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Register a new userUltraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state
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(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features show strong, broad absorption components when the X-ray source is behind the companion star and noticeably weaker absorption when the X-ray source is between us and the companion star. We fit the P Cygni profiles using the SEI method applied to a spherically symmetric stellar wind subject to X-ray photoionization from the black hole. The Si IV doublet provides the most reliable estimates of the parameters of the wind and X-ray illumination. The velocity $v$ increases with radius $r$ according to $v=v_infty(1-r_star/r)^beta$, with$betaapprox0.75$ and $v_inftyapprox1420$ km s$^{-1}$.The microturbulent velocity was $approx160$ km s$^{-1}$. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate of L$_{X,38}/dot M_{-6} approx 0.33$, measured at $dot M_{-6}$ = 4.8. Our models determine parameters that may be used to estimate the accretion rate onto the black hole and independently predict the X-ray luminosity. Our predicted L$_x$ matches that determined by contemporaneous RXTE ASM remarkably well, but is a factor of 3 lower than the rate according to Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the energy of accretion may go into powering a jet.
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We present results from Hubble Space Telescope UV spectroscopy of the massive X-ray binary system, HD226868 = Cyg X-1. The spectra were obtained at both orbital conjunction phases in two separate runs in 2002 and 2003 when the system was in the X-ray high/soft state. The stellar wind lines suffer large reductions in strength when the black hole is in the foreground due to the X-ray ionization of the wind ions. A comparison of HST and archival IUE spectra shows that similar photoionization effects occur in both the X-ray states. We constructed model UV wind line profiles assuming that X-ray ionization occurs everywhere in the wind except the zone where the supergiant blocks the X-ray flux. The good match between the observed and model profiles indicates that the wind ionization extends to near to the hemisphere of the supergiant facing the X-ray source. The H-alpha emission strength is generally lower in the high/soft state compared to the low/hard state, but the He II 4686 emission is relatively constant between states. The results suggest that mass transfer in Cyg X-1 is dominated by a focused wind flow that peaks along the axis joining the stars and that the stellar wind contribution is shut down by X-ray photoionization effects. The strong stellar wind from the shadowed side of the supergiant will stall when Coriolis deflection brings the gas into the region of X-ray illumination. This stalled gas component may be overtaken by the orbital motion of the black hole and act to inhibit accretion from the focused wind. The variations in the strength of the shadow wind component may then lead to accretion rate changes that ultimately determine the X-ray state.
We reported previously that for Cyg X-1 there is a settling period following the transition from hard to soft state (astro-ph/9610071). During the transiton, The low energy spectrum (below ~10 keV) varies significantly from observation to observation while the high energy portion changes little. The source reaches nominal soft-state brightness during the settling period. It can be characterized by a soft low-energy spectrum and significant low-frequency 1/f noise and white noise on the power density spectrum (PDS). The low-energy spectrum becomes even softer, and the PDS is completely dominated by the 1/f noise, when the ``true soft state is reached. In this paper, subsequent RXTE observations of Cyg X-1 in the soft state are examined, and the results confirm our earlier conclusions. Furthermore, we show the results from observations taken during a soft-to-hard transition. As expected, the white noise appears again, and accordingly, the 1/f noise becomes less dominant, similar to the settling period at the end of the hard-to-soft transition. The low-frequency 1/f noise has not been observed when Cyg X-1 is in the hard state. Therefore, it seems to be positively correlated with the disk mass accretion rate which is low in the hard state and high in the soft state. The difference in the observed spectral and timing properties between the hard and soft states is qualitatively consistent with a simple ``fluctuating corona model (astro-ph/9610071). Here we present more evidence for it.
Orbital variability has been found in the X-ray hardness of the black hole candidate Cygnus X-1 during the soft/high X-ray state using light curves provided by the Rossi X-ray Timing Explorers All Sky Monitor. We are able to set broad limits on how the mass-loss rate and X-ray luminosity vary between the hard and soft states. The folded light curve shows diminished flux in the soft X-ray band at phase 0 (defined as the time of of the superior conjunction of the X-ray source). Models of the orbital variability provide slightly superior fits when the absorbing gas is concentrated in neutral clumps and better explain the strong variability in hardness. In combination with the previously established hard/low state dips, our observations give a lower limit to the mass loss rate in the soft state (Mdot<2x10^{-6} Msun/yr) than the limit in the hard state (Mdot<4x10^{-6} Msun/yr). Without a change in the wind structure between X-ray states, the greater mass-loss rate during the low/hard state would be inconsistent with the increased flaring seen during the high-soft state.
We present results from quantitative modeling and spectral analysis of the high mass X-ray binary Vela X-1 obtained with the Chandra HETGS. The spectra exhibit emission lines from H-like and He-like ions driven by photoionization, as well as fluorescent emission lines from several elements in lower charge states. In order to interpret and make full use of the high-quality data, we have developed a simulator, which calculates the ionization and thermal structure of a stellar wind photoionized by an X-ray source, and performs Monte Carlo simulations of X-ray photons propagating through the wind. The emergent spectra are then computed as a function of the viewing angle accurately accounting for photon transport in three dimensions including dynamics. From comparisons of the observed spectra with the simulation results, we are able to find the ionization structure and the geometrical distribution of material in Vela X-1 that can reproduce the observed spectral line intensities and continuum shapes at different orbital phases remarkably well. It is found that a large fraction of X-ray emission lines from highly ionized ions are formed in the region between the neutron star and the companion star. We also find that the fluorescent X-ray lines must be produced in at least three distinct regions --(1)the extended stellar wind, (2)reflection off the stellar photosphere, and (3)in a distribution of dense material partially covering and possibly trailing the neutron star, which may be associated with an accretion wake. Finally, from detailed analysis of the emission lines, we demonstrate that the stellar wind is affected by X-ray photoionization.
Results of a 1999 July 8-10 BeppoSAX observation during an anomalous low-state of Her X-1 are presented. The standard on-state power-law and blackbody continuum model is excluded at high confidence unless partial covering is included. This gives a power-law photon index of 0.63 +/- 0.02 and implies that 0.28 +/- 0.03 of the flux undergoes additional absorption of (27 +/- 7) 10^22 atom/cm2. 11% of the observed 0.1-10 keV flux is from the 0.068 +/- 0.015 keV blackbody. 1.237747(2) s pulses with a semi-amplitude of 2.1 +/- 0.8% are detected at >99.5% confidence and confirmed by RXTE measurements. This implies that Her X-1 underwent substantial spin-down close to the start of the anomalous low-state. The spectral and temporal changes are similar to those recently reported from 4U1626-67. These may result from a strongly warped disk that produces a spin-down torque. The X-ray source is then mostly viewed through the inner regions of the accretion disk. A similar mechanism could be responsible for the Her X-1 anomalous low-states. Shadowing by such an unusually warped disk could produce observable effects in the optical and UV emission from the companion star.
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