Gamma-ray Burst 250331D

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 31 Mar 25 05:49:47 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      47
TRIGGER_NUM:     765092962
GRB_RA:           26.867d {+01h 47m 28s} (J2000),
                  27.288d {+01h 49m 09s} (current),
                  26.038d {+01h 44m 09s} (1950)
GRB_DEC:         +56.717d {+56d 43' 00"} (J2000),
                 +56.842d {+56d 50' 31"} (current),
                 +56.467d {+56d 28' 03"} (1950)
GRB_ERROR:       24.85 [deg radius, statistical plus systematic]
GRB_INTEN:       71 [cnts/sec]
DATA_SIGNIF:     4.80 [sigma]
INTEG_TIME:      2.048 [sec]
GRB_DATE:        20765 TJD;    90 DOY;   25/03/31
GRB_TIME:        20957.01 SOD {05:49:17.01} UT
GRB_PHI:         140.00 [deg]
GRB_THETA:        15.00 [deg]
DATA_TIME_SCALE: 2.0480 [sec]
HARD_RATIO:      1.36
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      93%  GRB
2nd_MOST_LIKELY:   4%  Solar Flare
DETECTORS:       0,0,0, 0,0,0, 1,0,0, 1,0,0, 0,0,
SUN_POSTN:         9.90d {+00h 39m 37s}   +4.26d {+04d 15' 49"}
SUN_DIST:         54.35 [deg]   Sun_angle= -1.2 [hr] (East of Sun)
MOON_POSTN:       32.67d {+02h 10m 40s}  +16.58d {+16d 34' 40"}
MOON_DIST:        40.47 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      130.60, -5.32 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       48.84, 41.94 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250331243/quicklook/glg_lc_medres34_bn250331243.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 121.47,2.63 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 31 Mar 25 05:50:05 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      57
TRIGGER_NUM:     765092962
GRB_RA:           57.420d {+03h 49m 41s} (J2000),
                  58.064d {+03h 52m 15s} (current),
                  56.155d {+03h 44m 37s} (1950)
GRB_DEC:         +69.620d {+69d 37' 12"} (J2000),
                 +69.695d {+69d 41' 42"} (current),
                 +69.467d {+69d 28' 03"} (1950)
GRB_ERROR:       10.51 [deg radius, statistical only]
DATA_SIGNIF:     5.60 [sigma]
DATA_INTERVAL:   4.096 [sec]
GRB_DATE:        20765 TJD;    90 DOY;   25/03/31
GRB_TIME:        20957.01 SOD {05:49:17.01} UT
GRB_PHI:         151.00 [deg]
GRB_THETA:        33.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4173 (Gnd S/W Version number)
SUN_POSTN:         9.90d {+00h 39m 37s}   +4.26d {+04d 15' 49"}
SUN_DIST:         72.51 [deg]   Sun_angle= -3.2 [hr] (East of Sun)
MOON_POSTN:       32.67d {+02h 10m 40s}  +16.58d {+16d 34' 44"}
MOON_DIST:        55.39 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      137.39, 11.92 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       73.72, 48.02 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250331243/quicklook/glg_lc_medres34_bn250331243.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_765092962.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        In the LAT Field-of-view.  
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:     Mon 31 Mar 25 05:51:12 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      0
TRIGGER_NUM:     765092962
GRB_RA:           51.140d {+03h 24m 34s} (J2000),
                  51.724d {+03h 26m 54s} (current),
                  49.993d {+03h 19m 58s} (1950)
GRB_DEC:         +67.090d {+67d 05' 24"} (J2000),
                 +67.178d {+67d 10' 39"} (current),
                 +66.913d {+66d 54' 47"} (1950)
GRB_ERROR:       8.86 [deg radius, statistical only]
DATA_SIGNIF:     5.80 [sigma]
DATA_INTERVAL:   5.120 [sec]
GRB_DATE:        20765 TJD;    90 DOY;   25/03/31
GRB_TIME:        20957.01 SOD {05:49:17.01} UT
GRB_PHI:         148.00 [deg]
GRB_THETA:        30.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:         9.91d {+00h 39m 37s}   +4.26d {+04d 15' 50"}
SUN_DIST:         69.10 [deg]   Sun_angle= -2.8 [hr] (East of Sun)
MOON_POSTN:       32.68d {+02h 10m 43s}  +16.58d {+16d 35' 01"}
MOON_DIST:        52.09 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      137.05,  8.51 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       69.25, 46.43 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250331243/quicklook/glg_lc_medres34_bn250331243.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_765092962.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        In the LAT Field-of-view.  
