The formation of the black hole in the X-ray binary system V404 Cyg

Using new and archival radio data, we have measured the proper motion of the black hole X-ray binary V404 Cyg to be  9.2 ± 0.3 mas yr⁻¹  . Combined with the systemic radial velocity from the literature, we derive the full three-dimensional heliocentric space velocity of the system, which we use to calculate a peculiar velocity in the range 47–102 km s⁻¹, with a best-fitting value of 64 km s⁻¹. We consider possible explanations for the observed peculiar velocity and find that the black hole cannot have formed via direct collapse. A natal supernova is required, in which either significant mass  (∼11 M_⊙)  was lost, giving rise to a symmetric Blaauw kick of up to ∼65 km s⁻¹, or, more probably, asymmetries in the supernova led to an additional kick out of the orbital plane of the binary system. In the case of a purely symmetric kick, the black hole must have been formed with a mass  ∼9 M_⊙  , since when it has accreted  0.5–1.5 M_⊙  from its companion.     

The radio spectrum of a quiescent stellar mass black hole

Observations of V404 Cyg performed using the Westerbork Synthesis Radio Telescope at four frequencies, over the interval 1.4–8.4 GHz, have provided us with the first broad-band radio spectrum of a 'quiescent' stellar mass black hole. The measured mean flux density is of 0.35 mJy, with a spectral index  α=+0.09 ± 0.19  (such that   S _ν∝ν^α  ). Synchrotron emission from an inhomogeneous partially self-absorbed outflow of plasma accounts for the flat/inverted radio spectrum, in analogy with hard-state black hole X-ray binaries, indicating that a steady jet is being produced between a few 10⁻⁶ and a few per cent of the Eddington X-ray luminosity.     

Hα flares from V404 Cyg in quiescence

We present a spectrophotometric study of short-term optical variability in the quiescent black hole X-ray transient V404 Cyg. This includes two nights of high-time-resolution H α spectroscopy with which we resolve much of the time variability, and a further six nights of archival spectroscopy with lower time resolution but higher spectral resolution. We find significant variability in most of the data considered, with both the H α line and the continuum often varying in a correlated way. This includes both dramatic flares lasting a few hours in which the line flux nearly doubles and lower-level flickering. The strongest flares involve development of asymmetry in the line profile, with the red wing usually strongest independent of orbital phase. It is unclear why this is the case, but we discuss several possible explanations. We consider the energetics of the flares and compare with plausible models including chromospheric activity on the companion star, local magnetic reconnection events within the disc and varying irradiation from close to the black hole. Based on the line profile changes during the flares, we conclude that the most likely origin for the variability is variable photoionization by the central source, although local flares within the disc cannot be ruled out.     

Multicolour observations of V404 Cyg with ULTRACAM

We present high-time-resolution multicolour observations of the quiescent soft X-ray transient V404 Cyg obtained with ULTRACAM. Superimposed on the ellipsoidal modulation of the secondary star are large flares on time-scales of a few hours, as well as several distinct rapid flares on time-scales of tens of minutes. The rapid flares, most of which show further variability and unresolved peaks, cover shorter time-scales than those reported in previous observations. The power density spectrum of the 5-s time-resolution data shows a quasi-periodic oscillation (QPO) feature at 0.78 mHz (=21.5 min). Assuming this periodicity represents the Keplerian period at the transition between the thin and advective disc regions, we determine the transition radius. We discuss the possible origins for the QPO feature in the context of the advection-dominated accretion flow model.
We determine the colour of the large flares and find that the i '-band flux per unit frequency interval is larger than that in the g ' band. The colour is consistent with optically thin gas with a temperature of ∼8000 K arising from a region with an equivalent blackbody radius of at least  2 R_⊙  , which covers 3 per cent of the surface of the accretion disc. Our timing and spectral analysis results support the idea that the rapid flares (i.e. the QPO feature) most likely arise from regions near the transition radius.     

The quiescent spectral energy distribution of V404 Cyg

We present a multiwavelength study of the black hole X-ray binary V404 Cyg in quiescence, focusing upon the spectral energy distribution (SED). Radio, optical, ultraviolet (UV) and X-ray coverage is simultaneous. We supplement the SED with additional non-simultaneous data in the optical through infrared where necessary. The compiled SED is the most complete available for this, the X-ray and radio brightest quiescent black hole system. We find no need for a substantial contribution from accretion light from the near-UV to the near-IR, and in particular the weak UV emission constrains published spectral models for V404 Cyg. We confirm that no plausible companion spectrum and interstellar extinction can fully explain the mid-IR, however, and an infrared (IR) excess from a jet or cool disc appears to be required. The X-ray spectrum is consistent with a  Γ∼ 2  power law as found by all other studies to date. There is no evidence for any variation in the hardness over a range of a factor of 10 in luminosity. The radio flux is consistent with a flat spectrum (in   f _ν  ). The break frequency between a flat and optically thin spectrum most likely occurs in the mid or far-IR, but is not strongly constrained by these data. We find the radio to be substantially variable but with no clear correlation with X-ray variability.     

