Contours of goodness of fit for a power-law fit to the bolometric flux data for the burst from 0918-549 (upper plot) and burst number 2 from SAX J1808.4-3658 (lower) as a function of t_s and α. Three contours are drawn, for with Δχ² = 1.0,2.3 and 4.6 (i.e., not per degree of freedom). The first contour delimits the single parameter 1σ region. The second and third contour delimit the 68% and 90% confidence regions. The contours show a coupling between both parameters.