Abstract: In this paper we discuss a variety of issues concerning the exciting and mysterious Galactic center gamma-ray sources 1E 1740.7-2942 and GRS 1758-258. We discuss the problem associated with the highly uncertain X-ray absorption column toward 1E 1740.7-2942 and use the recent Roentgen Satellite (ROSAT) results to narrow its range to 0.5-1 x 10(exp 23)/sq cm. Then the current upper limits from deep optical and near-IR searches of stellar objects at these source locations are plotted on an H-R diagram, from which we find the mass of a potential companion star of the (supposed) black hole in GRS 1758-258 to be less than 4 solar mass and in 1E 1740.7-2942 to be less than 9 solar mass. The observed well-collimated radio jets in 1E 1740.7-2942 require the existence of a stable accretion disk (presumably from binary accretion). The apparent association of 1E 1740.7-2942 with a high-density molecular cloud, on the other hand, points to possible accretion directly from the interstellar medium (ISM). We present an analysis of the energetics and kinematics of the radio jets in 1E 1740.7-2942. We present the long-term X-ray light curves of the two sources which include both the Granat/SIGMA's 3 yr monitoring data and all the data from previous imaging balloon and satellite observations over the last decade. The possible physical mechanisms responsible for producing both the long-term X-ray variations and the radio jets are postulated. We also consider Roche lobe-overflowing, low-mass X-ray binaries and Bondi-Hoyle accretion directly from a high-density surrounding medium. We propose a plausible scenario in which both sources are binary systems with a black hole primary and a low-mass companion and they are accreting mainly from the ISM at a rate self-regulated by the interaction between the accretion flow and the emerging hard X-ray flux.

Abstract: Binary systems represent the best examples of real time astrophysics. These lectures deal with the interface between observation and theory in comparatively normal close binary stars. The first half discusses the observational techniques for determining periods, radial velocities, light curve data, and surface maps of stars and accretion disks. The second portion deals with a range of specific physical processes. Although the observational data is slanted toward the ultraviolet, these lectures emphasize the need for multiwavelength and multi-technique approaches.

Abstract: Aql X-1 has been detected in five observations with ROSAT, at X-ray luminosities varying from L x (0.4-2.4 keV)∼4×10 32 erg s −1 to ∼2×10 36 erg s −1 . The characteristic temperature of the X-rays varies from kT bb ≃0.31 keV at the lowest to kT bb ≃0.55 keV at the highest flux level observed. This moderate temperature variation excludes models that explain the large range in X-ray luminosity as the consequence mainly of a variation in temperature, and implies that accretion onto the neutron star drops by five orders of magnitude between outburst maximum (at L x ∼10 37 erg s −1 ) and quiescence

Abstract: The problem of a crust of normal matter surrounding a strange star is considered. This crust is thinner by a factor 0.22--0.65 than the external crust of a neutron star.

Abstract: It is assumed that the compressed accretion matter on the crust of a neutron star with magnetic field has ferromagnetic permeability. The observed inverse correlation between field strength and accreted mass can be deduced, by means of the analytic calculation of a proposed crust screen model. The field-period relation P = 2.7 ms B-9(5/7) is also presented, which fits for the equilibrium period line of X-ray binary sources.

Abstract: Since the equilibrium composition of strange matter is crucial to the accurate calculation of its neutrino energy loss rates, we determine the composition by using the thermodynamic potentials (per unit volume) of the strange matter, and find that the electron abundance decreases with the strong interaction coupling constant or ba-ryon density. After deriving the formula of neutrino energy loss rates of the strange matter, we study the cooling of a homogeneous strange star with a thin crust in detail, and compare it with that of a neutron star. We conclude that the surface temperature of a young strange star is much less than that of a young neutron star of the same age, so one can distinguish between a strange star and a neutron star.

Abstract: Although very different in shape, Be-type and Be-shell-type spectral lines are thought to arise from the same kind of circumstellar environment, a disk-like envelope. The only discrimination between these two types of spectra is the inclination angle: If sufficiently high, a large column depth of circumstellar material causes partial obscuration of the stellar disk, i.e. shell absorption; otherwise a pure emission line is observed. A shell line can be considered as equivalent to P Cygni-type absorption trough for different geometry (disk instead of sphere) and kinematics (rotation instead of outflow; see Hanuschik, these proceedings).

