Imaging extended sources with coded mask telescopes: application to the INTEGRAL IBIS/ISGRI instrument

¹ Service d'Astrophysique, CEA-Saclay, 91191 Gif-Sur-Yvette Cedex, France e-mail: mrenaud@cea.fr
² APC-UMR 7164, 11 place M. Berthelot, 75231 Paris, France
³ Department of Physics, NC State University, 2700 Stinson Drive, Box 8202, Raleigh, NC 27695, USA
⁴ Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA
⁵ Sabancı University, Orhanlı – Tuzla, Istanbul, 34956, Turkey

Received: 7 March 2006
Accepted: 30 May 2006

Abstract

Context.In coded mask techniques, reconstructed sky images are pseudo-images: they are maps of the correlation between the image recorded on a detector and an array derived from the coded mask pattern.

Aims.The INTEGRAL /IBIS   telescope provides images where the flux of each detected source is given by the height of the local peak in the correlation map. As such, it cannot provide an estimate of the flux of an extended source. What is needed is intensity sky images giving the flux per solide angle as typically done at other wavelengths.

Methods.In this paper, we present the response of the INTEGRAL IBIS /ISGRI coded mask instrument to extended sources. We develop a general method based on analytical calculations in order to measure the intensity and the associated error of any celestial source and validated with Monte-Carlo simulations.

Results.We find that the sensitivity degrades almost linearly with the source extent. Analytical formulae are given as well as an easy-to-use recipe for the INTEGRAL user. We check this method on IBIS /ISGRI data but these results are general and applicable to any coded mask telescope.