Abstract: There are now about 50 known radio pulsars in binary systems, including at least five in double neutron star binaries. In some cases, the stellar masses can be directly determined from measurements of relativistic orbital effects. In others, only an indirect or statistical estimate of the masses is possible. We review the general problem of mass measurement in radio pulsar binaries and critically discuss all current estimates of the masses of radio pulsars and their companions. We find that significant constraints exist on the masses of 21 radio pulsars and on five neutron star companions of radio pulsars. All the measurements are consistent with a remarkably narrow underlying Gaussian mass distribution, m=1.35?0.04 M. There is no evidence that extensive mass accretion (?m0.1 M) has occurred in these systems. We also show that the observed inclinations of millisecond pulsar binaries are consistent with a random distribution, and thus find no evidence for either alignment or counteralignment of millisecond pulsar magnetic fields.

Abstract: A remarkable correlation between the centroid frequencies of quasi periodic oscillations, QPOs, (or peaked noise components) from low mass X-ray binaries, has been recently discovered by Psaltis, Belloni and van der Klis (1999). This correlation extends over nearly 3 decades in frequency and encompasses both neutron star and black hole candidate systems. We discuss this result in the light of the relativistic precession model, which has been proposed to interpret the kHz QPOs as well as some of the lower frequency QPOs of neutron star low mass X-ray binaries of the Atoll and Z classes. Unlike other models the relativistic precession model does not require the compact object to be a neutron star and can be applied to black hole candidates as well. We show that the predictions of the relativistic precession model match both the value and dependence of the correlation to a very good accuracy without resorting to additional assumptions.

Abstract: We study systematically the ^0.1¨1200 Hz quasi-periodic oscillations (QPOs) and broad noise com- ponents observed in the power spectra of nonpulsing neutron star and black hole low-mass X-ray binaries. We show that among these components we can identify two, occurring over a wide range of source types and luminosities, whose frequencies follow a tight correlation. The variability components involved in this correlation include neutron star kilohertz QPOs and horizontal-branch oscillations, as well as black hole QPOs and noise components. Our results suggest that the same types of variability may occur in both neutron star and black hole systems over 3 orders of magnitude in frequency and with coherences that vary widely but systematically. Con—rmation of this hypothesis will strongly con- strain theoretical models of these phenomena and provide additional clues to understanding their nature. Subject headings: accretion, accretion disksblack hole physicsstars: neutron ¨ stars: oscillationsX-rays: stars

Abstract: Hydrodynamic simulations of the merger of stellar mass black hole-neutron star binaries are compared with mergers of binary neutron stars. The simulations are Newtonian but take into account the emission and back-reaction of gravitational waves. The use of a physical nuclear equation of state allows us to include the effects of neutrino emission. For low neutron star-to-black hole mass ratios, the neutron star transfers mass to the black hole during a few cycles of orbital decay and subsequent widening before finally being disrupted, whereas for ratios near unity the neutron star is destroyed during its first approach. A gas mass between ~0.3 and ~0.7 M☉ is left in an accretion torus around the black hole and radiates neutrinos at a luminosity of several times 1053 ergs s-1 during an estimated accretion timescale of about 0.1 s. The emitted neutrinos and antineutrinos annihilate into e± pairs with efficiencies of 1%-3% and rates of up to ~2 × 1052 ergs s-1, thus depositing an energy E 1051 ergs above the poles of the black hole in a region that contains less than 10-5 M☉ of baryonic matter. This could allow for relativistic expansion with Lorentz factors around 100 and is sufficient to explain apparent burst luminosities Lγ ~ E/(fΩtγ) up to several times 1053 ergs s-1 for burst durations tγ ≈ 0.1-1 s, if the γ emission is collimated in two moderately focused jets in a fraction fΩ = 2δΩ/(4π) ≈ -(1/10) of the sky.

Abstract: A newly discovered instability in rotating neutron stars, driven by gravitational radiation reaction acting on the stars' r-modes, is shown here to set an upper limit on the spin rate of young neutron stars. We calculate the timescales for the growth of linear perturbations due to gravitational radiation reaction, and for dissipation by shear and bulk viscosity, working to second order in a slow-rotation expansion within a Newtonian polytropic stellar model. The results are very temperature-sensitive: in hot neutron stars (T>109 K), the lowest-order r-modes are unstable, while in colder stars they are damped by viscosity. These calculations have a number of interesting astrophysical implications. First, the r-mode instability will spin down a newly born neutron star to a period close to the initial period inferred for the Crab pulsar, probably between 10 and 20 ms. Second, as an initially rapidly rotating star spins down, an energy equivalent to roughly 1% of a solar mass is radiated as gravitational waves, which makes the process an interesting source for detectable gravitational waves. Third, the r-mode instability rules out the scenario in which millisecond pulsars are formed by accretion-induced collapse of a white dwarf; the new star would be hot enough to spin down to much slower rates. Stars with periods less than perhaps 10 ms must have been formed by spin-up through accretion in binary systems, where they remain colder than the Eddington temperature of about 108 K. More accurate calculations will be required to define the limiting spin period more reliably, and we discuss the importance of the major uncertainties in the stellar models, in the initial conditions after collapse, and in the physics of cooling, superfluidity, and the equation of state.

