Orbital, precessional and flaring variability of Cygnus X-1

We present the results of a 2.5-yr multiwavelength monitoring programme of Cygnus X-l, making use of hard and soft X-ray data, optical spectroscopy, U BVJHK photometry and radio data. In particular, we confirm that the 5.6-d o rbital period is apparent in all wavebands, and note the existence of a wav elength dependence to the modulation, in the sense that higher energies rea ch minimum first. We also find a strong modulation at a period of 142 +/- 7 d, which we suggest is caused by precession and/or radiative warping of th e accretion disc. Strong modulation of the hard and soft X-lay flux at this long period may not be compatible with simple models of an optically thin accretion flow and corona in the low state. We present the basic components required for more detailed future modelling of the system - including a pa rtially optically thick jet, quasi-continuous in the low state, the base of which acts as the Comptonizing corona. In addition, we find that there are a number of flares that appear to be correlated in at least two wavebands and generally in more. We choose two of these flares to study in further de tail, and find that the hard and soft X-rays are well correlated in the fir st, and that the soft X-rays and radio are correlated in the second. In gen eral, the optical and infrared show similar behaviour to each other, but ar e not correlated with the X-rays or radio.