Spectra Of X-ray Pulsars Research Articles

Dynamical and Radiative Properties of X-Ray Pulsar Accretion Columns: Phase-averaged Spectra

Abstract The availability of the unprecedented spectral resolution provided by modern X-ray observatories is opening up new areas for study involving the coupled formation of the continuum emission and the cyclotron absorption features in accretion-powered X-ray pulsar spectra. Previous research focusing on the dynamics and the associated formation of the observed spectra has largely been confined to the single-fluid model, in which the super-Eddington luminosity inside the column decelerates the flow to rest at the stellar surface, while the dynamical effect of gas pressure is ignored. In a companion paper, we have presented a detailed analysis of the hydrodynamic and thermodynamic structure of the accretion column obtained using a new self-consistent model that includes the effects of both gas and radiation pressures. In this paper, we explore the formation of the associated X-ray spectra using a rigorous photon transport equation that is consistent with the hydrodynamic and thermodynamic structure of the column. We use the new model to obtain phase-averaged spectra and partially occulted spectra for Her X-1, Cen X-3, and LMC X-4. We also use the new model to constrain the emission geometry, and compare the resulting parameters with those obtained using previously published models. Our model sheds new light on the structure of the column, the relationship between the ionized gas and the photons, the competition between diffusive and advective transport, and the magnitude of the energy-averaged cyclotron scattering cross-section.

Cyclotron resonant scattering feature simulations

Electron cyclotron resonant scattering features (CRSFs) are observed as absorption-like lines in the spectra of X-ray pulsars. A significant fraction of the computing time for Monte Carlo simulations of these quantum mechanical features is spent on the calculation of the mean free path for each individual photon before scattering, since it involves a complex numerical integration over the scattering cross section and the (thermal) velocity distribution of the scattering electrons. We aim to numerically calculate interpolation tables which can be used in CRSF simulations to sample the mean free path of the scattering photon and the momentum of the scattering electron. The tables also contain all the information required for sampling the scattering electron's final spin. The tables were calculated using an adaptive Simpson integration scheme. The energy and angle grids were refined until a prescribed accuracy is reached. The tables are used by our simulation code to produce artificial CRSF spectra. The electron momenta sampled during these simulations were analyzed and justified using theoretically determined boundaries. We present a complete set of tables suited for mean free path calculations of Monte Carlo simulations of the cyclotron scattering process for conditions expected in typical X-ray pulsar accretion columns (0.01<B/B_{crit}<=0.12, where B_{crit}=4.413x10^{13} G and 3keV<=kT<15keV). The sampling of the tables is chosen such that the results have an estimated relative error of at most 1/15 for all points in the grid. The tables are available online at http://www.sternwarte.uni-erlangen.de/research/cyclo.

Effective-action approach to wave propagation in scalar QED plasmas

A relativistic quantum field theory with nontrivial background fields is developed and applied to study waves in plasmas. The effective action of the electromagnetic 4-potential is calculated ab initio from the standard action of scalar QED using path integrals. The resultant effective action is gauge invariant and contains nonlocal interactions, from which gauge bosons acquire masses without breaking the local gauge symmetry. To demonstrate how the general theory can be applied, we study a cold unmagnetized plasma and a cold uniformly magnetized plasma. Using these two examples, we show that all linear waves well-known in classical plasma physics can be recovered from relativistic quantum results when taking the classical limit. In the opposite limit, classical wave dispersion relations are modified substantially. In unmagnetized plasmas, longitudinal waves propagate with nonzero group velocities even when plasmas are cold. In magnetized plasmas, anharmonically spaced Bernstein waves persist even when plasmas are cold. These waves account for cyclotron absorption features observed in spectra of X-ray pulsars. Moreover, cutoff frequencies of the two non-degenerate electromagnetic waves are redshifted by different amounts. These corrections need to be taken into account in order to correctly interpret diagnostic results in laser plasma experiments.

