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\subsection{Understanding Black Holes - Emission from Near the Event Horizon}


{\bf{
{Dr. Heino Falcke}\\
{Max-Planck-Institut f\"ur Radioastronomie}\\
{Auf dem H\"ugel 69}\\
{D-53121 BONN}\\
{Telefon: 0228/525217}\\
{Fax: 0228/525229}\\
{Email: hfalcke@mpifr-bonn.mpg.de}\\}}


\subsubsection{Abstract}
Black holes are important as most likely candidates for the dark mass
concentration at the centers of galaxies. They are intimately linked
to the evolution of galaxies and their energy output can contribute a
significant fraction of the energy budget of hot plasma in the
universe and galaxy clusters. In addition the plasma in jets and
accretion flows from black holes gives rise to a multitude of particle
physics processes, high energy emission, and cosmic ray
production. Emission from black holes is no longer found only in
powerful quasars and radio galaxies but also in a large fraction of
more or less normal galaxies, like our own Milky Way. We will further
develop already successful emission models for black holes, which
combine the major important concepts, like accretion disks, advection
dominated accretion flows, jets, radiation processes and particle
interactions. Parallel we will undertake surveys and multi-wavelength
studies of black holes in the Milky Way and other galaxies. By
combining theory and observations we will constrain the demographics
and basic parameters of black holes, such as accretion rates and power
output, and investigate the relevant physical processes near the event
horizon.


\subsubsection{Current status of research}
Supermassive black holes mark the central region of galaxies and
dominate the gravitational potential of galaxies on the smallest
scales. Dynamical evidence for black holes has been found in many
galaxies and most recent observations indicate that their masses scale
with that of the galaxy bulges and halos, thus suggesting a common
evolution (Ferrarese \& Merrit 2000, ApJ 539, L9; Gebhard et al.~2000,
ApJ 539, L13).

Fortunately, black holes are not really black.  In contrast to dark
matter distributed in halos and galaxy clusters, black holes are often
associated with luminous thermal and non-thermal emission.  Radio jets
in particular have become the ``smoking guns'' of black holes and
active galactic nuclei (AGN). Their energy output can in principle
substantially contribute to the heating of gas in the universe (e.g.,
En\ss{}lin et al. 1998, A\&A 333, L 47) and thus be important for
properly calculating the hydrostatics of hot X-ray gas in clusters.

AGN jets are also the site of many high-energy processes because
particles are accelerated to ultra-relativistic energies, leading to
radio through X-ray and $\gamma$-ray emission.  Today we know that
radio jets are relativistic, magnetized plasma flows ejected at
velocities close to the speed of light, likely from the immediate
vicinity of black holes (e.g. Zensus 1997, ARA\&A 35). On the smallest
scales, the radio emission from these jets appears as a very compact
radio core with a flat and variable radio spectrum. With Very Long
Baseline Inter\-ferometry (VLBI) this emission can often be resolved
into a core-jet structure sometimes showing apparent superluminal
motion caused by the relativistic ejection velocity. Currently, with
two major X-ray satellites in orbit, hard X-ray point sources found in
the nuclei of galaxies are an equally powerful tool for identifying
active black holes (e.g., Mushotzky et al. 2000, HEAD 32.2101).

Compact radio cores have also been found in several other sources,
including the center of the Milky Way (Sgr A*; Melia \& Falcke 2001,
ARA\&A 39, in press), X-ray binaries (stellar mass black holes and
neutron stars; Mirabel \& Rodrigues 1999, ARA\&A 37, 409), and many
nearby galaxies (e.g. Nagar et al., ApJ 542, 186). Despite being
interesting in their own right, none of these sources can compare in
violence and power output with the ultra-luminous quasar class of
AGN. On the other hand it is thought that the same engine -- black
holes -- are involved in these sources, though perhaps accreting at a
lower level than quasars.

Nevertheless, because of their proximity and large number, even the
faint non-thermal emission in low-luminosity AGN can be of great
interest. In the Galactic Center, for example, the radio and X-ray
emission most likely comes from the direct vicinity of the central
supermassive black hole's event horizon.  General relativistic
ray-tracing calculations have shown that a factor of two to three
improvement in the resolution of the VLBI-technique will make it
feasible to actually image the event horizon in the next few years
(Falcke et al. 2000, ApJ 528, L13). Current observations already
greatly constrain possible models for the emission processes on
the most compact scales: a combination of synchrotron radiation,
inverse Compton radiation, and perhaps also inelastic proton-proton
collisions seem to be required.

