"Harri Tavaila" writes:

"Allen Thomson" kirjoitti viestissä
om...

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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