Abstract: Massive stars (with mass m* > 8 solar masses M⊙) are fundamental to the evolution of galaxies, because they produce heavy elements, inject energy into the interstellar medium, and possibly regulate the star formation rate. The individual star formation time, t*f, determines the accretion rate of the star; the value of the former quantity is currently uncertain by many orders of magnitude1,2,3,4,5,6, leading to other astrophysical questions. For example, the variation of t*f with stellar mass dictates whether massive stars can form simultaneously with low-mass stars in clusters. Here we show that t*f is determined by the conditions in the star's natal cloud, and is typically ∼105 yr. The corresponding mass accretion rate depends on the pressure within the cloud—which we relate to the gas surface density—and on both the instantaneous and final stellar masses. Characteristic accretion rates are sufficient to overcome radiation pressure from ∼100M⊙ protostars, while simultaneously driving intense bipolar gas outflows. The weak dependence of t*f on the final mass of the star allows high- and low-mass star formation to occur nearly simultaneously in clusters.

Abstract: We observed 1E 1207.4-5209, a neutron star in the center of the supernova remnant PKS 1209-51/52, with the ACIS detector aboard the Chandra X-Ray Observatory and detected two absorption features in the source spectrum. The features are centered near 0.7 and 1.4 keV; their equivalent widths are about 0.1 keV. We discuss various possible interpretations of the absorption features and exclude some of them. A likely interpretation is that the features are associated with atomic transitions of once-ionized helium in the neutron star atmosphere with a strong magnetic field. The first clear detection of absorption features in the spectrum of an isolated neutron star provides an opportunity to measure the mass-to-radius ratio and to constrain the equation of state of the superdense matter.

Abstract: The binary star LS I +61303 is remarkable for its periodic radio outbursts every 26.5 days. We recently discovered a 4.4 year periodic modulation of the phase and amplitude of these outbursts using the Gregory-Loredo Bayesian algorithm for detecting periodic signals of unknown shape. In this paper we obtain improved estimates of both the orbital period (P1) and modulation period (P2) based on a larger data set and examine the behavior of the spectral index between 2.2 and 8.3 GHz, versus modulation period. The new estimates are P1 =2 6.4960 ± .0028 days and P2 = 1667 ± 8 days and our best estimate of the radio phase of periastron is 0.4. Analysis of the spectral index data indicates that the optical depth in the synchrotron emission region is always << 1a t 8.3 GHz, and can reach values 2.7 at 2.2 GHz. A test of the precessing Be star model of Lipunov and Nazin indicates that it is unlikely to be the correct mechanism to explain the 1667 day periodic modulation.

Abstract: We investigate an interesting new class of high-mass X-ray binaries (HMXBs) with long orbital periods (Porb > 30 days) and low eccentricities (e 0.2). The orbital parameters suggest that the neutron stars in these systems did not receive a large impulse, or kick, at the time of formation. After considering the statistical significance of these new binaries, we develop a self-consistent phenomenological picture wherein the neutron stars born in the observed wide HMXBs receive only a small kick (50 km s-1), while neutron stars born in isolation, in the majority of low-mass X-ray binaries, and in many of the well-known HMXBs with Porb 30 days receive the conventional large kicks, with a mean speed of ~300 km s-1. Assuming that this basic scenario is correct, we discuss a physical process that lends support to our hypothesis, whereby the magnitude of the natal kick to a neutron star born in a binary system depends on the rotation rate of its immediate progenitor following mass transfer—the core of the initially more massive star in the binary. Specifically, the model predicts that rapidly rotating precollapse cores produce neutron stars (NSs) with relatively small kicks, and vice versa for slowly rotating cores. If the envelope of the NS progenitor is removed before it has become deeply convective, then the exposed core is likely to be a rapid rotator. However, if the progenitor becomes highly evolved prior to mass transfer, then a strong magnetic torque, generated by differential rotation between the core and the convective envelope, may cause the core to spin down to the very slow rotation rate of the envelope. Our model has important implications for the dynamics of stellar core collapse, the retention of neutron stars in globular clusters, and the formation of double neutron star systems in the Galaxy.

