There is a wide range of fundamental physical problems directly related to how pulsars function. Some of these are independent of the specific pulsar mechanism. Others relate directly to the physics of the pulsar and already shed some light on the properties of matter at high density (\ensuremath{\sim}${10}^{15}$ g/cc) and in strong magnetic fields (\ensuremath{\sim}${10}^{12}$ G). Pulsars are assumed to be rotating neutron stars surrounded by strong magnetic fields and energetic particles. It is somewhere within this "magnetosphere" that the pulsar action is expected to take place. Currently there has been considerable difficulty in formulating an entirely self-consistent theory of the magnetospheric behavior and there may be rapid revisions in the near future, which is all the more surprising since many of the issues involve "elementary" problems in electromagnetism. One interesting discovery is that charge-separated plasmas apparently can support stable static discontinuities.

Theory of pulsar magnetospheres

Using a consistent set of models, we parameterized the X-ray spectra of all accreting pulsars in the Rossi X-Ray Timing Explorer (RXTE) database that exhibit cyclotron resonance scattering features (CRSFs, or cyclotron lines) These sources in our sample are Her X-1, 4U 0115+63, Cen X-3, 4U 1626-67, XTE J1946-274, Vela X-1, 4U 1907+09, 4U 1538-52, GX 301-2, and 4U 0352+309 (X Per) We searched for correlations among the spectral parameters, concentrating on how the cyclotron line energy relates to the continuum and therefore how the neutron star B-field influences the X-ray emission As expected, we found a correlation between the CRSF energy and the spectral cutoff energy However, with our consistent set of fits we found that the relationship is more complex than what has been reported previously Also, we found not only that the width of the cyclotron line correlates with the energy (as suggested by theory), but also that the width scaled by the energy correlates with the depth of the feature We discuss the implications of these results, including the possibility that accretion directly affects the relative alignment of the neutron star spin and dipole axes Finally, we comment on the current state of fitting phenomenological models to spectra in the RXTE/BeppoSAX era and the need for better theoretical models of the X-ray continua of accreting pulsars

https://iopscience.iop.org/article/10.1086/343033/pdf

Magnetic Fields of Accreting X-Ray Pulsars with the Rossi X-Ray Timing Explorer

Pion degrees of freedom in nuclear matter

We present a catalogue of low-mass X-ray binaries. The catalogue is an updated version of the catalogue of van Paradijs (1995). This new catalogue contains 150 sources, 31 new low-mass X-ray binaries in addition to the 119 sources listed in van Paradijs' catalogue. The aim of this catalogue is to help the reader gain easy access to the recent literature (up to about August 2000) on individual sources, and to provide of some basic information on the X-ray sources and their counterparts in other wavelength ranges (UV, optical, IR, and radio). In cases where there is some doubt about the low-mass nature of the X-ray binary, this is mentioned. In an appendix we list the Anomalous X-ray Pulsars (AXPs), which nowadays are no longer thought to be low-mass X-ray binaries.

/pdf/a-catalogue-of-low-mass-x-ray-binaries-tb6g9n68ps.pdf

A catalogue of low-mass X-ray binaries

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 broadened significantly our understanding in particular for the super-giant systems in the Milky Way, whose 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 into three categories: classical/obscured, eccentric and fast transient. The classical systems feature low eccentricity and variability factor of $${\sim }10^3$$
 , 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 progressed 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 in our Galaxy and to constrain some of the time scales and processes driving their birth and evolution.

High-mass X-ray binaries in the Milky Way

A spectral feature, apparently an absorption line, has been observed at an energy of 20.1 + or - 0.5 keV in the pulsed flux of the 3.61-s X-ray pulsar 4U 0115+63 by using the UCSD/MIT instrument on HEAO 1. The line strength, expressed as equivalent width, is 3.1 + or - 0.5 keV. Although essentially unresolved, the feature has a depth more than 60% of the continuum flux. If the feature arises by cyclotron resonance absorption near the magnetic poles of the neutron star, it implies a magnetic field of between approximately 1.8 and 2.5 x 10 to the 12th G, depending on the gravitational redshift (no more than about 5-40%).

An absorption feature in the spectrum of the pulsed hard X-ray flux from 4U0115 + 63

The results of hard X-ray observations of Sco X-1 by HEAO 1 and OSO 7 are reported. The X-ray spectrum between 20 and 70 keV was found consistent with thermal bremsstrahlung from a hot plasma at a temperature of 5.15 plus or minus 0.05 keV. Observations with the instruments of HEAO 1 on Sept. 6-7, 1978, and with those of OSO 7 during 1972, have not confirmed reports that the bremsstrahlung spectrum is accompanied by an extra component at energies greater than 40 keV - the reported observations yield a two-standard deviation upper limit to the flux in this region of 6.4 x 10 to the -6th photons/sq cm/s/keV.

F. A. Primini

Papers

An absorption feature in the spectrum of the pulsed hard X-ray flux from 4U0115 + 63

A high sensitivity determination of the hard X-ray spectrum of Sco X-1

Oao 1653-40