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Purpose Corneal endothelial cell loss is one of the possible complications associated with phakic iris-fixated intraocular lens (PIOL) implantation. We postulate that this might be connected to the alteration of corneal metabolism secondary to the lens implantation. Methods A mathematical model of transport and consumption/production of metabolic species in the cornea is proposed, coupled with a model of aqueous flow and transport of metabolic species in the anterior chamber. Results Results are presented both for open and closed eyelids. We showed that, in the presence of a PIOL, glucose availability at the corneal endothelium decreases significantly during sleeping. Conclusions Implantation of a PIOL significantly affects nutrient transport processes to the corneal endothelium especially during sleep. It must still be verified whether this finding has a clinical relevance.

The aim of this study is to investigate the characteristics of the aqueous humour flow in the anterior chamber of the eye in the presence of a perforated, phakic, iris-fixated intraocular lens (pIOL). Such pIOLs are implanted in the anterior chamber in front of the iris and therefore they interfere with aqueous motion. The aim of this work is to investigate whether a perforation in the body of the pIOL can improve its fluid dynamics performance. Numerical simulations are conducted using the free computational fluid dynamics program OpenFOAM. The aqueous humour is modelled as a Newtonian incompressible fluid and, when temperature effects are considered, the Navier–Stokes equations are coupled to the energy equation, using Boussinesq’s approach to account for fluid density changes associated temperature variations. The pressure drop across the anterior chamber is calculated considering perforations in the pIOL of various sizes and also studying the extreme case in which the passage between the iris and the pIOL gets plugged, thus leaving the hole in the pIOL as the only possible pathway for aqueous flow. The study shows that the presence of a hole in the pIOL can only have a significant role on the pressure in the eye if the normal aqueous flow in the region between the pIOL and the iris gets blocked and has a negligible effect on the wall shear stress on the cornea.

One of the possible risks associated with the implant of iris-fixated phakic intraocular lenses (pIOL) is loss of corneal endothelial cells. We hypothesize that this might be due to alterations in corneal metabolism secondary to the lens implantation. To verify the feasibility of this assumption, we propose a mathematical model of the transport and diffusion of metabolic species in the anterior chamber and the cornea, coupled to a model of aqueous flow. Results are obtained both with and without the pIOL in the case of closed eyelids. The results suggest that glucose availability may be significantly reduced at the corneal endothelium. However, it must still be verified whether this finding has clinical relevance.

Flow in the aqueous humour that fills the anterior chamber of the eye occurs in response to the production and drainage of the aqueous humour, and also due to buoyancy effects produced by thermal gradients. Phakic intraocular lenses (pIOLs) are manufactured lenses that are surgically inserted in the eyes of patients to correct refractive errors. Their presence has a dramatic effect on the circulation of the aqueous humour, resulting a very different flow in the anterior chamber, the effects of which have not been extensively investigated. In this article we use a simplified mathematical model to analyse the flow, in order to assess the effect of the implanted lens on the pressure drop required to drive the flow and also on the wall shear stress experienced by the corneal endothelial cells and the cells of the iris. A high pressure drop could result in an increased risk of glaucoma, whilst raised shear stress on the cornea could result in a reduction in the density of endothelial cells there, and on the iris it could result in the detachment of pigment cells, which block the outflow of the eye, also leading to glaucoma. Our results confirm those of previous fully numerical studies, and show that, although the presence of the lens causes significant differences in the flow topology and direction, the typical magnitudes of the shear stress are not significantly changed from the natural case. Our semi-analytical solution allows us to perform a thorough study of the dependence of the results on the controlling parameters and also to understand the basic physical mechanisms underlying flow characteristics.

The megacities arising around the planet are like the Internet where many events are taking place simultaneously. The urban scape today is becoming more a space of flows—migrants, trade, capital, information, microbes—than a space of places rooted in an historical identity. The megaurban condition today encompasses many realities, from the glittering generic city-state of Singapore to the slums climbing up the hillsides around Mexico City or Sao Paulo. In these spaces we work, love and live out the intimate moments of our lives. In these spaces we consume and spew out climate warming gases. In this section, two of the world's “star architects”—Rem Koolhaas and Frank Gehry—the visionary “arcologist” Paolo Soleri and the Turkish novelist and Nobel laureate, Orhan Pamuk, grasp at chronicling the reality of where we live.

