From: g_d_pusch_remove_underscores@xnet.com   
      
   "Harri Tavaila"  writes:   
      
   > "Allen Thomson"  kirjoitti viestissä   
   > news:501f9880.0404260844.4d8a7801@posting.google.com...   
   >>   
   >> One light year seems pretty ambitious, but has anybody   
   >> actually done a study of the signatures a large body would   
   >> generate moving through the interstellar medium at close   
   >> to the speed of light?   
   >   
   > 1987Natur.330..455Y   
   >   
   > Abstract   
   > A highly unusual radio source lying within 1 deg of the Galactic center   
   > has been discovered whose 'cometary' morphology suggests that it is a   
   > wake produced by a radio source moving supersonically with respect to   
   > the ambient interstellar medium. Maps of the source are shown, and its   
   > characteristics are discussed. Two possible models which might explain   
   > the wake are suggested.   
      
   Interesting, but probably not relevant to the question being posed, because   
   the speed of sound in the interstellar medium is only on the order of the   
   mean thermal velocity of the interstellar medium's gas molecules, which is   
   many orders of magnitude less than the speed of light.   
      
   It is difficult to predict how relativistic bulk condensed matter would   
   interact with interstellar gas; however, it is not unreasonable to expect   
   that a certain amount of X-ray and gamma radiation would be produced when   
   gas molecules hit a relativistic projectile, and if it had a high enough   
   gamma-factor, possibly even charged and neutral pions (which would decay   
   to muons and gammas, respectively, and the muons would decay to electrons   
   or positrons). Relative to the Earth frame, these produced particles would   
   be aberrated forward into a cone about its trajectory with an opening angle   
   ~1/(2*\gamma), and doppler-shifted upward to even higher energies.   
   Hence, an oncoming relativistic projectile _might_ appear as a rapidly   
   brightening "point" source of beamed high energy gamma radiation, and if   
   it was moving faster than the pion production threshold, there might also   
   be a "wake" of annhilation radiation from the pion-decay positrons...   
      
      
   -- Gordon D. Pusch   
      
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