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\title{What is Sgr~A*?}
\subtitle{The Starved Black Hole in the Center of the Milky Way}

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\author{H. FALCKE}
\institute{Max-Planck-Institut f\"ur Radioastronomie (MPIfR)\\
           Auf dem H\"ugel 69, D-53121 Bonn, Germany \\
           E-mail: HFalcke@mpifr-bonn.mpg.de (hfalcke@astro.umd.edu)}
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\runningtitle{What is Sgr~A*?}

\begin{document}

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\footnotetext[1]{
Invited Review, in ``Unsolved Problems of the Milky Way'', IAU Symp. 169,
L. Blitz \& P. Teuben (eds.), Kluwer Academic Press, p. 169 (1996)}

\section{Introduction}
Sgr~A*\index{Sgr A*} \index{Galactic Center, Sgr A*} \index{Galactic
Center, black hole} \index{black holes, supermassive} is the unique 1
Jy flat spectrum radio point source located at the dynamical center of
the Galaxy and in the very center of the central star cluster (Eckart
et al. 1993). Due to its unusual appearance it has long been
speculated that this source is powered by a supermassive black hole --
an object whose presence has been suspected to reside in the nuclei of
many other galaxies as well. Its mass is believed to be as large as
$M_\bullet\sim2\cdot10^6M_{\odot}$ (e.g. Genzel \& Townes 1987) while
a lower limit of $M_\bullet>200-2000M_{\odot}$ can be inferred from
the low proper motion of Sgr A* (Backer -- this volume). The currently
best dynamical arguments for the million solar mass black hole stem
from detailed stellar kinematic studies (Haller et al. 1995).

The enormous increase in observational data obtained for Sgr A* in
recent years has enabled us to develop, compare and constrain a
variety of models for the emission characteristics of this source.
Because of its relative proximity and further observational input to
come Sgr A* may therefore become one of the best laboratories for
studying supermassive black hole candidates and basic AGN
physics. This paper briefly summarizes our current understanding of
this enigmatic radio source.

\section{Observational Input}
\subsection{Radio-Submm Spectrum}
The radio spectrum of Sgr A* \index{Sgr A*, radio spectrum} has been
extensively studied in the range 1-600 GHz where it mostly shows
substantial variability. As there are only very few quasi-simultaneous
flux density measurements available (see Wright \& Backer 1993) an
exact description of the radio spectrum is very uncertain at the
moment. An averaged spectrum combined of various data sets available
in the literature (Duschl \& Lesch 1994) may be fitted by a single
powerlaw with spectral index $\alpha\sim 1/3$
($S_{\nu}\propto\nu^\alpha$). However, it appears as if the submm
regime is less variable than the radio regime (Zylka et al. 1995) and
there might even be a weak submm-excess (Zylka et al. 1992; compare
also Rogers et al. 1994 with Zylka et al. 1995). Sgr A* is not seen at
IR wavelength and hence the spectrum must cut-off towards $12\mu$m
(Zylka et al. 1992; Gezari -- this volume). The spectrum also cuts off
below 1 GHz.

\subsection{High energies}
\index{Sgr A*, X-rays} Evidence has grown substantially that Sgr A* is
also an X-ray emitter. Art-P/GRANAT detected an x-ray source
coinciding with the position of Sgr A* within $40''$ (Sunyaev et
al. 1991). It shows variability within a factor 2 over a period of
several months.  The identification as Sgr A* is corroborated by a
ROSAT detection of this source with a positional uncertainty of only
10'' (Predehl \& Tr\"umper 1994).  The ROSAT flux, however, was lower
than expected from the Art-P measurements and prompted the
interpretation of additional intrinsic absorption in Sgr A* -- if the
ROSAT and the Art-P source are identical. The spectrum in the Art-P
band (4-20 keV) is a hard powerlaw with $\alpha\sim-0.6$ and breaks
already in the range 35-100 keV (Goldwurm et al. 1994).  The
situation, however, seems to have changed now, after the launch of
ASCA. Koyama (1994) and Maeda et al. (1996) report that the hard
source at the Galactic Center (GC) and the soft ROSAT source are
offset by 1.3 arcmin. They claim that the hard souce -- which is most
likely the source detected by Art-P -- is not Sgr A*, but a transient
x-ray binary. This means that the total x-ray luminosity of Sgr A*
could be as low as $10^5L_\odot$ and lower.