Abstract: We study the viscous effects on the interaction between the coplanar Be-star disc and the neutron star in Be/X-ray binaries, using a three-dimensional, smoothed particle hydrodynamics code. For simplicity, we assume the Be disc to be isothermal at the temperature of half the stellar effective temperature. In order to mimic the gas ejection process from the Be star, we inject particles with the Keplerian rotation velocity at a radius just outside the star. Both the Be star and the neutron star are treated as point masses. We find that the Be-star disc is effectively truncated if the Shakura‐Sunyaev viscosity parameter αSS � 1, which confirms the previous semi-analytical result. In the truncated disc, the material decreted from the Be star accumulates, so that the disc becomes denser more rapidly than if around an isolated Be star. The resonant truncation of the Be disc results in a significant reduction of the amount of gas captured by the neutron star and a strong dependence of the mass-capture rate on the orbital phase. We also find that an eccentric mode is excited in the Be disc through direct driving as a result of a one-armed bar potential of the binary. The strength of the mode becomes greater in the case of a smaller viscosity. In a high-resolution simulation with αSS = 0.1, the eccentric mode is found to precess in a prograde sense. The mass-capture rate by the neutron star modulates as the mode precesses.

Abstract: We performed 3D numerical simulations of the merger of equal-mass binary neutron stars in full general relativity using a new large-scale supercomputer. We take the typical grid size as (505, 505, 253) for (x, y, z) and the maximum grid size as (633, 633, 317). These grid numbers enable us to put the outer boundaries of the computational domain near the local wave zone and hence to calculate gravitational waveforms of good accuracy (within ∼ 10% error) for the first time. To model neutron stars, we adopt a Γ -law equation of state in the form P =( Γ − 1)ρe, where P , ρ, e and Γ are the pressure, rest mass density, specific internal energy and adiabatic constant. It is found that gravitational waves in the merger stage have characteristic features that reflect the formed objects. In the case that a massive, transient neutron star is formed, its quasi-periodic oscillations are excited for a long duration, and this property is reflected clearly by the quasi-periodic nature of waveforms and the energy luminosity. In the case of black hole formation, the waveform and energy luminosity are likely damped after a short merger stage. However, a quasi-periodic oscillation can still be seen for a certain duration, because an oscillating transient massive object is formed during the merger. This duration depends strongly on the initial compactness of neutron stars and is reflected in the Fourier spectrum of gravitational waves. To confirm our results and to calibrate the accuracy of gravitational waveforms, we carried out a wide variety of test simulations, changing the resolution and size of the computational domain.

Abstract: We report on XMM-Newton observations performed on 2001 September 13-14 of the neutron star X-ray transient KS 1731-260 in quiescence. The source was detected at an unabsorbed 0.5-10 keV flux of only (4-8) × 10-14 ergs cm-2 s-1, depending on the model used to fit the data, which for a distance of 7 kpc implies a 0.5-10 keV X-ray luminosity of approximately (2-5) × 1032 ergs s-1. The 2001 September quiescent flux of KS 1731-260 is lower than that observed during the Chandra observation in 2001 March. In the cooling neutron star model for the quiescent X-ray emission of neutron star X-ray transients, this decrease in the quiescent flux implies that the crust of the neutron star in KS 1731-260 cooled down rapidly between the two epochs, indicating that the crust has a high conductivity. Furthermore, enhanced cooling in the neutron star core is also favored by our results.

Abstract: We report the discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624 490 during an XMM-Newton observation in 2001 February. The features are identied with the K ab- sorption lines of Fe xxv and Fe xxvi and have energies of 6:72 0:03 keV and 7:00 0:02 keV and equivalent widths (EWs) of 7:5 +1:7 6:3 eV and 16:6 +1:9 5:9 eV, respectively. The EWs show no obvious dependence on orbital phase, except during a dip, and correspond to a column of>10 17:3 Fe atom cm 2 . In addition, faint absorption features tentatively identied with Ni xxvii K and Fe xxvi K may be present. A broad emission feature at 6:58 +0:07 0:04 keV with an EW of 78 +19 6 eV is also evident. This is probably the 6.4 keV feature reported by earlier missions since tting a single Gaussian to the entire Fe-K region gives an energy of 6:39 +0:03 0:04 keV. A deep ab- sorption feature is present during the dip with an energy consistent with Fe xxv K. This is the second dipping LMXRB source from which narrow Fe absorption features have been observed. Until recently the only X-ray bina- ries known to exhibit narrow X-ray absorption lines were two superluminal jet sources and it had been suggested that these features are related to the jet formation mechanism. It now appears likely that ionized absorption features may be common characteristics of accreting systems with accretion disks.