We present optical monitoring of the black hole candidate XTE J1752-223 during its 2009 - 2010 outburst and decay to quiescence. The optical light curve can be described by an exponential decay followed by a plateau, then a more rapid fade towards quiescence. The plateau appears to be due to an extra component of optical emission that brightens and then fades over ~ 40 days. We show evidence for the origin of this optical 'flare' to be the synchrotron jet during the decaying hard state, and we identify and isolate both disc and jet components in the spectral energy distributions. The optical flare has the same morphology and amplitude as a contemporaneous X-ray rebrightening. This suggests a common origin, but no firm conclusions can be made favouring or disfavouring the jet producing the X-ray flare. The quiescent optical magnitudes are B >= 20.6, V >= 21.1, R >= 19.5, i' >= 19.2. From the optical outburst amplitude we estimate a likely orbital period of < 22 h. We also present near-infrared (NIR) photometry and polarimetry and rare mid-infrared imaging (8 - 12 microns) when the source is nearing quiescence. The fading jet component, and possibly the companion star may contribute to the NIR flux. We derive deep mid-IR flux upper limits and NIR linear polarization upper limits. With the inclusion of radio data, we measure an almost flat jet spectral index between radio and optical; F_nu ~ nu^(~ +0.05). The data favour the jet break to optically thin emission to reside in the infrared, but may shift to frequencies as high as the optical or UV during the peak of the flare.

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 the analysis of two Chandra observations of Circinus X-1 performed in 2007, for a total exposure time of ~50 ks. The source was observed with the High Resolution Camera during a long X-ray low-flux state of the source. Cir X-1 is an accreting neutron-star binary system that exhibits ultra-relativistic arcsec-scale radio jets and an extended arcmin-scale radio nebula. Furthermore, a recent paper has shown an X-ray excess on arcmin-scale prominent on the side of the receding radio jet. In our images we clearly detect X-ray structures both on the side of the receding and the approaching radio jet. The X-ray emission is consistent with being from synchrotron origin. Our detection is consistent with neutron-star binaries being as efficient as black-hole binaries in producing X-ray outflows, despite their shallower gravitational potential., Accepted for publication in MNRAS-Letters

We analyzed simultaneous archival XMM-Newton and RXTE observations of the X-ray binary and black hole candidate Swift J1753.5-0127. In a previous analysis of the same data a soft thermal component was found in the X-ray spectrum, and the presence of an accretion disk extending close to the innermost stable circular orbit was proposed. This is in contrast with the standard picture in which the accretion disk is truncated at large radii in the low/hard state. We tested a number of spectral models and we found that several of them fit the observed spectra without the need of a soft disk-like component. This result implies that the classical paradigm of a truncated accretion disk in the low/hard state can not be ruled out by these data. We further discovered a broad iron emission line between 6 and 7 keV in these data. From fits to the line profile we found an inner disk radius that ranges between ~6-16 gravitational radii, which can be in fact much larger, up to ~250 gravitational radii, depending on the model used to fit the continuum and the line. We discuss the implications of these results in the context of a fully or partially truncated accretion disk., Comment: 10 pages, 3 figures, 1 table, accepted for publication on MNRAS, using mn2e.cls style file

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 the results of the timing analysis of five Rossi X-ray Timing Explorer observations of the Black Hole Candidate GRS 1915+105 between 1996 September and 1997 December. The aim was to investigate the possible presence of a type-B quasi-periodic oscillation (QPO). Since in other systems this QPO is found to appear during spectral transitions from Hard to Soft states, we analyzed observations characterized by a fast and strong variability, in order to have a large number of transitions. In GRS 1915+105, transitions occur on very short time scales (~ sec): to single them out we averaged Power Density Spectra following the regular path covered by the source on a 3D Hardness-Hardness-Intensity Diagram. We identified both the type-C and the type-B quasi-periodic oscillations (QPOs): this is the first detection of a type-B QPO in GRS 1915+105. As the spectral transitions have been associated to the emission and collimation of relativistic radio-jets, their presence in the prototypical galactic jet source strengthens this connection.

