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GCN Circular 6432

GRB070521 - SDSS Pre-Burst Observations
2007-05-21T07:12:54Z (17 years ago)
Richard J. Cool at U.of AZ/Steward Obs <>
Richard J. Cool (Arizona), Daniel J. Eisenstein (Arizona), David W. Hogg
(NYU), Michael R. Blanton (NYU), David J. Schlegel (LBNL), J. Brinkmann
Donald Q. Lamb (Chicago), Donald P. Schneider (PSU), and Daniel E. Vanden
(PSU) report:

The Sloan Digital Sky Survey (SDSS) imaged the field of burst GRB070521
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

We supply FITS images in each of the 5 SDSS bands of a 8'x8' region centered
on the GRB position (ra=242.669 (16:10:40.6), dec=30.2410 (30:14:27.6);
Swift-BAT TRIGGER 279935), 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 GRB070521_sdss.calstar.dat, we report photometry and astrometry
of 558 bright stars (r<20.5) within 15' of the burst location.  The
presented in this file are asinh magnitudes as are standard in the SDSS
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 GRB070521_sdss.objects_flux.dat and
GRB070521_sdss.objects_magnitudes.dat, we report photometry of 1833
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 GRB070521_sdss.objects_flux.dat are in nanomaggies
while the magnitudes listed in GRB070521_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=0.145 mag, A_g=0.107 mag, A_r =
0.078 mag, A_i=0.059 mag, and A_z=0.042 mag.

The file GRB070521_sdss.spectro.dat contains a list of the 5 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:

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
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.
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