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:     Mon 31 Mar 25 05:58:41 UT
NOTICE_TYPE:     Fermi-GBM Final Position
RECORD_NUM:      0
TRIGGER_NUM:     765092962
GRB_RA:           21.130d {+01h 24m 31s} (J2000),
                  21.529d {+01h 26m 07s} (current),
                  20.346d {+01h 21m 23s} (1950)
GRB_DEC:         +55.730d {+55d 43' 48"} (J2000),
                 +55.861d {+55d 51' 39"} (current),
                 +55.470d {+55d 28' 11"} (1950)
GRB_ERROR:       13.95 [deg radius, statistical only]
GRB_DATE:        20765 TJD;    90 DOY;   25/03/31
GRB_TIME:        20957.01 SOD {05:49:17.01} UT
GRB_PHI:         146.00 [deg]
GRB_THETA:        12.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:         9.91d {+00h 39m 38s}   +4.27d {+04d 15' 57"}
SUN_DIST:         52.43 [deg]   Sun_angle= -0.8 [hr] (East of Sun)
MOON_POSTN:       32.75d {+02h 11m 01s}  +16.61d {+16d 36' 54"}
MOON_DIST:        40.17 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      127.61, -6.84 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       44.43, 42.65 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250331243/quicklook/glg_lc_medres34_bn250331243.gif
LOC_URL:         http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250331243/quicklook/glg_locplot_all_bn250331243.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:        This is likely a Long GRB.

GCN Circular #39961
The Fermi GBM team reports the detection of a likely LONG GRB
At 05:49:17 UT on 31 Mar 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250331D (trigger 765092962.014718 / 250331243).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 21.1, Dec = 55.7 (J2000 degrees, equivalent to J2000 01h 24m, 55d 42'), with a statistical uncertainty of 13.9 degrees.
The angle from the Fermi LAT boresight is 12.0 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250331243/quicklook/glg_skymap_all_bn250331243.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/bn250331243/quicklook/glg_healpix_all_bn250331243.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250331243/quicklook/glg_lc_medres34_bn250331243.gif
GCN Circular #39979
Samuele Ronchini (PSU), James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:
Swift/BAT did not localize GRB 250331D onboard (T0: 2025-03-31T05:49:17 UTC, Fermi GCN 39961).
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 burst occurred during a Swift slew. The imaging is performed using BatAnalysis tools (Parsotan et al. 2025 [https://arxiv.org/abs/2502.00278](https://arxiv.org/abs/2502.00278)). A confident location is found for the burst with SNR of 11.6
The BAT position is
RA, Dec = 48.52543, 62.30596 deg which is
```
   RA(J2000)  = 03h 14m 06.10s
   Dec(J2000) =  +62d 18’ 21.5″
```
with an estimated uncertainty of 5 arcmin.
This position is consistent with the Ferm/GBM localization (GCN 39961).
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 #39982
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 250331D. Automated analysis of the XRT data will be presented online at https://www.swift.ac.uk/ToO_GRBs/00021825 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 #39990
V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko, A.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, 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. 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-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) started inspect of the Fermi GRB 250331D ( Fermi GBM team, GCN 39961) errorbox 62486 sec after notice time and 62523 sec after trigger time at 2025-03-31 23:11:20 UT, with upper limit up to 17.9 mag. The observations began at zenith distance = 71 deg. The sun altitude is -35.8 deg.
The galactic latitude b = -7 deg., longitude l = 128 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2829033
We obtain a following upper limits.
```
Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   62554 | 2025-03-31 23:11:20 |   MASTER-Kislovodsk | (02h 54m 34.96s , +65d 33m 33.8s) |   C |    60 | 17.7 |        
   63431 | 2025-03-31 23:25:58 |   MASTER-Kislovodsk | (02h 54m 36.68s , +65d 33m 23.8s) |   C |    60 | 17.9 |        
   64508 | 2025-03-31 23:43:54 |   MASTER-Kislovodsk | (02h 53m 45.13s , +65d 44m 21.5s) |   C |    60 | 17.7 |        
```
Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.
GCN Circular #39998
A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) 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 250331D, collecting 5.3 ks of Photon Counting (PC) mode data between T0+45.2 ks and T0+68.1 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):  48.6572  =  03:14:37.72
  Dec (J2000.0): +62.2833  =  +62:16:59.7
  Error: 3.5 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: (7.4 [+1.6, -1.4])e-3 ct s^-1   
  Distance: 235 arcsec from Swift/BAT position.
  Flux: (3.18 [+0.67, -0.61])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)
```
Source 4:
```
  RA (J2000.0):  48.7705  =  03:15:4.93
  Dec (J2000.0): +62.3734  =  +62:22:24.1
  Error: 5.9 arcsec (radius, 90% conf.)
  Count-rate: (1.77 [+0.95, -0.72])e-3 ct s^-1	 
  Distance: 476 arcsec from Swift/BAT position.
  Flux: (1.19 [+0.64, -0.49])e-10 erg cm^-2 s^-1 (observed, 0.3-10 keV)
```
A catalogued source was also detected within the estimated 3-sigma Swift/BAT error region (493 arcsec).
Two uncatalogued sources were also detected too far from the GRB position to be likely afterglow candidates.
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/00021825.
