The Extinction Towards the GRB970228 Field

We determine the local galactic extinction towards the field of gamma-ray burst GRB970228 using a variety of methods. We develop a maximum likelihood method for measuring the extinction by comparing galaxy counts in the field of interest to those in a field of known extinction, and apply this method to the GRB970228 field. We also measure the extinction by comparing the observed stellar spectral energy distributions of stars in the GRB970228 field to the spectral energy distribution of library spectra of the same spectral type. Finally we estimate the extinction using the Balmer emission line ratios of a galaxy in the GRB970228 field, and the neutral hydrogen column density and amount of infrared dust emission toward this field. Combining the results of these methods, we find a best-fit galactic extinction in the optical of $A_V=1.19^{+0.10}_{-0.17}$, which implies a a substantial dimming and change of the spectral slope of the intrinsic GRB970228 afterglow.