Daniel Proga

Dynamics of line-driven disk winds in active galactic nuclei

We present the results of axisymmetric time-dependent hydrodynamic calculations of line-driven winds from accretion disks in active galactic nuclei (AGNs). We assume the disk is flat, Keplerian, geometrically thin, and optically thick, radiating according to the ?-disk prescription. The central engine of the AGN is a source of both ionizing X-rays and wind-driving UV photons. To calculate the radiation force, we take into account radiation from the disk and the central engine. The gas temperature and ionization state in the wind are calculated self-consistently from the photoionization and heating rate of the central engine. We find that a disk accreting onto a 108 M? black hole at the rate of 1.8 M? yr-1 can launch a wind at ~1016 cm from the central engine. The X-rays from the central object are significantly attenuated by the disk atmosphere so they cannot prevent the local disk radiation from pushing matter away from the disk. However, in the supersonic portion of the flow high above the disk, the X-rays can overionize the gas and decrease the wind terminal velocity. For a reasonable X-ray opacity, e.g., ?X = 40 g-1 cm2, the disk wind can be accelerated by the central UV radiation to velocities of up to 15,000 km s-1 at a distance of ~1017 cm from the central engine. The covering factor of the disk wind is ~0.2. The wind is unsteady and consists of an opaque, slow vertical flow near the disk that is bounded on the polar side by a high-velocity stream. A typical column density through the fast stream is a few times 1023 cm-2 so the stream is optically thin to the UV radiation. This low column density is precisely why gas can be accelerated to high velocities. The fast stream contributes nearly 100% to the total wind mass-loss rate of 0.5 M? yr-1.

DISCERNING THE PHYSICAL ORIGINS OF COSMOLOGICAL GAMMA-RAY BURSTS BASED ON MULTIPLE OBSERVATIONAL CRITERIA: THE CASES OF z=6.7 GRB 080913, z=8.2 GRB 090423, AND SOME SHORT/HARD GRBs

The two high-redshift gamma-ray bursts, GRB 080913 at z = 6.7 and GRB 090423 at z = 8.2, recently detected by Swift appear as intrinsically short, hard GRBs. They could have been recognized by BATSE as short/hard GRBs should they have occurred at z ≤ 1. In order to address their physical origin, we perform a more thorough investigation on two physically distinct types (Type I/II) of cosmological GRBs and their observational characteristics. We reiterate the definitions of Type I/II GRBs and then review the following observational criteria and their physical motivations: supernova (SN) association, specific star-forming rate (SFR) of the host galaxy, location offset, duration, hardness, spectral lag, statistical correlations, energetics and collimation, afterglow properties, redshift distribution, luminosity function, and gravitational wave signature. Contrary to the traditional approach of assigning the physical category based on the gamma-ray properties (duration, hardness, and spectral lag), we take an alternative approach to define the Type I and Type II Gold Samples using several criteria that are more directly related to the GRB progenitors (SN association, host galaxy type, and specific SFR). We then study the properties of the two Gold Samples and compare them with the traditional long/soft and short/hard samples. We find that the Type II Gold Sample reasonably tracks the long/soft population, although it includes several intrinsically short (shorter than 1 s in the rest frame) GRBs. The Type I Gold Sample only has five GRBs, four of which are not strictly short but have extended emission. Other short/hard GRBs detected in the Swift era represent the BATSE short/hard sample well, but it is unclear whether all of them belong to Type I. We suggest that some (probably even most) high-luminosity short/hard GRBs instead belong to Type II. Based on multiple observational criteria, we suggest that GRB 080913 and GRB 090423 are more likely Type II events. In general, we acknowledge that it is not always straightforward to discern the physical categories of GRBs, and re-emphasize the importance of invoking multiple observational criteria. We cautiously propose an operational procedure to infer the physical origin of a given GRB with available multiple observational criteria, with various caveats laid out.

MOMENTUM DRIVING: WHICH PHYSICAL PROCESSES DOMINATE ACTIVE GALACTIC NUCLEUS FEEDBACK?

The deposition of mechanical feedback from a supermassive black hole (SMBH) in an active galactic nucleus into the surrounding galaxy occurs via broad-line winds which must carry mass and radial momentum as well as energy. The effect can be summarized by the dimensionless parameter {eta}= M-dot{sub outf}/ M-dot{sub acc}=2{epsilon}{sub w}c{sup 2}/v{sub w}{sup 2} where {epsilon}{sub w} ({identical_to} E-dot{sub w}/(M-dot{sub acc}c{sup 2})) is the efficiency with which accreted matter is turned into wind energy in the disk surrounding the central SMBH. The outflowing mass and momentum are proportional to {eta}, and many prior treatments have essentially assumed that {eta} = 0. We perform one- and two-dimensional simulations and find that the growth of the central SMBH is very sensitive to the inclusion of the mass and momentum driving but is insensitive to the assumed mechanical efficiency. For example in representative calculations, the omission of momentum and mass feedback leads to a hundred-fold increase in the mass of the SMBH to over 10{sup 10} M{sub sun}. When allowance is made for momentum driving, the final SMBH mass is much lower and the wind efficiencies that lead to the most observationally acceptable results are relatively low with {epsilon}{sub w} {approx}< 10{sup -4}.

