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Astron. Astrophys. 323, 382-386 (1997)

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1. Introduction

Spherical flows have been studied in the past four decades rather extensively (Bondi, 1952; Shapiro, 1973; Shapiro & Salpeter, 1975; Park & Ostriker, 1989; see Chakrabarti, 1996a for a recent review). These zero angular momentum flows were not found to be very efficient radiators since they carry the total energy along with them, except at the boundary layer of objects with hard surfaces. Therefore, it was difficult to explain, for example, the quasar luminosity and the soft states of galactic and extra-galactic black hole candidates using spherical flow models around black holes. The Keplerian accretion disks (Shakura & Sunyaev, 1973), on the other hand, are very efficient radiators. They locally dissipate the entire amount of heat that is generated by viscosity. Generalized adiabatic Bondi flows which contained angular momentum (Liang & Thompson, 1980; Chakrabarti, 1989; Chakrabarti 1990a, hereafter C90a) could also be highly inefficient emitters. However, such flows with viscosity and cooling effects which may join a Keplerian or sub-Keplerian flow far away can have intermediate efficiencies (C90a; Chakrabarti, 1990b hereafter C90b; Chakrabarti 1996a, b, hereafter C96ab). In a black hole accretion, they partly carry along energy through the event horizon (see, Fig. 8a of C90a) and partly radiate the dissipated heat and thus successfully bridge the gap between a classical Bondi flow and a classical Shakura-Sunyaev type Keplerian disk. Shocks may also form just outside the horizon if the flow can pass through two sonic points. In a neutron star accretion, the flow dissipates the energy carried along in the boundary layer just outside the neutron star surface (Chakrabarti 1989; C96b). These disks which may contain both Keplerian and sub-Keplerian flows can explain most of the stationary and non-stationary spectral features of black hole and neutron star candidates as explained in detail in Chakrabarti & Titarchuk (1995, hereafter CT95); C96b; Molteni et al. (1996); Ryu et al. (1997).

In the present paper, we take a closer look at spherical accretion flows, not only on black holes (BH), but also on other compact objects, such as weakly magnetized and slowly rotating neutron stars (NS) and naked singularities (NSing). Naked singularities are not as widely considered to be astrophysically relevant as black holes and neutron stars, though recently their theoretical existence have been discussed, and serious studies of using them to explain astrophysical phenomena are being considered (Penrose, 1974; Ori & Piran 1990; Nakamura et al. 1993; Chakrabarti & Joshi, 1994). The adiabatic accretion flow onto black holes and naked singularities has to pass through a sonic surface and enters the horizon or the singularity supersonically, while on an unmagnetized neutron stars, the flow may or may not have standing shock waves as it rapidly lands on the surface. We discuss the fundamental differences between these solutions and present some possible spectral signatures by which these objects could be distinguished. This review of the subject was essential in view of the recognition that the quasi-spherical sub-Keplerian flows could be useful in explaining the quiescent states of black holes (Ebisawa et al, 1996), hard states of black holes and neutron stars and more importantly weak hard tails seen in the soft states of black hole candidates (CT95).

In the next Section, we present the basic equations, and the procedure to solve them. In § 3, we present the solutions of these equations. In § 4, we briefly discuss the spectral properties of these solutions. Finally, in § 5, we discuss importance of angular momentum and its effect on the results and make concluding remarks.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998