Gamma-ray Burst 250316A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sun 16 Mar 25 09:00:27 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      45
TRIGGER_NUM:     763808402
GRB_RA:          215.517d {+14h 22m 04s} (J2000),
                 215.819d {+14h 23m 17s} (current),
                 214.917d {+14h 19m 40s} (1950)
GRB_DEC:         +14.200d {+14d 11' 60"} (J2000),
                 +14.086d {+14d 05' 10"} (current),
                 +14.427d {+14d 25' 39"} (1950)
GRB_ERROR:       17.62 [deg radius, statistical plus systematic]
GRB_INTEN:       99 [cnts/sec]
DATA_SIGNIF:     7.50 [sigma]
INTEG_TIME:      2.048 [sec]
GRB_DATE:        20750 TJD;    75 DOY;   25/03/16
GRB_TIME:        32397.69 SOD {08:59:57.69} UT
GRB_PHI:         216.00 [deg]
GRB_THETA:       100.00 [deg]
DATA_TIME_SCALE: 2.0480 [sec]
HARD_RATIO:      0.36
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      96%  GRB
2nd_MOST_LIKELY:   3%  Generic Transient
DETECTORS:       0,0,0, 0,0,0, 0,1,1, 0,0,0, 0,0,
SUN_POSTN:       356.35d {+23h 45m 25s}   -1.58d {-01d 34' 41"}
SUN_DIST:        139.05 [deg]   Sun_angle= 9.4 [hr] (West of Sun)
MOON_POSTN:      196.57d {+13h 06m 17s}   -9.23d {-09d 13' 43"}
MOON_DIST:        30.14 [deg]
MOON_ILLUM:      96 [%]
GAL_COORDS:        6.01, 65.24 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      207.98, 26.69 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250316375/quicklook/glg_lc_medres34_bn250316375.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 357.47,3.68 [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 16 Mar 25 09:00:45 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      57
TRIGGER_NUM:     763808402
GRB_RA:          200.610d {+13h 22m 26s} (J2000),
                 200.917d {+13h 23m 40s} (current),
                 200.002d {+13h 20m 00s} (1950)
GRB_DEC:         +18.240d {+18d 14' 24"} (J2000),
                 +18.109d {+18d 06' 32"} (current),
                 +18.501d {+18d 30' 04"} (1950)
GRB_ERROR:       16.92 [deg radius, statistical only]
DATA_SIGNIF:     9.70 [sigma]
DATA_INTERVAL:   4.096 [sec]
GRB_DATE:        20750 TJD;    75 DOY;   25/03/16
GRB_TIME:        32397.69 SOD {08:59:57.69} UT
GRB_PHI:         201.00 [deg]
GRB_THETA:       102.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4173 (Gnd S/W Version number)
SUN_POSTN:       356.35d {+23h 45m 25s}   -1.58d {-01d 34' 41"}
SUN_DIST:        150.77 [deg]   Sun_angle= 10.4 [hr] (West of Sun)
MOON_POSTN:      196.57d {+13h 06m 17s}   -9.23d {-09d 13' 47"}
MOON_DIST:        27.67 [deg]
MOON_ILLUM:      96 [%]
GAL_COORDS:      343.32, 78.60 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      191.58, 24.84 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250316375/quicklook/glg_lc_medres34_bn250316375.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_763808402.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:     Sun 16 Mar 25 09:01:29 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      0
TRIGGER_NUM:     763808402
GRB_RA:          216.040d {+14h 24m 10s} (J2000),
                 216.340d {+14h 25m 22s} (current),
                 215.445d {+14h 21m 47s} (1950)
GRB_DEC:         +15.500d {+15d 30' 00"} (J2000),
                 +15.387d {+15d 23' 12"} (current),
                 +15.726d {+15d 43' 33"} (1950)
GRB_ERROR:       7.16 [deg radius, statistical only]
DATA_SIGNIF:     15.70 [sigma]
DATA_INTERVAL:   11.264 [sec]
GRB_DATE:        20750 TJD;    75 DOY;   25/03/16
GRB_TIME:        32397.69 SOD {08:59:57.69} UT
GRB_PHI:         216.00 [deg]
GRB_THETA:        99.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       356.36d {+23h 45m 25s}   -1.58d {-01d 34' 40"}
SUN_DIST:        138.23 [deg]   Sun_angle= 9.3 [hr] (West of Sun)
MOON_POSTN:      196.58d {+13h 06m 18s}   -9.23d {-09d 13' 57"}
MOON_DIST:        31.46 [deg]
MOON_ILLUM:      96 [%]
