{
  "bibcode": "2005GCN..3795....1E",
  "body": "Daniel J. Eisenstein (Arizona) and Richard Cool (Arizona)\nreport on behalf of the Sloan Digital Sky Survey (SDSS) Collaboration:\n\nThe Sloan Digital Sky Survey imaged the region of GRB 050813 on May \n8 & 10, 2005 (MJD 53499.42 and 53501.25, roughly 90 days before the burst\ntrigger; the XRT position itself was imaged on May 10).  These data will\nbe part of a future SDSS data release, but as they should be useful as\na pre-burst comparison image and for calibrating photometry, we are\nsupplying the images and photometry measurements for this GRB field.\nSDSS spectroscopy has not yet been performed here.\n\nData from the SDSS is being placed at \n    http://cmb.as.arizona.edu/~eisenste/grb050813\nThese include a JPG and 2 files of photometry and astrometry.  We plan\nto supply FITS image cutouts from the SDSS next week, but technical\nproblems have stalled us this weekend.\n\n\nInspecting the images and catalogs, the conclusion most immediately\nrelevant to the topic of short-hard bursts is that there is no bright\ngalaxy anywhere close to the GRB position.\n\nTo the limit of the SDSS imaging, there are only 2 objects within 15\" of\nthe XRT position.  One is a bright star discussed by Berger (GCN 3792):\nRA = 241.98915, Dec = 11.24521, u = 17.286, g = 16.112, r = 15.694,\ni = 15.552, z = 15.481.  The photon errors on this photometry are 1%\nor less in all bands.  The colors of this object are that of an F star\nand the morphology is point-like; it is extremely unlikely that this\nobject is extragalactic.\n\nThe other object is faint: RA = 241.98688, Dec = 11.25030, g = 22.36+-0.1,\nr = 21.34+-0.06, i = 20.87+-0.06, and essentially undetected in u and z.\nThe above are model magnitudes; the Petrosian magnitudes are brighter by\nseveral tenths of a magnitude but have much larger errors.  This object\nis also point-like in the SDSS imaging, and the g-r/r-i colors are that\nof a K star.  Hence, this object is probably also a star, but it is close\nenough to the SDSS detection limit that a deeper image would be needed to\nbe certain of this.\n\nGoing out to 19\" from the XRT position, there is a third point-like\nobject: RA = 241.98325, Dec = 11.24473, g=22.44+-0.1, r=20.95+-0.05,\ni=19.74+-03, z=19.18+-0.07.  This is very likely an M star.\n\nThere are 2 extended sources at r=21 within 1 arcminute of the XRT position,\na blue galaxy at RA=241.97903, Dec=11.24401 (33\" from the XRT position) \nand a red one at RA=241.97762, Dec=11.24578 (36\" from the XRT position).\nThere are no galaxies brighter than r=19.5 within 3 arcminutes of the XRT\nposition.\n\n\nThe file grb050813.jpg contains a simple JPG made from the SDSS gri\nimages.  North is *not* up in this image; rather, north is about 2:30\non the clock and east is at 5:30.  The bright star mentioned above is\nthe one close to the center.  The second object is the nearest faint\nobject to the upper right.  The XRT position is halfway in between these.\nThe image is about 4' on a side.\n\n\nIn the file grb050813.sdss_objects.dat, we report photometry of 201\nobjects detected by SDSS within 3' radius of the Swift BAT position.\nWe have removed saturated objects and objects fainter than 23.0 in the\nr-band model magnitude.\n\nAll quantities are standard SDSS photometry, meaning that they are\nvery close to AB zeropoints and are quoted in asinh magnitudes.\nPhotometric zeropoints are known to about 2% rms; photon noise can be\nmuch worse, of course.  See documentation for details.  None of this\nphotometry is corrected for dust extinction.  The Schlegel, Finkbeiner,\nand Davis (1998) predictions for this region are A_u = 0.288 mag, A_g =\n0.212 mag, A_r = 0.154 mag, A_i = 0.117 mag, and A_z = 0.083 mag.\n\nSDSS astrometry is generally better than 0.1 arcsecond per coordinate.\nUsers requiring high precision astrometry should take note that the SDSS\nastrometric system can differ from other systems, such as those used in\nGCN 3792; we have not checked the offsets in this region.\n\nSee the SDSS DR4 documentation for more details: http://www.sdss.org/dr4.\n\nThese data have been reduced with the DR4 data pipelines; however, they\nare not part of that data release.  We cannot guarantee that the values\nhere will exactly match those of the data release in which these data\nare included.\n\n\nTo aid with calibration of other data sets, we also report the\nastrometry and photometry of 147 brighter stars (r<20.5) in a wider\nregion (roughly 6') around the burst.  These data are in the file\ngrb050813.sdss_calibstars.dat.  Beware that some of these stars are not\nwell-detected in the u-band; use the errors to monitor this.\n\n\nThis note can be cited, but please also cite the SDSS data release paper,\nAbazajian et al. (AJ, 129, 1755, 2005), when using the data or referring\nto the technical documentation.",
  "circularId": 3795,
  "createdOn": 1123968402000,
  "email": "deisenstein@as.arizona.edu",
  "subject": "GRB050813, SDSS pre-burst observations",
  "submitter": "Daniel Eisenstein at U of Arizona  <deisenstein@as.arizona.edu>",
  "eventId": "GRB 050813"
}