  Cosmic-ray acceleration has been a long-standing mystery and despite more
than a century of study, we still do not have a complete census of acceleration
mechanisms. The collision of strong stellar winds in massive binary systems
creates powerful shocks, which have been expected to produce high-energy
cosmic-rays through Fermi acceleration at the shock interface. The accelerated
particles should collide with stellar photons or ambient material, producing
non-thermal emission observable in X-rays and gamma-rays. The supermassive
binary star eta Carinae drives the strongest colliding wind shock in the solar
neighborhood. Observations with non-focusing high-energy observatories indicate
a high energy source near eta Carinae, but have been unable to conclusively
identify eta Carinae as the source because of their relatively poor angular
resolution. Here we present the first direct focussing observations of the
non-thermal source in the extremely hard X-ray band, which is found to be
spatially coincident with the star within several arc-seconds. These
observations show that the source of non-thermal X-rays varies with the orbital
phase of the binary, and that the photon index of the emission is similar to
that derived through analysis of the gamma-ray spectrum. This is conclusive
evidence that the high-energy emission indeed originates from non-thermal
particles accelerated at colliding wind shocks.
