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1.
We present the results of the analysis of Rossi X-ray Timing Explorer ( RXTE ) observations of the new X-ray transient, SWIFT J1753.5−0127, during its outburst in 2005 July. The source was caught at the peak of the burst with a flux of 7.19e-09 erg s−1cm−2 in the 3–25 keV energy range and observed until it decreased by about a factor of 10. The photon index of the power-law component, which is dominant during the entire outburst, decreases from ∼1.76 to 1.6. However, towards the end of the observations the photon index is found to increase, indicating a softening of the spectra. The presence of an ultrasoft thermal component, during the bright phases of the burst, is clear from the fits to the data. The temperature associated with this thermal component is 0.4 keV. We believe that this thermal component could be due to the presence of an accretion disc. Assuming a distance of 8.5 kpc,   L X/ L Edd≃ 0.05  at the peak of the burst, for a black hole of mass  10 M⊙  . The source is found to be locked in the low/hard state during the entire outburst and likely falls in the category of the X-ray transients that are observed in the low/hard state throughout the outburst. We discuss the physical scenario of the low/hard state outburst for this source.  相似文献   

2.
The X-ray binary system GX 301−2 consists of a neutron star in an eccentric orbit accreting from the massive early-type star Wray 977. It has previously been shown that the X-ray orbital light curve is consistent with the existence of a gas stream flowing out from Wray 977 in addition to its strong stellar wind. Here, X-ray monitoring observations by the Rossi X-ray Timing Explorer ( RXTE )/All-Sky Monitor and pointed observations by the RXTE /Proportional Counter Array over the past decade are analysed. We analyse both the flux and column density dependence on orbital phase. The wind and stream dynamics are calculated for various system inclinations, companion rotation rates and wind velocities, as well as parametrized by the stream width and density. These calculations are used as inputs to determine both the expected accretion luminosity and the column density along the line-of-sight to the neutron star. The model luminosity and column density are compared to observed flux and column density versus orbital phase, to constrain the properties of the stellar wind and the gas stream. We find that the change between bright and medium intensity levels is primarily due to decreased mass loss in the stellar wind, but the change between medium and dim intensity levels is primarily due to decreased stream density. The mass-loss rate in the stream exceeds that in the stellar wind by a factor of ∼2.5. The quality of the model fits is better for lower inclinations, favouring a higher mass for Wray 977 in its allowed range of  40–60 M⊙  .  相似文献   

3.
We present X-ray/ γ -ray spectra of Cyg X-1 observed during the transition from the hard to the soft state and in the soft state by ASCA , RXTE and CGRO /OSSE in 1996 May and June. The spectra consist of a dominant soft component below ∼2 keV and a power-law-like continuum extending to at least ∼800 keV. We interpret them as emission from an optically thick, cold accretion disc and from an optically thin, non-thermal corona above the disc. A fraction f ≳0.5 of total available power is dissipated in the corona.
We model the soft component by multicolour blackbody disc emission taking into account the torque-free inner-boundary condition. If the disc extends down to the minimum stable orbit, the ASCA RXTE data yield the most probable black hole mass of M X≈10 M⊙ and an accretion rate,     , locating Cyg X-1 in the soft state in the upper part of the stable, gas-pressure-dominated, accretion-disc solution branch.
The spectrum of the corona is well modelled by repeated Compton scattering of seed photons from the disc off electrons with a hybrid, thermal/non-thermal distribution. The electron distribution can be characterized by a Maxwellian with an equilibrium temperature of kT e∼30–50 keV, a Thomson optical depth of τ ∼0.3 and a quasi-power-law tail. The compactness of the corona is 2≲ℓh≲7, and a presence of a significant population of electron–positron pairs is ruled out.
We find strong signatures of Compton reflection from a cold and ionized medium, presumably an accretion disc, with an apparent reflector solid angle, Ω/2π∼0.5–0.7. The reflected continuum is accompanied by a broad iron K α line.  相似文献   

4.
We have used the RXTE and INTEGRAL satellites simultaneously to observe the high-mass X-ray binary (HMXB) IGR J19140+0951. The spectra obtained in the 3–80 keV range have allowed us to perform a precise spectral analysis of the system along its binary orbit. The spectral evolution confirms the supergiant nature of the companion star and the neutron star nature of the compact object. Using a simple stellar wind model to describe the evolution of the photoelectric absorption, we were able to restrict the orbital inclination angle in the range 38°–75°. This analysis leads to a wind mass-loss rate from the companion star of  ∼5 × 10−8 M⊙ yr−1  , consistent with an OB I spectral type. We have detected a soft excess in at least four observations, for the first time for this source. Such soft excesses have been reported in several HMXBs in the past. We discuss the possible origin of this excess, and suggest, based on its spectral properties and occurrences around the superior conjunction, that it may be explained as the reprocessing of the X-ray emission originating from the neutron star by the surrounding ionized gas.  相似文献   

