Einstein observations of hot stars

Surveys with instruments on the $\text{Einstein Observatory}$ have shown that essentially all 0 and B main sequence stars are X-ray sources as are many, if not all, 0B supergiants and Wolf-Rayet stars. The X-ray luminosities are sufficient to explain broad lines from the superionization stages seen in the UV spectra of the stars. High energy resolution spectra from the Solid State Spectrometer are shown to place severe constraints on various models for the location of the X-ray sources in the outer atmospheres of the stars. Coronal and embedded shock models for the X-ray emission are discussed and each is found to have some problems in explaining the X-ray emission of 0B stars. X-ray line emission of Si XIII and S XV in ? Ori is discussed and interpreted as arising from magnetically confined loops.     

Coronal X-ray temperatures from Einstein and EXOSAT observations

Spectral analysis of coronal X-ray emission from stars observed with both the Einstein and EXOSAT Observatories is presented. Using computer codes developed by Raymond and Smith /1/ and Landini and Fossi /2/ to calculate the X-ray emission from optically thin plasma in collisional equilibrium we find that the derived coronal parameters depend only rather insensitively on the details of the calculated theoretical X-ray spectrum and demonstrate how both the Einstein Observatory IPC spectra and the EXOSAT LE filter ratios can be naturally and simultaneously explained by assuming an underlying continuous emission measure distribution as is the case in the solar corona.     

Evolution of the chromospheric and coronal activity of intermediate mass stars

Recent ultraviolet and X-ray observations pertaining to the outer atmospheric structure of intermediate mass (4–6 M₀) stars and the evolution of their structure are presented. A distance-limited (d ≤ 200 pc) IUE ultraviolet survey of early K bright giants shows that C IV emission commonly is present. These stars are almost evenly split between stars showing hybrid-chromospheric and coronal outer atmospheric structures. EXOSAT observations have been obtained for three hybrid stars, of which only α TrA, the nearest, is detected. The temperature of the emitting plasma is likely to be ∼10⁶K. Observations of six K II stars made with the Einstein satellite show no detections. The general conclusion from the available X-ray data is that early K bright giants are not strong X-ray sources.     

X-ray diagnostics of globular clusters

The presence of compact X-ray sources in globular clusters allows diagnostic studies of both the X-ray sources themselves and the globular clusters to be carried out. A review of much of this work, primarily based on Einstein X-ray observations and supporting studies of globular clusters at radio through UV wavelengths, is presented. The compact X-ray sources in globular clusters are found to be compact binaries containing neutron stars and - in a separate lower luminosity component of an apparently bimodal luminosity function - possibly white dwarfs. Implications for the formation and evolution of compact binary X-ray sources in globular clusters and in the galactic bulge are discussed. In particular, new evidence is presented that the galactic bulge sources may be compact binaries in the remnants of disrupted globular clusters.     

Evolution of X-ray binaries: Achievements and advances

Three recent developments in the field of formation and evolution of neutron stars and black holes in binaries are addressed:

• The finding that there is a class of neutron stars, formed in interacting binaries, that do not receive kick velocities in their birth events. This finding is particularly important for our understanding of the formation – and formation rates – of double neutron stars. It is argued that these low-kick neutron stars, which tend to have low masses, are formed by a different physical mechanism than the neutron stars that receive large kick velocities at birth.

• The occurrence of velocity kicks in the formation events of stellar black holes.

• The nature of the companions of millisecond X-ray pulsars.

The properties of hard X-ray emitting symbiotic stars

Hard X-ray emitting symbiotic stars are candidates for SN Ia progenitors. The importance of Type Ia SNe as standard candles for cosmology makes the study of their progenitor systems particularly important. Additionally, they provide one of the most promising laboratories for the study of astrophysical jets. Typically, the X-ray emission in these systems is modeled with a collisional plasma model, sometimes with an emission measure distribution taken from a cooling flow model. The lack of any coherent periods in both X-rays and optical wave band strongly suggests that the accreting white dwarfs in the hard X-ray symbiotic stars are non-magnetic. Although relatively few have been discovered to date, but we believe that there are very many of them in our galaxy and could be possible candidates for the Galactic Ridge X-ray Emissions (GRXE).     

Einstein observations of cool stars

Observations of cool stars with the $\text{Einstein}$ Observatory (HEAO-2) have brought about a fundamental change in our knowledge and understanding of stellar coronae. The existence of X-ray emission from stars throughout the H-R diagram, the wide range of X-ray luminosity within a given spectral and luminosity class, and the strong correlation of X-ray luminosity with stellar age and rotation are among the more significant $\text{Einstein}$ results. These results are strong evidence for the influence of stellar dynamo action on the formation and heating of stellar coronae. A discussion of relevant consortium and guest observations will be given. The Hyades cluster, in particular, will serve as an example to demonstrate the usefulness of X-ray observations in the study of stellar activity and coronal evolution.     