Abstract: The merger of compact binaries, especially black holes and neutron stars, is frequently invoked to explain gamma-ray bursts (GRBs). In this paper, we present three-dimensional hydrodynamical simulations of the relatively neglected mergers of white dwarfs and black holes. During the merger, the white dwarf is tidally disrupted and sheared into an accretion disk. Nuclear reactions are followed, and the energy release is negligible. Peak accretion rates are ~0.05 M☉ s-1 (less for lower mass white dwarfs) and last for approximately a minute. Many of the disk parameters can be explained by a simple analytic model that we derive and compare to our simulations. This model can be used to predict accretion rates for white dwarf and black hole (or neutron star) masses that are not simulated here. Although the mergers studied here create disks with larger radii and longer accretion times than those from the merger of double neutron stars, a larger fraction of the white dwarf's mass becomes part of the disk. Thus the merger of a white dwarf and a black hole could produce a long-duration GRB. The event rate of these mergers may be as high as 10-6 yr-1 per galaxy.

Abstract: We analyze three sets of infrared star counts in the inner ~0.5 pc of the Galactic center. We perform statistical tests on the star counts and model in detail the extinction field and the effects of dwarf-giant collisions on the luminosity function. We find that both the star counts and the depletion of the brightest stars in the inner ~0.05 pc can be explained by an n ∝ r-α stellar cusp with α in the range 3/2-7/4, in which the envelopes of the brightest giants are destroyed by stellar collisions. Such a cusp is consistent with the Bahcall-Wolf solution for the distribution of stars that have undergone two-body relaxation around a black hole. We show that systematic uncertainties due to variable extinction and unrelaxed stars are probably small, but deeper star counts are required to confirm these results. We estimate that the tidal disruption rate of cusp stars by the black hole is a few × 10-5 yr-1.

Abstract: Recently Andersson et al. and Bildsten have independently suggested that an r-mode instability might be responsible for stalling the neutron star spin-up in strongly accreting low-mass X-ray binaries (LMXBs). We show that if this does occur, there are two possibilities for the resulting neutron star evolution. If the r-mode damping is a decreasing function of temperature, then the star undergoes a cyclic evolution: (1) accretional spin-up triggers the instability near the observed maximum spin rate; (2) the r-modes become highly excited through gravitational radiation reaction, and in a fraction of a year (0.13 yr in a particular model that we have considered) they viscously heat the star up to T~2.5 × 109 K; (3) r-mode gravitational radiation reaction then spins the star down in tspindown0.08(ffinal/130 Hz)−6 yr to a limiting rotational frequency ffinal, whose exact value depends on the not fully understood mechanisms of r-mode damping; (4) the r-mode instability shuts off; and (5) the neutron star slowly cools and is spun up by accretion for ~5 × 106 yr, until it once again reaches the instability point, closing the cycle. The shortness of the epoch of r-mode activity makes it unlikely that r-modes are currently excited in the neutron star of any galactic LMXBs, and unlikely that advanced LIGO interferometers will see gravitational waves from extragalactic LMXBs. Nevertheless, this cyclic evolution could be responsible for keeping the rotational frequencies within the observed LMXB frequency range. If, on the other hand, the r-mode damping is temperature independent, then a steady state with constant angular velocity and Tcore 4 × 108 K is reached, in which r-mode viscous heating is balanced by neutrino cooling and accretional spin-up torque is balanced by gravitational radiation reaction spin-down torque. In this case (as Bildsten and Andersson et. al. have shown) the neutron stars in LMXBs could be potential sources of periodic gravitational waves, detectable by enhanced LIGO interferometers.

Abstract: Using the most recent realistic effective interactions for nuclear matter with a smooth extrapolation to high densities including causality, we constrain the equation of state and calculate maximum masses of rotating neutron stars. First- and second-order phase transitions to, e.g., quark matter at high densities are included. If neutron star masses of ~2.3 M☉ from quasi-periodic oscillations in low-mass X-ray binaries are confirmed, a soft equation of state as well as strong phase transitions can be excluded in neutron star cores.

