Daniel Proga

1) cases with simplified microphysics (applicable to low luminosity AGN and the galactic center, for instance):

Some of the most dramatic phenomena of astrophysics, such as quasars and powerful radio galaxies, are most likely powered by accretion onto supermassive black holes (SMBHs). Nevertheless, SMBHs appear to spend most of their time in a remarkably quiescent state. SMBHs are embedded in the relatively dense environments of galactic nuclei and it is natural to suppose that the gravity due to an SMBH will draw in matter at high rates, leading to a high system luminosity. However, this simple prediction often fails as many systems are much dimmer than one would expect.

One of the key effects that could reduce the mass accretion is gas rotation. Even a relatively slow rotation can result in the centrifugal force strong enough to affect the gas dynamics. In Proga & Begelman ( 2003a ), we considered inviscid accretion flows with a spherically symmetric density distribution at the outer boundary, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum. We studied accretion flows by means of numerical 2D, axisymmetric, hydrodynamical simulations. Later, in the follow-up papers we studied the effects of the adiabatic index, temperature, and 3D, in the hydrodynamical and magnetohydrodynamical limits:

-- HD in 2D - Moscibrodzka & Proga ( 2008);

-- HD in 3D - Janiuk, Proga, & Kurosaw ( 2008), and Janiuk, et al. ( 2009)

-- MHD in 2D - Proga & Begelman ( 2003b), Proga ( 2005), and Moscibrodzka & Proga ( 2009).

We also computed synthetic broad-band spectra based on Proga & Begelman's 2-D MHD simulations for direct comparison of the observations of Sgr A* (Moscibrodzka et al. 2007).

Our main result from the 2-D inviscid case, is that the properties of the accretion flow do not depend as much on the outer boundary conditions (i.e., the amount as well as distribution of the angular momentum) as on the geometry of the non-accreting matter. The material that has too much angular momentum to be accreted forms a thick torus near the equator (see movie 1 and its zoom-in version movie 2). Consequently, the geometry of the polar region, where material is accreted (the funnel), and the mass accretion rate through it are constrained by the size and shape of the torus. Our results show one way in which the mass accretion rate of slightly rotating gas can be significantly reduced compared to the accretion of non-rotating gas (i.e., the Bondi rate), and set the stage for calculations that will take into account the transport of angular momentum and energy.

Figure above shows some results from simulations of a magnetized, slowly rotating flow presented in Proga & Begelman ( 2003b). Specifically, it shows the density maps overplotted with the direction of the velocity field for four generic stages of the inner accertion flow close to the black hole: (i) the initial stage when both the equatorial torus and the polar funnel accrete (the top left panel; note a torus corona between the torus and the funnel), (ii) the stage when the accretion occurs only through the torus (the top righ panel), (iii) the stage when there is practically no accretion because the torus is pushed away by a very strong poloidal magnetic field forming a cylinder with the black holes inside it (the bottom left panel; this stage is recurrent yet very short-lived), and (iv) the accretion occurs through the torus and through one of the polar region where the low angular momentum material manages to get into the inner flow (the bottom right panel). The last stage is similar to the first stage but there are the following differences: during the fourth stage the polar funnel accretion is only on one side of the equator whereas during the first stage it in on both sides; the fourth stage lasts relatively long at the beginning of the simulations and even repeats whereas the fourth stage is recurrent and last a shorter period of time (much longer than the third stage though). This movie shows how the inner flow changes between the second, third, and fourth stages. Broad-band spectra predicted by these simulations are presented in Moscibrodzka et al. ( 2007).

2) cases with sophisticated microphysics of very high density and temperature fluids (applicable to gamma-ray bursts):

As a illustration of a universal nature of accretion onto a black hole, we can consider gamma-ray bursts (GRBs). In Proga, MacFadyen , Armitage , & Begelman ( 2003) we present results from axisymmetric, time-dependent magnetohydrodynamic (MHD) simulations of the collapsar model for gamma-ray bursts. We begin the simulations after the 1.7 solar mass iron core of a 25 solar mass presupernova star has collapsed and study the ensuing accretion of the 7 solar mass helium envelope onto the central black hole formed by the collapsed iron core. We consider a spherically symmetric progenitor model, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum and a weak radial magnetic field. Our MHD simulations include a realistic equation of state, neutrino cooling, photodisintegration of helium, and resistive heating. Our main conclusion is that, within the collapsar model, MHD effects alone are able to launch, accelerate and sustain a strong polar outflow. We also find that the outflow is Poynting flux-dominated, and note that this provides favorable initial conditions for the subsequent production of a baryon-poor fireball. These simulations are relevant to the early as well as late evolution of GRBs (e.g., Proga & Zhang 2006 , Janiuk & Proga 2008 , and Janiuk, Moderski, & Proga 2008 ).

Maps of logarithmic density and toroidal magnetic field overplotted with the direction of the poloidal velocity at t=0.2735 s. The length scale is in units of the BH radius (i.e., r'=r/R_S and z'=z/R_S)

As above, but a zoom-in version.

Movie1 ,movie2, and movie3 show the time evolution (of the density) in our entire computational domain (out to 1000 black hole radii), in the inner part of the domain (out to 100 black hole radii), and the innermost part of the domain (out to 40 black hole radii), respectively.