Accreting millisecond X-ray pulsars: 10 years of INTEGRAL observations

During the last 10 years, INTEGRAL made a unique contribution to the study of accreting millisecond X-ray pulsars (AMXPs), discovering three of the 14 sources now known of this class. Besides increasing the number of known AMXPs, INTEGRAL also carried out observations of these objects above 20 keV, substantially advancing our understanding of their behaviour. We present here a review of all the AMXPs observed with INTEGRAL and discuss the physical interpretation of their behaviour in the X-ray domain. We focus in particular on the lightcurve profile during outburst, as well as the timing, spectral, and thermonuclear type-I X-ray bursts properties.Comment: 8 pages, 8 figures. Proceedings of "An INTEGRAL view of the high-energy sky (the first 10 years)" the 9th INTEGRAL Workshop, October 15-19, 2012, Paris, Franc

kHz Quasi Periodic Oscillations in Low Mass X-ray Binaries as Probes of General Relativity in the Strong Field Regime

We consider the interpretation of a pair of kHz Quasi Periodic Oscillations (QPOs) in the Fourier spectra of two Low Mass X-Ray Binaries, Sco X-1 and 4U1608-52, hosting an old accreting neutron star. The observed frequency difference of these QPOs decreaseas as their frequency increases, contrary to simple beat frequency models, which predict a constant frequency difference. We show that the behaviour of these QPOs is instead well matched in terms of the fundamental frequencies (in the radial and azimuthal directions) for test particle motion in the gravitational field of the neutron star, for reasonable star masses, and nearly independent of the star spin. The radial frequency must be much smaller than the azimuthal one, testifying that kHz QPOs are produced close to the innermost stable orbit. These results are not reproduced through the post--Newtonian (PN) approximation of General Relativity (GR). kHz QPOs from X-ray binaries likely provide an accurate laboratory for strong field GR.Comment: to appear in Physical Review Letters, PRL Latex plus 2 figures in standard PostScript forma

Future X-ray timing missions

Thanks to the Rossi X-ray Timing Explorer (RXTE), it is now widely recognized that fast X-ray timing can be used to probe strong gravity fields around collapsed objects and constrain the equation of state of dense matter in neutron stars. We first discuss some of the outstanding issues which could be solved with an X-ray timing mission building on the great successes of RXTE and providing an order of magnitude better sensitivity. Then we briefly describe the 'Experiment for X-ray timing and Relativistic Astrophysics' (EXTRA) recently proposed to the European Space Agency as a follow-up to RXTE and the related US mission 'Relativistic Astrophysics Explorer' (RAE).Comment: To be published in `Proceedings of the Third Microquasar Workshop: Granada Workshop on galactic relativistic jet sources', Eds A. J. Castro-Tirado, J. Greiner and J. M. Paredes, Astrophysics and Space Science, in press. More about EXTRA can be found at: http://www.cesr.fr/~barret/extra.htm

Branching Transition of a Directed Polymer in Random Medium

A directed polymer is allowed to branch, with configurations determined by global energy optimization and disorder. A finite size scaling analysis in 2D shows that, if disorder makes branching more and more favorable, a critical transition occurs from the linear scaling regime first studied by Huse and Henley [Phys. Rev. Lett. 54, 2708 (1985)] to a fully branched, compact one. At criticality clear evidence is obtained that the polymer branches at all scales with dimension ${\bar d}_c$ and roughness exponent $\zeta_c$ satisfying $({\bar d}_c-1)/\zeta_c = 0.13\pm 0.01$, and energy fluctuation exponent $\omega_c=0.26 \pm0.02$, in terms of longitudinal distanceComment: REVTEX, 4 pages, 3 encapsulated eps figure

Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar

The relativistic plasma flows onto neutron stars that are accreting material from stellar companions can be used to probe strong-field gravity as well as the physical conditions in the supranuclear-density interiors of neutron stars. Plasma inhomogeneities orbiting a few kilometres above the stars are observable as X-ray brightness fluctuations on the millisecond dynamical timescale of the flows. Two frequencies in the kilohertz range dominate these fluctuations: the twin kilohertz quasi-periodic oscillations (kHz QPOs). Competing models for the origins of these oscillations (based on orbital motions) all predict that they should be related to the stellar spin frequency, but tests have been difficult because the spins were not unambiguously known. Here we report the detection of kHz QPOs from a pulsar whose spin frequency is known. Our measurements establish a clear link between kHz QPOs and stellar spin, but one not predicted by any current model. A new approach to understanding kHz QPOs is now required. We suggest that a resonance between the spin and general relativistic orbital and epicyclic frequencies could provide the observed relation between QPOs and spin.Comment: Published in the 2003 July 3 issue of Natur

Young Crab-like pulsars and luminous X-ray sources in starbursts and optically dull galaxies