Abstract: We consider the unusual evolutionary state of the secondary star in Cygnus X-2. Spectroscopic data give a low mass (M2 . 0:5 2 0:7 M() and yet a large radius (R2 . 7 R() and high luminosity (L2 . 150 L(). We show that this star closely resembles a remnant of early massive Case B evolution, during which the neutron star ejected most of the , 3 M( transferred from the donor (initial mass M2i , 3:6 M() on its thermal timescale , 10 yr. As the system is far too wide to result from common-envelope evolution, this strongly supports the idea that a neutron star efficiently ejects the excess inflow during super-Eddington mass transfer. Cygnus X-2 is unusual in having had an initial mass ratio qi ˆ M2i=M1 in a narrow critical range near qi . 2:6. Smaller qi lead to long-period systems with the former donor near the Hayashi line, and larger qi to pulsar binaries with shorter periods and relatively massive white dwarf companions. The latter naturally explain the surprisingly large companion masses in several millisecond pulsar binaries. Systems like Cygnus X-2 may thus be an important channel for forming pulsar binaries. Key words: binaries: close ± stars: evolution ± stars: individual: Cygnus X-2 ± pulsars: general ± X-rays: stars. 1 I N T R O D U C T I O N Cygnus X-2 is a persistent X-ray binary with a long orbital period (P ˆ 9:84 d; Cowley, Crampton & Hutchings 1979). The observation of unambiguous type I X-ray bursts (Smale 1998) shows that the accreting component is a neutron star rather than a black hole. The precise spectroscopic information found by Casares, Charles & Kuulkers (1998), and the parameters that can be derived from it, are summarized in Table 1. The mass ratio q ˆ M2=M1 . 0:34 implies that mass transfer widens the system, and is therefore probably driven by expansion of the secondary star. Normally in long-period low-mass X-ray binaries (LMXBs) this occurs because of the nuclear evolution of a subgiant secondary along the Hayashi line, with typical effective temperatures Teff;2 . 3000±4000 K. However, Casares et al.'s observations show that this cannot be the case for Cygnus X-2. The secondary is in the Hertzsprung gap (spectral type A9 III): use of Roche geometry and the Stefan±Boltzmann law gives L2 . 150 L( with Teff;2 . 7330 K (see Table 1). Moreover, the mass ratio q . 0:34, and the assumption that the primary is a neutron star and thus obeys M1 & 2 M(; implies that the secondary has a low mass (M2 ˆ qM1 & 0:68 M(). In contrast, an isolated A9 III star would have a mass of about 4 M(. More recently, Orosz & Kuulkers (1999) have modelled the ellipsoidal variations of the secondary and thereby derived a model-dependent inclination of i ˆ 628: 5 ^ 48 which translates into component masses …M1 ˆ 1:78 ^ 0:23†M( and (M2 ˆ 0:60 ^ 0:13†M(: In this paper we consider explanations for the unusual nature of the secondary in Cygnus X-2. We find only one viable possibility, namely that this star is currently close to the end of early massive Case B mass transfer, and thus that the neutron star has somehow managed to reject most of the mass (, 3 M() transferred to it in the past. In support of this idea, we show that this type of evolution naturally explains the surprisingly large companion masses in several millisecond pulsar binaries. 2 M O D E L S F O R C Y G N U S X 2 In this section we consider four possible explanations for the unusual nature of the secondary in Cygnus X-2. We shall find that three of them are untenable, and thus concentrate on the fourth possibility. 2.1 A normal star at the onset of Case B mass transfer? The simplest explanation is that the position of the secondary in the Hertzsprung±Russell (HR) diagram is just that of a normal star crossing the Hertzsprung gap. Because such a star no longer burns hydrogen in the core, this is a massive Case B mass transfer as defined by Kippenhahn & Weigert (1967, hereafter KW). Provided that the initial mass ratio qi & 1 the binary always expands on mass transfer, which occurs on a thermal time-scale. Kolb (1998) investigated this type of evolution systematically and

