Neutrino emissivity under neutral kaon condensation

Neutrino emissivity from neutron star matter with neutral kaon condensate is considered. It is shown that a new cooling channel is opened, and what is more, all previously known channels acquire the greater emissivity reaching the level of the direct URCA cycle in normal matter.Comment: 7 pages, 1 figure, to be published in Phys.Rev.C, revised version: the sectioning changed and more discussion adde

Stability of the mixed phase in hybrid stars

The transition from hadronic matter to quark matter in the core of neutron stars is likely to be associated with the appearance of a mixed phase, leading to a smooth variation of the star density profile. We discuss the results of a systematic study of the properties of the mixed pase upon Coulomb and surface effects. A state of the art nonrelativistic equation of state of nuclear matter has been used for the low density phase, while quark matter has been described within the MIT bag model, including the effect of perturbative one-gluon exchange interactions. The implications for neutron star structure are discussed.Comment: Submitted to Astronomy & Astrophysic

Witnessing the magnetospheric boundary at work in Vela X-1

We present an analysis of the Vela X-1's "off-states" based on Suzaku observations taken in June 2008. Defined as states in which the flux sudden decreases below the instrumental sensitivity, these "off-states" have been interpreted by several authors as the onset of the "propeller regime". For the first time ever, however, we find that the source does not turn off and, although the flux drops by a factor of 20 during the three recorded "off-states", pulsations are still observed. The spectrum and the pulse-profiles of the "off-states" are also presented. Eventually, we discuss our findings in framework of the "gated accretion" scenario and conclude that most likely the residual flux is due to the accretion of matter leaking through the magnetosphere by means of Kelvin-Helmholz instabilities (KHI).Comment: 4 pages 4 figures; accepted for publication in A&A letters (20/02/2011); v1.1 - some changes in language + added 3 reference

Transient jets in V617 Sagittarii

Some of the luminous Compact Binary Supersoft X-Ray sources (CBSS) have shown indications of jets, also called satellites due to their appearance in the spectra. In V Sagittae (V Sge) stars, the galactic counterparts of the CBSS, such features have been reported only for WX Cen. If V Sge stars are indeed the analogs of CBSS, one may expect transient jet emission in other objects of this class. Spectroscopic observations of the V Sge star V617 Sgr have been made, both at high photometric state and at decline. We show that V617 Sgr presents Halpha satellites at high photometric state with velocities of +/-780 km/s. This feature confirms, once more, the CBSS nature of the V Sge stars, however the details of the spectral characteristics also suggest that the two groups of stars display some intrinsic spectroscopic differences, which are likely to be due to a selection effect related to chemical abundance.Comment: Four pages, accepted to be published as a Letter in A&

Viscous timescale in high mass X-ray binaries

Context: Low mass X-ray binaries were found to have very low frequency breaks in their power density spectra below which the power density spectra are nearly in white noise structure and at higher frequencies they approximately follow the $P_\nu \propto \nu^{-1.3}$ law. Aims: In 2005, Gilfanov and Arefiev studied X-ray variability of persistent LMXBs in the $10^{-8}-10^{-1}$ Hz frequency range and To determine whether high mass X-ray binary power density spectra have similar properties and the findings for low mass X-ray binaries are also valid for high mass binaries, we analyzed the time series of high mass X-ray binary sources produced by All Sky Monitor of Rossi X-ray Timing Explorer. Method: We obtained the power density spectra of the high mass X-ray binaries using the cosine transform of autocorrelation function. Results: We identified break frequencies for seven sources, namely OAO 1657-415, SS 433, Vela X-1, SMC X-1, 4U 1700-377, GX 301-2, and LMC X-1. The normalized break frequencies with respect to the orbital frequency ($f_{break}/f_{orbit}$) for sources OAO 1657-415, SS 433, SMC X-1 and LMC X-1 are consistent with those of Roche lobe overflow systems. The other high mass X-ray binary systems, Vela X-1, GX 301-2, and 4U 1700-377, however, have larger break frequency ratios, $f_{break}/f_{orb}$, which are indicative of short viscous times. These are all wind-accreting sources and the stellar winds in the systems allow the formation of only short radius discs. Consequently, we qualitatively distinguished the Roche lobe overflow binaries from the wind accreting system by comparing their normalized break frequencies.Comment: 9 pages, 5 figures, accepted by A&

