The investigation of binary blazars – proposal for the GA of the CAS.
Blazars represent physically extremely important case of Active Galactic Nuclei, AGN. They exhibit violent activity and
variability over a very wide range of energy and time. As first extragalactic objects, they show hints for the possible periodicity
(OJ287). This periodicity can be caused e.g. by double nature of the blazar, and/or the processing jet. The searches for periodic
light changes in blazars are very limited so far, since such analyses require well sampled and very prolonged time intervals
covered, typically tens of years. Such data bases are very rare to find. We propose an innovative approach how to extract such
long-term light curves, namely by analyzing very large amounts of plates located in large astronomical plate collections. The
recent progress in computer power and digitization methods of the plate material makes such study possible. The generated long
term light curves will be then carefully examined for periodicity and character of light changes in order to reveal and study the
underlying physical processes and their models.
1. A new era of astronomy and unanswered questions
Previous centuries and decades were dedicated to the study of the Solar system and stars in our own Galaxy. With new
technologies, telescopes and instruments new exciting, yet unexplored phenomena emerged. It is the Universe of gamma-ray
bursts, quasars, black holes, galaxies and cosmology. AGN and blazars are a part of this new era of astronomy, too. Many
fundamental questions remain open in this new branch – questions that are waiting to be uncovered.
2. AGN and blazars
1.2.1. AGN
There exist “normal” galaxies in the Universe and galaxies that are called AGN. The abbreviation AGN stands for Active
Galactic Nuclei and suggests that inside these galaxies an “active” nucleus is present.
This nucleus is seen in the spectra of AGN galaxies. Unlike normal galaxies whose spectra are mostly thermal, coming from the
constituent stars in the galaxy, the spectra of AGN galaxies are non-thermal having origin in non-thermal processes.
The AGN are usually very luminous and the theory assumes a black hole, typically of 10 7 to 109 solar masses, to be present in the
center of an AGN galaxy. The surrounding matter, attracted by the gravitation of this massive black hole, is pulled towards it,
forming an accretion disk, loses angular momentum through viscous and turbulent processes and spirals down to the black hole.
Frictional processes that take place during the fall of matter onto the black hole lead to the emergence of plasma emitting mainly
in the UV and X-ray region. Clouds of gas in the vicinity of the black hole (either in BLR – broad line region – or in NLR –
narrow line region) are excited and produce strong emission lines. BLR is situated closer to the black hole; the clouds move fast
and emit broad lines. NLR is more extended, clouds of NLR move slower and emit narrow lines.
A dust torus is usually supposed to be present surrounding the black hole at the distance of about 1 pc and a jet of energetic
particles is assumed to exist perpendicular to the dust torus, being responsible for the observed radio emission of some AGN.
Most of the luminosity of AGN is emitted in radio, IR, UV, X-ray and gamma-ray wavelengths rather than in the optical band.
Nowadays different types of AGN are observed – radio galaxies, quasars, blazars, Seyfert galaxies, LINERs, nuclear HII regions
and starburst galaxies. This is a division from an observational point of view. However, in the unified model (Antonucci, 1993)
different classes of AGN (radio galaxies, quasars, blazars . . .) are supposed to be intrinsically the same, all having the same
environment outlined above. The observed variety of different types of AGN is just assumed to be a result of an orientation effect
of the nuclei relative to the observer. Only the radio power of jets divides AGN intrinsically into radio loud and radio quiet ones.
The former class has strong radio jets, whereas in the latter class the radio jets are weak.
2.2. Blazars
When the observer’s line of sight is close to the direction of a powerful radio jet, a blazar is observed. Blazars appear point-like in
the sky and have apparently a non-stellar continuum. Blazars usually have a compact core emitting at radio frequencies with a flat
or inverted spectrum. They exhibit rapid (often flare-like) variability at all wavelengths and a strong and variable optical
polarization. A relativistic jet is pointing almost towards us and the plasma in the jet is moving at relativistic speeds in the
magnetic field. Hence superluminal motion, relativistic beaming and a very energetic emission at gamma frequencies are
observed in many blazars.