There is also a gamma-ray detection of the GC with EGRET (Mattox et
al. 1992) but at present it is not clear whether this is a point
source or extended emission.

\subsection{Luminosity constraints}
\index{Sgr A*, luminosity} The bolometric optical-UV luminosity of Sgr
A* can be estimated from the fact that a luminous point source should
contribute to the heating of the surrounding dust and thus be visible
in submm-IR data (Falcke et al. 1993a) -- which is not the
case. Hence, we estimated that Sgr A* can not be very luminous with
$L_{\rm UV}\le$ a few $10^5L_{\odot}$. Recently Zylka et al. (1995)
have updated their submm measurements of the Sgr A region and
concluded from the low temperature gradients in the dust that the dust
heating can not be dominated by a single point source but is more
likely due to a cluster of luminous stars (e.g.  Krabbe et
al. 1991). This would also indicate that Sgr~A* has a bolometric
luminosity of not more than a few $10^5L_\odot$, but this estimate
could be uncertain by a factor 10. A lower limit derived from the
claimed detection of Sgr A* at NIR wavelengths (Eckart et al. 1992)
has become uncertain as this source was now resolved into a cluster of
stars (Genzel -- this volume; Eckart et al. 1995) making it difficult
to identify Sgr~A* with the present uncertainties between the radio
and optical reference frame. Provided the separation of the 5
sources is real, the upper limit for Sgr~A* in the K band could be
$L_\nu<3\cdot 10^{21}$ erg/sec/Hz. In the IR Gezari (this volume) has
reported an upper limit at $12.4\mu$m of $0.9\cdot10^{22}$
erg/sec/Hz. With all these upper limits one starts to wonder whether
Sgr~A* is actually there, and without the compact radio source we
would have not the slightest clue that there is something. However, we
have argued earlier that in order to explain the radio emission, one
needs $L_{\rm UV}>10^4L_\odot$ (Falcke et al. 1993b) and there is
still  hope that finally Sgr A* will reveal itself unambiguously
at other wavelengths as well.

\subsection{Source Size}
\index{Sgr A*, source size}
The mm-submm size of Sgr A* is constrained at least within one order
of magnitude. From the absence of refractive scintillation Gwinn et
al. (1991) have argued that Sgr A* must be larger than $10^{12}$ cm at
$\lambda1.3$ and $\lambda0.8$ mm.  Krichbaum et al. (1993 \& 1994)
obtained source sizes for Sgr A* of $4.2\cdot10^{13}$ cm at 86 GHz and
$9.5\cdot10^{13}$cm at 43 GHz with VLBI -- the latter well above the
expected scattering size as extrapolated from lower frequencies. This
claim is challenged by Rogers et al. (1994) who only get $2\cdot
10^{13}$ cm at 86 GHz in an experiment with a factor 2 shorter
baseline. Krichbaum et al. (1993) also found additional weak
components and a somewhat elongated source structure at 43 GHz VLBI
not seen by Backer et al. (1993). A possibility to reconcile the
results could be source variability and elongation of the internal
structure which would lead to different sizes if observed with
differently oriented baselines.  It will be very interesting to see
the results of further mm-VLBI experiments.

