Two illustrative cases for the estimate of the continuum power law and the escape fraction. The upper panels show a QSO of redshift 3.815 with a fesc,q ∼ 1, the lower panels a QSO of redshift 3.726 with a fesc,q ∼ 0. Left-hand column: the two spectra are plotted in black. Note that the region blueward of the Lyman α has been divided by the average transmission of the IGM estimated (see Section 4) according to Inoue et al. (2014) in order to show where the average continuum should be located. The green line shows the uncertainty of the flux. The red line shows the fitted power-law continuum. Right-hand column: the two observed spectra are plotted in black, the expected position of the power-law continuum multiplied by the average IGM transmission is in red and the uncertainty of the observed flux in green. The cyan portion of the spectrum is the region where the escape fraction of the UV photons produced by the QSO has been estimated.
