Swift XRT and VLT Observations of the Afterglow of GRB 041223
David N. Burrows, Joanne E. Hill, Guido Chincarini, Gianpiero Tagliaferri, Sergio Campana, Alberto Moretti, Patrizia Romano, Daniele Malesani, Judith L. Racusin, Shiho Kobayashi, Bing Zhang, Peter Meszaros, Paul T. O'Brien, Richard Willingale, Julian P. Osborne, Giancarlo Cusumano, Paolo Giommi, Lorella Angelini, Antony F. Abbey, L. Angelo Antonelli, Andrew P. Beardmore, Milvia Capalbi, Stefano Covino, Paolo D'Avanzo, Michael R. Goad, Jamie A. Kennea, David C. Morris, Claudio Pagani, Kim L. Page, Luigi Stella, John A. Nousek, Alan A. Wells, Neil Gehrels
Abstract
The Swift Gamma-Ray Burst Explorer, launched on 2004 November 20, is a multiwavelength, autonomous, rapid-slewing observatory for gamma-ray burst (GRB) astronomy. On 2004 December 23, during the activation phase of the mission, the Swift X-Ray Telescope (XRT) was pointed at a burst discovered earlier that day by the Swift Burst Alert Telescope. A fading, uncataloged X-ray source was discovered by the XRT and was observed over a period of about 3 hours, beginning 4.6 hours after the burst. The X-ray detection triggered a VLT observation of the optical/NIR counterpart, located about 1.1 arcseconds from the XRT position. The X-ray counterpart faded rapidly, with a power law index of -1.72 +/- 0.20. The average unabsorbed X-ray flux 4.6-7.9 hours after the burst was 6.5 x 10^{-12} erg 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 hours after the burst, and faded with a power-law index of -1.14 +/- 0.08 with a reddening-corrected SED 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, with an early jet break in the narrow component and an underlying electron index of 1.8-2.0.