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First Detection of a Foreground Damped Ly-Alpha Absorber Along a GRB Line of Sight?We present a VLT spectrum of the optical afterglow of GRB 991216, taken 1.5 days after the burst, and HST (Hubble Space Telescope) imaging of the host galaxy, obtained four months later. The spectrum contains three metal absorption-line systems with redshifts z = 1.024, z = 0.803, and z = 0.771, where the highest redshift most likely reflects the distance to the host galaxy. For the z = 1.024 and z = 0.803 systems we tentatively detect MgI which suggests a dense environment at these redshifts. This and the strength of the z = 0.803 Fe lines indicate that this system very likely is a damped Ly-alpha absorber (DLA), which would be the first foreground DLA to be detected along a GRB afterglow sight line. The HST images are consistent with these findings: they show two blobs of light, one underneath the projected OT position, the presumed host galaxy, and the other 0.6" away, which is probably responsible for the absorption lines at z = 0.803. The lowest redshift system can be explained by either one of the two galaxies that are located roughly 2" away from the transient. Including these newly found systems, the total number of DLAS and Lyman limit systems along GRB afterglow sight lines is consistent with the number expected from QSO (quasi-stellar object) absorption line studies. We expect early spectroscopy of GRB afterglows to significantly increase the number of detected foreground absorption systems, and we discuss some advantages over QSO lines of sight.
Document ID
20010039411
Acquisition Source
Marshall Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Authors
Vreeswijk, P. M.
(Amsterdam Univ. Netherlands)
Fruchter, A. S.
(Space Telescope Science Inst. Baltimore, MD United States)
Pian, E.
(Consiglio Nazionale delle Ricerche Bologna, Italy)
Rol, E.
(Amsterdam Univ. Netherlands)
Wijers, R. A. M. J.
(State Univ. of New York Stony Brook, NY United States)
Kouveliotou, C.
(NASA Marshall Space Flight Center Huntsville, AL United States)
Kaper, L.
(Amsterdam Univ. Netherlands)
Palazzi, E.
(Consiglio Nazionale delle Ricerche Bologna, Italy)
Masetti, N.
(Consiglio Nazionale delle Ricerche Bologna, Italy)
Frontera, F.
(Consiglio Nazionale delle Ricerche Bologna, Italy)
Rose, M. Franklin
Date Acquired
August 20, 2013
Publication Date
January 1, 2001
Subject Category
Astronomy
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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