01
GRB_TIME:        62975.54 SOD {17:29:35.54} UT
GRB_PHI:         161.00 [deg]
GRB_THETA:        22.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4173 (Gnd S/W Version number)
SUN_POSTN:        69.94d {+04h 39m 45s}  +22.15d {+22d 09' 15"}
SUN_DIST:         79.42 [deg]   Sun_angle= -1.5 [hr] (East of Sun)
MOON_POSTN:      148.29d {+09h 53m 09s}  +15.51d {+15d 30' 27"}
MOON_DIST:        84.54 [deg]
MOON_ILLUM:      36 [%]
GAL_COORDS:      263.15,-27.66 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       97.29,-78.20 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250601729/quicklook/glg_lc_medres34_bn250601729.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_770491780.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        In the LAT Field-of-view.  
COMMENTS:        Bright hard burst in the GBM.  
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:     Sun 01 Jun 25 17:31:23 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      0
TRIGGER_NUM:     770491780
GRB_RA:           91.300d {+06h 05m 12s} (J2000),
                  91.451d {+06h 05m 48s} (current),
                  91.002d {+06h 04m 01s} (1950)
GRB_DEC:         -50.940d {-50d 56' 23"} (J2000),
                 -50.943d {-50d 56' 35"} (current),
                 -50.934d {-50d 56' 03"} (1950)
GRB_ERROR:       3.38 [deg radius, statistical only]
DATA_SIGNIF:     13.00 [sigma]
DATA_INTERVAL:   6.144 [sec]
GRB_DATE:        20827 TJD;   152 DOY;   25/06/01
GRB_TIME:        62975.54 SOD {17:29:35.54} UT
GRB_PHI:         159.00 [deg]
GRB_THETA:        18.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:        69.94d {+04h 39m 45s}  +22.15d {+22d 09' 16"}
SUN_DIST:         75.52 [deg]   Sun_angle= -1.4 [hr] (East of Sun)
MOON_POSTN:      148.30d {+09h 53m 11s}  +15.50d {+15d 30' 11"}
MOON_DIST:        82.85 [deg]
MOON_ILLUM:      36 [%]
GAL_COORDS:      258.67,-27.80 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       93.04,-74.37 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250601729/quicklook/glg_lc_medres34_bn250601729.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_770491780.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        In the LAT Field-of-view.  
COMMENTS:        Bright hard burst in the GBM.  
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:     Sun 01 Jun 25 17:38:56 UT
NOTICE_TYPE:     Fermi-GBM Final Position
RECORD_NUM:      0
TRIGGER_NUM:     770491780
GRB_RA:           86.490d {+05h 45m 58s} (J2000),
                  86.609d {+05h 46m 26s} (current),
                  86.257d {+05h 45m 02s} (1950)
GRB_DEC:         -55.710d {-55d 42' 35"} (J2000),
                 -55.701d {-55d 42' 04"} (current),
                 -55.728d {-55d 43' 38"} (1950)
GRB_ERROR:       4.45 [deg radius, statistical only]
GRB_DATE:        20827 TJD;   152 DOY;   25/06/01
GRB_TIME:        62975.54 SOD {17:29:35.54} UT
GRB_PHI:         152.00 [deg]
GRB_THETA:        23.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:        69.94d {+04h 39m 46s}  +22.16d {+22d 09' 18"}
SUN_DIST:         79.14 [deg]   Sun_angle= -1.1 [hr] (East of Sun)
MOON_POSTN:      148.36d {+09h 53m 26s}  +15.48d {+15d 28' 32"}
MOON_DIST:        87.90 [deg]
MOON_ILLUM:      36 [%]
GAL_COORDS:      263.74,-31.20 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       79.56,-79.02 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250601729/quicklook/glg_lc_medres34_bn250601729.gif
LOC_URL:         http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250601729/quicklook/glg_locplot_all_bn250601729.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:        In the LAT Field-of-view.  
COMMENTS:        Bright hard burst in the GBM.  
COMMENTS:        This is likely a Long GRB.

