GRB 250504.98, GRB 250504A

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) was pointed to the Swift GRB250504.98 (trigger No 1310284,17h 58m 26.88s , -40d 20m 56.4s, R=0.05) errorbox  14 sec after notice time and 40 sec after trigger time at 2025-05-04 23:26:38 UT, with upper limit up to  16.7 mag. The observations began at zenith distance = 29 deg. The sun  altitude  is -70.3 deg. 

The galactic latitude b = -9 deg., longitude l = 352 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2860822

We obtain a following upper limits.  

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

      46 |         MASTER-SAAO |   C |    10 | 16.7 |        
      75 |         MASTER-SAAO |   C |    10 | 16.7 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

The Fermi GBM team reports the detection of a likely LONG GRB

At 23:25:54 UT on 4 May 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250504A (trigger 768093959.78286 / 250504976).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 265.4, Dec = -40.9 (J2000 degrees, equivalent to J2000 17h 41m, -40d 53'), with a statistical uncertainty of 2.1 degrees.

The angle from the Fermi LAT boresight is 113.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250504976/quicklook/glg_skymap_all_bn250504976.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/bn250504976/quicklook/glg_healpix_all_bn250504976.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250504976/quicklook/glg_lc_medres34_bn250504976.gif

M. J. Moss (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),
J. A. Kennea (PSU), D. M. Palmer (LANL) and M. H. Siegel (PSU) report
on behalf of the Neil Gehrels Swift Observatory Team:

At 23:25:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250504A (trigger=1310284).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 269.612, -40.349 which is 
   RA(J2000) = 17h 58m 27s
   Dec(J2000) = -40d 20' 57"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 60 sec.  The peak count rate
was ~4000 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 23:27:15.3 UT, 77.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 269.61691,
-40.36642 which is equivalent to:
   RA(J2000)  = 17h 58m 28.06s
   Dec(J2000) = -40d 21' 59.1"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 64 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.12
x 10^21 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
137 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the
XRT error circle. Because of the density of catalogued stars, further analysis
is required to report an upper limit for any afterglow in the sub-image. The
8'x8' region for the list of sources generated on-board covers 0.00% of the XRT
error circle. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.191.

Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)

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 250504A ( Fermi GBM team, GCN 40342
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 ) errorbox  1 sec after notice time and 43 sec after trigger time at 2025-05-04 23:26:38 UT, with upper limit up to  17.3 mag. The observations began at zenith distance = 29 deg. The sun  altitude  is -70.3 deg. 

The galactic latitude b = -6 deg., longitude l = 350 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2860892

We obtain a following upper limits.  

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

      49 | 2025-05-04 23:26:38 |         MASTER-SAAO | (17h 56m 41.66s , -40d 35m 44.3s) |   C |    10 | 16.7 |        
      77 | 2025-05-04 23:27:07 |         MASTER-SAAO | (17h 56m 41.67s , -40d 35m 44.8s) |   C |    10 | 16.7 |        
     111 | 2025-05-04 23:27:36 |         MASTER-SAAO | (17h 56m 41.71s , -40d 35m 45.1s) |   C |    20 | 17.0 |        
     151 | 2025-05-04 23:28:15 |         MASTER-SAAO | (17h 56m 41.72s , -40d 35m 45.3s) |   C |    20 | 17.0 |        
     249 | 2025-05-04 23:29:43 |         MASTER-SAAO | (17h 56m 41.65s , -40d 35m 44.1s) |   C |    40 | 17.2 |        
     313 | 2025-05-04 23:30:42 |         MASTER-SAAO | (17h 56m 41.63s , -40d 35m 43.6s) |   C |    50 | 17.2 |        
     387 | 2025-05-04 23:31:51 |         MASTER-SAAO | (17h 56m 41.55s , -40d 35m 43.1s) |   C |    60 | 17.3 |        
     466 | 2025-05-04 23:33:10 |         MASTER-SAAO | (17h 56m 41.50s , -40d 35m 42.3s) |   C |    60 | 17.3 |        
     545 | 2025-05-04 23:34:29 |         MASTER-SAAO | (17h 56m 41.40s , -40d 35m 41.4s) |   C |    60 | 17.3 |        
     624 | 2025-05-04 23:35:48 |         MASTER-SAAO | (17h 56m 41.29s , -40d 35m 40.5s) |   C |    60 | 17.3 |        
     703 | 2025-05-04 23:37:08 |         MASTER-SAAO | (17h 56m 41.19s , -40d 35m 39.7s) |   C |    60 | 17.3 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 940 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 250504A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 269.61806, -40.36676 which is equivalent
to:

RA (J2000): 17h 58m 28.34s
Dec (J2000): -40d 22' 00.3"

with an uncertainty of 4.0 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

S. Bala (USRA) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:


"At 23:25:54.78 UT on 04 May 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250504A (trigger 768093959/250504976),
which was also detected by Swift BAT (M. J. Moss et al. 2025, GCN 40343
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 ).
The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 111 degrees.

