1998A&A...339L..37O

We present an analysis of the X-ray absorption properties of 6 gamma-ray burst (GRB) afterglows measured with BeppoSAX. Between 8 hrs and 20 hrs after the initial GRB trigger, individual spectra can be described by a power-law with a photon index of ∼2 and absorption, N_H, marginally consistent with the galactic value. Taken collectively, the data are inconsistent with zero N_H at the >99.999% confidence level. The data are only marginally consistent with a distribution of column densities varying as the total galactic N_H in the direction of each of the bursts (χ²=9.6 for 6 degrees of freedom). The data are consistent with cosmological models in which GRB occur within host galaxies. By simultaneously fitting a power-law spectral model with N_H fixed at the galactic value and additional, redshifted, absorption to all 6 afterglow spectra, the best-fit average N_H within the host objects is found to be (1.01±^0.28_0.51)x10²²atom/cm². This value is compatible with the host galaxy column densities inferred from optical data for GRB970508, GRB971214 and GRB980329, supporting the hypothesis that GRB occur within heavily absorbed star forming regions of their host galaxies.