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GCN Circular 6950

Subject
GRB071020 - SDSS Pre-burst Observations
Date
2007-10-20T07:30:59Z (17 years ago)
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 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
GRB071020 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/GRB071020

We supply FITS images in each of the 5 SDSS bands of a 8'x8'
region centered on the GRB position (ra=119.664 (07:58:39.4),
dec=32.8610 (32:51:39.6); Swift-BAT TRIGGER 294835), 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 GRB071020_sdss.calstar.dat, we report photometry
and astrometry of 741 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 GRB071020_sdss.objects_flux.dat and
GRB071020_sdss.objects_magnitudes.dat, we report photometry
of 975 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 GRB071020_sdss.objects_flux.dat are in nanomaggies while
the magnitudes listed in GRB071020_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=0.335 mag, A_g=0.246 mag,
A_r = 0.179 mag, A_i=0.135 mag, and A_z=0.096 mag.

The file GRB071020_sdss.spectro.dat contains a list of the
3 objects with SDSS spectroscopy within 6 arcminutes of the
GRB position.  In addition to the redshift and 1-sigma error
for each object, this file also lists the object spectroscopic
classification.

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, PASP 118, 733).  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, 172, 634),
when using the data or referring to the technical documentation.
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