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1.
We present phase resolved optical spectroscopy and X-ray timing of the neutron star X-ray binary EXO 0748−676 after the source returned to quiescence in the autumn of 2008. The X-ray light curve displays eclipses consistent in orbital period, orbital phase and duration with the predictions and measurements before the return to quiescence. Hα and He  i emission lines are present in the optical spectra and show the signature of the orbit of the binary companion, placing a lower limit on the radial velocity semi-amplitude of   K 2 > 405 km s−1  . Both the flux in the continuum and the emission lines show orbital modulations, indicating that we observe the hemisphere of the binary companion that is being irradiated by the neutron star. Effects due to this irradiation preclude a direct measurement of the radial velocity semi-amplitude of the binary companion; in fact, no stellar absorption lines are seen in the spectrum. Nevertheless, our observations place a stringent lower limit on the neutron star mass of   M 1 > 1.27 M⊙  . For the canonical neutron star mass of   M 1= 1.4 M⊙  , the mass ratio is constrained to  0.075 < q < 0.105  .  相似文献   

2.
We present high-resolution spectra (1.0 km s−1 FWHM) of the circumstellar Ca K line towards β Pictoris obtained on 1997 June 19 and 20. On the former date a strong absorption component was found at a heliocentric velocity of v helio = +8 km s−1, that is blueshifted by 14 km s−1 with respect to the main, 'stable', circumstellar component at v helio = +22 km s−1. To our knowledge, this is the first detection of a blueshifted Ca  ii component with a strength comparable to the more frequently observed redshifted events. On the following night a blueshifted component was still present, but its strength had decreased significantly; in addition, a strong redshifted component had appeared at v helio = +54 km s−1 which was absent on the previous night. The implications of these observations for the evaporating 'comet' model of spectral variations in the β Pictoris disc are discussed.  相似文献   

3.
The optical spectrum of the early B hydrogen-deficient star LS IV+6°2 has been analysed. With T eff = 31000 K, log  g  = 4.05, n H ∼ 10−4 and n c ∼ 0.1 (relative abundances by number), it is the hottest high-gravity extreme helium star (EHe) yet studied. The He  i spectrum shows all predicted permitted and forbidden transitions in absorption. LS IV+6°2 is a comparatively metal-rich EHe star; abundances of C, N, O, Ne, Mg, Al and P are typical of other EHes, whilst Si and S are somewhat deficient. With the surface parameters given, LS IV+6°2 lies close to the boundary of the helium star pulsation instability finger near T eff ∼ 27000 K. Available data indicate that the radial velocity is variable, but give no indication of amplitude or period.  相似文献   

4.
We present phase resolved optical spectroscopy and photometry of V4580 Sagittarii, the optical counterpart to the accretion powered millisecond pulsar SAX J1808.4−3658, obtained during the 2008 September/October outburst. Doppler tomography of the N  iii λ4640.64 Bowen blend emission line reveals a focused spot of emission at a location consistent with the secondary star. The velocity of this emission occurs at  324 ± 15 km s−1  ; applying a ' K -correction', we find the velocity of the secondary star projected on to the line of sight to be  370 ± 40 km s−1  . Based on existing pulse timing measurements, this constrains the mass ratio of the system to be  0.044+0.005−0.004  , and the mass function for the pulsar to be  0.44+0.16−0.13 M⊙  . Combining this mass function with various inclination estimates from other authors, we find no evidence to suggest that the neutron star in SAX J1808.4−3658 is more massive than the canonical value of  1.4 M⊙  . Our optical light curves exhibit a possible superhump modulation, expected for a system with such a low mass ratio. The equivalent width of the Ca  ii H and K interstellar absorption lines suggest that the distance to the source is ∼2.5 kpc. This is consistent with previous distance estimates based on type-I X-ray bursts which assume cosmic abundances of hydrogen, but lower than more recent estimates which assume helium-rich bursts.  相似文献   

5.
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 H α 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 high-velocity, 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 (+430 km s−1 after correcting for Galactic rotation) is the highest velocity known for an X-ray binary.  相似文献   

6.
We explore the hypothesis that some high-velocity runaway stars attain their peculiar velocities in the course of exchange encounters between hard massive binaries and a very massive star (either an ordinary  50–100 M⊙  star or a more massive one, formed through runaway mergers of ordinary stars in the core of a young massive star cluster). In this process, one of the binary components becomes gravitationally bound to the very massive star, while the second one is ejected, sometimes with a high speed. We performed three-body scattering experiments and found that early B-type stars (the progenitors of the majority of neutron stars) can be ejected with velocities of  ≳200–400 km s−1  (typical of pulsars), while  3–4 M⊙  stars can attain velocities of  ≳300–400 km s−1  (typical of the bound population of halo late B-type stars). We also found that the ejected stars can occasionally attain velocities exceeding the Milky Ways's escape velocity.  相似文献   

