Structure function in bins of X-ray luminosity, represented as points connected by continuous lines. Straight lines with different dash styles: least-squares fits weighted according to the number of points in each bin of time lag. 10^43.5   erg/s < L_X < 10⁴⁴   erg/s: circles, dotted lines; 10⁴⁴   erg/s < L_X < 10^44.5   erg/s: squares, short-dashed lines; 10^44.5   erg/s < L_X < 10⁴⁵   erg/s: triangles, long-dashed lines; 10⁴⁵   erg/s < L_X < 10^45.5   erg/s: crosses, dot-dashed lines.