Letter to the Editor

INTEGRAL-RXTE observations of Cygnus X-1^*

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85748 Garching, Germany
² INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland
³ Institut für Astronomie und Astrophysik – Abt. Astronomie, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁴ Astro Space Center of the PN Lebedev Physical Institute, 84/32 Profsoyuznaya Street, Moscow 117997, Russia
⁵ MIT-CXC, NE80-6077, 77 Massachusetts Ave., Cambridge, MA 02139, USA
⁶ DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), CEA Saclay, 91191 Gif-sur-Yvette, France
⁷ Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warszawa, Poland
⁸ Astrophysics, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK
⁹ GACE, Instituto de Ciencia de los Materiales, Universidad de Valencia, PO Box 22085, 46071 Valencia, Spain
¹⁰ Danish Space Research Institute, Juliane Maries Vej 30, 2100 Copenhagen Ø., Denmark
¹¹ Observatoire de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland

Corresponding author: K. Pottschmidt, katja.pottschmidt@obs.unige.ch

Received: 14 July 2003
Accepted: 18 August 2003

Abstract

We present first results from contemporaneous observations of Cygnus X-1 with INTEGRAL and RXTE, made during INTEGRAL's performance verification phase in 2002 November and December. Consistent with earlier results, the  keV data are well described by Comptonization spectra from a Compton corona with a temperature of  keV and an optical depth of plus reflection from a cold or mildly ionized slab with a covering factor of . A soft excess below 10 keV, interpreted as emission from the accretion disk, is seen to decrease during the 1.5 months spanned by our observations. Our results indicate a remarkable consistency among the independently calibrated detectors, with the remaining issues being mainly related to the flux calibration of INTEGRAL.