7.
A spectroscopic study of the binary Wolf–Rayet (WR)+O system WR 145 is performed, in order to determine the radial velocity orbits of the individual stars, the angle of orbital inclination and the stellar masses. The emission and absorption components are separated from the original spectra, allowing us to confirm the spectral classification WN 7o/CE of the hybrid WR component and to derive a spectral classification O7V((f)) for the O star. A study of the wind-collision properties is performed. Fitting the radial velocity and full width at half-maximum of the excess emission with Lührs' model results in an inclination angle of   i = 63°  , leading to estimates of the stellar masses:   M WR= 18 M⊙  and   M O= 31 M⊙  . Both of these masses are compatible with those of other stars of similar types.  相似文献   

8.
The first orbital solution for the spectroscopic pair in the multiple star system σ Scorpii, determined from measurements with the Sydney University Stellar Interferometer, is presented. The primary component is of β Cephei variable type and has been one of the most intensively studied examples of its class. The orbital solution, when combined with radial velocity results found in the literature, yields a distance of  174+23−18 pc  , which is consistent with, but more accurate than the Hipparcos value. For the primary component we determine  18.4 ± 5.4 M⊙, −4.12 ± 0.34 mag  and  12.7 ± 1.8 R⊙  for the mass, absolute visual magnitude and radius, respectively. A B1 dwarf spectral type and luminosity class for the secondary is proposed from the mass determination of  11.9 ± 3.1 M⊙  and the estimated system age of 10 Myr.  相似文献   

9.
In this paper we report on optical spectroscopic observations of the low-mass X-ray binary 2S 0921–630 obtained with the Very Large Telescope. We found sinusoidal radial velocity variations of the companion star with a semi-amplitude of  99.1 ± 3.1 km s−1  modulated on a period of 9.006 ± 0.007 d, consistent with the orbital period found previously for this source, and a systemic velocity of  44.4 ± 2.4 km s−1  . Owing to X-ray irradiation, the centre of light measured by the absorption lines from the companion star is probably shifted with respect to the centre of mass. We try to correct for this using the so-called K -correction. Conservatively applying the maximum correction possible and using the previously measured rotational velocity of the companion star, we find a lower limit to the mass of the compact object in 2S 0921–630 of   MX sin3 i > 1.90 ± 0.25 M⊙  (1σ errors). The inclination in this system is well constrained since partial eclipses have been observed in X-ray and optical bands. For inclinations in the range  60° < i < 90°  we find  1.90 ± 0.25 < MX < 2.9 ± 0.4 M⊙  . However, using this maximum K -correction we find that the ratio between the mass of the companion star and that of the compact object, q , is 1.32 ± 0.37, implying super-Eddington mass-transfer rates; however, evidence for that has not been found in 2S 0921–630. We conclude that the compact object in 2S 0921–630 is either a (massive) neutron star or a low-mass black hole.  相似文献   

10.
Spectra of the spreading layers on the neutron star surface are calculated on the basis of the Inogamov–Sunyaev model taking into account general relativity correction to the surface gravity and considering various chemical composition of the accreting matter. Local (at a given latitude) spectra are similar to the X-ray burst spectra and are described by a diluted blackbody. Total spreading layer spectra are integrated accounting for the light bending, gravitational redshift and the relativistic Doppler effect and aberration. They depend slightly on the inclination angle and on the luminosity. These spectra also can be fitted by a diluted blackbody with the colour temperature depending mainly on a neutron star compactness. Owing to the fact that the flux from the spreading layer is close to the critical Eddington, we can put constraints on a neutron star radius without the need to know precisely the emitting region area or the distance to the source. The boundary layer spectra observed in the luminous low-mass X-ray binaries, and described by a blackbody of colour temperature   T c= 2.4 ± 0.1 keV  , restrict the neutron star radii to   R = 14.8 ± 1.5 km  (for a  1.4-M⊙  star and solar composition of the accreting matter), which corresponds to the hard equation of state.  相似文献   

