[vsnet-grb-info 3202] GRB060428B - SDSS Pre-Burst Observations

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5037
SUBJECT: GRB060428B - SDSS Pre-Burst Observations
DATE:    06/04/29 17:10:01 GMT
FROM:    Richard J. Cool at U.of AZ/Steward Obs  <rcool at as.arizona.edu>

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 GRB060428B 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/GRB060428B

We supply FITS images in each of the 5 SDSS bands of a
8'x8' region centered on the GRB position (ra=235.357
(15:41:25.7), dec=62.0238 (62:01:25.7); GCN 5017),
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 GRB060428B_sdss.calstar.dat, we report
photometry and astrometry of 303 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 GRB060428B_sdss.objects_flux.dat and
GRB060428B_sdss.objects_magnitudes.dat, we report
photometry of 501 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 GRB060428B_sdss.objects_flux.dat
are in nanomaggies while the magnitudes listed in
GRB060428B_sdss.objects_magnitudes.dat are asinh
magnitudes.

**Be aware that at least a portion of the photometry
provided in this release has been flagged as
non-photometric. As photometry for objects with this flag
set may have non-optimal calibration, we do not recommend
these objects be used for photometric calibration.
Non-photometric imaging may still be valuable as a
pre-burst comparison and for astrometric calibration.

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=0.073 mag, A_g=0.054 mag, A_r =
0.039 mag, A_i=0.030 mag, and A_z=0.021 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/dr4.

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, in
press, astro-ph/0507711), when using the data or referring
to the technical documentation.