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We have developed a new method, close in philosophy to the photometricredshift technique, which can be applied to spectral data of very lowsignal-to-noise ratio. Using it we intend to measure redshifts while minimisingthe dangers posed by the usual extraction techniques. GRB afterglows havegenerally very simple optical spectra over which the separate effects ofabsorption and reddening in the GRB host, the intergalactic medium, and our ownGalaxy are superimposed. We model all these effects over a series of templateafterglow spectra to produce a set of clean spectra that reproduce what wouldreach our telescope. We also model carefully the effects of thetelescope-spectrograph combination and the properties of noise in the data,which are then applied on the template spectra. The final templates arecompared to the two-dimensional spectral data, and the basic parameters(redshift, spectral index, Hydrogen absorption column) are estimated usingstatistical tools. We show how our method works by applying it to our data ofthe NIR afterglow of GRB090423. At z ~ 8.2, this was the most distant objectever observed. We use the spectrum taken by our team with the TelescopioNazionale Galileo to derive the GRB redshift and its intrinsic neutral Hydrogencolumn density. Our best fit yields z=8.4^+0.05/-0.03 and N(HI)<5x10^20 cm^-2,but with a highly non-Gaussian uncertainty including the redshift range z [6.7,8.5] at the 2-sigma confidence level. Our method will be useful to maximise therecovered information from low-quality spectra, particularly when the set ofpossible spectra is limited or easily parameterisable while at the same timeensuring an adequate confidence analysis.

Spectrophotometric Redshifts. A New Approach to the Reduction of Noisy Spectra and its Application to GRB090423