GRB 060923B

M. Stamatikos (NASA/ORAU), S. D. Barthelmy (GSFC),
D. N. Burrows (PSU), M. Capalbi (ASDC), M.L. Conciatore (ASDC),
N. Gehrels (NASA/GSFC), C. Guidorzi (Univ Bicocca&INAF-OAB),
S. T. Holland (GSFC/USRA), J. A. Kennea (PSU),
C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL), M. Perri (ASDC),
T. Sakamoto (NASA/ORAU), D. E. Vanden Berk (PSU) and
H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team:

At 11:38:06 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 060923B (trigger=230702).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA,Dec 238.201, -30.907 {15h 52m 48s, -30d 54' 25"} (J2000)
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 10 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~4 sec after the trigger. 

The XRT began observing the field at 11:40:00 UT, 114 seconds after
the BAT trigger. XRT centroided on a cosmic ray on-board, so the
position sent out in our first, automated XRT Position Notice is
incorrect. From preliminary ground analysis of photon-counting mode
data, XRT did find an uncatalogued X-ray source located at RA(J2000)
= 15h 52m 46.83s, Dec(J2000) = -30d 54' 12.6", with an estimated
uncertainty of 6.5 arcseconds (90% confidence radius). This position
was sent out via BACODINE in a second XRT Position Notice with the
correct position. This location is 21 arcseconds from the BAT
on-board position, within the BAT error circle. The initial flux in
the 2.5s image was 2.4e-11 erg/cm2/s (0.2-10 keV). 

UVOT took a finding chart exposure of nominal 200 seconds with the
White (160-650 nm) filter starting 122 seconds after the BAT trigger. 
No afterglow candidate has been found in the initial data products. 
Image catalog data are not available at this time. 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. The overlap of the sub-image and the XRT error circle is 100%. 
No source is seen in the XRT error circle to a magnitude of
approximately 18.5.  No correction has been made for the expected
extinction corresponding to E(B-V) of 0.15.

L. Barbier (GSFC), S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC),
E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (BYU-Idaho),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL),
A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
on behalf of the Swift-BAT team:
 
Using the data set from T-240 to T+754 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 060923B (trigger #230702)
(Stamitikos, et al., GCN Circ. 5590
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 ).  The BAT ground-calculated position
is RA,Dec = 238.193, -30.907 deg {15h 52m 46.3s, -30d 54' 25.8"} (J2000)
+- 1.6 arcmin, (radius, sys+stat, 90% containment).  The partial coding was 36%.
 
The mask-weighted lightcurve has a single peak starting at T-1 sec
and ending at T+10 sec.  T90 (15-350 keV) is 8.8 +- 0.1 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-0.1 to T+9.9 is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum
is 2.53 +- 0.25.  The fluence in the 15-150 keV band is 4.8 +- 0.6 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+4.93 sec in the 15-150 keV band
is 1.5 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90%
confidence level.

M. Capalbi, M. Perri , M.L. Conciatore (ASDC), D.N. Burrows (PSU) and M. 
Stamatikos (NASA/ORAU) report on behalf of the Swift XRT team:


We have analysed the first 2 orbits of XRT data from GRB 060923B 
(Stamatikos et al., GCN Circ. 5590
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 ) with a total exposure of 2.1ks in 
photon counting mode. The refined position is:

RA(J2000)  =  15h 52m 46.83s,
Dec(J2000) = -30d 54m 11.9s

with an uncertainty of 6 arcsec (90% containment). This position is 0.7 
arcsec from the one calculated from early downlinked data (Stamatikos et 
al., GCN Circ. 5590
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 ).

The 0.3-10.0 keV X-ray light curve can be fit with a single power-law 
with a slope of -0.7+/-0.1.

A power-law fit to the X-ray spectrum from T + 125s to T + 365s gives a 
photon index of 1.7+/-0.5, fixing the hydrogen column density to the 
galactic value in the direction of the burst (8.8e20cm**-2). The 
0.3-10.0 keV observed flux is 8.5e-12 ergs cm**-2 s**-1, which 
corresponds to an unabsorbed flux of 8.9e-12 ergs cm**-2 s**-1.

If the burst continues decaying at the current rate we estimate an XRT 
count rate of 0.014 counts/s at T + 24hr, which corresponds to an 
observed 0.3-10.keV flux of  5.6e-13 ergs cm**-2 s**-1.

This circular is an official product of the Swift XRT Team.

S. Holland and A. Cucchiara report on behalf of the
Swift/UVOT team:

The Swift UVOT began observing GRB 060923B (trigger #230702),
M. Stamatikos et al., GCN Circ. 5590
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 ) 122 seconds after the
BAT trigger. No optical afterglow is detected in the XRT
refined error circle (M. Capalbi et al., GCN Circ.5602
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 ).

The 3-sigma limiting magnitudes for the finding chart and the
coadded images for the UVOT filters are listed below:

Finding chart:

Filter        T_range (s)    Exposure (s)    3sigma UL
White         122-220         98              19.42
White         5163-5360       197             19.79

V             225-387         162             18.52

Coadded images:

Filter        T_range (s)    Exposure (s)    3sigma UL
V             5572-5768       196             18.61
B             4959-5156       197             19.61
U             4754-4950       197             19.28
W1            4550-6170       386             19.45
M2            4345-5974       394             19.70
W2            5368-5565       197             19.92

T_range is calculated from the time of the burst. No correction has
been made for the Galactic reddening of E(B-V) = 0.15 mag
(Schlegel et al. 1998) along the line of sight to the burst.