High-mass X-ray binaries in the Milky Way

High-mass X-ray binaries are fundamental in the study of stellar evolution, nucleosynthesis, structure and evolution of galaxies and accretion processes. Hard X-rays observations by INTEGRAL and Swift have broad- ened significantly our understanding in particular for the super-giant systems in the Milky Way, which number has increased by almost a factor of three. INTEGRAL played a crucial role in the discovery, study and understanding of heavily obscured systems and of fast X-ray transients. Most super-giant systems can now be classified in three categories: classical/obscured, eccentric and fast transient. The classical systems feature low eccentricity and variability factor of about 1000, mostly driven by hydrodynamic phenomena occurring on scales larger than the accretion radius. Among them, systems with short orbital periods and close to Roche-Lobe overflow or with slow winds, appear highly obscured. In eccentric systems, the variability amplitude can reach even higher factors, because of the contrast of the wind density along the orbit. Four super-giant systems, featuring fast outbursts, very short orbital periods and anomalously low accretion rates, are not yet understood. Simulations of the accretion processes on relatively large scales have pro- gressed and reproduce parts of the observations. The combined effects of wind clumps, magnetic fields, neutron star rotation and eccentricity ought to be included in future modelling work. Observations with INTEGRAL in combination with other observatories were also important for detecting cyclotron resonant scattering features in spectra of X-ray pulsars, probing their variations and the geometry of the accretion column and emission regions. Finally, the unique characteristics of INTEGRAL and its long life time played a fundamental role for building a complete catalogue of HXMBs, to study the different populations of these systems ...

Transient X-ray pulsar V 0332+53: pulse-phase-resolved spectroscopy and the reflection model

We present the results of the pulse phase- and luminosity-resolved spectroscopy of the transient X-ray pulsar V0332+53, performed for the first time in a wide luminosity range (1-40)x10^{37} erg/s during a giant outburst observed by the RXTE observatory in Dec 2004 - Feb 2005. We characterize the spectra quantitatively and built the detailed "three-dimensional" picture of spectral variations with pulse phase and throughout the outburst. We show that all spectral parameters are strongly variable with the pulse phase, and the pattern of this variability significantly changes with luminosity directly reflecting the associated changes in the structure of emission regions and their beam patterns. Obtained results are qualitatively discussed in terms of the recently developed reflection model for the formation of cyclotron lines in the spectra of X-ray pulsars.

On the origin of cyclotron lines in the spectra of X-ray pulsars

Cyclotron resonance scattering features are observed in the spectra of some X-ray pulsars and show significant changes in the line energy with the pulsar luminosity. In a case of bright sources, the line centroid energy is anti-correlated with the luminosity. Such a behaviour is often associated with the onset and growth of the accretion column, which is believed to be the origin of the observed emission and the cyclotron lines. However, this scenario inevitably implies large gradient of the magnetic field strength within the line-forming region, and it makes the formation of the observed line-like features problematic. Moreover, the observed variation of the cyclotron line energy is much smaller than could be anticipated for the corresponding luminosity changes. We argue that a more physically realistic situation is that the cyclotron line forms when the radiation emitted by the accretion column is reflected from the neutron star surface. The idea is based on the facts that a substantial part of column luminosity is intercepted by the neutron star surface and the reflected radiation should contain absorption features. The reflection model is developed and applied to explain the observed variations of the cyclotron line energy in a bright X-ray pulsar V 0332+53 over a wide range of luminosities.

IONIZED REFLECTION SPECTRA FROM ACCRETION DISKS ILLUMINATED BY X-RAY PULSARS

X-ray reflection signatures are observed around multiple classes of accreting compact objects. Modelling these features yield 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 (\Gamma < 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 K line and the soft excess. The ionized reflector shows strong evidence for significant Doppler broadening and is redshifted by ~10^4 km/s. 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.