Recent Chandra observations of nearby galaxies have revealed a number of ultraluminous X-ray sources (ULXs) with super-Eddington luminosities, away from the central regions of non-active galaxies. The nature of these sources is still debated. We argue that a fraction of them could be young, Crab-like pulsars, the X-ray luminosity of which is powered by rotation. We use the pulsar birth parameters estimated from radio pulsar data to compute the steady-state pulsar X-ray luminosity distribution as a function of the star formation rate (SFR) in the galaxy. We find that ~10% of optically dull galaxies are expected to have a source with L_x >~ 10^{39} erg/s, while starbursts galaxies should each have several of these sources. We estimate that the X-ray luminosity of a few percents of galaxies is dominated by a single bright pulsar with L_x >~10^{39} erg/s, roughly independently of its SFR. We discuss observational diagnostics that can help distinguish the young pulsar population in ULXs.Comment: 17 pages, 4 figures, accepted to Ap

Highly neurotic never-depressed students have negative biases in information processing

BACKGROUND: Cognitive theories associate depression with negative biases in information processing. Although negatively biased cognitions are well documented in depressed patients and to some extent in recovered patients, it remains unclear whether these abnormalities are present before the first depressive episode. METHOD: High neuroticism (N) is a well-recognized risk factor for depression. The current study therefore compared different aspects of emotional processing in 33 high-N never-depressed and 32 low-N matched volunteers. Awakening salivary cortisol, which is often elevated in severely depressed patients, was measured to explore the neurobiological substrate of neuroticism. RESULTS: High-N volunteers showed increased processing of negative and/or decreased processing of positive information in emotional categorization and memory, facial expression recognition and emotion-potentiated startle (EPS), in the absence of global memory or executive deficits. By contrast, there was no evidence for effects of neuroticism on attentional bias (as measured with the dot-probe task), over-general autobiographical memory, or awakening cortisol levels. CONCLUSIONS: These results suggest that certain negative processing biases precede depression rather than arising as a result of depressive experience per se and as such could in part mediate the vulnerability of high-N subjects to depression. Longitudinal studies are required to confirm that such cognitive vulnerabilities predict subsequent depression in individual subjects

Are There Magnetars in High Mass X-ray Binaries? The Case of SuperGiant Fast X-Ray Transients

In this paper we survey the theory of wind accretion in high mass X-ray binaries hosting a magnetic neutron star and a supergiant companion. We concentrate on the different types of interaction between the inflowing wind matter and the neutron star magnetosphere that are relevant when accretion of matter onto the neutron star surface is largely inhibited; these include the inhibition through the centrifugal and magnetic barriers. Expanding on earlier work, we calculate the expected luminosity for each regime and derive the conditions under which transition from one regime to another can take place. We show that very large luminosity swings (~10^4 or more on time scales as short as hours) can result from transitions across different regimes. The activity displayed by supergiant fast X-ray transients, a recently discovered class of high mass X-ray binaries in our galaxy, has often been interpreted in terms of direct accretion onto a neutron star immersed in an extremely clumpy stellar wind. We show here that the transitions across the magnetic and/or centrifugal barriers can explain the variability properties of these sources as a results of relatively modest variations in the stellar wind velocity and/or density. According to this interpretation we expect that supergiant fast X-ray transients which display very large luminosity swings and host a slowly spinning neutron star are characterized by magnetar-like fields, irrespective of whether the magnetic or the centrifugal barrier applies. Supergiant fast X-ray transients might thus provide a new opportunity to detect and study magnetars in binary systems.Comment: Accepted for publication in ApJ. 16 pages, 6 figure

Assessment of disk MHD generators for a base load powerplant

Results from a study of the disk MHD generator are presented. Both open and closed cycle disk systems were investigated. Costing of the open cycle disk components (nozzle, channel, diffuser, radiant boiler, magnet and power management) was done. However, no detailed costing was done for the closed cycle systems. Preliminary plant design for the open cycle systems was also completed. Based on the system study results, an economic assessment of the open cycle systems is presented. Costs of the open cycle disk conponents are less than comparable linear generator components. Also, costs of electricity for the open cycle disk systems are competitive with comparable linear systems. Advantages of the disk design simplicity are considered. Improvements in the channel availability or a reduction in the channel lifetime requirement are possible as a result of the disk design

Critical Droplets and Phase Transitions in Two Dimensions

In two space dimensions, the percolation point of the pure-site clusters of the Ising model coincides with the critical point T_c of the thermal transition and the percolation exponents belong to a special universality class. By introducing a bond probability p_B<1, the corresponding site-bond clusters keep on percolating at T_c and the exponents do not change, until p_B=p_CK=1-exp(-2J/kT): for this special expression of the bond weight the critical percolation exponents switch to the 2D Ising universality class. We show here that the result is valid for a wide class of bidimensional models with a continuous magnetization transition: there is a critical bond probability p_c such that, for any p_B>=p_c, the onset of percolation of the site-bond clusters coincides with the critical point of the thermal transition. The percolation exponents are the same for p_c<p_B<=1 but, for p_B=p_c, they suddenly change to the thermal exponents, so that the corresponding clusters are critical droplets of the phase transition. Our result is based on Monte Carlo simulations of various systems near criticality.Comment: Final version for publication, minor changes, figures adde