873 research outputs found

    Search for pulsations at high radio frequencies from accreting millisecond X-ray pulsars in quiescence

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    It is commonly believed that millisecond radio pulsars have been spun up by transfer of matter and angular momentum from a low-mass companion during an X-ray active mass transfer phase. A subclass of low-mass X-ray binaries is that of the accreting millisecond X-ray pulsars, transient systems that show periods of X-ray quiescence during which radio emission could switch on. The aim of this work is to search for millisecond pulsations from three accreting millisecond X-ray pulsars, XTE J1751-305, XTE J1814-338, and SAX J1808.4-3658, observed during their quiescent X-ray phases at high radio frequencies (5 - 8 GHz) in order to overcome the problem of the free-free absorption due to the matter engulfing the system. A positive result would provide definite proof of the recycling model, providing the direct link between the progenitors and their evolutionary products. The data analysis methodology has been chosen on the basis of the precise knowledge of orbital and spin parameters from X-ray observations. It is subdivided in three steps: we corrected the time series for the effects of (I) the dispersion due to interstellar medium and (II) of the orbital motions, and finally (III) folded modulo the spin period to increase the signal-to-noise ratio. No radio signal with spin and orbital characteristics matching those of the X-ray sources has been found in our search, down to very low flux density upper limits. We analysed several mechanisms that could have prevented the detection of the signal, concluding that the low luminosity of the sources and the geometric factor are the most likely reasons for this negative result.Comment: 5 pages, 3 figures. Accepted for publication by A&

    On Low Mass X-ray Binaries and Millisecond Pulsar

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    The detection, in 1998, of the first Accreting Millisecond Pulsar, started an exciting season of continuing discoveries in the fashinating field of compact binary systems harbouring a neutron star. Indeed, in these last three lustres, thanks to the extraordinary performances of astronomical detectors, on ground as well as on board of satellites, mainly in the Radio, Optical, X-ray, and Gamma-ray bands, astrophysicists had the opportunity to thoroughly investigate the so-called Recycling Scenario: the evolutionary path leading to the formation of a Millisecond Radio Pulsar. The most intriguing phase is certainly the spin-up stage during which, because of the accretion of matter and angular momentum, the neutron star accumulates an extraordinary amount of mechanical rotational energy, up to one percent of its whole rest-mass energy. These millisecond spinning neutron stars are truly extreme physical objects: General and Special Relativity are fully in action, since their surfaces, attaining speeds close to one fifth of the speed of light, are located just beyond their Schwartzscild Radius, and electrodynamical forces, caused by the presence of huge surface magnetic fields of several hundred million Gauss, display their spectacular properties accelerating electrons up to such energies to promote pair creation in a cascade process responsible for the emission in Radio and Gamma-ray. The rotational energy is swiftly converted and released into electromagnetic power which, in some cases, causes the neutron star to outshine with a luminosity of one hundred Suns. In this paper I will review some of the most recent discoveries on (accreting) millisecond pulsars.Comment: 7 pages, 1 table, proceedings of the conference: "Reading the book of Globular Clusters with the lens of stellar evolution", Rome Astronomical Observatory, 26-28 November 201

    Probing the Equation of State of Ultradense Matter with a Submillisecond Pulsar Search Experiment

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    Current ideas about the equation of state for the ultradense matter constituting neutron stars provide models with a range of neutron star radii for a given mass. This implies different estimates for the maximum angular velocity that such an object could attain. The fastest and the slowest angular velocity differ by a significant amount, depending on the equation of state adopted. In particular, the identification of a submillisecond pulsar would allow us to constrain the equation of state of dense matter. In this paper, we discuss a possible evolutionary scenario resulting in a submillisecond pulsar, taking into account current ideas about the evolution of the magnetic field of neutron stars. Pulsar luminosities and lifetimes in the submillisecond period range, derived on the basis of phenomenological considerations, suggest that the effort of searching for such an object would be worthwhile. All the pulsar searches conducted up to now have been prevented by instrumental selection effects from probing the submillisecond range. We discuss the feasibility of a submillisecond pulsar search experiment in the context of current hardware and software capabilities

    Quantum clock: A critical discussion on spacetime

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    We critically discuss the measure of very short time intervals. By means of a Gedankenexperiment, we describe an ideal clock based on the occurrence of completely random events. Many previous thought experiments have suggested fundamental Planck-scale limits on measurements of distance and time. Here we present a new type of thought experiment, based on a different type of clock, that provide further support for the existence of such limits. We show that the minimum time interval Δt\Delta t that this clock can measure scales as the inverse of its size Δr\Delta r. This implies an uncertainty relation between space and time: Δr\Delta r Δt\Delta t >Gℏ/c4> G \hbar / c^4; where G, ℏ\hbar and c are the gravitational constant, the reduced Planck constant, and the speed of light, respectively. We outline and briefly discuss the implications of this uncertainty conjecture.Comment: 10 pages, published in Physical Review

    Modelling the closest double degenerate system RXJ0806.3+1527 and its decreasing period