5.
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   

6.
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   

7.
We study in a systematic way the quality factor of the lower and upper kilohertz quasi-periodic oscillations (kHz QPOs) in a sample of low-luminosity neutron star X-ray binaries, showing both QPOs varying over a wide frequency range. The sample includes 4U 1636−536, 4U 1608−522, 4U 1735−44, 4U 1728−34, 4U 1820−303 and 4U 0614+09. We find that all sources except 4U 0614+09 show evidence of a drop in the quality factor of their lower kHz QPOs at high frequency. For 4U 0614+09 only the rising part of the quality factor versus frequency curve has been sampled so far. At the same time, in all sources but 4U 1728−34, the quality factor of the upper kHz QPO increases all the way to the highest detectable frequencies. We show that the high-frequency behaviours of both the lower and the upper kHz QPO quality factors are consistent with what is expected if the drop is produced by the approach of an active oscillating region to the innermost stable circular orbit: the existence of which is a key feature of general relativity in the strong field regime. Within this interpretation, our results imply gravitational masses around 2 M⊙ for the neutron stars in those systems.  相似文献   

8.
We present phase resolved optical photometry and spectroscopy of the accreting millisecond pulsar HETE J1900.1−2455. Our R -band light curves exhibit a sinusoidal modulation, at close to the orbital period, which we initially attributed to X-ray heating of the irradiated face of the secondary star. However, further analysis reveals that the source of the modulation is more likely due to superhumps caused by a precessing accretion disc. Doppler tomography of a broad Hα emission line reveals an emission ring, consistent with that expected from an accretion disc. Using the velocity of the emission ring as an estimate for the projected outer disc velocity, we constrain the maximum projected velocity of the secondary to be 200 km s−1, placing a lower limit of  0.05 M⊙  on the secondary mass. For a  1.4 M⊙  primary, this implies that the orbital inclination is low, ≲20°. Utilizing the observed relationship between the secondary mass and the orbital period in short-period cataclysmic variables, we estimate the secondary mass to be ∼0.085  M⊙  , which implies an upper limit of  ∼2.4 M⊙  for the primary mass.  相似文献   

9.
We present simultaneous high-resolution optical spectroscopy and X-ray data of the X-ray binary system GR Mus (XB 1254–690), obtained over a full range of orbital phases. The X-ray observations are used to re-establish the orbital ephemeris for this source. The optical data include the first spectroscopic detection of the donor star in this system through the use of the Doppler Tomography technique on the Bowen fluorescence blend (∼4630–4650 Å). In combination with an estimate for the orbital parameters of the compact object using the wings of the He  ii λ4686 emission line, dynamical mass constraints of  1.20 ≤ M X /M⊙≤ 2.64  for the neutron star and  0.45 ≤ M 2/M⊙≤ 0.85  for the companion are derived.  相似文献   

10.
A new means of incorporating radiative transfer into smoothed particle hydrodynamics (SPH) is introduced, which builds on the success of two previous methods – the polytropic cooling approximation as devised by Stamatellos et al. and flux-limited diffusion. This hybrid method preserves the strengths of its individual components, while removing the need for atmosphere matching or other boundary conditions to marry optically thick and optically thin regions. The code uses a non-trivial equation of state to calculate temperatures and opacities of SPH particles, which captures the effects of H2 dissociation, H0 ionization, He0 and He+ ionization, ice evaporation, dust sublimation, molecular absorption, bound-free and free–free transitions and electron scattering. The method is tested in several scenarios, including (i) the evolution of a  0.07 M⊙  protoplanetary disc surrounding a  0.5 M⊙  star; (ii) the collapse of a  1 M⊙  protostellar cloud and (iii) the thermal relaxation of temperature fluctuations in a static homogeneous sphere.  相似文献   

11.
We present X-ray, broad-band optical and low-frequency radio observations of the bright type IIP supernova SN 2004et. The Chandra X-ray Observatory observed the supernova at three epochs, and the optical coverage spans a period of ∼470 d since explosion. The X-ray emission softens with time, and we characterize the X-ray luminosity evolution as   L X∝ t −0.4  . We use the observed X-ray luminosity to estimate a mass-loss rate for the progenitor star of  ∼2 × 10−6 M⊙ yr−1  . The optical light curve shows a pronounced plateau lasting for about 110 d. Temporal evolution of photospheric radius and colour temperature during the plateau phase is determined by making blackbody fits. We estimate the ejected mass of 56Ni to be  0.06 ± 0.03 M⊙  . Using the expressions of Litvinova & Nadëzhin we estimate an explosion energy of  (0.98 ± 0.25) × 1051 erg  . We also present a single epoch radio observation of SN 2004et. We compare this with the predictions of the model proposed by Chevalier, Fransson & Nymark. These multiwavelength studies suggest a main-sequence progenitor mass of  ∼20 M⊙  for SN 2004et.  相似文献   

