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1%%
2% International Cosmic Ray Conference 2007 Merida Yucatan Mexico
3%%
4
5%Class Requeried
6\documentclass[dvips,openbib]{article}
7%The ICRC Style
8\usepackage{icrctc07}
9
10%The paper title
11\title{Long-term VHE $\gamma$-ray monitoring of bright blazars with a dedicated Cherenkov telescope}
12%Short title to print in the headers to the final publication (Not showed in this print).
13\shorttitle{Blazar monitoring with a dedicated IACT}
14%All paper authors
15\authors{T.~Bretz$^{1}$, M.~Backes$^{2}$, W.~Rhode$^{2}$, K.~Mannheim$^{1}$, J.~Becker$^{2}$, D.~Dorner$^{1}$, T.~Kneiske$^{2}$, M.~Meyer$^{1}$.}
16%Short title to print in the headers to the final puplication (Not showed in this print).
17\shortauthors{T. Bretz and M. Backes and et al}
18%All the affiliations.
19\afiliations{$^1$Universit\"{a}t W\"{u}rzburg, Am Hubland, 97074 W\"{u}rzburg, Germany\\
20$^2$Universit\"{a}t Dortmund, Otto-Hahn-Stra{\ss}e 4, 44227 Dortmund, Germany}
21\email{tbretz@astro.uni-wuerzburg.de}
22
23%The abstract.
24\abstract{High-peaked BL Lacertae objects are a prime source
25population for studies with Cherenkov telescopes. It is obvious that
26monitoring observations of strong blazars are orthogonal to the mission
27of the larger Cherenkov telescopes, as H.E.S.S. and MAGIC with their
28discovery potential for new sources (luminosity function, redshift
29distribution). We propose to set up a Cherenkov telescope with low-cost
30but high performance design for robotic operation. The goal is to
31achieve long-term monitoring of bright blazars which will unravel the
32origin and nature of their variability. The telescope design is based
33on a technological upgrade of one of the former telescopes of the HEGRA
34collaboration on the Canarian Island La Palma (Spain). A first study
35is presented.}
36
37%%%%%%%%%%%%%%%%%%%% B E G I N D O C U M E N T%%%%%%%%%%%%%%%%%%%%%%%
38\begin{document}
39
40\input icrc0974.def
41
42\maketitle
43%Begin the section.
44
45\paragraph{Introduction}
46
47Since \cite{Chandrasekhar:1931} the termination of the HEGRA observations, the succeeding
48experiments MAGIC and H.E.S.S.\ have impressively extended the physical
49scope of gamma ray observations by detecting tens of formerly unknown
50gamma ray sources and analyzing their energy spectra and temporal
51behavior. This became possible by lowering the energy threshold from
52700 GeV to less than 100 GeV and increasing at the same time the
53sensitivity by a factor of five.
54
55To fully exploit the discovery potential of the improved sensitivity,
56the discovery of new, faint objects has become the major task for the
57new telescopes. A diversity of astrophysical source types such as
58pulsar wind nebulae, supernova remnants, microquasars, pulsars, radio
59galaxies, clusters of galaxies, gamma ray bursts, and blazars can be
60studied with these telescopes and limits their availability for
61monitoring purposes of well-known bright sources.
62
63But there are strong reasons to make an effort for the continuous
64monitoring of the few exceptionally bright blazars. This can be
65achieved by operating a dedicated monitoring telescope of the
66HEGRA-type, referred to in the following as DWARF (Dedicated
67multiWavelength Agn Research Facility). The reasons are outlined in
68detail below.
69
70\paragraph{Science case}
71The variability of blazars, seen across the entire electromagnetic
72spectrum, arises from the dynamics of relativistic jets and the
73particle acceleration going on in them. The jets are launched in the
74vicinity of accreting supermassive black holes. Theoretical models
75predict variability arising from the interplay between jet expansion,
76particle injection, acceleration and cooling.
