THE Be/X-RAY BINARY SWIFT J1626.6−5156 AS A VARIABLE CYCLOTRON LINE SOURCE

Abstract

Swift J1626.6−5156 is a Be/X-ray binary that was in outburst from 2005 December until 2008 November. We have examined Rossi X-ray Timing Explorer/Proportional Counter Array (PCA) and High Energy X-ray Timing Explorer 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⁻¹ PCU⁻¹, 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) × 10¹¹ 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.