Jets from black hole X-ray binaries: testing, refining and extending empirical models for the coupling to X-rays

In this paper we study the relation of radio emission to X-ray spectral and variability properties for a large sample of black hole X-ray binary systems. This is done to test, refine and extend – notably into the timing properties – the previously published 'unified model' for the coupling of accretion and ejection in such sources. In 14 outbursts from 11 different sources we find that in every case the peak radio flux, on occasion directly resolved into discrete relativistic ejections, is associated with the bright hard to soft state transition near the peak of the outburst. We also note the association of the radio flaring with periods of X-ray flaring during this transition in most, but not all, of the systems. In the soft state, radio emission is in nearly all cases either undetectable or optically thin, consistent with the suppression of the core jet in these states and 'relic' radio emission from interactions of previously ejected material and the ambient medium. However, these data cannot rule out an intermittent, optically thin, jet in the soft state. In attempting to associate X-ray timing properties with the ejection events we find a close, but not exact, correspondence between phases of very low integrated X-ray variability and such ejections. In fact the data suggest that there is not a perfect one-to-one correspondence between the radio, X-ray spectral or X-ray timing properties, suggesting that they may be linked simply as symptoms of the underlying state change and not causally to one another. We further study the sparse data on the reactivation of the jet during the transition back to the hard state in decay phase of outbursts, and find marginal evidence for this in one case only. In summary we find no strong evidence against the originally proposed model, confirming and extending some aspects of it with a much larger sample, but note that several aspects remain poorly tested.     

Origin of X-ray emission from transient black hole candidates in quiescence

We report results from a systematic study of X-ray emission from black hole transients in quiescence. In this state, mass accretion is thought to follow the geometry of an outer optically thick, geometrically thin disc and an inner optically thin, geometrically thick radiatively inefficient accretion flow (RIAF). The inner flow is likely also coupled to the jets near the black hole that are often seen in such systems. The goal of the study is to see whether the X-ray emission in the quiescent state is mainly powered by the accretion flow or by the jets. Using data from deep XMM – Newton observations of selected black hole transients, we have found that the quiescent X-ray spectra are, to a high precision, of power-law shape in the cases of GRO J1655-40 and V404 Cyg. Such spectra deviate significantly from the expected X-ray spectrum of the RIAF at very low-accretion rates. On the other hand, they can naturally be explained by emission from the jets, if the emitting electrons follow a power-law spectral distribution (as is often assumed). The situation remains ambiguous in the case of XTE J1550-564, due to the relatively poorer quality of the data. We discuss the implication of the results.     

Charge-coupled device observations of V388 Cyg

We present BVR full-light curves of V388 Cyg to provide a complete photometric solution for the first time. The light curves show a high degree of asymmetry (O'Connell effect). The maxima at 0.25 phase (Max I) are 0.023, 0.018 and 0.012 mag higher than those at 0.75 phase (Max II) in B , V and R , respectively. Three possible spot models are applied to fit the asymmetric light curves of V388 Cyg, in order to explain the O'Connell effect. We conclude that the model of the cool spot on the cooler star is the most reasonable model for V388 Cyg. The continuous period variation is confirmed by recently collected times of minima, including one minimum that is determined in this paper. The period decrease rate is estimated as d p /d t =−2.055×10⁻⁷ d yr⁻¹ .     

Constraints on black hole accretion in V Puppis

In light of the recent suggestion that the nearby eclipsing binary star system V Puppis has a dark companion on a long orbit, we present the results of radio and X-ray observations of it. We find an upper limit on its radio flux of about 300 μJy and a detection of it in the X-rays with a luminosity of about  3 × 10³¹  erg s⁻¹, a value much lower than what had been observed in some of the low angular resolution surveys of the past. These data are in good agreement with the idea that the X-ray emission from V Puppis comes from mass transfer between the two B stars in the system, but can still accommodate the idea that the X-ray emission comes from the black hole accreting stellar wind from one or both of the B stars.     

A classification of the X-ray and radio states of Cyg X-3 and their long-term correlations

We present a detailed classification of the X-ray states of Cyg X-3 based on the spectral shape and a new classification of the radio states based on the long-term correlated behaviour of the radio and soft X-ray light curves. We find a sequence of correlations, starting with a positive correlation between the radio and soft X-ray fluxes in the hard spectral state, changing to a negative one at the transition to soft spectral states. The temporal evolution can be in either direction on that sequence, unless the source goes into a very weak radio state, from which it can return only following a major radio flare. The flare decline is via relatively bright radio states, which results in a hysteresis loop on the flux–flux diagram. We also study the hard X-ray light curve, and find its overall anticorrelation with the soft X-rays. During major radio flares, the radio flux responds exponentially to the level of a hard X-ray high-energy tail. We also specify the detailed correspondence between the radio states and the X-ray spectral states. We compare our results to those of black hole and neutron star binaries. Except for the effect of strong absorption and the energy of the high-energy break in the hard state, the X-ray spectral states of Cyg X-3 closely correspond to the canonical X-ray states of black hole binaries. Also, the radio/X-ray correlation closely corresponds to that found in black hole binaries, but it significantly differs from that in neutron star binaries. Overall, our results strongly support the presence of a black hole in Cyg X-3.     