Abstract: We present new optical and infrared (IR) observations of Cir X-1 taken near apastron. Both sets of spectra show asymmetric emission lines. Archival optical observations show that an asymmetric Ha emission line has been in evidence for the past 20 years, although the shape of the line has changed significantly. We present an eccentric (e , 0:7–0:9† low-mass binary model, where the system consists of a neutron star orbiting around a (sub)giant companion star of 3–5 M(. We suggest that the broad components of the emission lines arise in a highvelocity, optically thick flow near the neutron star, while the narrow components of the optical and the IR lines arise near the companion star and a heated ejecta shell surrounding the binary respectively. In this model, the velocity of the narrow component reflects the space velocity of the binary; the implied radial velocity (1430 km s after correcting for Galactic rotation) is the highest velocity known for an X-ray binary. Key words: binaries: spectroscopic – stars: individual: Cir X-1 – X-rays: stars. 1 I N T RO D U C T I O N Cir X-1 is one of the most puzzling X-ray binaries known. Like the peculiar systems SS 433 and Cyg X-3, it cannot easily be classified into any of the major categories of X-ray binaries. Indeed, there is even doubt as to whether it is a high-mass X-ray binary (HMXB) or a low-mass X-ray binary (LMXB). Since its discovery in the early 1970s, Cir X-1 has been studied intensively at X-ray wavelengths. The X-ray properties of Cir X-1 were found to differ dramatically each time it was observed (see, e.g., Kaluzienski et al. 1976; Tennant 1988; Tsunemi et al. 1989; Shirey et al. 1996). Periodic modulation of the X-ray flux was found at a period of 16.6 d (Kaluzienski et al. 1976). A radio counterpart was detected (Clark, Parkinson & Caswell 1975), and was found to flare at the same period as the X-ray modulation (Haynes et al. 1978). These flares were initially detected at peak flux levels of .1 Jy; since the 1970s, the flux of the source has decreased dramatically, and it has only occasionally been detected above 50 mJy (Stewart et al. 1991). This radio source is located 25 arcmin from the centre of the supernova remnant G 321.920.3, and is apparently connected to the remnant by a radio nebula (Haynes et al. 1986). Stewart et al. (1993) have imaged arcmin-scale collimated structures within the surrounding nebula, suggesting an outflow from the X-ray binary. Fender et al. (1998) have imaged an arcsec-scale asymmetric jet aligned with these larger structures, raising the possibility that the outflow from the system is relativistic. Recently, Case & Bhattacharya (1998) have revised the estimated distance to G 321.920.3 (and hence to Cir X-1, assuming that they are associated) to 5.5 kpc, which is substantially smaller than the original suggested distance to Cir X-1 of 10 kpc (Goss & Mebold 1977). The discovery of Type I X-ray bursts (Tennant, Fabian & Shafer 1986b) suggests that the compact object is probably a weakly magnetized neutron star. The close association of Cir X-1 with the supernova remnant suggests that the system may be a young (,105 yr old) runaway system from a supernova explosion (Stewart et al. 1993). The optical counterpart to Cir X-1 was identified as a highly reddened star with strong Ha emission (Whelan et al. 1977). This object was later shown to consist of three stars within a radius of 1.5 arcsec, the southernmost of which is the true counterpart (Moneti 1992; Duncan, Stewart & Haynes 1993). The long orbital period and periodic X-ray activity suggested a high-mass system in an eccentric orbit (Murdin et al. 1980); however, the variability of the optical emission, the faintness of the optical counterpart, and several of its X-ray characteristics suggest that the companion is a low-mass star. The lack of spectroscopic studies in the optical band means that most of the fundamental orbital parameters of Cir X-1 have not been determined. Moreover, Cir X-1 shows very different properties q 1999 RAS E-mail: hmj@aaoepp.aao.gov.au 416 H. M. Johnston, R. Fender and K. Wu from time to time. This make it difficult to construct a coherent picture for the system from observations of different wavelengths at different epochs. Here we present new spectroscopic observations of Cir X-1, and use these, together with analysis of archival observations of the system, to suggest a more coherent model for the system. 2 O B S E RVAT I O N S A N D DATA R E D U C T I O N 2.1 New optical observations Cir X-1 was observed on 1997 June 4 using the 3.9-m AngloAustralian Telescope (AAT). The mean orbital phase of the observation was 0.51, calculated according the ephemeris of Stewart et al. (1991). The RGO Spectrograph was used in combination with the TEK 1k CCD in the 82-cm camera and a grating of 270 groove mm in first order, resulting in a dispersion of ,1:08 A pixel21 over a wavelength range 6060–7165 A. The spatial scale was 0.25 arcsec pixel; the spectral resolution, measured from the arc lines, was 5.4 A. A 1.5-arcsec-wide slit was used, oriented north–south so that both Cir X-1 and star 2 of Moneti (1992) were in the slit. The atmospheric seeing was about 1 arcsec. Five 1800-s integrations were taken, interspersed with CuAr arc-lamp exposures, before cloud prevented the acquisition of any more data. The bias and pixel-to-pixel gain variations were removed from each exposure using standard procedures in iraf. Cosmic rays were removed using the method of Croke (1995) to compare adjacent frames. Because of the presence of the nearby confusing star (Moneti’s star 2), special care needed to be taken to measure the flux from our object. At every position along the dispersion direction, we fit two Gaussian profiles, with fixed widths …FWHM ˆ 5:6 pixels† and separation (6 pixels), to the skysubtracted frames. The amplitude of these Gaussians was used as the estimate of the flux from Cir X-1 and star 2 at each wavelength. We then determined the wavelength calibration using the CuAr arc lamp exposures. We fitted a low-order polynomial to the arc line wavelengths as a function of pixel number: the rms scatter of the fits was ,1=4 of a pixel. A rough flux-calibration was performed by comparing with the spectrum of the observed flux standard LTT 4364 although, since the night was nonphotometric, this flux calibration should be considered only approximate. 2.2 Infrared observations K-band spectroscopy of Cir X-1 was obtained using the Cryogenic Array Spectrometer/Imager (CASPIR) on the ANU 2.3-m telescope at Siding Spring Observatory on the night of 1997 June 20. The K grism was used with the SBRC 256 256 InSb array, giving a dispersion of 21.5 A pixel over a wavelength range of 1.94–2.49mm. The spatial scale was 0.5 arcsec pixel. A 5-arcsec slit was used, oriented east–west: note that this means that Moneti’s stars 2 and 3 both contributed light in our spectrum. The telescope was nodded by ^12 arcsec along the slit to provide sky frames at the same position as the object. Argon lamp spectra were taken to perform wavelength calibration, and two nearby bright stars (BS 5699 and 5712) were observed in order to remove atmospheric spectral features and perform flux calibration. Standard data reduction procedures were followed, using the local caspir package running in iraf. Bias and dark frames were used to linearize all frames, the sky background was subtracted from the object frames, and pixel-to-pixel variations were corrected. The chip distortion was corrected in order to align the dispersion and spatial directions along rows and columns of the chip; the sky background was then subtracted and spectra extracted. A low-order polynomial was fitted to the argon lines, and these calibrations applied to the object spectra. Flux calibration was achieved by dividing the observed spectra by the spectrum of a nearby mid-G-type star and then multiplying by a model for the absolute flux distribution of the calibrator. Residual terrestrial atmospheric features were then corrected using an early-type star.