Superluminal motion describes proper motion of some source structure whose apparent speed projected to the sky is higher than
the speed of light. This phenomenon occurs when emitting regions are moving at very high speeds and for small viewing angles.
Relativistic beaming, on the other hand, strongly influences the luminosities. It is believed to explain rapid variations of
luminosity, polarization, featureless continuum and high luminosities of blazars. As blazar jets are viewed at small angles their
observed luminosity can be amplified by a factor of thousands.
3. Black hole pairs in AGN
3.1. The OJ 287 case
One of the most interesting blazars ever observed is the blazar called OJ 287. This blazar is an easy target both for radio and
optical telescopes. Moreover, it was also observed to be a gamma-ray source. In 1988 Sillanpaa et al. (1988) studied the optical
light curve of this blazar and noticed a 12-year period within the major outbursts in the optical band of this blazar. The outbursts
have a double peak structure. Consequently, Sillanpaa et al. predicted the next outburst to come in 1994. In order to get a good
coverage of the 1994 outburst, in autumn 1993 a project – the OJ-94 project – was set up (Pursimo et al., 2000) for monitoring of
OJ 287. The first observations within the OJ-94 collaboration started in autumn 1993. Besides optical telescopes, also infrared
and radio observations were collected. Moreover, a small amount of UV, X-ray and gamma-ray data were gained, too. The
expected brightening had a double-peak structure. The first optical peak observed in 1994 lacked a radio counterpart. On the other
hand, the second optical peak in 1995 was accompanied by a radio outburst peaking at the same time as the optical one but being
faster.
After the data points were collected and the light curve analyzed, several models have been suggested to explain the behavior of
OJ 287. Most of these models assume that the engine of OJ 287 is a pair of supermassive black holes. Sillanpaa et al. (1988)
assume that the light variations are produced by mass inflows from accretion disk into the black hole, caused by tidal force of the
secondary black hole. In the Lehto and Valtonen’s model (1996) the outbursts are associated with the companion black hole’s
crossing of the accretion disk of the primary. On the other hand, Katz (1997) assumes that the secondary black hole exerts a
torque on the accretion disk of the primary and thus causes the relativistic jet to sweep across our line of sight, which may
consequently result in the periodic change of this behavior. Villata et al. (1998) came up with a new model - a lighthouse model
with a pair of bent jets.
All of these models face several problems explaining some specific features observed within OJ 287. Some of these problems are
mentioned e.g. in Valtaoja et al. (2000), where a new model is suggested instead. The authors (Valtaoja et al, 2000) assume that
the first optical flare in the double maximum structure is connected with the presence of the secondary black hole, originates in
the vicinity of the black hole, and thus is not accompanied by a radio flare. On the contrary, the second optical flare has its origin
in the relativistic synchrotron emitting jet and thus has a radio counterpart.
Some of the forthcoming papers (Valtonen et al., 2004) make use of all the so far available data and finally try to get the right
model of OJ 287.
3.2. The importance of a new insight
OJ 287 is the first object, and it is a unique one – it is a candidate for a double black hole system and the only one that has such an
excellent suggestion in its light curve for its binary nature. However, we are not completely sure that there is a double pair of
black holes inside OJ 287. Alternative models may suggest only one black hole, with quasi-periodic oscillation of the accretion
disk causing the quasi-periodic behavior of major outbursts. It is hard to decide for the right model of OJ 287. The only thing that
rejects or confirms theoretical models is a comparison of theory predictions with observations and measured data points.
However, in the case of OJ 287 as well the data sampling has to be very dense and has to cover a long time back to the past, so
that we have enough information to build and verify a viable theory and model. One has to realize the time scales for periodic
variations are very long.
Therefore, not only future dense measurements are necessary to find the right model of OJ 287. A tough work has already been
done to obtain historical data points. Only through this way we are able to make any progress in our knowledge of OJ 287.
Moreover, measurements in other wavelengths are valuable to get a better picture of the source.
The double black hole nature, in the case of OJ 287, has been inferred mainly from its periodic behavior. There are other
(Conway & Wrobel, 1995) possible pieces of observational evidence for double nuclei in some other blazars.
