  Swift J1626.6-5156 is a Be/X-ray binary that was in outburst from December
2005 until November 2008. We have examined RXTE/PCA and HEXTE spectra of three
long observations of this source taken early in its outburst, when the PCA 2-20
keV count rate was >70 counts/s/PCU, as well as several combined observations
from different stages of the outburst. The spectra are best fit with an
absorbed cutoff power law with a ~6.4 keV iron emission line and a Gaussian
optical depth absorption line at ~10 keV. We present strong evidence that this
absorption-like feature is a cyclotron resonance scattering feature, making
Swift J1626.6-5156 a new candidate cyclotron line source. The redshifted energy
of ~10 keV implies a magnetic field strength of ~8.6(1+z) x 10^11 G in the
region of the accretion column close to the magnetic poles where the cyclotron
line is produced. Analysis of phase averaged spectra spanning the duration of
the outburst suggests a possible positive correlation between the fundamental
cyclotron energy and source luminosity. Phase resolved spectroscopy from a long
observation reveals a variable cyclotron line energy, with phase dependence
similar to a variety of other pulsars, as well as the first harmonic of the
fundamental cyclotron line.