\section{Properties of the radio source}
\subsection{A homogenous blop?}
Recent submm measurements (see Zylka et al. 1995) indicate that the
radio spectrum of Sgr A* continues up to several hundred GHz with peak
fluxes around 3.5 Jy and a sharp cut-off towards the IR. The submm
spectrum can no longer be explained by thermal dust emission as this
would require extremely cold dust ($\sim15$K) which is very unlikely
because of the intense (stellar) radiation field in the Galactic
Center. To explain the flat submm spectrum with synchrotron emission
one needs either a combination of self-absorbed components (requiring
high compactness) or an electron distribution where the bulk of the
electron energy is concentrated in a narrow energy interval.  The
latter could be either a very flat electron powerlaw distribution
($dN/d\gamma\propto\gamma^{-p}$) with $p<1/3$ and sharp high-energy
cut-off, a steep powerlaw with low-energy cut-off, a monoenergetic
(e.g. an electron beam) or a thermal distribution.

Duschl \& Lesch (1994, also this volume) suggested that the radio
emission of Sgr A* can simply be explained with a single homogenous
blob of monoenergetic electrons. Although this can not be quite true
in its most rigorous formulation, as argued below, one can use this
approach to get a fairly good idea of the basic parameters of the Sgr
A* radio source: the required model parameters are the magnetic field
$B$, the Lorentz factor $\gamma_{\rm e}$, the electron density $n_{\rm
e}$, the volume $V=\pi R^2 Z$ (assumed to be cylindric) and the
distance set to 8.5 kpc. On the other side we have three measurable
input parameters: the peak frequency $\nu_{\rm max}\sim\nu_{\rm
c}/3.5$ of a monoenergetic synchrotron spectrum, the peak flux
$S_{\nu_{\rm max}}$ and the VLBI source size (see above). A fourth
parameter can be gained if one assumes that magnetic field and
relativistic electrons are in equipartition, i.e. $B^2/8\pi=k n_{\rm
e} \gamma_{\rm e} m_{\rm e} c^2$ with $k\sim1$. With this condition we
obtain (averaged over pitch angle) that
\index{synchrotron emission, monoenergetic electrons}
\index{monoenergetic electrons, synchrotron emission}

$$
\gamma_{\rm e}=326 \; k^{1/7}\left({F_{\nu_{\max}}\over 3.5 {\rm
Jy}}\right)^{-1/7}  
\left({\nu_{\rm max}\over 10^{12} 
{\rm Hz}}\right)^{3/7} 
\left({R\over 10^{13}{\rm cm}}\right)^{2/7}
\left({Z\over 4\cdot 10^{13}{\rm cm}}\right)^{1/7}
$$


$$
B=10\,{\rm G}\; k^{-2/7}\left({F_{\nu_{\max}}\over 3.5 {\rm
Jy}}\right)^{2/7}  
\left({\nu_{\rm max}\over 10^{12} 
{\rm Hz}}\right)^{1/7}
\left({R\over 10^{13}{\rm cm}}\right)^{-4/7}
\left({Z\over 4\cdot 10^{13}{\rm cm}}\right)^{-2/7}
$$


$$
n_{\rm e}={ 1.4\!\cdot\!10^4\over {\rm cm^{3}}}k^{2/7}\!\left({F_{\nu_{\max}}\over 3.5 {\rm
Jy}}\right)^{5/7}  \!\!
\left({\nu_{\rm max}\over 10^{12} 
{\rm Hz}}\right)^{-1/7}\!\!
\left({R\over  10^{13}{\rm cm}}\right)^{-10/7}\!\!
\left({Z\over 4\!\cdot\! 10^{13}{\rm cm}}\right)^{-5/7}\!\!\!.
$$

Apparently the `non'-equipartition parameter $k$ enters only weakly
 and as long as one is not very far from equipartition the parameters
 are basically fixed: $\nu_{\max}$ is known within a factor
 three, $S_{\nu_{\max}}$ within $50\%$ and the source size within a
 factor 10. This means that models advocating very high electron
 Lorentz factors ($\gamma_{\rm e}\sim10^4$, Kundt 1990) deviate from
 equipartition by $\sim10$ orders of magnitude!