Hard spectra of X-ray pulsars from INTEGRAL data

We present spectra for 34 accretion-powers X-ray pulsars and one millisecond pulsar that were within the field of view of the INTEGRAL observatory over two years (December 2002–January 2005) of its in-orbit operation and that were detected by its instruments at a statistically significant level (> 8σ in the energy range 18–60 keV). There are seven recently discovered objects of this class among the pulsars studied: 2RXP J130159.6-635806, IGR/AX J16320-4751, IGR J16358-4726, AX J163904-4642, IGR J16465-4507, SAX/IGR J18027-2017, and AX J1841.0-0535. We have also obtained hard X-ray (>20 keV) spectra for the accretion-powered pulsars RX J0146.9+6121, AX J1820.5-1434, and AX J1841.0-0535 for the first time. We analyze the evolution of spectral parameters as a function of the intensity of the sources and compare these with the results of previous studies.

The cyclotron spectrum of anisotropic ultrarelativistic electrons: Interpretation of X-ray pulsar spectra

The spectrum of cyclotron radiation produced by electrons with a strongly anisotropic velocity distribution is calculated taking into account higher harmonics. The motion of the electrons is assumed to be ultrarelativistic along the magnetic field and nonrelativistic across the field. One characteristic feature of the resulting spectrum is that harmonics of various orders are not equally spaced. The physical properties and observed spectra of four X-ray pulsars displaying higher cyclotron harmonics are analyzed. It was shown that at least in one of them, the cyclotron feature can apparently be only an emission line. Moreover, the observed harmonics are not equidistant, and display certain other properties characteristic of emission by strongly anisotropic ultrarelativistic electrons. In addition, there are indirect theoretical arguments that the electrons giving rise to cyclotron features in the spectra of X-ray pulsars are ultrarelativistic and characterized by strongly anisotropic distributions. As a result, estimates of the magnetic fields of X-ray pulsars (which are usually derived from the energies of cyclotron lines) and certain other physical parameters require substantial revision.

Harmonic Coupling of the Red Noise in X‐Ray Pulsars

The power spectra of X-ray pulsars often show the presence of a red-noise component. This noise is produced by aperiodic variability believed to be associated with instabilities that seem to occur in accretion flows onto compact objects. In this paper we discuss how, independently of the details of the physical processes that generate these instabilities, a careful analysis of the power spectra can furnish some constraints on the distance from the stellar surface at which the sudden energy release associated with the instabilities occurs. In particular, any aperiodic variability coming from the accretion flow funneled toward the magnetic poles should be modulated at the pulsar spin period (coupling). We show how, in the power spectra, this coupling results in a broadening at the base of the harmonics. To investigate this effect, we have adopted a mathematical description of the noise in order to produce simulated light curves and the resulting power spectra. A comparison of power spectra from simulations with real data allows the detection or exclusion of the broadening effect. As an application of this method we have compared simulated power spectra with one obtained from a Ginga observation of the X-ray pulsar SMC X-1. For this source the coupling effect is evident.

Photon Bubble Oscillations in Accretion-powered Pulsars

We describe time-dependent, two-dimensional axisymmetric radiation hydrodynamic calculations of locally super-Eddington accretion onto highly magnetized neutron stars appropriate to the flow onto the polar caps of high-luminosity X-ray pulsars. Our calculations show the development and nonlinear evolution of bubble instabilities in the settling plasma below the radiation-dominated shock that terminates free fall along the magnetic field. The bubbles develop as elongated, very low density, optically thin regions of local outflow forming typically a few kilometers above the neutron star's surface with outflow velocities of order 0.1c. These holes in the plasma reduce the total opacity of the accretion mound and through advection significantly enhance the efficiency of radiation transport. The number of bubbles varies from a few to 20 and results in significant fluctuations (4%-10%) in the emitted luminosity. These fluctuations appear as reasonably high quality (Q) photon bubble (PBOs) in the power spectrum of the emergent luminosity time series on timescales between 0.1 and 1 ms, a phenomenon not previously observed. The discovery of PBOs would provide a powerful probe into the physical characteristics of super-Eddington flows. We use our results to exhibit power spectra of the predicted luminosity oscillations and discuss the feasibility of their detection by the X-Ray Timing Explorer (XTE). We also use our results to calculate the emergent spectrum and show that it is a highly depleted modified blackbody distribution roughly consistent with known rotation phase-averaged observations in the 10-100 keV range. Thus we have shown that the spectra of X-ray pulsars are a natural consequence of the dynamics of polar cap accretion.