GAL_COORDS:        9.29, 65.54 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      207.97, 28.08 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250316375/quicklook/glg_lc_medres34_bn250316375.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_763808402.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:     Sun 16 Mar 25 09:03:17 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      1
TRIGGER_NUM:     763808402
GRB_RA:          210.260d {+14h 01m 02s} (J2000),
                 210.554d {+14h 02m 13s} (current),
                 209.678d {+13h 58m 43s} (1950)
GRB_DEC:         +22.530d {+22d 31' 48"} (J2000),
                 +22.409d {+22d 24' 32"} (current),
                 +22.771d {+22d 46' 16"} (1950)
GRB_ERROR:       7.01 [deg radius, statistical only]
DATA_SIGNIF:     16.00 [sigma]
DATA_INTERVAL:   12.288 [sec]
GRB_DATE:        20750 TJD;    75 DOY;   25/03/16
GRB_TIME:        32397.69 SOD {08:59:57.69} UT
GRB_PHI:         208.00 [deg]
GRB_THETA:        95.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       356.36d {+23h 45m 26s}   -1.58d {-01d 34' 39"}
SUN_DIST:        140.79 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      196.59d {+13h 06m 22s}   -9.24d {-09d 14' 22"}
MOON_DIST:        34.48 [deg]
MOON_ILLUM:      96 [%]
GAL_COORDS:       20.50, 73.57 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      199.00, 32.46 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250316375/quicklook/glg_lc_medres34_bn250316375.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_763808402.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:     Sun 16 Mar 25 09:09:07 UT
NOTICE_TYPE:     Fermi-GBM Final Position
RECORD_NUM:      0
TRIGGER_NUM:     763808402
GRB_RA:          208.700d {+13h 54m 48s} (J2000),
                 208.993d {+13h 55m 58s} (current),
                 208.119d {+13h 52m 29s} (1950)
GRB_DEC:         +24.190d {+24d 11' 24"} (J2000),
                 +24.067d {+24d 04' 02"} (current),
                 +24.435d {+24d 26' 06"} (1950)
GRB_ERROR:       10.23 [deg radius, statistical only]
GRB_DATE:        20750 TJD;    75 DOY;   25/03/16
GRB_TIME:        32397.69 SOD {08:59:57.69} UT
GRB_PHI:         206.00 [deg]
GRB_THETA:        94.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       356.36d {+23h 45m 26s}   -1.58d {-01d 34' 33"}
SUN_DIST:        141.25 [deg]   Sun_angle= 9.8 [hr] (West of Sun)
MOON_POSTN:      196.63d {+13h 06m 32s}   -9.26d {-09d 15' 41"}
MOON_DIST:        35.45 [deg]
MOON_ILLUM:      96 [%]
GAL_COORDS:       24.83, 75.43 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      196.63, 33.38 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250316375/quicklook/glg_lc_medres34_bn250316375.gif
LOC_URL:         http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250316375/quicklook/glg_locplot_all_bn250316375.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 #39741
The Fermi GBM team reports the detection of a likely LONG GRB
At 08:59:57 UT on 16 Mar 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250316A (trigger 763808402.691648 / 250316375).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 208.7, Dec = 24.2 (J2000 degrees, equivalent to J2000 13h 54m, 24d 11'), with a statistical uncertainty of 10.2 degrees.
The angle from the Fermi LAT boresight is 94.0 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250316375/quicklook/glg_skymap_all_bn250316375.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/bn250316375/quicklook/glg_healpix_all_bn250316375.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250316375/quicklook/glg_lc_medres34_bn250316375.gif
GCN Circular #39744
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 250316A onboard (T0: 2025-03-16T08:59:57.690 UTC, Fermi GCN 39741).
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 BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), detects the burst in a 16.384 s analysis time bin starting at T0 + 8.192 s with a sqrt(TS) of 17.7. An arcminute localization is found with DeltaLLHOut of 72.75 and a DeltaLLHPeak of 42.6.
