  The X-ray light curves of accreting black holes and neutron stars in binary
systems show various types of quasi-periodic oscillations (QPOs), the origin of
which is still debated. The Relativistic Precession Model identifies the QPO
frequencies with fundamental time scales from General Relativity, and has been
proposed as a possible explanation of certain types of such oscillations. Under
specific conditions (i.e., the detection of a particular QPOs triplet) such a
model can be used to obtain self-consistent measurements of the mass and spin
of the compact object. So far this has been possible only in the black hole
binary GRO J1655-40. In the RXTE/PCA data from the 1999-2000 outburst of the
black hole transient XTE J1859+226 we found a QPO triplet, and used the the
Relativistic Precession Model to obtain high-precision measurements of the
black hole mass and spin - M = (7.85+/-0.46) Msun, a* = 0.149+/-0.005 - the
former being consistent with the most recent dynamical mass determination from
optical measurements. Similarly to what has been already observed in other
black hole systems, the frequencies of the QPOs and broad-band noise components
match the general relativistic frequencies of particle motion close to the
compact object predicted by the model. Our findings confirm previous results
and further support the validity of the Relativistic Precession Model, which is
the only electromagnetic-measurement-based method that so far has consistently
yielded spins close to those from the gravitational waves produced by merging
binary black holes.
