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The accretion/ejection coupling in accreting black hole binaries has been described by empirical relations between the X-ray/radio and X-ray/optical-infrared luminosities. These correlations were initially supposed to be universal. However, recently many sources have been found to produce jets that, given certain accretion-powered luminosities, are fainter than expected from the correlations. This shows that black holes with similar accretion flows can produce a broad range of outflows in power. Here we discuss whether typical parameters of the binary system, as well as the properties of the outburst, produce any effect on the energy output in the jet. We also define a jet-toy model in which the bulk Lorentz factor becomes larger than ~1 above ~0.1% of the Eddington luminosity. We finally compare the "radio quiet" black holes with the neutron stars.

We present the results of simultaneous X-ray and radio observations of the peculiar Z-type neutron star X-ray binary Cir X-1, observed with the Rossi X-ray timing explorer satellite and the Australia Telescope Compact Array in 2000 October and 2002 December. We identify typical Z source behaviour in the power density spectra as well as characteristic Z patterns drawn in an X-ray hardness-intensity diagram. Power spectra typical of bright atoll sources have also been identified at orbital phases after the periastron passage, while orbital phases before the periastron passage are characterized by power spectra that are typical neither of Z nor of atoll sources. We investigate the coupling between the X-ray and the radio properties, focusing on three orbital phases when an enhancement of the radio flux density has been detected, to test the link between the inflow (X-ray) and the outflow (radio jet) to/from the compact object. In two out of three cases we associate the presence of the radio jet to a spectral transition in the X-rays, although the transition does not precede the radio flare, as detected in other Z sources. An analogous behaviour has recently been found in the black hole candidate GX 339-4. In the third case, the radio light curve shows a similar shape to the X-ray light curve. We discuss our results in the context of jet models, considering also black hole candidates., Comment: Accepted for publication in MNRAS

We present our monitoring campaign of the outburst of the black-hole candidate Swift J1753.5-0127, observed with the Rossi X-ray Timing Explorer and the Swift satellites. After ~4.5 years since its discovery, the source had a transition to the hard intermediate state. We performed spectral and timing studies of the transition showing that, unlike the majority of the transient black holes, the system did not go to the soft states but it returned to the hard state after a few months. During this transition Swift J1753.5-0127 features properties which are similar to those displayed by the black hole Cygnus X-1. We compared Swift J1753.5-0127 to one dynamically confirmed black hole and two neutron stars showing that its power spectra are in agreement with the binary hosting a black hole. We also suggest that the prolonged period at low flux that followed the initial flare is reminiscent of that observed in other X-ray binaries, as well as in cataclysmic variables., Accepted for publication in MNRAS

The appearance and time variability of accreting millisecond X-ray pulsars (hereafter AMXPs, e.g. Wijnands & van der Klis 1998) depends strongly on the accretion rate, the effective viscosity and the effective magnetic diffusivity of the disk-magnetosphere boundary. The accretion rate is the main parameter which determines the location of the magnetospheric radius of the star for a given stellar magnetic field. We introduce a classification of accreting neutron stars as a function of the accretion rate and show the corresponding stages obtained from our global 3D magnetohydrodynamic (MHD) simulations and from our axisymmetric MHD simulations. We discuss the expected variability features in each stage of accretion, both periodic and quasi-periodic (QPOs). We conclude that the periodicity may be suppressed at both very high and very low accretion rates. In addition the periodicity may disappear when ordered funnel flow accretion is replaced by disordered accretion through the interchange instability., 10 pages, 6 figures, Invited review, to appear in the proceedings of the workshop 'A Decade of Accreting Millisecond X-ray Pulsars' (Amsterdam, April 2008); see animations at http://www.astro.cornell.edu/~romanova/projects.htm

The spin frequency distribution of accreting millisecond X-ray pulsars cuts off sharply above 730 Hz, well below the breakup spin rate for most neutron star equations of state. I review several different ideas for explaining this cutoff. There is currently considerable interest in the idea that gravitational radiation from rapidly rotating pulsars might act to limit spin up by accretion, possibly allowing eventual direct detection with gravitational wave interferometers. I describe how long-term X-ray timing of fast accreting millisecond pulsars like the 599 Hz source IGR J00291+5934 can test the gravitational wave model for the spin frequency limit., 8 pages with 2 figures, to appear in the proceedings of "A Decade of Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds. R. Wijnands et al. (AIP Conf. Proc.)

