I assume that can only be guessed at by the assumed energy of the
event and the 1/r^2 law.  So, if the 1/r^2 law is incorrect (assume
some unknown material [dark matter??] inhibits Gamma Ray propagation),
could it be possible that we are actually seeing much less energetic
events happening much closer to us?  The even distribution could
be caused by the characteristic propagation distance of gamma rays 
being shorter then 1/2 the thickness of the disk of the galaxy.

Just some idle babbling,
-- 

   Jim Batka  | Work Email:  BATKAJ@CCMAIL.DAYTON.SAIC.COM | Elvis is
              | Home Email:  JBATKA@DESIRE.WRIGHT.EDU      |   DEAD!