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تسجيل مستخدم جديدThe spotty donor star in the X-ray transient Cen X-4
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T. Shahbaz
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We accurately determine the fundamental system parameters of the neutron-star X-ray transient Cen X-4 solely using phase-resolved high-resolution UVES spectroscopy. We first determine the radial-velocity curve of the secondary star and then model the shape of the phase-resolved absorption line profiles using an X-ray binary model. The model computes the exact rotationally broadened phase-resolved spectrum and does not depend on assumptions about the rotation profile, limb-darkening coefficients and the effects of contamination from an accretion disk. We determine the secondary star-to-neutron star binary mass ratio to be 0.1755+/-0.0025, which is an order of magnitude more accurate than previous estimates. We also constrain the inclination angle to be 32 (+8; -2) degrees, Combining these values with the results of the radial velocity study gives a neutron star mass of 1.94 (+0.37; -0.85) Msun consistent with previous estimates. Finally, we perform the first Roche tomography reconstruction of the secondary star in an X-ray binary. The tomogram reveals surface inhomogeneities that are due to the presence of cool starspots. A large cool polar spot, similar to that seen in Doppler images of rapidly-rotating isolated stars is present on the Northern hemisphere of the K7 secondary star and we estimate that about 4 per cent of the total surface area of the donor star is covered with spots. This evidence for starspots supports the idea that magnetic braking plays an important role in the evolution of low-mass X-ray binaries.
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We analyze the ASCA spectrum of the Cen X-3 X-ray binary system in eclipse using atomic models appropriate to recombination-dominated level population kinetics in an overionized plasma. In order to estimate the wind characteristics, we first fit the
eclipse spectrum to a single-zone photoionized plasma model. We then fit spectra from a range of orbital phases using global models of photoionized winds from the companion star and the accretion disk that account for the continuous distribution of density and ionization state. We find that the spectrum can be reproduced by a density distribution of the form derived by Castor, Abbot, & Klein (1975) for radiation-driven winds with with the value of the mass-loss rate divided by the terminal velocity consistent with values for isolated stars of the same stellar type. This is surprising because the neutron star is very luminous (~10^38 erg/s) and the X-rays from the neutron star should ionize the wind and destroy the ions that provide the opacity for the radiation-driven wind. Using the same functional form for the density profile, we also fit the spectrum to a spherically symmetric wind centered on the neutron star, a configuration chosen to represent a disk wind. We argue that the relatively modest orbital variation of the discrete spectrum rules out a disk wind hypothesis.
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el. The X-ray light curve shows a very low level of emission, punctuated by a single, faint flare. Analysis of spectra measured during the flare and during quiescence is performed. The quiescent state shows a continuum spectrum well deconvolved to three spectral models: two components are from a collisionally-ionized plasma (with temperatures kT1=0.24 keV and kT2=0.76 keV), together with a power law model (photon index of 2.55), dominating above 2 keV. The X-ray flux emitted at this lowest level is 3.2$times10^{-13}$ erg/cm2/s (0.5-10 keV, corrected for the interstellar absorption), implying an X-ray luminosity of 1.85$times10^{32}$ erg/s (at 2.2 kpc). The two temperature collisionally-ionized plasma is intrinsic to the stellar wind of the donor star, while the power law can be interpreted as emission due to residual, low level accretion onto the compact object. The X-ray luminosity contributed by the power law component only, in the lowest state, is (4.8$pm{1.4})times10^{31}$ erg/s, the lowest quiescent luminosity detected from the compact object in an SFXT. Thanks to this very faint X-ray state caught by XMM-Newton, X-ray emission from the wind of the donor star LM Vel could be well-established and studied in detail for the first time, as well as a very low level of accretion onto the compact object. The residual accretion rate onto the compact object in IGR J08408-4503 can be interpreted as the Bohm diffusion of (possibly magnetized) plasma entering the neutron star magnetosphere at low Bondi capture rates from the supergiant donor wind at the quasi-spherical radiation-driven settling accretion stage.
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