Because of the high compactness of Sgr A* synchrotron self-absorption
\index{synchrotron self-absorption}
becomes another important point to be considered. Using an absorption
coefficient of $\kappa_{\rm sync}=1.4\cdot10^{-9} {\rm cm}^{-1}
(n_{\rm e}/{\rm cm}^{-3}) (B/{\rm G})
\gamma_{\rm e}^{-5} \left(\nu/\nu_{\rm c}\right)^{-5/3}$ 
one finds the synchrotron self-absorption frequency to be
$$
\nu_{\rm ssa}={2.5\,{\rm GHz}\over k^{0.09}}\left({F_{\nu_{\max}}\over 3.5 {\rm
Jy}}\right)^{0.69}  
\left({\nu_{\rm max}\over 10^{12} 
{\rm Hz}}\right)^{-.46}
\left({R\over 10^{13}{\rm cm}}\right)^{-.77}
\left({Z\over 4\cdot 10^{13}{\rm cm}}\right)^{-.69}\!.
$$ Here we took the {\em maximum} sizes allowed by mm-VLBI; if further
studies show that Sgr A* is even more compact at submm then $\nu_{\rm
ssa}$ will increase further making it completely impossible to
describe the whole spectrum with a single component.

\subsection{Submm Source Size}
\index{Sgr A*, source size}
We can now make very solid arguments about the possible source size of
Sgr A* at submm wavelengths. As VLBI measurements are only available
at higher wavelengths one could still postulate arbitrarily large
submm source sizes.  However, if Sgr A*(submm) were optically thin and
larger than $4\cdot10^{13}$ cm we should have seen the low frequency
$\nu^{1/3}$ part of its spectrum with 3mm VLBI already. This could only be
avoided if the submm component becomes optically thick
below $\sim100$ GHz. As shown above this is possible only for a very
compact source where the dimensions of Sgr A* at submm wavelengths are
substantially {\it smaller} than at $\lambda$3mm. {\em Consequently Sgr A* has
to be equal or smaller at submm wavelengths than at $\lambda$3mm}.

Once $\nu_{\rm ssa}$ can be determined, e.g. from broadband
variability studies, we can specify the compactness of Sgr A* from its
spectral characteristics alone.  Arguing that the bulk of the emission at
submm and mm wavelengths comes from two separate components,
i.e. requiring $\nu_{\rm ssa}\sim 100$ GHz for the submm component,
would imply a source size of only $$ R\sim1.5\cdot10^{12}{\rm cm}\quad
k^{-1/17}
\left({F_{\nu_{\max}}\over 3.5 {\rm
Jy}}\right)^{8/17}  
\left({\nu_{\rm max}\over 10^{12} 
{\rm Hz}}\right)^{-16/51}
\left({\nu_{\rm ssa}\over 100{\rm GHz}}\right)^{-35/51}
$$ for $R\sim Z$. This corresponds to $5 R_{\rm
g}\;(=5{GM_\bullet\over c^2})$ of a $2\cdot 10^6M_\odot$ black hole
and hence to the innermost parts of an accretion disk or the very base
of a jet. The fact that the non-thermal spectrum cuts-off towards the
IR indicates that the submm regime indeed corresponds to the smallest
spatial scale. \index{Sgr A*, mass} Do we touch the supermassive black hole at these
wavelengths directly?

\subsection{Multiple Components}
Although the single, monoenergetic, homogenous blob hypothesis clearly
is the simplest description it appears not to be sufficient to explain
Sgr A* and there are several observational indications 
suggesting a non-homogenous source structure, i.e.
\begin{itemize}
\item[$\rhd$] different core sizes at $\lambda$7mm and $\lambda$3mm (Krichbaum et
al. 1994)
\item[$\rhd$] different variability at radio and submm
(Zylka et al. 1995)
\item[$\rhd$] varying simultaneous spectral indices (Wright \& Backer 1993).
\end{itemize}
Thus inhomogenous models (with gradients in size, $B$ and $n_{\rm e}$,
e.g. in a jet or an accretion disk) are required to describe Sgr A*.


