Neutron stars isolated

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. 

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. 

Table 1. Local ( ) < I kpc) population of young (age < 4.25 Myrs) isolated neutron stars...

Bignami, G.F., Caraveo, P.A., de Luca, A., Mereghetti, S. (2003), The magnetic field of an isolated neutron star from X-ray cyclotron absorption lines , Nature 423, 725. 

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

Haberl, F., Schwope, A.D., Hambaryan, V., Hasinger, G., Motch, C. (2003), A broad absorption feature in the X-ray spectrum of the isolated neutron star RBS1223 (1RXS J130848.6+212708) , A A403, L19. 

Motch, C. Zavlin, V. Haberl, F. (2003), The proper motion and energy distribution of the isolated neutron star RX J0720.4-3125 , A A 408, 323. 

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. 

Pons, J.A. et al. (2001), Toward a mass and radius determination of the nearby isolated neutron star RX J185635-3754 , ApJ, 564, 981. 

Popov, S.B., Treves, A., Turolla, R. (2003b). Radioquiet isolated neutron stars old and young, nearby and far away, dim and very dim , in Proc. of the 4th AGILE workshop (in press), [astro-ph/0310416]. 

In Fig. 6, we report the radius an the mass of the compact star RX J1856.5-3754 inferred by Walter Lattimer (2002) (see also Kaplan et al. 2002) from the fit of the full spectral energy distribution for this isolated radio-quite neutron star , after a revised parallax determination (Kaplan et al. 2002) which implies a distance to the source of 117 12 pc. Comparing the mass-radius box for RX J1856.5-3754 reported in Fig. 6 with the theoretical determination of the MR relation for different equations of state, one concludes that RX J1856.5-3754 could be (see e.g. Fig. 2 in Walter Lattimer, 2002) either an hadronic star or an hybrid or strange star (see also Drake et al. 2002). 

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. 

Figure 2. Left GW frequencies depending on the gravitational mass in isolation at the ISCO. Right compactness of an isolated neutron star with respect to the gravitational mass.

Rutledge, R.E., Fox, D.W., Bogosavljevic, M., Mahabal, A. (2003). A limit on the number of isolated neutron stars detected in the ROSAT Bright Source Catalogue , astro-ph/0302107. 

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