I review the basic observational properties of accreting millisecond pulsars that are important for understanding the physics involved in formation of their pulse profiles. I then discuss main effects responsible for shaping these profiles. Some analytical results that help to understand the results of simulations are presented. Constraints on the pulsar geometry and the neutron star equation of state obtained from the analysis of the pulse profiles are discussed., Invited review, to appear in the proceedings of the workshop 'A Decade of Accreting Millisecond X-ray Pulsars' (Amsterdam, April 2008)

We present a comprehensive timing study of the 2.5 ms pulsar SAX J1808.4—3658, a soft X‐ray transient with a recurrence time of roughly 2 years. Using data from the Rossi X‐ray Timing Explorer, we construct a phase‐connected pulse timing solution for each of four observed outbursts during 1998–2005. We measure a long‐term spin down with better than 6σ confidence. The rate is (−5.6±2.0)×10−16 Hz s−1, with most of the change occurring during quiescence, and most likely is due to the magnetic dipole torques from a B = 1.5×108 G field. We also find that the 2 hr binary orbital period is increasing at a rate of (3.5±0.2)×10−12 s s−1, although it is unclear whether this change reflects secular evolution or short‐term variability.

The detailed timing analysis of the accreting X‐ray millisecond pulsar XTE J1807—294 during the 2003 outburst observed by RXTE, will be summed up here. A timing technique was developed in order to obtain the complete set of orbital parameters. The phase delays derived from the pulse profile show a strong erratic behavior superposed to what appears as a global spin‐up trend. The erratic behavior of the pulse phase delays is strongly anti‐correlated to rapid variations of the flux. A separate analysis of the phase delays of the first and second harmonic of the spin frequency shows that the latter are far less affected by the erratic behavior. In the hypothesis that the second harmonic pulse phase delays are a good tracer of the spin frequency evolution, we derive a precise estimate of the spin frequency and its derivative.

It is ten years since the first accretion‐driven millisecond X‐ray pulsar was discovered and I take this perfect opportunity to tell the story of its discovery.

We consider the currently observed spin distributions of various types of neutron stars, including isolated and binary radio millisecond pulsars in the Galactic plane and globular cluster system as well as neutron stars in low-mass X-ray binary systems where the spin rate is known either through coherent pulsations or burst oscillations. We find that the spin distributions of isolated and binary radio millisecond pulsars are statistically different, at least for those residing in globular clusters, with the binary pulsars being on average faster spinning. This result is likely to hold despite observational biases still affecting the observed spin distribution. A possible explanation for this is that the isolated radio millisecond pulsars are on average older than those in binary systems., 5 pages, 1 figure; appears in the proceedings of the workshop "A Decade of Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds. R. Wijnands et al. (AIP Conf. Proc.)

The key contribution of the discovery of nuclear-powered pulsations from the accretion-powered millisecond pulsars (AMPs) has been the establishment of burst oscillation frequency as a reliable proxy for stellar spin rate. This has doubled the sample of rapidly-rotating accreting neutron stars and revealed the unexpected absence of any stars rotating near the break-up limit. The resulting `braking problem' is now a major concern for theorists, particularly given the possible role of gravitational wave emission in limiting spin. This, however, is not the only area where burst oscillations from the AMPs are having an impact. Burst oscillation timing is developing into a promising technique for verifying the level of spin variability in the AMPs (a topic of considerable debate). These sources also provide unique input to our efforts to understand the still-elusive burst oscillation mechanism. This is because they are the only stars where we can reliably gauge the role of uneven fuel deposition and, of course, the magnetic field., Invited review, to appear in the proceedings of the workshop 'A decade of accreting millisecond x-ray pulsars' (Amsterdam, April 2008)

Intermittent accreting millisecond X-ray pulsars are an exciting new type of sources. Their pulsations appear and disappear either on timescales of hundreds of seconds or on timescales of days. The study of these sources add new observational constraints to present models that explain the presence or not of pulsations in neutron star LMXBs. In this paper we present preliminary results on spectral and aperiodic variability studies of all intermittent AMSPs, with a particular focus on the comparison between pulsating and non pulsating periods., 4 pages, 2 figures; to appear in the proceedings of the workshop "A Decade of Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds. R. Wijnands et al. (AIP Conf. Proc.)