\section{Spherical wind accretion models}
\index{Sgr A*, wind accretion}
If we now want to go beyond a mere description of Sgr A*, we have to
ask how this source is powered and what the underlying engine
producing the radio and x-ray emission actually is?  One idea is that
if Sgr A* is a black hole it should swallow some fraction of the
strong stellar winds seen in the GC through Bondi-Hoyle accretion.
\index{Bondi-Hoyle accretion}
\index{spherical accretion}

The rate of infall depends only on the mass of the black hole and the
wind parameters. Once we know the latter we can determine the black
hole mass from the estimated accretion rate, which in turn could be
derived from the spectrum of Sgr A*. The general validity of the
Bondi-Hoyle accretion (without angular momentum) under these
assumptions was recently demonstrated by 3D numerical calculations
(Ruffert \& Melia 1994) and the main uncertainties are related to the
plasmaphysical effects associated with the infall. It is usually
assumed that the magnetic field in the accreted plasma is amplified by
compression up to a point where it reaches the equipartition value.
Beyond this point the excess magnetic field is assumed to be
dissipated and used to heat the plasma.  The electron temperature is
determined by the equilibrium between heating and cooling via
cyclo-synchrotron radiation where one has to consider two domains for
the solution of this problem: (1) hot electrons, where the typical
electron Lorentz factors
\index{electron Lorentz factor}
 are of the order 100-1000 and (2) warm
electrons, where the electron Lorentz factor is still close to unity.

The first domain is in a regime where synchrotron emission is
important and also very effective. This requires only low accretion
rates ($\dot M\sim10^{-10} M_{\odot}/{\rm yr}$) and hence permits only
moderately high black hole masses of the order $M_{\bullet}\simeq10^3
M_{\odot}$ (Ozernoy 1992). The second domain is in the transition
regime between cyclotron and synchrotron radiation, which is less
effective than pure synchrotron radiation and hence requires higher
accretion rates ($\dot M\sim10^{-4} M_{\odot}/{\rm yr}$) and a higher
black hole mass of the order $M_{\bullet}\simeq10^6M_{\odot}$ (Melia
1992 \& 1994). \index{Sgr A*, accretion rate}  \index{Sgr A*, mass}
\index{Galactic Center, accretion rate}

The big advantage of the wind-accretion approach is that it, firstly,
appears unavoidable and, secondly, self-consistently ties observable
parameters and accretion rate to the mass of the central object. The
radio spectrum is well reproduced and initially Melia also was able to
account for the x-ray flux.

On the other hand there are several counter arguments to be
considered: Firstly, it is not at all clear that the wind has zero
angular momentum, which would diminish the accretion rate and lead to
a circularization of the accretion flow further away from the central
object. There also could be residual angular momentum in Sgr A*
itself, e.g. because of a fossil accretion disk which could catch the
\index{accretion disk, fossil} inflow further out, filling a reservoir
of rather dense matter instead of directly feeding the black hole. The
viscous time scales of such a disk can be very long -- up to $10^7$
years (Falcke \& Heinrich 1994). \index{Sgr A*, accretion disk} More
detailed calculation (Falcke \& Melia 1996), show that in fact an
intrinsically large angular momentum in the wind is required -- a
fossil disk alone is not sufficent, as the disk/wind interaction would
produce too much NIR emission -- to stop (and circularize) the infall
at a scale of $10^{16}$ cm and to avoid a strong luminosity output.

There are also problems specific to each model. Ozernoy predicts a
very compact source which, as shown above, would become self-absorbed
already at high radio frequencies and hence requires the presence of
other emission components. Melia on the other hand needs a very high
accretion rate and, as Ruffert \& Melia (1994) have shown,
fluctuations will always lead to the formation of an accretion disk
close to the black hole even for the case of initially zero angular
momentum. As most of the energy of an accretion disk is produced very
close to the black hole it seems impossible to avoid a high luminosity
output from this accretion process. The luminosity produced by a
Schwarzschild hole ($R_{\rm in}=6 R_{\rm g}$) is $L_{\rm
disk}=0.8\cdot10^8L_{\odot} \dot M/(10^{-4} M_\odot/{\rm yr})$ and
even if the outer disk radius is only two times larger than $R_{\rm
in}$, $L_{\rm disk}$ reduces only by a factor 3. Given the strong
limits on the luminosity of Sgr A* of $L_{\rm disk}\ll10^6L_{\odot}$
it is very unlikely that such a high accretion rate is {\em currently}
flowing onto the black hole.  \index{Sgr A*, accretion rate} Finally,
the recent SIGMA results (Goldwurm et al. 1994) and especially the
ASCA results seem to be in contradiction with the predicted X-ray
spectrum of the Melia model.

