TITLE:   GCN CIRCULAR
NUMBER:  11843
SUBJECT: GRB 110328A / Swift J164449.3+573451: beamed emission
DATE:    11/03/30 17:23:02 GMT
FROM:    Sergio Campana at INAF-OAB  <sergio.campana@brera.inaf.it>

S. Campana (INAF-OAB), S. Covino (INAF-OAB), G. Tagliaferri (INAF-OAB),
V. D'Elia (ASI-ASDC), L. Stella (INAF-OAR), R. Salvaterra (Insubria 
University):

GRB 110328A/ Swift J164449.3+573451 is characterized by a very fast
variability.  A doubling time less than 500 s can be easily recovered
from the Swift X-ray data. Assuming that the entire source is varying,
this poses a limit on the mass of the varying object of M1<5x10^5 solar
masses, based on the arguments of Cavallo & Rees (1978) and assuming
a conversion efficiency of ~10%, typical of accretion onto a black hole.

On the other side if this source is really at z=0.354 (Levan et al. GCN
11833; Thoene et al. GCN 11834), as confirmed by the radio position
of the counterpart (Zauderer et al. GCN 11836), its peak flux of ~10^-8
erg/cm2/s (0.3-10 keV based on the Swift Burst analyser, Evans et al.
2010, A&A 509 A102) implies a luminosity of ~5x10^48 erg/s.
In order not to overcome the Eddington limit a mass of M2>3x10^10
solar masses is needed.

The two mass estimates strongly disagree providing clear evidence for
a highly beamed emission.