GCN Circular 12214

G. Bower, S. B. Cenko, J. S. Bloom (UC Berkeley) and B. D. Metzger
(Princeton) report on behalf of a larger collaboration:

We have obtained a second epoch of long-baseline interferometry with the
VLBA of the unusual high-energy transient Sw J1644+57 (GRB 110328A; Levan
et al, Science, 333, 199, 2011; Bloom et al, Science, 333, 203, 2011;
Burrows et al., astro-ph/1104.4787; Zauderer et al., astro-ph/1106.3568).
Observations were obtained on 2011 July 17 at 8.4 and 22 GHz with
recording bandwidth of 512 Mbps. Preliminary analysis of images at both
frequencies reveals a compact (i.e., unresolved) source with flux
densities of 15 and 12 mJy, respectively. Errors in the flux density are
set by the amplitude scale and are estimated at 10%. The 8.4 GHz
localization is consistent with the position obtained from our previous
epoch of VLBA observations on 2011 April 1 and 3 (Levan et al., Science
333, 199, 2011; see also Zauderer et al., astro-ph/1106.3568) at the level
of ~ 300 uas. If we assume expansion with a constant speed from the time
of the initial high-energy detections of this source, this places an upper
limit on the average outflow Lorentz factor of Gamma <~ 5.