{
  "bibcode": "2011GCN.11854....1B",
  "body": "E. Berger (Harvard), A. Levan (U. Warwick), N. R. Tanvir\n(U. Leicester), A. Zauderer, A. M. Soderberg (Harvard), and\nD. A. Frail (NRAO) report:\n\n\"We performed absolute and relative astrometry on radio and optical\nimages of GRB 110328A / Swift J164449.3+573451 (GCNs 11823, 11824) in\nan atempt to locate the radio transient (GCNs 11836, 11848) relative\nto the quiescent optical source at z=0.35 (GCNs 11827, 11830, 11833,\n11834).  We used EVLA observations at a mean frequency of 5.8 GHz\n(GCNs 11836, 11848), a Gemini-N/GMOS r-band image, and a UKIRT/WFCAM\nJ-band image (GCN 11846).\n\nThe Gemini and UKIRT images were astrometrically aligned relative to\n2MASS using 20 and 55 objects in common, respectively, leading to an\nrms scatter of 0.14\" and 0.10\" in each coordinate, respectively.  For\nthe EVLA image we used the native astrometric solution relative to the\nICRS.  The accuracy of the astrometry is verified by imaging and\nmeasuring the position of the phase calibrator, which agrees with the\nICRS position to better than 0.4 mas.  We note that the 2MASS\npoint-source-catalog positions used to align the Gemini and UKIRT\nimages are reconstructed onto the ICRS.\n\nWe find the following coordinates for the optical source in the Gemini\nimage (J2000):\n      RA = 16:44:49.939\n      DEC = +57:34:59.64\nwith an uncertainty of 0.14\" in each coordinate, dominated by the\nastrometric solution uncertainty.\n\nThe position of the faint NIR counterpart in the UKIRT image is:\n      RA = 16:44:49.958\n      DEC = +57:35:00.00\nwith an uncertainty of about 0.3\" in the source centroid, and an\nadditional 0.1\" due to astrometric solution uncertainty.\n\nFinally, the ICRS position of the radio transient is:\n      RA = 16:44:49.925\n      DEC = +57:34:59.68\nwith negligible centroid uncertainty of about 0.01\".\n\nThe relative offset between the radio and optical positions is\ntherefore 0.12+/-0.20\", or about 0.6+/-1.0 kpc (at z=0.35).  The\noffset relative to the NIR position is 0.42+/-0.42\".  Thus, the radio\ntransient position is consistent with arising in the nucleus of the\nhost galaxy (GCN 11847).\n\nWe find an additional common source between the radio, optical, and\nNIR images with the following positions:\n\noptical:\n      RA = 16:44:48.047\n      DEC = +57:32:16.83\nnear-IR:\n      RA = 16:44:48.093\n      DEC = +57:32:17.03\nradio:\n      RA = 16:44:48.139\n      DEC = +57:32:16.74\n\nThis source is a galaxy with extended structure in the optical/NIR\nimages, but it is unresolved in the radio.  We therefore associate the\nradio position with the nucleus of the galaxy (positions given above).\nThe resulting positional offsets are 0.75+/-0.20\" (radio-to-optical)\nand 0.47+/-0.15\" (radio-to-NIR).  We caution that the error bars do\nnot include a systematic uncertainty in centroiding on the galaxy\nnucleus (~0.1\"), and do not account for potential distortions due to\nthe location of the object near the edge of the Gemini image.  Taken\nat face value, the offset between these positions potentially reflects\na relative shift between the optical/NIR and radio astrometric\nsystems.  If applied to the radio counterpart of GRB 110328A / Swift\nJ164449.3+573451, this shift would lead to a significant offset\n(~3-sigma confidence level) between the radio position and the center\nof the host galaxy.\n\nTo conclude, absolute astrometry indicates that the radio transient is\nconsistent with arising in the nucleus of the host galaxy (0.6+/-1.0\nkpc); the relative radio-optical/NIR positions of an additional common\nsource allow for an offset of up to ~3 kpc.  Additional wide-field\n(~10') deep NIR imaging may provide a larger number of common objects\nwith the EVLA images, thereby impoving the relative astrometric\nsolution.",
  "circularId": 11854,
  "createdOn": 1301597220000,
  "email": "eberger@cfa.harvard.edu",
  "subject": "GRB 110328A / Swift J164449.3+573451: Radio-optical/NIR Astrometry",
  "submitter": "Edo Berger at Harvard  <eberger@cfa.harvard.edu>",
  "eventId": "GRB 110328A"
}