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The development of techniques to measure accurately black hole spins iscrucial to study the physics and astrophysics of these objects. X-rayreflection spectroscopy is currently the most popular method to estimate thespins of accreting black holes; so far it has provided a spin measurement ofabout 40 stellar-mass black holes in X-ray binaries and 40 supermassive blackholes in active galactic nuclei. The relativistic precession model (RPM) isanother method to measure the spins of stellar-mass black holes: it requiresthe measurement of the frequencies of three simultaneous quasi-periodicoscillations and can potentially provide precise estimates of the black holemass and spin. However, the two methods do not seem to provide consistentresults when applied to the same sources, which questions the reliability andaccuracy of these measurements. Recently, the RPM has been applied to infer thespin of the black hole in XTE J1859+226. The authors found $a_* = 0.149 \pm0.005$ (68% CL). There are no other spin measurements of this source. We lookedfor archived RXTE observations of XTE J1859+226 with blurred reflectionfeatures and found 23 spectra suitable for measuring the spin. We employed twodifferent models with relxill and relxillD and obtained a higher spin valuefrom all these fits. From simultaneous fitting of 7 spectra of higher quality,we found $a_* = 0.986^{+0.001}_{-0.004}$ and $a_* =0.987 \pm 0.003$ (90% CL,statistical) with relxill and relxillD, respectively. Our results confirm thediscrepancy between the spin measurements inferred from the two techniques.

Measuring Black Hole Spins through X-ray Reflection Spectroscopy and the Relativistic Precession Model: the case of XTE J1859+226