Abstract: Ordinary radio pulsars1 are neutron stars with magnetic fields of ∼1012 gauss and spin periods in the range 0.1 to 3 seconds. In contrast, millisecond radio pulsars2 have much weaker fields (∼109 gauss) and faster, millisecond spin rates. For both types of pulsar, the energy driving the radio pulsations is thought to be derived from the rotation of the neutron star. The star gradually ‘spins down’ as energy is radiated away. Millisecond radio pulsars are often located in binary systems3. In a widely accepted theoretical model4,5, they started as ordinary pulsars which lost most of their magnetic field and were ‘spun up’ to millisecond periods by the accretion of matter from a companion star in an X-ray binary system. Evidence6,7,8,9,10,11 for this model has gradually mounted, but direct proof—in the form of the predicted coherent millisecond X-ray pulsations in the persistent flux of an X-ray binary has been lacking, despite many searches12,13,14,15. Here we report the discovery16 of such a pulsar, confirming theoretical expectations. The source will probably become a millisecond radio pulsar when the accretion turns off completely.

Abstract: Gravitational radiation drives an instability in the r-modes of young rapidly rotating neutron stars. This instability is expected to carry away most of the angular momentum of the star by gravitational radiation emission, leaving a star rotating at about 100 Hz. In this paper we model in a simple way the development of the instability and evolution of the neutron star during the year-long spindown phase. This allows us to predict the general features of the resulting gravitational waveform. We show that a neutron star formed in the Virgo cluster could be detected by the LIGO and VIRGO gravitational wave detectors when they reach their “enhanced” level of sensitivity, with an amplitude signal-to-noise ratio that could be as large as about 8 if near-optimal data analysis techniques are developed. We also analyze the stochastic background of gravitational waves produced by the r-mode radiation from neutron-star formation throughout the universe. Assuming a substantial fraction of neutron stars are born with spin frequencies near their maximum values, this stochastic background is shown to have an energy density of about 10^(−9) of the cosmological closure density, in the range 20 Hz to 1 kHz. This radiation should be detectable by “advanced” LIGO as well.

Abstract: Nuclear reactions occurring deep in the crust of a transiently accreting neutron star efficiently maintain the core at a temperature >5e7 K. When accretion halts, the envelope relaxes to a thermal equilibrium set by the flux from the hot core, as if the neutron star were newly born. For the time-averaged accretion rates typical of low-mass X-ray transients, standard neutrino cooling is unimportant and the core thermally re-radiates the deposited heat. The resulting luminosity has the same magnitude as that observed from several transient neutron stars in quiescence. Confirmation of this mechanism would strongly constrain rapid neutrino cooling mechanisms for neutron stars. Thermal emission had previously been dismissed as a predominant source of quiescent emission since blackbody spectral fits implied an emitting area much smaller than a neutron star's surface. However, as with thermal emission from radio pulsars, fits with realistic emergent spectra will imply a substantially larger emitting area. Other emission mechanisms, such as accretion or a pulsar shock, can also operate in quiescence and generate intensity and spectral variations over short timescales. Indeed, quiescent accretion may produce gravitationally redshifted metal photoionization edges in the quiescent spectra (detectable with AXAF and XMM). We discuss past observations of Aql~X-1 and note that the low luminosity X-ray sources in globular clusters and the Be star/X-ray transients are excellent candidates for future study.

Abstract: Beginning from massive binaries in the Galaxy, we evolve black hole-neutron star (BH-NS) binaries and binary neutron stars, such as the Hulse-Taylor system PSR 1913+16. The new point in our evolution is a quantitative calculation of the accretion of matter by a neutron star in a common-envelope evolution that sends it into a black hole. We calculate the mass of the latter to be ~2.4 M☉. The black hole fate of the first neutron star can only be avoided if the neutron star does not go through common-envelope evolution. This can be realized if the two massive binaries are sufficiently close in mass, not more than ~5% apart, so that they burn helium at the same time. Then their common hydrogen envelope is expelled by the tightening in the double He star system, with attendant hydrodynamical coupling to the envelope. In this way, we obtain a rate of 10-5 per yr per galaxy for production of the narrow neutron star binaries, such as Hulse-Taylor 1913+16 or Wolszczan 1934+12. This is in agreement with estimates based on the observed number of such systems extrapolated to the entire Galaxy, with beaming factors and corrections for the ~90% of binary pulsars estimated to be unobservable. Our chief conclusion is that the production rate for BH-NS binaries (in which the neutron star is unrecycled) is ~10-4 per yr per galaxy, an order of magnitude greater than that of neutron star binaries. Not only should this result in a factor of ~10 more mergings for gravitational wave detectors such as LIGO, but the signal should also be larger. We include some discussion of why BH-NS binaries have not been observed, but conclude that they should be actively searched for.