The distribution of misalignments of radio jet position angles on parsec and kiloparsec scales appears to be bimodal. There are
two classes of objects. In the first class radio jets on parsec and kiloparsec scales are aligned. In the second class, radio jets on
parsec and kiloparsec scales are approximately perpendicular.
Conway and Murphy (1993) suggested that selection effects due to Doppler boosting and a helically distorted jet could explain
this phenomenon of the misaligned jets; with the helical jet structure being a result of the orbital motion of a binary black hole
system.
The orthogonally misaligned jets are observed within about 50% of core-dominated sources. That would suggest that a binary
black hole system could be common. Does that mean that many other blazars are in fact double supermassive black hole systems?
This fact has to be confirmed and supported by other means – e.g. by long-term light curve analysis.
Also AGN morphology, double peak emission lines or elliptical accretion disks suggest other AGN are possible candidates for
binary systems (Eracleous, 1995).
We see here a new picture of AGN. The confirmation of a black hole pair in the center of blazars and AGN may put a new insight
into the model of AGN that was depicted above. If the so far generally accepted unified model by Antonucci (1993) proves to be
quite incomplete – and we may call it incomplete if a second black hole turns out to dwell in the center – then we may expect a
new trigger in theories and observations and a revision of previous results.
The proof of existence of double black hole nuclei in the center of AGN and their distribution with redshift would play a crucial
role in cosmology and in the theory of formation, evolution and merging of galaxies.
But the proof of existence of double black holes in blazars and AGN offers much more – it also gives a chance to verify the
general theory of relativity. According to this theory massive binary systems at eccentric orbits should emit gravitational waves.
In the future gravitational wave interferometers will be built in the space. A good database of possible candidate sources is
needed. We can look for these candidates also within AGN and blazars, if they prove to contain double black hole systems inside.
Good light curves and their analyses, in which periodicity behavior is detected, will not only suggest the double black hole nature
of some AGN but also put constraints on the possible detection of a signal coming from these objects via gravitational waves.
These objects, in which the double black hole nature is confirmed, may thus become important candidate sources for detection of
gravitational radiation with future interferometers built in the space, such as LISA and ASTROD (Paolis et al., 2003). A good
database of possible binary black hole systems is crucial for a testing of general relativity.
Recently several other blazars have showed suggestion for possible black hole pair inside – Mrk 421, Mrk 501, Mrk 766, PKS
0420-014 – however the database is still very poor.
As we have just shown, the confirmation of the binary black hole nature of some blazars and AGN would put new foundations in
the AGN science and would play a crucial role in cosmology as well as in general relativity. However, demonstrated on the case
of OJ 287, we see that dense data points reaching long to the history are strongly needed. Amazingly, at the present we are not
only dependent upon better future projects and instruments but we are also limited by the lack of data points in the past.
Therefore, if it is possible to extract these historical data points, we should do this. Otherwise a further progress will be difficult
and much more costly to realize.
4. Hunt for more binary black holes
Naturally, question arises whether these objects as well as the related physics are representative for the whole blazar sample. It is
obvious that better analysis and understanding of additional blazars is of great physical value.
In the last few years there has been a revolution in cosmology: the dark matter and dark energy have been confirmed, and there
are very strong reasons to believe in the “bottom-up” view of galaxy evolution. In this view galactic halos of a million of solar
mass form first, and they have about one thousand solar mass black holes inside each of them. The present day galactic halos and
central supermassive black holes result from numerous mergers between halos and black holes, respectively. One major problem
with this view has been the lack of observed binary black holes. The evolutionary calculations lead to a large number of galaxies
with binary black holes (eg. Volonteri et al. 2003). Even though many binaries are thrown out of galaxies by the gravitational
slingshot, and others are forced to merge in the same process, there still should be a sizable binary black hole population. Why do
we not observe them?
Yu (2002) suggests an answer. The periods of the expected binary black holes are too long to be within reach of the observational
records. Also the mass ratios are expected to be high which makes it hard to establish a binary nature. In the period/mass ratio
distribution plane there is only a small corner region where binary black holes can be detected. This would explain why detections
of binary black holes are so rare, even though the phenomenon should be very common.