GCN Circular #40596
The Fermi GBM team reports the detection of a likely LONG GRB
At 17:29:35 UT on 1 Jun 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250601A (trigger 770491780.538054 / 250601729).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 86.5, Dec = -55.7 (J2000 degrees, equivalent to J2000 05h 46m, -55d 42'), with a statistical uncertainty of 4.5 degrees.
The angle from the Fermi LAT boresight is 23.0 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250601729/quicklook/glg_skymap_all_bn250601729.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/bn250601729/quicklook/glg_healpix_all_bn250601729.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250601729/quicklook/glg_lc_medres34_bn250601729.gif
GCN Circular #40599
P. A. Evans (U. Leicester) reports on behalf of the Swift team:
Swift has initiated a ToO observation of the Swift/BAT-detected event GRB 250601A. Automated analysis of the XRT data will be presented online at https://www.swift.ac.uk/ToO_GRBs/00021844 Any uncatalogued X-ray sources detected in this analysis will be reported on this website and via GCN COUNTERPART notices. These are not necessarily related to the Swift/BAT event. Any X-ray source considered to be a probable afterglow candidate will be reported via a GCN Circular after manual consideration.
Details of the XRT automated analysis methods are detailed in Evans et al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).
This circular is an official product of the Swift-XRT team.
GCN Circular #40600
James DeLaunay (PSU), Samuele Ronchini (PSU), Aaron Tohuvavohu (Caltech), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:
Swift/BAT did not localize GRB 250601A onboard (T0: 2025-06-01T17:29:35.54 UTC, Fermi GCN 40596).
The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).
Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.
The position is found with standard imaging, with an SNR = 8.7. The duration of the burst is ~1 sec.
The BAT position is
RA, Dec = 90.0709, -57.2892 deg which is
```
   RA(J2000)  = 06h 00m 17.02s
   Dec(J2000) = -57d 17’ 21.1″
```
with an estimated uncertainty of 3 arcmin radius.
More details about this burst can be found on the trigger report page here: https://guano.swift.psu.edu/trigger_report?id=770491811
XRT and UVOT follow-up has been requested. Results of follow-up observations will be reported in future circulars.
GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.
A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/

GCN Circular #40603
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-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB 250601A ( Fermi GBM team, GCN 40596) errorbox 31266 sec after notice time and 31297 sec after trigger time at 2025-06-02 02:11:13 UT, with upper limit up to 18.8 mag. The observations began at zenith distance = 80 deg. The sun altitude is -41.7 deg.
The galactic latitude b = -31 deg., longitude l = 264 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2888853
We obtain a following upper limits.

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   31328 | 2025-06-02 02:11:13 |         MASTER-SAAO | (05h 23m 33.62s , -61d 25m 21.7s) |   C |    60 | 18.1 |        
   31328 | 2025-06-02 02:11:13 |         MASTER-SAAO | (05h 27m 12.25s , -61d 40m 47.0s) |   C |    60 | 16.8 |        