11.
We have studied the problem of an all-sky search in reference to a continuous gravitational wave particularly for such sources whose wave-form are known in advance. We have made an analysis of the number of templates required for matched-filter analysis as applicable to these sources. We have employed the concept of fitting factor (FF); treating the source location as the parameters of the signal manifold and have studied the matching of the signal with templates corresponding to different source locations. We have investigated the variation of FF with source location and have noticed a symmetry in template parameters,  θT  and  φT  . It has been found that the two different template values in source location, each in  θT  and  φT  , have the same FF. We have also computed the number of templates required assuming the noise power spectral density   S n ( f )  to be flat. It is observed that higher FF requires an exponentially increasing, large number of templates.  相似文献   

12.
Atmospheres and spectra of strongly magnetized neutron stars   总被引：1，自引：0，他引：1  
We construct atmosphere models for strongly magnetized neutron stars with surface fields     and effective temperatures     . The atmospheres directly determine the characteristics of thermal emission from isolated neutron stars, including radio pulsars, soft gamma-ray repeaters, and anomalous X-ray pulsars. In our models, the atmosphere is composed of pure hydrogen or helium and is assumed to be fully ionized. The radiative opacities include free–free absorption and scattering by both electrons and ions computed for the two photon polarization modes in the magnetized electron–ion plasma. Since the radiation emerges from deep layers in the atmosphere with     , plasma effects can significantly modify the photon opacities by changing the properties of the polarization modes. In the case where the magnetic field and the surface normal are parallel, we solve the full, angle-dependent, coupled radiative transfer equations for both polarization modes. We also construct atmosphere models for general field orientations based on the diffusion approximation of the transport equations and compare the results with models based on full radiative transport. In general, the emergent thermal radiation exhibits significant deviation from blackbody, with harder spectra at high energies. The spectra also show a broad feature     around the ion cyclotron resonance     , where Z and A are the atomic charge and atomic mass of the ion, respectively; this feature is particularly pronounced when     . Detection of the resonance feature would provide a direct measurement of the surface magnetic fields on magnetars.  相似文献   

13.
In an attempt to model the accretion on to a neutron star in low-mass X-ray binaries, we present 2D hydrodynamical models of the gas flow in close vicinity of the stellar surface. First, we consider a gas pressure-dominated case, assuming that the star is non-rotating. For the stellar mass we take   M star= 1.4 × 10−2 M⊙  and for the gas temperature   T = 5 × 106 K  . Our results are qualitatively different in the case of a realistic neutron star mass and a realistic gas temperature of T ≃ 108 K, when the radiation pressure dominates. We show that to get the stationary solution in a latter case, the star most probably has to rotate with the considerable velocity.  相似文献   

14.
Young, rapidly rotating neutron stars could accelerate ions from their surfaces to energies of ∼1 PeV. If protons reach such energies, they will produce pions (with low probability) through resonant scattering with X-rays from the stellar surface. The pions subsequently decay to produce muon neutrinos. Here, we calculate the energy spectrum of muon neutrinos, and estimate the event rates at Earth. The spectrum consists of a sharp rise at ∼50 TeV, corresponding to the onset of the resonance, above which the flux drops with neutrino energy as  ε−2ν  up to an upper energy cut-off that is determined by either kinematics or the maximum energy to which protons are accelerated. We estimate event rates as high as 10–100 km−2 yr−1 from some candidates, a flux that would be easily detected by IceCube. Lack of detection would allow constraints on the energetics of the poorly understood pulsar magnetosphere.  相似文献   

15.
We have defined a new Hβ absorption index definition,  Hβo  , which has been optimized as an age indicator for old and intermediate age stellar populations. Rather than using stellar spectra, we employed for this purpose a library of stellar population spectral energy distributions of different ages and metallicities at moderately high resolution.  Hβo  provides us with improved abilities for lifting the age–metallicity degeneracy affecting the standard Hβ Lick index definition. The new index, which has also been optimized against photon noise and velocity dispersion, is fully characterized with wavelength shift, spectrum shape, dust extinction and [α/Fe] abundance ratio effects.  Hβo  requires spectra of similar qualities as those commonly used for measuring the standard Hβ Lick index definition. Aiming at illustrating the use and capabilities of  Hβo  as an age indicator we apply it to Milky Way globular clusters and to a well selected sample of early-type galaxies covering a wide range in mass. The results shown here are particularly useful for applying this index and understand the involved uncertainties.  相似文献   