PURPOSE: Phakic intraocular lenses (pIOLs) are used for correcting vision; in this paper we investigate the fluid dynamical effects of an iris-fixated lens in the anterior chamber. In particular, we focus on changes in the wall shear stress (WSS) on the cornea and iris, which could be responsible for endothelial and pigment cell loss, respectively, and also on the possible increase of the intraocular pressure, which is known to correlate with the incidence of secondary glaucoma. METHODS: We use a mathematical model to study fluid flow in the anterior chamber in the presence of a pIOL. The governing equations are solved numerically using the open source software OpenFOAM. We use an idealized standard geometry for the anterior chamber and a realistic geometric description of the pIOL. RESULTS: We consider separately the main mechanisms that produce fluid flow in the anterior chamber. The numerical simulations allow us to obtain a detailed description of the velocity and pressure distribution in the anterior chamber, and indicated that implantation of the pIOL significantly modifies the fluid dynamics in the anterior chamber. However, lens implantation has negligible influence on the intraocular pressure and does not produce a significant increase of the shear stress on the cornea, while the shear stress on the iris, although increased, is not enough to cause detachment of cells. CONCLUSIONS: We conclude that alterations in the fluid dynamics in the anterior chamber as a result of lens implantation are unlikely to be the cause of medical complications associated with its use.

We present a spectral analysis of the black hole candidate and X-ray transient source Swift J1753.5 0127 making use of simultaneous observations of XMM-Newton and Rossi X-ray Timing Explorer (RXTE) in 2006, when the source was in outburst. The aim of this paper is to test whether a thermal component due to the accretion disc is present in the X-ray spectrum. We fit the data with a range of spectral models, and we find that for all of these models the fits to the X-ray energy spectra significantly require the addition of the disc black-body component. We also find a broad iron emission line at around 6.5 keV, most likely due to iron in the accretion disc. Our results confirm the existence of a cool inner disc extending near or close to the innermost circular orbit (ISCO).We further discovered broad emission lines of NVII and OVIII at ~ 0.52 keV and 0.65 keV, respectively in the RGS spectrum of Swift J1753.5-0127., Comment: 17 pages, 6 figures, 4 tables, Accepted 2013 February 19 for publication in Monthly Notices of the Royal Astronomical Society Main Journal

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

In studies of accreting black holes in binary systems, empirical relations have been proposed to quantify the coupling between accretion processes and ejection mechanisms. These processes are probed respectively by means of X-ray and radio/optical-infrared observations. The relations predict, given certain accretion conditions, the expected energy output in the form of a jet. We investigated this coupling by studying the black hole candidate Swift J1753.5-0127, via multiwavelength coordinated observations over a period of ~4 years. We present the results of our campaign showing that, all along the outburst, the source features a jet that is fainter than expected from the empirical correlation between the radio and the X-ray luminosities in hard spectral state. Because the jet is so weak in this system the near-infrared emission is, unusually for this state and luminosity, dominated by thermal emission from the accretion disc. We briefly discuss the importance and the implications of a precise determination of both the slope and the normalisation of the correlations, listing some possible parameters that broadband jet models should take into account to explain the population of sources characterized by a dim jet. We also investigate whether our data can give any hint about the nature of the compact object in the system, since its mass has not been dynamically measured.