An alternative to the models mentioned above was proposed by Narayan
et al. (1995), who explain the discrepancy between high accretion rate
and low luminosity by the effects of an advection dominated disk. In
this model more than $99\%$ of the energy is not radiated but
transported through the disk by advection and finally swallowed by the
black hole. And in fact it appears as if advection is non-negligible
in many accretion disks, but whether indeed such a high fraction of
the energy is transported by advection alone is not at all clear. One
also has to make sure that not a substantial fraction of the energy is
released in the inner parts of the disk and the energy is swallowed
quietly by the black hole. Even if an advection dominated disk is not
the whole story, it may be an interesting part of it.

\section{Jet-disk models}
\subsection{The basic idea}
Already in 1980 Reynolds and McKee argued that it is very difficult to
confine the synchrotron emitting particles in Sgr A* and proposed a
wind or jet model to explain the radio spectrum.  Rees (1982) tried to
explain Sgr A* by accretion from the interstellar matter as discussed
in the previous section, however, invoking an accretion disk where the
synchrotron emission stems from a relativistic electron gas in its
inner parts.

\index{Sgr A*, accretion disk}
\index{Sgr A*, jet}
\index{radio jets}
\index{accretion disk}
\index{active galactic nuclei}
We recently suggested to consider a coupled jet disk system for Sgr A*
(Falcke et al. 1993a\&b, Falcke \& Biermann 1994\&1995). The basic
concept behind this approach -- which has also successfully been
applied to AGN -- is to postulate a fundamental symbiosis between jets
and disks around compact objects, i.e. that both always exist and both
are energetically important. As the typical escape speed close to a
black hole is scale invariant and always a large fraction of $c$ we
expect at least mildly relativistic outflows irrespective of the black
hole mass. The power of the jet should be mainly governed by the
accretion rate.

We extended the classic Blandford \& K\"onigl (1979) jet-emission
model by adding mass and energy conservation in a jet-disk system also
defining scale invariant paramters for the plasma flow.  A more
refined model spectrum which includes the effects of adiabatic losses
and non-conical jet geometry (see Reynolds 1982) but uses the same
basic principles is shown in Fig. 1. Here we also accounted for the
presence of a cylindrical region at the base of the jet which we
termed 'nozzle', \index{jets, nozzle} assuming that this is the region
where the jet is accelerated and the electrons are injected. Hence the
spectrum consists of three regions:

\begin{itemize}
\item[a)] the  {\it nozzle}, dominated by a single, quasi
monoenergetic electron distribution producing the {\it submm} bump; 
\item[b)]  the
{\it jet} itself, producing an inverted radio spectrum at {\it cm} wavelengths where the
exact spectral index depends on the jet shape and 
\item[c)] an {\it intermediate}
region at {\it  mm} wavelengths where both contribute equally.
\end{itemize}

The turnover frequencies between those regions depend on the
self-absorption frequency of the submm component and as discussed
above on the source size of jet and nozzle. Therefore one expects
these parameters to be fixed by either mm-submm VLBI or simultaneous
variability studies at cm-submm wavelengths.  