This circular is an official product of the Swift-XRT team.
GCN Circular #40004
E. Bissaldi (Politecnico and INFN Bari), C. Meegan (UAH) and A. Holzmann Airasca (Univ. Trento and INFN Bari) report on behalf of the Fermi GBM Team:
"At 05:49:17.01 UT on 31 March 2025, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 250331D (trigger 765092962 / 250331243), which was also detected by the Swift/BAT-GUANO (Ronchini et al. 2025, GCN 39979). The GBM on-ground location is consistent with the Swift position.
The angle from the Fermi LAT boresight at the GBM trigger time is 27 degrees.
The GBM light curve consists of a single emission episode with a duration (T90) of about 9 s (50-300 keV). The time-averaged spectrum from T0-2.5 s to T0+6.4 s is adequately fit by a power law function with an exponential high-energy cutoff. The power law index is -1.12 +/- 0.27 and the cutoff energy, parameterized as Epeak, is 91 +/- 20 keV.
The event fluence (10-1000 keV) in this time interval is (6.1 +/- 0.8)E-07 erg/cm^2. The 1-sec peak photon flux measured starting from T0-0.64 s in the 10-1000 keV band is 2.2 +/- 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 #40005
N. Pankov (HSE, IKI), R. Ya. Inasaridze (AbAO), A. Pozanenko (IKI) report on behalf of IKI GRB-FuN:
We observed the entire localisation region of BAT-GUANO/Swift of GRB250331D (Ronchini et al., GCN 39979), initially detected by GBM/Fermi (Fermi GBM team, GCN 39961) with AS-32 telescope of AbAO starting on 2025-03-31 (UT) 18:56:37. We report the results of optical transient searches in five XRT/Swift localisations reported in https://www.swift.ac.uk/ToO_GRBs/00021825/ and (Melandri et al., GCN 39998). Details of the preliminary photometry of the stacked image and the sources detected so far are given below.

Date UT start t-T0 Filter Exp. UL(3sigma) Telescope
(mid, d) (s)
2025-03-31 18:56:37 0.56447 R 61x60 20.8 AS-32

XRT #1
------
RA = 48.26026d
Dec = 62.30756d
Err90 = 5.1"

Inside the XRT error circle the single source is detected:
RA = 48.25957d +/- 0.05"
Dec = 62.30756 +/- 0.02"
R(AbAO) = 13.24 +/- 0.05
USNO-B1.0 R2 = 13.39

XRT #2
------
RA = 48.65718d
Dec = 62.28326d
Err90 = 3.5"

Inside the XRT error circle the single source is detected:
RA = 48.66142d +/- 0.14"
Dec = 62.28324d +/- 0.07"
R(AbAO) = 18.72 +/- 0.07
USNO-B1.0 R2 = 18.97

XRT #3
------
RA = 48.86152d
Dec = 62.34017d
Err90 = 7.0"

Inside the XRT error circle the single source is detected:
RA = 48.85897d +/- 0.12"
Dec = 48.85897d +/- 0.06"
R(AbAO) = 18.64 +/- 0.07
USNO-B1.0 R2 = 18.93

XRT #4
------
RA = 48.77053d
Dec = 62.37337d
Err90 = 5.9"

Inside the XRT error circle the single source is detected:
RA = 48.76831d +/- 0.3"
Dec = 62.37292d +/- 0.3"
R(AbAO) = 19.68 +/- 0.26
USNO-B1.0 R2 = 19.94

XRT #5
------
RA = 48.3703896d
Dec = 62.10607d
Err90 = 6.9"

Inside the XRT error circle the single source is detected:

RA = 48.37039d +/- 0.3"
Dec = 62.10664d +/- 0.3"
R(AbAO) ~ 21.0
PS1 r = 21.17 +/- 0.15, i = 20.21 +/- 0.06

We did not identify any variable sources in any of the XRT localisations ( https://www.swift.ac.uk/ToO_GRBs/00021825/).

GCN Circular #40079
A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has conducted further observations of the field of the Swift/BAT-detected burst GRB 250331D. The observations now extend from T0+45.3 ks to T0+406.9 ks and have a total exposure time of 8.6 ks.
Of the sources previously reported, "Source 2" is fading with >3-sigma significance, and is therefore likely the GRB afterglow. Using 3064 s of PC mode data and 2 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 48.65718, +62.28326 which is equivalent to:
```
RA (J2000): 03h 14m 37.72s
Dec(J2000): +62d 16' 59.8"
```
with an uncertainty of 3.5 arcsec (radius, 90% confidence). This position is 3.9 arcmin from the Swift/BAT position. There is evidence that the source may be fading, however we cannot constrain the decay at the current time.
The results of the XRT-team automatic analysis of the likely afterglow are at https://www.swift.ac.uk/ToO_GRBs/00021825/Source2.php. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021825.
This circular is an official product of the Swift-XRT team.

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