FEEDBACK FROM CENTRAL BLACK HOLES IN ELLIPTICAL GALAXIES. III. MODELS WITH BOTH RADIATIVE AND MECHANICAL FEEDBACK

We find, from high-resolution hydro simulations, that winds from active galactic nuclei effectively heat the inner parts (100 pc) of elliptical galaxies, reducing infall to the central black hole; and radiative (photoionization and X-ray) heating reduces cooling flows at the kpc scale. Including both types of feedback with (peak) efficiencies of 3 ? 10?4 w 10?3 and of EM 10?1.3 respectively, produces systems having duty cycles, central black hole masses, X-ray luminosities, optical light profiles, and E+A spectra in accord with the broad suite of modern observations of massive elliptical systems. Our main conclusion is that mechanical feedback (including energy, momentum, and mass) is necessary but the efficiency, based on several independent arguments, must be a factor of 10 lower than is commonly assumed. Bursts are frequent at z > 1 and decline in frequency toward the present epoch as energy and metal-rich gas are expelled from the galaxies into the surrounding medium. For a representative galaxy of final stellar mass 3 ? 1011 M ?, roughly 3 ? 1010 M ? of recycled gas has been added to the interstellar medium (ISM) since z 2 and, of that, roughly 63% has been expelled from the galaxy, 19% has been converted into new metal-rich stars in the central few hundred parsecs, and 2% has been added to the central supermassive black hole (SMBH), with the remaining 16% in the form of hot X-ray emitting ISM. The bursts occupy a total time of 170 Myr, which is roughly 1.4% of the available time. Of this time, the central supermassive black hole would be seen as a UV or optical source for 45% and 71% of the time, respectively. Restricting to the last 8.5 Gyr, the bursts occupy 44 Myr, corresponding to a fiducial duty cycle of 5 ? 10?3.

Accretion of Low Angular Momentum Material onto Black Holes: Two-dimensional Magnetohydrodynamic Case

We report on the second phase of our study of slightly rotating accretion flows onto black holes. We consider magnetohydrodynamical (MHD) 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 and a weak radial magnetic field. We study accretion flows by means of numerical two-dimensional, axisymmetric, MHD simulations with and without resistive heating. Our main result is that the properties of the accretion flow depend mostly on an equatorial accretion torus that is made of the material that has too much angular momentum to be accreted directly. The torus accretes, however, because of the transport of angular momentum due to the magnetorotational instability (MRI). Initially, accretion is dominated by the polar funnel, as in the hydrodynamic inviscid case, where material has zero or very low angular momentum. At the later phase of the evolution, the torus thickens toward the poles and develops a corona or an outflow or both. Consequently, the mass accretion through the funnel is stopped. The accretion of rotating gas through the torus is significantly reduced compared with the accretion of nonrotating gas (i.e., the Bondi rate). It is also much smaller than the accretion rate in the inviscid, weakly rotating case. Our results do not change if we switch on or off resistive heating. Overall our simulations are very similar to those presented by Stone, Pringle, Hawley, and Balbus despite different initial and outer boundary conditions. Thus, we confirm that MRI is very robust and controls the nature of radiatively inefficient accretion flows. Although the time-averaged properties of our models approach a steady state, we find that the instantaneous mass-accretion rate in the latter stages of our simulations is highly time-dependent, with the inner flow displaying three generic flow patterns.

NUMERICAL SIMULATIONS OF MASS OUTFLOWS DRIVEN FROM ACCRETION DISKS BY RADIATION AND MAGNETIC FORCES

We study the two-dimensional, time-dependent magnetohydrodynamics (MHD) of radiation-driven winds from luminous accretion disks initially threaded by a purely axial magnetic field. The radiation force is mediated primarily by spectral lines and is calculated using a generalized multidimensional formulation of the Sobolev approximation. We use ideal MHD to compute numerically the evolution of Keplerian disks, varying the magnetic field strengths and the luminosity of the disk, the central accreting object, or both. We find that the magnetic fields very quickly start deviating from purely axial because of the magnetorotational instability. This leads to fast growth of the toroidal magnetic field as field lines wind up because of the disk rotation. As a result the toroidal field dominates over the poloidal field above the disk and the gradient of the former drives a slow and dense disk outflow, which conserves specific angular momentum. Depending on the strength of the magnetic field relative to the system luminosity, the disk wind can be radiation or MHD driven. The pure radiation-driven wind consists of a dense, slow outflow that is bounded on the polar side by a high-velocity stream. The mass-loss rate is mostly due to the fast stream. As the magnetic field strength increases, first the slow part of the flow is affected; namely, it becomes denser and slightly faster and begins to dominate the mass-loss rate. In very strong magnetic field or pure MHD cases, the wind consists of only a dense, slow outflow without the presence of the distinctive fast stream so typical of pure radiation-driven winds. Our simulations indicate that winds launched by the magnetic fields are likely to remain dominated by the fields downstream because of their relatively high densities. The radiation force due to lines may not be able to change a dense MHD wind because the line force strongly decreases with increasing density. Subject headings: accretion, accretion disks — binaries: close — galaxies: nuclei — methods: numerical — MHD