The GBM light curve consists of multiple peaks with a duration (T90)
of about 49 s (50-300 keV). The time-averaged spectrum
from T0+0.003 to T0+59.393 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.06 +/- 0.08 and the cutoff energy,
parameterized as Epeak, is 621 +/- 130 keV.

The event fluence (10-1000 keV) in this time interval is
(1.20 +/- 0.06)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+10 s in the 10-1000 keV band is 3.6 +/- 0.3 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/"

S. de Wet (DTU Space), P.M. Vreeswijk (Radboud) and P.J. Groot (Radboud/UCT/SAAO) report on behalf of the MeerLICHT consortium:

The 0.6 m wide-field MeerLICHT optical telescope located in Sutherland, South Africa, obtained a repeating series of 60 s exposures in the q,u,g,r,i,z bands of GRB 250504A following the Swift detection (Moss et al., GCN 40343
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 ). Observations started at 23:29:27 UT on 2025 May 5 (210 seconds post-trigger) and continued for a further 3 hours, following the filter sequence quqgqrqiqz. 

We find a new transient candidate in our first q-band exposure at the following coordinates:
   
RA (J2000) = 17:58:28.38 (269.61827d)
Dec (J2000) = -40:21:59.58 (-40.36655d)

calibrated against Gaia DR2. The astrometric uncertainty is ~0.3" in each coordinate. This position is outside the initial XRT error circle reported by Moss et al. (GCN 40343
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 ) but within the updated XRT error circle reported by Osborne et al. (GCN 40345
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 ). We regard this object as the optical afterglow to GRB250504A. We report the following AB magnitudes and non detections from our first exposures:

q = 19.32 +/- 0.05 at 23:29:59 UT
u > 19.34 (3sigma) at 23:31:20 UT
g = 19.69 +/- 0.10 at 23:34:04 UT
r = 19.52 +/- 0.11 at 23:36:48 UT
i = 19.32 +/- 0.13 at 23:39:31 UT
z = 18.85 +/- 0.20 at 23:42:14 UT

MeerLICHT is built and run by a consortium consisting of Radboud University, University of Cape Town, the South African Astronomical Observatory, the University of Oxford, the University of Manchester and the University of Amsterdam. 

R. Brivio, M. Ferro, P. D'Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:

We observed the field of GRB 250504A detected by Fermi/GBM (Fermi GBM team, GCN 40342
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 ) and Swift/BAT (Moss et al., GCN 40343
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 ) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, J, H, and K bands, started on 2025 May 05 at 01:09:20 UT (i.e. 1.7 hr after the burst), and lasted for about 1 hour.

From preliminary inspection, we do not detect any counterpart at the position of the optical afterglow (de Wet et al., GCN. 40347) down to the following 3sigma limits:

r > 19.3 (AB; calibrated against the SkyMapper catalogue),
at a mid-time of 2.2 hours after the trigger;

H > 15.7 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 2.1 hours after the trigger.

L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Z. H. Yao, Y. N. Ma, D.H. Zhao, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC) report on behalf of the SVOM/VT Instrument Center:

SVOM/VT conducted ToO follow-up observations of GRB 250504A detected by Fermi (Fermi/GBM team, GCN 40342
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 ; Bala et al., GCN 40346
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 ) and Swift (Moss et al., GCN 40343
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 ). The observation started on 2025-05-05T01:11:01 UT, 2025 May 05, ~1.75 hr after the burst.

Our preliminary analysis shows no detection of an optical counterpart in VT_B-band stacked images (3975 s total effective exposure) at both the de Wet et al. (GCN 40347
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 ) position and the enhanced XRT position (4.0 arcsec error radius; Osborne et al. GCN 40345
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 ), with a 3 sigma upper limit of VT_B > 22.0 mag (AB) at 2.74 hr post-burst.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC),CAS.

A. Saccardi (CEA/Irfu), M. Garnichey (LUX-Paris Obs.), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), A. Martin-Carrillo (UCD), B. Schneider (LAM), N. R. Tanvir (Leicester), A. de Ugarte Postigo (LAM) report on behalf of the Stargate collaboration:

We observed the field of GRB 250504A detected by Fermi/GBM (Fermi GBM team, GCN 40342
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 ; Bala et al., GCN 40346
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 ) and Swift/BAT (Moss et al., GCN 40343
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 ) using the ESO VLT UT3 (Melipal) with the X-shooter acquisition camera.

