GRB 110328A: VLBA Observations

ATel #3269; Andreas Brunthaler (MPIfR), Alicia Soderberg (Harvard), Michael Rupen (NRAO), Ashley Zauderer, Edo Berger (Harvard), Dale Frail (NRAO), And Michael Bietenholz (York)
on 8 Apr 2011; 19:49 UT
Credential Certification: Alicia M. Soderberg (asoderberg@cfa.harvard.edu)

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We observed the variable radio counterpart (GCNs 11836, 11848) of GRB110328A / Swift J164449.3+573451 (GCNs 11823, 11824) with the Very Long Baseline Array (VLBA) and Effelsberg Radio Telescope for 7 hours beginning on April 2 at 6:00 UT. Observations at both telescopes were carried out at a central frequency of 8.46 GHz. Based on a preliminary analysis of the VLBA baselines alone, we report a significant detection (SNR=29) of the source at position:
RA: 16 44 49.9313
DEC: 57 34 59.6895
with a conservative error estimate of 0.5 mas dominated by the positional uncertainty of the phase calibrator, J1638+5720. This is the most precise position available for the transient. It is coincident with the measured positions for the variable radio, NIR, and X-ray counterparts (GCNs 11836, 11848, 11853, 11854, 11886) and the host galaxy nucleus (GCN 11881). The source is not resolved in our VLBA observation; this constrains the size to be smaller than that of the beam: 1.9 x 0.7 mas at a position angle of 18 deg. Further VLBI observations are planned to set limits on the proper motion and source structure.
We thank the NRAO and Effelsberg scheduling staff for enabling these rapid response observations.