Swift X-Ray Telescope and Very Large Telescope Observations of the Afterglow of GRB 041223

Abstract

GRB 041223 was the first gamma-ray burst detected by the Swift X-Ray Telescope (XRT), beginning 4.6hr after the burst. The X-ray detection triggered a VLT observation of the optical/near-infrared(NIR) counterpart, located about 1 ##IMG## [http://ej.iop.org/icons/Entities/farcs.gif] {farcs} 1from the XRT position. The X-ray counterpart faded rapidly, with a power-law index of -1.72 ± 0.20,with an average unabsorbed X-ray flux of 6.5 × 10 -12 ergs cm -2 s -1 in the 0.5-10 keV band for apower-law spectrum of photon index 2.02 ± 0.13 with Galactic absorption. The NIR counterpart wasobserved at three epochs between 16 and 87 hr after the burst and faded with a power-law index of-1.14 ± 0.08 with a reddening-corrected spectral energy distribution power-law slope of -0.40 ±0.03. We find that the X-ray and NIR data are consistent with a two-component jet in a wind medium.