GRB 250509A

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

At 22:33:29 UT on 9 May 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250509A (trigger 768522814.285632 / 250509940).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 46.0, Dec = -35.3 (J2000 degrees, equivalent to J2000 03h 04m, -35d 17'), with a statistical uncertainty of 3.0 degrees.

The angle from the Fermi LAT boresight is 89.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250509940/quicklook/glg_skymap_all_bn250509940.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/bn250509940/quicklook/glg_healpix_all_bn250509940.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250509940/quicklook/glg_lc_medres34_bn250509940.gif

M. H. Siegel (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC) and
B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 22:33:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250509A (trigger=1311764). Swift slewed immediately to the burst.
The BAT on-board calculated location is RA, Dec 46.455, -38.853 which is 
   RA(J2000) = 03h 05m 49s
   Dec(J2000) = -38d 51' 11"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi peak
structure with a duration of about 60 sec.  The peak count rate
was ~2800 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 22:34:41.7 UT, 71.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 46.45032, -38.84173 which
is equivalent to:
   RA(J2000)  = 03h 05m 48.08s
   Dec(J2000) = -38d 50' 30.2"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 42 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 in excess of the Galactic value (2.03 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 9.2
(+3.98/-3.41) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 7.21e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of nominal 150.000 seconds with the White
filter starting 80 seconds after the BAT trigger. No credible afterglow
candidate has been found in the initial data products. Data from the 2.7'x2.7'
sub-image are not available at this time. The 8'x8' region for the list of
sources generated on-board covers 100% of the XRT error circle. The list of
sources is typically complete to about 18 mag. No correction has been made for
the expected extinction corresponding to E(B-V) of 0.021.

Burst Advocate for this burst is M. H. Siegel (siegel AT swift.psu.edu).
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-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 250509A ( M. H. Siegel et al., GCN 40407
Loading...
 
 ) errorbox  1296 sec after notice time and 1332 sec after trigger time at 2025-05-09 22:55:43 UT, with upper limit up to  18.4 mag. Observations started at twilight.  The observations began at zenith distance = 76 deg. The sun  altitude  is -14.1 deg. 

The galactic latitude b = -59 deg., longitude l = 244 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2866479

We obtain a following upper limits.  

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

    1423 |         MASTER-OAFA |   C |   180 | 18.2 |        
    1609 |         MASTER-OAFA |   C |   180 | 18.4 |        
Filter C is a clear (unfiltred) band. 


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

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-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB 250509A ( Fermi GBM team, GCN 40406
Loading...
 
 ) errorbox  7 sec after notice time and 64 sec after trigger time at 2025-05-09 22:34:33 UT, with upper limit up to  18.4 mag. Observations started at twilight.  The observations began at zenith distance = 70 deg. The sun  altitude  is -9.7 deg. 

The galactic latitude b = -60 deg., longitude l = 237 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2866536

We obtain a following upper limits.  

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

      70 | 2025-05-09 22:34:33 |         MASTER-OAFA | (03h 18m 00.28s , -38d 23m 19.1s) |   C |    10 | 14.1 |        
     104 | 2025-05-09 22:35:03 |         MASTER-OAFA | (03h 18m 00.24s , -38d 23m 19.1s) |   C |    20 | 14.5 |        
     138 | 2025-05-09 22:35:42 |         MASTER-OAFA | (03h 18m 00.14s , -38d 23m 18.9s) |   C |    10 | 14.4 |        
     174 | 2025-05-09 22:36:18 |         MASTER-OAFA | (03h 18m 00.11s , -38d 23m 18.5s) |   C |    10 | 14.4 |        
     215 | 2025-05-09 22:36:53 |         MASTER-OAFA | (03h 18m 00.10s , -38d 23m 18.6s) |   C |    20 | 14.7 |        
     265 | 2025-05-09 22:37:39 |         MASTER-OAFA | (03h 18m 00.12s , -38d 23m 19.3s) |   C |    30 | 14.8 |        
     865 | 2025-05-09 22:46:44 |         MASTER-OAFA | (03h 18m 00.32s , -38d 23m 28.0s) |   C |   140 | 14.8 |        
    1041 | 2025-05-09 22:49:30 |         MASTER-OAFA | (03h 18m 00.52s , -38d 23m 31.2s) |   C |   160 | 16.3 |        
    1227 | 2025-05-09 22:52:36 |         MASTER-OAFA | (03h 18m 00.73s , -38d 23m 34.3s) |   C |   160 | 16.5 |        
    1414 | 2025-05-09 22:55:42 |         MASTER-OAFA | (03h 18m 01.03s , -38d 23m 38.0s) |   C |   160 | 17.0 |        
    1424 | 2025-05-09 22:55:43 |         MASTER-OAFA | (03h 06m 24.43s , -38d 43m 44.4s) |   C |   180 | 18.2 |        
    1600 | 2025-05-09 22:58:49 |         MASTER-OAFA | (03h 18m 01.35s , -38d 23m 42.3s) |   C |   160 | 17.1 |        
    1610 | 2025-05-09 22:58:49 |         MASTER-OAFA | (03h 06m 24.67s , -38d 43m 47.0s) |   C |   180 | 18.4 |        
    1786 | 2025-05-09 23:01:55 |         MASTER-OAFA | (03h 18m 01.74s , -38d 23m 46.7s) |   C |   160 | 17.1 |        
    1796 | 2025-05-09 23:01:55 |         MASTER-OAFA | (03h 06m 24.96s , -38d 43m 50.0s) |   C |   180 | 18.3 |        
    1972 | 2025-05-09 23:05:01 |         MASTER-OAFA | (03h 18m 02.05s , -38d 23m 50.9s) |   C |   160 | 17.1 |        
    1983 | 2025-05-09 23:05:02 |         MASTER-OAFA | (03h 06m 25.27s , -38d 43m 53.2s) |   C |   180 | 18.2 |        
    2159 | 2025-05-09 23:08:08 |         MASTER-OAFA | (03h 18m 02.30s , -38d 23m 55.2s) |   C |   160 | 17.1 |        
    2169 | 2025-05-09 23:08:08 |         MASTER-OAFA | (03h 06m 25.58s , -38d 43m 56.5s) |   C |   180 | 18.1 |        
    2356 | 2025-05-09 23:11:14 |         MASTER-OAFA | (03h 06m 25.97s , -38d 44m 00.1s) |   C |   180 | 18.0 |        
    2531 | 2025-05-09 23:14:20 |         MASTER-OAFA | (03h 18m 02.98s , -38d 24m 03.8s) |   C |   160 | 17.1 |        
    2542 | 2025-05-09 23:14:20 |         MASTER-OAFA | (03h 06m 26.35s , -38d 44m 03.6s) |   C |   180 | 18.0 |        
    2718 | 2025-05-09 23:17:26 |         MASTER-OAFA | (03h 18m 03.37s , -38d 24m 09.0s) |   C |   160 | 17.2 |        
    2728 | 2025-05-09 23:17:27 |         MASTER-OAFA | (03h 06m 26.66s , -38d 44m 06.7s) |   C |   180 | 18.0 |        
    2904 | 2025-05-09 23:20:33 |         MASTER-OAFA | (03h 18m 03.73s , -38d 24m 14.1s) |   C |   160 | 17.1 |        
    2914 | 2025-05-09 23:20:33 |         MASTER-OAFA | (03h 06m 26.94s , -38d 44m 08.8s) |   C |   180 | 17.6 |        
    3090 | 2025-05-09 23:23:39 |         MASTER-OAFA | (03h 18m 04.07s , -38d 24m 19.9s) |   C |   160 | 16.9 |        
    3100 | 2025-05-09 23:23:39 |         MASTER-OAFA | (03h 06m 27.22s , -38d 44m 12.4s) |   C |   180 | 17.4 |        
Filter C is a clear (unfiltred) band. 