Abstract: We show that the model proposed by Esin, McClintock, & Narayan for the low state, intermediate state, and high state of the black hole soft X-ray transient, Nova Muscae 1991, is consistent with the spectral evolution of the black hole X-ray binary, Cyg X-1, during the hard-to-soft state transition observed in 1996. We also apply the model to the outbursts of two other black hole X-ray transients, GRO J0422+32 and GRO J1719-24.

Abstract: Soft X-ray Transients (SXRTs) have long been suspected to contain old, weakly magnetic neutron stars that have been spun up by accretion torques. After reviewing their observational properties, we analyse the different regimes that likely characterise the neutron stars in these systems across the very large range of mass inflow rates, from the peak of the outbursts to the quiescent emission. While it is clear that close to the outburst maxima accretion onto the neutron star surface takes place, as the mass inflow rate decreases, accretion might stop at the magnetospheric boundary because of the centrifugal barrier provided by the neutron star. For low enough mass inflow rates (and sufficiently short rotation periods), the radio pulsar mechanism might turn on and sweep the inflowing matter away. The origin of the quiescent emission, observed in a number of SXRTs at a level of ~10^(32)-10^(33) erg/s, plays a crucial role in constraining the neutron star magnetic field and spin period. Accretion onto the neutron star surface is an unlikely mechanism for the quiescent emission of SXRTs, as it requires very low magnetic fields and/or long spin periods. Thermal radiation from a cooling neutron star surface in between the outbursts can be ruled out as the only cause of the quiescent emission. We find that accretion onto the neutron star magnetosphere and shock emission powered by an enshrouded radio pulsar provide far more plausible models. In the latter case the range of allowed neutron star spin periods and magnetic fields is consistent with the values recently inferred from the properties of kHz QPO in LMXRBs. If quiescent SXRTs contain enshrouded radio pulsars, they provide a missing link between X-ray binaries and millisecond pulsars.

Abstract: We present a model for gamma-ray bursts (GRBs) in which a stellar mass black hole acquires a massive accretion disk by merging with the helium core of its red giant companion. The black hole enters the helium core after it, or its neutron star progenitor, first experiences a common envelope phase that carries it inward through the hydrogen envelope. Accretion of the last several solar masses of helium occurs on a timescale of roughly a minute and provides a neutrino luminosity of approximately 1051-1052 ergs s−1. Neutrino annihilation, 0.01%-0.1% efficient, along the rotational axis then gives a baryon-loaded fireball of electron-positron pairs and radiation (about 1050 ergs total) whose beaming and relativistic interaction with the circumstellar material makes the GRB (see, e.g., Rees & Meszaros). The useful energy can be greatly increased if energy can be extracted from the rotational energy of the black hole by magnetic interaction with the disk. Such events should occur at a rate comparable to that of merging neutron stars and black hole neutron star pairs and may be responsible for long complex GRBs but not short hard ones.

Abstract: We show that the model proposed by Esin, McClintock, & Narayan for the low state, intermediate state, and high state of the black hole soft X-ray transient, Nova Muscae 1991, is consistent with the spectral evolution of the black hole X-ray binary, Cyg X-1, during the hard-to-soft state transition observed in 1996. We also apply the model to the outbursts of two other black hole X-ray transients, GRO J0422)32 and GRO J1719(24. Subject headings: accretion, accretion disks E black hole physics E stars: individual (Cygnus X-1, GRO J0422)32, GRO J1719(24) E X-rays: stars

