GRB 250809A

D. Sadaula (GSFC/UMBC), 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), T. Parsotan (GSFC),
T. Sakamoto (AGU) (i.e. the Swift-BAT team):
 
Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250809A (trigger #1340954)
(Parsotan, et al., GCN Circ. 41303
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 ).  The BAT ground-calculated position is
RA, Dec = 233.034, -53.314 deg which is 
   RA(J2000)  =  15h 32m 08.2s 
   Dec(J2000) = -53d 18' 50.6" 
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 96%.
 
The BAT light curve showed a complex structure with a duration of about 32 sec.
T90 (15-350 keV) is 17.79 +- 2.77 sec (estimated error including systematics).
 
The time-averaged spectrum from T+1.01 to T+20.94 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.83 +- 0.20.  The fluence in the 15-150 keV band is 3.9 +- 0.5 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T+11.58 sec in the 15-150 keV band
is 0.6 +- 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/1340954

N.P.M. Kuin (UCL/MSSL) and T. M. Parsotan (GSFC) report on behalf of the
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250809A
128 s after the BAT trigger (Parsotan et al., GCN Circ. 41303
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 ). A source
consistent with the XRT position ( Evans et al. GCN Circ. 41304
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  ) is
detected in the initial UVOT exposures.

Preliminary detections and 3-sigma upper limits using the UVOT photometric
system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early
exposures are:

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

v                  435          755           58         17.62 +/- 0.21
b                  384         1038           97         18.86 +/- 0.19
u                  128         1013          452         19.57 +/- 0.21
w1                 484          805           58        >18.4
m2                 459          780           58        >20.1
w2                 410         1064           78        >18.8

The earliest exposures were fainter; these detections are near the peak of
the light curve.

The magnitudes in the table are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 1.919 in the direction of the burst (Schlegel
et al. 1998).

K.L. Page (U. Leicester) reports on behalf of the Swift-XRT team:

GCN Circ. 41313
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  provided an incomplete automatic modelling of the XRT
light-curve of GRB 250809A. The first snapshot of data is dominated by a
large flare, peaking at 240 s after the BAT trigger. The underlying decay
can be modelled as a power-law with alpha = 3.0 +/- 0.2.

If the light curve continues to decay with a power-law decay index of
3.0, the count rate at T+24 hours will be 2.3 x 10^-8 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.3 x
10^-20 (1.5 x 10^-19) erg cm^-2 s^-1.

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

J.P. Osborne (U. Leicester), A. D'Ai (INAF-IASFPA), S. Campana
(INAF-OAB), A. Melandri (INAF-OAR), M.A. Williams (PSU), S. Dichiara
(PSU), J.A. Kennea (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 6.7 ks of XRT data for GRB 250809A, from 113 s to 51.4
ks after the   trigger. The data comprise 910 s in Windowed Timing (WT)
mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. The best available XRT position
 (using the promptly downlinked event data, the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec =
233.00043, -53.29626 which is equivalent to:

RA (J2000): 15h 32m 00.10s
Dec(J2000): -53d 17' 46.5"

with an uncertainty of 3.0 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial rise, with a power-law
index of alpha=-1.5 (NaN, NaN), followed by a break at T+281 s	to an
alpha of 8.0 (NaN, NaN).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.41 (+/-0.13). The
best-fitting absorption column is  2.05 (+0.18, -0.17) x 10^22 cm^-2,
in excess of the Galactic value of 8.0 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 4.9 x 10^-11 (1.6 x 10^-10) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     2.05 (+0.18, -0.17) x 10^22 cm^-2
Galactic foreground: 8.0 x 10^21 cm^-2
Excess significance: 11.9 sigma
Photon index:	     2.41 (+/-0.13)

If the light curve continues to decay with a power-law decay index of
2.96, the count rate at T+24 hours will be 3.7 x 10^-8 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x
10^-18 (6.1 x 10^-18) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01340954.

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

R. Strausbaugh (Eastern Illinois University), A. Cucchiara (NASA) report on behalf of a larger collaboration:

We observed the Swift GRB 250809A field (Parsotan et al., GCN 41303
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 )  with the LCOGT 1-meter Sinistro instrument at the Cerro Tololo Interamerican Observatory, Chile site, on August 9, from 23:10 to 23:42 UT (corresponding to 6.28 to 6.85 hours after the GRB trigger time) with the Bessel R and I filters.

We performed a series of 3x300s exposures in I-band and R-band.  We do not detect an uncatalogued source within the Prompt Swift XRT error region (Evans et al., GCN 41304
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 ) in either band.

The following 3-sigma upper limits are calculated using the USNO-B1.0 catalog as reference:

R > 20.6
I > 20.3

These magnitudes are not corrected for galactic extinction.

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

Using 923 s of promptly downlinked XRT event data for GRB 250809A, we
find an enhanced XRT position of the afterglow: RA, Dec: 233.00043,
-53.29626 which is equivalent to:
   RA (J2000)  = 15 32 00.10
   Dec (J2000) = -53 17 46.5
with an uncertainty of 3.0 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/1340954.

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.

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

At 16:53:19 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250809A (trigger=1340954).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 233.015, -53.289 which is 
   RA(J2000) = 15h 32m 04s
   Dec(J2000) = -53d 17' 18"
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 40 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~11 sec after the trigger.

The XRT began observing the field at 16:55:25.0 UT, 125.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 233.00282, -53.29640 which
is equivalent to:
   RA(J2000)  = 15h 32m 00.68s
   Dec(J2000) = -53d 17' 47.0"
with an uncertainty of 4.3 arcseconds (radius, 90% containment). This
location is 37 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 8.04
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 5.16e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
128 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. Because of the density of catalogued stars, further analysis
is required to report an upper limit for any afterglow in the sub-image. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. Because of the density of catalogued stars, further analysis is
required to report an upper limit for any afterglow in the region. No
correction has been made for the large, but uncertain, extinction expected.

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/)