GRB 060923

M. Stamatikos (NASA/ORAU), 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),
D. M. Palmer (LANL), M. Perri (ASDC), T. Sakamoto (NASA/ORAU) and
D. E. Vanden Berk (PSU) report on behalf of the Swift Team:

At 05:12:15 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 060923 (trigger=230662).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA,Dec 254.623, +12.379 {16h 58m 30s, +12d 22' 45"} (J2000)
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed at least two peaks,
lasting 12 seconds.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began taking data at 05:13:36 UT, 81 seconds after the BAT
trigger.  The XRT found a previously uncatalogued X-ray source in 
the field of view at the following coordinates:
   RA(J2000) =  16 58 28.27
   Dec(J2000)=  +12 21 43.3
with an uncertainty of 6.9 arcseconds radius (90% containment). 
This position lies 67 arcseconds from the center of the BAT error circle. 
The approximate initial flux is 1.7e-11 erg/cm2/sec (0.2-10 keV). 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 85 seconds after the BAT trigger. No
afterglow candidate has been found in the initial data products. The
overlap of the sub-image and the XRT error circle is 100%. The
overlap of the 8'x8' region for the list of sources generated
on-board and the XRT error circle is 100%.  The limiting magnitude of
the finding chart image is approximately 18.5.   No correction has
been made for the expected extinction corresponding to E(B-V) of
0.06.

W. Li, N. Butler, J. S. Bloom, and A. V. Filippenko, University of
California, Berkeley, on behalf of the KAIT GRB team, report:

The robotic 0.76-m Katzman Automatic Imaging Telescope (KAIT)
at Lick Observatory observed GRB 060923, detected with Swift
(Trigger 230662; Stamatikos et al. GCN 5583
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 ). The automatic sequence
started at 05:12:33, 18 s after the burst. The first two images,
however, failed due to some technical issues. In subsequent images,
Our image processing pipeline did not find any new objects, and
reported the following photometry limits (calibrated with USNO B1.0):

======================================================================

Start UT   t(GRB)   Filter    Exp(s)  3sigma-limit      

05:13:57    102s    I         15.0     <16.9
05:14:21    126s    clear     20.0     <17.9
05:14:52    157s    V         45.0     <16.9
05:15:48    213s    I         45.0     <17.5
05:16:43    268s    clear     45.0     <18.5
05:17:39    324s    V         60.0     <17.2

========================================================================

More analyses are in progress.

Richard J. Cool (Arizona), Daniel J. Eisenstein (Arizona), David
W. Hogg (NYU), Michael R. Blanton (NYU), David J. Schlegel (LBNL),
J. Brinkmann (APO), Donald Q. Lamb (Chicago), Donald P. Schneider
(PSU), and Daniel E. Vanden Berk (PSU) report:

The Sloan Digital Sky Survey (SDSS) imaged the field of burst
GRB060923 prior to the burst.  As these data should be useful
as a pre-burst comparison and for calibrating photometry, we are
supplying the images and photometry measurements for this GRB field
to the community.

Data from the SDSS, including 5 FITS images, 3 JPGS, and
3 files of photometry and astrometry, are being placed at
http://mizar.as.arizona.edu/~grb/public/GRB060923

We supply FITS images in each of the 5 SDSS bands of a 8'x8' region
centered on the GRB position (ra=254.618 (16:58:28.3), dec=12.3620
(12:21:43.2); GCN 5583
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 ), as well as 3 gri color-composite JPGs (with
different stretches). The units in the FITS images are nanomaggies
per pixel.  A pixel is 0.396 arcsec on a side. A nanomaggie is
a flux-density unit equal to 10^-9 of a magnitude 0 source or,
to the extent that SDSS is an AB system, 3.631e-6 Jy.  The FITS
images have WCS astrometric information.

In the file GRB060923_sdss.calstar.dat, we report photometry
and astrometry of 1668 bright stars (r<20.5) within 15' of the
burst location.  The magnitudes presented in this file are asinh
magnitudes as are standard in the SDSS (Lupton 1999, AJ, 118,
1406). Beware that some of these stars are not well-detected in
the u-band; use the errors and object flags to monitor data quality.

In the files GRB060923_sdss.objects_flux.dat and
GRB060923_sdss.objects_magnitudes.dat, we report photometry
of 2573 objects detected within 6' of the GRB position.
We have removed saturated objects and objects with model
magnitudes fainter than 23.0 in the r-band.  The fluxes listed
in GRB060923_sdss.objects_flux.dat are in nanomaggies while the
magnitudes listed in GRB060923_sdss.objects_magnitudes.dat are
asinh magnitudes.

