Hans-Peter Nollert, Penn State

nollert@phys.psu.edu

Modern astrophysics is unthinkable without the input of general relativity. Therefore, the German Astronomical Society (Astronomische Gesellschaft) joined forces with the `Gravitation and Relativity Theory' section of the German Physical Society (DPG) in organizing this school on selected topics in relativistic astrophysics, such as gravitational lensing, gravitational waves, neutron stars and collapsing binaries, and accretion phenomena. The school took place in the physics center of the Deutsche Physikalische Gesellschaft in Bad Honnef from August 19 to 23 1996.

** Jürgen Ehlers** brought the lectures off to a great start with his
comprehensive overview over the basic concepts of general relativity,
with emphasis on physical interpretation, on astrophysical relevance,
especially for lensing and gravitational radiation, and on the initial
value problem for time evolution calculations. He found ways to help
even old experts in the field see many things in a new light.

** Peter Schneider** discussed gravitational lensing: its history in
the context of astrophysics, the basic concepts and the wealth of
information that can be gained from observations of weak lensing: Mass
profiles of galaxies, dark mass concentration, mean distribution of
galaxies, even the Hubble constant - and much more. With new
telescopes soon becoming operational, he foresees a bright future for
his field.

** Joachim Wambsganss** described the searches for microlensing
events. He presented the theoretical background and an overview over
the history of the MACHO, EROS, and OGLE projects. The search for dark
matter objects, for binaries and planets is the main objective in
studying galactic microlensing events. About three times as many
events as expected are observed in the galactic bulge, but fewer than
expected towards the large Magellanic cloud. A preliminary conclusion
states that the galactic halo almost certainly does not consist of
brown dwarfs. The focus of attention for extragalactic events is on
the determination of size and brightness profile of the sources, and
on the detection of compact objects for the lenses and the
determination of their masses.

** Ute Kraus** discussed theory and consequences of light deflection
near neutron stars. Geometric effects, such as increased visibility of
the star's surface, can have drastic effects for the pulse profiles of
radiation emitted on or near the surface of the star. Since her main
concern were light curves of X-ray pulsars, it is sufficient to
consider photon trajectories in a Schwarzschild metric. In addition to
the geometric effects, changes of photon energy and intensity
radiation have to be taken into account.

** Karsten Danzmann's** guide on ``How to build a GEO600 interferometric
gravitational wave detector in your back yard with spare change found
under your couch cushions'' covered every aspect from using recycling
to make your laser light go further, to reducing noise of nearby
tractors, to the proper way of welding the vacuum tubes. If you can
spare a little more, go for LISA, the heavenly version - you'll be
first on the block to have one, and you will be guaranteed a variety
of spectacular sources, such as coalescence of massive black holes
anywhere in the universe, or white dwarf binaries.

** Ed Seidel** reported that the Grand Challenge community is getting
ready to tackle a new challenge: the fully relativistic,
three dimensional treatment of the merger of neutron star binaries. The
plan is to use post-Newtonian techniques for the pre-coalescence
phase, and then take the results of this as initial data (at a
separation of about **8M**) for the general relativistic hydrodynamical
calculation. The relativistic field equations will be even more
difficult to handle than the hydrodynamic equations, requiring the
development of suitable algorithms, of adaptive mesh techniques,
finding the best gauge conditions, and an effective use of parallel
algorithms.

** Heinz Herold** discussed the effects of various equations of state
and of rapid rotation on the equilibrium state of neutron stars. The
structure equations can be solved using a variational principle in the
form of a minimum surface problem. The numerical treatment is based on
a finite element discretization. It turns out that higher mass models
allow higher angular velocities. The deformation of the surface of the
star was visualized using isometric embedding (for its internal
geometry) or ray-tracing (for a view from the outside).

An excursion to the Drachenfels, a nearby hill featuring ancient ruins of a fortress, a grand view over the Rhine river, and a restaurant, provided some welcome diversion for the participants on Tuesday afternoon.

Instabilities of rotating stars can be quite frightening: ** Lee
Lindblom** pointed out that in principle, every star, even the earth,
shows rotational instability. Luckily, they are usually countered by
dissipative effects. Using a Newtonian two-potential technique, he
found that the balance may be in favor for the instabilities in the
case of neutron stars. However, it is not clear if they can prevail in
a relativistic context, since they will be damped by gravitational
radiation. At least for realistic equations of state, they may turn
out not to be an issue.

** Hans-Peter Nollert** discussed treating collisions of black holes
and neutron stars without the help of supercomputers. He pretended
that the two colliding bodies are like a perturbation of the single
final object: . The gravitational radiation emitted
during and after the collision can then be obtained from linear
equations. For black holes, the comparison with the full numerical
calculations is remarkably good. He wishes he could do the same trick
for neutron stars - if only a good fairy would take care of the
initial conditions...