A radio-emitting outflow in the quiescent state of A0620−00: implications for modelling low-luminosity black hole binaries

Deep observations with the Very Large Array of A0620–00, performed in 2005 August, resulted in the first detection of radio emission from a black hole binary at X-ray luminosities as low as 10^−8.5 times the Eddington limit. The measured radio flux density, of  51 ± 7 μJy  at 8.5 GHz, is the lowest reported for an X-ray binary system so far, and is interpreted in terms of partially self-absorbed synchrotron emission from outflowing plasma. Making use of the estimated outer accretion rate of A0620−00 in quiescence, we demonstrate that the outflow kinetic power must be energetically comparable to the total accretion power associated with such rate, if it was to reach the black hole with the standard radiative efficiency of 10 per cent. This favours a model for quiescence in which a radiatively inefficient outflow accounts for a sizable fraction of the missing energy, and, in turn, substantially affects the overall dynamics of the accretion flow. Simultaneous observations in the X-ray band, with Chandra , confirm the validity of a non-linear radio/X-ray correlation for hard state black hole binaries down to low quiescent luminosities, thereby contradicting some theoretical expectations. Taking the mass term into account, the A0620−00 data lie on the extrapolation of the so-called Fundamental Plane of black hole activity, which has thus been extended by more than two orders of magnitude in radio and X-ray luminosity. With the addition of the A0620−00 point, the plane relation provides an empirical proof for the scale invariance of the jet-accretion coupling in accreting black holes over the entire parameter space observable with current instrumentation.     

Absolute and geometric parameters of the W UMa‐type contact binary V404 Pegasi

We present the results of our investigation of the geometrical and physical parameters of the W UMa‐type binary V404 Peg from analysis of CCD (BVRI) light curves and radial velocity data. The photometric data were obtained during 2010 at Ankara University Observatory (AUO). Light and radial velocity observations were analyzed simultaneously by using the well‐known Wilson‐Devinney (2007 revision) code to obtain absolute and geometrical parameters. Our solution indicates that V404 Peg is an A‐type overcontact binary with a mass ratio of q = 0.243 and an overcontact degree of f = 32.1 %. Combining our light curves with the radial velocity curves from Maciejewski & Ligeza (2004), we determined the absolute parameters of this system as follows: a = 2.672 R_⊙, M₁ = 1.175 M_⊙, M₂ = 0.286 M_⊙, R₁ = 1.346 R_⊙, and R₂ = 0.710 R_⊙. Finally, we discuss the evolutionary condition of the system (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Opacity effects and shock-in-jet modelling of low-level activity in Cygnus X-3

We present simultaneous dual-frequency radio observations of Cygnus X-3 during a phase of low-level activity. We constrain the minimum variability time-scale to be 20 min at 43 GHz and 30 min at 15 GHz, implying source sizes of 2–4 au. We detect polarized emission at a level of a few per cent at 43 GHz which varies with the total intensity. The delay of ∼10 min between the peaks of the flares at the two frequencies is seen to decrease with time, and we find that synchrotron self-absorption and free–free absorption by entrained thermal material play a larger role in determining the opacity than absorption in the stellar wind of the companion. A shock-in-jet model gives a good fit to the light curves at all frequencies, demonstrating that this mechanism, which has previously been used to explain the brighter, longer lived giant outbursts in this source, is also applicable to these low-level flaring events. Assembling the data from outbursts spanning over two orders of magnitude in flux density shows evidence for a strong correlation between the peak brightness of an event, and the time-scale and frequency at which this is attained. Brighter flares evolve on longer time-scales and peak at lower frequencies. Analysis of the fitted model parameters suggests that brighter outbursts are due to shocks forming further downstream in the jet, with an increased electron normalization and magnetic field strength both playing a role in setting the strength of the outburst.     

Opening angles, Lorentz factors and confinement of X-ray binary jets

We present a collation of the available data on the opening angles of jets in X-ray binaries, which in most cases are small (≲10°). Under the assumption of no confinement, we calculate the Lorentz factors required to produce such small opening angles via the transverse relativistic Doppler effect. The derived Lorentz factors, which are in most cases lower limits, are found to be large, with a mean >10, comparable to those estimated for active galactic nuclei (AGN) and much higher than the commonly assumed values for X-ray binaries of 2–5. Jet power constraints do not, in most cases, rule out such high Lorentz factors. The upper limits on the opening angles show no evidence for smaller Lorentz factors in the steady jets of Cygnus X-1 and GRS 1915+105. In those sources in which deceleration has been observed (notably  XTE J1550−564  and Cygnus X-3), some confinement of the jets must be occurring, and we briefly discuss possible confinement mechanisms. It is however possible that all the jets could be confined, in which case the requirement for high bulk Lorentz factors can be relaxed.