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C.S. Pilman, J.F. Sepinsky, E.F. Guinan, G.P. McCook (Villanova University), I. Ribas (Universidad de Barcelona)
Cygnus X-1 is one of the best studied variable X-ray sources. About 30 yrs. ago this strong X-ray source was identified with a 9th mag spectroscopic binary (HDE 226868) having an orbital period of P = 5.60 days. This binary system shows complicated, low amplitude ellipsoidal light variations and consists of a O9.7 Iab star and a probable 7-10 M\odot black hole companion with an x-ray emitting accretion disk. Long-term X-ray and radio observations of the system show it to change on times scales of seconds (flickering), minutes-hours (flares), days (orbital), months-years (low/hard states and high/soft x-ray states). Some of the long-term x-ray and radio variations appear to be related to the semi-periodic changes in the accretion disk and also may arise from a precessing disk and jet.
We report on Stromgren uvby and H\alpha narrow and intermediate band photometry of Cyg X-1 that is being carried out with the Four College Consortium 0.8m Automatic Photometric Telescope (FFCAPT). The observations started in September 2000 and are continuing through the Fall 2001. Light curves have been formed from the photometry, and show an show the underlying the 5.60 day low amplitude ( y-amp = 0.045mag) light variations expected to arise from the tidal and rotational distortion of the luminous supergiant. However, near phase 0.20P to 0.35P persistent brightness enhancements of ~0.02 mag are frequently observed. The least active orbital phases occur near 0.45 - 0.65P. Also present are apparently random light variations of \approx 0.01mag. These most likely arise from small light variations (non-radial pulsations) of the O9 supergiant. The H\alpha photometry reveals a weak variation of H\alpha emission/absorption as a function of orbital phase. We have downloaded ongoing X-ray observations of Cyg X-1 made by RXTE to search for correlations. Also we have carried out an analysis of the light curves using the Wilson-Devinney Binary Code to compute refined values of the orbital inclination and mass ratio. We discuss the results of the modeling of the optical and x-ray observations and present an integrated model of the system in which the light enhancements observed near 0.3 phase appear to arise from the impact of the stellar wind on the accretion disk.
This research is supported by NSF/RUI Grant 00-71260 and by the Undergraduate Summer Research Assistance Program Grant from the Delaware Space Grant College Consortium.
The author(s) of this abstract have provided an email address for comments about the abstract: edward.guinan@villanova.edu