Superorbital modulation of X-ray emission from gamma-ray binary LSI +61
  303
  We report the discovery of a systematic constant time lag between the X-ray
and radio flares of the gamma-ray binary LSI +61 303, persistent over long,
multi-year, time scale. Using the data of monitoring of the system by RXTE we
show that the orbital phase of X-ray flares from the source varies from
$\phi_X\simeq 0.35$ to $\phi_X\simeq 0.75$ on the superorbital 4.6 yr time
scale. Simultaneous radio observations show that periodic radio flares always
lag the X-ray flare by $\Delta\phi_{X-R}\simeq 0.2$. We propose that the
constant phase lag corresponds to the time of flight of the high-energy
particle filled plasma blobs from inside the binary to the radio emission
region at the distance ~10 times the binary separation distance. We put forward
a hypothesis that the X-ray bursts correspond to the moments of formation of
plasma blobs inside the binary system.