Abstract: We have analysed the kinematical parameters of Cir X-1 to constrain the nature of its companion star, the eccentricity of the binary and the pre-supernova parameter space. We argue that the companion is most likely to be a low-mass …& 2:0 M(† unevolved star and that the eccentricity of the orbit is 0:94 ^ 0:04. We have evaluated the dynamical effects of the supernova explosion and we find it must have been asymmetric. On average, we find that a kick of , 740 km s21 is needed to account for the recently measured radial velocity of ‡430 km s21 (Johnston, Fender & Wu) for this extreme system. The corresponding minimum kick velocity is , 500 km s21. This is the largest kick needed to explain the motion of any observed binary system. If Cir X-1 is associated with the supernova remnant G321.9-0.3 then we find a limiting minimum age of this remnant of , 60 000 yr. Furthermore, we predict that the companion star has lost , 10 per cent of its mass as a result of stripping and ablation from the impact of the supernova shell shortly after the explosion. Key words: stars: individual: Cir X-1 – stars: neutron – supernovae: general. 1 I N T RO D U C T I O N Cir X-1 is a unique binary in many aspects. Despite intensive X ray observations since its discovery in the early 1970s very little is known about its stellar components besides an orbital period of 16.6 d (Kaluzienski et al. 1976). It is still uncertain whether the binary is a high-mass (HMXB) or low-mass X-ray binary (LMXB). The discovery of Type I X-ray bursts (Tennant, Fabian & Shafer 1986) demonstrates that the accreting compact object must be a neutron star. In a recent paper Johnston, Fender & Wu (1999) presented new optical and infrared observations of Cir X-1 which show asymmetric emission lines. Combined with 20 years of archival Ha line emission data, they interpreted that the narrow components of the lines imply a radial velocity of ‡430 km s21 for the system (including a small correction for the Galactic rotation). This is the highest velocity known for any X ray binary. Cir X-1 is located 25 0 from the centre of the supernova remnant G321.9-0.3 and is apparently connected by a radio nebula (Haynes et al. 1986). Furthermore, there is some evidence for an association from observations of arcmin-scale collimated structures within the nebula (Stewart et al. 1993) that are likely to originate from an arcsec-scale jet which has been observed (Fender et al. 1998) to be aligned with these larger structures. If the suggested association is correct then Cir X-1 is a young (,105 yr† runaway system and, in combination with the recent estimated distance to the remnant of 5.5 kpc (Case & Bhattacharya 1998), the inferred minimum transverse velocity (390 km s) yields a resulting 3D space velocity of .580 km s21: In this paper we investigate what can be learned about Cir X-1 and the effects of the supernova explosion which gave rise to this high runaway velocity and orbital period of 16.6 d. 2 DY N A M I C A L E F F E C T S O F A N A S Y M M E T R I C S U P E R N OVA E X P L O S I O N We use the analytical formulae by Tauris & Takens (1998) to calculate the dynamical effects of an asymmetric supernova (SN) in Cir X-1. These formulae also include the effect of shell impact on the companion star. We assume that the exploding He-star with mass MHe (the progenitor of the neutron star with mass MNS) is at the origin of the cartesian coordinate system which we shall use in our description. The positive z-axis points in the direction of the pre-SN orbital angular momentum. The positive y-axis points towards the companion star (with mass M2) and the positive x-axis points in the direction of v, which is the pre-SN relative velocity vector of the He-star with respect to the companion star. We assume the pre-SN orbit is circular and r0 is the pre-SN separation between the two stellar components. The systemic velocity of a binary which survives the SN is jvsysj ˆ  Dpx ‡ Dpy ‡ Dpz q =…MNS ‡M2f† …1†

Abstract: Rapidly spinning neutron stars, recycled in low-mass binaries, may have accreted a substantial amount of mass. The available relativistic measurements of neutron star masses, all clustering around 1.4 M☉, however, refer mostly to slowly rotating neutron stars that accreted a tiny amount of mass during evolution in a massive binary system. We develop a semianalytical model for studying the evolution of the spin period P of a magnetic neutron star as a function of the baryonic mass load Mac; evolution is followed down to submillisecond periods, and the magnetic field is allowed to decay significantly before the end of recycling. We use different equations of state and include rotational deformation effects and the presence of a strong gravitational field and of a magnetosphere. For the nonmagnetic case, comparison with numerical relativistic codes shows the accuracy of our description. The minimum accreted mass requested to spin up a magnetized 1.35 M☉ neutron star at a few milliseconds is ~0.05 M☉, while this value doubles for an unmagnetized neutron star. Below 1 ms, the request is for at least ~0.25 M☉. Only highly nonconservative scenarios for the binary evolution could prevent the transfer of such a mass to the compact object. Unless a physical mechanism limits the rotational period, there may exist a yet undetected population of massive submillisecond neutron stars. The discovery of a submillisecond neutron star would imply a lower limit for its mass of about 1.7 M☉.

Abstract: We have discovered a ~1 Hz quasi-periodic oscillation (QPO) in the persistent emission, the dips, and the type I X-ray bursts of the low-mass X-ray binary 4U 1323-62. The rms amplitude of the QPO is approximately 9%, only weakly depending on photon energy. The amplitude is consistent with being constant throughout the persistent emission, the dips, and the bursts in all but one observation, where it is much weaker during one dip. These properties suggest that we have observed a new type of QPO, which is caused by quasi-periodic obscuration of the central X-ray source by a structure in the accretion disk. This can only occur when the binary inclination is high, consistent with the fact that 4U 1323-62 is a dipping source. The quasi-periodic obscuration could take place by partial covering of an extended central X-ray source by a near-opaque medium or by covering of a point source by a medium having suitable characteristics to produce the relatively energy-independent oscillations.

