The catalog of variable sources detected by INTEGRAL

I. Catalog and techniques^*

¹ DESY Zeuthen, Platanenallee 6, 15738 Zeuthen, Germany
e-mail: telezhinsky@gmail.com
² Astronomical Observatory of Kiev University, Observatorna 3, 04058 Kiev, Ukraine
³ INAF/IASF Milano, via E. Bassini 15, 20133 Milano, Italy
⁴ ISDC Data Centre for Astrophysics, Chemin d’Écogia 16, 1290 Versoix, Switzerland
⁵ Observatoire Astronomique de l’Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland

Received: 27 March 2009
Accepted: 27 April 2010

Abstract

Context. During its 6 years of operation, INTEGRAL/ISGRI has detected more than 500 sources. Many of these sources are variable. Taking into account that nearly half of INTEGRAL/ISGRI sources are new and many of them remain unidentified, the variability properties of the sources can provide additional constraints to help us to classify and identify the unknown sources.

Aims. To study the variability properties of the sources detected by INTEGRAL/ISGRI, we develop a method to quantify the variability of a source. We describe here our techniques and compile a catalog of the sources that fit our criteria of variability.

Methods. We use the natural time binning of INTEGRAL observations called Science Window (≈2000 s) and test the hypothesis that the detected sources are constant using a χ² all-sky map in three energy bands (20–40, 40–100, 100–200 keV). We calculate an intrinsic variance of the flux in individual pixels and use it to define the fractional variability of a source. The method is sensitive to the source variability on timescales of one Science Window and higher. We concentrate only on the sources that were already reported to be detected by INTEGRAL.

Results. We present a catalog of 202 sources found to be significantly variable. For the catalog sources, we give the measure of variability and fluxes with corresponding errors in the 20–40, 40–100 and 100–200 keV energy bands, and we present some statistics about the population of variable sources. The description of the physical properties of the variable sources will be given in a forthcoming paper.