Structure function in bins of X-ray luminosity. Black lines and crosses denote 10⁴³ erg / s <L_X ≤ 10^43.5 erg / s; blue lines and triangles denote 10^43.5 erg / s <L_X ≤ 10⁴⁴ erg / s; green lines and squares indicate 10⁴⁴ erg / s <L_X ≤ 10^44.5 erg / s; yellow lines and hexagons represent 10^44.5 erg / s <L_X ≤ 10⁴⁵ erg / s; and red lines and circles indicate 10⁴⁵ erg / s <L_X ≤ 10^45.5 erg / s.