Abstract: We report the discovery of a quasi-periodic oscillation (QPO) in data obtained with the Rossi X-ray Timing Explorer of the dipping and eclipsing low-mass X-ray binary EXO 0748-676. The QPO had a frequency between 0.58 and 2.44 Hz changing on time scales of a few days, an rms amplitude between 8% and 12%, and was detected in the persistent emission, during dips and during type I X-ray bursts. During one observation, when the count rate was a factor 2 to 3 higher than otherwise, the QPO was not detected. The strength of the QPO did not significantly depend on photon energy, and is consistent with being the same in the persistent emission, both during and outside the dips, and during type I X-ray bursts. Frequency shifts were observed during three of the four X-ray bursts. We argue that the QPO is produced by the same mechanism as the QPO recently found by Jonker et al. (1999) in 4U 1323-62. Although the exact mechanism is not clear, it is most likely related to the high inclination of both systems. An orbiting structure in the accretion disc that modulates the radiation from the central source seems the most promising mechanism.

Abstract: The authors report the discovery of kHz fluctuations, including quasi-periodic oscillations (QPO) at {approximately}330 Hz and {approximately}760 Hz and a broadband kHz continuum in the power density spectrum of the high mass X-ray binary pulsar Centaurus X-3 (Jernigan, Klein and Arons 2000). These observations of Cen X-3 were carried out with the Rossi X-ray Timing Explorer (RXTE). The fluctuation spectrum is flat from mHz to a few Hz, then steepens to f{sup {minus}2} behavior between a few Hz and {approximately}100 Hz. Above a hundred Hz, the spectrum shows the QPO features, plus a flat continuum extending to {approximately}1200 Hz and then falling out to {approximately}1800 Hz. Multi-dimensional radiation hydrodynamics simulations of optically thick plasma flow onto a magnetized neutron star show that the fluctuations at frequencies above 100 Hz are the likely consequence of the photon bubble turbulence and oscillations (PBO) previously predicted (Klein et al. 1996) to be observable in this source. The authors show that previous observations of Cen X-3 constrain the models to depend on only one parameter, the size of the polar cap. For a polar cap opening angle of 0.25 radians (polar cap radius {approximately}2.5 km and area {approximately}20 km{sup 2}, for a neutron star radius of 10 km), the authors show that the spectral form above 100 Hz is reproduced by the simulations, including the frequencies of the QPO and the relative power in the QPO and the kHz continuum. This has resulted in the first measurement of the polar cap size of an X-ray pulsar.

Abstract: The typical product of the star formation process is a binary star. Binaries have provided the first dynamical measures of the masses of pre-main-sequence (PMS) stars, providing support for the calibrations of PMS evolutionary tracks. Surprisingly, in some star-forming regions PMS binary frequencies are higher than among main-sequence solar-type stars. The difference in PMS and main-sequence binary frequencies is apparently not an evolutionary effect; recent attention has focussed on correlations between binary frequency and stellar density or cloud temperatures. Accretion disks are common among young binary stars. Binaries with separations between 1 AU and 100 AU have substantially less submillimeter emission than closer or wider binaries, suggesting that they have truncated their disks. Evidence of dynamical clearing has been seen in several binaries. Remarkably, PMS binaries of all separations show evidence of circumstellar disks and continued accretion. This suggests that the circumstellar disks are replenished from circumbinary disks or envelopes. The frequent presence of disks suggests that planet formation can occur in binary environments, and formation of planets in wide binaries is already established by their discovery. Circumbinary disk masses around very short period binaries are ample to form planetary systems such as our own. The nature of planetary systems among the most common binaries, with separations between 10 AU and 100 AU, is less clear given the observed reduction in disk mass, though they may have disk masses adequate for the formation of terrestrial-like planets.

Abstract: The progenitor evolution of the massive X-ray binary Wray 977 is investigated using new models of massive close binary evolution. These models yield constraints on the mass limit for neutron star/black hole formation in single stars, MBH. We argue for quasi-conservative evolution in this system, and we find MBH > 13::21M from the existence of a neutron star in Wray 977, with the uncertainty being due to uncertainties in the treatment of convection. Our results revise earlier published much larger values of MBH derived from the parameters of Wray 977. Then, on the basis of a grid of 37 evolutionary models for massive close binaries with various initial masses, mass ratios and periods, we derive primary initial-final mass, initial mass- final helium core mass, and initial mass-final CO-core mass relations for the various mass transfer cases of close binary evolution. From these models we derive for single stars that MBH < 25M, independent of whether most black hole bina- ries formed through the Case A/B or the Case C binary channel. Using our grid of binary models, we obtain a consistent scenario for the formation of black holes in binary systems. We emphasize that in binaries the critical initial mass limits for neutron star/black hole formation and for white dwarf/neutron star formation are very different from the cor- responding values in single stars. While the first may well be above 100M in Case A/B binaries, the latter is found to be in the range 12...15M instead of the canonical value of 8...10 M usually quoted for single stars. This effect should not be ne- glected in population synthesis studies of massive binary sys- tems. Also, neutron star and black hole mass functions obtained for single stars can not per se compared to the masses of compact objects in binary systems. Massive close binaries produce also Type Ib and Ic super- novae. We find two different types of supernova progenitor structure in our models, one with remaining helium masses of the order of 1M which stems from an intermediate progeni- tor initial mass range (about 16...25 M), and another with one order of magnitude smaller remaining helium masses from ini- tial masses above and below this. A possible connection to the