Abstract: We study the timing properties of the low mass X-ray binary 4U 1728-34 using recently released data from the Rossi X-Ray Timing Explorer. This binary, like many others with accreting neutron stars, is known to exhibit strong quasi-periodic oscillations (QPOs) of its X-ray flux near 1 kHz. In addition to the kilohertz QPOs, the Fourier power spectra show a broken power law noise component, with a break frequency between 1 and 50 Hz, and a Lorentzian between 10 and 50 Hz. We find that the frequencies of the break and the low-frequency Lorentzian are well correlated with the frequencies of the kilohertz QPOs. The slope of the correlation is similar to that expected if the oscillations are due to relativistic frame dragging (Lense-Thirring precession) in the inner accretion disk (Stella & Vietri 1998). The correlation is also nearly identical to the one found in Z-sources between the the well known QPOs on the horizontal branch and the kilohertz QPOs, suggesting that the low frequency oscillations are a similar phenomenon in these sources. The frequency of the break in the power spectra is also correlated with the frequencies of the kilohertz QPOs. As previously noted for the similar binaries 4U 1608-50 and 4U 1705-44, this broken power law component closely resembles that of black hole candidates in the low state, where the break frequency is taken as an indicator of mass accretion rate. The relation between break frequency and kilohertz QPO frequency thus provides additional proof that the frequency of the kilohertz QPOs increases with mass accretion rate.

Abstract: From considerations of spin evolution and kinematics in the galactic potential, we argue that the pulsars B1913+16, B1534+12, and B2127+11C may be younger than previously assumed, and we find that a lower bound on the formation and merger rate of close double neutron star binaries is $10^{-6.7} f_b^{-1} f_d^{-1}$ yr$^{-1}$, where $f_b$ is the beaming fraction and $f_d$ accounts for the possibility that the known NS-NS binaries are atypical of the underlying population (e.g., if most such binaries are born with shorter orbital periods). If we assume no prior knowledge of the detectable lifetimes of such systems, the rate could be as large as $\simeq 10^{-5.0} f_b^{-1} f_d^{-1}$ yr$^{-1}$. From both plausible bounds on $f_b$ and $f_d$, and a revision of the independently derived limit proposed by Bailes (1996), we argue that a firm upper bound is $10^{-4}$ yr$^{-1}$. We also present a unifying empirical overview of the spin-up of massive binary pulsars based on their distribution in spin period $P$ and spin-down rate $\dot P$, finding evidence for two distinct spin-up processes, one dominated by disk accretion, the other by wind accretion. We argue that the positions of binary pulsars in the $P$-$\dot P$ diagram can be understood if (1) there exists a Galactic population of pulsars in double neutron star systems with combinations of spin and orbital periods that have prevented their detection in surveys to date; (2) recycled pulsars in wide-orbit binaries are not born near the canonical spin-up line in the $P$-$\dot P$ diagram because they were predominantly spun up through wind accretion; and (3) there exists a disfavored evolutionary endpoint for radio pulsars with spin periods 5--30 ms and $\dot P > 10^{-19}$ s-s$^{-1}$.

Abstract: We present the results of Newtonian hydrodynamic simulations of the coalescence of a binary consisting of a black hole with a neutron star. The calculations show that for a wide range of initial conditions the core of the neutron star survives the initial mass transfer episode. We therefore identify black hole-neutron star binaries as the astrophysical production site of low-mass neutron stars unstable to explosion. The relevance of the simulations to the theory of gamma-ray bursts is also discussed.

Abstract: The formation and evolution of binaries which con- tain two neutron stars or a neutron star with a black hole com- panionarediscussedindetail.Theevolutionofthedistributions in orbital period and eccentricity for neutron star binaries are studied as a function of time. In the model which ts the obser- vationsofhighmassbinarypulsarsbestthedepositionoforbital energyintocommonenvelopeshastobeveryefcientandakick velocity distribution has to contain a signicant contribution of low velocity kicks. The estimated age of the population has to be between several 100Myr and 1 Gyr. The birthrate of neutron starsbinaryis 3:410 5 yr 1 (assuming100%binarity)and theirmergerrateis 210 5 yr 1 .Themergerrateofneutron starbinariesisconsistentwiththeestimatedrateof-raybursts, if the latter are beamed into an opening angle of a few degrees. We argue that PSR B2303+46 is possibly formed in a scenario in which the common envelope is avoided while for the other three known high-mass binary pulsars a common envelope is required to explain their orbital period.

Abstract: In observations with the Rossi X-ray Timing Explorer we have discovered quasi-periodic oscillations (QPOs) near 1 kHz from 4U 1705-44, a low--mass X-ray binary with a neutron star classified as an atoll source. In six separate observations, we detect one QPO with a frequency ranging between 770 and 870 Hz and a 4% rms fraction in the full detector energy band. There is evidence for a second QPO at 1073 Hz in one interval. The separation in frequency of the two QPOs is 298+-11 Hz. The QPOs are present only in observations where the mass accretion rate is inferred to be at an intermediate level, based on the atoll source phenomenology. At the highest accretion rates, the QPOs are not detected with upper limits to the rms fraction of about 2%. At the lowest accretion rates the upper limits are about 4%. The QPO frequency increases with inferred mass accretion rate. This is expected in models where the QPO frequency is generated by motion at an inner edge of the accretion disk. An increased mass accretion rate causes the disk edge to move in, increasing the orbital frequency. Five Type-I X-ray bursts are observed with no detectable oscillations.

