  This work is devoted to the study of the broadband 0.8-79 keV spectral and
timing properties of the poorly studied X-ray pulsar XTE J1859+083 during its
2015 outburst based on the data from the NuSTAR and Swift observatories. We
show that the source pulse profile has complex shape that depends on the energy
band. Pulse fraction of XTE J1859+083 has constant value around 35% in the
broad energy band, this behaviour is atypical for X-ray pulsars. At the same
time its energy spectrum is typical of this class of objects and has a
power-law shape with an exponential cutoff at high energies. No cyclotron
absorption line was discovered in the source spectrum. On the basis of indirect
method and the absence of a cyclotron line, an estimation was made for the
magnetic field strength as less than $5\times10^{11}$ G or belonging to the
interval from $5\times10^{12}$ to $2.0^{+0.9}_{-1.2}\times10^{13}$ G. Data from
the NOT and SALT telescopes as well as optical and IR sky surveys allowed us
also to study the nature of its optical companion. We have proposed and studied
new possible candidates for the optical companion of XTE J1859+083 and the most
likely candidate was identified. The results of the optical and IR photometry
and spectroscopy of these possible companions showed that the system is a Be
X-ray binary, showing Br$\gamma$, He I and strong H$\alpha$ spectral lines.