The radial velocity curve of HD153919 (4U1700-37) revisited

We have re-analysed all available high-resolution ultraviolet IUE spectra of the high-mass X-ray binary HD153919/4U1700-37. The radial velocity semi-amplitude of 20.6 +/- 1.0 km/s and orbital eccentricity of 0.22 +/- 0.04 agree very well with the values obtained earlier from optical spectra. They disagree with earlier conclusions for the same data reduced by Heap & Corcoran (1992) and by Stickland & Lloyd (1993).Comment: 6 pages, latex, figure included, Astronomy & Astrophysics, in pres

Neutron star properties and the equation of state of neutron-rich matter

We calculate total masses and radii of neutron stars (NS) for pure neutron matter and nuclear matter in beta-equilibrium. We apply a relativistic nuclear matter equation of state (EOS) derived from Dirac-Brueckner-Hartree-Fock (DBHF) calculations. We use realistic nucleon-nucleon (NN) interactions defined in the framework of the meson exchange potential models. Our results are compared with other theoretical predictions and recent observational data. Suggestions for further study are discussed.Comment: 13 pages, 9 figures, 1 table; Revised version, accepted for publication in Physical Review

Hadron Masses in Medium and Neutron Star Properties

We investigate the properties of the neutron star with relativistic mean field models. We incorporate in the quantum hadrodynamics and in the quark-meson coupling models a possible reduction of meson masses in nuclear matter. The equation of state for neutron star matter is obtained and is employed in Oppenheimer-Volkov equation to extract the maximum mass of the stable neutron star. We find that the equation of state, the composition and the properties of the neutron stars are sensitive to the values of the meson masses in medium.Comment: 18 pages, 5 figures and 2 tables. To be published in EPJ

X-Ray Wind Tomography of the highly absorbed HMXB IGR J17252-3616

Our goal is to understand the specificities of highly absorbed sgHMXB and in particular of the companion stellar wind, thought to be responsible for the strong absorption. We have monitored IGR J17252-3616, a highly absorbed system featuring eclipses, with XMM-Newton to study the vari- ability of the column density and of the Fe K{\alpha} emission line along the orbit and during the eclipses. We also built a 3D model of the structure of the stellar wind to reproduce the observed variability. We first derived a refined orbital solution built from INTEGRAL, RXTE and XMM data. The XMM monitoring campaign revealed significant variation of intrinsic absorbing column density along the orbit and of the Fe K{\alpha} line equivalent width around the eclipses. The origin of the soft X-ray absorption is modeled with an dense and extended hydrodynamical tail, trailing the neutron star. This structure extends along most of the orbit, indicating that the stellar wind is strongly disrupted by the neutron star. The variability of the absorbing column density suggests that the terminal velocity of the wind is smaller (~400 km/s) than observed in classical systems. This can also explain the much stronger density perturbation inferred from the observations. Most of the Fe K{\alpha} emission is generated in the most inner region of the hydrodynamical tail. This region, that extends over a few accretion radii, is ionized and does not contribute to the soft X-ray absorption. We have built a qualitative model of the stellar wind of IGR J17252-3616 that can represent the observations and suggest that highly absorbed systems have a lower wind velocity than classical sgHMXB. This proposal could be tested with de- tailed numerical simulations and high-resolution infrared/optical observations. If confirmed, it may turn out that half of the persistent sgHMXB have low stellar wind speeds.Comment: 9 pages, 8 figure