The correlations between the twin kHz QPOs, as well as their relations to neutron star spins, are summarized. For most sources power law correlations between the frequencies of the twin kHz QPOs are preferred. It is difficult to reconcile a spin frequency with the kHz QPO frequency difference. The kHz QPO frequencies of two accreting millisecond pulsars, SAX J1808.4‐3658 and XTE J1807‐294, are generally lower than those of other sources, and there exists a 1.5 times shift factor in frequency if we plot the kHz QPO data points of both pulsars to agree with the tendencies of the other sources. Furthermore, the kHz QPO model by Stella and Vietri needs to assume the stellar mass to be 3 solar mass to reconcile their model with the data of SAX J1808.4‐3658 and XTE J1807‐294.

We discuss an idea for how accreting millisecond pulsars could contribute to the understanding of the QCD phase transition in the high-density nuclear matter equation of state (EoS). It is based on two ingredients, the first one being a ``phase diagram'' of rapidly rotating compact star configurations in the plane of spin frequency and mass, determined with state-of-the-art hybrid equations of state, allowing for a transition to color superconducting quark matter. The second is the study of spin-up and accretion evolution in this phase diagram. We show that the quark matter phase transition leads to a characteristic line in the Omega-M plane, the phase border between neutron stars and hybrid stars with a quark matter core. Along this line a change in the pulsar's moment of inertia entails a waiting point phenomenon in the accreting millisecond X-ray pulsar (AMXP) evolution: most of these objects should therefore be found along the phase border in the Omega-M plane, which may be viewed as the AMXP analog of the main sequence in the Hertzsprung-Russell diagram for normal stars. In order to prove the existence of a high-density phase transition in the cores of compact stars we need population statistics for AMXP's with sufficiently accurate determination of their masses and spin frequencies., Comment: 8 pages, 3 figures, to appear in Proceedings of the Conference on "A Decade of Accreting Millisecond X-Ray Pulsars, Amsterdam, April 14-18, 2008

Millisecond radio pulsars are magnificent tools for performing a variety of physical and astrophysical investigations, ranging from tests of general relativity to nuclear matter physics, from cosmology to globular cluster dynamics, from plasma physics to binary evolution. The basic properties of these sources, as well as the nature and evolution of their possible progenitors will be first reviewed. Then the focus will be on the main still unanswered issues, with particular emphasis on the link with the accreting millisecond X‐ray pulsars.

Accreting millisecond pulsars provide a glimpse into the thermal, chemical, and magnetic evolution of accreting neutron stars. In this contribution I review how accretion‐induced nuclear processes alter the composition of the neutron star crust and heat the neutron star core; how observations of quasi‐persistent transient neutron stars probe the equation of state of dense matter; and how these observations compare with complimentary studies of quasi‐persistent transients and superbursting neutron stars.

We discuss the effect of accretion on the evolution of the magnetic field of a neutron star and highlight the main unresolved issues. Charged, accreted matter is funneled towards the magnetic poles where it heats the stellar surface and alters its magnetic structure resulting in an overall reduction of the magnetic dipole moment. Mechanisms for accretion‐induced field reduction include accelerated Ohmic decay, vortex‐fluxoid interactions, and magnetic burial or screening. We discuss how these can be integrated into a global model and detail recent self‐consistent, three‐dimensional, magneto‐hydrodynamic, calculations (using analytic Grad‐Shafranov methods and the numerical solver ZEUS‐MP) which incorporate global resistive instabilities. These models can explain why neutron stars in binaries have systematically lower magnetic dipole moments than isolated neutron stars. Finally we discuss applications including the evolution of accreting millisecond pulsars and type‐I X‐ray bursts, magnetars, and gravitational waves.

We summarize here the main results of the timing analysis performed on the pulsed X‐ray emission of XTE J1751—305 and XTE J1814—338. These two sources encompass some of the main issues regarding the rotational evolution of Accreting Millisecond Pulsars: they have an opposite rotational reaction to the accretion of mass and only the phases of XTE J1751—305 evolve smoothly, while in XTE J1814—338 a modulation around an average trend appears in anti correlation with variations of the emitted X‐ray flux. We explore the possibility that this phenomenon is related to variations of the mass accretion rate on each spot, but only the behavior of the fundamental Fourier component could be fully reproduced according to the considered model. The most favorable explanation of this effect therefore appears to be still in terms of motions of the accretion path from the disk to the Neutron Star.