ROSAT observations of the Chamaeleon star forming cloud

Two soft X-ray images of the Chamaeleon I star forming cloud obtained with the ROSAT Position Sensitive Proportional Counter are presented. Seventy reliable, and perhaps 19 additional, X-ray sources are found. Up to Ninety percent of these sources are certainly or probably identified with T Tauri stars formed in the cloud. Twenty to 35 are probably previously unrecognized ‘weak’ T Tauri (WTT) stars. T Tauri X-ray luminosities range from log , or 10² – 10⁴ times solar levels, with mean in the 0.2–2.5 keV band. The X-ray luminosities of well-studied Chamaeleon cloud members are correlated with a complex of four stellar properties: effective temperature, mass, radius and bolometric luminosity. The spatial distribution, H-R diagram locations of the stars indicate WTT and CTT are coeval. The total premain sequence population of the cloud is likely to be > 100 stars, with WTT stars outnumbering ‘classical’ T Tauri (CTT) stars by 2:1.     

X-ray flaring activity of flare stars and RS CVn binaries

In recent times, X-ray flares have been observed in several flare stars and RS CVn binaries. From an analysis of the X-ray flare observations we find a correlation between the average energy emitted in the X-ray band in flares and the bolometric luminosity of the stars. This relation is similar to the relation obtained in the optical band specifically for the flare stars and hence we suggest a similarity in the flaring nature of the flare stars and the RS CVn binaries. Further, a correlation is also found between the X-ray flare decay time and the peak X-ray luminosity for the flare stars and the RS CVn binaries. We show that this relation can be explained by the solar flare loop models.     

Coronal heating mechanisms

As a result of the large body of data available from solar and stellar coronae, our understanding of the mechanisms responsible for the heating of coronal plasmas to temperatures of the order of ～ 10⁸ K has changed. The solar corona is highly structured by magnetic fields and the acoustic shocks which, according to early theories, should have acted as the coronal energy source have not been observed. Einstein Observatory data show moreover that coronae are present in most regions of the H-R diagram. The observed relationship between X-ray luminosity and rotational velocity in dwarf stars from spectral types F to M again suggests an active role for the magnetic fields.The basic picture which is emerging is that coronae in stellar types from F to M are produced because of the interaction of the magnetic field with the convective velocity fields generated in the photosphere resulting in MHD waves or currents which dissipate in the corona. X-ray emission in early type stars cannot be explained with this mechanism and the models which have been proposed for these stars are not yet completely satisfactory.     

XMM-Newton observations of low luminosity Be/X-ray candidates

A small number of early Be stars exhibit X-ray luminosities intermediate between those typical of early type stars and those radiated by Be/X-ray binaries in the quiescent state. We report on XMM-Newton observations of two such Be stars, HD 161103 and SAO 49725 which were originally discovered in a systematic cross-correlation between the ROSAT all-sky survey and SIMBAD. The new observations confirm the X-ray luminosity detected by ROSAT (L_X  10³² erg s⁻¹) and the hardness of their X-ray spectra (thin thermal with kT  8–10 keV or power law with photon index of 1.7) which are both unusual for normal early type stars. We discuss the possible origin of this excess X-ray emission in the light of the models proposed for γ-Cas, magnetic disc-star interaction or accretion onto a compact companion object, neutron star or white dwarf, and compare the properties of these two sources with those of the new massive systems discovered in the XMM- Newton/SSC survey of the Galactic plane.     

Nature of gamma-ray burst sources

Observational evidence suggests that gamma ray bursts have a local galactic origin involving neutron stars. In this light we make a critical review of physics of the thermonuclear runaway model placing emphasis on self-consistency. We further show that some of the proposed models can be observationally excluded in the light of existing data from the Einstein Observatory. The possibility of gamma bursts arising in low mass binaries is finally discussed in the light of evolutionary scenarios leading to low luminosity systems.     

EXOSAT observations of the Perseus cluster

EXOSAT CMA and GSPC observations of the Perseus Cluster of galaxies are reported. The soft X-ray image data confirm the earlier Einstein Observatory results in terms of the density distribution radially outward from NGC1275. We have used the GSPC spectral data at higher energies to study the strength of the X-ray continuum and of the Fe line emission over the cluster. An asymmetry is found in the distribution of the hot gas around NGC1275, which is consistent with the E-W direction of the ellipticity discovered at soft X-rays by the Einstein Observatory.     