\begin{figure}
%\vspace{5cm}  % amount of vertical space needed
\centerline{
{\psfig{figure=hfalcke.ps,height=5cm,bbllx=2.6cm,bblly=8.1cm,bburx=19.2cm,bbury=18.7cm}}}
\caption{\index{Sgr A*, radio spectrum}
Model spectrum for jet and nozzle coupled to an accretion disk in Sgr
A*. Parameters are: $R_{\rm nozz}=3\cdot 10^{11}$ cm, $Z_{\rm
nozz}=4.25\cdot 10^{12}$ cm, $\gamma_{\rm e}=70$, $q_{\rm j/l}=0.35$,
$L_{\rm disk}=10^{39}$ erg/sec, $x_{\rm e}=1$, $\gamma_{\rm j}=2$,
$i=60^\circ$ (see Falcke \& Biermann 1995). We included adiabatic
losses, a nozzle where electrons are injected monoenergetically and a
jet with shape $R_{\rm j}=R_{\rm nozz}+(Z_{\rm j}/Z_{\rm
nozz})^{0.55}/{\cal M}$ slowly reaccelerating electrons into a $p=2.5$
powerlaw. }
\end{figure}

The main finding of this kind of model is that size and flux of Sgr A*
are compatible with it being a radio jet, i.e. the low accretion rate
results in a very compact jet but still can yield a 1 Jy
source. Although the overall power of the jet is fairly low due to the
low accretion rate, the ratio between jet power $Q_{\rm jet}$ and
$L_{\rm disk}$ appeared relatively high $(\sim0.3-1)$. This can 
easily be checked by crudely estimating the magnetic luminosity of Sgr
A* which is $L_{\rm B}\sim 0.125(10\,{\rm G})^2 (10^{13} {\rm cm})^2
c\sim10^4L_{\odot}$. Now, one only has to remember that the total jet
power including relativistic particles and kinetic energy is at least
3-4 times higher and that probably $L_{\rm disk}\le10^5L_{\odot}$.


\subsection{The AGN connection - the case for hadronic cascades}
We found that the same kind of model can not only explain Sgr A* but
also the jets in AGN and even account for the tight UV-radio
correlation in radio weak quasars (Falcke et al. 1995b).
\index{active galactic nuclei, UV-bump}
\index{active galactic nuclei, jets}

Once more the limits imposed by the accretion disk played a crucial
r\^ole. Again one infers injection of relativistic electrons
(positrons) at high energies above $\gamma_{\rm e}=100$ for radio loud
jets and we argued that perhaps the difference between radio loud and
radio weak quasars could be understood by the lack of this efficient
injection mechanism in radio weak quasars (Falcke, Gopal-Krishna,
Biermann 1995a). 
\index{quasars, radio loud \& radio weak}
\index{electron Lorentz factor}
Anyway, the similarity of the high electron Lorentz
factors found (directly) in Sgr A* and (indirectly) in AGN is more
than striking.  Hence we suggested that this typical Lorentz factor
has a basic physical reason, namely the $\pi$-decay following hadronic
cascades initiated by $pp$-collisions between relativistic protons in
the jet and thermal protons surrounding the jet. 
\index{hadronic cascades}
\index{proton-proton collisions}
Because of the high
rest mass of the $\pi$ the secondary pairs produced in the cascade
will have a characteristic energy of $>35 MeV$ ($\gamma_{\rm
e}>70$) (see also Biermann et al. 1995). 
\index{electrons, secondaries}
\index{electrons, injection}
\index{pion-decay}
Jets interacting with a dense medium can inject additional
high energy secondary electrons and become radio loud, while those
which do not interact remain radio weak with only primary electrons
injected at thermal energies -- in this respect Sgr A* is {\it radio
loud}. The latter remains true if one extends the $L_{\rm disk}$-radio
correlation of AGN to lower luminosities and includes nearby Galaxies
with detected radio cores and even stellar mass black holes (Falcke
1994, Falcke \& Biermann 1996): again one finds something like a
radio loud/radio weak dichotomy, smoothly connecting to AGN, with Sgr
A* beeing fairly loud.