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    In the hypothesis that the 5.4m binary RXJ0806.3+1527 consists of a low mass helium white dwarf (donor) transferring mass towards its more massive white dwarf companion (primary), we consider as possible donors white dwarfs which are the result of common envelope evolution occurring when the helium core mass of the progenitor giant was still very small (~ 0.2Msun), so that they are surrounded by a quite massive hydrogen envelope (~1/100Msun or larger), and live for a very long time supported by proton--proton burning. Mass transfer from such low mass white dwarfs very probably starts during the hydrogen burning stage, and the donor structure will remain dominated by the burning shell until it loses all the hydrogen envelope and begins transferring helium. We model mass transfer from these low mass white dwarfs, and show that the radius of the donor decreases while they shed the hydrogen envelope. This radius behavior, which is due to the fact that the white dwarf is not fully degenerate, has two important consequences on the evolution of the binary: 1) the orbital period decreases, with a timescale consistent with the period decrease of the binary RXJ0806.3+1527; 2) the mass transfer rate is a factor of about 10 smaller than from a fully degenerate white dwarf, easing the problem connected with the small X-ray luminosity of this object. The possibility that such evolution describes the system RXJ0806.3+1527 is also consistent with the possible presence of hydrogen in the optical spectrum of the star, whose confirmation would become a test of the model.Comment: 13 pages, 4 figures, accepted for publication on ApJ, main journa

    The spin and orbit of the newly discovered pulsar IGR J17480-2446

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    We present an analysis of the spin and orbital properties of the newly discovered accreting pulsar IGR J17480-2446, located in the globular cluster Terzan 5. Considering the pulses detected by the Rossi X-ray Timing Explorer at a period of 90.539645(2) ms, we derive a solution for the 21.27454(8) hr binary system. The binary mass function is estimated to be 0.021275(5) Msun, indicating a companion star with a mass larger than 0.4 Msun. The X-ray pulsar spins up while accreting at a rate of between 1.2 and 1.7E-12 Hz/s, in agreement with the accretion of disc matter angular momentum given the observed luminosity. We also report the detection of pulsations at the spin period of the source during a Swift observation performed ~2 d before the beginning of the RXTE coverage. Assuming that the inner disc radius lies in between the neutron star radius and the corotation radius while the source shows pulsations, we estimate the magnetic field of the neutron star to be within ~2E8 G and ~2.4E10 G. From this estimate, the value of the spin period and of the observed spin-up rate, we associate this source with the still poorly sampled population of slow, mildly recycled, accreting pulsars.Comment: 5 pages, accepted by A&A Letters on 2010 Nov 30. Timing solution derived on a longer time interval with respect to the previous versio

    A Firm Upper Limit to the Radius of the Neutron Star in SAX J1808.4-3658

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    We show that observations of X-ray pulsing from SAX J1808.4-3658 place a firm upper limit of 13.8 m^{1/3} km on the radius of the neutron star, where m is its mass in solar units. The limit is independent of distance or assumptions about the magnetospheric geometry, and could be significantly tightened by observations of the pulsations in the near future. We discuss the implications for the equation of state and the possible neutron star mass.Comment: (7 pages, 1 figure, accepted for publication in ApJ Letters

    The Broad Band Spectrum of MXB 1728-34 Observed by BeppoSAX

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    We report on the results of a broad band (0.1-100 keV) spectral analysis of the bursting atoll source MXB 1728-34 observed by the BeppoSAX satellite. Three bursts were present during this observation. The spectrum during the bursts can be fitted by a blackbody with a temperature of 2 keV. From the bursts we also estimate a distance to the source of 5.1 kpc. MXB 1728-34 was in a rather soft state during the BeppoSAX observation. The persistent spectrum is well fitted by a continuum consisting of a soft blackbody emission and a comptonized spectrum. We interpreted the soft component as the emission from the accretion disk. Taking into account a spectral hardening factor of 1.7, we estimated that the inner disk radius is Rincos⁥i∌20R_{\rm in} \sqrt{\cos i} \sim 20 km, where i is the inclination angle. The comptonized component could originate in a spherical corona, with temperature of 10 keV and optical depth of 5, surrounding the neutron star. A broad gaussian emission line at 6.7 keV is observed in the spectrum, probably emitted in the ionized corona or in the inner part of the disk. Another emission line is present at 1.66 keV.Comment: 12 pages, accepted by Ap

    Spectral changes during six years of Scorpius X-1 monitoring with BeppoSAX Wide Field Cameras

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    We analyse a sample of fifty-five observations of Scorpius X-1 available in the BeppoSAX Wide Field Camera public archive and spanning over the six years of BeppoSAX mission life. Spectral changes are initially analysed by inspection of colour-colour and colour-intensity diagrams, we also discuss the shift of the Z tracks in these diagrams. Then we select two long observations for spectral fitting analysis, a secular shift is evident between the tracks in these observations. We finally extract spectra along the tracks and discuss the best fit model, the parameter variations along the track and between tracks, and their link to the accretion rate.Comment: 6 pages, 11 postscrpt figures.To appear in the conference proceedings of `Interacting Binaries: Accretion, Evolution & Outcomes' (Cefalu', July 4-10 2004