Active galactic nuclei

Group members:

Elina Lindfors (PhD, Tuorla)
Kari Nilsson (PhD, FINCA)
Pauli Pihajoki (PhD, Tuorla)
Juri Poutanen (Professor, Tuorla)
Aimo Sillanpää (PhD, Tuorla)     
Leo Takalo (PhD, Tuorla)
Mauri Valtonen (Professor emeritus, FINCA/Tuorla)   
Kaj Wiik (PhD, Tuorla)   
Riho Reinthal (PhD student, Tuorla)
Vandad Fallah Ramazani (PhD student, Tuorla)


Recent Master and PhD theses on AGNs defended in the group:

1.      Pauli Pihajoki, PhD thesis “The supermassive binary black hole system OJ 287”, University of Turku, 2014

2.      Carolin Villforth, PhD thesis “Variability in Active Galactic Nuclei: understanding emission mechanisms and unification models”, University of Turku, 2011

3.      Elizaveta Rastorgueva, PhD thesis “Multifrequency VLBI Observations of Selected Active Galactic Nuclei”, University of Turku, 2011



1. Very-high energy astrophysics with Cherenkov telescopes 

Tuorla observatory is a member of Major Atmospheric Gamma-ray Imaging Cherenkov Telescopes (MAGIC) as well as Cherenkov Telescope Array (CTA) collaborations. MAGIC experiment is located in the Roque de los Muchachos European North Observatory (2200m a.s.l.) on the Canary island of La Palma. The system of two MAGIC telescopes, each of 17m in diameter, is currently measuring very high energy (VHE) gamma-rays from cosmic sources in the energy range 25 GeV−50 TeV. The neutral gamma rays penetrate into Earth’s atmosphere and produce avalanches of secondary particles that emit Cherenkov light. MAGIC is studying gamma-rays from galactic and extragalactic sources by recording and analyzing stereoscopic pictures of these Cherenkov flashes. Tuorla group is responsible for the optical monitoring support of VHE sources such as blazars which are among the main targets for MAGIC (see the project site for recent lightcuves).  E.Lindfors is currently coordinating activity of the MAGIC AGN group.

CTA is a multinational, world-wide project to build a new generation ground-based gamma-ray instrument in the energy range extending from some tens of GeV to above 100 TeV. It is proposed as an open observatory and will consist of two arrays of IACTs, a first array at the Northern Hemisphere with emphasis on the study of extragalactic objects at the lowest possible energies, and a second array at the Southern Hemisphere, which is to cover the full energy range and concentrate on galactic sources. CTA intends to improve the flux sensitivity of the current generation of IACTs such as MAGIC, HESS, and VERITAS by an order of magnitude.


Figure 1:  The radio image of the central part of the radio galaxy IC 310. MAGIC has observed extraordinary short variations in the gamma-ray flux from this object at 5 min timescale, which is four times smaller than the light-crossing time of the event horizon of the supermassive black hole which is the engine behind this activity. The data can be explained only of the gamma-ray emission happens in an extremely narrow region located near the event horizon of the black hole and permeated by strong electromagnetic fields.

Selected publications:

1.    MAGIC Collaboration, 2014, Black hole lightning due to particle acceleration at sub-horizon scales, Science Express,,

2.       Acharya B. S., et al. 2013, Introducing the CTA concept, Astroparticle Physics, 43, 3-18,

3.       Sol H., et al., 2013, Active Galactic Nuclei under the scrutiny of CTA, Astroparticle Physics, 43, 215-240,,

4.       Actis M., et al. 2011, Design concepts for the Cherenkov Telescope Array CTA: an advanced facility for ground-based high-energy gamma-ray astronomy, Experimental Astronomy, 32, 193-31,,

5.       MAGIC Collaboration, 2008, Observation of Pulsed γ-Rays Above 25 GeV from the Crab Pulsar with MAGIC, Science, 322, 1221-1224,,

6.       MAGIC Collaboration, 2008, Very-High-Energy gamma rays from a Distant Quasar: How Transparent Is the Universe? Science, 320, 1752-175,,

7.       MAGIC Collaboration, 2007, Variable Very High Energy γ-Ray Emission from Markarian 501, ApJ, 669, 862-88,,

8.       MAGIC Collaboration, 2007, Variable Very-High-Energy Gamma-Ray Emission from the Microquasar LS I +61 303, Science, 312, 1771-1773,,


2. Supermassive binary black hole in OJ 287

Tuorla Observatory has a special relationship with one particular AGN,  OJ 287. It is one of the targets of the combined optical and radio monitoring program of Tuorla Observatory and Metsähovi Radio Observatory. Combining this data with archival data, researchers at Tuorla discovered that OJ 287 brightened periodically in the optical wavelengths, with a period of approximately 12 years (Sillanpää et al. 1988). This discovery led to the publication of a supermassive binary black hole model for OJ 287. From this time onwards, OJ 287 has been intensively studied at Tuorla with a variety of approaches including Post-Newtonian black hole dynamics modelling, accretion disk research, comprehensive time series analysis, multifrequency and polarization studies, etc.  If any single AGN could be nominated as the official Tuorla mascot, it would without question be OJ 287.