So far, there is only one reasonably well established case of a binary black hole, OJ287. It just happens that the parameters of this
binary fit into to the small coner established by Yu: period 12 yr, mass ratio 150 (Lehto and Valtonen 1996). Prior to Yu’s work it
was often argued that the parameters established for OJ287 were too unlikely, and that therefore the model cannot be correct.
Now the argument has turned around completely; one should look for more systems like OJ287. There should be plenty of them if
current cosmological theories are correct. OJ287 provides a guide how to discover supermassive binary black holes. Its binary
nature was revealed by the historical record of light variations which goes as far back as to 1891 (eg. Hudec et al 2001). The
record can still be perfected and this work is under way. In the case of this blazar, model predictions have been made both with
regard to past and to future outbursts. In the past, the 1934 outburst has not yet been recorded, in the future the next one is
expected in 2006. The detections of both outbursts are part of this proposal. But a serious search should be made for other cases
as well.
5. Proposed method
Following procedures are proposed for selected candidates in order to detect and analyse more binary blazars:
extracting the well sampled light curves of selected objects from archival astronomical plates
detailed analysis of these data by powerful computers and dedicated software (period analysis etc.)
There are nearly 3 millions astronomical archival plates located at different observatories. These plates represent a unique
database for various scientific projects. The archives can easily provide thousands of exposures for any celestial position,
reaching monitoring intervals of up to few years of continuous monitoring – i.e. tens of thousands of hours.
The recent efforts to digitize the plates and the corresponding software development significantly facilitate the extraction of
unique scientific data from archival records and related reductions and analysis.
The photographic sky monitoring is available for more than 100 years. However, only the recent development of photographic
scanners and powerful computers allows an efficient extraction of scientific data. Some of the archives have very high quality
plates achieving limiting magnitudes of up to 20–23 (direct imaging) and /or 17–19 (spectral with objective prism).
Archival and sky patrol plates represent a valuable tool in investigations of various types of high energy sources such as
blazars/quasars, X-ray binaries, X-and gamma-ray transients, etc. It is obvious that the automated evaluation of sky patrol plates
has large potential in:
providing extended monitoring intervals with good sampling,
allowing long-term evolution and changes to be studied
searching for optically variable AGNs-QSOs-blazars and other objects
providing their light curves with good sampling
searching for their flares
providing simultaneous and quasisimultaneous optical data for satellite campaigns, even back in time
monitoring of objects as base for proposals for ToO (Target of Opportunity) for satellite high energy observations
providing extended database for identification and classification of sources
The detection and investigation of very large amplitude flares from AGN may serve as an example. There is increasing evidence
that some AGN may exhibit very large amplitude flares exceeding mag 10 (Hudec et al. 1997). These large flares are however so
rare that very long monitoring times (of order of thousands hrs or more) are required to detect them. They can be obtained easily
on plates but hardly by other methods.
For the proposed project, we suggest the following plate archives to be used: Sonneberg Observatory, Germany (280 000 plates),
Harvard College Observatory USA (600 000 plates), UKSTU plate collection ROE Edinburgh UK (18 000 very deep plates),
Observatory Leiden NL (40 000 plates). We will also exploit possible other sources of archival material. We will exploit all
available archives on-line and check if there are plates covering the positions of objects (candidates).
We expect to analyse part of the archival data by plate scanning and computer analysis. The remaining part will be analysed by
classical methods on site (measuring the magnitudes directly on plates).
5. Light curves analyses – proposed methods
The long-term light curves of blazars will be a starting point for periodicity analysis. Periods will be searched on time scales of
days up to tens of years. Stellingwerf's method, Deeming’s method and the CLEAN algorithm will be the tools used for these
analyses. Real periods will be detected and false alarms filtered. The rate of change of periods will be controlled with the help of
wavelet analysis.
A ratio of blazars with long-term periods and blazars with no long-term periods will be established. The redshift distribution of
period ranges will be established. Well-sampled light curves will possibly be used to build models for individual blazars and
determine their orbital parameters.
Individual components in the light curves will be analyzed – quiescent background, periodic components and flares. This will
consequently enable to determine the environment present in each blazar.
