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 4.6 hr after the burst. The X-ray detection triggered a VLT observation of the optical/near-infrared (NIR) counterpart, located about 1″.1 from 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 a power-law spectrum of photon index 2.02 ± 0.13 with Galactic absorption. The NIR counterpart was observed 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. © 2005. The American Astronomical Society. All rights reserved.