GRB 250603A

T. M. Parsotan (GSFC), R. Brivio (INAF-OAB), R. Gupta (NASA GSFC),
M. J. Moss (GSFC), D. M. Palmer (LANL) and C. Salvaggio (INAF-OAB)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 01:10:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250603A (trigger=1320335).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 129.918, -39.268 which is 
   RA(J2000) = 08h 39m 40s
   Dec(J2000) = -39d 16' 05"
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 18 sec with a possible short 
spike at T0+16 sec.  The peak count rate was ~1,300 counts/sec 
(15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 01:12:04.7 UT, 101.7 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. We are waiting for the full dataset to detect and localise the
XRT counterpart.

Although the initial 2.5s of XRT data do not show a clear X-ray
afterglow, the high significance of the BAT image (7.82 sigma)
gives us confidence that this is an astrophysical source.
Further analysis awaits the full downlinked dataset.

We note the presence of Fermi-GBM event at ~T+10 minutes
(GBM trigger: 770606412) with a position consistent (to its
25° error radius) with this source location.

Given the location of the BAT detection at 1.42° from the Galactic
plane, persistent flux over 10 minutes would suggest a Galactic transient.
If so, we would name it Swift J0839.7-3916 .

Burst Advocate for this burst is T. M. Parsotan (tyler.parsotan AT nasa.gov).
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/)

K.L. Page and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT
team:

The XRT began observing the field of GRB 250603A (GCN Circ. 40613
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 ) at
01:12:04.7 UT, 101.7 seconds after the BAT trigger. Using
promptly-collected data (although downlinked after a delay), we find a
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
129.93750, -39.29289 which is equivalent to:
   RA(J2000)  = 08h 39m 45.00s
   Dec(J2000) = -39d 17' 34.4"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 104 arcseconds from the BAT onboard position, within the
BAT error circle.

Given the nature of the fading X-ray light-curve, we confirm this trigger
as GRB 250603A, and not a new Galactic transient.

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 1415 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 250603A, 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 = 129.93745, -39.29287 which is equivalent
to:

RA (J2000): 08h 39m 44.99s
Dec (J2000): -39d 17' 34.3"

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.

S. Campana (INAF-OAB), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB),
D.N. Burrows (PSU), M.A. Williams (PSU), S. Dichiara (PSU), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester) and P.A. Evans report
on behalf of the Swift-XRT team:

We have analysed 7.3 ks of XRT data for GRB 250603A, from 92 s to 34.7
ks after the   trigger. The data comprise 3 s in Windowed Timing (WT)
mode (taken while Swift was slewing), 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=1.17 (+0.09, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.48 (+0.32, -0.30). The
best-fitting absorption column is  1.6 (+0.5, -0.4) x 10^22 cm^-2, in
excess of the Galactic value of 6.9 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 7.6 x 10^-11 (1.1 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.6 (+0.5, -0.4) x 10^22 cm^-2
Galactic foreground: 6.9 x 10^21 cm^-2
Excess significance: 3.3 sigma
Photon index:	     1.48 (+0.32, -0.30)

If the light curve continues to decay with a power-law decay index of
1.17, the count rate at T+24 hours will be 8.8 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.7 x
10^-14 (9.8 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/01320335.

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

S. R. Oates (Lancaster U.) and T. M. Parsotan (GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250603A
105 s after the BAT trigger (Parsotan et al., GCN Circ. 40613
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 ).
No optical afterglow consistent with the enhanced XRT position
(Beardmore et al., GCN Circ. 40616
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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           105          255          147         >20.4
u_FC               318          567          246         >19.7
white              105         1531          382         >21.0
v                  647         5129          190         >19.0
b                  573         1516           97         >19.6
u                  318         1491          324         >19.8
w1                 697         1467           78         >18.7
m2                 672         1442           97         >18.5
w2                 623         5029          294         >19.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 1.89 in the direction of the burst
(Schlegel et al. 1998).