Abstract: Properties of non-rotating and rapidly rotating pro- toneutron stars and neutron stars are investigated. Protoneutron stars are hot, lepton rich neutron stars which are formed in Type- II supernovae. The hot dense matter is described by a realistic equation of state which is obtained by extending a recent ap- proach of Myers and ´ Swiatecki to the nuclear mass formula. We investigate the properties of protoneutron stars and neutron stars at different evolutionary stages in order to emphasize the differ- ences between very young and old neutron stars. The numerical calculations are performed by means of an exact description of rapid, uniform rotation in the framework of general relativity. We show that the minimal marginally stable protoneutron star mass is much higher than the corresponding minimum mass of a cold neutron star. The minimum gravitational (baryonic) mass of 0.89-1.13 M (0.95-1.29 M) of a neutron star is there- fore determined at the earliest stages of its evolution. We also show that the use of different temperature profiles in the enve- lope as well as different shapes of the neutrino sphere change the properties of protoneutron stars and hot neutron stars by up to 20 %. A preliminary analysis indicates that even the most massive protoneutron stars rotating with Kepler frequency are secularly stable. Under the assumption of conserved angular momentum and baryonic mass, the maximum rotational fre- quency of an evolved neutron star is determined by the Kepler frequency of the protoneutron star. We can thus derive a lower limit, Pmin 1:56 2:22 ms, to the rotational period of young neutron stars with a canonical gravitational mass of 1:35 M. This result furtherly supports the assumption that millisecond pulsars are accelerated due to accretion onto a cold neutron star.

Abstract: We develop an exploratory model for the outer, grav- itationally unstable regions of accretion disks around massive black holes. We consider black holes of mass 10 6 to 10 10 M, and primeval or solar abundances. In a first step we study star formation and evolution in a purely gaseous marginally unsta- ble disk, and we show that unstable fragments should collapse rapidly and give rise to compact objects (planets or protostars), which then accrete at a high rate and in less than 10 6 years acquire a mass of a few tens of M, according to a mecha- nism first proposed by Artymowicz et al. (1993). When these stars explode as supernovae, the supernova shells break out of the disk, producing strong outflows. We show that the gaseous disk is able to support a large number of massive stars and su- pernovae while staying relatively homogeneous. An interesting aspect is that the residual neutron stars can undergo other accre- tion phases, leading to other (presumably powerful) supernova explosions. In a second step we assume that the regions at the periphery of the disk provide a quasi stationary mass inflow dur- ing the lifetime of quasars or of their progenitors, i.e. 10 8 yrs, and that the whole mass transport is ensured by the supernovae, which induce a transfer of angular momentum towards the ex- terior, as shown by the numerical simulations of Rozyczka et al. (1995). Assuming that the star formation rate is proportional to the growth rate of the gravitational instability, we solve the disk structure and determine the gas and the stellar densities, the heating being provided mainly by the stars themselves. We find self-consistent solutions in which the gas is maintained in a state very close to gravitational instability, in a ring located between 0.1 and 10 pc for a black hole mass of 10 6 M, and between 1 and 100 pc for a black hole mass of10 8 M or larger, whatever the abundances, and for relatively low accretion rates ( 10% of the critical accretion rate). For larger accretion rates the number of stars becomes so large that they inhibit any fur- ther star formation, and/or the rate of supernovae is so high that they distroy the homogeneity and the marginal stability of the disk. We postpone the study of this case. Several consequences of this model can be envisioned, be- sides the fact that it proposes a solution to the problem of the Send offprint requests to: Suzy Collin (Observatoire de Meudon) mass transport in the intermediate region of the disk where global instabilities do not work. As a first consequence, it could explain the high velocity metal enriched outflows implied by the presence of the broad absorption lines in quasars. As a sec- ond consequence it could account for a pregalactic enrichment of the intergalactic medium, if black holes formed early in the Universe. Finally it could provide a triggering mechanism for starbursts in the central regions of galaxies. A check of the model would be to detect a supernova exploding within a few parsecs from the center of an AGN, an observation which can be per- formed in the near future.

Abstract: X 1908+075 is an optically unidentified and highly absorbed X-ray source that appears in early surveys such as Uhuru, OSO-7, Ariel V, HEAO-1, and the EXOSAT Galactic Plane Survey. These surveys measured a source intensity in the range of 2-12 mCrab at 2-10 keV, and the position was localized to ~ 0.5 degrees. We use the Rossi X-ray Timing Explorer (RXTE) All Sky Monitor (ASM) to confirm our expectation that a particular Einstein IPC detection (1E 1908.4+0730) provides the correct position for X 1908+075. The analysis of the coded mask shadows from the ASM for the position of 1E 1908.4+0730 yields a persistent intensity ~ 8 mCrab (1.5-12 keV) over a 3 year interval beginning in 1996 February. Furthermore, we detect a period of 4.400 +- 0.001 days with a false alarm probability < 1.0e-7 . The folded light curve is roughly sinusoidal, with an amplitude that is 22 % of the mean flux. The X-ray period may be attributed to the scattering and absorption of X-rays through a stellar wind combined with the orbital motion in a binary system. We suggest that X 1908+075 is an X-ray binary with a high mass companion star.

Abstract: We report on the discovery of X-ray pulsations from the Be/X-ray system LS 992/RX J0812.4-3114 during an RXTE observation. From a timing analysis of the source we obtained a barycentric pulse period of 31.8851 \pm 0.0004 s. The pulse profile is highly structured and departs from a pure sinusoidal shape. It shows a sharp dip that may indicate absorption by the accretion flow. The energy spectrum from 3-30 keV can be fitted by a power-law model with an exponential cut-off in accordance with other X-ray pulsars. The X-ray luminosity is estimated to be $\sim 1.1 \times 10^{36} erg/s$ in the energy range 3-30 keV, assuming a distance of $\sim 9 kpc$.

