GRB 250628A

M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), J. J. DeLaunay (PSU),
R. Gupta (NASA GSFC) and D. M. Palmer (LANL) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 00:55:56.71 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250628A (trigger=1328546). Swift slewed immediately to the burst.
The BAT on-board calculated location is RA, Dec 38.551, -38.081 which is 
   RA(J2000) = 02h 34m 12s
   Dec(J2000) = -38d 04' 50"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed two emission pulses with 
the first brighter pulse starting from T0-20 sec to T0+20 sec and 
the second fainter pulse from T0+25 end T0+60 sec.  The peak count rate
was ~1066 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 00:57:57.7 UT, 121.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 38.55240,
-38.07909 which is equivalent to:
   RA(J2000)  = 02h 34m 12.58s
   Dec(J2000) = -38d 04' 44.7"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 7.9 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.  We cannot determine whether the source
is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (2.30 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.1
(+4.01/-3.17) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 9.47e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 129 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. The typical 3-sigma upper limit has been about 19.6 mag.
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
0.023.

Burst Advocate for this burst is M. Ferro (matteo.ferro AT inaf.it).
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/)

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

Using 1647 s of XRT Photon Counting mode data and 4 UVOT
images for GRB 250628A, 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 = 38.55311, -38.07894 which is equivalent
to:

RA (J2000): 02h 34m 12.75s
Dec (J2000): -38d 04' 44.2"

with an uncertainty of 2.4 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.

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 250628A ( M. Ferro et al., GCN 40853
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 ) errorbox  17683 sec after notice time and 17722 sec after trigger time at 2025-06-28 05:51:18 UT, with upper limit up to  16.0 mag. The observations began at zenith distance = 81 deg. The sun  altitude  is -72.0 deg. 

The galactic latitude b = -66 deg., longitude l = 246 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2918453

We obtain a following upper limits.  

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

   17812 |         MASTER-OAFA |   C |   180 | 16.0 |        
Filter C is a clear (unfiltred) band. 


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

A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M. Perri
(SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti
(INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M.A. Williams (PSU)
and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 8.2 ks of XRT data for GRB 250628A, from 127 s to 27.6
ks after the   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 series of power-law decays. The
initial decay index is alpha=5.0 (+0.6, -0.5). At T+358 s  the decay
flattens to an alpha of -0.01 (+0.11, -0.13) before breaking again at
T+14.4 ks to a final decay with index alpha=2.6 (+/-0.6).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.92 (+0.17, -0.16). The
best-fitting absorption column is  3.2 (+0.8, -0.7) x 10^21 cm^-2, in
excess of the Galactic value of 2.3 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 4.0 x 10^-11 (5.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.2 (+0.8, -0.7) x 10^21 cm^-2
Galactic foreground: 2.3 x 10^20 cm^-2
Excess significance: 6.8 sigma
Photon index:	     1.92 (+0.17, -0.16)

If the light curve continues to decay with a power-law decay index of
2.6, the count rate at T+24 hours will be 1.6 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.5 x
10^-14 (9.3 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01328546.

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

Paul Kuin (MSSL/UCL) and M Ferro (INAF-OAB) report on behalf of
the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of
GRB 250628A  130 s after the BAT trigger (Ferro et al.,
GCN Circ. 40853
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 ).
No optical afterglow consistent with the enhanced XRT position
(Evans et al. GCN Circ. 40854
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 ) is detected in the initial UVOT
exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           130          280          147         >20.8
u_FC               342          591          246         >19.3
white              130          641          167         >20.8
v                  671          691           19         >17.8
b                  597          617           19         >18.5
u                  342          591          246         >19.3
w1                 720          740           19         >18.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.023 in the direction of the burst
(Schlegel et al. 1998).

R. Brivio (INAF-OABr), A. Martin-Carrillo (UCD), G. Corcoran (UCD), L. Izzo (INAF/OAC), A. Rossi (INAF/OAS), D. B. Malesani (DAWN/NBI and Radboud), N. R. Tanvir (Leicester) report on behalf of the Stargate collaboration:

We observed the field of GRB 250628A detected by Swift/BAT (Ferro et al., GCN 40853
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 ) using the ESO VLT UT3 (Melipal) equipped with the X-shooter acquisition camera.

A short sequence of imaging was secured in the r (3x30 s), g (3x40 s), and z (3x60 s) bands. No afterglow is detected within the Swift/XRT enhanced error region (Evans et al., GCN 40854
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 ).
From preliminary analysis, we measure the following 3sigma upper limits, calibrated against nearby stars from the Legacy Survey (DR9):

g > 23.7 (AB) on Jun 28 at 08:51:48 UT (7.93 hr after the GRB trigger),
r > 23.6 (AB) on Jun 28 at 08:45:04 UT (7.82 hr after the GRB trigger),
z > 22.8 (AB) on Jun 28 at 08:48:21 UT (7.87 hr after the GRB trigger).

We note the detection of significant excess absorption in the X-ray spectrum (Evans, GCN 40857
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 ; https://www.swift.ac.uk/xrt_spectra/01328546/). This argues against a high-redshift origin for GRB 250628A and likely indicates some dust extinction.

We acknowledge excellent support from the ESO observing staff in Paranal, in particular Boris Häußler.

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 250628A detected by Swift/BAT (Ferro et al., GCN 40853
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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, and H bands, started on 2025 June 28 at 06:17:20 UT (i.e. 5.4 hr after the burst), and lasted for about 1 hour.
From preliminary inspection, we do not detect any possible counterpart within the Swift/XRT error region (Evans et al., GCN 40854
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 ) down to the following 3sigma limits:

r > 20.1 (AB; calibrated against the SkyMapper catalogue),
at a mid-time of 5.9 hr after the trigger;

H > 14.8 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 5.4 hr after the trigger.