Radio surveys of the Magellanic Clouds at 843 MHz have been made with the Molonglo Observatory Synthesis Telescope. An initial catalogue (Mills et al. 1984) presented details of 38 supernova remnants detected by a combination of X-ray, optical and radio observations. The subsequent completion of the radio survey has revealed at least a further 17 remnants, mainly of large diameter and undetected by the Einstein X-ray Observatory. Though a few remnants have their radio emission concentrated towards their centre there is no evidence in the Magellanic Clouds for a Crab-like plerion (without an associated shell). The two well-established plerions 0540-693 (with optical and X-ray pulsar) and 0538-691 (N157B) appear to be connected with partial shells of strong radio emission which are relatively weak in optical emission lines (0538-691 is superposed on an HII region). Four sources which have a central concentration show larger optical and X-ray shells. The optical spectra of three of these (0505-679, 0509-675 and 0519-690) are dominated by the emission lines of hydrogen and Tuohy et al. (1982) argue that they are remnants of Type I supernovae. The fourth source (0453-685) which has detectable [OIII] emission may be an older but similar type of remnant. A further remnant 0509-687 (N103B) is compact but its radio diameter of 6 pc agrees with the published X-ray result.

Based on the statistical studies of external galaxies, our Galaxy is expected to produce Type I and Type II supernovae in closely equal number. There are as of this writing 162 known galactic SNRs.

The basis for our investigation is the observations that SN II are tightly correlated with spiral galaxies and that SN I do not correlate with spiral arms, but are roughly of old disk population.

We seek a method by which a correlation of SNR with spiral arms can be determined. For this preliminary exercise we use only information on the positions of the SNR in galactic coordinates.

As a test, we compared the angular distribution of SNR and giant HII regions which are presumed to define the location of the spiral arms. To the eye, there did seem to have a correlation.

We thus faced a host of questions.

Do SN I still occur in spiral arms, implying that they have massive progenitors?

Is the rate of formation of SN I in Galaxy (and M31) considerably less than that of SN II?

Do SN I not produce long lived radio SNRs?

We have developed a quantitative approach to investigate these questions. Using the observed angular distributions of SNRs and giant HII region, we form a cumulative distribution with respect to angle. Two observed distributions can be compared then, and the Kolmogrov-Smirnov statistics was used to determine the probability that the two samples are not drawn from the same distribution.

In addition, we have constructed Monte-Carlo models in which sample objects are distributed in a prescribed pattern in the spiral arms and the galactic disk. The parameters of the Monte-Carlo simulation, such as the opening angle of the spiral arms and the fraction of objects in the disk vs arm population, can be adjusted to obtain the best fit to the observed distribution.

We also study the influence of the selection effects, if the surface brightness of SNR falls below a threshold for detectability (as 1/r2).

This paper reviews accreting neutron stars in X-ray binaries, with particular emphasis on how variations in magnetic field strength may be responsible for explaining the spectral and temporal properties observed from the various systems. This includes a review of X-ray pulsars in both low and high mass systems, and a discussion of the spectral properties of the low mass X-ray binaries.

The X-ray observations of the Milky Way galaxy with non-imaging X-ray satellites (e.g. UHURU, HEAO1) has revealed a number of discrete, point -like bright sources clustered around the Galaxy's center (in the bulge region) and on the plane of the Galaxy (see for example Tanambaum and Tucker, 1984). The brightermost ones have been associated with close accreting binary systems, containing an evolved object (a white dwarf, a neutron star or a black hole) and a companion visible star. For sources with X-ray luminosities Lx≦1038 erg s−1, the compact object needs to be a neutron star or a black hole.

Recent observational results on X-ray bursts and burst sources are reviewed. Two distinct types of bursts, Type I and Type II bursts, are discussed in relation to the mass and radius of neutron stars and to the problems of unusual mass accretion in some burst sources.