   32079 | 2025-06-02 02:23:44 |         MASTER-SAAO | (05h 23m 32.93s , -61d 25m 26.3s) |   C |    60 | 18.2 |        
   32079 | 2025-06-02 02:23:44 |         MASTER-SAAO | (05h 27m 11.81s , -61d 40m 51.9s) |   C |    60 | 16.9 |        
   32832 | 2025-06-02 02:36:17 |         MASTER-SAAO | (05h 23m 35.76s , -61d 24m 34.2s) |   C |    60 | 18.2 |        
   32832 | 2025-06-02 02:36:17 |         MASTER-SAAO | (05h 27m 14.94s , -61d 39m 58.9s) |   C |    60 | 16.9 |        
   32926 | 2025-06-02 02:37:51 |         MASTER-SAAO | (05h 01m 57.98s , -57d 37m 48.3s) |   C |    60 | 18.3 |        
   32926 | 2025-06-02 02:37:51 |         MASTER-SAAO | (05h 05m 13.56s , -57d 53m 13.1s) |   C |    60 | 16.9 |        
   33492 | 2025-06-02 02:47:17 |         MASTER-SAAO | (05h 23m 29.69s , -61d 25m 33.6s) |   C |    60 | 18.3 |        
   33492 | 2025-06-02 02:47:17 |         MASTER-SAAO | (05h 27m 09.27s , -61d 40m 57.8s) |   C |    60 | 16.9 |        
   33586 | 2025-06-02 02:48:51 |         MASTER-SAAO | (05h 01m 58.51s , -57d 38m 06.3s) |   C |    60 | 18.6 |        
   33586 | 2025-06-02 02:48:51 |         MASTER-SAAO | (05h 05m 14.44s , -57d 53m 30.3s) |   C |    60 | 17.1 |        
   33679 | 2025-06-02 02:50:24 |         MASTER-SAAO | (05h 17m 04.95s , -57d 38m 12.0s) |   C |    60 | 18.3 |        
   33679 | 2025-06-02 02:50:24 |         MASTER-SAAO | (05h 20m 20.73s , -57d 53m 36.1s) |   C |    60 | 17.0 |        
   34244 | 2025-06-02 02:59:49 |         MASTER-SAAO | (05h 23m 26.94s , -61d 25m 56.2s) |   C |    60 | 18.2 |        
   34348 | 2025-06-02 03:01:33 |         MASTER-SAAO | (05h 01m 56.88s , -57d 38m 26.6s) |   C |    60 | 18.6 |        
   34348 | 2025-06-02 03:01:33 |         MASTER-SAAO | (05h 05m 13.07s , -57d 53m 50.3s) |   C |    60 | 17.2 |        
   34441 | 2025-06-02 03:03:06 |         MASTER-SAAO | (05h 17m 02.63s , -57d 38m 31.9s) |   C |    60 | 18.3 |        
   34441 | 2025-06-02 03:03:06 |         MASTER-SAAO | (05h 20m 18.63s , -57d 53m 56.0s) |   C |    60 | 17.0 |        
   35007 | 2025-06-02 03:12:32 |         MASTER-SAAO | (05h 23m 27.32s , -61d 26m 18.0s) |   C |    60 | 18.4 |        
   35101 | 2025-06-02 03:14:06 |         MASTER-SAAO | (05h 01m 55.99s , -57d 38m 46.4s) |   C |    60 | 18.6 |        
   35101 | 2025-06-02 03:14:06 |         MASTER-SAAO | (05h 05m 12.55s , -57d 54m 10.0s) |   C |    60 | 17.2 |        
   35194 | 2025-06-02 03:15:39 |         MASTER-SAAO | (05h 17m 02.39s , -57d 38m 52.0s) |   C |    60 | 18.4 |        
   35194 | 2025-06-02 03:15:39 |         MASTER-SAAO | (05h 20m 18.72s , -57d 54m 16.2s) |   C |    60 | 17.0 |        
   35288 | 2025-06-02 03:17:13 |         MASTER-SAAO | (05h 02m 24.56s , -54d 08m 10.8s) |   C |    60 | 17.0 |        
   35381 | 2025-06-02 03:18:46 |         MASTER-SAAO | (05h 12m 59.55s , -53d 50m 37.6s) |   C |    60 | 18.5 |        
   35381 | 2025-06-02 03:18:46 |         MASTER-SAAO | (05h 15m 57.34s , -54d 06m 02.3s) |   C |    60 | 16.7 |        
   35661 | 2025-06-02 03:23:26 |         MASTER-SAAO | (05h 11m 01.98s , -51d 56m 40.6s) |   C |    60 | 18.5 |        
   35661 | 2025-06-02 03:23:26 |         MASTER-SAAO | (05h 13m 52.04s , -52d 12m 05.7s) |   C |    60 | 16.8 |        
   35757 | 2025-06-02 03:25:02 |         MASTER-SAAO | (05h 01m 55.75s , -57d 41m 01.1s) |   C |    60 | 18.6 |        