A short sequence of imaging was secured in the r (3x30 s), g (3x60 s), and z (3x40 s) bands. No afterglow is detected consistent with the optical position reported by de Wet et al. (GCN 40347
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 ) with the following 5 sigma upper limit calibrated against nearby SkyMapper stars:

r > 23.4 (AB) on 2025 May 05 at 03:21:58 UT (3.93 hr after the burst).

We acknowledge excellent support from the ESO observing staff in Paranal, in particular Abigail Frost, Florian Rodler, Marco Berton, and Rodrigo Palominos.

J.A. Kennea (PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester),
J.P. Osborne (U. Leicester), A. Melandri (INAF-OAR), T. Sbarrato
(INAF-OAB), P. D'Avanzo (INAF-OAB), M. A. Williams (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 6.9 ks of XRT data for GRB 250504A, from 81 s to 56.7
ks after the  BAT trigger. The data comprise 47 s in Windowed Timing
(WT) mode with the remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.26 (+0.05, -0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.60 (+0.24, -0.22). The
best-fitting absorption column is  2.9 (+1.4, -0.7) x 10^21 cm^-2,
consistent with the Galactic value of 2.1 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 4.8 x 10^-11 (6.0 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.9 (+1.4, -0.7) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.60 (+0.24, -0.22)

If the light curve continues to decay with a power-law decay index of
1.26, the count rate at T+24 hours will be 1.7 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.3 x
10^-14 (1.0 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01310284.

This circular is an official product of the Swift-XRT team.

D. M. Palmer (LANL), S. D. Barthelmy (GSFC), 
R. Gupta (GSFC), H. A. Krimm (NSF),
S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa),
C. B. Markwardt (GSFC), M. J. Moss (GSFC),
T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC),
T. Sakamoto (AGU) (i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250504A (trigger #1310284)
(Moss, et al., GCN Circ. 40343
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 ).  The BAT ground-calculated position is
RA, Dec = 269.605, -40.345 deg which is 
   RA(J2000)  =  17h 58m 25.3s 
   Dec(J2000) = -40d 20' 41.8" 
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 98%.
 
The mask-weighted light curve shows a complex light curve with several pulses.
T90 (15-350 keV) is 52.39 +- 2.31 sec (estimated error including systematics).
 
The time-averaged spectrum from T-2.96 to T+55.10 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.27 +- 0.08.  The fluence in the 15-150 keV band is 2.3 +- 0.1 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+7.70 sec in the 15-150 keV band
is 1.6 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 
 
The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1310284

P. McDermott, D. Murphy, C. McKenna, C. de Barra, A. Ulyanov, G. Finneran, G. Corcoran, L. Cotter, A. Empey, J. Fisher, F. Gibson Kiely, J. Thompson, D. McKeown, A. Martin-Carrillo, L. Hanlon, S. McBreen, on behalf of the EIRSAT-1 team: 

EIRSAT-1 reports the detection of the long gamma-ray burst GRB 250504A by the Gamma-ray Module (GMOD) instrument, which was also detected by Fermi GBM (GCN [40342](https://gcn.nasa.gov/circulars/40342
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 ) and [40346](https://gcn.nasa.gov/circulars/40346
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 )) and Swift-BAT (GCN [40343](https://gcn.nasa.gov/circulars/40343
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 )).  The GMOD detection was made starting at 2025-05-04 23:25:56.5 UTC.

The GMOD light curve for GRB 250405A, with 1.2s binning, shows multiple peaks consistent with the detections by Fermi GBM and Swift-BAT.

The spacecraft location at time of detection was  9.886º S, 164.567° E and an altitude of 386 km.

The light curve for this event as measured by GMOD can be found here:
https://grb.eirsat1.ie/250504A/250504A_LC_onboard_preliminary.png

EIRSAT-1 is Ireland’s first satellite (Doyle et al. Proceedings of the 4th SSEA, 2022). It is a 2U CubeSat and carries onboard a number of experiments including the Gamma-Ray Module (GMOD), a novel, compact, gamma-ray detector (Murphy et al, Experimental Astronomy, 53, 961–990, 2022). GMOD consists of a 25 mm × 25 mm × 40 mm Cerium Bromide scintillator coupled to SiPMs and is designed to detect gamma-ray bursts in the ~ 60 keV - 1.5 MeV range. EIRSAT-1 was developed in University College Dublin with support from ESA’s Fly Your Satellite! programme and was launched on 1st December 2023.