The formation of neutron stars in globular clusters is discussed in light of a number of recent results and, in particular, studies of the origin and evolution of the high luminosity x-ray binaries found in globular clusters. We argue that the neutron stars most probably arise from the accretion-induced collapse of white dwarfs in compact binary systems, themselves detectable as low luminosity cluster x-ray sources. The white dwarfs which can collapse are probably the remnants of relatively more massive stars than those presently found in globulars. This can account for the predominant occurrence of the high luminosity cluster sources in clusters of relatively high metallicity, since those clusters have recently been found to probably have flatter mass functions of their component stars.

We discuss formation mechanisms for low-mass X-ray binaries in globular clusters. We apply the most efficient mechanism, tidal capture in close two-body encounters between neutron and main-sequence stars, to the clusters of our galaxy. The observed number of X-ray sources in these can be explained if the birth velocities of neutron stars are higher than estimated from velocity measurements of radiopulsars, or if the initial mass function steepens at high masses. We perform a statistical test on the distribution of X-ray sources with respect to the number of close encounters in globular clusters, and find satisfactory agreement between the tidal capture theory and observation, apart from the presence of low-mass X-ray binaries in four clusters with a very low encounter rate: Ter 1, Ter 2, Gr 1 and NGC 6712.

EXOSAT observations indicate that some dim globular cluster sources may be less luminous than hitherto assumed, and support the view that the brighter dim sources may be soft X-ray transients in quiescence.

Spectral observations of low-mass X-ray binaries (LMXBs) show that the soft component usually dominates over the hard one. These results provide additional support to an interpretation based on models of LMXBs in which the neutron star while, on the average, spinning up, is also experiencing a spinning down torque. Under these conditions, a fraction of the luminosity associated with the gravitational release of energy on the surface of the accreting neutron star may manifest itself as luminosity originating in the inner part of the accretion disk. It is probably possible to separate the two contributions; the stellar luminosity can be associated with the hard component of the spectrum and the disk luminosity, related to the exchange of energy due to the torque between the rapidly spinning neutron star and the accretion disk, can be associated with the soft spectral component.

A dozen of X-ray pulsars have been observed with the Japanese X-ray astronomy satellites Hakucho and Tenma between 1979 and 1984. The obsetvations revealed remarkable pulse period changes both in disk-fed and wind-fed pulsars. From the histories of the pulse period changes so far measured, we are able to classify the X-ray pulsars into three categories:

1. (1) Pulsars which exhibit a steady spin-up with a constant rate of period change (e.g., 4U 1626-67 and GX 1+4).

2. (2) Disk-fed pulsars in which considerable period changes are superposed on the secular trend of spin-up (e.g., Her X-1 and Cen X-3).

3. (3) Wind-fed pulsars whose pulse periods fluctuate randomly about the average pulse period on time scales from days to years (e.g., Vela X-1 and GX 301-2).

Among the dozen X-ray pulsars observed by Hakucho and Tenma, Vela X-1 was most extensively studied. Timing studies of Vela X-1 revealed an episode of secular spin-down of the pulsar between 1979 and 1982, and large rates of short-term period fluctuations superimposed on the secular trend. Recent studies of the wind-fed pulsar suggest that the angular momentum transfer expected from the wind accretion is too small to account for the observed rates. Spectroscopic studies of Vela X-1 by Tenma observations provided a variety of phenomena related to the binary system and detailed properties of matter surrounding the system, especially using an iron emission line fluoresced thereby as a probe.

Formation of a disk in the wind-fed pulsar is proposed to accommodate a variety of phenomena observed from Vela X-1. A disk is expected to be formed through the interaction of the spherically accreting matter with the rotating magnetosphere, under the assumption that the closed magnetosphere extends to the corotation radius. Application of this model to Vela X-1 requires a large magnetic moment of μ ≈ 1032 gauss cm3 (1013 ≲ B ≲ 1014 gauss, depending on the stellar parameters), but once the large value is accepted, this model fits well with the large rates of period change and phenomena which would otherwise be regarded as strange. If this model works for Vela X-1, it will be also applied to some of other wind-fed pulsars.