All quantities reported are standard SDSS photometry, meaning that
they are very close to AB zeropoints and magnitudes are quoted
in asinh magnitudes.  Photometric zeropoints are known to about
2% rms.  None of the photometry is corrected for dust extinction.
The Schlegel, Finkbeiner, and Davis (1998) predictions for this
region are A_U=0.287 mag, A_g=0.211 mag, A_r = 0.153 mag, A_i=0.116
mag, and A_z=0.082 mag.

There are currently no objects within 6 arcminutes of the GRB
position in the SDSS spectroscopic database.


SDSS astrometry is generally better than 0.1 arcsecond per
coordinate.  Users requiring high precision astrometry should take
note that the SDSS astrometric system can differ from other systems
such as those used in other notices; we have not checked the offsets
in this region.

More detailed information pertaining to our SDSS GRB releases
can be found in our initial data release paper (Cool et al. 2006,
astro-ph/0601218).  See the SDSS DR4 documentation for more details:
http://www.sdss.org/dr5.

These data have been reduced using a slightly different pipeline
than that used for SDSS public data releases.  We cannot guarantee
that the values here will exactly match those in the data release
in which these data are included.  In particular, we expect the
photometric calibrations to differ by of order 0.01 mag.

This note may be cited, but please also cite the SDSS data release
paper, Adelman-McCarthy et al. (2006, ApJS, 162, 38), when using
the data or referring to the technical documentation.
grb@mizar:/home/grb/public_html/private/sdss/GRB060923

A. Melandri (Liverpool JMU) and A. Gomboc (University of Ljubljana) report
on behalf of the RoboNet-1.0 collaboration:

The 2m Faulkes North Telescope (Hawaii) automatically reacted to the Swift
burst GRB060923 (trigger 230662, Stamatikos et al. GCN 5583
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 ). Observations
started 2.8 min after the trigger time. In first images we do not detect
any new source brighter than R~19.

Further analysis are ongoing.

N. Tanvir (U. Leicester), A. Levan (U. Hertfordshire), M. Jarvis
(U. Oxford), T. Wold (JACH)

We observed the field of GRB 060923 with UFTI on UKIRT, beginning
at 06:07 (UT), approximately 55 minutes post-burst.  We detect a
faint source in the K-band image at approximately:

 16 58 28.18  +12 21 38.9

This position (based on provisional astrometry) is only 4.6 arcsec
from the reported XRT position (Stamatikos et al. GCN 5583
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 ).
The source appears to have faded in another (shallower) K-band
observation made 30 mins later, and therefore is likely to be
the afterglow of this burst.

Further analysis is ongoing.

G. G. Williams (MMTO) and P. A. Milne (Steward Observatory), on behalf of 
the Super-LOTIS Collaboration, report:

The robotic 0.6-m Super-LOTIS telescope began observing the error box of 
GRB 060923 (Swift Trigger 230662, Stamatikos et al. GCN 5583
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 ) at 
05:12:56.1 UT, 41.1 seconds after the trigger.  Our initial observations 
include 5 x 10s exposures, 5 x 20s exposures, and 30 x 60s exposures, all 
in the R-band.  The observations began at a high airmass of 2.92.

We do not detect any variable sources or afterglow candidates within the 
XRT error box in our earliest exposure or in the sum of our first five 
exposures to the following 3-sigma limiting magnitudes determined from 
nearby USNO-B1.0 stars:

t_start (UT)	exp t (s)	t_start-t_0 (s)	Limit
--------------------------------------------------------
05:12:56.1	10.0 		41.1 		R > 16.6 +/- 0.15
05:12:56.1      5 x 10.0        41.1            R > 18.3 +/- 0.15

The errors are dominated by scatter in the USNO-B1.0 stars.

J. Tueller (GSFC) 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)
on behalf of the Swift-BAT team:
 
Using the data set from T-240 to T+879 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 060923 (trigger #230662)
(Stamatikos, et al., GCN Circ. 5583
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 ).  The BAT ground-calculated position
is RA,Dec = 254.623, 12.341 deg {16h 58m 29.6s, 12d 20' 28.1"} (J2000)
+- 2.1 arcmin, (radius, sys+stat, 90% containment).  The partial coding was 33%.
 
The burst started with a 6-sec wide peak at T-42 sec.  The main peak starts
at T-5 sec, peaks at T+1 sec, and ends at T+20 sec.  There is a possible
third peak at T+19 sec.  T90 (15-350 keV) is 51.7 +- 1 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-42.3 to T+11.5 is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.69 +- 0.23.  The fluence in the 15-150 keV band is 8.7 +- 1.3 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+0.03 sec in the 15-150 keV band
is 1.3 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90% confidence level.