The X-ray Spectra of Accreting Pulsars: Studies of Three Sources Using Empirical and Phenomenological Models
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Author(s): Hemphill, Paul Britton | Advisor(s): Rothschild, Richard E | Abstract: Accreting X-ray pulsars are a class of astrophysical objects consisting of a neutron star in a binary system with a stellar companion. Matter expelled by the companion star is captured by the neutron star's gravity; as this matter falls towards the neutron star's surface, is is compressed and heated, giving off X-rays. As the matter falls the last few miles above the neutron star surface, a number of physical processes compete for dominance, resulting in a highly complex environment governed by the interplay of magnetic, hydrodynamical, and radiative processes. The resulting spectrum often shows broad absorption-like features called cyclotron lines, which provide the only direct measurement of the magnetic field of a neutron star and act as probes of the properties of the accretion column, and their behavior with respect to changes in the accretion rate onto the neutron star has been of interest in recent years. My work in this dissertation brings together nearly 20 years of data from three X-ray satellites to study the X-ray emission from accreting pulsars, with a focus on the hard X-ray continuum and cyclotron lines. I present results for the accreting pulsars 4U 1538-522 and 4U 1907+09, examining the behavior of their cyclotron lines with respect to their luminosity, finding evidence for a positive correlation between the line energy and luminosity in 4U 1907+09. A combined analysis of most of the available X-ray data for the accreting pulsar 4U 1538-522 shows no such correlation in this source, either positive or negative. However, I do present evidence that the cyclotron line energy in 4U 1538-522 has shifted upwards by ~5% in recent years compared to measurements from 10-20 years ago. I additionally carry out an extensive analysis of the environment around 4U 1538-522 using the soft X-ray detectors aboard the satellite Suzaku. I finally present a set of new results from the transient X-ray pulsar V 0332+53, which I fit with a new physics-based model for the accretion column. These fits allow me to constrain the size and temperature of the accretion column, as well as the relative contributions of different processes in the column to the overall observed spectrum.Keywords:
X-ray pulsar
X-ray binary
Intermediate polar
Compact star
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ABSTRACT The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was observed by the Nuclear Spectroscope Telescope Array in the 3–79 keV energy band for a net exposure time of 50 ks. We present the results of this observation which enabled the discovery of a cyclotron resonant scattering feature with a centroid energy of keV. This allowed us to measure the magnetic field strength of the neutron star for the first time: B = (2.5 ± 0.1) × 10 12 G. The known pulsation period is now observed at 904.0 ± 0.1 s. Since 2006, the neutron star has undergone a long-term spin-up trend at a rate of s s −1 (−0.6 s per year, or a frequency derivative of Hz s −1 ). In the power density spectrum, a break appears at the pulse frequency which separates the zero slope at low frequency from the steeper slope at high frequency. This addition of angular momentum to the neutron star could be due to the accretion of a quasi-spherical wind, or it could be caused by the transient appearance of a prograde accretion disk that is nearly in corotation with the neutron star whose magnetospheric radius is around 2 × 10 8 cm.
X-ray binary
X-ray pulsar
Compact star
Vela
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Accreting X-ray pulsars are among the most luminous objects in the X-ray sky. In highly magnetized neutron stars (B~10^12 G), the flow of matter is dominated by the strong magnetic field. The general properties of accreting X-ray binaries are presented, focusing on the spectral characteristics of the systems. The use of cyclotron lines as a tool to directly measure a neutron star's magnetic field and to test the theory of accretion are discussed. We conclude with the current and future prospects for accreting X-ray binary studies.
X-ray pulsar
X-ray binary
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High amplitude X-ray brightness oscillations during thermonuclear X-ray bursts were discovered with the Rossi X-ray Timing Explorer (RXTE) in early 1996. Spectral and timing evidence strongly supports the conclusion that these oscillations are caused by rotational modulation of the burst emission and that they reveal the spin frequency of neutron stars in low mass X-ray binaries. The recent discovery of X-ray burst oscillations from two accreting millisecond pulsars has confirmed this basic picture and provided a new route to measuring neutron star properties and constraining the dense matter equation of state. I will briefly summarize the current observational understanding of accreting millisecond pulsars, and describe recent attempts to determine the mass and radius of the neutron star in XTE J1814-338.
