Neutron stars accretion

The site of the r-process is also not clear, but it seems that the conditions needed to reproduce Solar-System r-process abundances may hold in the hot bubble caused by neutrino winds in the immediate surroundings of a nascent neutron star in the early stages of a supernova explosion (see Fig. 6.10). Circumstantial evidence from Galactic chemical evolution supports an origin in low-mass Type II supernovae, maybe around 10 M (Mathews, Bazan Cowan 1992 Pagel Tautvaisiene 1995). Another possibility is the neutrino-driven wind from a neutron star formed by the accretion-induced collapse of a white dwarf in a binary system (Woosley Baron 1992) leading to a silent supernova (Nomoto 1986). In stars with extreme metal-deficiency, the heavy elements sometimes display an abundance pattern characteristic of the r-process with little or no contribution from the s-process, and the... 

However, it is impossible to isolate the matter in the core of a neutron star for detailed study. It is thus necessary to identify observable aspects of neutron stars that can be, in some sense, mapped to the equation of state of high-density material. In this review we discuss various constraints on the equation of state from astronomical observations. We focus on observations of accreting binary systems. 

The short lifetimes of high-mass stars means that HMXBs are not thought to be long-lived, typically only a few million years. As a result, even at an Eddington accretion rate of M = 1018 g s 1, only a few hundredths of a solar mass is transferred to the neutron star. Therefore, the mass of a neutron star in an HMXB is expected to be close to its birth mass. In contrast, LMXBs can last for tens of millions of years and in principle accrete tenths of a solar mass. 

However, one must be careful because in an LMXB the optical emission from the accretion disk (whether in the outer, cool regions or as reprocessed X-ray emission) can outshine the companion by a large factor. This makes spectral lines difficult to measure and also complicates the ellipsoidal light curve technique. The ideal systems to study are therefore transient systems, which undergo periods of active mass transfer (often for a few weeks to a few months) before lapsing into quiescence, where there is little to no mass transfer. During quiescence, the companion is still distorted by the gravity of the neutron star, hence the flux variations still occur, but without any contamination by the accretion disk. There is a relatively new approach similar to this that... 

A more model-dependent way to constrain neutron star structure has to do with measurements of orbital frequencies in the accretion disk near the neutron star. Suppose that the frequency of some observed phenomenon could be identified with an orbital frequency vor >, and that this phenomenon lasted many cycles. The orbital radius f 0rb is clearly greater than the stellar radius R. In... 

In just the last year, several observations have allowed new constraints on neutron star structure (1) a mass of M > 1.6 M (at >95% confidence) has been measured for a neutron star (Nice et al. 2003) (2) the first surface redshift, 2 = 0.35, has been detected from a neutron star (Cottam et al. 2002), and (3) the first non-sinusoidal light curve has been measured from an accreting millisecond neutron star (Strohmayer et al 2003). These observations, along with many previously available data, hold out good hope for strong constraints on high-density matter in the next few years. 

Abstract We briefly review selected results in astrophysics of neutron stars (NSs) obtained during the last two years, focusing on isolated radioquiet objects. We discuss in some detail the population synthesis of close-by isolated NSs (INSs), the spectra of INSs, the detection of spectral features in these sources (including cyclotron features), and the recent results on velocity distribution of NSs and accretion onto INSs from the interstellar medium. 

Alpar, A. (2003), Accretion models for young neutron stars , in Pulsars, AXPs and SGRs observed with BeppoSAX and other observatories . Edited by G. Cusumano, E. Massaro, T. Mineo. p. 197 [astro-ph/0306179]. 

Blaes, O., Blandford, R.D., Rajagopal, M. (1995), Accreting isolated neutron stars. III. Preheating of infalling gas and cometary HII regions , ApJ 454, 370. 

Perna, R., Narayan, R., Rybicki, G., Stella, L., Treves, A. (2003), Bondi accretion and the problem of the missing isolated neutron stars , ApJ 594, 936. 

The interaction of mass transfer, gravitational wave backreaction and the reaction of the neutron star radius to the mass loss leads to a very complicated accretion dynamics in a neutron star black hole system. We find in all of our simulations (apart from an extreme test case with mass ratio q = 0.93, i.e. a... 

The influence of the appearance of such exotic states like quarks in stellar matter is topic of the study of quasi-stationary simulations of the evolution of isolated compact stars [15, 12, 7, 23] and accreting systems, where one companion is a superdense compact object [9,27], In this work we investigate the observability of the hadron-quark deconfinement phase transition in the dynamical evolution of a neutron star merger. 

SUMMARY We investigate the unsteady motion of mass reservoir formed by the accretion onto the magnetosphere around rotating neutron stars. The unsteady motion of the reservoir induces secondary accretion to neutron star by R-T instability. The nonperiodic or quasiperiodic phenomena of X-ray bursters seems to be related to this property of mass reservoir on the magnetosphere. We classify the typical dynamical state of the reservoir into three types with the parameters which are accretion rate M and angular velocity of neutron star ft. They are nonsequential oscillation sequential periodic (quasi-periodic) oscillation, and chaotic oscillation states. 

Physical Model We propose a symplified model for the non-linear-like phenomena in X-ray sources, considering the properties of Rayleigh-Taylor instability on the magnetopause which is formed by the accreting matter to the neutron star, and then obtain the motion of this magnetopause. ... 

Abstract We have simulated the helium shell flashes on an 1.3 M0 white dwarf and estimated the amount of mass loss. Our results may suggest a serious difficulty for the theories of the formation of Type I supernovae and of the accretion-induced formation of neutron stars because a significant amount of envelope mass is ejected during a helium shell flash. 

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