The late time evolution of gamma-ray bursts: ending hyperaccretion and producing flares

We consider the properties of a hyperaccretion model for gamma-ray bursts (GRBs) at late times when the mass supply rate is expected to decrease with time. We point out that the region in the vicinity of the accretor and the accretor itself can play an important role in determining the rate of accretion, and its time behaviour, and ultimately the energy output. Motivated by numerical simulations and theoretical results, we conjecture that the energy release can be repeatedly stopped and then restarted by the magnetic flux accumulated around the accretor. We propose that the episode or episodes when the accretion resumes correspond to X-ray flares discovered recently in a number of GRBs.

FEEDBACK FROM CENTRAL BLACK HOLES IN ELLIPTICAL GALAXIES. I. MODELS WITH EITHER RADIATIVE OR MECHANICAL FEEDBACK BUT NOT BOTH

The importance of the radiative feedback from massive black holes at the centers of elliptical galaxies is not in doubt, given the well-established relations among electromagnetic output, black hole mass, and galaxy optical luminosity. In addition, feedback due to mechanical and thermal deposition of energy from jets and winds emitted by the accretion disk around the central black hole is also expected to occur and has been included in the work of several investigators. In this paper, we improve and extend the accretion and feedback physics explored in our previous papers to include also a physically motivated model of mechanical feedback, in addition to radiative effects. In particular, we study the evolution of an isolated elliptical galaxy with the aid of a high-resolution one-dimensional hydrodynamical code, where the cooling and heating functions include photoionization and Compton effects, and restricting to models which include only radiative or only mechanical feedback (in the form of nuclear winds). We confirm that for Eddington ratios above 0.01 both the accretion and radiative output are forced by feedback effects to be in burst mode, so that strong intermittencies are expected at early times, while at low redshift the explored models are characterized by smooth, very sub-Eddington mass accretion rates punctuated by rare outbursts. However, the explored models always fail some observational tests. If we assume the high mechanical efficiency of 10?2.3 adopted by some investigators, we find that most of the gas is ejected from the galaxy, the resulting X-ray luminosity is far less than is typically observed and little supermassive black hole (SMBH) growth occurs. But models with low enough mechanical efficiency to accommodate satisfactory SMBH growth tend to allow too strong cooling flows and leave galaxies at z = 0 with E+A spectra more frequently than is observed. In a surprising conclusion, we find that both types of feedback are required. Radiative heating over the inner few kiloparsecs is needed to prevent calamitous cooling flows, and mechanical feedback from active galactic nucleus winds, which affects primarily the inner few hundred parsecs, is needed to moderate the luminosity and growth of the central SMBH. Models with combined feedback are explored in a forthcoming paper.

Dynamics of accretion flows irradiated by a quasar

We present results from axisymmetric time-dependent hydrodynamical calculations of gas flows under the influence of the gravity of black holes in quasars. We assume that the flows are nonrotating and exposed to quasar radiation. We take into account X-ray heating and the radiation force due to electron scattering and spectral lines. To compute the radiation field, we consider an optically thick, geometrically thin, standard accretion disk as a source of UV photons and a spherical central object as a source of X-rays. The gas temperature and ionization state in the flow are calculated self-consistently from the photoionization and heating rate of the central object. We find that for a 108 M☉ black hole with an accretion luminosity of 0.6 of the Eddington luminosity, the flow settles into a steady state and has two components: (1) an equatorial inflow and (2) a bipolar inflow/outflow with the outflow leaving the system along the disk rotational axis. The inflow is a realization of a Bondi-like accretion flow. The second component is an example of a nonradial accretion flow becoming an outflow once it is pushed close to the rotational axis where thermal expansion and radiation pressure accelerate it outward. Our main result is that the existence of the above two flow components is robust to the outer boundary conditions and the geometry and spectral energy distribution of the radiation field. However, the flow properties are not robust. In particular, the outflow power and collimation is higher for the radiation dominated by the UV/disk emission than for the radiation dominated by the X-ray/central engine emission. Our most intriguing result is that a very narrow outflow driven by radiation pressure on lines can carry more energy and mass than a broad outflow driven by thermal expansion.

Axisymmetric Magnetohydrodynamic Simulations of the Collapsar Model for Gamma-Ray Bursts

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 M? iron core of a 25 M? presupernova star has collapsed and study the ensuing accretion of the 7 M? 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.