Hard X-ray studies with a balloon-borne large area xenon detector telescope

Large area multi-layer proportional chambers filled with Xenon have been developed for the study of cosmic X-ray sources in 20 – 100 keV range. The X-ray telescope consists of two such detectors, each having an effective area of 1200 cm 2 and equipped with a 5° x 5° FWHM collimator, mounted on a servo-controlled orientation platform for making pointed mode observations. Superior energy resolution of the Xenon detectors, being 10 % FWHM at 22 keV, makes them well suited for detection of cyclotron emission lines in the spectra of X-ray pulsars. The X-ray light curves and energy spectrum of the 4.8 hour binary Cyg X-3, observed in a balloon flight in March 1986 is described.

The X-ray pulsar A0535+26 in hard X-rays - Average spectrum, pulse-phase spectroscopy, and spectral time variability

A hard X-ray (30-190 keV) observation of A0535+26 in outburst was performed during a balloon flight on October 4, 1980. Detailed spectral analysis shows that the average photon spectrum of A0535+26 can be well fitted by a Wein law with kT = 7.68 + or - 0.035 keV. This spectral law is in good agreement with theoretical model spectra of X-ray pulsars. The energy spectra over 51 phase bins are also well fitted by a Wein law with a kT variable with pulse phase from 7.3 to about 8.5 keV. Apart from this spectral variability, photon spectra averaged over a pulse period strongly suggest that chaotic spectral variability is also present. A simple geometrical model for the X-ray emission and modulation with pulse phase from A0535+26 is presented. This model gives good qualitative agreement with the present observations. 29 references.

Cyclotron radiation in astrophysics (review)

The authors review the research uses and sources of cosmic electron cyclotron radiation under the following categories: cyclotron radiation in interstellar plasma; cyclotron radiation from active solar regions; radio emission of magnetic stars; cyclotron absorption bands in the optical spectra of magnetic white dwarfs and neutron stars; and cyclotron lines in the spectra of x-ray pulsars and gamma bursters.

Spectra of radiation from a strongly magnetized plasma

Radiation from an optically thick, tenuous, isothermal and magnetized plasma is considered under conditions typical for X-ray pulsars, in the approximation of coupled diffusion of normal modes. The spectra are calculated of the fluxes and specific intensities of outgoing radiation, their dependences on the plasma densityN, temperatureT and magnetic fieldB are analysed with due regard to the vacuum polarization by a strong magnetic field. Simple analytical expressions are obtained in the limiting cases for the fluxes and intensities. It is shown that atEB »Ea (EB=11.6B12 keV,Ea≃0.1N221/2T1−3/4 keV,B12=B/1012 G,N22=N/1022 cm−3,T1=T/10 keV) the magnetic field strongly intensifies the flux and changes its spectrum in the regionEa ≲E ≲EB. AtE ≲T the spectrum of the energy flux is almost flat in the region\(\sqrt {E_a E_B } \lesssim E \lesssim E_B \). For homogeneous plasma without Comptonization the cyclotron line atE≃=EB appears in emission, though in many other cases it may appear in absorption. The vacuum polarization may produce the ‘vacuum feature’ atE≃EW≃13N221/2B12−1 keV, which, as a rule, appears in absorption. The intensity spectra vary noticeably with the direction of radiation, in particular, at some directions nearB, the spectra become harder than in other directions. Quantization of the magnetic field (EB>T) strongly increases the plasma luminosity (∝EB/T for homogeneous plasma). The results obtained explain a number of basic features in the observed X-ray pulsar spectra.