We present the discovery of fast infrared/X-ray correlated variability in the black-hole transient GX 339-4. The source was observed with sub-second time resolution simultaneously with VLT/ISAAC and RXTE/PCA in August 2008, during its persistent low-flux highly variable hard state. The data show a strong correlated variability, with the infrared emission lagging the X-ray emission by 100 ms. The short time delay and the nearly symmetric cross-correlation function, together with the measured brightness temperature of ~2.5 x 10^6 K, indicate that the bright and highly variable infrared emission most likely comes from a jet near the black hole. Under standard assumptions about jet physics, the measured time delay can provide us a lower limit of Gamma > 2 for the Lorentz factor of the jet. This suggests that jets from stellar-mass black holes are at least mildly relativistic near their launching region. We discuss implications for future applications of this technique., 5 pages, 3 figures, accepted for publication in MNRAS Letters

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

We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It produces tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, $\Theta\lesssim30^\circ$, between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hot spots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermittent pulsations from accreting systems., Comment: 5 pages, 3 figures, proceedings of the conference "A Decade of Accreting Millisecond Pulsars," Amsterdam, April 2008

The primary concern of this contribution is that accreting millisecond pulsars (AMXPs) show a much larger amount of information than is commonly believed. The three questions to be addressed are: 1. Is the apparent spin torque observed in AMXPs real ? 2. Why do we see correlations and anti-correlations between fractional amplitudes and timing residuals in some AMXPs ? 3. Why the timing residuals, the lightcurve and the 1Hz QPO in SAX J1808.4$-$3658 are related ?, To be published in the proceedings of the workshop 'A Decade of Accreting Millisecond X-ray pulsars' (Amsterdam 14-18 April 2008; Eds. Wijnands et al.)

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.

We have carried out a search for radio pulsed emission from the Accreting Millisecond X‐ray pulsars (AMXPs) XTE J0929‐314, XTE J1751‐305 and XTE J1814‐338 at 6.5 and 8.5 GHz, observed during their X‐Ray quiescent phase. The detection of radio signals, in the quiescent phase of this kind of transient systems, would give a ultimate proof of the recycling model, unambiguously establishing that the AMXPs are the progenitors of the Radio millisecond pulsars. For the first source we can conclude that no radio pulsation is present in the analyzed data down to a limit of 68 μJy at 6.4 GHz and 26 μJy at 8.5 GHz. We discuss several mechanisms that could prevent the detection and suggest that the free‐free absorption cannot be responsible for the lack of a detection, given the relatively high used radio frequencies, while the beaming factor and the intrinsic low luminosity are viable explanations for the non detection of radio pulsation. For the other two sources the work is now in progress.

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.

At present, ten years after they were first discovered, ten accreting millisecond pulsars are known. I present a study of the aperiodic X-ray variability in three of these systems, which led to the discovery of simultaneous kHz quasi periodic oscillations in XTE J1807-294 and extremely strong broadband noise at unusually low variability frequencies in IGR J00291+5934. Furthermore, we classified SWIFT J1756.9-2508 as an atoll source and measured in its 2007 outburst spectral and variability properties typical of the extreme island state. I also give detailed estimates of the total fluence during the studied outbursts., Comment: To be published in the proceedings of the workshop 'A Decade of Accreting Millisecond X-ray pulsars' (Amsterdam 14-18 April 2008; Eds. Wijnands et al.)

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 report here on the orbital evolution of the accreting millisecond pulsar SAX J1808.4—3658. In particular, we find for this source an estimate of the orbital period derivative, Ṗorb = (3.40±0.12)×10−12 s/s. This derivative is positive and is more than one order of magnitude higher than what is expected from secular evolution driven by angular momentum losses caused by gravitational radiation under conservative mass transfer. In the hypothesis that the measured derivative of the orbital period reflects the secular evolution of the system, we propose a simple explanation of this puzzling result assuming that during X‐ray quiescence the source is ejecting matter (and angular momentum) from the inner Lagrangian point. The proposed orbital evolution of the system suggests a degenerate or fully convective companion star and indicates that this kind of sources are capable to efficiently ablate the companion star, and therefore are black widows visible in X‐rays during transient mass accretion episodes.