Abstract: The X-ray source RX J0806+15 was discovered using ROSAT , and shows an X-ray light curve with a prominent modulation on a period of 321.5 s. We present optical observations in which we report the detection of its optical counterpart. We find an optical period consistent with the X-ray period. We do not find convincing evidence for a second period in the data: this implies the 321.5-s period is the orbital period. As such it would be the shortest period stellar binary system yet known. We discuss the nature of this system. We conclude that an isolated neutron star and an intermediate polar interpretation is unlikely and that a double degenerate interpretation is the most likely.

Abstract: We present results from a short series of blue, moderate-resolution spectra of the microquasar binary SS 433. The observations were made at a time optimized to find the spectrum of the donor star, i.e., when the donor was in the foreground and well above the plane of the obscuring disk. In addition to the well-known stationary and jet emission lines, we find evidence of a weak absorption spectrum that resembles that of an A-type evolved star. These lines display radial velocity shifts opposite to those associated with the disk surrounding the compact star, and they appear strongest when the disk is maximally eclipsed. All these properties suggest that these absorption lines form in the atmosphere of the hitherto unseen mass donor star in SS 433. The radial velocity shifts observed are consistent with a mass ratio MX/MO = 0.57 ± 0.11 and masses of MO = 19 ± 7 M☉ and MX = 11 ± 5 M☉. These results indicate that the system consists of an evolved, massive donor and a black hole mass gainer.

Abstract: Nearly all of the initial angular momentum of the matter that goes into each forming star must somehow be removed or redistributed during the formation process The possible transport mechanisms and the possible fates of the excess angular momentum are discussed, and it is argued that transport processes in discs are probably not sufficient by themselves to solve the angular momentum problem, while tidal interactions with other stars in forming binary or multiple systems are likely to be of very general importance in redistributing angular momentum during the star formation process Most, if not all, stars probably form in binary or multiple systems, and tidal torques in these systems can transfer much of the angular momentum from the gas around each forming star to the orbital motions of the companion stars Tidally generated waves in circumstellar discs may contribute to the overall redistribution of angular momentum Stars may gain much of their mass by tidally triggered bursts of rapid accretion, and these bursts could account for some of the most energetic phenomena of the earliest stages of stellar evolution, such as jet-like outflows If tidal interactions are indeed of general importance, planet-forming discs may often have a more chaotic and violent early evolution than in standard models, and shock heating events may be common Interactions in a hierarchy of subgroups may play a role in building up massive stars in clusters and in determining the form of the upper initial mass function (IMF) Many of the processes discussed here have analogues on galactic scales, and there may be similarities between the formation of massive stars by interaction-driven accretion processes in clusters and the buildup of massive black holes in galactic nuclei

Abstract: We have modeled X-ray burst oscillations observed with the Rossi X-Ray Timing Explorer from two low-mass X-ray binaries (LMXB): 4U 1636-53 with a frequency of 580 Hz and 4U 1728-34 at a frequency of 363 Hz. We have computed least-squares fits to the oscillations observed during the rising phase of bursts using a model that includes emission from either a single circular hot spot or a pair of circular antipodal hot spots on the surface of a neutron star. We model the spreading of the thermonuclear hot spots by assuming that the hot spot angular size grows linearly with time. We calculate the flux as a function of rotational phase from the hot spots and take into account photon deflection in the relativistic gravitational field of the neutron star, assuming the exterior spacetime is the Schwarzschild metric. We find acceptable fits with our model in a χ2 sense and use these to place constraints on the compactness of the neutron stars in these sources. For 4U 1636-53, in which detection of a 290 Hz subharmonic supports the two-spot model, we find that the compactness (i.e., mass/radius ratio) is constrained to be M/R < 0.163 at 90% confidence (G = c = 1). This requires a relatively stiff equation of state (EOS) for the stellar interior. For example, if the neutron star has a mass of 1.4 M☉, then its radius must be greater than 12.8 km. Fits using a single hot spot model are not as highly constraining. We discuss the implications of our findings for recent efforts to calculate the EOS of dense nucleon matter and the structure of neutron stars.

