Effects of the stellar wind on X-ray spectra of Cygnus X-3

We study X-ray spectra of Cyg X-3 from BeppoSAX , taking into account absorption and emission in the strong stellar wind of its companion. We find the intrinsic X-ray spectra are well modelled by disc blackbody emission, its upscattering by hot electrons with a hybrid distribution, and by Compton reflection. These spectra are strongly modified by absorption and reprocessing in the stellar wind, which we model using the photoionization code cloudy . The form of the observed spectra implies the wind is composed of two phases. A hot tenuous plasma containing most of the wind mass is required to account for the observed features of very strongly ionized Fe. Small dense cool clumps filling ≲0.01 of the volume are required to absorb the soft X-ray excess, which is emitted by the hot phase but not present in the data. The total mass-loss rate is found to be  (0.6–1.6) × 10⁻⁵ M_⊙ yr⁻¹  . We also discuss the feasibility of the continuum model dominated by Compton reflection, which we find to best describe our data. The intrinsic luminosities of our models suggest that the compact object is a black hole.     

Frequency-resolved spectroscopy of Cyg X-1: fast variability of the reflected emission in the soft state

Using RXTE /PCA data, we study the fast variability of the reflected emission in the soft spectral state of Cyg X-1 by means of Fourier frequency-resolved spectroscopy. We find that the rms amplitude of variations of the reflected emission has the same frequency dependence as the primary radiation down to time-scales of ≲30–50 ms. This might indicate that the reflected flux reproduces, with nearly flat response, variations of the primary emission. Such behaviour differs notably from that of the hard spectral state, in which variations of the reflected flux are significantly suppressed in comparison with the primary emission, on time-scales shorter than ∼0.5–1 s.
If related to the finite light-crossing time of the reflector, these results suggest that the characteristic size of the reflector, presumably an optically thick accretion disc, in the hard spectral state is larger by a factor of ≳5–10 than in the soft spectral state. Modelling the transfer function of the disc, we estimate the inner radius of the accretion disc to be R _in∼100 R _g in the hard state and R _in≲10 R _g in the soft state for a 10-M_⊙ black hole.     

The violent past of Cygnus X-2

Cygnus X-2 appears to be the descendant of an intermediate-mass X-ray binary (IMXB). Using Mazzitelli's stellar code we compute detailed evolutionary sequences for the system and find that its prehistory is sensitive to stellar input parameters, in particular the amount of core overshooting during the main-sequence phase. With standard assumptions for convective overshooting a case B mass transfer starting with a 3.5-M_⊙ donor star is the most likely evolutionary solution for Cygnus X-2. This makes the currently observed state rather short-lived, of order 3 Myr, and requires a formation rate > 10⁻⁷–10⁻⁶ yr⁻¹ of such systems in the Galaxy. Our calculations show that neutron star IMXBs with initially more massive donors (≳4 M_⊙) encounter a delayed dynamical instability; they are unlikely to survive this rapid mass transfer phase. We determine limits for the age and initial parameters of Cygnus X-2 and calculate possible dynamical orbits of the system in a realistic Galactic potential, given its observed radial velocity. We find trajectories which are consistent with a progenitor binary on a circular orbit in the Galactic plane inside the solar circle that received a kick velocity ≤200 km s⁻¹ at the birth of the neutron star. The simulations suggest that about 7 per cent of IMXBs receiving an arbitrary kick velocity from a standard kick velocity spectrum would end up in an orbit similar to Cygnus X-2, while about 10 per cent of them reach yet larger Galactocentric distances.     

The aperiodic broad-band X-ray variability of Cygnus X-3

We study the soft X-ray variability of Cygnus X-3. By combining data from the All-Sky Monitor and Proportional Counter Array instruments on the RXTE satellite with EXOSAT /Medium Energy (ME) detector observations, we are able to analyse the power density spectrum (PDS) of the source from 10⁻⁹ to 0.1 Hz, thus covering time-scales from seconds to years. As the data on the longer time-scales are unevenly sampled, we combine traditional power spectral techniques with simulations to analyse the variability in this range. The PDS at higher frequencies  (≳10⁻³ Hz)  are for the first time compared for all states of this source. We find that it is for all states well described by a power law, with index  ∼−2  in the soft states and a tendency for a less steep power law in the hard state. At longer time-scales, we study the effect of the state transitions on the PDS, and find that the variability below  ∼10⁻⁷ Hz  is dominated by the transitions. Furthermore, we find no correlation between the length of a high/soft-state episode and the time since the previous high/soft state. On intermediate time-scales, we find evidence for a break in the PDS at time-scales of the order of the orbital period. This may be interpreted as evidence for the existence of a tidal resonance in the accretion disc around the compact object, and constraining the mass ratio to   M ₂/ M ₁≲ 0.3  .     