We have observed the accreting millisecond X-ray pulsar SAX J1808.4-3658 (1808) in quiescence during two 50 ksec XMM-Newton observations, and acquired near-simultaneous photometry with Gemini South. We find 1808's X-ray spectrum to be hard, describable with an absorbed power-law of photon index 1.7-1.9 and unabsorbed X-ray luminosity Lx = 5.2-7.9×10(31) ergs s-1. No thermal neutron star (NS) component is seen, with a limit on any possible NS component of LNS(0.01-10 keV)The near-simultaneous Gemini observations find a large sinusoidal flux modulation on 1808's orbital period, consistent with predictions from an irradiated secondary star. We model the contributions of the disk and donor star, and find that the donor must be irradiated by an external flux of Lirr = 1.15-1.78×10(34) ergs/s, much larger than observed in the X-ray band. This irradiation may be in the form of relativistic particles from the NS turning on as a radio pulsar when not accreting, as suggested by Burderi et al. The amplitude and color dependence of the optical modulation constrain the system inclination and donor radius. These constraints, through the pulsar mass function, deliver constraints on the NS mass of MNS>2.2 Msolar, or for a distance uncertainty 10% larger, of MNS>1.8 Msolar. Such a heavy NS is consistent with the accelerated neutrino cooling found from the X-ray observations.

Accretion‐powered pulsars with slight ellipticites are expected to precess, producing characteristic variations in their pulse properties. We compute light curves for various geometric configurations of a precessing pulsar and predict the periodic modulation of the mean flux, pulse‐phase residuals and fractional amplitudes of the pulse profiles. We then search for these signatures in 37 days of X‐ray timing data from the accreting millisecond pulsar XTE J1814—338. We analyze a 12.2‐d modulation observed previously and show that it is consistent with a freely precessing neutron star only if the inclination angle is

The possibility of millisecond neutron star spin periods in LMXBs has been discussed in earnest since the recycling scenario for the formation of millisecond radio pulsars was first proposed in the early 1980’s. A beat frequency interpretation of the then newly discovered low‐frequency QPOs in Z sources involving millisecond spins was proposed in 1985. After the discovery of the kHz QPOs in LMXBs with RXTE in 1996, the possibility of a link with the neutron star spin has been discussed extensively again. The discoveries of the accreting millisecond pulsars from 1998 onward and of kHz QPOs in several of these from 2003 gave further impetus to the debate about to what extent aperiodic phenomena in LMXBs have a direct link with the spin. I review the current status of this discussion. Beat frequency models that were originally proposed for the kHz QPOs are no longer viable. If a relation of the QPO frequencies with the spin exists, then this requires a rather specific, presumably resonant, interaction of the spin with the accretion flow forcing matter to move in step with the spin somewhere in the flow. The evidence for a link of the kHz QPOs with neutron star spin has not over the years become more convincing despite much more data, yet connections do seem to exist. I suggest that this provides a hint as to how the kHz and other QPO frequencies arise and what is the optimal approach to satisfactorily describing them as a physical phenomenon.

Millisecond pulsars, both during and after accretion, stand a class apart from the general pulsar population due to the much weaker field strength at their surface. The prevailing view is that accretion of matter on to a neutron star causes a decay of its magnetic field strength, as viewed by an external observer. In this paper we review some of the promising mechanisms that may be responsible for such decay.

The discovery of millisecond X-ray pulsations in neutron-star low-mass X-ray binaries has until now only been done with the Rossi X-ray Timing Explorer. However, XMM-Newton has also pointed at some of these systems during an outburst and shown that it can clearly detect the pulsations as well. In fact, XMM-Newton detects the pulsations with high enough significance to have discovered them, had they still been unknown. Thus, XMM-Newton can also be used to search for pulsations. Moreover, for the faintest systems XMM-Newton is even more sensitive to detect pulsations than RXTE due to the significantly less background count rate for XMM-Newton.