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 present ongoing Rossi X-ray Timing Explorer (RXTE) monitoring observations of the 377.3 Hz accretion-powered pulsar, HETE J1900.1-2455 Activity continues in this system more than 3 years after discovery, at a mean luminosity of 4.4e36 erg/s (for d=5 kpc), although pulsations were present only within the first 70 days. X-ray variability has increased each year, notably with a brief interval of nondetection in 2007, during which the luminosity dropped to below 1e-3 of the mean level. A deep search of data from the intervals of nondetection in 2005 revealed evidence for extremely weak pulsations at an amplitude of 0.29% rms, a factor of ten less than the largest amplitude seen early in the outburst. X-ray burst activity continued through 2008, with bursts typically featuring strong radius expansion. Spectral analysis of the most intense burst detected by RXTE early in the outburst revealed unusual variations in the inferred photospheric radius, as well as significant deviations from a blackbody. We obtained much better fits instead with a comptonisation model., Comment: 8 pages, 3 figures, one table; 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.)

XTE J1701-407 is a new transient X-ray source discovered on June 8th, 2008. More than one month later it showed a rare type of thermonuclear explosion: a long type I X-ray burst. We report herein the results of our study of the spectral and flux evolution during this burst, as well as the analysis of the outburst in which it took place. We find an upper limit on the distance to the source of 6.1 kpc by considering the maximum luminosity reached by the burst. We measure a total fluence of 3.5*10^{-6} erg/cm^2 throughout the ~20 minutes burst duration and a fluence of 2.6*10^{-3} erg/cm^2 during the first two months of the outburst. We show that the flux decay is best fitted by a power law (index ~1.6) along the tail of the burst. Finally, we discuss the implications of the long burst properties, and the presence of a second and shorter burst detected by Swift ten days later, for the composition of the accreted material and the heating of the burning layer., Comment: MNRAS-Letters, accepted. Minor changes according to referee's report. 5 pages, 3 figures

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.

Using data obtained with the ESO‐Very Large Telescope, we have performed a deep optical and NIR photometric study of the fields of three Accreting Millisecond X—ray Pulsars (XTE J1814—338, XTE J0929—314 and XTE J1807—294) during quiescence in order to look for the presence of a variable counterpart. If suitable candidates are found, we also carried out optical spectroscopy. We present here the first multi‐band (VR) detection of the optical counterpart of XTE J1814—338 in quiescence together with its optical spectrum. The optical light curve shows variability in both bands consistent with a sinusoidal modulation at the known 4.3 hr orbital period. The marginal detection of the very faint counterpart of XTE J0929—314 and deep upper limits for the optical/NIR counterpart of XTE J1807—294 are also reported. We also briefly discuss the results reported in the literature for the optical/NIR counterpart of XTE J1751—305. Our findings are consistent with AMXPs being systems containing an old, weakly magnetized ne...

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.

We present X-ray observations of the Black-Hole Binary GRS 1915+105 made with the RXTE (Rossi X-ray Timing Explorer). We concentrated on timing analysis of the strong variability focusing on its aperiodic variability on short (, 8 pages, 5 figures, accepted for publication in the proceedings of VI Microquasar Workshop: Microquasars and Beyond, 18-22 September 2006, Como (Italy), ed: T. Belloni (2006)

We report the results of a systematic timing analysis of RXTE observations of GRS 1915+105 when the source was in its variability class theta, characterized by alternating soft and hard states on a time scale of a few hundred seconds. The aim was to examine the high-frequency part of the power spectrum in order to confirm the hecto-Hertz Quasi-Periodic Oscillations (QPO) previously reported from observations from mixed variability behaviours. During the hard intervals (corresponding to state C in the classification of Belloni et al., 2000, A&A, 35, 271), we find a significant QPO at a frequency of ~170 Hz, although much broader (Q~2) than previously reported. No other significant peak is observed at frequencies >30 Hz. A time-resolved spectral analysis of selected observations shows that the hard intervals from class theta show a stronger and steeper (Gamma=2.8-3.0) power-law component than hard intervals from other classes. We discuss these results in the framework of hecto-Hertz QPOs reported from GRS 1915+105 and other black-hole binaries., 7 pages, 8 figures, accepted for publication on MNRAS