Abstract: We propose that when neutron stars in low-mass X-ray binaries accrete sufficient mass and become millisecond pulsars, the interiors of these stars may undergo phase transitions, exciting stellar radial oscillations. We show that the radial oscillations will be mainly damped by gravitational-wave radiation instead of internal viscosity. Such gravitational waves may be detected by the advanced Laser Interferometer Gravitational-wave Observatory.

Abstract: Gravitation wave noise associated with unresolved binary stars in the Galaxy is studied with the special aim of determining the upper frequency at which it stops contributing to the rms noise level of the proposed space-borne interferometer (LISA). The upper limit to this background is derived from the statistics of SN Ia explosions, some of which can be triggered by binary white dwarf coalescences. The upper limiting frequency at which binary stochastic noise crosses LISA rms sensitivity is found to lie within the range ≈ 0.03-0.07 Hz, depending on the Galactic binary white dwarf coalescence rate. To be reliably detectable by LISA, the energy density of the relic cosmological background per logarithmic frequency interval should be ΩGW h2100 > 10−8 at f > 0.03 Hz.

Abstract: We analyze the effect of the quadrupole component in the mass distribution of a rapidly rotating neutron star on energy release in the boundary layer on the surface of the accreting star and in the accretion disk in the cases where the stellar radius is smaller (or larger) than the radius of the marginally stable circular orbit. We calculate the velocities and trajectories of the particles that fall on the stellar surface from the marginally stable orbit for a low- luminosity accreting source. The corresponding external gravitational field of the star is modeled by a new exact solution of the Einstein equations in vacuum. The parameters of this solution are adjusted by reconciling the numerical data for the radius of the marginally stable orbit and the gravitational redshift of Cook et al. (1994) with the corresponding data in the analytical solution. For various equations of state, we consider 1.4 solar mass normal sequences and maximum mass normal sequences.

Abstract: We calculate stochastic gravitational wave back- ground produced by extragalactic merging binary white dwarfs at the LISA frequencies 10 3 10 2 Hz with account of a strong evolution of global star formation rate in the Universe recently established observationally. We show that for the ob- served global star formation history and modern cosmological models the extragalactic background is an order of magnitude smaller than the mean Galactic value. An early star formation burst at high redshifts can bring it at a higher level but still a few times lower than the mean Galactic one.

Abstract: ABSTRA C T I present pointed ROSAT PSPC observations of the pre-cataclysmic binary V471 Tauri. The hard X-ray emission (>0.4 keV) is not eclipsed by the K star, demonstrating conclusively that this component cannot be emitted by the white dwarf. Instead I show that its spectrum and luminosity are consistent with coronal emission from the tidally spun-up K star. The star is more active than other K stars in the Hyades, but equally active as K stars in the Pleiades with the same rotation periods, demonstrating that rotation ‐ and not age ‐ is the key parameter in determining the level of stellar activity. The soft X-ray emission (<0.4 keV) is emitted predominately by the white dwarf and is modulated on its spin period. I find that the pulse profile is stable on time-scales of hours and years, supporting the idea that it is caused by the opacity of accreted material. The profile itself shows that the magnetic field configuration of the white dwarf is dipolar and that the magnetic axis passes through the centre of the star. There is an absorption feature in the light curve of the white dwarf, which occurs at a time when our line of sight passes within a stellar radius of the K star. The column density and duration of this feature imply a volume and mass for the absorber that are similar to those of coronal mass ejections of the Sun. Finally I suggest that the spin‐orbit beat period detected in the optical by Clemens et al. may be the result of the interaction of the K-star wind with the magnetic field of the white dwarf.

Abstract: We report the discovery of a bright and unresolved radio counterpart to the massive X-ray binary LS 5039. The opti- cal position of this early type star is in excellent agreement with that measured in the radio. The observed spectrum is clearly of non-thermal synchrotron nature and some evidences of vari- ability, although with moderate amplitude, have been detected at both radio and optical wavelengths. All the available data strongly support that LS 5039 is a new member in the reduced family of radio loud massive X-ray binaries.

