GRB 250905A

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) (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 250905A (trigger #1347265)
(Brivio, et al., GCN Circ. 41714
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 ).  The BAT ground-calculated position is
RA, Dec = 232.943, 0.789 deg which is 
   RA(J2000)  =  15h 31m 46.3s 
   Dec(J2000) = +00d 47' 19.6" 
with an uncertainty of 3.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 74%.
 
The BAT light curve showed a complex structure with multiple peaks. The burst lasted about 70 sec. T90 (15-350 keV) is 48.00 +- 50.60 sec (estimated error including systematics).
 
The time-averaged spectrum from T-15.10 to T+64.89 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.65 +- 0.67.  The fluence in the 15-150 keV band is 4.1 +- 1.4 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T+8.40 sec in the 15-150 keV band
is 0.27 +- 0.16 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/1347265

M.G. Bernardini (INAF-OAB), E. Ambrosi	(INAF-IASFPA) , M. Capalbi
(INAF-OAR), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU),
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 4.7 ks of XRT data for GRB 250905A, from 140 s to 27.9
ks after the   trigger. The data comprise 26 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=5.2 (+0.4, -0.3).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+0.8, -0.6). The
best-fitting absorption column is  1.8 (+3.2, -1.2) x 10^21 cm^-2,
consistent with the Galactic value of 5.5 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 4.1 x 10^-11 (5.2 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.8 (+3.2, -1.2) x 10^21 cm^-2
Galactic foreground: 5.5 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.9 (+0.8, -0.6)

If the light curve continues to decay with a power-law decay index of
5.2, the count rate at T+24 hours will be 2.5 x 10^-14 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x
10^-24 (1.3 x 10^-24) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01347265.

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

S. R. Oates (Lancaster Uni.) and R. Brivio (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250905A
140 s after the BAT trigger (Brivio et al., GCN Circ. 41714
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 ).
No optical afterglow consistent with the XRT position
(Osborne et al., GCN Circ. 41722
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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/3sigUL
white  (FC)     140          290             147           >21.0
   u    (FC)     300           405            103            >19.8
   v              4973       16058          1082            >20.3
   b              5794         5978            181            >20.3
   u              5588       27905          1036            >21.1
uvw1          5383       27038           1082           >20.4
uvm2          5178       16867            989            >20.9

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.054 in the direction of the burst
(Schlegel et al. 1998).

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

Using 233 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 250905A, 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 = 233.00557, +0.79370 which is equivalent
to:

RA (J2000): 15h 32m 01.34s
Dec (J2000): +00d 47' 37.3"

with an uncertainty of 2.5 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.

WeiKang Zheng (UCB) and Alexei V. Filippenko (UCB) report on

behalf of the KAIT GRB team:


The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at

Lick Observatory, observed the field of Swift GRB 250905A (Brivio

et al., GCN 41714
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 ) starting at 03:14 UT, ~64 minutes after the burst.

A set of 34x60s clear (roughly R) filter images were obtained under

twilight condition. We do not detect any new optical counterpart

candidate within the XRT error circles (Brivio et al., GCN 41714
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 ),

down to an upper limit of about 20.0 (Vega) mag at a mid time of

94 minutes after the burst.

R. Brivio (INAF-OAB), S. Dichiara (PSU), D. M. Palmer (LANL),
C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB),
B. Sbarufatti (INAF-OAB) and M. H. Siegel (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 02:09:54 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250905A (trigger=1347265).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 233.047, +0.777 which is 
   RA(J2000) = 15h 32m 11s
   Dec(J2000) = +00d 46' 39"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Due to a telemetry gap, the BAT lightcurve
is not immediately available.

The XRT began observing the field at 02:12:11.4 UT, 136.6 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 233.00629, 0.79395
which is equivalent to:
   RA(J2000)  = 15h 32m 01.51s
   Dec(J2000) = +00d 47' 38.2"
with an uncertainty of 3.4 arcseconds (radius, 90% containment). This
location is 158 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 consistent with the Galactic value of 5.50
x 10^20 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 140 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 none of
the XRT error circle. 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 0.054.

Burst Advocate for this burst is R. Brivio (riccardo.brivio 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/)