The network program combines new strategies in empirical and theoretical research towards an understanding of structure and radiation processes of quasars. Linear dimensions of important entities below the angular resolution that can be achieved with any direct imaging technique will be revealed through densely sampled monitoring. Causality relates characteristic time-scales to characteristic length scales, high order moments in statistical analyses shall be used to constrain the geometry and spatial structure of entities on different scales. Combined with regular direct imaging studies at the highest angular resolution achievable, this program will bridge the gap to structural studies possible up to date. State-of-the art numerical models will be tested against the empirical results in this crucial overlapping range where different approaches meet. Likewise, the conclusions on spatial structure will provide new clues for future MHD simulations. Radiation processes will be studied in the extreme cases, involving the highest energies and the highest photon densities. This will involve coordinated, multifrequency monitoring together with a new generation of spacecrafts and ground-based instruments, which - by their overlapping operation for the period of this network - will provide a unique opportunity for multifrequency studies throughout the entire high-frequency part of the electromagnetic spectrum. Likewise, the joint efforts shall be used to solve the long-standing question on the highest photon densities in quasars. Solutions on both problems are expected to provide insight into the fundamental astrophysical problem of particle acceleration and will lead to a new understanding of energy transport in quasars.
The key element in exploring quasar structure through monitoring is to construct homogeneous data-banks of densely sampled, high precision flux measurements covering a wide range in timescales. Information on quasar structure shall be extracted using innovative statistical means. Results shall be tested in a range of linear scales which can be probed with those techniques which provide the highest resolution in direct imaging in a small number of sources, and will be amalgamated with direct imaging studies through interferometry for general conclusions.
An essential aspect in the construction of the data sets is the combination of the leading teams for obtaining sufficiently dense coverage and overcome the problem of low duty cycles. Tools shall be developed to make the archives homogeneous and ensure homogeneous data-taking in future studies. In a first phase we expect to homogenize data taking and reduction strategies, set up joint target lists and arrange collaborative campaigns. In a second phase, a network of fully robotic systems shall replace these labor-intensive approach. Future progress can only be ensured if data taking can be automated to guarantee long-term continuity. Development of common standards to robotic observing and automated telescope operation and data analysis of high-quality flux determinations is essential. Homogeneous archives of the Members are essential to provide adequate data for long-term coverage (probing long time-scales and large linear scales). Adequate data-mining tools to combine distributed archives will be developed. The data-sets will thus become large enough to use new statistical tools which could not be applied to such studies in the past which had been constrained to smaller data sets. The statistical tools will be developed in close cooperation with theoretical models on quasar structure to allow optimum extraction of astrophysical information.
The parameter-space available for studies of quasar structure shall also be extended in the opposite way, towards faster variability. This will involve optimized use of fast photometry for quasar variability and exploration of the regime of rapid variations. We will explore the high temporal frequency regime of parameter space and to study the rare and very fast events which may ultimately resolve the particle acceleration in AGN. It will determine the regime where variability time-scales are set by acceleration and cooling processes rather than source geometry.
While variability is the only tool to probe substructure in quasars on the important linear scales that cannot be studies with interferometric techniques, even elaborate statistical means cannot avoid that results are more ambiguous than those obtained through direct imaging. Fortunately, there is a small subset of sources where the highest resolution that can be achieved through direct imaging overlaps with the largest scales that can be probed through variability. Reconstructing structure from variability can hence in principle be tested against direct imaging in those few cases. We shall carry out the necessary campaigns jointly among the members. The cross-check enabled through these sources will then allow us to amalgamate the results on structural variations from variability with those of VLBI imaging for the larger set of sources that shall be studies within out program to cover the required range in quasar properties.
Understanding radiation processes requires simultaneous studies at different bands. In case of high photon densities optical and radio regimes are most important, and the prime question in the separation of intrinsic and extrinsic effects in radio variability. This shall be solved by coordinating the observations in the two bands that will be obtained to determine structural properties to guarantee simultaneous coverage on different temporal scales. We will also use the coordinated coverage of flux variability and interferometric imaging to determine the role of extrinsic variability.
