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% International Cosmic Ray Conference 2007 Merida Yucatan Mexico
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%The paper title
\title{Long-term VHE $\gamma$-ray monitoring of bright blazars with a dedicated Cherenkov telescope}
%Short title to print in the headers to the final publication (Not showed in this print).
\shorttitle{Blazar monitoring with a dedicated IACT}
%All paper authors
\authors{T.~Bretz$^{1}$, M.~Backes$^{2}$, W.~Rhode$^{2}$, K.~Mannheim$^{1}$, J.~Becker$^{2}$, D.~Dorner$^{1}$, T.~Kneiske$^{2}$, M.~Meyer$^{1}$.}
%Short title to print in the headers to the final puplication (Not showed in this print).
\shortauthors{T. Bretz and M. Backes and et al}
%All the affiliations.
\afiliations{$^1$Universit\"{a}t W\"{u}rzburg, Am Hubland, 97074 W\"{u}rzburg, Germany\\
$^2$Universit\"{a}t Dortmund, Otto-Hahn-Stra{\ss}e 4, 44227 Dortmund, Germany}
\email{tbretz@astro.uni-wuerzburg.de}

%The abstract.
\abstract{High-peaked BL Lacertae objects are a prime source
population for studies with Cherenkov telescopes. It is obvious that
monitoring observations of strong blazars are orthogonal to the mission
of the larger Cherenkov telescopes, as H.E.S.S. and MAGIC with their
discovery potential for new sources (luminosity function, redshift
distribution). We propose to set up a Cherenkov telescope with low-cost
but high performance design for robotic operation. The goal is to
achieve long-term monitoring of bright blazars which will unravel the
origin and nature of their variability. The telescope design is based
on a technological upgrade of one of the former telescopes of the HEGRA
collaboration on the Canarian Island La Palma (Spain). A first study
is presented.}

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\paragraph{Introduction}

Since \cite{Chandrasekhar:1931} the termination of the HEGRA observations, the succeeding
experiments MAGIC and H.E.S.S.\ have impressively extended the physical
scope of gamma ray observations by detecting tens of formerly unknown
gamma ray sources and analyzing their energy spectra and temporal
behavior. This became possible by lowering the energy threshold from
700 GeV to less than 100 GeV and increasing at the same time the
sensitivity by a factor of five.

To fully exploit the discovery potential of the improved sensitivity,
the discovery of new, faint objects has become the major task for the
new telescopes. A diversity of astrophysical source types such as
pulsar wind nebulae, supernova remnants, microquasars, pulsars, radio
galaxies, clusters of galaxies, gamma ray bursts, and blazars can be
studied with these telescopes and limits their availability for
monitoring purposes of well-known bright sources.

But there are strong reasons to make an effort for the continuous
monitoring of the few exceptionally bright blazars. This can be
achieved by operating a dedicated monitoring telescope of the
HEGRA-type, referred to in the following as DWARF (Dedicated
multiWavelength Agn Research Facility). The reasons are outlined in
detail below.

\paragraph{Science case}
The variability of blazars, seen across the entire electromagnetic
spectrum, arises from the dynamics of relativistic jets and the
particle acceleration going on in them. The jets are launched in the
vicinity of accreting supermassive black holes. Theoretical models
predict variability arising from the interplay between jet expansion,
particle injection, acceleration and cooling.

Long-term monitor observations of bright blazars are the key to obtain
a solid data base for variability investigations. 

An understanding of this variability will deepen our knowledge about
\begin{itemize}
\item the composition and generation of the jets, intimately connected
to the physics of the ergosphere of rapidly spinning black holes
embedded into the hot plasma from the accretion flow.
\item  the plasma physics responsible for highly efficient particle
acceleration, bearing similarities to plasma physics of the interaction
between extremely intense laser beams and matter.
\item  the orbital modulation of jets due to binary black holes
expected from galaxy merger models.
\end{itemize}
Assuming conservatively the performance of a single HEGRA-type telescope,
long-term monitoring of at least the following blazars is possible:
Mrk421, Mrk501, 1ES\,2344+514, 1ES\,1959+650, H\,1426+428, PKS\,2155-304.
We emphasize that DWARF will run as a facility dedicated to these
targets only, providing a maximum observation time for the program.
\begin{itemize}
\item Flux variations will be determined and compared with variability
properties in other wavelength ranges.
\item Hadronic emission processes and possible coincidences between
VHE-gamma and neutrino-emission will be investigated.
\item The search for signatures of binary black hole systems from
orbital modulation of VHE gamma ray emission will be performed.
\end{itemize}
Furthermore, we seek to obtain know-how for the operation of future
networks of Cherenkov telescopes (e.g. a monitoring array around the
globe or CTA) or telescopes at inaccessible sites.

