GRB 250605A

R. Gupta (NASA GSFC), J. J. DeLaunay (PSU), S. Dichiara (PSU),
C. Gronwall (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII),
H. A. Krimm (NSF), M. J. Moss (GSFC), K. L. Page (U Leicester),
T. M. Parsotan (GSFC), M. H. Siegel (PSU) and M. A. Williams (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 18:52:31.27 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250605A (trigger=1321323).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 166.977, -19.795 which is 
   RA(J2000) = 11h 07m 54s
   Dec(J2000) = -19d 47' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a complex light curve 
with one strong pulse between T0-2 sec to T0+10 sec and a second dimmer but 
broad pulse between T0+40 sec to T0+100 sec.  The peak count rate was 4,000
 counts/sec (15-350 keV), at ~2 sec after the trigger.

The XRT began observing the field at 18:54:09.2 UT, 98.0 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 166.9571, -19.7882 which is equivalent to:
   RA(J2000)  = 11h 07m 49.70s
   Dec(J2000) = -19d 47' 17.5"
with an uncertainty of 5.5 arcseconds (radius, 90% containment). This
location is 71 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy.

UVOT took a finding chart exposure of 144 seconds with the White filter
starting 106 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
0.039.

Burst Advocate for this burst is R. Gupta (rahulbhu.c157 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/)

B. P. Gompertz (U. Birmingham), Dimple (U. Birmingham), A. Martin-Carrillo (UCD), G. Corcoran (UCD) and D. B. Malesani (DAWN/NBI and Radboud), report on behalf of a larger collaboration:

We initiated follow-up observations of GRB 250605A (Gupta et al., GCN 40633
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 ) with the IO:O camera on the 2m robotic Liverpool Telescope (LT). Observations began at 20:48:52.89 UT, ~2 hours after the Swift/BAT trigger, and consisted of 10x60 s exposures in the SDSS i filter. Observations were taken close to twilight, at high airmass, and the target was 28 deg from the moon at 77% illumination.

Within the Swift/XRT error box, we detect a possible optical counterpart, not present in archival imaging, with an AB magnitude of i ~ 21.1 at RA = 11:07:49.7, Dec = -19:47:15.4. However, this detection is only significant at the 2-sigma level and further, deeper observations are required to confirm its reality. Formally, we measure a 3-sigma upper limit of i > 20.6 within the XRT localisation region.

We also note the presence of a faint source visible in the Legacy Survey archival imaging with an AB magnitude of i = 23.54. The source position is consistent with the possible optical counterpart, and may be the host galaxy of GRB 250605A.

Magnitudes are calibrated against nearby Pan-STARRS stars and not corrected for galactic extinction.

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

Using 3740 s of XRT Photon Counting mode data and 7 UVOT
images for GRB 250605A, 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 = 166.95715, -19.78841 which is equivalent
to:

RA (J2000): 11h 07m 49.72s
Dec (J2000): -19d 47' 18.3"

with an uncertainty of 2.0 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.

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

We have analysed 7.0 ks of XRT data for GRB 250605A, from 90 s to 50.2
ks after the   trigger. The data comprise 149 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=4.36 (+0.19, -0.15). At T+164 s  the decay
flattens to an alpha of 3.19 (+0.29, -0.32) before breaking again at
T+326 s to a final decay with index alpha=0.79 (+/-0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.12 (+0.06, -0.05). The
best-fitting absorption column is  2.59 (+0.20, -0.19) x 10^21 cm^-2,
in excess of the Galactic value of 4.3 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.10 (+0.13, -0.12)
and a best-fitting absorption column of 2.9 (+/-0.5) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (5.5 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.9 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 4.3 x 10^20 cm^-2
Excess significance: 8.7 sigma
Photon index:	     2.10 (+0.13, -0.12)

If the light curve continues to decay with a power-law decay index of
0.79, the count rate at T+24 hours will be 0.085 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x
10^-12 (4.7 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01321323.

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

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

SVOM/VT conducted ToO follow-up observations of the GRB 250605A (Gupta et al., GCN 40633
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 , Melandri et al., GCN 40644
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 ). The observation started on 2025-06-06T05:02:39 UT in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 

No uncatalogued sources are detected in single or stacked images within the Swift/XRT error box (Evans et al., GCN 40642
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 ) and the possible optical candidate reported by Gompertz et al. (GCN 40634
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 ). The 3 sigma limit magnitudes are derived as below:

Mid time    | Band | Exposure Time | 3 sigma limit magnitude
11.28 hour    VT_B      59*70 sec         >23.6 mag    
11.28 hour    VT_R      59*70 sec         >23.6 mag


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.

M. H. Siegel (PSU) and R. Gupta (NASA GSFC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250605A 107 s after the BAT trigger (Gupta et al., GCN Circ. 40633
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 ). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 40642
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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           107          251          141            >20.9
white             5225        11166          933            >21.41
v                16403        17310          885            >20.07
b                 5020         5220          196            >19.83
u                 4815        22332          204            >19.32
uvw1             21418        22318          885            >20.16
uvm2             17315        18088          761            >20.44
uvw2             11173        11858          674            >20.47

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.039 in the direction of the burst
(Schlegel et al. 1998).

R. Gupta (GSFC), S. D. Barthelmy (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),
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 250605A (trigger #1321323)
(Gupta, et al., GCN Circ. 40633
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 ).  The BAT ground-calculated position is
RA, Dec = 166.963, -19.786 deg which is 
   RA(J2000)  =  11h 07m 51.1s 
   Dec(J2000) = -19d 47' 10.5" 
with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 35%.
 
The BAT light curve shows a complex light curve with multiple emission episodes:
a period of very dim extended emission between ~T0-100 sec to T0-10 leading up to  
one strong pulse between T0-10 sec to T0+10 sec and followed by a dim broad pulse 
between T0+40 sec to ~T0+125 sec. T90 (15-350 keV) is 176.05 +- 5.15 sec (estimated 
error including systematics).
 
The time-averaged spectrum from T-100.30 to T+126.69 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.50 +- 0.22, 
and Epeak of 76.7 +- 27.6 keV (chi squared 71.54 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.2 +- 0.0 x 10^-05 erg/cm2
and the 1-sec peak flux measured from T-0.00 sec in the 15-150 keV band is
6.1 +- 0.4 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.83 +- 0.05 (chi squared 79.05 for 57 d.o.f.).  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/1321323