Long term variability of Cygnus X-1

III. Radio–X-ray correlations

¹ Institut für Astronomie und Astrophysik – Abt. Astronomie, Universität Tübingen, Sand 1, 72076 Tübingen, Germany e-mail: gleiss@astro.uni-tuebingen.de
² Mullard Radio Astronomy Observatory, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK
³ Massachusetts Institute of Technology, Center for Space Research, 77 Massachusetts Ave., Cambridge MA 02139, USA
⁴ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85748 Garching, Germany
⁵ INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland
⁶ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, and Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands

Received: 16 February 2004
Accepted: 24 June 2004

Abstract

Long time scale radio–X-ray correlations in black holes during the hard state have been found in many sources and there seems to emerge a universal underlying relationship which quantitatively describes this behavior. Although it would appear only natural to detect short term emission patterns in the X-ray and – with a certain time lag – in the radio, there has been little evidence for this up to now. The most prominent source for radio–X-ray correlations on short time scales (minutes) so far remains GRS 1915+105 where a single mass ejection could be detected successively in X-ray, IR, and radio wavebands. We analyze a database of more than 4 years of simultaneous radio–X-ray data for Cygnus X-1 from the Ryle Telescope and RXTE PCA/HEXTE. We confirm the existence of a radio–X-ray correlation on long time scales, especially at hard energies. We show that apparent correlations on short time scales in the lightcurves of Cygnus X-1 are most likely the coincidental outcome of white noise statistics. Interpreting this result as a breakdown of radio–X-ray correlations on shorter time scales, this sets a limit to the speed of the jet.