16.
The z  = 2.286  IRAS galaxy F10214 + 4724 remains one of the most luminous galaxies in the Universe, despite its gravitational lens magnification. We present optical and near-infrared spectra of F10214 + 4724, with clear evidence for three distinct components: lines of width ∼ 1000 km s−1 from a Seyfert 2 nucleus; ≲ 200 km s−1 lines which are likely to be associated with star formation; and a broad (∼ 4000 km s−1) C  III ] 1909-Å emission line which is blueshifted by ∼ 1000 km s−1 with respect to the Seyfert 2 lines. Our study of the Seyfert 2 component leads to several new results. (i) From the double-peaked structure in the Lyα line, and the lack of Lyβ, we argue that the Lyα photons have emerged through a neutral column of N H ∼ 2.5 × 1025 m−2, possibly located within the AGN narrow-line region, as proposed for several high-redshift radio galaxies. (ii) The resonant O  VI 1032, 1036-Å doublet (previously identified as Lyβ) is in an optically thick (1:1) ratio. At face value this implies an extreme density ( n e ∼ 1017 m−3) more typical of broad-line region clouds. However, we attribute this instead to the damping wings of Lyβ from the resonant absorption. (iii) A tentative detection of He  II 1086 suggests little extinction in the rest frame ultraviolet.  相似文献   

17.
The X-ray spectra of the magnetar candidates are customarily fitted with an empirical, two component model: an absorbed blackbody and a power-law. However, the physical interpretation of these two spectral components is rarely discussed. It has been recently proposed that the presence of a hot plasma in the magnetosphere of highly magnetized neutron stars might distort, through efficient resonant cyclotron scattering, the thermal emission from the neutron star surface, resulting in production of non-thermal spectra. Here we discuss the Resonant Cyclotron Scattering (RCS) model, and present its XSPEC implementation, as well as preliminary results of its application to Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters. N.R. is supported by an NWO Post-Doctoral Fellowship. S.Z. thanks the Particle Physics and Astronomy Research Council, PPARC, for support through an Advanced Fellowship.  相似文献   

18.
We report new radial velocity observations of V779 Cen, the optical companion to the X-ray pulsar Cen X-3. Two sets of results at two epochs yield very different radial velocity amplitudes. We demonstrate there are problems with the first set, not least that they are incompatible with the observed duration of the X-ray eclipse for all inclination angles. The anomalously high radial velocities are probably a result of changes in the outflow behaviour of the companion star. Although there is no reason to doubt the results from the second epoch when viewed in isolation, given the anomalous radial velocities of the first epoch, they must be treated with caution. Using these data, the semi-amplitude of the resulting radial velocity curve is found to be 24.4±4.1 km s−1. Given the accurately measured semi-amplitude of the orbit of the pulsar, 414.3±0.9 km s−1, the mass ratio of the system is 0.059±0.010. The inclination of the system is found to be 702±27, assuming that the optical component fills its Roche lobe, and that the system is in synchronous rotation. Hence the mass of the neutron star is 1.21±0.21 M⊙, and the mass of the optical companion is 20.5±0.7 M⊙. This is a smaller uncertainty than previously reported values, and is consistent with the canonical neutron star mass of 1.4 M⊙.
In addition, we use our spectra to determine the spectral class of V779 Cen to be O6-7II-III.  相似文献   

19.
We have measured the radial velocity variation of the white dwarf secondary in the binary system containing the millisecond pulsar PSR J 1012 + 5307. Combined with the orbital parameters of the radio pulsar, we infer a mass ratio q (≡ M 1/ M 2) = 10.5 ± 0.5. Our optical spectroscopy has also allowed us to determine the mass of the white dwarf companion by fitting the spectrum to a grid of DA model atmospheres: we estimate M 2 = 0.16 ± 0.02 M⊙, and hence the mass of the neutron star is 1.64 ± 0.22 M⊙, where the error is dominated by that of M 2. The orbital inclination is 52 ± 4°. For an initial neutron star mass of ∼ 1.4 M⊙, only a few tenths of a solar mass at most has been successfully accreted over the lifetime of the progenitor low-mass X-ray binary. If the initial mass of the secondary was ∼ 1 M⊙, our result suggests that the mass transfer may have been non-conservative.  相似文献   

20.
We present spectroscopic observations from the Spitzer Space Telescope of six carbon-rich asymptotic giant branch (AGB) stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C2H2 and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the Large Magellanic Cloud, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the dusty radiative transfer model and determine their dust mass-loss rates to be in the range  1.0–3.3 × 10−8 M⊙ yr−1  . The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars shows the strongest SiC feature in our present Local Group sample.  相似文献   

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