77
78Long-term monitor observations of bright blazars are the key to obtain
79a solid data base for variability investigations.
80
81An understanding of this variability will deepen our knowledge about
82\begin{itemize}
83\item the composition and generation of the jets, intimately connected
84to the physics of the ergosphere of rapidly spinning black holes
85embedded into the hot plasma from the accretion flow.
86\item the plasma physics responsible for highly efficient particle
87acceleration, bearing similarities to plasma physics of the interaction
88between extremely intense laser beams and matter.
89\item the orbital modulation of jets due to binary black holes
90expected from galaxy merger models.
91\end{itemize}
92Assuming conservatively the performance of a single HEGRA-type telescope,
93long-term monitoring of at least the following blazars is possible:
94Mrk421, Mrk501, 1ES\,2344+514, 1ES\,1959+650, H\,1426+428, PKS\,2155-304.
95We emphasize that DWARF will run as a facility dedicated to these
96targets only, providing a maximum observation time for the program.
97\begin{itemize}
98\item Flux variations will be determined and compared with variability
99properties in other wavelength ranges.
100\item Hadronic emission processes and possible coincidences between
101VHE-gamma and neutrino-emission will be investigated.
102\item The search for signatures of binary black hole systems from
103orbital modulation of VHE gamma ray emission will be performed.
104\end{itemize}
105Furthermore, we seek to obtain know-how for the operation of future
106networks of Cherenkov telescopes (e.g. a monitoring array around the
107globe or CTA) or telescopes at inaccessible sites.
108
109At least one of the proposed targets will be visible any time of the
110year. For calibration purposes, some time will be scheduled for
111observations of the Crab nebula, which is the brightest known VHE
112emitter with constant flux.
113
114In detail the following investigations are planned:
115\begin{itemize}
116\item As a direct result of the measurements, the duty cycle, the
117baseline emission, and the power spectrum of flux variations will be
118determined and compared with variability properties in other wavelength
119ranges.
120\item The lightcurves will be interpreted using models for the
121nonthermal emission from relativistically expanding plasma jets.
122\item The black hole mass and accretion rate will be determined from
123the emission models. Estimates of the black hole mass from emission
124models, a possible orbital modulation, and the Magorrian relation
125(relating the black hole mass with the stellar bulge mass of the host
126galaxy) will be compared.
127\item When flaring states will be discovered during the monitor
128program, MAGIC will issue a Target of Opportunity observation to obtain
129better time resolution.
130\item Correlating the arrival times of neutrinos detected by the
131neutrino telescope IceCube with simultaneous measurements of DWARF will
132allow to test the hypothesis that flares in blazar jets are connected
133to hadronic emission processes and thus to neutrino emission from these
134sources. The investigation proposed here is complete for both neutrino
135and gamma observations, and can therefore lead to conclusive results.
136\item The diffuse flux of escaping UHE cosmic rays obtained from
137AUGER or flux limits of neutrinos from IceCube, respectively, will be
138used to constrain models of UHE cosmic ray origin and large-scale
139magnetic fields.
140\item Multi-frequency observations together with the Metsähovi Radio
141Observatory and the optical Tuorla Observatory are planned. The
142measurements will be correlated with INTEGRAL and GLAST results, when
143available. X-ray monitoring using the SWIFT and Suzaku facilities will
144be proposed.
145\item The most ambitious scientific goal of this proposal is the search
146for signatures of binary black hole systems from orbital modulation of
147VHE gamma ray emission. In case of a confirmation of the present hints
148in the temporal behaviour of Mrk501, gravitational wave templates could
149be computed with high accuracy to establish their discovery with LISA.
150\end{itemize}
151
152\paragraph{Technical setup}
153
154At the Observatorio del Roque de los Muchachos (ORM), at the MAGIC site, the
155mount of the former HEGRA telescope CT3 now owned by the MAGIC
156collaboration is still operational.
