We observed the accreting millisecond pulsar SAX J1808.4−3658 with Gemini-South in g′ and i′ bands, nearly simultaneous with XMM–Newton observations. A clear periodic flux modulation on the system's ...orbital period is present, consistent with the varying aspect of the donor star's heated face. We model the contributions of a disc and donor star to these optical bands. To produce the observed modulation amplitudes, we conclude that the donor must be irradiated by an external flux two orders of magnitude greater than provided by the measured X-ray luminosity. A possible explanation for this irradiation is that the radio pulsar mechanism becomes active during the quiescent state as suggested by Burderi et al., with relativistic particles heating the donor's day side face. Our modelling constrains the binary inclination to be 36°–67°. We obtain estimates for the pulsar mass of >2.2 M⊙ (although this limit is sensitive to the source's distance), consistent with the accelerated neutron star cooling in this system indicated by X-ray observations. We also estimate the donor mass to be 0.07–0.11 M⊙, providing further indications that the system underwent non-standard binary evolution to reach its current state.
The condensation of matter from a corona to a cool, optically thick inner disk is investigated for black hole X-ray transient systems in the low/hard state. A description of a simple model for the ...exchange of energy and mass between corona and disk originating from thermal conduction is presented, taking into account the effect of Compton cooling of the corona by photons from the underlying disk. It is found that a weak, condensation-fed inner disk can be present in the low/hard state of black hole transient systems for a range of luminosities that depends on the magnitude of the viscosity parameter. For alpha similar to 0.1-0.4, an inner disk can exist for luminosities in the range similar to (0.001-0.02)L sub(Edu). The model is applied to the X-ray observations of the black hole candidate sources GX 339-4 and SWIFT J1753.5-0127 in their low/hard state. It is found that Compton cooling is important In the condensation process, leading to the maintenance of cool inner disks in both systems. As the results of the evaporation/condensation model are independent of the black hole mass, it is suggested that such inner cool disks may contribute to the optical and ultraviolet emission of low-luminosity active galactic nuclei.
The population synthesis of cataclysmic variable binary systems at short orbital periods (<2.75 hr) is investigated. A grid of detailed binary evolutionary sequences has been calculated and included ...in the simulations to take account of additional angular momentum losses beyond that associated with gravitational radiation and mass loss, due to nova outbursts, from the system. As a specific example, we consider the effect of a circumbinary disk to gain insight into the ingredients necessary to reproduce the observed orbital period distribution. The resulting distributions show that the period minimum lies at about 80 minutes, with the number of systems monotonically increasing with increasing orbital period to a maximum near 90 minutes. There is no evidence for an accumulation of systems at the period minimum, which is a common feature of simulations in which only gravitational radiation losses are considered. The shift of the peak to about 90 minutes is a direct result of the inclusion of systems formed within the period gap. The period distribution is found to be fairly flat for orbital periods ranging from about 85 to 120 minutes. The steepness of the lower edge of the period gap can be reproduced, for example, by an input of systems at periods near 2.25 hr due to a flow of cataclysmic variable binary systems from orbital periods longer than 2.75 hr. The good agreement with the cumulated distribution function of observed systems within the framework of our model indicates that the angular momentum loss by a circumbinary disk or a mechanism that mimics its features, coupled with a weighting factor to account for selection effects in the discovery of such systems and a flow of systems from above the period gap to below the period gap, is an important ingredient for understanding the overall period distribution of cataclysmic variable binary systems.