Abstract: This paper presents analysis and interpretation of OGLE photometric data of four X-ray binary pulsar systems in the Small Magellanic Cloud: 1WGA J0054.9-7226, RX J0050.7-7316, RX J0049.1-7250, and 1SAX J0103.2-7209. In each case, the probable optical counterpart is identified on the basis of its optical colours. In the case of RX J0050.7-7316 the regular modulation of its optical light curve appears to reveal an ellipsoidal modulation with a period of 1.416 days. Using reasonable masses for the neutron star and the B star, we show that the amplitude and relative depths of the minima of the I-band light curve of RX J0050.7-7316 can be matched with an ellipsoidal model where the B star nearly fills its Roche lobe. For mass ratios in the range of 0.12 to 0.20, the corresponding best-fitting inclinations are about 55 degrees or larger. The neutron star would be eclipsed by the B star at inclinations larger than 60 degrees for this particular mass ratio range. Thus RX J0050.7-7316 is a good candidate system for further study. In particular, we would need additional photometry in several colours, and most importantly, radial velocity data for the B star before we could draw more quantitative conclusions about the component masses.

Abstract: The Low Mass X-ray Binary (LMXB) X1822-330 in NGC 6652 is one of 12 bright, or transient, X-ray sources to have been discovered in Globular Clusters. We report on a serendipitous ASCA observation of this Globular Cluster LMXB, during which a Type I burst was detected and the persistent, non-burst emission of the source was at its brightest level recorded to date. No orbital modulation was detected, which argues against a high inclination for the X1822-330 system. The spectrum of the persistent emission can be fit with a power law plus a partial covering absorber, although other models are not ruled out. Our time-resolved spectral analysis through the burst shows, for the first time, clear evidence for spectral cooling from kT=2.4+/-0.6 keV to kT=1.0+/0.1 keV during the decay. The measured peak flux during the burst is ~10% of the Eddington luminosity for a 1.4 Msun neutron star. These are characteristic of a Type I burst, in the context of the relatively low quiescent luminosity of X1822-330.

Abstract: Various surveys of low-mass binaries in star forming regions have been performed in recent years. They reached opposite conclusions concerning possible binary excesses in some of these associations. I develop a consistent method to reanalyze all these studies, so that I can compare all data consistently, and understand the previous findings. I also report the detection of five new companions to Taurus members. It appears that binary fraction in Taurus exceeds the main sequence value by a factor of 1.7 in the range 4– 2000AU. The companion star fraction in this separation range is the same as the overall main sequence fraction. Ophiuchus, Chameleon, and possibly Lupus show similar excesses, although with lower confidence levels. Binaries in Ophiuchus seem to have larger flux ratios (towards faint companions) than in Taurus. It appears very unlikely that all very young star forming regions have binary excesses. The binary fraction seems to be established after ∼ 1Myr, but the precise nature of the difference between various regions is still unclear (overall binary fraction, orbital period distribution). It is not currently possible to put constraints on the binary formation models: higher angular resolution and larger sample sizes will be required.

Abstract: We present sub-arcsecond images of the mm dust emission and13CO J=2–1 line emission in the young quadruple system GG Tau. These observations unambiguously resolve the circumbinary disk of the close ( ∼ 0.3′′) binary system into two distinct components: an extremely dense, sharp-edged ring, surrounded by an extended disk. Continuum emission is also detected from the center of this structure; it probably arises in the small circumstellar disk or disks of the binary. The kinematic data show that the ring+disk system is in Keplerian rotation and yield the estimateM = (1.28 ± 0.07)(D/140 pc)M for the mass of the binary stars. We derive the physical parameters of the ring and disk from these data and from new 2 ′′ resolution images of the HCO+ J=1–0 line and 3.4 mm continuum emission. The temperature in the ring + disk system is consistent with heating by the stellar light (including the IR excess coming from the inner disks). Comparison with the optical/NIR images indicates a disk thickness compatible with an hydrostatic equilibrium.

Abstract: The study of star formation often concentrates on the easily detectable product of this process: the young stars. These stars are typically not in isolation but are found in groups ranging in sizes from binary systems to clusters containing thousands of stars. In this chapter we discuss the mechanisms for, and implications of, star formation in groups.

Abstract: The Low Mass X-ray Binary (LMXB) X1832-330 in NGC 6652 is one of about 10 bright X-ray sources to have been discovered in Globular Clusters. We report on a serendipitous ASCA observation of this Globular Cluster LMXB, during which a Type I burst was detected and the persistent, non-burst emission of the source was at its brightest level recorded to date. No orbital modulation was detected, which argues against a high inclination for the X1832-330 system. The spectrum of the persistent emission can be fit with a power law plus a partial covering absorber, although other models are not ruled out. Our time-resolved spectral analysis through the burst shows, for the first time, clear evidence for spectral cooling from kT = 2.4 +/- 0.6 keV to kT = 1.0 +/- 0.1 keV during the decay. The measured peak flux during the burst is approximately 10% of the Eddington luminosity for a 1.4 Solar Mass neutron star. These are characteristic of a Type I burst, in the context of the relatively low quiescent luminosity of X1832-330.