TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4777
SUBJECT: GRB060218 : SDSS pre-burst observations
DATE:    06/02/18 11:27:26 GMT
FROM:    Richard J. Cool at U.of AZ/Steward Obs  <rcool@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 P. Schneider
(PSU), and Daniel E. Vanden Berk (PSU) report:

The Sloan Digital Sky Survey (SDSS) imaged the field of
burst GRB060218 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/GRB060218

We supply FITS images in each of the 5 SDSS bands of a
8'x8' region centered on the GRB position (ra=50.4154
(03:21:39.7), dec=16.8670 (16:52:01.3); GCN 4776),
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 GRB060218_sdss.calstar.dat, we report
photometry and astrometry of 308 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 GRB060218_sdss.objects_flux.dat and
GRB060218_sdss.objects_magnitudes.dat, we report photometry
of 947 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 GRB060218_sdss.objects_flux.dat
are in nanomaggies while the magnitudes listed in
GRB060218_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=0.749 mag, A_g=0.551 mag, A_r =
0.400 mag, A_i=0.303 mag, and A_z=0.215 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.