Orbital, precessional and flaring variability of Cygnus X-1

Abstract

We present the results of a 2.5-year multiwavelength monitoring programme of Cygnus X-1, making use of hard and soft X-ray data, optical spectroscopy, UBVJHK photometry and radio data. In particular we confirm that the 5.6-day orbital period is apparent in all wavebands and note the existence of a wavelength-dependence to the modulation, in the sense that higher energies reach minimum first. We also find a strong modulation at a period of 142 ± 7 days, which we suggest is due to precession and/or radiative warping of the accretion disc. Strong modulation of the hard and soft X-ray 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 partially optically thick jet, quasi-continuous in the low state, the base of which acts as the Comptonising corona. In addition, we find that there are a number of flares which appear to be correlated in at least two wavebands and generally in more. We choose two of these flares to study in further detail and find that the hard and soft X-rays are well-correlated in the first and that the soft X-rays and radio are correlated in the second. In general, the optical and infrared show similar behaviour to each other but are not correlated with the X-rays or radio.

Long term variability of Cygnus X-1

“…The orbital period is P = 5.599829 ± 0.000016 d with T 0 = 52 872.288 ± 0.009 ). The source also shows a superorbital period that has been reported as ∼300 d (Priedhorsky et al 1983), then as ∼150 d (Brocksopp et al 1999;Benlloch et al 2004;Ibragimov et al 2007), and lately as having changed back to ∼300 d (Zdziarski et al 2011).…”

Orbital, Precessional and Flaring Variability in Cygnus X-1

“…Indeed, we show in Brocksopp et al (1999) and Lyuty (1999 in prep. ) that the small deviations from orbital modulation are a result of the wind, although we have shown in section 3.2 that there is also a small contribution from the accretion disc.…”

“…We were surprised to see that the optical photometry did not fold onto this period satisfactorily, in contradiction to Brocksopp et al (1999) when a much longer dataset was used. We therefore separated the data according to the Xray state and, for the hard state, removed flares and scatter above ∼ 3σ from all datasets.…”

“…We use the ephemeris of Brocksopp et al (1999) with P orb =5.599829±0.000016 days. The starlink package period was used to fold all data on the orbital period with superior conjunction of the black hole corresponding to the zero point (JD 2441874.707).…”

Evolutionary Constraints on the Masses of the Components of HDE 226868/Cyg X-1 Binary System

“…The most recent determination of the mass function is f (Mx) = 0.251 ± 0.007 M⊙ (Gies et al 2003), which differs only slightly from the earlier values f (Mx) = 0.244 ± 0.005 M⊙ (Brocksopp et al 1999) and f (Mx) = 0.252 M⊙ (Gies & Bolton 1982). The corresponding projected radius of the optical component orbit is a1sini = 8.36 ± 0.08 R⊙.…”