Sw J1644+57 (GRB 110328A): Continued VLBA Observations
ATel #3522; G. Bower, S. B. Cenko, J. S. Bloom (UC Berkeley) and B. D. Metzger (Princeton) report on behalf of a larger collaboration
on 30 Jul 2011; 19:22 UT
Credential Certification: S. Bradley Cenko (cenko@astro.berkeley.edu)
Subjects: Radio, AGN, Black Hole, Blazar, Gamma-Ray Burst, Transient
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.
