The Be/x-ray Binary Swift J1626.6−5156 as a Cyclotron Line Source

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 sPCU, as well as several combined observations from different stages of the source’s life. The spectra are best fit with an absorbed cut-off 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 feature is actually 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)×1011G in the region of the accretion column close to the magnetic poles where the cyclotron line is produced. Phase-resolved spectroscopy reveals the second harmonic of the fundamental cyclotron line. There is a significant positive correlation between the fundamental cyclotron energy and the source luminosity.