SED of GRB 050904 at redshift z = 6.29 (Kawai et al. 2006) using the detection from PdBI 5.896 days after the burst (Castro-Tirado et al., in prep.). The X-ray flux is estimated from the XRT light curve, the radio data are from Frail et al. (2006), while optical/NIR are extrapolations from Tagliaferri et al. (2005); Haislip et al. (2006). For the rough synchrotron spectrum drawn in blue, we have assumed an electron energy distribution index of p = 2.30, while an SMC extinction with A_V = 0.12 (consistent with the value derived by Stratta et al. 2011; but see also Zafar et al. 2010) and Lyman blanketing (Madau 1995) have been considered in the red dotted line to match the optical/NIR observations. With these data we can constrain the position of ν_a < 2 × 10¹¹ Hz and 2 × 10¹¹ < ν_c < 10¹³ Hz.