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Astron. Astrophys. 339, L37-L40 (1998)

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Letter to the Editor

The absorption properties of gamma-ray burst afterglows measured by BeppoSAX

Alan Owens 1, M. Guainazzi 1, T. Oosterbroek 1, A. Orr 1, A.N. Parmar 1, E. Costa 2, M. Feroci 2, L. Piro 2, P. Soffitta 2, D. Dal Fiume 3, F. Frontera 3, 4, E. Palazzi 3, E. Pian 3, J. Heise 5, J.J.M. in 't Zand 5, M.C. Maccarone 6 and L. Nicastro 6

1 Astrophysics Division, Space Science Department of ESA, ESTEC, 2200 AG Noordwijk, The Netherlands
2 Instituto Astrofisica Spaziale, CNR., Via Fosso del Cavaliere, I-00131 Roma, Italy
3 ITESRE/CNR, via Gobetti 101, I-40129 Bologna, Italy
4 Dipartimento di Fisica, Universita di Ferrara, Via Paradiso 12, I-44100 Ferrara, Italy
5 Space Research Organization Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
6 IFCAI/CNR, Via U. La Malfa 153, I-90146 Palermo, Italy

Received 8 September 1998 / Accepted 28 September 1998

Abstract

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 [FORMULA]2 and absorption, [FORMULA], marginally consistent with the galactic value. Taken collectively, the data are inconsistent with zero [FORMULA] at the [FORMULA]99.999% confidence level. The data are only marginally consistent with a distribution of column densities varying as the total galactic [FORMULA] in the direction of each of the bursts ([FORMULA]=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 [FORMULA] fixed at the galactic value and additional, redshifted, absorption to all 6 afterglow spectra, the best-fit average [FORMULA] within the host objects is found to be (1.01[FORMULA]) [FORMULA] 1022 atom cm-2. 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.

Key words: gamma-rays: bursts – X-rays: general

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© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
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