See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.
The BAT position is RA, Dec = 219.63, 14.886 deg which is
```
   RA(J2000)  = 14h 38m 31.20s
   Dec(J2000) = 14d 53’ 09.60″
```
with an estimated uncertainty of 5 arcmin radius.
XRT and UVOT follow-up has been requested. Results of follow-up observations will be reported in future circulars.
More details about this burst can be found on the trigger report page here:
https://guano.swift.psu.edu/trigger_report?id=763808432
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 #39745
P. A. Evans (U. Leicester) reports on behalf of the Swift team:
Swift has initiated a ToO observation of the Swift/BAT GRB 250316A. Automated analysis of the XRT data will be presented online at https://www.swift.ac.uk/ToO_GRBs/00021823
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 #39747
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 250316A ( Fermi GBM team, GCN 39741) errorbox 36240 sec after notice time and 36276 sec after trigger time at 2025-03-16 19:04:34 UT, with upper limit up to 17.3 mag. The observations began at zenith distance = 55 deg. The sun altitude is -38.8 deg.
The galactic latitude b = 75 deg., longitude l = 24 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2813565
We obtain a following upper limits.

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

   36307 | 2025-03-16 19:04:34 |   MASTER-Kislovodsk | (13h 45m 52.80s , +24d 09m 52.7s) |   C |    60 | 16.2 |        
   36387 | 2025-03-16 19:05:54 |   MASTER-Kislovodsk | (13h 54m 40.28s , +24d 10m 54.0s) |   C |    60 | 16.5 |        
   36527 | 2025-03-16 19:08:14 |   MASTER-Kislovodsk | (13h 41m 50.76s , +26d 03m 19.9s) |   C |    60 | 17.3 |        
   36609 | 2025-03-16 19:09:36 |   MASTER-Kislovodsk | (13h 50m 50.40s , +26d 03m 08.2s) |   C |    60 | 17.0 |        
   36689 | 2025-03-16 19:10:56 |   MASTER-Kislovodsk | (13h 58m 47.69s , +26d 02m 55.6s) |   C |    60 | 17.1 |        
   36769 | 2025-03-16 19:12:16 |   MASTER-Kislovodsk | (14h 07m 45.91s , +26d 04m 05.9s) |   C |    60 | 16.8 |        
   36849 | 2025-03-16 19:13:36 |   MASTER-Kislovodsk | (14h 02m 29.99s , +24d 10m 37.1s) |   C |    60 | 16.8 |        
   36928 | 2025-03-16 19:14:55 |   MASTER-Kislovodsk | (14h 11m 21.21s , +24d 09m 17.3s) |   C |    60 | 16.4 |        
   37235 | 2025-03-16 19:20:02 |   MASTER-Kislovodsk | (13h 45m 55.17s , +24d 10m 08.5s) |   C |    60 | 16.9 |        
   37315 | 2025-03-16 19:21:22 |   MASTER-Kislovodsk | (13h 54m 37.47s , +24d 11m 22.2s) |   C |    60 | 16.8 |        
   37394 | 2025-03-16 19:22:41 |   MASTER-Kislovodsk | (13h 41m 51.58s , +26d 04m 40.2s) |   C |    60 | 17.0 |        
   37474 | 2025-03-16 19:24:01 |   MASTER-Kislovodsk | (13h 50m 48.05s , +26d 05m 34.6s) |   C |    60 | 16.9 |        
   37553 | 2025-03-16 19:25:21 |   MASTER-Kislovodsk | (13h 58m 42.08s , +26d 04m 34.8s) |   C |    60 | 16.8 |

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

GCN Circular #39750
J.A. Kennea (PSU), A.P. Beardmore (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , D.N. Burrows (PSU), M. A. Williams (PSU), J.P. Osborne (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 GUANO-detected burst GRB 250316A (GCN #39744), collecting 4.3 ks of Photon Counting (PC) mode data between T0+21.5 ks and T0+46.5 ks.
