02 04 0 v 1 6 F eb 1 99 5 Grand Unified Model of Accretion Disks : The Sub - Keplerian Paradigm a

In the 1970s, the standard accretion disk models were constructed to largely explain observations from the binary systems. In these systems, angular momentum supplied at the outer edge is necessarily Keplerian. Viscosity drives the inflow by removing angular momentum outwards and keeping the entire disk Keplerian in the process. Since then these binary disk models are being used also to explain big blue bump observed in the continuum of active galaxies and quasars and with some success. Occasionally, one invokes additional components along with the standard thin disks, such as corona, warm absorbers, etc. in order to explain continuum as well as variable components of X-rays and γ-rays. There are several recent observations of almost zero time-lag correlated variabilities between X-rays and optical which cannot be explained by simple Keplerian disk models. Temporal variation of line profiles from objects, such as, ARP102B and 3C390.3 is impossible to explain by using axisymmetric disk models. Recent HST observation of M87 shows clear evidence of non-axisymmetry in the ionized disk around the black hole and the association of the spiral shock in NGC4258 with the accretion disk cannot be ruled out. These examples suggest the existence of large scale spiral shock waves in both M87 and NGC 4258. It is also generally believed that the radiation from our own galactic center is most likely coming from a low efficiency, quasi-spherical, accretion flow. That the disks need not be of ‘standard’ type was sensed by theoreticians even in late ’70s and throughout the ’80s. Thick accretion disk, transonic accretion disk and slim accretion disk models (For a general discussion and references, see Chakrabarti) came about. In thick disks, angular momentum is assumed to be almost constant but the radial motion is ignored. In transonic disks, the flow is thin but the radial motion is included. In slim disks, matter is allowed to pass through the inner sonic point, thus improving on the standard disk model. However, in all these models, unsuccessful attempts were made to match the flow with Keplerian disks at some distance and the disk structure depended strongly upon the matching radius and other parameters invoked. This intrinsic problem with these theoretical models, as well as problems in explaining a large number observations with Keplerian disks, particularly when applied to active galaxies, disappear with the realization that the accretion disks in active galaxies need not be Keplerian anywhere including the outer boundary! The outer boundary condition here is completely different from that of the binary systems, since matter is largely supplied by winds from