[vsnet-grb-info 1956] GRB050813, SDSS pre-burst observations

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3795
SUBJECT: GRB050813, SDSS pre-burst observations
DATE:    05/08/13 21:26:42 GMT
FROM:    Daniel Eisenstein at U of Arizona  <deisenstein at as.arizona.edu>

Daniel J. Eisenstein (Arizona) and Richard Cool (Arizona)
report on behalf of the Sloan Digital Sky Survey (SDSS) Collaboration:

The Sloan Digital Sky Survey imaged the region of GRB 050813 on May 
8 & 10, 2005 (MJD 53499.42 and 53501.25, roughly 90 days before the burst
trigger; the XRT position itself was imaged on May 10).  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 
    http://cmb.as.arizona.edu/~eisenste/grb050813
These include a JPG and 2 files of photometry and astrometry.  We plan
to supply FITS image cutouts from the SDSS next week, but technical
problems have stalled us this weekend.


Inspecting the images and catalogs, the conclusion most immediately
relevant to the topic of short-hard bursts is that there is no bright
galaxy anywhere close to the GRB position.

To the limit of the SDSS imaging, there are only 2 objects within 15" of
the XRT position.  One is a bright star discussed by Berger (GCN 3792):
RA = 241.98915, Dec = 11.24521, u = 17.286, g = 16.112, r = 15.694,
i = 15.552, z = 15.481.  The photon errors on this photometry are 1%
or less in all bands.  The colors of this object are that of an F star
and the morphology is point-like; it is extremely unlikely that this
object is extragalactic.

The other object is faint: RA = 241.98688, Dec = 11.25030, g = 22.36+-0.1,
r = 21.34+-0.06, i = 20.87+-0.06, and essentially undetected in u and z.
The above are model magnitudes; the Petrosian magnitudes are brighter by
several tenths of a magnitude but have much larger errors.  This object
is also point-like in the SDSS imaging, and the g-r/r-i colors are that
of a K star.  Hence, this object is probably also a star, but it is close
enough to the SDSS detection limit that a deeper image would be needed to
be certain of this.

Going out to 19" from the XRT position, there is a third point-like
object: RA = 241.98325, Dec = 11.24473, g=22.44+-0.1, r=20.95+-0.05,
i=19.74+-03, z=19.18+-0.07.  This is very likely an M star.

There are 2 extended sources at r=21 within 1 arcminute of the XRT position,
a blue galaxy at RA=241.97903, Dec=11.24401 (33" from the XRT position) 
and a red one at RA=241.97762, Dec=11.24578 (36" from the XRT position).
There are no galaxies brighter than r=19.5 within 3 arcminutes of the XRT
position.


The file grb050813.jpg contains a simple JPG made from the SDSS gri
images.  North is *not* up in this image; rather, north is about 2:30
on the clock and east is at 5:30.  The bright star mentioned above is
the one close to the center.  The second object is the nearest faint
object to the upper right.  The XRT position is halfway in between these.
The image is about 4' on a side.


In the file grb050813.sdss_objects.dat, we report photometry of 201
objects detected by SDSS within 3' 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 = 0.288 mag, A_g =
0.212 mag, A_r = 0.154 mag, A_i = 0.117 mag, and A_z = 0.083 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 other systems, such as those used in
GCN 3792; we have not checked the offsets in this region.

See the SDSS DR4 documentation for more details: http://www.sdss.org/dr4.

These data have been reduced with the DR4 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.


To aid with calibration of other data sets, we also report the
astrometry and photometry of 147 brighter stars (r<20.5) in a wider
region (roughly 6') around the burst.  These data are in the file
grb050813.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 referring
to the technical documentation.