Abstract: After almost 2.5 years of actively accreting, the neutron star X-ray transient and eclipsing binary MXB 1659-29 returned to quiescence in 2001 September. We report on a Chandra observation of this source taken a little over a month after this transition. The source was detected at an unabsorbed 0.5-10 keV flux of only (2.7 - 3.6) x 10^{-13} ergs/s/cm/cm, which implies a 0.5-10 keV X-ray luminosity of approximately (3.2 - 4.3) x 10^{33} (d/10 kpc)^2 erg/s, with d the distance to the source in kpc. Its spectrum had a thermal shape and could be well fitted by either a blackbody with a temperature kT of 0.3 keV or a neutron star atmosphere model with a kT of ~0.1 keV. The luminosity and spectral shape of MXB 1659-29 are very similar to those observed of the other neutron star X-ray transients when they are in their quiescent state. The source was variable during our observation, exhibiting a complete eclipse of the inner part of the system by the companion star. Dipping behavior was observed before the eclipse, likely due to obscuration by an extended feature in the outer part of a residual accretion disk. We discuss our observation in the context of the cooling neutron star model proposed to explain the quiescent properties of neutron star X-ray transients.

Abstract: We present the results of a Chandra/HST study of the point sources of the NGC 4472. We identify 144 X-ray sources, 72 with HST matches. The optical data show 1102 sources, 829 with globular cluster colors. Thirty matches are found - likely to be low mass X-ray binaries in globular clusters, while 42 have no optical counterparts to V~25 and I~24 - likely predominantly LMXBs in the field star population. Thus approximately 40% of the X-ray sources are in globular clusters and ~4% of the globular clusters contain X-ray sources. The blue GC sources may have harder X-ray spectra than the red GC sources. No significant differences are found between the X-ray properties of the field sources and of the GC sources. This study, along with our previous result from Paper I in this series on the similarity of the spatial profile of the field LMXBs, globular cluster LMXBs, and the globular clusters themselves suggest that a significant fraction of the observed low mass X-ray binaries in the field may be created in a globular cluster then ejected into the field by stellar interaction s; however, by comparing the results for NGC 4472 with those in several other galaxies, we find tentative evidence for a correlation be tween the globular cluster specific frequency and the fraction of LMXBs in globular clusters, a correlation which would be most easily explained if some of the field sources were generated in situ. We show that isolated accreting very massive black holes are unlikely to be observable with current X-ray instrumentation and that these sources hence do not contaminate the LMXB population. We discuss the possibility that several equatorial point sources may indicate the presence of a disk wind responsible for the low radiative efficiency observed in the nucleus of this source. (abridged)

Abstract: The properties of tectonic earthquake sources are compared with those deduced here for fault planes in solid neutron-star matter. The conclusion that neutron-star matter cannot exhibit brittle fracture at any temperature or magnetic field is significant for current theories of pulsar glitches, and of the anomalous X-ray pulsars and soft-gamma repeaters.

Abstract: We report the discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624-490 during an XMM-Newton observation in 2001 February. The features are identified with the K alpha absorption lines of Fe xxv and Fe xxvi and have energies of 6.72 +/- 0.03 keV and 7.00 +/- 0.02 keV and equivalent widths (EWs) of -7.5 +1.7 -6.3 eV and -16.6 +1.9 -5.9 eV, respectively. The EWs show no obvious dependence on orbital phase, except during a dip, and correspond to a column of greater than 10^17.3 Fe atom /cm2. In addition, faint absorption features tentatively identified with Ni xxvii K alpha and Fe xxvi K beta may be present. A broad emission feature at 6.58 +0.07 -0.04 keV with an EW of 78 +19 -6 eV is also evident. This is probably the 6.4 keV feature reported by earlier missions since fitting a single Gaussian to the entire Fe-K region gives an energy of 6.39 +0.03 -0.04 keV. A deep absorption feature is present during the dip with an energy consistent with Fe xxv K alpha. This is the second dipping LMXRB source from which narrow Fe absorption features have been observed. Until recently the only X-ray binaries known to exhibit narrow X-ray absorption lines were two superluminal jet sources and it had been suggested that these features are related to the jet formation mechanism. It now appears likely that ionized absorption features may be common characteristics of accreting systems with accretion disks.