The ROSAT view of the cataclysmic variable sky

The ROSAT All-Sky Survey has for the first time permitted a synoptic view of the soft X-ray sky with high sensitivity. In this paper, we discuss the X-ray properties of known cataclysmic variables (CVs) as observed in the Survey and present a status report on programs to identify CVs among the newly discovered ROSAT X-ray sources. Of 170 CVs with known orbital period, 92 were detected in the Survey and 22 of these fall in the bright-source category with more than 0.5 PSPC cts/s. Among the new bright sources, so far 19 have been identified as CVs and 3 as CV candidates, about doubling the census. We present spectra and light curves of known and new systems and discuss the origin of the X-ray emission in the different subclasses of CVs.     

Accretion-disc radius variations in close binaries

Outer radius variations play an important role in the disc structure and evolution. We consider theoretical and observational consequences of such variations in cataclysmic binaries and low-mass X-ray binaries. We find that the action of tidal torques must be important well inside the tidal radius. We also conclude that it is doubtful that the tidal–thermal instability is responsible for the superoutburst/superhump phenomena.     

Einstein X-ray observations of clusters of galaxies

The X-ray imaging capability of the Einstein Observatory has provided new observational material in many branches of astrophysics. In this contribution we will review the implications of the X-ray observations for the classification of clusters, the formation of SO galaxies, and the interaction of the galaxies with the intracluster gas.     

Einstein observatory results on active galactic nuclei

The Einstein Observatory, by virtue of its increased sensitivity and improved angular resolution, has increased substantially the number of known X-ray emitting active galactic nuclei. This has made possible the detailed study of the relation of X-ray flux to both the continuum flux in the optical and radio bands, and to the line emission properties of these objects. In addition, the Einstein imaging instruments have detected morphology in AGN X-ray emission, in particular from jetlike structures in Cen-A, M87, and 3C273.The improved energy resolution and sensitivity of the spectrometers onboard the Observatory has provided detailed information on the geometry and ionization structure of the region responsible for the broad optical emission lines in a few AGN's. Such information, combined with detailed theoretical modeling and IUE and optical observations, have allowed us to construct a moderately detailed picture of the broad line region in these objects.     

Exploring the Dynamic X-ray Universe:Scientific Opportunities for the Einstein Probe Mission

Time-domain astrophysics will enter a golden era towards the end of this decade with the advent of major facilities across the electromagnetic spectrum and in the multi-messenger realms of gravitational wave and neutrino. In the soft X-ray regime, the novel micro-pore lobster-eye optics provides a promising technology to realise, for the first time, focusing X-ray optics for wide-angle monitors to achieve a good combination of sensitivity and wide field of view. In this context Einstein Probe, a soft X-ray all-sky monitor mission, was proposed and selected as a candidate mission of priority in the space science programme of the Chinese Academy of Sciences. This paper reviews the most important science developments and key questions in this field towards 2020 and beyond, and how to achieve them technologically. It also introduces the Einstein Probe mission, including its key science goals and mission definition, as well as some of the key technological issues.      

Bow shocks around pulsars and neutron stars

Pulsar wind nebulae are now well-established as important probes both of neutron stars’ relativistic winds and of the surrounding interstellar medium. Amongst this diverse group of objects, pulsar bow shocks have long been regarded as an oddity, only seen around a handful of rapidly moving neutron stars. However, recent efforts at optical, radio and X-ray wavelengths have identified many new pulsar bow shocks, and these results have consequently motivated renewed theoretical efforts to model these systems. Here, I review the new results and ideas which have emerged on these spectacular systems, and explain how bow shocks and “Crab-like” nebulae now form a consistent picture within our understanding of pulsar winds.     

The ROSAT mission

A primary scientific objective of the ROSAT mission is to perform the first all-sky survey with an imaging X-ray telescope leading to an improvement in sensitivity by several orders of magnitude compared with previous surveys. A large number of new sources (? 10⁵) will be discovered and located with an accuracy of 1 arcmin or better. These will comprise almost all astronomical objects from nearby normal stars to distant quasistellar objects. After completion of the survey which will take half a year the instrument will be used for detailed observations of selected sources with respect to spatial structure, spectra and time variability. In this mode which will be open for guest observers ROSAT will provide substantial improvement over the imaging instruments of the Einstein observatory.The main ROSAT telescope consists of a fourfold nested mirror system with 83 cm aperture having three focal plane instruments. Two of them will be imaging proportional counters (0.1 – 2 keV) providing a field of view of 2°, an angular resolution of ≈ 30″ in the pointing mode and a spectral resolution ^ΔE/E ≈ 45% FWHM at 1 keV. The third focal instrument will be a high resolution imager (≈ 3″). The main ROSAT telescope will be complemented by a parallel looking Wide Field camera which extend the spectral coverage into the XUV band.