The general properties of the nonthermal nonpulsed X-ray emission of rotation-powered pulsars are investigated in the context of a pulsar wind nebula model. An examination of the observed X-ray ...emission from a sample of 23 pulsars in the energy range between 2 and 10 keV reveals that the relation of X-ray luminosity, L sub(X), to the pulsar spin-down power, E, is steeper for the nonpulsed component than for the pulsed component. Specifically, L super(n) sub(X) super(pul) 8 E super(1.4c0.1) for the nonpulsed component, whereas L super(p) sub(X) super(ul) 8 E super(1.2c0.08) for the pulsed component. The former relation is consistent with emission from a pulsar wind nebula model in which L super(n) sub(X) super(pul) 8 E super(p/2), where p is the power-law index of the electron energy distribution. The relation for the pulsed component, on the other hand, is consistent with a magnetospheric emission model. In addition, the photon spectral index, , was found to be correlated with the conversion efficiency of spin-down power into nonpulsed X-ray emission, with greater efficiencies for 6 2-2.5 than for 6 1.5-2. Such a relation can be naturally understood within the framework of a pulsar wind nebula model, with the former relation corresponding to the emission of X-rays in the fast-cooling regime and the latter relation corresponding to emission in the slow-cooling regime. The X-ray properties of pulsar wind nebulae are sensitive to the physical conditions (e.g., the density and magnetic field) of the interstellar medium, which can lead to important differences between the X-ray emission characteristics (luminosity, photon spectral index, and emission morphology) of pulsars in various environments. Such wind nebulae can contribute to the nonthermal symmetric emission morphology (pointlike) and elongated emission morphology (tail-like) from sources similar to Geminga and PSR B1757-24.
We calculate the full stellar structural evolution of donors in AM CVnAM Canum Venaticorum (AM CVn) systems formed through the white dwarf (WD) channel coupled to the binary's evolution. Contrary to ...assumptions made in prior modelling, these donors are not fully convective over much of the AM CVn phase and do not evolve adiabatically under mass loss indefinitely. Instead, we identify three distinct phases of evolution: a mass-transfer turn-on phase (during which Porb continues to decrease even after contact, the donor contracts, and the mass-transfer rate accelerates to its maximum), a phase in which the donor expands adiabatically in response to mass loss, and a cooling phase beginning at Porb≈ 45–55 min during which the donor contracts. The physics that determines the behaviour in the first and third phases, both of which are new outcomes of this study, are discussed in some detail. We find the overall duration of the turn-on phase to be between ∼104 and ∼106 yr, significantly longer than prior estimates. We predict the donor's luminosity, L, and effective temperature, Teff. During the adiabatic expansion phase (ignoring irradiation effects), L≈ 10−6–10−4 L⊙ and Teff≈ 1000–1800 K. However, the flux generated in the accretion flow dominates the donor's intrinsic light at all times. The impact of irradiation on the donor extends the phase of adiabatic expansion to longer Porb, slows the contraction during the cooling phase, and alters the donor's observational characteristics. Irradiated donors during the adiabatic phase can attain surface luminosities up to ≈10−2 L⊙. We argue that the turn-on and cooling phases both will leave significant imprints on the AM CVn population's Porb-distribution. Finally, we show that the eclipsing AM CVn system SDSS J0926+3624 provides evidence that WD channel systems with non-zero entropy donors contribute to the AM CVn population, and we discuss the observational signature of the donor in this system.
Recent deep Chandra surveys of the Galactic center region have revealed the existence of a faint, hard X-ray source population. While the nature of this population is unknown, it is likely that ...several types of stellar objects contribute. For sources involving binary systems, accreting white dwarfs and accreting neutron stars with main-sequence companions have been proposed. Among the accreting neutron star systems, previous studies have focused on stellar wind-fed sources. In this paper, we point out that binary systems in which mass transfer occurs via Roche lobe overflow (RLOF) can also contribute to this X-ray source population. A binary population synthesis study of the Galactic center region has been carried out, and it is found that evolutionary channels for neutron star formation involving the accretion-induced collapse of a massive ONeMg white dwarf, in addition to the core collapse of massive stars, can contribute to this population. The RLOF systems would appear as transients with quiescent luminosities, above 2 keV, in the range from 10 super(31) to 10 super(32) ergs s super(-1). The results reveal that RLOF systems primarily contribute to the faint X-ray source population in the Muno et al. survey and that wind-fed systems can contribute to the less sensitive Wang et al. survey. However, our results suggest that accreting neutron star systems are not likely to be the major contributor to the faint X-ray source population in the Galactic center.
