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Register a new userTemporal Properties of Cygnus X-1 During the Spectral Transitions
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Wei Cui
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We report the results from our timing analysis of 15 RXTE observations of Cygnus X-1 throughout its 1996 spectral transitions. The entire period can be divided into 3 distinct phases: (1) transition from the hard to soft state, (2) soft state, and (3) transition from the soft state back to the hard state. The observed X-ray properties in phases 1 and 3 are remarkably similar, suggesting that the same physical processes are likely involved in triggering such transitions. The power density spectrum (PDS) during the transition can be characterized by a red noise component, followed by a white noise component which extends to roughly 1-3 Hz where it is cut off, and a steeper power law at higher frequencies. The X-ray flux also exhibits apparent quasi-periodic oscillation (QPO) with the centroid frequency varying in the range of 4-12 Hz. The QPO shows no correlation with the source flux, but becomes more prominent at higher energies. This type of PDS bears resemblance to that of other black hole candidates often observed in a so-called very high state, although the origin of the observed QPO may be very different. The low-frequency red noise has not been observed in the hard state, thus seems to be positively correlated with the disk mass accretion rate which is presumably low in the hard state and high in the soft state; in fact, it completely dominates the PDS in the soft state. In the framework of thermalComptonization models, Cui et al. (see astro-ph/9610071 and astro-ph/9610072) speculated that the difference in the observed spectral and timing properties between the hard and soft states is due to the presence of a ``fluctuating Comptonizing corona during the transition. Here we present the measured hard X-ray time lags and coherence functions between various energy bands, and show that the results strongly support such a scenario.
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We present the long term evolution of the frequency-dependent X-ray time lags of the black hole candidate Cygnus X-1 as measured in 1996 and 1998 with the Rossi X-ray Timing Explorer (RXTE). Lag spectra measured during the 1996 June soft state are very similar to those seen during 1996 December and most of 1998 while Cyg~X-1 was in its hard state. During state transitions, however, the shape and magnitude of the X-ray lag is highly variable and tends to be much larger than outside of the state transitions. This behavior is most obvious in the 1--10 Hz band. The increase of the X-ray lag during the state transitions might be related to the formation and destruction of the synchrotron radiation emitting outflows present during the hard state.
We present the results of the analysis of the broad-band spectrum of Cygnus X-1 from 3.0 to 200 keV, using data from a 10 ksec observation by the Rossi X-ray Timing Explorer. The spectrum can be well described phenomenologically by an exponentially cut-off power law with a photon index Gamma = 1.45 +/- 0.02 (a value considerably harder than typically found), e-folding energy E_fold = 162 +/- 9 keV, plus a deviation from a power law that formally can be modeled as a thermal blackbody with temperature kT_bb = 1.2 +/1 0.2 keV. Although the 3 - 30 keV portion of the spectrum can be fit with a reflected power law with Gamma = 1.81 +/- 0.01 and covering fraction f = 0.35 +/- 0.02, the quality of the fit is significantly reduced when the HEXTE data in the 30 - 100 keV range is included, as there is no observed hardening in the power law within this energy range. As a physical description of this system, we apply the accretion disc corona models of Dove, Wilms & Begelman (1997) --- where the temperature of the corona is determined self-consistently. A spherical corona with a total optical depth tau = 1.6 +/- 0.1 and an average temperature kT_c = 87 +/- 5 keV, surrounded by an exterior cold disc, does provide a good description of the data (reduced chi-squared = 1.55). These models deviate from the data by up to 7% in the 5 - 10 keV range, and we discuss possible reasons for these discrepancies. However, considering how successfully the spherical corona reproduces the 10 - 200 keV data, such ``photon-starved coronal geometries seem very promising for explaining the accretion processes of Cygnus X-1.
In 2005 February we observed Cygnus X-1 over a period of 10 days quasi-continuously with the Rossi X-ray Timing Explorer and the Ryle telescope. We present the results of the spectral and timing analysis on a timescale of 90 min and show that the behavior of Cyg X-1 is similar to that found during our years long monitoring campaign. As a highlight we present evidence for a full transition from the hard to the soft state that happened during less than three hours. The observation provided a more complete picture of a state transition than before, especially concerning the evolution of the time lags, due to unique transition coverage and analysis with high time resolution.
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 report the results of an observation of Cygnus X-1 with INTEGRAL, that we combine with simultaneous radio observations with the Ryle telescope. Both spectral and variability properties of the source indicate that Cygnus X-1 was in an Intermediate State. The INTEGRAL spectrum shows a high-energy cut-off or break around 100 keV. The shape of this cut-off differs from pure thermal Comptonisation, suggesting the presence of a non-thermal component at higher energies.The average broad band spectrum is well represented by hybrid thermal/non-thermal Comptonisation models. During the 4 day long observation the source showed an important spectral and flux variability. A principal component analysis demonstrates that most of this variability occurs through 2 independent modes. The first mode consists in changes in the overall luminosity on time scale of hours with almost constant spectra that are strikingly uncorrelated with the variable radio flux. We interpret this variability mode as variations of the dissipation rate in the corona. The second variability mode consists in a pivoting of the spectrum around 10 keV. It acts on a longer time-scale: initially soft, the spectrum hardens in the first part of the observation and then softens again. This pivoting pattern is strongly correlated with the radio (15 GHz) emission: radio fluxes are stronger when the INTEGRAL spectrum is harder. We propose that the pivoting mode represents a mini state transition from a nearly High Soft State to a nearly Low Hard State, and back. This mini-transition would be caused by changes in the soft cooling photons flux in the hot Comptonising plasma associated with an increase of the temperature of the accretion disc. The jet power then appears to be anti-correlated with the disc luminosity and unrelated to the coronal power.
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