Letter to the Editor

A refined position catalogue of the Swift XRT afterglows

¹ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy e-mail: moretti@merate.mi.astro.it
² ASI Science Data Center, via G. Galilei, 00044 Frascati, Italy
³ NASA/Goddard Space Flight Center, Greenbelt Road, Greenbelt, MD 20771, USA
⁴ Department of Physics and Astronomy, Johns Hopkins University, 3400 Charles Street, Baltimore, MD 21218, USA
⁵ Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD 21044-3432, USA
⁶ Department of Astronomy & Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
⁷ X-Ray Observational Astronomy Group, Department of Physics & Astronomy, University of Leicester, LE1 7RH, UK
⁸ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Sezione di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
⁹ Università degli Studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, 20126 Milano, Italy

Received: 18 November 2005
Accepted: 19 January 2006

Abstract

We present a catalogue of refined positions of 68 gamma ray burst (GRB) afterglows observed by the Swift X-ray Telescope (XRT) from the launch up to 2005 Oct. 16. This is a result of the refinement of the XRT boresight calibration. We tested this correction by means of a systematic study of a large sample of X-ray sources observed by XRT with well established optical counterparts. We found that we can reduce the systematic error radius of the measurements by a factor of two, from 6.5´´ to 3.2´´ (90% of confidence). We corrected all the positions of the afterglows observed by XRT in the first 11 months of the Swift mission. This is particularly important for the 37 X-ray afterglows without optical counterpart. Optical follow-up of dark GRBs, in fact, will be more efficient with the use of the more accurate XRT positions.