Abstract: We have combined Rossi X-ray Timing Explorer observations of X2127+119, the low-mass X-ray binary in the globular cluster M15, with archival X-ray lightcurves to study the stability of the 17.1 hr orbital period. We find that the data cannot be fit by the Ilovaisky (1993) ephemeris, and requires either a 7sigma change to the period or a period derivative Pdot/P~9x10e-7 per year. Given its remarkably low L_X/L_opt such a Pdot lends support to models that require super-Eddington mass transfer in a q~1 binary.

Abstract: We report the discovery of a fairly bright transient during observations with the Wide Field Cameras on board the BeppoSAX satellite in September 1996. It was detected at a peak intensity of 0.1 Crab (2 to 10 keV) and lasted between 6 and 40 days above a detection threshold of 2 mCrab. Two very bright type I X-ray bursts were detected from this transient in the same observations. These almost certainly identify this X-ray transient as a low-mass X-ray binary with a neutron star as compact component. The double-peaked time history of both bursts at high energies suggests a peak luminosity close to the Eddington limit. Assuming this to be true implies a distance to this object of 4 kpc.

Abstract: We report evidence of type-I X-ray bursting activity from the transient 1RXS J170930.2-263927 when it was in outburst early 1997. This identifies the source as a probable low-mass X-ray binary containing a neutron star. The error boxes of the detected bursts and of the persistent emission, as obtained with the Wide Field Cameras on board the BeppoSAX satellite, rule out an association with the proposed radio counterpart (Hjellming & Rupen 1997).

Abstract: The evolution of young compact star clusters is studied using N-body simulations in which both stellar evolution and physical collisions between stars are taken into account. The initial conditions are chosen to represent R136, a compact star cluster in the 30 Doradus region of the Large Magellanic Cloud. The present runs do not include the effects of primordial binaries. We find that physical collisions between stars in these models are frequent, and that the evolution of the most massive stars and the dynamical evolution of the cluster are closely coupled. In all cases, a single star grows steadily in mass through mergers with other stars, forming a very massive (>100 Msun) star in less than 3-4 Myr. The growth rate of this runaway merger is much larger than estimates based on simple cross-section arguments, mainly because the star is typically found in the core and tends to form binaries with other massive stars there. The runaway is ``rejuvenated'' by each new collision, and its lifetime is extended considerably as a consequence. Observationally, such a star will appear in the Hertzsprung-Russell diagram as a blue straggler. When the runaway forms a black hole, the binary in which it is found is usually dissociated.

Abstract: We present a simple model illustrating how a highly relativistic, compact object which is stable in isolation can be driven dynamically unstable by the tidal field of a binary companion. Our compact object consists of a test-particle in a relativistic orbit about a black hole; the binary companion is a distant point mass. Our example is presented in light of mounting theoretical opposition to the possibility that sufficiently massive, binary neutron stars inspiraling from large distance can collapse to form black holes prior to merger. Our strong-field model suggests that first order post-Newtonian treatments of binaries, and stability analyses of binary equilibria based on orbit-averaged, mean gravitational fields, may not be adequate to rule out this possibility.

Abstract: We identify the periodic transient hard X-ray source GRO J1849-03 with the transient x-ray pulsar GS 1843-02 = X1845-024 based on the detection of x-ray outbursts from X1845-024 coincident with hard x-ray outbursts of GRO J1849-03 Based on its spin period of 948 s and its orbital period of 241 days, we classify the system as a Be/X-ray binary

Abstract: We report on extensive V, R, and i band photometry of the black-hole candidate Nova Sco 1994 (GRO J1655{40) obtained during March 1996, when the source was close to its quiescent pre-outburst optical brightness (V=17.3). From our observations and data taken from the literature we derive a re- ned ephemeris for inferior conjunction of the secondary star: HJD 2449838:4198(52) + 2:62168(14)N. We have modeled the V, R, and i band light curves in terms of a Roche lobe ll- ing secondary and an accretion disk around the compact star, the latter described as a flat cylinder with a radial temperature distribution. From the shape of the light curve we constrain the binary inclination and mass of the secondary star to lie in the ranges 63:7{70:7 and 1.60{3.10 M, respectively. This lim- its the mass of the black-hole to the range 6.29{7.60 M. The mass range we obtained for the secondary star is supported by the results of stellar evolution calculations.