On hard X-ray spectra of accreting neutron stars

Formation of the spectra of X-ray pulsars and gamma bursters is investigated. Interpretation of a hard X-ray spectrum of pulsars containing cyclotron lines is feasible on the basis of an isothermal model of a polar spot heated due to accretion to a neutron star. It has been ascertained that in the regions responsible for the formation of continuum radiation and lines the mode polarization is determined by a magnetized vacuum rather than by a plasma. Bearing this in mind, the influence of the magnetic field of a star on the wide wings of the cyclotron line and on its depth is discussed. The part played by the accreting column in the case of strong accretion (≃1019 el cm−3) needed for long sustaining of the high level of X-rays from a neutron star-pulsar is studied. There occur the gaps in spectrum at frequencies close to the electron gyro-frequency and its harmonics due to the screening of the hot spot by the opaque gyro-resonant layers located within the accreting column. These gaps ensure the formation of cyclotron lines in absorption irrespective of the presence of such lines in the X-ray spectrum of a polar hot spot. The spectra of gamma-bursters recorded by ‘Venus 11’ and ‘Venus 12’ are interpreted in terms of a two-layer model of a polar hot spot. The estimates are given of the distance to some of the bursters, of the emission measure from a high-temperature layer responsible for continuum radiation and of the dispersion measure of a colder layer forming cyclotron lines in absorption. It is noted that the action of an accreting column leads generally to the radiation depression at frequencies below cyclotron lines. By the observed depression for one of the bursters the electron density of near-star accreting plasma during the burst has been directly estimated (≃4×10−14 el cm−3). Possible appearance of ‘false’ cyclotron lines associated with cyclotron scattering in accreting column has been revealed. The problem of measuring the magnetic fields of neutron stars taking account of the gravitational redshift and the quantum recoil effect in emission and in absorption is discussed. Possibilty for a more precise measurement of the magnetic fields of those bursters whose spectrum contains both a cyclotron and an annihilation lines is noted.

A model for X-ray pulsar spectra

The transfer of X-rays through a hot, magnetized, scattering atmosphere is studied using a Monte Carlo technique. The process considered is Compton scattering, including cyclotron resonance scattering. Unlike previous work on the subject, the effects of frequency shifts, relativistic corrections, recoil, and polarization mode changes have been included. It is found that the emergent X-ray spectra harden toward the magnetic equator, are cut off at a frequency less than the fundamental cyclotron frequency, and depend on the magnetic field strength. The relevance of the results to observed pulsar spectra is indicated.

Possible vacuum signature in the spectra of X-ray pulsars

view Abstract Citations (67) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Possible vacuum signature in the spectra of X-ray pulsars. Ventura, J. ; Nagel, W. ; Meszaros, P. Abstract The vacuum polarization effect expected in the strong magnetic fields of pulsars should dominate the behavior of the opacities and the radiative transfer at X-ray frequencies. A sharp feature in the cross sections, which is peculiar to vacuum effects, is present at a particular frequency. In a radiative-transfer calculation for a simplified model atmosphere, these cross sections give rise to a line feature at this frequency, in addition to the cyclotron feature. This should serve as a diagnostic for the presence of vacuum effects, as well as for the determination of the plasma density. A change in the plane of polarization is also predicted at the frequency of the sharp feature. Publication: The Astrophysical Journal Pub Date: November 1979 DOI: 10.1086/183090 Bibcode: 1979ApJ...233L.125V Keywords: Astronomical Spectroscopy; Pulsars; Vacuum Spectroscopy; X Ray Sources; Atmospheric Models; Plasma Spectra; Polarization Characteristics; Radiative Transfer; Astrophysics; Polarization:X-Ray Pulsars full text sources ADS |