A transient relativistic radio jet from Cygnus X-1

We report the first observation of a transient relativistic jet from the canonical black hole candidate, Cygnus X-1, obtained with the Multi-Element Radio-Linked Interferometer Network (MERLIN). The jet was observed in only one of six epochs of MERLIN imaging of the source during a phase of repeated X-ray spectral transitions in 2004 Jan–Feb, and this epoch corresponded to the softest 1.5–12 keV X-ray spectrum. With only a single epoch revealing the jet, we cannot formally constrain its velocity. Nevertheless, several lines of reasoning suggest that the jet was probably launched 0.5–4.0 d before this brightening, corresponding to projected velocities of  0.2 c ≲ v _app≲ 1.6 c   , and an intrinsic velocity of  ≳0.3 c   . We also report the occurrence of a major radio flare from Cyg X-1, reaching a flux density of ∼120 mJy at 15 GHz, and yet not associated with any resolvable radio emission, despite a concerted effort with MERLIN. We discuss the resolved jet in terms of the recently proposed 'unified model' for the disc–jet coupling in black hole X-ray binaries, and tentatively identify the 'jet line' for Cyg X-1. The source is consistent with the model in the sense that a steady jet appears to persist initially when the X-ray spectrum starts softening, and that once the spectral softening is complete the core radio emission is suppressed and transient ejecta/shock observed. However, there are some anomalies, and Cyg X-1 clearly does not behave like a normal black hole transient in progressing to the canonical soft/thermal state once the ejection event has happened.     

The orbital period of HDE 226868/Cyg X-1

We present epoch 1996, high-quality radial velocity data for HDE 226868, the optical counterpart of Cygnus X-1. Combining our results with all published historical data, we have derived a new ephemeris for the system of HJD 245 0235.29 + n  × 5.5998, which allows accurate orbital phase calculations to be made for any X-ray observations over the last 30 years. We find no evidence for any period change such as that suggested by Ninkov, Walker &38; Yang. We discuss the shortcomings of previous work in establishing the period and orbital elements.     

X-ray timing behaviour of Cygnus X-2 at low intensities

It is known that the overall (mean) intensity of the low-mass X-ray binary Cyg X-2 varies on time-scales from a day to months, independently of the variations on time-scales of hours to a day by which the source moves between the horizontal, normal and flaring branches.
We present RXTE PCA observations of Cyg X-2, taken when its overall intensity was near its lowest values, in 1996 October and 1997 September. For the first time we perform a study of the fast timing behaviour at such low intensities. During the 1996 October observations, the source was in the left part of the horizontal branch, and during the 1997 September observations was most likely in the lower parts of the normal branch and flaring branch.
We find that the properties of the very low-frequency noise during the 1997 September observations are consistent with a monotonic decrease in its strength and power-law index as a function of overall intensity. In contrast, the strength of the ∼6 Hz normal branch quasi-periodic oscillations does not vary monotonically with overall intensity. They are strongest at medium overall intensity and weaker both when the overall intensity is low and when the overall intensity is high.     

A good long look at the black hole candidates LMC X-1 and LMC X-3

LMC X-1 and LMC X-3 are the only known persistent stellar-mass black-hole candidates that have almost always shown spectra that are dominated by a soft, thermal component. We present here results from 170-ks-long Rossi X-ray Timing Explorer ( RXTE ) observations of these objects, taken in 1996 December, where their spectra can be described by a disc blackbody plus an additional soft     high-energy power law (detected up to energies of 50 keV in LMC X-3). These observations, as well as archival Advanced Satellite for Cosmology and Astrophysics ( ASCA ) observations, constrain any narrow Fe line present in the spectra to have an equivalent width ≲90 eV. Stronger, broad lines (≈150 eV EW,     are permitted. We also study the variability of LMC X-1. Its X-ray power spectral density (PSD) is approximately proportional to     between 10⁻³ and 0.3 Hz with a root-mean-square (rms) variability of ≈7 per cent. At energies >5 keV, the PSD shows evidence of a break at     possibly indicating an outer disc radius of ≲1000  GM c ² in this likely wind-fed system. Furthermore, the coherence function     a measure of the degree of linear correlation between variability in the >5 keV band and variability in the lower energy bands, is extremely low (≲50 per cent). We discuss the implications of these observations for the mechanisms that might be producing the soft and hard X-rays in these systems.     

Damped harmonic oscillator interpretation of the soft-state power spectra of Cyg X-1

We develop a model of an accretion disc in which the variability induced at a given radius is governed by a damped harmonic oscillator at the corresponding epicyclic frequency. That variability induces both linear and non-linear responses in the locally emitted radiation. The total observed variability of a source is the sum of these contributions over the disc radius weighted by the energy dissipation rate at each radius. It is shown that this simple model, which effectively has only three parameters including the normalization, can explain the range of the power spectra observed from Cyg X-1 in the soft state. Although a degeneracy between the black hole mass and the strength of the damping does not allow a unique determination of the mass, we can still constrain it to  ≲(16–20) M_⊙  . We also show that our model preserves the observed linear rms–flux relationship even in the presence of the non-linear flux response.     

Echoes from the companion star in Sco X-1

We present simultaneous X-ray ( RXTE ) and optical (ULTRACAM) narrow-band (Bowen blend/He  ii and nearby continuum) observations of Sco X-1 at 2–10 Hz time resolution. We find that the Bowen/He  ii emission lags the X-ray light curves with a light traveltime of     s which is consistent with reprocessing in the companion star. The echo from the donor is detected at orbital phase ∼0.5 when Sco X-1 is at the top of the flaring branch (FB). Evidence of echoes is also seen at the bottom of the FB but with time-lags of 5–10 s which are consistent with reprocessing in an accretion disc with a radial temperature profile. We discuss the implication of our results for the orbital parameters of Sco X-1.