Investigations of radiation processes at the highest photon energies rely on determinations of the correlation between variations that are seen at the high-frequency end of the electro-magnetic spectrum. Simultaneous multi-frequency monitoring will allow us to trace the evolution of flares throughout the entire waveband regime and hence determine the nature of the dominant radiation processes. The degree of correlation will clarify the degree of radiative reprocessing and will thus identify the primary radiation mechanism. The specific spectral evolution will be reconstructed from the comparison of variability patterns at different frequencies. This will allow us to disentangle geometric and radiative effects, which can then be modeled individually. In order to compare the behavior at different frequencies, observations will be coordinated to achieve simultaneous coverage, making use of the unprecedented opportunities that will be available in Europe in the forthcoming years thanks to a unique set of new gamma-ray instruments and facilities (INTEGRAL, AGILE, HESS and MAGIC). The following years offer the unprecedented chance of complete wavelength coverage above 1 eV. We will construct a multifrequency archive to combine all data available from the studies of the best-observed sources.
All of the key elements described above involve methodological, empirical and theoretical research in a dynamic fashion. Theoretical modeling of the instantaneous spectral energy distributions extracted from the simultaneous monitoring efforts at an early stage will identify the crucial regimes which require denser sampling in subsequent monitoring campaigns. Empirical results will continuously be compared with analytical results via numerical modeling in all fields involved. Combining information about sub-structure on the smallest scales probed by variability with requirements on particle distribution functions derived from information on radiation mechanisms, particle acceleration mechanisms and thus fundamental MHD dynamics in Quasars will be probed. This will provide the necessary information to study the energy transport and loss channels within Quasar jets and outflows.
The science activities of this network are addressed in six different research
themes. They are closely connected to each other. While all of the empirical
themes drive the requirements of the hardware/software theme, the empirical
projects aim at maximizing efficiency by teaming up observational resources
and
coordinated campaigns.
The two theoretical themes have many links among
themselves and obvious connections to the three empirical subjects.
Currently most optical monitoring programs are run by observers
specifying individual exposures, achieving accuracies of about 1%
with sampling times of a few 100 to 10 000 sec. In order to improve
quality and quantity of optical monitoring the network aims at
determining a better understanding of the parameters that affect the
quality of the photometry of point sources in differential photometry
and implementing programs that allow measurements with
accuracies close to the photon flux limit. In parallel
it aims at an assessment of the technological requirements for robotic
telescopes and practical implementation of a network of
robotic telescopes that shall be operated by several teams of the
network.
Nearly 15 years ago rapid radio variability was discovered. Fast changes of
flux density could be due to interstellar scattering, but would then be
restricted to low radio frequencies. If they are intrinsic in nature,
very high apparent brightness temperatures are required. Both explanations
are linked, and it is important to separate intrinsic effects from
interstellar scintillation to determine plasma properties and radiation
mechanisms of intrinsic IDV and to make use of RISS induced variability
as an ultra-high resolution interferometer. The network plans to
follow several routes to separate intrinsic and extrinsic effects:
We intend to carry out several pioneering experiments involving
monitoring at cm, sub-mm, IR, and optical frequencies to separate the
role of interstellar scintillation (which would be negligible in the
sub-mm regime), followed by the implementation of observing
strategies for optimized dedicated programs which will be possible using
large-scale European facilities. A second attempt aims
at determining brightness temperatures at optical and IR wavelengths from
very fast optical fluctuations.
We plan to arrange and carry out coordinated multi-frequency campaigns,
making use of the first-time availability of a complete wavelength coverage,
including radio-, mm-, near-IR, optical, X-ray, and gamma-ray
instrumentation.
We want to combine the existing data sets in the different institutions
which have been acquired to study long term variations. Combinations of these
data will provide a much denser coverage and enable us to study the
low-frequency end of the important fast variations.
We want to exploit the huge data-base assembled by several of the teams
on the peculiar object OJ 287, which is the best case known for periodic
variability. The origin of the periodicity is unknown but likely to hold
important clues to quasar variability in general.
a) How to transform bulk kinetic energy of jets into radiation?
b) The relationship of jet power and power of accretion.
c) The origin of the FR-I/FR-II dichotomy.
Concerning the first aspect, there is a general agreement that the
luminosity produced by jets is
the result of the transformation of part of their bulk kinetic
energy into random energy of electron which can then radiate.
While details of this conversion have been unclear so far,
we aim at a significant advance in our knowledge in this respect
due to the refinement of the theory and to the numerical simulations.
We will explore whether intermittent flows where different parts of the
plasma go at slightly different bulk velocities may produce shocks which
then accelerate particles to relativistic random velocities.
Numerical simulations will test whether this can account for the observed SED
and whether correlations among variations between different wavebands are
expected. A more advanced step are numerical simulation which
test the collision process itself, jet stability and the amount of
jet power transferred to electrons, protons, and to the magnetic field .
