  Tunneling density of states of both the massless and massive (gapped)
particles in metallic carbon nanotubes is known to have anomalous energy
dependence. This is the result of coupling to multiple low-energy bosonic
excitation (plasmons). For both kinds of particles the ensuing effect is the
suppression of the density of states by electron-electron interactions. We
demonstrate that the optical absorption between gapless and gapped states is
affected by the many-body effects in the opposite way. The absorption
probability is enhanced compared with the non-interacting value and develops a
power-law frequency dependence with the exponent -0.2 for typical nanotubes.