Millisecond
Thermonuclear Fusion
X-ray burster
X-ray binary
X-ray pulsar
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Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation D. Dal Fiume, Filippo Frontera, Nicola Masetti, Mauro Orlandini, Eliana Palazzi, Stefano Del Sordo, Andrea Santangelo, Alberto Segreto, Tim Oosterbroek, Arvind N. Parmar; Cyclotron lines in X-ray pulsars as a probe of relativistic plasmas in superstrong magnetic fields. AIP Conf. Proc. 12 April 2000; 510 (1): 183–187. https://doi.org/10.1063/1.1303199 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search
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I. Rotation-Powered Pulsars.- The Population.- Pulsar Surveys.- Millisecond Pulsar Surveys.- The Kinematics of Pulsars.- Interstellar Scintillations and Neutron Star Kinematics.- A Deep Search for Young Pulsars in the Galactic Plane at 1400 MHz (abstract).- Progenitors of the Local Pulsars: Lower Mass Limit and Beaming Factor (abstract).- The Relation Between Radio Luminosity and Magnetic Field in Rotation-Powered Pulsars (abstract).- Does the Radio Luminosity of Pulsar Grow up in its Later Stage? (abstract).- A Few Remarks on the Two Types of Pulsars (abstract).- The Kinematic Properties of Two Pulsar Types (abstract).- Toward an Empirical Theory of Pulsar Emission (abstract).- Radio Emission Mechanisms for Two Types of Pulsars abstract).- Pulse Asymmetry of Millisecond Pulsars (abstract).- Pulsar Polarization Limiting Radii and the Evolution of Pulsar Beams (abstract).- Polarization Angle Swings Rediscussed (abstract).- Triplicity of Pulsar Profiles and Orthogonal Polarization Modes (abstract).- The Effects of Inverse Compton Scattering on the Pulsars' Radiation (abstract).- The Geometric Study of Drifting Subpulses (abstract).- Pair Production in Intense Electromagnetic Fields of Pulsars (abstract).- A Modified Pulsar Model, Green Function, Period Distribution (abstract).- A Massive Glitch in PSR 0355+54 (abstract).- A Large Timing Discontinuity in the Vela Pulsar, July 1985 (abstract).- Pulsars and Supernova Remnants.- The Galactic Pulsar Population and Neutron Star Birth.- Neutron Star Coupling to its Environment.- Crab-Like Supernova Remnants.- Neutron Stars in Twelve Supernova Remnants.- The Progenitors of Pulsars.- Distribution of Two Types of Pulsars in Comparison with that of SNRs (abstract).- The Structure of Pulsar Nebulae (abstract).- High-Resolution Radio Observations of the Crablike Supernova Remnant 3C 58 (abstract).- Search for Plerions in the Direction of Two Young Pulsars (abstract).- Supernova Remnants with Radio Jets (abstract).- The Evolution of Young Supernova Remnants (abstract).- On the Origin of Kepler's Supernova Remnant (abstract).- The X-ray Spectra of Tycho and SN1006 (abstract).- The Galactic Sources G5.4-1.2 and G5.27-0.90 (abstract).- Supernova Remnants in the Magellanic Clouds Observed at the Molonglo Observatory (abstract).- A Statistical Study of the Correlation Between Galactic SNRs and Spiral Arms (abstract).- II. Accretion Powered Pulsars.- The Population.- Accreting Neutron Stars.- Population of Accreting Neutron Stars in External Galaxies.- Observations of X-ray Burst Sources.- On the Origin of Neutron Stars in Globular Clusters.- The Globular Cluster Population of X-ray Binaries.- Hardness Ratio in Evolving Low-Mass X-ray Binary Systems (abstract).- Results of the Timing Analyses of X-ray Pulsars Observed by Hakucho and Tenma (abstract).- Herculis XI :Results and Interpretation (abstract).- A Hard X-ray Observation of Cyg X-1 in 1985 (abstract).- The X-ray Transient EXO 2030+375 (abstract).- Analysis of X-ray Spectrum for Globular Cluster M15 X-ray Source (abstract).- Theoretical Considerations.- Accretion onto Magnetized Neutron Stars: Polar Cap Flow and Centrifugally Driven Winds.- Line Radiation from Accreting Magnetized Neutron Stars.- X-ray Bursting Neutron Stars.- Polar Cap Accretion onto Magnetized Neutron Stars: An Analytic Solution (abstract).- Radiation Gas Dynamics of Polar Cap Accretion onto Magnetized Neutron Stars (abstract).- X-ray Irradiated Accretion Disk and Bimodal States (abstract).