Figure 2: A binary black hole model for OJ 287. (credit S&T: Gregg Dinderman)

Selected publications:

1.       Pihajoki P., Valtonen M., Ciprini S., 2013, Short time-scale periodicity in OJ 287, MNRAS, 434, 3122-3129,,

2.       Pihajoki P., Valtonen M., et al., 2013, Short time-scale periodicity in OJ 287, ApJ, 764, 5,,

3.       Valtonen M., Pihajoki P., 2013, A helical jet model for OJ287, A&A, 557, A28,,

4.       Valtonen M.J., Ciprini S., Lehto H.J. 2012, On the masses of OJ287 black holes, MNRAS, 427, 77-8,,

5.       Valtonen M.J., Wiik K., 2012, Optical polarization angle and VLBI jet direction in the binary black hole model of OJ287, MNRAS, 421, 1861-1867,,

6.       Valtonen M.J., Mikkola S., Lehto H.J., Gopakumar A., Hudec R., Polednikova J., 2011, Testing the Black Hole No-hair Theorem with OJ287, ApJ, 742, 22,,

7.       Valtonen M.J., Mikkola S., Merritt D., Gopakumar A., Lehto H.J., Hyvönen T., Rampadarath H., Saunders R., Basta M., Hudec R., 2010, Measuring the Spin of the Primary Black Hole in OJ287, ApJ, 709, 725-73,,

8.       Valtonen M.J., Nilsson K., Villforth C., Lehto H.J.,  Takalo L.O., Lindfors E., Sillanpää A., Hentunen V.-P., Mikkola S., et al. 2009, Tidally Induced Outbursts in OJ 287 during 2005-2008, ApJ, 698, 781-78,

9.       Valtonen M.J., Lehto H.J.,  Nilsson K., Heidt J., Takalo L.O., Sillanpää A., Villforth C., et al. 2008, A massive binary black-hole system in OJ287 and a test of general relativity, Nature, 452, 851-85,,

10.    Valtonen M.J., Kidger M., Lehto H., Poyner G., 2008, The structure of the October/November 2005 outburst in OJ287 and the precessing binary black hole model, A&A, 477, 407-412,

11.    Lehto H., Valtonen M.J., 1996, OJ 287 Outburst Structure and a Binary Black Hole Model, ApJ, 460, 207-21,

12.    Sillanpää A., Haarala S., Valtonen M.J., Sundelius B., Byrd G.G., 1988, OJ 287 - Binary pair of supermassive black holes, ApJ, 325, 628-634,

3. Physics of jets and their gamma-ray emission 

In radio- and gamma-ray loud AGNs, the emission from a relativistic jet beamed in the direction of motion dominates the energy output. In spite of many years of research, the basic questions on the location of the gamma-ray emitting region and on the particle acceleration mechanisms remain unanswered. We have recently used the Fermi/LAT data to study GeV spectra on bright blazars, demonstrating that they consistently show breaks at similar energies (Stern & Poutanen 2014; see Fig. 3), which can be interpreted as a result of gamma-gamma pair production absorption within the broad-line region (BLR; see Poutanen & Stern 2010). This has profound implication on the location of the gamma-ray emitting region, univocally proving it being with the BLR. We also conduct theoretical studies of gamma-ray emission mechanisms and have proposed a novel particle acceleration mechanism"photon breeding"operating in relativistic jets in blazars (Stern & Poutanen 2006, 2008).  


Figure 3: Redshift corrected Fermi/LAT spectra of individual bright blazars. The dashed line shows the log-normal model, while the solid line includes also the photon-photon pair production absorption on BLR  photons.

Selected publications:


1.       Stern B.E., Poutanen J., 2014, The mystery of spectral breaks: Lyman continuum absorption by photon-photon pair production in the Fermi GeV spectra of bright blazars, ApJ, 794,,

2.       Stern B.E., Poutanen J., 2011,Variation of the gamma-gamma opacity by the He II Lyman continuum constrains the location of the gamma-ray emission region in the blazar 3C 454.3, MNRAS, 417, L11-L15,,

3.       Poutanen J., Stern B.E., 2011, Fermi Observations of Blazars: Implications for Gamma-ray Production, in AGN Physics in the CTA Era, PoS(AGN 2011)015,

4.       Poutanen J., Stern B.E., 2010, GeV breaks in blazars as a result of gamma-ray absorption within the broad-line region, ApJL, 717, L118-L121,,

5.       Stern B.E., Poutanen J., 2008, Radiation from relativistic jets in blazars and the efficient dissipation of their bulk energy via photon breeding, MNRAS, 383, 1695-1712,,

6.       Poutanen J., Stern B.E., 2008, Photon breeding mechanism in relativistic jets: astrophysical implications, Int. J. Mod. Phys. D., 17, 1619-1628,,

7.       Stern B.E., Poutanen J., 2008, Gamma-ray emission of relativistic jets as a supercritical process, Int. J. Mod. Phys. D., 17, 1611-1617,, 

8.       Stern B.E., Poutanen J., 2006, A photon breeding mechanism for the high-energy emission of relativistic jets, MNRAS, 372, 1217-12,,



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