12.
We report on the discovery of a binary pulsar, PSR J1740−3052, during the Parkes multibeam survey. Timing observations of the 570-ms pulsar at Jodrell Bank and Parkes show that it is young, with a characteristic age of 350 kyr, and is in a 231-d, highly eccentric orbit with a companion whose mass exceeds 11 M⊙. An accurate position for the pulsar was obtained using the Australia Telescope Compact Array. Near-infrared 2.2-μm observations made with the telescopes at the Siding Spring observatory reveal a late-type star coincident with the pulsar position. However, we do not believe that this star is the companion of the pulsar, because a typical star of this spectral type and required mass would extend beyond the orbit of the pulsar. Furthermore, the measured advance of periastron of the pulsar suggests a more compact companion, for example, a main-sequence star with radius only a few times that of the Sun. Such a companion is also more consistent with the small dispersion measure variations seen near periastron. Although we cannot conclusively rule out a black hole companion, we believe that the companion is probably an early B star, making the system similar to the binary PSR J0045−7319.  相似文献   

13.
Recently discovered quasi-periodic oscillations in the X-ray brightness of low-mass X-ray binaries are used to derive constraints on the mass of the neutron star component and the equation of state of neutron star matter. The observations are compared with models of rapidly rotating neutron stars which are calculated by means of an exact numerical method in full relativity. For the equations of state we select a broad collection of models representing different assumptions about the many-body structure and the complexity of the composition of superdense matter. The mass constraints differ from their values in the approximate treatment by ∼10 per cent. Under the assumption that the maximum frequency of the quasi-periodic oscillations originates from the innermost stable orbit, the mass of the neutron star is in the range M ∼1.92–2.25 M⊙. The quasi-periodic oscillation in the Atoll-source 4U 1820−30 in particular is only consistent with equations of state that are rather stiff at high densities, which is explainable, so far, only with pure nucleonic/leptonic composition. This interpretation contradicts the hypothesis that the protoneutron star formed in SN 1987A collapsed to a black hole, since this would demand a maximum neutron star mass below 1.6 M⊙. The recently suggested identification of quasi-periodic oscillations with frequencies of about 10 Hz with the Lense–Thirring precession of the accretion disc is found to be inconsistent with the models studied in this work, unless it is assumed that the first overtone of the precession is observed.  相似文献   

14.
The accretion-induced neutron star (NS) magnetic field evolution is studied through considering the accretion flow to drag the field lines aside and dilute the polar-field strength, and as a result the equatorial field strength increases, which is buried inside the crust on account of the accretion-induced global compression of star crust. The main conclusions of model are as follows: (i) the polar field decays with increase in the accreted mass; (ii) the bottom magnetic field strength of about 108 G can occur when the NS magnetosphere radius approaches the star radius, and it depends on the accretion rate as     ; and (iii) the NS magnetosphere radius decreases with accretion until it reaches the star radius, and its evolution is little influenced by the initial field and the accretion rate after accreting  ∼0.01 M⊙  , which implies that the magnetosphere radii of NSs in low-mass X-ray binaries would be homogeneous if they accreted the comparable masses. As an extension, the physical effects of the possible strong magnetic zone in the X-ray NSs and recycled pulsars are discussed. Moreover, the strong magnetic fields in the binary pulsars PSR 1831−00 and PSR 1718−19 after accreting about  0.5 M⊙  in the binary-accretion phase,  8.7 × 1010  and  1.28 × 1012 G  , respectively, can be explained through considering the incomplete frozen flow in the polar zone. As an expectation of the model, the existence of the low magnetic field  (∼3 × 107 G)  NSs or millisecond pulsars is suggested.  相似文献   

15.
In this study we present and re-analyse the historical, 1889–1998, light curve (LC) of the eclipsing symbiotic binary AR Pav. For the first time, we show that the timing of mid-points of eclipses observed during a quiescent phase obeys a quadratic ephemeris, with an initial orbital period P 0=605.18 d and a rate of period change     .
We determined a distance to the system of 5.8±1.5 kpc, the mass ratio of the giant to the hot star, M g M h=0.4±0.1, the mass of the giant, M g=1.8+1/−0.5 M⊙ and its radius, R g=167±15 R⊙.
During quiescence, the LC has characteristic features similar to those observed in cataclysmic variables (CVs). It can be well reproduced by a model of a large accretion disc surrounding the hot star. However, it is probable that the geometry of the transferred material in the Roche lobe of the accretor in AR Pav is different from that of CVs.
During active phases the shape of the LC changes considerably. A complex wave-like variation developed as a function of the orbital phase with an amplitude of ∼1 mag. It is interpreted in terms of a collisionally heated emission region located on the giant surface and arising from the hot star eruption.  相似文献   