At least one of the proposed targets will be visible any time of the
year. For calibration purposes, some time will be scheduled for
observations of the Crab nebula, which is the brightest known VHE
emitter with constant flux.

In detail the following investigations are planned:
\begin{itemize}
\item As a direct result of the measurements, the duty cycle, the
baseline emission, and the power spectrum of flux variations will be
determined and compared with variability properties in other wavelength
ranges.
\item The lightcurves will be interpreted using models for the
nonthermal emission from relativistically expanding plasma jets.
\item The black hole mass and accretion rate will be determined from
the emission models. Estimates of the black hole mass from emission
models, a possible orbital modulation, and the Magorrian relation
(relating the black hole mass with the stellar bulge mass of the host
galaxy) will be compared.
\item When flaring states will be discovered during the monitor
program, MAGIC will issue a Target of Opportunity observation to obtain
better time resolution.
\item Correlating the arrival times of neutrinos detected by the
neutrino telescope IceCube with simultaneous measurements of DWARF will
allow to test the hypothesis that flares in blazar jets are connected
to hadronic emission processes and thus to neutrino emission from these
sources. The investigation proposed here is complete for both neutrino
and gamma observations, and can therefore lead to conclusive results.
\item The diffuse flux of escaping UHE cosmic rays obtained from
AUGER or flux limits of neutrinos from IceCube, respectively, will be
used to constrain models of UHE cosmic ray origin and large-scale
magnetic fields.
\item Multi-frequency observations together with the Metsähovi Radio
Observatory and the optical Tuorla Observatory are planned. The
measurements will be correlated with INTEGRAL and GLAST results, when
available. X-ray monitoring using the SWIFT and Suzaku facilities will
be proposed.
\item The most ambitious scientific goal of this proposal is the search
for signatures of binary black hole systems from orbital modulation of
VHE gamma ray emission. In case of a confirmation of the present hints
in the temporal behaviour of Mrk501, gravitational wave templates could
be computed with high accuracy to establish their discovery with LISA.
\end{itemize}

\paragraph{Technical setup}

At the Observatorio del Roque de los Muchachos (ORM), at the MAGIC site, the
mount of the former HEGRA telescope CT3 now owned by the MAGIC
collaboration is still operational.
Basic support from the  shift crew of MAGIC is guaranteed, although
robotic operation is the primary goal. Robotic operation is necessary
to reduce costs and man power demands. Furthermore, we seek to obtain
know-how for the operation of future networks of Cherenkov telescopes.
>From the experience with the construction and operation of MAGIC or
HEGRA, respectively, the proposing groups consider the planned focused
approach (small number of experienced scientists) as optimal for
achieving the project goals. The available automatic analysis package
developed for MAGIC is modular and flexible, and can thus be used with
minor changes for the DWARF project.

To complete the mount to a functional Cherenkov telescope, the following steps are necessary:

{\em Camera} For long-term observations stability of the camera is a
major criterion. To keep the systematic errors small good background
estimation is mandatory. The only possibility for a synchronous
determination of the background is the determination from the night-sky
observed in the same field-of-view with the same instrument. To achieve
this the observed position is moved out of the camera center which
allows the estimation of the background from positions symmetric with
respect to the camera center (so called wobble-mode). This observation
mode increases the sensitivity by at least a factor of two because
spending observation for dedicated background observations becomes
obsolete, which also ensures a better time coverage of the observed
sources. Having a camera large enough allowing more than one
independent position for background estimation increases sensitivity
further by better background statistics. This is the case if the source
can be shifted 0.6$^\circ$-0.7$^\circ$ out of the camera center.
To decrease the dependence of the background measurement on the camera
geometry, a camera layout as symmetric as possible will be chosen.
Consequently a camera allowing for wobble-mode observations should be
round and have a diameter of 4.5$^\circ$-5.0$^\circ$ to completely
contain shower images of events in the TeV energy range. To achieve
this requirements a 313 pixel camera can be build
based on the experience with HEGRA and MAGIC. Photomultipliers with a 
diameter of 19\,mm and with a quantum efficiency improved by 20\% with 
respect to the old CT3 system are considered.

They ensure a granularity which is enough to guarantee good results
even below the flux peak energy. Each individual pixel has to be
equipped with a preamplifier, an active high-voltage supply and
control. If development of G-APDs ($QE \ge50\%$) will be fast enough,
respectively the price low enough, and their long term stability is
proven well in time, their usage will be considered. For a transition
time one of the old HEGRA cameras might be used. With a special
coating (wavelength shifter) its quantum efficiency might be improved
by ~8$\%$.

