Variation in Emission and Absorption Lines and Continuum Flux by Orbital Phase in Vela X-1

Abstract

High-resolution spectral studies were undertaken at orbital phases (ϕ) 0, 0.25, and 0.5 on the high-mass X-ray binary Vela X-1 using archival Chandra data. We present (1) the first detailed analysis of the multiple strong narrow emission lines present in ϕ = 0.5, (2) an analysis of the absorption of the continuum in ϕ = 0.5, and (3) the first detection of narrow emission and absorption lines in ϕ = 0.25. Multiple fluorescent and H- and He-like emission lines in the band 1.6–20 Å in eclipse are partially obscured at ϕ = 0.25 by the X-ray continuum. The ϕ = 0.25 spectrum displays three triplets, two with a blueshifted resonance line in absorption and the intercombination and forbidden lines in emission, and shows in absorption other blueshifted lines seen in emission in eclipse. At ϕ = 0.5 the soft X-ray continuum diminishes revealing an "eclipse-like" spectrum; however, line flux values are around 13-fold those in eclipse. We conclude the narrow emission lines in Vela X-1 become apparent when the continuum is blocked from the line of sight, either by eclipse or by scattering and/or absorption from a wake or cloud. The H- and He-like lines arise in warm photoionized regions in the stellar wind, while the fluorescent lines (including a Ni Kα line) are produced in cooler clumps of gas outside these regions. Absorption of the 5–13 Å continuum at ϕ = 0.5 may be caused by an accretion wake composed of dense stagnant photoionized plasma inside a Stromgren zone. Multiple fluorescent emission lines may be a common feature of the supergiant category of HMXBs.