Abstract: Type I X-ray bursts are very common in neutron star X-ray binaries, but no Type I burst has been seen in the dozen or so binaries in which the accreting compact star is too massive to be a neutron star and therefore is identified as a black hole candidate. We have carried out a global linear stability analysis of the accumulating fuel on the surface of a compact star to identify the conditions under which thermonuclear bursts are triggered. Our analysis, which improves on previous calculations, reproduces the gross observational trends of bursts in neutron star systems. It further shows that, if black hole candidates have surfaces, they would very likely exhibit instabilities similar to those that lead to Type I bursts on neutron stars. The lack of bursts in black hole candidates is thus significant, and indicates that these objects have event horizons. We discuss possible caveats to this conclusion.

Abstract: We present a compilation of spectroscopic observations of the sgB[e] star CI Cam, the optical counterpart of XTE J0421+560. This includes data from before, during, and after its 1998 outburst, with quantitative results spanning 37 years. The object shows a rich emission line spectrum originating from circumstellar material, rendering it difficult to determine the nature of either star involved or the cause of the outburst. We collate all available pre-outburst data to determine the state of the system before this occurred and provide a baseline for comparison with outburst and post-outburst data. During the outburst all lines become stronger, and hydrogen and helium lines become significantly broader and asymmetric. After the outburst, spectral changes persist for at least three years, with FeII and [N II] lines still a factor of ~ 2 above the pre-outburst level and He I, HeII, and N II lines suppressed by a factor of 2–10. We find that the spectral properties of CI Cam are similar to other sgB[e] stars and therefore suggest that the geometry of the circumstellar material is similar to that proposed for the other objects: a two component outflow, with a fast, hot, rarefied polar wind indistinguishable from that of a normal supergiant and a dense, cooler equatorial outflow with a much lower velocity. Based on a comparison of the properties of CI Cam with the other sgB[e] stars we suggest that CI Cam is among the hotter members of the class and is viewed nearly pole-on. The nature of the compact object and the mechanism for the outburst remain uncertain, although it is likely that the compact object is a black hole or neutron star, and that the outburst was precipitated by its passage through the equatorial material. We suggest that this prompted a burst of supercritical accretion resulting in ejection of much of the material, which was later seen as an expanding radio remnant. The enhanced outburst emission most likely originated either directly from this supercritical accretion, or from the interaction of the expanding remnant with the equatorial material, or from a combination of both mechanisms.

Abstract: The isolated neutron star RX J0806.4-4123 was observed with XMM-Newton in November 2000. The data from the three EPIC instruments allowed us (i) to derive an improved X-ray position to an accuracy of 2-3 00 , (ii) to accumulate the first medium-resolution soft X-ray spectra of high statistical quality and (iii) to find a candidate for the neutron star rotation period. Although this period of 11.3714 s is formally detected at a 3.5 level in the EPIC-pn data, the similar pulse profiles deduced from all three EPIC instruments increase the confidence that the period is real. The pulsed fraction of6% would then be the weakest X-ray flux modulation detected from dim isolated neutron stars. We fitted the X-ray spectra with blackbody and neutron star atmosphere models and discuss the results with respect to the brightness limit placed by optical images. The reduced size of the error circle on the X-ray position should allow deeper searches for an optical counterpart.

Abstract: Observations of the 297 s X-ray pulsar 1E1145.1-6141 with the Rossi X-Ray Timing Explorer have revealed its 14.4 d eccentric orbit around its B supergiant companion. The best-fit orbital elements are: P_orb = 14.365(2) d, a_x sin i=99.4(18) light-s, e=0.20(3). No eclipses are detected, indicating that the binary inclination is less than 55 deg.

Abstract: Using phase resolved spectroscopic observations obtained with the Ultraviolet and Visual Echelle Spectrograph on ESO's Kueyen Very Large Telescope supplemented by spectroscopic observations obtained with the Boller and Chivens Spectrograph on the Walter Baade Magellan telescope, we found sinusoidal radial-velocity variations with a semi-amplitude 327+-17 km/s. From previous observations and from the fact that the epoch of minimum velocity arrived early with respect to the epoch calculated from pulse timing we know that the companion star is suffering from irradiation. Since we most likely observed primarily the side of the companion star facing the observer at phase ~0.75 the velocity quoted above is not the true radial velocity semi-amplitude of the companion star. Assuming a uniform contribution to the line profile from this hemisphere yields a radial velocity semi-amplitude of 280+-26 km/s for a systemic velocity of 54+-24 km/s; if the contribution is instead weighted somewhat more towards the side of the companion facing the X-ray source then the true semi-amplitude is larger than this value. Together with the well constrained inclination (81

