Dipartimento di Scienze Fisiche ed Astronomiche Universitá di Palermo, Italy

**Gábor Tóth**

Department of Atomic Physics, Eötvös Univ., Hungary

**Oleg A. Kuznetsov**

Keldysh Institute of Applied Mathematics, Moscow, Russia

*accepted by the Astrophysical Journal*

Earlier analytical studies and numerical simulations of time dependent axially symmetric flows onto black holes have shown that it is possible to produce stationary shock waves with a stable position both for ideal inviscid and for moderately viscous accretion disks.

We perform several two dimensional numerical simulations of accretion flows
in the equatorial plane to study shock stability against *
non-axisymmetric azimuthal perturbations*. We find a peculiar new result. A
very small perturbation seems to produce an instability as it crosses the
shock, but after some small oscillations, the shock wave suddenly transforms
into an asymmetric closed pattern, and it stabilizes with a finite radial
extent, despite the inflow and outflow boundary conditions are perfectly
symmetric.

The main characteristics of the final flow are: 1) The deformed shock rotates steadily without any damping. It is a permanent feature and the thermal energy content and the emitted energy vary periodically with time. 2) This behavior is also stable against further perturbations. 3) The average shock is still very strong and well defined, and its average radial distance is somewhat larger than that of the original axially symmetric circular shock. 4) Shocks obtained with larger angular momentum exhibit more frequencies and beating phenomena. 5) The oscillations occur in a wide range of parameters, so this new effect may have relevant observational consequences, like (quasi) periodic oscillations, for the accretion of matter onto black holes. Typical time scales for the periods are 0.01 and 1000 seconds for black holes with 10 and 10^6 solar mass, respectively.