   35757 | 2025-06-02 03:25:02 |         MASTER-SAAO | (05h 05m 12.76s , -57d 56m 25.1s) |   C |    60 | 17.3 |        
   35851 | 2025-06-02 03:26:36 |         MASTER-SAAO | (05h 17m 02.21s , -57d 39m 55.6s) |   C |    60 | 18.5 |        
   35851 | 2025-06-02 03:26:36 |         MASTER-SAAO | (05h 20m 18.88s , -57d 55m 20.0s) |   C |    60 | 17.2 |        
   36038 | 2025-06-02 03:29:43 |         MASTER-SAAO | (05h 12m 58.98s , -53d 52m 22.2s) |   C |    60 | 18.4 |        
   36038 | 2025-06-02 03:29:43 |         MASTER-SAAO | (05h 15m 56.70s , -54d 07m 48.0s) |   C |    60 | 17.0 |        
   36318 | 2025-06-02 03:34:23 |         MASTER-SAAO | (05h 11m 01.70s , -51d 56m 54.3s) |   C |    60 | 18.7 |        
   36318 | 2025-06-02 03:34:23 |         MASTER-SAAO | (05h 13m 51.53s , -52d 12m 20.2s) |   C |    60 | 17.0 |        
   36411 | 2025-06-02 03:35:56 |         MASTER-SAAO | (05h 24m 08.15s , -51d 57m 10.8s) |   C |    60 | 18.4 |        
   36411 | 2025-06-02 03:35:56 |         MASTER-SAAO | (05h 26m 57.92s , -52d 12m 36.4s) |   C |    60 | 17.0 |        
   36504 | 2025-06-02 03:37:29 |         MASTER-SAAO | (05h 05m 10.11s , -57d 56m 43.4s) |   C |    60 | 17.4 |        
   36504 | 2025-06-02 03:37:29 |         MASTER-SAAO | (05h 01m 53.17s , -57d 41m 19.2s) |   C |    60 | 18.7 |        
   36597 | 2025-06-02 03:39:02 |         MASTER-SAAO | (05h 20m 17.51s , -57d 55m 46.9s) |   C |    60 | 17.3 |        
   36597 | 2025-06-02 03:39:02 |         MASTER-SAAO | (05h 17m 00.88s , -57d 40m 22.3s) |   C |    60 | 18.6 |        
   36784 | 2025-06-02 03:42:09 |         MASTER-SAAO | (05h 15m 55.94s , -54d 07m 49.4s) |   C |    60 | 17.0 |        
   36784 | 2025-06-02 03:42:09 |         MASTER-SAAO | (05h 12m 57.97s , -53d 52m 23.8s) |   C |    60 | 18.6 |        
   36973 | 2025-06-02 03:45:18 |         MASTER-SAAO | (05h 04m 12.35s , -56d 01m 02.3s) |   C |    60 | 17.5 |        
   36973 | 2025-06-02 03:45:18 |         MASTER-SAAO | (05h 01m 05.36s , -55d 45m 37.7s) |   C |    60 | 18.8 |        
   37066 | 2025-06-02 03:46:51 |         MASTER-SAAO | (05h 13m 57.11s , -52d 13m 36.4s) |   C |    60 | 17.2 |        
   37066 | 2025-06-02 03:46:51 |         MASTER-SAAO | (05h 11m 06.80s , -51d 58m 10.9s) |   C |    60 | 18.7 |        
   37159 | 2025-06-02 03:48:24 |         MASTER-SAAO | (05h 26m 51.17s , -52d 13m 53.2s) |   C |    60 | 17.1 |

Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.

GCN Circular #40604
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the Swift/BAT-detected burst GRB 250601A, collecting 4.6 ks of Photon Counting (PC) mode data between T0+17.2 ks and T0+35.3 ks.
Two uncatalogued X-ray sources have been detected within the estimated 3-sigma Swift/BAT error region (493 arcsec), however none of them is above the RASS limit or shows definitive signs of fading. Therefore, at the present time we cannot identify which, if any, is the afterglow. Details of these sources are given below:
Source 2:
```
  RA (J2000.0):  90.2023  =  06:00:48.56
  Dec (J2000.0): -57.2379  =  -57:14:16.6
  Error: 7.3 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: (7.8 +/- 1.9)e-3 ct s^-1	 
  Distance: 315 arcsec from Swift/BAT position.
  Flux: (4.2 +/- 1.0)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)
```
Source 3:
```
  RA (J2000.0):  90.0936  =  06:00:22.47
  Dec (J2000.0): -57.2778  =  -57:16:40.0
  Error: 5.7 arcsec (radius, 90% conf.)