Abstract: Employing the most recent parametrization of the baryon-baryon interaction of the Nijmegen group, we investigate, in the framework of the Brueckner--Bethe--Goldstone many-body theory at zero temperature, the influence of neutrino trapping on the composition, equation of state, and structure of neutron stars, relevant to describe the physical conditions of a neutron star immediately after birth (protoneutron star). We find that the presence of neutrinos changes significantly the composition of matter delaying the appearance of hyperons and making the equation of state stiffer. We explore the consequences of neutrino trapping on the early evolution of a neutron star and on the nature of the final compact remnant left by the supernova explosion.

Abstract: We report on several pointed Rossi X-ray Timing Explorer observations of the enigmatic low-mass X-ray binary (LMXB) 4U 1957+11 at different X-ray luminosities. The luminosity of the source varied by more than a factor of 4 on time-scales of months to years. The spectrum of the source tends to become harder when its luminosity increases. Only very weak (1–2 per cent rms amplitude, 0.001–10 Hz, 2–60 keV) rapid X-ray variability was observed during the observations. A comparison of the spectral and temporal behaviour of 4U 1957+11 with other X-ray binary systems, in particular LMC X-3, indicates that 4U 1957+11 is likely to be a persistent LMXB harbouring a black hole and it is persistently in the black hole high state. If confirmed, it would be the only such system known.

Abstract: We have obtained Hubble Space Telescope/Space Telescope Imaging Spectrograph low-resolution ultraviolet spectra of the X-ray pulsar 4U 1626-67 (=KZ TrA); 4U 1626-67 is unusual even among X-ray pulsars because of its ultrashort binary period (P = 41.4 minutes) and remarkably low mass function (≤1.3 × 10-6 M⊙). The far-UV spectrum was exposed for a total of 32 ks and has sufficient signal-to-noise ratio to reveal numerous broad emission and prominent narrower absorption lines. Most of the absorption lines are consistent in strength with a purely interstellar origin. However, there is evidence that both C I and C IV require additional absorbing gas local to the system. In emission, the usual prominent lines of N V and He II are absent, while both O IV and O V are relatively strong. We further identify a rarely seen feature at ~1660 A as the O III] multiplet. Our ultraviolet spectra therefore provide independent support for the recent suggestion that the mass donor is the chemically fractionated core of either a CONe or ONeMg white dwarf; this was put forward to explain the results of Chandra high-resolution X-ray spectroscopy. The velocity profiles of the ultraviolet lines are in all cases broad and/or flat topped or perhaps even double peaked for the highest ionization cases of O; in either case, the ultraviolet line profiles are in broad agreement with the Doppler pairs found in the X-ray spectra. Both the X-ray and far-UV lines are plausibly formed in (or in an corona just above) a Keplerian accretion disk; the combination of ultraviolet and X-ray spectral data may provide a rich data set for follow-on detailed models of the disk dynamics and ionization structure in this highly unusual low-mass X-ray pulsar system.

Abstract: We have obtained Hubble Space Telescope/STIS low-resolution ultraviolet spectra of the X-ray pulsar 4U 1626-67 (=KZ TrA); 4U 1626-67 is unusual even among X-ray pulsars due to its ultra-short binary period (P=41.4 min) and remarkably low mass-function (<1.3e-6 Msun). The far-UV spectrum was exposed for a total of 32ks and has sufficient signal-to-noise to reveal numerous broad emission and prominent narrower absorption lines. Most of the absorption lines are consistent in strength with a purely interstellar origin. However, there is evidence that both CI and CIV require additional absorbing gas local to the system. In emission, the usual prominent lines of NV and HeII are absent, whilst both OIV and OV are relatively strong. We further identify a rarely seen feature at ~1660A as the OIII] multiplet. Our ultraviolet spectra therefore provide independent support for the recent suggestion that the mass donor is the chemically fractionated core of either a C-O-Ne or O-Ne-Mg white dwarf; this was put forward to explain the results of Chandra high-resolution X-ray spectroscopy. The velocity profiles of the ultraviolet lines are in all cases broad and/or flat-topped, or perhaps even double-peaked for the highest ionization cases of O; in either case the ultraviolet line profiles are in broad agreement with the Doppler pairs found in the X-ray spectra. Both the X-ray and far-UV lines are plausibly formed in (or in an corona just above) a Keplerian accretion disc; the combination of ultraviolet and X-ray spectral data may provide a rich data set for follow-on detailed models of the disk dynamics and ionization structure in this highly unusual low-mass X-ray pulsar system.

