  The radiation from accreting X-ray pulsars was expected to be highly
polarized, with some estimates for the polarization degree of up to 80%.
However, phase-resolved and energy-resolved polarimetry of X-ray pulsars is
required in order to test different models and to shed light on the emission
processes and the geometry of the emission region. Here we present the first
results of the observations of the accreting X-ray pulsar Vela X-1 performed
with the Imaging X-ray Polarimetry Explorer (IXPE). Vela X-1 is considered to
be the archetypal example of a wind-accreting high-mass X-ray binary system,
consisting of a highly magnetized neutron star accreting matter from its
supergiant stellar companion. The spectro-polarimetric analysis of the
phase-averaged data for Vela X-1 reveals a polarization degree (PD) of
2.3$\pm$0.4% at the polarization angle (PA) of -47.3$\pm$5.4 deg. A low PD is
consistent with the results obtained for other X-ray pulsars and is likely
related to the inverse temperature structure of the neutron star atmosphere.
The energy-resolved analysis shows the PD above 5 keV reaching 6-10%, and a 90
deg difference in the PA compared to the data in the 2-3 keV range. The
phase-resolved spectro-polarimetric analysis finds a PD in the range 0-9% with
the PA varying between -80 and 40 deg.