The hard X-ray spectrum of Her X1 was measured for the first time with a high-resolution (1.4 keV FWHM) germanium spectrometer (LEGS: GSFC/Saclay collaboration). The observation was performed near the peak of the 0B state in the 35 day cycle and 1.24 s pulsations were observed between the energies of 20 keV and 70 keV. The best fit energies are 35 keV for an absorption line and 39 keV for an emission line. These are significantly lower energies than those derived from previous experiments.

On September 22, 1985, a hard X-ray observation of Cyg X-1 was performed by using a balloon-borne CsI-NaI phoswich telescope HAPI-2 at Xianghe Balloon Facility in China. The main detector is CsI(T1) with a thickness of 0.4cm and an area of 140cm2. The energy range is 20–200KeV. The telescope reached a float altitude of about 38Km(4g/cm2). The photon's arrival time and energy loss spectrum were measured for both background and active tracking on-source observations.

EXOSAT has observed a bright transient X-ray pulsar EXO 2030+375 that decayed in intensity by a factor ∼5000 between 1985 May and August. The variations in 42s pulse period enable an orbital period of 37.9±1.3 days with an eccentricity of 0.31±0.02 to be determined. The spin-up timescale of ∼30 years suggests that this is a very large outburst of order a few 1038 ergs/s. The mass function of 5Mo is consistent with the unidentified companion being a Be star, similar to many other pulsing X-ray binaries. The position of EXO 2030+375, obtained with the EXOSAT Imaging Telescope, is RA: 20 30 21.25, Decl. +37 27 51 (1950; with an uncertainty radius of 10″).

Some basic concepts of accretion onto the polar caps of magnetized neutron stars are reviewed. Preliminary results of new, multidimensional, time–dependent calculations of polar cap flow are outlined, and are used to suggest the possible observability of fluctuations in the X–ray intensity of accretion powered pulsars on time scales of 10–100 msec. The possible relevance of such fluctuations to Quasi–Periodic oscillations is suggested. Basic concepts of the interaction between a disk and the magnetosphere of a neutron star are also discussed. Some recent work on the disk–magnetosphere interaction is outlined, leading to the suggestion that a neutron star can lose angular momentum by driving some or all of the mass in the disk off as a centrifugally driven wind. The relevance of such mass loss to the orbital evolution of the binary is pointed out.

A new non-thermal model on cyclotron line radiation from accreting magnetized neutron stars have been reviewed. Above the magnetic polar cap of accreting neutron stars, the maser instability may be excited by a part of non-thermal electrons, and the radiation with the frequency near electron cyclotron frequency and its second harmonic may be emitted as well. According to this model, the intensity and energy of line radiation will vary with pulsation phase of X-ray pulsars. These phenomen have been observed from Her X-1.

The maximum peak luminosity of the X-ray bursts from a burster is most likely interpreted as the Eddington luminosity of a helium-rich envelope surrounding a neutron star. If this interpretation is true, we can obtain a relation between the mass and the radius of the neutron star in terms of the maximum effective temperature of bursts. On the other hand, the most naive understanding of the origin of the 4.1 keV absorption line often detected in X-ray burst spectra gives us another relation of the neutron star mass with its radius. By solving two simultaneous equations, we can determine the values of the mass and the radius of the neutron star, respectively. However, the result is critical to every neutron star model currently considered.

The persistent emissions from X-ray bursters are also discussed.