The second topic concerns the relation between the
kinetic power carried by the jet and the luminosity emitted through the
accretion process. A main uncertainty is the nature of the matter content
of the jet: are they dominated by a normal electron-proton plasma or is
the contribution of electron--positron pairs important?
We expect that our observations simultaneous with AGILE will
give a major contribution to this problem, since it will allow
estimates of the jet power for a large number of blazars.
Recent advances in our understanding the massive black holes are ubiquitous
with a wide variety of black hole masses leads to the question
whether the two main type of radio-galaxies, namely FR I and FR II
are characterized by different black hole masses. Indications are
expected from different variability characteristics, jet structure,
and our understanding of the MHD flows. Using advances in these fields,
we will explore why the black hole mass determines structural
differences on the tens of kiloparsec scale,
which shall lead to new ideas to explain this stunning
weak/strong radio-galaxy dichotomy.
In addition to these six specific themes, a joint effort towards establishing a
data-bank of densely sampled, long-term, high quality photometric data
is pursued by all teams. This effort is in close contact with each of
the six specific themes. It will provide the basis for exploration of
extended parts of parameter space (short time-scales and long time-scales).
1) Towards automated, fast, and accurate photometry:
Convener: N. Smith, CIT, Ireland, Depute: S. Wagner, LSW, Germany2) Separating intrinsic and extrinsic Intraday Variability:
Convener: C. Raiteri, OAT, Italy, Depute: A. Witzel, MPIfR, Germany3) Radiation processes at high energies:
Coordinated multi-frequency monitoring is an essential
for the understanding of radiation mechanisms. The high-energy end
of the synchrotron branch and the Compton-scattered emission are
of special importance. The network shall develop efficient
techniques for the long-term operation of a network of robotic
stations. First steps include the establishment of an archive,
and development of efficient statistical tools
for detailed analysis of variability data.
It will set up strategies for coordinated long-term monitoring
programs, which will be used to carry out such long-term simultaneous
observations in parallel with the European Missions AGILE and INTEGRAL
and which will act as a trigger to the European
TeV facilities HESS and MAGIC. Detailed studies
shall be carried out for periods of about two weeks together with
XMM, INTEGRAL, and ground-based Cerenkov telescopes about twice
a year on sources of different overall properties.
The results of short-term and long-term monitoring will be used to
improve our understanding of radiation mechanisms and particle
acceleration in different environments.
Convener: L. Takalo, Tuorla, Finland, Depute: S. Wagner, LSW, Germany4) Variations of Source Structure and Flux:
Imaging studies at high resolution are only possible in the radio domain.
They exhibit structure on all linear scales that have been probed so far.
Substructure on small scales is obviously associated with variability on
time scales comparable to the travel time through the source. The details
of this relation are linked to the kinematics of substructure in the jets,
and hence with the MHD properties. They can best be explored in the
overlapping region where the fastest changes in structural variations
correspond to well-sampled long-tern flux-density monitoring.
This requires a dramatic increase in sampling rate and dynamic range
of VLBI campaigns. We intend to carry out a pioneering experiment
to determine optimum strategies for several campaigns.
In parallel we plan to set up an international data-base
for well-sampled sources in parallel with long-term studies.
Convener: A. Witzel, MPIfR Bonn, Germany, Depute: M. Tornikoski, HUT,
Finland
References: A. Zensus, 1997, Ann. Rev. Astron. Astrophys., 35, 607.
5) Particle acceleration in MHD outflows:
The observations provided by the empirical programs allow detailed
investigations of physical properties that are crucial for the
observational signature: Particle acceleration and kinematic
properties of the MHD outflows. Two obvious areas of research are
studies of the saturation of particle acceleration at a shock front in
the presence of synchro-Compton losses for the accelerated, high
energy electrons as well as studies of the radiative
signatures produced in such a picture, application to Blazar jets, and
comparison with the multi-wavelength observations on the
one hand. On the other hand we will study the jet formation in
two-component relativistic and non-relativistic MHD outflows from AGN
and study of the radiation produced from particles
accelerated in shock fronts arising naturally in discontinuities in
these flows.
Convener: K. Tsinagnos, IASA, Greece, Depute: S. Wagner, LSW, Germany
The detailed physics studied in topic 5 shall be used to understand global
properties of AGN jets. The research and training aims are divided into
three parts:
Convener: G. Ghisellini, OAB, Italy, Depute: S. Wagner, LSW, Germany