Fig. 2
Light curves and hardness ratio of Vela X-1 (Chandra ObsID 1928) in the 0.5–3 keV and 3–10 keV energy bands, binned to ~283 s. The hardness is defined as the quotient of the count rates in the 3–10 keV and 0.5–3 keV bands. Gray shaded areas indicate the time periods used to extract the high-hardness spectrum, and unshaded regions are used for the low-hardness spectrum.
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