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GCN Circular 3418

GRB050509b, SDSS pre-burst observations
2005-05-12T16:38:13Z (19 years ago)
Daniel Eisenstein at U of Arizona <>
Daniel J. Eisenstein (Arizona), David W. Hogg (NYU), and Nikhil
Padmanabhan (Princeton) report on behalf of the Sloan Digital Sky Survey
(SDSS) Collaboration:

The Sloan Digital Sky Survey imaged the region of GRB 050509b on Dec 15,
2004 (MJD 53354.48, 145 days before the burst trigger).  These data will
be part of a future SDSS data release, but as they should be useful as
a pre-burst comparison image and for calibrating photometry, we are
supplying the images and photometry measurements for this GRB field.
SDSS spectroscopy has not yet been performed here.

Data from the SDSS is being placed at
These include 5 FITS images, 3 JPGs, and 2 files of photometry and 

We supply FITS images in each of the 5 SDSS bands of a 8'x8' region
centered on the BAT position (ra=189.073, dec=28.991; Barthelmy et
al. GCN 3385), 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 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.

The cluster NSC J123610+285901 (Gal et al., AJ, 125, 2064, 2003) is
clearly visible in the JPGs.

The bright galaxy close to the XRT error circle (ra=189.0537, dec=28.9830)
has SDSS photometry of r_Petrosian = 17.18 +- 0.02 mag and model colors
of u-g = 1.78 +- 0.13 mag, g-r = 1.42 +- 0.02 mag, r-i = 0.52 +- 0.01,
i-z = 0.35 +- 0.02 mag.  The galaxy is selected by both the Luminous
Red Galaxy and Main galaxy sample targeting algorithms, and at z=0.225
(Prochaska et al. GCN 3390 & 3399), this is a rather luminous galaxy.
Using the r-band Petrosian magnitude, we compute a rest-frame g-band
luminosity M_g = -21.90, applying evolution corrections to z=0.3
(not z=0! see Zehavi et al. ApJ, 621, 22, 2005 for an explanation).
This is about 1.5 magnitudes brighter than L* (Blanton et al., ApJ, 582,
819, 2003).  The comoving number density of galaxies more luminous than
this is only about 2x10^-5 h^3 Mpc^-3.  This is all in keeping with the
large velocity dispersion reported by Prochaska et al. (GCN 3399).

A posteriori statistics are always suspicious, but we note that the sky
density of galaxies this bright is about 40 per square degree, while
that of galaxies this luminous at z<0.3 is about 1 per square degree.
These are far smaller than 40,000 deg^-2, the inverse of the area of a
10 arcsecond circle.  Hence, the chance association with a low-redshift
galaxy is unlikely (at the 10^-3 level), although unless additional
evidence for the association is found, it will be hard to draw a firm
conclusion from this one object.

In the file grb050509b.sdss_objects.dat, we report photometry of 335
objects detected by SDSS within 4' radius of the Swift BAT position.
We have removed saturated objects and objects fainter than 23.0 in the
r-band model magnitude.

All quantities are standard SDSS photometry, meaning that they are
very close to AB zeropoints and are quoted in asinh magnitudes.
Photometric zeropoints are known to about 2% rms; photon noise can be
much worse, of course.  See documentation for details.  None of this
photometry is corrected for dust extinction.  The Schlegel, Finkbeiner,
and Davis (1998) predictions for this region are A_u = 0.098 mag, A_g =
0.072 mag, A_r = 0.052 mag, A_i = 0.040 mag, and A_z = 0.028 mag.

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 the 2MASS system used in GCN 3392;
we have not checked the offsets in this region.  

See the SDSS DR3 documentation for more details:

These data have been reduced with the DR3 data pipelines; however, they
are not part of that data release.  We cannot guarantee that the values
here will exactly match those of the data release in which these data
are included.

Moreover, the calibration of the FITS image may differ by a small amount
(~1%) from the calibration of the quoted photometry; these came from
different pipelines.

To aid with calibration of other data sets, we also report the photometry
of 90 brighter stars (r<20.5) in a wider region (0.3 by 0.2 degrees) around 
the burst.  These data are in the file grb050509b.sdss_calibstars.dat.  
Beware that some of these stars are not well-detected in the u-band;
use the errors to monitor this.

This note can be cited, but please also cite the SDSS data release paper,
Abazajian et al. (AJ, 129, 1755, 2005), when using the data or refering
to the technical documentation.
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