Abstract: Coherent millisecond x‐ray pulsations are expected from low‐mass x‐ray binaries (LMXB), but remain undetected. Using the single‐parameter Quadratic Coherence Recovery Technique (QCRT) to correct for unknown binary orbital motion we have performed Fourier transform searches for coherent oscillations in all long, continuous segments of data obtained at 1 millisecond time resolution during Ginga observations of LMXB. We have searched the six known Z‐sources (GX 5‐1, Cyg X‐2, Sco X‐1, GX 17+2, GX 340+0, and GX 349+2), seven of the ten known atoll sources (GX 3+1, GX 9+1, GX 9+9, 1728‐33, 1820‐30, 1636‐53, and 1608‐52), the ‘‘peculiar’’ source Cir X‐1, and the high mass binary Cyg X‐3. We find no evidence for coherent pulsations in any of these sources, with 99% confidence limits on the pulsed fraction between 0.3% and 5.0% at frequencies below the Nyquist frequency of 512 Hz.

Abstract: Neutron star binaries, such as the one observed in the famous binary pulsar PSR 1916+13, end their life in a catastrophic merge event (denoted here NS$^2$M). The merger releases $\approx 5 \cdot 10^{53}$ergs, mostly as neutrinos and gravitational radiation. A small fraction of this energy suffices to power $\gamma$-ray bursts (GRBs) at cosmological distances. Cosmological GRBs must pass, however, an optically thick fireball phase and the observed $\gamma$-rays emerge only at the end of this phase. Hence, it is difficult to determine the nature of the source from present observations (the agreement between the rates of GRBs and NS$^2$Ms being only an indirect evidence for this model). In the future a coinciding detection of a GRB and a gravitational radiation signal could confirm this model.

Abstract: Two of the major contending theories for binary and multiple star formation are fragmentation driven by rotation during cloud collapse and gravitational instability of massive protostellar disks. Implicit in the literature on these two mechanisms is that the parameter space available for binary formation in a star forming region varies with the cloud temperature. The sense of the difference is that binary formation is more likely in low-temperature clouds. Observations of cloud conditions and of binary fractions among young stellar objects are approaching the necessary level of detail to test this prediction; and there are hints in recent observations that such differences exist

Abstract: The structure and stability of the inner regions of accretion disks surrounding neutron stars and black holes have been studied. Within the framework of the α viscosity prescription for optically thick disks, we assume the viscous stress scales with gas pressure only, and the α parameter, which is less than or equal to unity, is formulated as α0(h/r)n, where h is the local scale height and n and α0 are constants. We neglect advective energy transport associated with radial motions and construct the vertical disk structures by assuming a Keplerian rotation law and local hydrostatic and thermal equilibrium. The vertical structures have been calculated with and without convective energy transport, and it is found that convection is important especially for mass accretion rates greater than about 0.1 times the Eddington value, MEdd. Although convective efficiency is low, convection does help to stabilize the disk. The results show that the disk can be locally unstable and that for n≳0.75, an S‐shaped relatio...

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Abstract: The accreting binary pulsar 4U 1626-67 was detected in the soft X-rays by ROSAT in the course of the All-Sky survey at an average luminosity of 1.5×10 35 d 5kpc 2 erg s −1 , the lowest observed so far. The ROSAT spectrum is better fitted by a bremsstrahlung model rather than a blackbody spectrum. The source spectrum appears to be absorbed by a column density consistent with the interstellar one. The pulse period determined from the survey data indicates a decrease in the spin-up rate. A previous pointed EXOSAT observation performed in 1983 is also analysed. Its light curve is dominated by a series of flares, characteristic to the source. Excess flux in the low energies is present in the EXOSAT GSPC spectrum of the source. There is no indication for significant photoelectric absorption in the hard X-ray spectrum

Abstract: The modulated light in the Intermediate Polar FO Aqr at the three periods P(sub spin) = 20.9 min, P(sub orb) = 4.85 hr and P(sub beat) = 22.5 min is studied in different spectral ranges to derive information on their nature. In this system the accretion geometry, with or without an accretion disk, is still a matter of debate (Hellier 1991; Norton et al. 1992). The different orbital behavior of phase coherence between the spin and beat pulses in the X-rays (Norton et al. 1992) and in the optical/IR regions cannot be easily accounted for by only a diskless dominated geometry where the accretion flow is switching from one pole to the other each half of the beat period. We therefore propose an accretion scenario where a non-axisymmetric disk is present. In such a non-standard accretion disk an azimuthal structure provides not only the source of variable mass transfer to the white dwarf, but also a reprocessing site which is mainly viewing the X-ray emission from the lower pole. Our spectral analysis shows that reprocessing is also occurring at the surface of the secondary star. The spin pulsation in the optical and IR continua can be explained by the so-called 'accretion curtain' model (Rosen et al. 1988) though an additional reprocessing component at the spin period cannot be excluded. In contrast to the X-rays, the beat optical/IR modulation is not intrinsic. Reprocessing at the surface of the secondary star and at the thickened part of the disk can also account for the orbital modulation in the UV, optical and IR regions.