Abstract: The Anomalous X-ray Pulsars (AXP) are a small group of pulsars characterized by periods of several seconds and by the absence of massive companion stars. Three of them are associated with supernova remnants. It is unclear whether they are accreting from very low mass companions or consist of isolated neutron stars. In the latter case, their rotational energy loss is too small to provide the observed luminosity, and other powering mechanisms must be invoked. The discovery of similar periodicities and spin-down values in the Soft Gamma-Ray-repeaters suggests that also the AXP could be strongly magnetized neutron stars in which the X-ray emission is powered by the dissipation of the magnetic energy. However, also models based on accretion onto isolated neutron stars have been proposed.

Abstract: We used optical spectroscopy of the neutron star X-ray transient XTE J2123-058 to measure the rotational broadening of the binary companion's stellar absorption lines and determined that the companion's projected rotational velocity is vsini = 121 (+21)(-29) km/s. This value is considerably larger than the measurements of vsini obtained previously for three other neutron star systems where the values are between 34 and 65 km/s. The larger value is likely due to the combination of high binary inclination and short (6 hr) orbital period for XTE J2123-058. Along with previously measured parameters, the rotational broadening measurement allowed us to determine the binary parameters, including the ratio of the neutron star mass to the companion mass, q = M_1/M_2 = 2.7 (+1.3)(-0.9), the neutron star mass, M_1 = 1.46 (+0.30)(-0.39) solar masses, and the companion mass, M_2 = 0.53 (+0.28)(-0.39) solar masses, assuming a Roche lobe filling companion synchronously rotating with the binary orbit. These values are consistent with a previous measurement where the H alpha emission line was used to determine the semiamplitude of the neutron star's radial velocity curve (K_1). It is a significant result that the two methods give consistent values. We also report the first measurement of the XTE J2123-058 companion's radius. Assuming synchronous rotation, we obtain R_2 = 0.62 (+0.11)(-0.15) solar radii, which is in-line with that expected for a late K-type star on or close to the main sequence. Finally, we report the first precise spectroscopic determination of the binary orbital period (P_orb = 21442.3 +/- 1.8 seconds).

Abstract: We have obtained spectroscopy with the Far Ultraviolet Spectroscopic Explorer (FUSE) of the supersoft X-ray binary RX J0513.9-6951 over a complete binary orbital cycle. The spectra show a hot continuum with extremely broad O VI emission and weak Lyman absorptions. He II emission is weak and narrow, while N III and C III emissions are undetected, although lines from these ions are prominent at optical wavelengths. The broad O VI emission and Lyman absorption show radial velocity curves that are approximately antiphased and have semiamplitudes of ~117 +- 40 and 54 +- 10 km/s, respectively. Narrow emissions from He II and O VI show small velocity variations with phasing different from the broad O VI, but consistent with the optical line peaks. We also measure considerable changes in the FUV continuum and O VI emission line flux. We discuss the possible causes of the measured variations and a tentative binary interpretation.

Abstract: We present the results of optical and near-IR spectroscopic and broadband multicolour photometric observations of the emission-line object V669 Cep. We nd evidence that it contains a hot, low luminosity, B4{ B6 star and a cool companion (most likely late-type giant). Signicant variations of the H line strength are detected on a timescale of months. The emission-line spectrum and strong IR-excess indicate a large amount of circumstellar gas and dust in the system. The spectral energy distribution in the near-IR region and the absence of late-type star features in the optical spectrum indicates that the cool star is heavily obscured by circumstellar dust, while the hot star is much less aected by reddening. The system is located at 1{1.5 kpc from the Sun in the local spiral arm. We suggest that V669 Cep is an evolved and probably mass exchanging binary system, a member of the group of Be stars with warm dust.