  Count-rate: (3.6 [+1.6, -1.3])e-3 ct s^-1   
  Distance: 60 arcsec from Swift/BAT position.
  Flux: (1.00 [+0.45, -0.35])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)
```
Another uncatalogued source was also detected, however this was too far from the GRB position to be the afterglow.
The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/ToO_GRBs/00021844.
This circular is an official product of the Swift-XRT team.
GCN Circular #40606
C. Salvaggio (INAF-OAB), S. B. Cenko (GSFC), B. Sbarufatti (INAF-OAB) report on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the Fermi/GBM, Swift/BAT-GUANO detected burst GRB 250601A (GCN 40596, 40600).
Two XRT sources have been previously reported in GCN 40604. One of the two sources (source 2) has an eROSITA counterpart, while XRT source 3 is a new transient. Therefore we consider source 3 a candidate afterglow for GRB 250601A.
Source 3:
RA (J2000.0): 90.0936 = 06:00:22.47
Dec (J2000.0): -57.2778 = -57:16:40.0
Error: 5.7 arcsec (radius, 90% conf.)
Count-rate: (3.6 [+1.6, -1.3])e-3 ct s^-1
Distance: 60 arcsec from Swift/BAT position.
Flux: (1.00 [+0.45, -0.35])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)
More Swift/XRT observations are planned to (un)confirm the afterglow detection.
The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/ToO_GRBs/00021844.
This circular is an official product of the Swift-XRT team.
GCN Circular #40607
Q. C. Liu (THU), X. Y. Zhou (PRIC), H. L. Peng (NNU), B. B. Zhang (BNU) and Y. Liu (NAO,CAS) report on behalf of the Einstein Probe (EP) team :
We performed a follow-up observation of GRB 250601A (detected through on-broad trigger of Fermi/GBM, Fermi, GCN #40596, and localized by Swift/BAT, DeLaunay, GCN #40600; followed by Swift/XRT, Page et al., GCN #40604), with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The observation began at 2025-06-02T02:10:26 (T-TGRB ~ 8.7 hrs) with an exposure time of about 5.8 ks. Two uncatalogued sources were detected by FXT-A and FXT-B in the 3-sigma Swift/BAT error circle with a radius of 493 arcsecond centered at RA, DEC = 90.0709 deg, -57.2892 deg, as listed below (the FXT flux is taken from FXT-B module). EP J060023.3-571631 has a separation of 11.6 arcsecond from the XRT source 3, and EP J060048.7-571413 has a separation of 4.8 arcsecond from the XRT source 2.
```
Source name | RA | DEC | Estimated Flux | SNR | Dist from Swift/BAT |
| deg | deg | (erg/s/cm^2) |         | offset (in arcmin) |

EP J060023.3-571631 | 90.0971 | -57.2752 | 3.2(+/-1.2) x 10^-14 | 4.1 | 1.19 | 
EP J060048.7-571413 * | 90.2037 | -57.2368 | 1.0(+/-0.2) x 10^-13 | 9.4 | 5.33 | 
```
Note: * EP J060048.7-571413 was also detected by Swift/XRT (GCN #40604) in a ~4.6 ks observation conducted at T-TGRB ~ 4.8 hrs, with a flux of ~ 4e-13 erg/s/cm^2. The source thus exhibits a decreasing trend in the X-ray flux within the two epochs.
The above observation was made with the EP-FXT instrument. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.
GCN Circular #40621
V. Sharma (NASA-GSFC/UMBC) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 17:29:35 UT on 1 Jun 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250601A (trigger 770491780 / 250601729). which was also detected by Swift/BAT-GUANO (J. DeLaunay et al. 2025, GCN 40600) and Swift/XRT (K.L. Page et al. 2025, GCN 40604). The Fermi GBM on-ground location is consistent with the Swift/BAT-GUANO position.
The angle from the Fermi LAT boresight is 24 degrees.
The GBM light curve consists of main emission episode with a duration (T90) of about 3 s (50-300 keV). The time-averaged spectrum from T0-0.5 to T0+2.4 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.58 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 873 +/- 142 keV.