16.
There is an apparent dichotomy between the metal-poor  ([Fe/H]≤−2)  yet carbon-normal giants and their carbon-rich counterparts. The former undergo significant depletion of carbon on the red giant branch after they have undergone first dredge-up, whereas the latter do not appear to experience significant depletion. We investigate this in the context that the extra mixing occurs via the thermohaline instability that arises due to the burning of  3He  . We present the evolution of [C/Fe], [N/Fe] and  12C/13C  for three models: a carbon-normal metal-poor star, and two stars that have accreted material from a  1.5 M⊙  AGB companion, one having received  0.01 M⊙  of material and the other having received  0.1 M⊙  . We find the behaviour of the carbon-normal metal-poor stars is well reproduced by this mechanism. In addition, our models also show that the efficiency of carbon-depletion is significantly reduced in carbon-rich stars. This extra-mixing mechanism is able to reproduce the observed properties of both carbon-normal and carbon-rich stars.  相似文献   

17.
We use deep Hubble Space Telescope photometry of the rich, young (∼20- to 45-Myr old) star cluster NGC 1818 in the Large Magellanic Cloud to derive its stellar mass function (MF) down to  ∼0.15 M⊙  . This represents the deepest robust MF thus far obtained for a stellar system in an extragalactic, low-metallicity  ([Fe/H]≃−0.4 dex)  environment. Combining our results with the published MF for masses above  1.0 M⊙  , we obtain a complete present-day MF. This is a good representation of the cluster's initial MF (IMF), particularly at low masses, because our observations are centred on the cluster's uncrowded half-mass radius. Therefore, stellar and dynamical evolution of the cluster will not have affected the low-mass stars significantly. The NGC 1818 IMF is well described by both a lognormal and a broken power-law distribution with slopes of  Γ= 0.46 ± 0.10  and  Γ≃−1.35  (Salpeter-like) for masses in the range from 0.15 to  0.8 M⊙  and greater than  0.8 M⊙  , respectively. Within the uncertainties, the NGC 1818 IMF is fully consistent with both the Kroupa solar neighbourhood and the Chabrier lognormal mass distributions.  相似文献   

18.
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   

19.
Phase-resolved medium-resolution VLT spectroscopy of the low-mass X-ray binary GX 9+9 has revealed narrow C  iii emission lines that move in phase relative to our new estimate of the ephemeris, and show a velocity amplitude of 230 ± 35 km s−1. We identify the origin of these lines as coming from the surface of the donor star, thereby providing the first estimate of the mass function of   f ( M 1) ≥ 0.22 M⊙  . Rotational broadening estimates together with assumptions for the mass donor give  0.07 ≤ q ≤ 0.35  and  182 ≤ K 2≤ 406 km s−1  . Despite a low-mass ratio, there is no evidence for a superhump in our data set. Doppler maps of GX 9+9 show the presence of a stream overflow, either in the form of material flowing downward along the accretion disc rim or in a similar fashion as occurs in high mass transfer rate cataclysmic variables known as the SW Sex stars. Finally, we note that the Bowen region in GX 9+9 is dominated by C  iii instead of N  iii emission as has been the case for most other X-ray binaries.  相似文献   

20.
We present a detailed calculation of the evolution of low-mass (<0.25 M⊙) helium white dwarfs. These white dwarfs (the optical companions to binary millisecond pulsars) are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with mass between 0.01 and 0.06 M⊙. As a result of mixing between the core and outer envelope, the surface hydrogen content ( X surf) is 0.5–0.35 , depending on the initial value of the heavy element Z and the initial secondary mass. We found that the majority of our computed models experience one or two hydrogen shell flashes. We found that the mass of the helium dwarf in which the hydrogen shell flash occurs depends on the chemical composition. The minimum helium white dwarf mass in which a hydrogen flash takes place is 0.213 M⊙ ( Z =0.003), 0.198 M⊙ ( Z =0.01), 0.192 M⊙ ( Z =0.02) or 0.183 M⊙ ( Z =0.03). The duration of the flashes (independent of chemical composition) is between a few ×106 and a few ×107 yr. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. We show that the transition from a cooling white dwarf with a temporarily stable hydrogen-burning shell to a cooling white dwarf in which almost all residual hydrogen is lost in a few thermal flashes (via Roche lobe overflow) occurs between 0.183 and 0.213 M⊙ (depending on the heavy element value).  相似文献   

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