Current models for active black holes -- across the astrophysical
spectrum -- are based on combinations of standard $\alpha$-disks,
where most energy liberated during the infall into the potential well
is radiated, advection dominated accretion flows (ADAFs), where energy
is stored and advected into the event horizon, and jet models, where a
significant fraction of the energy escapes in a relativistic outflow.
With the amount of new observational data pouring in, especially in
the radio and X-ray regime, a major goal of research will be to
understand the physical mechanisms governing the very vicinity of
black holes and the associated energetic phenomena. This requires
developing and improving emission models that help to constrain
fundamental parameters for accretion, mass ejection and particle
acceleration and identify underlying emission mechanisms. This is
critical for our understanding of black hole evolution, as well as the
energetics of the universe and the origin of ultra-high energy cosmic
rays.


\subsubsection{Personal experience}
During the last couple of years, we have developed a basic model for
jets and accretion flows explaining the non-thermal emission in
accreting black holes (Falcke, Biermann, Markoff).  Based on the idea
of a ``jet-disk symbiosis'' the model successfully describes the
emission of black holes in very different sources, like low-luminosity
AGN, X-ray binaries, and quasars with the accretion rate being the
main parameter. Falcke and collaborators have written numerous reviews
on this topic and especially their work on the emission of Sgr A* (the
Galactic Center black hole) is widely recognized.  Markoff \& Biermann
have a strong particle physics background and thus strengthen the
connection to other parts of the SFB.

In addition, Falcke, Nagar, \& Wilson have been involved in a large
survey of radio emission from low-luminosity AGN at various
wavelengths and resolutions. This allowed them to compile a large
database of (partially newly discovered) black holes with detectable
emission. This extends the black hole census to low powers and
provides powerful means for discerning between various models.

A strength of this collaboration will be a close connection between
theory and observations of black holes.

\subsubsection{Goals of the project}
The goal of the project will be to develop a comprehensive emission
model of black holes on all mass scales, comprising the major ideas
currently discussed. We will further combine and integrate accretion
flow models such as standard disk and ADAF with jet emission models
and compare them with observations. As much as possible, the model
will include empirical and observational constraints. Parallel to this
we will expand our survey to discover more supermassive black hole
candidates (mainly in low-luminosity AGN and XRBs) and constrain their
emission spectra across the electromagnetic spectrum.

The final questions we hope to answer with this are: What are the
demographics of black holes? What is their power output in mechanical
energy through jets and relativistic particles (cosmic rays)? What is
the accretion rate onto black holes in various phases of their
life-cycle, given current radio and observational constraints? What
physical processes can be identified as being important near the event
horizon and in jets and accretion flows? We will also discuss the
possible effects of accreting, dissipative dark matter on the emission
spectra of black holes.

\subsubsection{Estimate of resources expected from DFG}
We will request two postdoc and two PhD-positions. One half will be
dedicated to the observational issues and the other half to
theoretical modeling. We will also require traveling funds for the
students and postdocs, in order for them to attend conferences and
conduct observations.  Finally, we will request funds for computer
equipment, and page charges for some journals.

\subsubsection{Members of research group relevant for the project}
Dr. Heino Falcke, MPIfR Bonn (PI)\\
Dr. Sera Markoff, MPIfR Bonn (PostDoc)\\
Dr. A. Zensus, MPIfR Bonn (Director)\\
Prof. Peter L. Biermann, MPIfR Bonn\\
Dr. Neil Nagar, Arcetri Observatory (PostDoc)\\
Prof. Andrew Wilson, University of Maryland \& Space Telescope Science 
Institute\\

\subsubsection{Connections to other projects of current SFB proposal}

Black holes are at the focal point of various lines of research
important for the proposed SFB. In a separate project, the common
evolution and growth of black holes and galactic bulges is
investigated. Our modeling of black hole emission processes, and the
application of this model to data will provide crucial information
about the magnitude of the central accretion rate in various phases of
AGN activity. Similarly, the energy output of black holes will help to
improve our understanding of the energetics of X-ray halos and thus,
for example, significantly affect the estimate for baryonic and dark
matter in galactic halos. Finally, the power of jets is a fundamental
parameter for estimating the (ultra-high energy) cosmic ray
contribution of AGN which, in turn has implications for the energy
budget in the universe and astro-particle physics developed in various
other projects within the SFB (e.g. the question whether dark matter
is dissipative).
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