Whatever the central source of a gamma ray burster actually is, there
has to be a fireball - unless gamma ray bursters are local, i.e. less
than 200pc away. With this premise, ** Peter Mészáros** gave
theoretical explanations for many observed features of these elusive
objects, based on the expansion of the fireball and the dissipation of
its energy.

** Harald Riffert** provided the necessary ingredients for a model of
thin accretion disks around black holes: Solve the gas dynamics in the
equatorial plane of a Kerr background metric, build the
energy-momentum tensor from an ideal fluid, viscous stress, and
radiation flux, assume the disk to be stationary and rotationally
symmetric, with velocities dominant in the
direction. Integrating over the height of the disk, the vertical
structure equations decouple from the radial part. The radial disk
structure can be solved analytically, and the vertical equations have
the same form as in the Newtonian case. The resulting model spectra
can be fit to the UV-soft X-ray continuum of AGN.

When it comes to rapidly rotating relativistic systems, most work has
concentrated on black holes and neutron stars, with little attention
payed to other systems. ** Jim Ipser** studied rapidly rotating
accretion disks around compact objects, using a quasinormal mode
analysis for perturbations of a simple equilibrium model. As a clever
trick, he uses the perturbed Euler equation to eliminate the velocity
perturbations. Relying on the Cowling approximation eliminates the
metric perturbations, resulting in a single equation for a
potential-like fluid variable. Taking into account frame dragging, his
model provides a possible source for quasi-periodic oscillations in
black hole X-ray binaries, giving a counter-argument to the objection
that sources showing QPO's cannot be candidates for black holes.

Are quasars supermassive black holes or star clusters at the centers
of galaxies? After reviewing an impressive collection of observational
data, ** Max Camenzind** favored the black hole scenario. In order
to explain the central machine providing the power and accelerating
the observed jets, magnetic fields are required. Consequently, the
magnetohydrodynamics of disks in the background field of rapidly
rotating stars was the topic of the second part of his lecture.

** Fred Rasio** presented a three dimensional Newtonian treatment of the
merger phase of binary neutron star coalescence, using smoothed
particle hydrodynamics. A Newtonian treatment is interesting in its
own right: The hydrodynamics contain enough challenging physics, and
they dominate the dynamics of the merger. The results can thus serve
as preliminary estimates for the gravitational radiation emitted
during the merger. When fully numerical codes become available, the
Newtonian results can serve as a test case. In the future, nuclear
physics, strong relativistic effects, and turbulent viscosity should
be included for a more realistic treatment.

** Pablo Laguna** studied the evolution of matter in curved spacetime,
using a smoothed particle approach on a fixed relativistic
background. The SPH simulation reproduces the results of length scale
estimates if the artificial viscosity is suitably adjusted. In
particular, he examined the tidal disruption of stars by massive black
holes. This scenario can be regarded as the fuelling process of active
galactic nuclei: A dense star cluster in the vicinity of a central,
supermassive black hole provides the necessary raw material.

A cosmic perspective was provided by ** Andreas Tammann**, who
reviewed observations determining the Hubble constant. Since
measurements of redshifts are generally undisputed, most of his talk
concentrated on determining cosmic distances. He used SNe Ia
supernovae calibrated by cepheids, the Virgo cluster, and field
galaxies. Including independent methods such as growth of supernovae
shells, gravitational lenses, or fluctuations of the microwave
background, he arrived at a value of . He warned the
audience to be critical of headlines which will soon appear in popular
newspapers, claiming that a new distance determination of the Fornax
cluster in the southern hemisphere leads to , since this
value may be based on improper identification of distances. He
discussed estimates for the age of the universe, which he puts at yrs, compatible with his favored value of .

** Michael Soffel** reviewed experiments relating to gravity: Is there
a fifth force (pronounced dead), does the gravitational constant
depend on time (not to within one part in ), and what is its
numerical value (the worst known physical constant)? He discussed the
weak equivalence principle, the Einstein EP, and the strong EP. All
are very well confirmed by various experiment; improved measurements
are desirable with respect to some quantizations of gravity, which
might cause tiny deviations (). With regard to
general relativity, he discussed perihelion advance, light deflection,
timing delay, and the Lense-Thirring effect.

The proceedings of the school will be published by Vieweg in early 1997.

The organizers, Hanns Ruder, Harald Riffert, and Hans-Peter Nollert for the Astronomische Gesellschaft and Friedrich Hehl for the Deutsche Physikalische Gesellschaft, wish to acknowledge the generous financial support from the WE-Heraeus Foundation which made this school possible.

We wish to point out that the names of the organizers of last year's school on ``Relativity and scientific computing'', Friedrich Hehl and Roland Puntigam for the Deutsche Physikalische Gesellschaft and Hanns Ruder for the Astronomische Gesellschaft, were inadvertently left out of the report on this school in the last issue of Matters of Gravity.

Sun Sep 1 16:45:26 EDT 1996