157Basic support from the shift crew of MAGIC is guaranteed, although
158robotic operation is the primary goal. Robotic operation is necessary
159to reduce costs and man power demands. Furthermore, we seek to obtain
160know-how for the operation of future networks of Cherenkov telescopes.
161>From the experience with the construction and operation of MAGIC or
162HEGRA, respectively, the proposing groups consider the planned focused
163approach (small number of experienced scientists) as optimal for
164achieving the project goals. The available automatic analysis package
165developed for MAGIC is modular and flexible, and can thus be used with
166minor changes for the DWARF project.
167
168To complete the mount to a functional Cherenkov telescope, the following steps are necessary:
169
170{\em Camera} For long-term observations stability of the camera is a
171major criterion. To keep the systematic errors small good background
172estimation is mandatory. The only possibility for a synchronous
173determination of the background is the determination from the night-sky
174observed in the same field-of-view with the same instrument. To achieve
175this the observed position is moved out of the camera center which
176allows the estimation of the background from positions symmetric with
177respect to the camera center (so called wobble-mode). This observation
178mode increases the sensitivity by at least a factor of two because
179spending observation for dedicated background observations becomes
180obsolete, which also ensures a better time coverage of the observed
181sources. Having a camera large enough allowing more than one
182independent position for background estimation increases sensitivity
183further by better background statistics. This is the case if the source
184can be shifted 0.6$^\circ$-0.7$^\circ$ out of the camera center.
185To decrease the dependence of the background measurement on the camera
186geometry, a camera layout as symmetric as possible will be chosen.
187Consequently a camera allowing for wobble-mode observations should be
188round and have a diameter of 4.5$^\circ$-5.0$^\circ$ to completely
189contain shower images of events in the TeV energy range. To achieve
190this requirements a 313 pixel camera can be build
191based on the experience with HEGRA and MAGIC. Photomultipliers with a
192diameter of 19\,mm and with a quantum efficiency improved by 20\% with
193respect to the old CT3 system are considered.
194
195They ensure a granularity which is enough to guarantee good results
196even below the flux peak energy. Each individual pixel has to be
197equipped with a preamplifier, an active high-voltage supply and
198control. If development of G-APDs ($QE \ge50\%$) will be fast enough,
199respectively the price low enough, and their long term stability is
200proven well in time, their usage will be considered. For a transition
201time one of the old HEGRA cameras might be used. With a special
202coating (wavelength shifter) its quantum efficiency might be improved
203by ~8$\%$.
204
205{\em Camera support} The camera chassis must be water tight. An
206automatic lid protecting the PMs at day-time will be installed. For
207further protection a plexi-glass window will be installed in the front
208of the camera. By over-coating the window with an anti-reflex layer of
209magnesium-fluoride a gain in transmission of 5$\%$ is expected. Each PM
210will be equipped with a light-guide. The current design will be
211improved by using a high reflectivity mirror-foil, to reach a
212reflectivity in the order of 98$\%$. In total this will gain another ~15$\%$ in
213light-collection efficiency compared to the old CT3 system.
214
215For this setup the camera holding has to be redesigned. An electric and
216optical shielding of the individual PMs is planned.
217
218{\em Data acquisition} For the data acquisition system a low-cost
219hardware readout based on an analog ring buffer (Domino II/III),
220currently developed for the MAGIC II readout, will be used.
221The low power consumption will allow to include the digitization near
222the signal source which makes an analog signal transfer obsolete. The
223advantage is less pick-up noise and less signal dispersion. By high
224sampling rates (0.5\,GHz-1.2\,GHz), additional information about the
225pulse shape can be obtained. This increases the over-all sensitivity
226further, because the short integration time allows for almost perfect
227suppression of noise due to night-sky background photons. The estimated
228trigger-rate of the telescope is below 100\,Hz (HEGRA:
229$<$10\,Hz) which allows to use a low-cost industrial solution for readout
230of the system like USB2.0. As for the HEGRA telescopes, a simple
231multiplicity trigger is enough, but also a simple three-next-neighbors
232(closed package) could be programmed.