One uncatalogued X-ray source has been detected within the estimated 3-sigma Swift/BAT error region (296 arcsec), it is below the RASS limit and shows only moderate signs of fading. Therefore, at the present time we cannot confirm this as the afterglow. Details of this source are given below:
Source 1:
```
  RA (J2000.0):  219.5964  =  14:38:23.15
  Dec (J2000.0): +14.8912  =  +14:53:28.2
  Error: 3.1 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: 0.0180 [+0.0025, -0.0024] ct s^-1   
  Distance: 118 arcsec from Swift/BAT position.
  Flux: (5.29 [+0.72, -0.70])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)
```
Four 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/00021823.
This circular is an official product of the Swift-XRT team.
GCN Circular #39751
S. Bala (USRA) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 08:59:57.69 UT on 16 March 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250316A (trigger 763808402/250316375). which was also detected by Swift/BAT-GUANO (Ronchini et al. 2025, GCN 39744). The Fermi GBM on-ground location is consistent with the Swift/BAT-GUANO position.
The angle from the Fermi LAT boresight is 98 degrees.
The GBM light curve consists of multiple peaks with a duration (T90) of about 38.7 s (50-300 keV). The time-averaged spectrum from T0-3.1 to T0+46.1 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.2 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 110 +/- 10 keV.
The event fluence (10-1000 keV) in this time interval is (4.7 +/- 0.3)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+16 s in the 10-1000 keV band is 3.5 +/- 0.3 ph/s/cm^2.
A Band function fits the spectrum equally well with Epeak= 80 +/- 10 keV, alpha = -1 +/- 0.2 and beta = -2.3 +/- 0.2.
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 #39758
A.P. Beardmore (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , J.A. Kennea (PSU), D.N. Burrows (PSU), M. A. Williams (PSU), J.P. Osborne (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 250316A. The observations now extend from T0+21.7 ks to T0+51.0 ks and have a total exposure time of 4.6 ks. . The source previously reported, "Source 1", is believed to be the afterglow. Using 3006 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 = 219.59644, +14.89108 which is equivalent to:
```
RA (J2000): 14h 38m 23.15s
Dec(J2000): +14d 53' 27.9"
```
with an uncertainty of 3.0 arcsec (radius, 90% confidence). This position is 118 arcsec from the Swift/BAT position. The source is fading with alpha >0.9.
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.7 (+0.9, -0.7). The best-fitting absorption column is 5.1 (+4.0, -2.8) x 10^21 cm^-2, in excess of the Galactic value of 1.5 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 3.0 x 10^-11 (8.4 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     5.1 (+4.0, -2.8) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^20 cm^-2
Excess significance: 2.9 sigma
Photon index:	     2.7 (+0.9, -0.7)
```
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021823. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021823.
This circular is an official product of the Swift-XRT team.
GCN Circular #39760
N. Pankov (HSE, IKI), R. Ya. Inasaridze (AbAO), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:
We performed optical observations of the field of GRB 250316A (The Fermi GBM team, GCN 39741; Ronchini et. al, GCN 39744; Evans, GCN 39745; Lipunov et. al, GCN 39747; Kennea et. al, GCN 39750; Bala & Meegan, GCN 39751; Beardmore et. al, GCN 39758) in the R filter with the 0.7-meter AS-32 telescope of the Abastumani Astrophysical Observatory (AbAO). The observations started on (UT) 2025-03-17 00:17:41, i.e. about 0.67 days since trigger. We do not find any new optical source within the error circle of the most probable candidate afterglow Swift XRT #1 (Beardmore et. al, GCN 39758). The preliminary upper limit is as follows:
```
Date       UT start  t-T0         Exp.    Filter OT  UL(3sigma)
                     (mid, days)  (s)
2025-03-17 00:17:41  0.67447      107*60  R      n/d 19.7
```
The photometry is based on nearby stars from the USNO-B1.0 catalog (R2 magnitudes) and has not been corrected for the Galactic extinction.
GCN Circular #39762
M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team:
Swift/UVOT observed the field of GRB250316A (GCN 39737) for a total of 3.3 ks in the white filter from 21.8 ks to 29 ks after the Fermi trigger. We detect no optical/UV source at the position of the XRT afterglow (Beardmore et al., GCN Circ. 39758) to an upper limit of white > 22.53. The magnitude is not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.017 in the direction of the burst (Schlegel et al. 1998).

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Jochen Greiner, last update: 25-Mar-2025 [Disclaimer]