- An Inverse Corapton Scattering Model for the Spectra of X-ray Pulsars (abstract).- The X-ray Radiation Mechanism of the Compact (Neutron) Binary Stars (abstract).- X-ray Spectra and Atmospheric Structures of Bursting Neutron Stars (abstract).- Model Atmospheres for X-ray Bursting Neutron Stars (abstract).- III. Neutron Star Formation in Theoretical Supernovae.- The Birth of Neutron Stars.- Numerical Experiments and Neutron Star Formation.- Neutron Star Formation in Theoretical Supernovae - Low Mass Stars and White Dwarfs.- Ancient Guest Stars as Harbingers of Neutron Star Formation.- IV. Neutron Stellar Evolution.- Quasi Periodic Oscillations as a Clue to Field and Spin Evolution.- Quasi-Periodic Oscillations in Low-Mass X-ray Binaries.- Observations of Quasi-Periodic Oscillations in Cyg X-2.- The Beat Frequency Model for QPOs.- Quasi-Periodic Oscillations.- On the Long Term Stability of Neutron Star Magnetic Fields (abstract).- On the Evolution of Magnetic Inclination with Age (abstract).- Timescale for the Decay of Magnetic Fields of Pulsars (abstract).- Tortion Influence on the Magnetic Field of Rotating Neutron Stars (abstract).- Evolution in Binaries.- Binary Pulsars: Observations and Implications.- Millisecond Pulsar Formation and Evolution.- Secondary Components of Binary Pulsars and Magnetic Field Decay in Neutron Stars (abstract).- Monte Carlo Simulations of Radio Pulsars and Their Progenitors (abstract).- Constraints to Possible Progenitor Systems of PSR 1831-00 (abstract).- Geodetic Precession in Binary Pulsars (abstract).- V. Neutron Star Physics.- Neutron Star Interiors.- Two Types of Pulsars.- Neutron Star Cooling: Criticical Test of Dense Matter Physics.- Neutron Stars Formed from Supernova Explosion and Quark Matter (abstract).- The Problem of Solidification in Neutron Stars (abstract).- Interior Structures for Two Types of Pulsars (abstract).- Modes of Energy Loss from Isolated Magnetized Neutron Star (abstract).- Effects of Magnetization on Structure Parameters of Neutron Stars (abstract).- Synchro-Curvature Radiation and Magnetic Pair Production of Relativistic Electrons in Strong Curved Magnetic Fields (abstract).- Inverse Compton Scattering in Strong Magnetic Fields: Applied to the Radiation Mechanism of PSR 0531+21 (abstract).- The Nutations of Neutron Stars and Core-Crust Coupling (abstract).- The Spin-Torsion Coupling Precession of Spin and its Effects on Single Pulses of Pulsars (abstract).- Neutron Star Cooling and the Vela Pulsar (abstract).- Einstein Observatory Limits on Neutron Star Surface Temperatures (abstract).- Thermal Radiation from a Radio Pulsar: PSRI055-52 (abstract).- Model Atmospheres for Cooling Neutron Stars (abstract).- The Neutron Stars with Magnetic Charges (abtract).- The Nonlinear Dispersion Relation and the Relationship of the Forming Soliton Area to the Evolution of Pulsars (abstract).- VI: Other Manifestations of Neutron Stars and their Place in Neutron Star Evolution.- Neutron Stars and Gamma-Rays.- A Review of Gamma-Ray Burst Observations.- Some Constraints on Neutron Star Properties from Gamma Ray Burster Observations.- Theory of Gamma Ray Bursters.- Cygnus X-3 and Other Ultra-High-Energy Gamma-Ray Sources.- A New Aspect of Galactic Ridge X-ray Emission - SNRs in a Tenuous Medium?.- EXOSAT News on Geminga (abstract).- The Origin of the Enhanced Dissipation alpha in Accretion Discs and its Relation to Gamma Bursts (abstract).- Inverse Compton Model of Gamma Ray Burst Spectra (abstract).- A Model for the 1979 March 5 Gamma-Ray Transient (abstract).- New Distance Limit for the 3-5-79 Source (abstract).- What Type of Binary System is Cygnus X-3? (abstract).- On the Phases of Period Variation of SS433 (abstract).- Production and Interaction of High Energy Neutrinos in Close X-ray Binaries (abstract).- An Experiment for Observing VHE Gamma Ray Sources (abstract).- High Energy Cosmic Rays from Young Neutron Stars (abstract).- Cosmic Ray Particle Acceleration in Pulsar Magnetospheres (abstract).- Observations of Neutron Stars Planned by the High Speed Photometer Team Using Space Telescope (abstract).- Epilogue.- Where Neutron Stars Come From, How Neutron Stars Evolve, and Where Neutron Stars Go.
