Coordinated observations of Cyg X-1 (V1357 Cyg) from 1994–1998 in the commonwealth of independent states

Abstract

We present observations of Cyg X-1 obtained during the coordinated international campaign “Optical Monitoring of Unique Astrophysical Objects,” carried out in observatories of CIS countries (Georgia, Kazakhstan, Russia, Uzbekistan, and Ukraine) in 1994–1998. The data are presented as a single set, taking into account systematic differences between individual data sets. In total, 2258 UBVR observations were obtained during 407 nights. The observations were carried out simultaneously with X-ray observations at 2–10 keV (ASM/RXTE) and 20–100 keV (BATSE/CGRO), and also with radio observations (in 1997). Our optical data partially overlap the soft X-ray outburst in 1996 (JD 2450200-320) and subsequent hard state, which displayed various types of increases and decreases of the X-ray flux. The 1996 outburst was not accompanied by any optical brightening. However, an orbital light curve corresponding to a tidally distorted star was superimposed on individual flares and brightness dips. For example, in two independent sets of observations, an intense optical flare with amplitude $0\mathop .\limits^m 04$ above the quiescent level was detected on JD 2450988, preceding the X-ray flare on JD 2451000-51040. In this time interval, the system displayed chaotic intrinsic variability on a timescale of a day, and flares and brightness dips with amplitude $0\mathop .\limits^m 10$ lasting up to several days. The variability power spectra display a predominant secondary period at 147^d±2^d, which is half the 294^d “precessional” period previously found at both X-ray and optical wavelengths. We show that the 147^d period dominates in X-ray data (ASM/RXTE) in the quiescent state, or after subtraction of the 1996 outburst radiation from the X-ray light curve. This period is also close to the period of 142^d±7^d derived from recent radio data. Cross-correlation analysis confirms a significant correlation between the long-term optical and 2–10 keV X-ray variations (without the soft X-ray outburst), with the X-ray variations lagging by 12±2 days.