This abstract should be read in conjunction with the papers by Arons and by Klein and Arons in these proceedings. In the context of the accretion models described there, one can find an analytic solution for the flow down the polar field lines if a number of simplifying assumptions are made. These are (1) steady flow in the co–rotating frame; (2) radiation pressure large compared to gas pressure; (3) pure scattering for the Rosseland opacity, with the magnetic corrections set equal to constants instead of using the actual functions of temperature; (4) diffusion flux of radiative energy proportional to the gradient of the energy density alone, instead of the correct sum of terms proportional to the photon energy density and the number density gradients; and (5) below a radiative shock, subsonic flow in approximate hydrostatic equilibrium. We assumed dipole geometry, and also assume the mass flux is independent of distance from the magnetic axis. The essential trick is to use (1), (2) and (5) to write the advective contribution to the radiation transfer equation as Mg/area = rate at which gravity does work on a fluid element, and use (3) and (4) to write the nonlinear diffusion flux as the ratio of gradients in the energy density. Then the multidimensional diffusion equation can be cast in a separable, linear form by using the logarithmic radial gradient of the energy density as the basic variable (see also Kirk, J., 1985, Astron. and Astrophys., 142, 430). The result is exponential stratification of the energy density, velocity and mass density along B with scale height R*[L(EDeff)/4Lco-p]; the effective Eddington luminosity is discussed by Arons, these proceedings. This result can be understood as the result of almost exact balance between upward diffusion and downward advection of photons in the optically thick medium. The same fluid quantities are stratified in a Gaussian manner across B, with angular half width at half maximum Δθ = [L(EDeff)/Lcap](r/R*)3/2. These distributions agree well with more sophisticated computational results, during times when the flow is steady. When used as a basis for calculations of the radiative entropy, the calculated emergent spectra are not dissimilar to the spectra of high luminosity, accretion powered pulsars.

We present results of the first self-consistent, time-dependent, 2-D calculations of the accretion of plasma onto polar caps of high luminosity (L*>1036erg-s−1) magnetized neutron stars. We follow the temporal and spatial evolution of three fluids, electrons, ions and photons in a superstrong (B=3×1012 Gauss) dipole magnetic field where radiation pressure dominates plasma pressure by solving coupled 2-D equations of radiation hydrodynamics. We have included several physical processes in the radiation-plasma coupling in superstrong magnetic fields (Klein, et al., 1984, Santa Cruz Workshop on High Energy Transients, and Arons, this conference). We solve the resulting system of coupled 2-D PDEs on a Cray XMP-48 by applying implicit finite-difference techniques with iterative operator splitting methods. We present results for two models of 5×1037 erg-s and 1.5×1038 erg-s−1 super-Eddington luminosity on one polar cap, each having initial mass flux independent of co-latitude of a field lines footprint. We find (a) Radiation develops a broad transverse fan beam that emerges from an annulus 0.2–0.5km above the polar cap. (b) The beam profile is determined by advective trapping of radiation in optically thick (τ11,τ⊥ ≈103) flow. Here the time for diffusion of radiation up through the accretion column is ≫ the time for downward advection. (c) There is a three fluid nonequilibrium with Ti≫Tγ≥Te. (d) Maximum photon temperature of ≈ 10–20 keV in the fan beam is in the observed range. (e) Cyclotron emission ≫ bremsstrahlung as a source of photons. (f) At early times (≪lms) radiation pressure strongly decelerates flow to 10−3 of freefall in central regions of accretion column resulting in a density mound, but plasma freefalls down the sides of the column. (g) Analytical models have reasonable agreement with numerical calculations; velocity and energy density roughly Gaussian transversally and exponential vertically, until the onset of “photon bubbles” after several dynamical times (∼lms). (h) Multiple “photon bubbles” rising subsonically in the accretion column form in the high luminosity model. We believe the photon bubbles to be a possible consequence of overstable convection in super-Eddington flows. These photon bubbles could be observable as 10–100μs fluctuations in the emergent flux and, thus, be an important diagnostic for inhomogeneous structure of the column.

It is well known that black hole candidates, Cyg X-1 and GX 339-4, have distinct high and low states, known as bimodal states. Detailed spectroscopic studies of these X-ray sources have revealed high and low states corresponding to optically thick and thin states of the surrounding accretion disks.

Many observations have been reported in the field of X-ray pulsars, but the mechanism for X-ray emission is not well understood. The X-ray spectra can not be simply described in terms of blackbody or thermal bremsstralung. The high-energy cutoff could be due to cyclotron absorption in high (≧1012 Gauss) magnetic fields. For the lower energy it can be fitted by a power law with energy index α.