Blazar flares will be studied, particularly, the rising and declining parts. Power law, broken power-law and exponential function
will be fitted to the profile of flares and compared with theoretical assumptions (e.g. Marscher and Gear, 1985).
Statistical conclusions will be reached. For the first time a large database of long-term blazar light curves will be made enabling
general conclusions for blazar theory.
6. Instrumentation/data available
We have the access to the Sonneberg Observatory Sky Patrol plates, as well as to other plate-data archives. The plate analysis are
expected to be based on the digitized data obtained by the plate scanner available at the Sonneberg Observatory and other sites.
The alternative is the estimation method and plate photometer. The collaborating Tuorla Observatory will provide additional data
(CCD magnitudes) within their blazar monitoring program . Further additional CCD photometry will be provided by Ondrejov
18cm, 25 cm and 50cm CCD telescopes.
7. Expertise available
Experience is available in experimental optical as well as high-energy astronomy and astrophysics with related data analyses and
interpretation methods. Important for the proposed study is the large expertise in analysis on archival astronomical plates.
8. Team
Rene Hudec – project management, measurements on plates,data analyses, interpretations, drawing physical conclusions
Milan Basta – flare fitting, color evolution studies, period searches and analyses
Filip Munz – theoretical background, comparison with models
Filip Hroch – data reductions and analyses, optical data analyses
Vojtech Simon – light curve analyses, period analyses
Vera Hudcova – related software developments
Cyril Polasek – 50 cm CCD telescope and related observations and data analyses
Petr Sobotka - 25cm CCD BART telescope observations and analyses
8. Collaborations
Tuorla Observatory, Piikkio, Finland – optical CCD observations, data analyses, physical conclusions, Magic project data
Sonneberg Observatory, Germany – providing and digitizing the archival plates
ROE Edinburgh, UK - providing and digitizing the archival plates
Brno Observatory – CCD observations
Abastumani Observatory, Georgia – optical CCD observations
University of Trinidad – theoretical backgroud, models of binary blazars, light curves fittings
9. Time schedule
2005
selection of further binary blazar candidates, CCD observations, data reductions and analyses, digitization of archival plates,
measurements on plates, analyses of light curves, color analyses
2006
CCD observations and data reductions and analyses, further plates analyses and interpretations, period searches and analyses
2007
More CCD and plate measurements, physical interpretations of the obtained results, drawing physical conclusions, presentations
of obtained results, publications, drawing final conclusions
10. Source candidates
The following blazars seems to be suitable for the analysis described above. But our goal is to add more candidates in the course
of the project.
OJ287
Mkn 501
PKS 0420-014
Mrk 421
Mrk 766
ON231 (W Comae)
0109+224
0735+178
3C66A
AO 0235+164
3C279
3C345
0716+714 (this can be sometimes really bright, just now 12.3 in R)
11. Funding
Funding is required especially for analyses on astronomical archival plates at observatories abroad (Sonneberg, Edinburgh,
Leiden, Harvard,...), i.e. for travels, and also for upgrades of computing and digitizing systems (the management of numerous and
large image files), innovative software for period analyses, as well as for archiving the extended image data.
References:
[1.] Antonucci, R.: Unified Models for Active Galactic Nuclei and Quasars; 1993, ARA&A, vol. 31, p. 473
[2.] Sillanpaa, A. et al.: OJ 287 - Binary Pair of Supermassive Black Holes; 1988, ApJ, vol. 325, p. 628
[3.] Pursimo, T. et al.: Intensive Monitoring of OJ 287; 2000, A&AS, vol. 146, p. 141
[4.] Lehto H. J.; Valtonen, M. J.: OJ 287 Outburst Structure and a Binary Black Hole Model; 1996, ApJ, vol. 460, p. 207
[5.] Katz, J. I.: A Precessing Disk in OJ 287?; 1997, ApJ, vol. 478, p. 527
[6.] Villata, M. et al.: A Beaming Model for the OJ 287 Periodic Optical Outbursts; 1998, MNRAS, vol. 293, p. L13
[7.] Valtaoja, E. et al.: Radio Monitoring of OJ 287 and Binary Black Hole Models for Periodic Outbursts; 2000, ApJ, vol. 531, p.
744
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