Other
Scientific paper
Nov 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002xrb..confe..25m&link_type=abstract
X-ray Binaries in the Chandra and XMM-Newton Era (with an emphasis on Targets of Opportunity), Abstracts of the conference held
Other
Optical Monitoring, The Optical/X-Ray Connection
Scientific paper
The H-alpha emission strength in Cygnus X-1 provides an important probe of the mass loss rate in the base of the stellar wind of the supergiant star. Optical spectra of the system from 1998 to 2002 show large variations in H-alpha over both long (years) and short (hours to days) timescales. When the spectra are grouped by H-alpha equivalent width, coherent orbital phase related variations in the profiles are seen. There is also an anti-correlation between the H-alpha emission strength and X-ray flux (from the Rossi X-ray Timing Explorer All Sky Monitor instrument) in the sense that when the H-alpha emission is strong the X-ray flux is weaker and the spectrum harder. On the other hand, there is no correlation between H-alpha emission strength and X-ray flux when H-alpha is weak. This relationship is not likely to be caused by wind X-ray absorption nor by the reduction in H-alpha emissivity by X-ray heating. Instead, the H-alpha variations probably track changes in wind density and strength near the photosphere. The density of the wind determines the size of X-ray ionization zones surrounding the black hole, and these in turn control the acceleration of the wind in the direction of the black hole. During the low/hard X-ray state, the strong wind is fast and the accretion rate is relatively low, while in the high/soft state the weaker, highly ionized wind attains only a moderate velocity and the accretion rate increases. It is also possible that X-rays from near the black hole cause radiative driving away from the accretion disk, further slowing the wind and enabling more accretion. The X-ray transitions from the normal low/hard to the rare high/soft state are probably triggered by episodes of decreased mass loss rate in the supergiant donor star.
High resolution Chandra spectra can help us understand the complex relationship between the variable stellar winds from the supergiant star and the X-ray flux from near the black hole. Schulz et al. (2001) took the first high-resolution spectrum of Cygnus X-1, at orbital phase phi = 0.93, during a transitional period of X-ray flaring within its low/soft state. Miller et al. (2002) and Marshall et al. (2001) also observed Cygnus X-1 at orbital phases phi = 0.76 and phi = 0.84 respectively, and they found distinct differences in the X-ray spectrum that could be attributed to either orbital phase or the longer term state of the system. Further observations during a constant high/soft state and low/hard state, combined with simultaneous H-alpha observations, would improve our knowledge of both long and short term changes in the ionization structure of the wind and accretion disk as the mass loss rate of the supergiant star changes. Chandra spectra during the high/soft state would also allow us to model the deceleration of the stellar winds due to X-ray radiation driving as they approach the black hole.
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