Thin accretion disks around neutron and quark stars

There are several proposed mechanisms for the formation of quark stars. Quark stars are expected to form during the collapse of the core of a massive star, after the supernova explosion, as a result of a first or second order phase transition, resulting in deconfined quark matter (Dai et al. 1995). The protoneutron star core or the neutron star core is a favorable environment for the conversion of ordinary matter to strange quark matter (Cheng et al. 1998b, Chan et al. 2009). Another possibility is that some neutron stars in low-mass X-ray binaries can accrete sufficient mass to undergo a phase transition to become strange stars (Cheng & Dai 1996). This mechanism has also been proposed as a source of radiation emission for cosmological γ-ray bursts (Cheng & Dai 1998).

Based on numerical integration of the general relativistic hydrostatic equilibrium equations a complete description of the basic astrophysical properties (mass, radius, eccentricity, Keplerian frequency etc.) of both static and rotating strange stars has been obtained, for different values of the bag constant and for different equations of state of the strange star (Witten 1984, Haensel et al. 1986, Gondek-Rosinska et al. 2000, Dey et al. 1998, Harko & Cheng 2002). Rotational proper-ties can discriminate between neutron and quark stars. Strange stars can reach much shorter periods than neutron stars, of the order of 0.5 ms (Cheng et al. 1998a). r-mode instabilities in rapidly rotating strange stars lead to specific signatures in the evolution of pulsars with periods below 2.5 ms, and some data on pulsar properties are consistent with this assumption (Madsen 2000). Strange stars could have a radius significantly less than that of neutron stars (Cheng et al. 1998a).

Photon emissivity is the basic parameter for determining macroscopic properties of stellar type objects. Because of the very high plasma frequency ω p near the strange matter edge, photon emissivity of strange matter is very low (Alcock et al. 1986). The spectrum of equilibrium photons is very hard, with ω > 20 MeV. The problem of the soft photon emissivity of quark matter at the surface of strange stars has also been considered (Cheng & Harko 2003, Harko & Cheng 2005). By taking into account the Landau-Pomeranchuk-Migdal effect and the absorption of the radiation in the external electron layer, the emissivity of the quark matter can be six orders of magnitude lower than the equilibrium black body radiation.

The Coulomb barrier at the quark surface of a hot strange star may also be a powerful source of e + e -pairs, which are created in the extremely strong electric field of the barrier. At surface temperatures of around 10 11 K, the luminosity of the outflowing plasma may be of the order ∼ 10 51 ergs -1 (Usov 1998a, Usov 1998b, Harko & Cheng 2006). Moreover, for about one day for normal quark matter and for up to a hundred years for superconducting quark matter, the thermal luminosity from the star’s surface, due to both photon emission and e + e -pair pro-duction, may be orders of magnitude higher than the Eddington limit (Page & Usov 2002).

However, despite these very specific signatures for quark stars, a definite method for discriminating them with respect to the neutron stars is still missing.

It is generally expected that most of the astrophysical objects grow substantially in mass via accretion. Accretion discs are flattened astronomical objects made of rapidly rotating gas which slowly spirals onto a central gravitating body, with its gravitational energy degraded to heat. A fraction of the heat converts into radiation, which partially escapes, and cools down the accretion disc. The only information that we have about accretion disk physics comes from this radiation, when it reaches radio, optical and X-ray telescopes, allowing astronomers to analyze its electromagnetic spectrum, and its time variability. The efficient cooling via the radiation over the disk surface prevents the disk from cumulating the heat generated by stresses and dynamical friction. In turn, this equilibrium causes the disk to stabilize its thin vertical size. The thin disk has an inner edge at the marginally stable orbit of the compact object potential, and the accreting plasma has a Keplerian motion in higher orbits (Page & Thorne 1974, Thorne 1974).

Recent astrophysical observations have shown that around many compact objects, as well as around black hole candidates, there are gas clouds surrounding the c

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