Where exactly those $pp$-collisions might occur in Sgr A* is still
uncertain: they may happen in an interaction zone between the jet and
infalling wind or the dense absorbing material discovered by ROSAT
(Predehl \& Tr\"umper 1994), but even the disk or the wind
(Mastichiadis \& Ozernoy 1994) itself could be a site for proton
(shock-)acceleration. If $pp$-collisions are the dominant cooling
\index{shock-acceleration, protons}
process for relativistic protons being accelerated in a dense medium
this would naturally yield monoenergetic secondary electrons.  Below
the $\pi$-production threshhold at 140 MeV $pp$-collisions are
inelastic and neither produce secondaries nor lead to cooling of the protons. Once
the protons are accelerated above the threshhold energy for
$\pi$-production, they will instantaneously cool by $pp$-collisions
until they fall below the threshhold energy thus bouncing back and
forth around this energy.  The resulting secondary electrons would be
injected in a narrow energy interval at roughly 1/4 of the the {\it
threshold} energy yielding $\gamma_{\rm e}\ge70$.

\section{Summary}
Considering the dynamical and spectral evidences I have no doubt that
indeed Sgr A* is the very center of the Galaxy and hence will have the
coordinates $l_3=0$ and $b_3=0$ after the next revision of the galactic
coordinate system (to be proposed at a future IAU assembly). Current
\index{galactic coordinates}
observational data constrain models for Sgr A* already much stronger
than for any other galactic nucleus -- we will never get closer to
a supermassive black hole. Although many question are still
\index{black holes, supermassive}
disputed, there is now some consensus that Sgr A* is currently put on
a starvation diet -- despite its high mass and strong stellar winds in
\index{starvation diet, black holes}
\index{black holes, starvation diet}
the surroundings. A coupled jet/disk system can explain the spectral
and structural characteristics of Sgr A* quite well and its smallest
source size is close to the typical size of a black hole of mass
$M_\bullet\sim10^6$, while the typical electron Lorentz factor of
$\gamma_{\rm e}\sim100$ may be indicative of hadronic
cascades. Crucial future experiments will be simultaneous variability
studies and mm-submm VLBI observations. Both, however, will require
joined efforts to face a single but promising challenge -- {\em
understanding Sgr A*}.


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\section*{Questions}
\parindent0cm

\medskip
{\bf C. Townes:} There are strong stellar winds in the GC. How can you
avoid a high mass inflow -- like in the Melia model -- if Sgr A* is a supermassive black hole?

\medskip
{\bf Answer:} Given the bolometric luminosity constraints for UV and
X-rays, I think that accretion rates as high as $10^{-4} M_\odot/$yr are
already ruled out by observations. Why Sgr A* does not accrete more
matter remains a mystery. Obviously we do not yet understand angular
momentum distribution and transport in the inner 0.1 pc around Sgr A*. On
the other hand I can not completely rule out that Sgr A* is less
massive than we think it is.


\medskip
{\bf C. Townes:} Does the mass of Sgr A* play an important r\^ole in your models.

\medskip
{\bf Answer:} Not really. Besides the dynamical estimates, only the
fact that the limits for the submm source size -- giving the smallest
scale -- are so close to what we expect for a $10^{6}M_\odot$ black hole
seems very suspicious. A sign for a low mass black hole would be
thermal x-ray emission from an accretion disk and heating of the
ambient gas.

\medskip
{\bf T. Hasegawa:} 
Do you have any comments on the accretion history of the black hole?
Do we see any signs of episodes of higher accretion rate in the past,
or has it been starving from the very beginning of its formation?

\medskip
{\bf Answer:} There is a weak feature -- the so called GC spur (Sofue,
Reich \& Reich 1989, ApJ 341, L47) -- which could be the smoke trail
of past jet\index{jet, GC spur}
activity. A single giant molecular can turn the GC into a
Seyfert nucleus\index{Seyfert galaxies}
at any time and this could have happened already in
the past. If the winds of the surrounding stars really are captured by
a fossil accretion disk around Sgr A* and are stored in a close orbit
than this could also lead to recurrent activity on a time scale of
$10^5-10^7$ years.

\end{document}