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

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using 426 s of promptly downlinked XRT event data for GRB 250509A, we
find an enhanced XRT position of the afterglow: RA, Dec: 46.44960,
-38.84128 which is equivalent to:
   RA (J2000)  = 03 05 47.90
   Dec (J2000) = -38 50 28.6
with an uncertainty of 1.9 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/1311764.

Position enhancement is described by Goad et al. (2007, A&A, 476, 1401)
and Evans et al. (2009, MNRAS, 397, 1177).

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

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

Using 877 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 250509A, 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 = 46.44965, -38.84201 which is equivalent
to:

RA (J2000): 03h 05m 47.92s
Dec (J2000): -38d 50' 31.2"

with an uncertainty of 2.3 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.

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M.G.
Bernardini (INAF-OAB), E. Ambrosi  (INAF-IASFPA) , M. Capalbi
(INAF-IASFPA), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU)
and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.8 ks of XRT data for GRB 250509A, from 63 s to 28.3
ks after the   trigger. The data comprise 156 s in Windowed Timing (WT)
mode (the first 6 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.20 (+/-0.09). At T+349 s  the decay
flattens to an alpha of -1.1 (+0.7, -0.4) before breaking again at
T+1377 s to a final decay with index alpha=1.44 (+0.10, -0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.02 (+0.11, -0.10). The
best-fitting absorption column is  6.6 (+/-0.7) x 10^21 cm^-2, in
excess of the Galactic value of 2.0 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.13 (+/-0.15) and a
best-fitting absorption column of 6.0 (+1.0, -0.9) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.1 x 10^-11 (7.5 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.0 (+1.0, -0.9) x 10^21 cm^-2
Galactic foreground: 2.0 x 10^20 cm^-2
Excess significance: 10.7 sigma
Photon index:	     2.13 (+/-0.15)

If the light curve continues to decay with a power-law decay index of
1.44, the count rate at T+24 hours will be 9.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.9 x
10^-13 (7.1 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/01311764.

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

A. Myers (NPP/GSFC), C. Meegan (UAH) and A. Holzmann Airasca (UniTrento and INFN Bari) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 22:33:29.29 UT on 09 May 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250509A (trigger 768522814/250509940).
which was also detected by Swift BAT (M. H. Siegel et al. 2025, GCN 40407
Loading...
 
 ).
The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 89 degrees.

The GBM light curve consists of main emission episode with a duration (T90)
of about 69 s (50-300 keV). The time-averaged spectrum
from T0+0.000 to T0+69.120 s is best fit by
a power law function.
The power law index is -2.19 +/- 0.03.

The event fluence (10-1000 keV) in this time interval is
(6.18 +/- 0.29)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+6.2 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:

T. Parsotan (GSFC), 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),
D. M. Palmer (LANL), D. Sadaula (GSFC/UMBC),
T. Sakamoto (AGU), M. H. Siegel (PSU)
(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 250509A (trigger #1311764)
(Siegel, et al., GCN Circ. 40407
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 46.451, -38.854 deg which is 
   RA(J2000)  =  03h 05m 48.3s 
   Dec(J2000) = -38d 51' 15.1" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 96%.
 
The mask-weighted BAT light curve shows a sharp rise and exponential decay profile. 
T90 (15-350 keV) is 55.43 +- 10.47 sec (estimated error including systematics).
 
The time-averaged spectrum from T-2.85 to T+71.46 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.54 +- 0.08.  The fluence in the 15-150 keV band is 2.4 +- 0.1 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+0.42 sec in the 15-150 keV band
is 1.9 +- 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/1311764