{\em Camera support} The camera chassis must be water tight. An
automatic lid protecting the PMs at day-time will be installed. For
further protection a plexi-glass window will be installed in the front
of the camera. By over-coating the window with an anti-reflex layer of
magnesium-fluoride a gain in transmission of 5$\%$ is expected. Each PM
will be equipped with a light-guide. The current design will be
improved by using a high reflectivity mirror-foil, to reach a
reflectivity in the order of 98$\%$. In total this will gain another ~15$\%$ in
light-collection efficiency compared to the old CT3 system.

For this setup the camera holding has to be redesigned. An electric and
optical shielding of the individual PMs is planned.

{\em Data acquisition} For the data acquisition system a low-cost
hardware readout based on an analog ring buffer (Domino II/III),
currently developed for the MAGIC II readout, will be used.
The low power consumption will allow to include the digitization near
the signal source which makes an analog signal transfer obsolete. The
advantage is less pick-up noise and less signal dispersion. By high
sampling rates (0.5\,GHz-1.2\,GHz), additional information about the
pulse shape can be obtained. This increases the over-all sensitivity
further, because the short integration time allows for almost perfect
suppression of noise due to night-sky background photons. The estimated
trigger-rate of the telescope is below 100\,Hz (HEGRA:
$<$10\,Hz) which allows to use a low-cost industrial solution for readout
of the system like USB2.0. As for the HEGRA telescopes, a simple
multiplicity trigger is enough, but also a simple three-next-neighbors
(closed package) could be programmed. 
Additional data reduction and preprocessing in the readout hardware or
the readout computer is provided. Assuming conservatively storage of
raw-data at a readout rate of 30\,Hz the storage space needed is less
than 250\,GB/month or 3\,TB/year.

{\em On-site computing} For on-site computing three standard
PCs are needed. This includes readout and storage, preprocessing, and
telescope control. 
The data will be transmitted as soon as possible after data taking via
Internet to the Datacenter in W\"urzburg. Absolute timing necessary for an
accurate source tracking will be achived by a GPS clock.

{\em Mount and Drive} The present mount is used. Only smaller
changes for safety, corrosion protection, cable ducts, etc. is
needed. For movement motors, shaft encoders and control electronics
have to be bought. The drive system should allow for relatively fast
repositioning for three reasons:
\begin{itemize} 
\item Fast movement is in most cases mandatory for future ToO observations.
\item Wobble-mode observations will be done changing the
wobble-position continuously (each 20\,min) for symmetry reasons. 
\item To ensure good time coverage of more than one source visible at
the same the observed source will be changed in constant time intervals
($\sim$20min).  
\end{itemize}
Therefore three 150\,Watt servo motors are intended. A microcontroller
based motion control unit (SPS) similar to the one of the current MAGIC
II drive system will be used. For communication with the readout-system
a standard Ethernet connection based on the TCP/IP- and UDP-protocol is
applied.

{\em Mirrors} The existing mirrors are replaced by new plastic mirrors.
The cheap and light-weight material has formerly been used for Winston
cones flown in balloon experiments. The mirrors are copied from a
master, coated with a reflecting and a protective material. 
By a change of the mirror geometry the mirror area can be increased
from 8.5\,m$^2$ to 13\,m$^2$; this includes an increase
of $\sim$10$\%$ per mirror by using a hexagonal layout. 
To keep track of the alignment, reflectivity and optical quality of the
individual mirrors, and the point-spread function of the total mirror,
during long-term observations the application of an automatic mirror
adjustment system, as successfully operated on the MAGIC telescope, is
intended. 

{\em Pointing calibration} To correct for axis misalignments
a pointing correction algorithm as used in the MAGIC tracking system
will be applied. It is calibrated by measuring the reflection of
bright guide stars on the camera surface and ensures a pointing
accuracy well below the pixel diameter. Therefore a high sensitive
low-cost video camera, as already in operation for MAGIC I and II, will
be installed.

{\em PM Gain calibration} For the calibration of the PM gain a
calibration system as used for the MAGIC telescope is build.

\paragraph{Conclusion}
The setup of a small telescope dedicated for long-term AGN monitoring 
is easily feasible. Such an activity is motivated by a variety of physical 
questions to be answered by the integration of this instrument in 
multiwavelength observations.

\paragraph{Future extensions}
The known duty cycle of 10\% ($\sim$1000h/year) for a Cherenkov
telescope operated at La Palma limits the time-coverage of the
observations. Therefore we propose a worldwide network of ($<10$) small
scale Cherenkov telescopes to be build in the future allowing 24h
monitoring of the bright AGNs. Such a system is so far completely
unique in this energy range. 

\section{Acknowledgements}
We would like to thank Eckart Lorenz, Riccardo Paoletti, Adrian Biland, 
Maria Victoria Fonseca and Jos$\acute{e}$ Luis Contreras for intense discussions 
and Christian Spiering, Brenda Dingus, Maria Magdalena Gonzalez Sanchez 
and the Magic collaboration for helpful support.\\

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