In this review we shall comment on a few recent findings which strengthen the view that the black hole accretion has substantial amount of sub-Keplerian component. The manifestation of this component is many fold. We discuss some of them. A general outline of the complex structure that emerges from the multitude of observations is presented. A detailed outline of what might be going on in outburst sources is also discussed. The relationship amount the spectral and timing properties can be best understood by this picture. We claim that the sub-Keplerian advective disk paradigm is a complete package. Since signatures of sub-Keplerian motion is already increasing in the literature, the whole package must be correct.
Deep Dive into Black Hole Accretion: From Quasars to Nano-Quasars.
In this review we shall comment on a few recent findings which strengthen the view that the black hole accretion has substantial amount of sub-Keplerian component. The manifestation of this component is many fold. We discuss some of them. A general outline of the complex structure that emerges from the multitude of observations is presented. A detailed outline of what might be going on in outburst sources is also discussed. The relationship amount the spectral and timing properties can be best understood by this picture. We claim that the sub-Keplerian advective disk paradigm is a complete package. Since signatures of sub-Keplerian motion is already increasing in the literature, the whole package must be correct.
The black hole accretion process is very simple in the sense that the flow must be transonic and thus must cross at least one sonic point. The inner sonic point r i is located between 1 to 6 Kerr radius r K = GM/c 2 (depending on the spin parameter) and it is as good as the point of no return as far as the flow is concerned. On the horizon, the in-fall velocity is the same as the velocity of light independent of the spin and mass parameter of the hole.
If the flow has ‘some’ angular momentum (λ ∼ λ ms to λ mb ) the number of physical sonic points would be more than one and for a significant region of the parameter space, the entropy at the inner sonic point r i is higher compared with that at the outer sonic point r o . In these cases, a stable or an oscillating shock at a mean radius r s is allowed at r i < r s < r o . The post-shock region between the shock and the inner sonic point r i is known as the CENtrifugal pressure dominated BOundary Layer or simply CENBOL where the flow is generally subsonic and hot. The difference in entropy at the sonic points is generated by the shock through turbulent processes in the immediate vicinity of the shock. The reason of the shock formation is simple: the centrifugal force λ 2 /r 3 increases more rapidly than the gravitational force ∼ 1/r 2 as the flow comes closer to the hole. This causes the matter to pile up at the centrifugal barrier and form a shock. For a steady shock, the well known Rankine-Hugoniot conditions, suitably modified to take care of the flow model, are satisfied (Chakrabarti, 1989ab). When the shock is nonsteady, the size of the CENBOL changes rapidly. The CENBOL region is the Compton cloud which inverse Comptonizes soft photons (be they from the synchrotron emission or from the Keplerian disk, if any). The spectral and timing properties of a black hole candidate (whether in a quasar with mass a few ×10 9 M ⊙ to a nano-quasar with mass a few ×M ⊙ ) is largely governed by the number density of electrons in the cloud (i.e., the size and the accretion rate Ṁh of the sub-Keplerian halo) and the relative supply of soft-photons from the synchrotron radiation and/or a Keplerian disk having a rate of Ṁd .
While the steady disk produces the observed spectrum, the oscillating shock produces the quasi-periodic oscillations in the photon counts. There could be at least three different ways that the CENBOL can oscillate: (a) when the cooling time-scale roughly agrees with the infall time-scale (Molteni, Sponholz & Chakrabarti, 1996) (b) when the outflow is important and takes away matter from the CENBOL (Ryu, Chakrabarti & Molteni, 1996) and (c) when the CENBOL is at the threshold of the optical depth τ ∼ 1, i.e., the net accretion rate ( Ṁd + Ṁh ∼ 1) (Chakrabarti, 1996, unpublished). In the last case, the CENBOL can oscillate crisscrossing τ ∼ 1. This is expected to be of high frequency. In general, one can have combinations of both (a) and (b) types as seen in Chakrabarti, Acharyya & Molteni (2004).
Since the inverse of the QPO frequency scales with the mass of the black hole mass, in quasars and milli-quasars the frequency would be very low (∼ nano to micro Hertz). Furthermore, in these objects the accretion rate is generally not high enough to produce (c) type QPOs, thus we do not see high-frequency QPO equivalent in these objects. The QPOs are likely to have more rms value for higher energy photons since the Comptonized photons in CENBOL participate in it. The definition of ‘high’ and ’low’ energy photons, of course, depend on the mass of the black hole. For quasars, the seed photons are in UV range while for nano-quasars they are in the soft X-ray range.
As in other compact objects where the boundary layers exist the jets are also produced from CENBOL, (Chakrabarti & Titarchuk, 1995;Chakrabarti et al. 1996;Chakrabarti, 1999). This is because the Keplerian disks are unable to produce accelerated and collimated jets. The outflow rates naturally depend on the shock strength (Chakrabarti, 1999;Das & Chakrabarti, 1999). For a no-shock case, the outflow rate is negligible as in the soft state, while in the very strong shock case, the outflow rate is low but steady as in the hard states. In the case of intermediate shock strengths, the outflow rate could be very high and depending on the optical depth of the base of the jet below the sonic radius, the jet may or may not be formed. These will be akin to burst-on and burst-off states (Chakrabarti & Nandi, 2000). When the optical depth of the base of the jet is high enough, there could be return flow bringing matter down to the disk, momentarily increasing the local accretion rate. This variation of the accretion rate causes the variation of the nature of the light curves as in GRS 1915+105. Here, the accretion rate is high enough so that the subsonic part of the jet can crisscross an optical depth of unity.
Out of these consistent scenarios, one grand picture of the accretion/outflow process emerges (Fig. 1)
…(Full text truncated)…
This content is AI-processed based on ArXiv data.