233Additional data reduction and preprocessing in the readout hardware or
234the readout computer is provided. Assuming conservatively storage of
235raw-data at a readout rate of 30\,Hz the storage space needed is less
236than 250\,GB/month or 3\,TB/year.
237
238{\em On-site computing} For on-site computing three standard
239PCs are needed. This includes readout and storage, preprocessing, and
240telescope control.
241The data will be transmitted as soon as possible after data taking via
242Internet to the Datacenter in W\"urzburg. Absolute timing necessary for an
243accurate source tracking will be achived by a GPS clock.
244
245{\em Mount and Drive} The present mount is used. Only smaller
246changes for safety, corrosion protection, cable ducts, etc. is
247needed. For movement motors, shaft encoders and control electronics
248have to be bought. The drive system should allow for relatively fast
249repositioning for three reasons:
250\begin{itemize}
251\item Fast movement is in most cases mandatory for future ToO observations.
252\item Wobble-mode observations will be done changing the
253wobble-position continuously (each 20\,min) for symmetry reasons.
254\item To ensure good time coverage of more than one source visible at
255the same the observed source will be changed in constant time intervals
256($\sim$20min).
257\end{itemize}
258Therefore three 150\,Watt servo motors are intended. A microcontroller
259based motion control unit (SPS) similar to the one of the current MAGIC
260II drive system will be used. For communication with the readout-system
261a standard Ethernet connection based on the TCP/IP- and UDP-protocol is
262applied.
263
264{\em Mirrors} The existing mirrors are replaced by new plastic mirrors.
265The cheap and light-weight material has formerly been used for Winston
266cones flown in balloon experiments. The mirrors are copied from a
267master, coated with a reflecting and a protective material.
268By a change of the mirror geometry the mirror area can be increased
269from 8.5\,m$^2$ to 13\,m$^2$; this includes an increase
270of $\sim$10$\%$ per mirror by using a hexagonal layout.
271To keep track of the alignment, reflectivity and optical quality of the
272individual mirrors, and the point-spread function of the total mirror,
273during long-term observations the application of an automatic mirror
274adjustment system, as successfully operated on the MAGIC telescope, is
275intended.
276
277{\em Pointing calibration} To correct for axis misalignments
278a pointing correction algorithm as used in the MAGIC tracking system
279will be applied. It is calibrated by measuring the reflection of
280bright guide stars on the camera surface and ensures a pointing
281accuracy well below the pixel diameter. Therefore a high sensitive
282low-cost video camera, as already in operation for MAGIC I and II, will
283be installed.
284
285{\em PM Gain calibration} For the calibration of the PM gain a
286calibration system as used for the MAGIC telescope is build.
287
288\paragraph{Conclusion}
289The setup of a small telescope dedicated for long-term AGN monitoring
290is easily feasible. Such an activity is motivated by a variety of physical
291questions to be answered by the integration of this instrument in
292multiwavelength observations.
293
294\paragraph{Future extensions}
295The known duty cycle of 10\% ($\sim$1000h/year) for a Cherenkov
296telescope operated at La Palma limits the time-coverage of the
297observations. Therefore we propose a worldwide network of ($<10$) small
298scale Cherenkov telescopes to be build in the future allowing 24h
299monitoring of the bright AGNs. Such a system is so far completely
300unique in this energy range.
301
302\section{Acknowledgements}
303We would like to thank Eckart Lorenz, Riccardo Paoletti, Adrian Biland,
304Maria Victoria Fonseca and Jos$\acute{e}$ Luis Contreras for intense discussions
305and Christian Spiering, Brenda Dingus, Maria Magdalena Gonzalez Sanchez
306and the Magic collaboration for helpful support.\\
307
308\bibliography{icrc0974}
309%This in the bibtex style, is ok.
310\bibliographystyle{plain}
311References will be added in the final version.
312\end{document}
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