  PSR B1259-63 is a 48 ms radio pulsar in a highly eccentric 3.4 year orbit
with a Be star SS 2883. Unpulsed gamma-ray, X-ray and radio emission components
are observed from the binary system. It is likely that the collision of the
pulsar wind with the anisotropic wind of the Be star plays a crucial role in
the generation of the observed non-thermal emission. The 2007 periastron
passage was observed in unprecedented details with Suzaku, Swift, XMM-Newton
and Chandra missions. We present here the results of this campaign and compare
them with previous observations. With these data we are able, for the first
time, to study the details of the spectral evolution of the source over a 2
months period of the passage of the pulsar close to the Be star. New data
confirm the pre-periastron spectral hardening, with the photon index reaching a
value smaller than 1.5, observed during a local flux minimum. If the observed
X-ray emission is due to the inverse Compton (IC) losses of the 10 MeV
electrons, then such a hard spectrum can be a result of Coulomb losses, or can
be related to the existence of the low-energy cut-off in the electron spectrum.
Alternatively, if the X-ray emission is a synchrotron emission of very high
energy electrons, the observed hard spectrum can be explained if the high
energy electrons are cooled by IC emission in Klein-Nishina regime.
Unfortunately the lack of simultaneous data in the TeV energy band prevents us
from making a definite conclusion on the nature of the observed spectral
hardening and, therefore, on the origin of the X-ray emission.