Abstract: Energy spectra of the black hole candidate GX339‐4 in the low intensity state were observed on four occasions through 1989 to 1991 with the Large Area Counters onboard the GINGA satellite. The spectra showed significant deviations from a power‐law, with an iron Kα emission line at ∼6.4 keV and broad iron K‐edge structure above ∼7 keV. The energy spectra above 4 keV were successfully explained with a reflection model, in which a part of the incident x‐rays with a power‐law spectrum is Compton reflected by optically thick matter. The line equivalent width with respect to the reflection component decreases as the source flux increases, consistent with an increase in the ionization state of the material so that resonant absorption followed by Auger ionization depletes the line.

Abstract: The status of observational research into the properties of X-ray binaries is briefly reviewed, with emphasis on the similarities and differences between systems containing accreting neutron stars and those that likely contain an accreting black hole. X-ray spectroscopic as well as X-ray timing differences exist between these two groups, but there is no hard evidence for the existence of any “black hole signature” that by itself alone allows identification of an accreting black hole in an X-ray binary. The similarities in the properties between the two groups might be as interesting as their differences. If these systems really contain neutron stars and black holes, respectively, they can be expected to have strong qualitative as well as quantitative differences, such as, e.g., the presence/absence of a surface and differences in mass or magnetic field strength. By studying similar phenomena in such different systems knowledge can be gained on both the nature of the studied phenomena and the physical differences between the accreting compact objects. It seems likely that the X-ray spectra of accreting low magnetic-field neutron stars can, when they are faint, become as hard as those of black-hole candidates in the 1–30 keV and 10–100 keV, perhaps even in the 20–500 keV ranges. There are also striking similarities in some of the timing properties of neutron stars and black-hole candidates. A possible synthesis is discussed of the rapid X-ray variability properties of high and low magnetic-field neutron stars and black-hole candidates.

Abstract: This paper reviews some of the recent observational results on stellar mass black hole candidates. Over the last decade, much of the progress in this field has been achieved through the study of transient X--ray binary sources which shine only sporadically in the X--ray sky and undergo much larger luminosity variations then most persistent sources. There are currently six stellar--mass black hole candidates for which the mass of the compact object is estimated to be well above the maximum neutron star mass. The three most recently identified black hole candidates of this kind belong to a class of transient X--ray sources with peculiar spectral properties. Several new transients of this class are being discovered and studied every year thanks to more continous monitoring of the X--ray sky. Many of them are likely to contain other black hole candidates. The growing number of candidates and the large luminosity variations (and therefore, accretion rate variations) in transient sources will allow to study the chacteristics of accreting black holes with an unprecedented detail.

Abstract: The observations of X-ray Type II bursts from the low-mass X-ray binary MXB 1730-335 can be explained by a particular form of magnetic gating in the presence of steady external accretion. The requirements are a strong magnetic field of the neutron star (7×1011–2×1012 gauss at the surface), rotational symmetry and alignment of the field axis with the axis of a steadily accreting disk to within 6°.

Abstract: Rapidly rotating magnetic neutron stars in eccentric binary systems containing an early type star provide a unique opportunity to investigate the interplay between radio pulsar, stellar wind and accretion phenomena. We summarise the radio pulsar-dominated and the accretion-dominated regimes, discussing how the transition from one regime to another can take place as a result of the varying orbital distance and relative velocity along the orbit, as well as changes of the wind characteristics. We derive the conditions under which the two known B star/radio pulsar binaries (PSR 1259-63 and PSR J0045-7319) can undergo a transition to the accreting regime. A strong increase of the mass loss ouflow from the companion is required, just to cause the onset of accretion onto the magnetospheric boundary. We also show that the X--ray transient A0538-66 is likely to undergo transitions from the accreting neutron star regime, to the regime of accretion onto the magnetosphere. These two regimes might correspond to the high (> 10^{38} erg/s) and the low-luminosity (< 10^{38} erg/s) outbursts observed from this source. A radio pulsar might become detectable in the long quiescent states of A0538-66. A new model of the enigmatic high-energy binary LS I +61 303 involving accretion onto the magnetosphere is also presented.

Abstract: Recent results concerning the determination of the masses of five black hole candidates are described. It is very significant that there are no pulsars among five massive (Mx > 3M>) X-ray sources in binary systems.

Abstract: We analyze the gravitational stability of a shocked interstellar gas layer and show how such a layer fragments into protostellar condensations whilst it is still confined mainly by ram pressure. As a consequence, the resulting protostars are massive and well separated. Our analysis is completely general and applies both to layers resulting from collisions between molecular cloud clumps, and to shells swept up by expanding nebulae. We present a numerical simulation of the former scenario, which produces a cluster of 35 massive stars resembling an OB subgroup, with most of the stars in binary systems.