The nonthermal, nonpulsed X-ray emission of MSPs is investigated. As in young pulsars, MSPs emit a relativistic wind, which in interacting with the ISM and/or a binary companion can significantly ...contribute to the nonpulsed emission of these pulsars. An application and extension of a simple model developed for young pulsars is applied to the old recycled MSP B1957+20. It is found that the pulsar wind can indeed contribute to both the resolved and unresolved X-ray emission. For other MSPs in the Galactic field for which the spectral index of the nonpulsed component has been measured (i.e., PSR B1937+21, PSR J0218+4232) the contribution of the pulsar wind to the nonpulsed X-ray luminosity is estimated. For the MSPs in the core regions of globular clusters, the pulsar wind nebula is likely affected by its interaction with the dense stellar environment, possibly leading to a diminished contribution to the total X-ray emission. In this case, the existence of nonthermal nonpulsed X-ray emission is more likely for binary than for isolated MSPs, with the emission arising from the interaction of the relativistic pulsar wind and a binary companion. Our study suggests that the magnetization parameter in the pulsar wind nebulae of MSPs is significantly larger than that of the Crab Nebula, by about a factor of 10. The nebulae powered by rapidly rotating neutron stars either in isolation or in the quiescent state of soft X-ray transients can contribute to the faint X-ray source population associated with nonaccreting neutron stars. The emission from MSPs moving at high velocities (>100 km s super(-1)) through regions of the ISM characterized by magnetic field strengths <0.1 mG may appear spatially extended with a tail-like morphology. Thus, MSPs may also contribute to the faint filamentary X-ray source subpopulation in the Galaxy.
The common envelope phase of binary star evolution plays an essential role
in the formation of short period systems containing a compact object. In this
process, significant mass and angular momentum ...are lost, transforming a wide
progenitor system into a close remnant binary. The pathways leading to this
phase and the outcomes are described. Emphasis is placed on the conditions that
are required for survival of the binary according to the results of
three-dimensional hydrodynamics calculations. The evolution of high-mass
systems containing neutron stars is discussed, including double neutron stars,
binary pulsars, Thorne-Zytkow objects, and high- and low-mass X-ray
binaries.
Celotno besedilo
Dostopno za:
CMK, DOBA, FMFMET, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The low-mass X-ray binary (LMXB) system FIRST J102347.6+003841 hosts a newly born millisecond pulsar (MSP) PSR J1023+0038 that was revealed as the first and only known rotation-powered MSP in a ...quiescent LMXB. While the system is shown to have an accretion disk before 2002, it remains unclear how the accretion disk has been removed in order to reveal the radio pulsation in 2007. In this Letter, we report the discovery of Delta *g-rays spatially consistent with FIRST J102347.6+003841, at a significance of seven standard deviations, using data obtained by the Fermi Gamma-ray Space Telescope. The Delta *g-ray spectrum can be described by a power law (PL) with a photon index of 2.9 ? 0.2, resulting in an energy flux above 200 MeV of (5.5 ? 0.9) X 10--12 erg cm--2 s--1. The Delta *g-rays likely originate from the MSP PSR J1023+0038, but also possibly from an intrabinary shock between the pulsar and its companion star. To complement the Delta *g-ray study, we also re-investigate the XMM-Newton data taken in 2004 and 2008. Our X-ray spectral analysis suggests that a broken PL with two distinct photon indices describes the X-ray data significantly better than a single PL. This indicates that there exists two components and that both components appear to vary with the orbital phase. The evidence for Delta *g-ray emission conforms with a recent suggestion that Delta *g-rays from PSR J1023+0038 may be responsible for ejecting the disk material out of the system.
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