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We have analyzed the time variability of the wide-band X-ray spectrum of Vela X-1, the brightest wind-fed accreting neutron star, on a short timescale of 2 ks by using {\it Suzaku} observations with an exposure of 100 ks. During the observation, the object showed strong variability including several flares and so-called "low states", in which the X-ray luminosity decreases by an order of magnitude. Although the spectral hardness increases with the X-ray luminosity, the majority of the recorded flares do not show any significant changes of circumstellar absorption. However, a sign of heavy absorption was registered immediately before one short flare that showed a significant spectral hardening. In the low states, the flux level is modulated with the pulsar spin period, indicating that even at this state the accretion flow reaches the close proximity of the neutron star. Phenomenologically, the broad-band X-ray spectra, which are integrated over the entire spin phase, are well represented by the "NPEX" function (a combination of negative and positive power laws with an exponential cutoff by a common folding energy) with a cyclotron resonance scattering feature at 50 keV. Fitting of the data allowed us to infer a correlation between the photon index and X-ray luminosity. Finally, the circumstellar absorption shows a gradual increase in the orbital phase interval 0.25--0.3, which can be interpreted as an impact of a bow shock imposed by the motion of the compact object in the supersonic stellar wind.
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We review the physical processes occurring in the magnetosphere of accreting X-ray pulsars, with emphasis on those processes that give rise to observable effects in their high (E>10 keV) energy spectra. In the second part we compare the empirical spectral laws used to fit the observed spectra with theoretical models, at the light of the BeppoSAX results on the broad-band characterization of the X-ray pulsar continuum, and the discovery of new (multiple) cyclotron resonance features.
Cyclotron resonance
Spectral Analysis
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X-ray reflection signatures are observed around multiple classes of accreting compact objects. Modeling these features yields important constraints on the physics of accretion disks, motivating the development of X-ray reflection models appropriate for a variety of systems and illumination conditions. Here, constant density ionized X-ray reflection models are presented for a disk irradiated with a very hard power-law X-ray spectrum (Γ < 1) and a variable high-energy cutoff. These models are then applied to the Suzaku data of the accreting X-ray pulsar LMC X-4, where very good fits are obtained with a highly ionized reflector responsible for both the broad Fe Kα line and the soft excess. The ionized reflector shows strong evidence for significant Doppler broadening and is redshifted by ∼104 km s−1. These features indicate that the reflecting material is associated with the complex dynamics occurring at the inner region of the magnetically truncated accretion disk. Thus, reflection studies of X-ray pulsar spectra may give important insights into the accretion physics at the magnetospheric radius.
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The discovery of millisecond pulsars switching between states powered either by the rotation of their magnetic field or by the accretion of matter has recently proved the tight link shared by millisecond radio pulsars and neutron stars in low-mass X-ray binaries. Transitional millisecond pulsars also show an enigmatic intermediate state in which the neutron star is surrounded by an accretion disk and emits coherent X-ray pulsations, but is sub-luminous in X-rays with respect to accreting neutron stars, and is brighter in gamma-rays than millisecond pulsars in the rotation-powered state. Here, we model the X-ray and gamma-ray emission observed from PSR J1023+0038 in such a state based on the assumptions that most of the disk in-flow is propelled away by the rapidly rotating neutron star magnetosphere, and that electrons can be accelerated to energies of a few GeV at the turbulent disk–magnetosphere boundary. We show that the synchrotron and self-synchrotron Compton emission coming from such a region, together with the hard disk emission typical of low states of accreting compact objects, is able to explain the radiation observed in the X-ray and gamma-ray bands. The average emission observed from PSR J1023+0038 is modeled by a disk in-flow with a rate of 1–3 × 10−11 M⊙ yr−1, truncated at a radius ranging between 30 and 45 km, compatible with the hypothesis of a propelling magnetosphere. We compare the results we obtained with models that assume that a rotation-powered pulsar is turned on, showing how the spin-down power released in similar scenarios is hardly able to account for the magnitude of the observed emission.
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