Physics
Scientific paper
Jan 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21530301h&link_type=abstract
American Astronomical Society, AAS Meeting #215, #303.01; Bulletin of the American Astronomical Society, Vol. 42, p.299
Physics
1
Scientific paper
The dark burst problem remains a major mystery of the gamma-ray burst (GRB) phenomenon. Without the sub-arcsecond position of an optical afterglow it is usually not possible to identify a host galaxy with confidence, so the burst's distance and environment are usually not known. GRB 090417B was an optically-dark burst detected and localized by Swift. It had a bright X-ray afterglow, but upper limits on the optical flux are strongly inconsistent with a synchrotron spectrum, making GRB 090417B formally a dark GRB. GRB 090417B has a high-precision X-ray position that is coincident with a z = 0.345 SSDS galaxy, which we take as the host galaxy. Broadband colours suggests that the host is an Sa or irregular galaxy with a 9 ± 2 Gyr stellar population and little ongoing star formation. The host's ultraviolet and optical spectral energy distribution suggests that the galaxy has a Milky Way extinction law with a mean extinction of AV = 3.5 mag. The spectrum of the X-ray afterglow yields a neutral hydrogen column density that is consistent with an extinction of AV = 11 mag along the line of sight within the host. GRB 090417B's X-ray afterglow shows no evidence for unusual physics before 70 ks. However, after 70 ks the X-ray spectrum becomes significantly softer than predicted by the standard fireball model. This softening can be explained using a dust scattering model if we assume a sheet of dust located 30-80 pc from the burst. This model predicts a host extinction along the line of sight of AV = 15-40 mag, which is consistent with that determined from the early-time X-ray spectrum. We suggest that GRB 090417B is a dark burst because of a dense, dusty region along the line-of-sight in the host galaxy.
Cummings Jay R.
Fonseca Emmanuel
Fynbo Johan P. U.
Holland Stephen
Roming Pete
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