The event fluence (10-1000 keV) in this time interval is (2.7 +/- 0.1)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+1 s in the 10-1000 keV band is 4.9 +/- 0.2 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/"
GCN Circular #40622
Roberto Ricci (U Rome), Yu-Han Yang (U Rome), Rosa L. Becerra (U Rome) and Eleonora Troja (U Rome) report on behalf of the ERC BHianca team:
We observed the field of GRB 250601A (Fermi GBM team GCN 40596; DeLaunay et al., GCN 40600) with the FORS2 imager on the ESO VLT UT1 (Antu). Observations began 29.8 hours after the trigger and were carried out in the R filter with an average airmass of ~2.3.
Our field covers the candidates reported by Swift/XRT (Page et al. GCN 40604, Salvaggio et al., GCN 40606) and EP/FXT (Liu et al., GCN 40607). Compared with the Legacy Survey DR10 (Dey et al. 2019) catalogue, we do not detect any uncatalogued source down to R>24 AB (3-sigma).
Further observations are planned.
We thank the staff at the VLT for the rapid execution of these observations.
GCN Circular #40624
N. Passaleva (U Rome), O. Guiffreda (UMD), J. Durbak (UMD), M. Elkabir (U Rome), E. Troja (U Rome), A. S. Kutyrev (NASA/GSFC), S. B. Cenko (NASA/GSFC)
Following the Fermi Gamma-ray Burst Monitor (GBM) detection (GCN 40596), and the Swift/BAT-GUANO arcminute localization of same event, we observed the transient field in J filter with PRIME ~23.5 hours after the initial Fermi detection.
Using nearby VISTA Hemispherical Survey (VHS) we do not detect any uncatalogued source down to J>20.3 AB (3-sigma) at the position of the candidates reported by Swift/XRT (Page et al. GCN 40604, Salvaggio et al., GCN 40606) and EP/FXT (Liu et al., GCN 40607).
PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024).
We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations.
GCN Circular #40626
N. Passaleva (U Rome), O. Guiffreda (UMD), J. Durbak (UMD), M. Elkabir (U Rome), E. Troja (U Rome), A. S. Kutyrev (NASA/GSFC), S. B. Cenko (NASA/GSFC)
Following the Fermi Gamma-ray Burst Monitor (GBM) detection (GCN 40596), and the Swift/BAT-GUANO arcminutes localization of same event, we observed the transient field in H filter with PRIME ~47.5 hours after the initial Fermi detection.
Using Two Micron All Sky Survey (2MASS) for preliminary calibration we do not detect any uncatalogued source down to H>20.6 AB (3-sigma) not corrected for galactic extinction at the position of the candidates reported by Swift/XRT (Page et al. GCN 40604, Salvaggio et al., GCN 40606) and EP/FXT (Liu et al., GCN 40607).
PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024).
We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations.
GCN Circular #40631
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report:
The short-duration GRB 250601A (Fermi GBM detection: Fermi GBM team, GCN 40596; Sharma et al., GCN 40621; Swift/BAT-GUANO localization: DeLaunay et al., GCN 40600) triggered Konus-Wind at T0=62975.326 s UT (17:29:35.326).
The burst light curve shows a multi-peaked pulse, which starts at ~T0-0.5 s and has a total duration of ~0.8 s. The emission is seen up to ~10 MeV.
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB250601_T62975/
As observed by Konus-Wind, the burst had a fluence of 2.02(-0.28,+0.31)x10^-6 erg/cm2, and a 16-ms peak flux, measured from T0-0.004 s, of 7.00(-2.01,+2.06)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range).
Since the brightest peak of the burst light curve was detected before the trigger, the spectral analysis was performed using the KW 3-channel light curve data.
Modelling the KW 3-channel time-integrated spectrum (measured from T0-0.492 s to T0+0.276 s) by a power law with exponential cutoff (CPL) model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep), yields alpha = 0.31(-0.37,+0.52) and Ep = 436(-62,+76) keV.
All the quoted errors are at the 68% confidence level. All the quoted values are preliminary.

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Jochen Greiner, last update: 05-Jun-2025 [Disclaimer]