THE SUPERMASSIVE BINARY BLACK HOLE SYSTEM OJ287 Pauli Pihajoki TURUN YLIOPISTON JULKAISUJA – ANNALES UNIVERSITATIS TURKUENSIS Sarja - ser. AI osa - tom. 492 | Astronomica - Chemica - Physica - Mathematica | Turku 2014 University of Turku Faculty of Mathematics and Natural Sciences Department of Physics and Astronomy University of Turku FI-20014 Turku Finland Supervised by Prof. Mauri Valtonen FINCA University of Turku Finland Reviewed by Dr. Tuomas Savolainen Dr. Peter Johansson Max Planck Institute for Radio Astronomy Department of Physics Max Planck Society University of Helsinki Germany Finland Opponent Prof. Jochen Heidt Landessternwarte Königstuhl University of Heidelberg Germany The originality of this thesis has been checked in accordance with the University of Turku quality assurance system using the Turnitin OriginalityCheck service. ISBN 978-951-29-5790-3 (PRINT) ISBN 978-951-29-5791-0 (PDF) ISSN 0082-7002 Painosalama Oy - Turku, Finland 2014 Acknowledgements This thesis, both the introduction and the research articles, have my name on them and are considered to be my work. In actuality, this thesis is a synthesis of efforts, not only by those whose names appear on the introduc- tion and the articles, but also many others who have contributed indirectly, since no man is an island after all. Many of these numerous contributors will be left out by necessity of brevity, but for these unnamed contributions I am no less grateful. Foremost, I extend my gratitude to my supervisor, Prof. Mauri Val- tonen. He presented me with the topic of my research, contributed sig- nificantly to the articles, and advised me whenever I had misunderstood something, had a gap in my knowledge or had blundered something in general. I wish to thank my esteemed opponent, Prof. Jochen Heidt, for agree- ing to travel to Finland to present his assessment of my thesis. Likewise, my thanks to Dr. Tuomas Savolainen and Dr. Peter Johansson for their valuable effort in reviewing this work. I am also grateful for the work done by all of my collaborators, without which major parts of the research could not have been done. I am par- ticularly indebted to all the observers for the data on OJ287 that I have had the pleasure to use. My thanks to Esko Gardner as well, for valiantly trying to catch all my grammar mistakes. I would also like to thank Seppo Mikkola, who supervised my Master’s thesis, and ignited the interest I now have in things both numerical and theoretical. I also thank Juha-Pekka Pellonpää, who was similarly involved in my Master’s thesis, and to whom I owe much of the occasional moments of clarity concerning a multitude of theoretical issues. My thanks also to Heikki Salo for his advice on numerical simulations. Major financial support for this thesis was provided by the Turku Uni- versity Foundation and Magnus Ehrnrooth Foundation, and for this I am very grateful. I also thank University of Turku and Aalto University for 3 employment as a scientist. The work on this thesis was not done in vacuo, but rather in the comfort- able atmosphere of Tuorla Observatory. The fine people at the Observatory contributed much to the quality of my working environment, especially via the coffee table and lunch discussions on a great variety of esoteric topics. My thanks to everyone who have partaken in these small events. Likewise, the company and support of my circle of friends has been invaluable. My thanks to the old crew and the new crew. You know who you are. I also wish to express my general gratitude for all the giants, some listed in the bibliography, whose shoulders I have clambered onto to steal a peek. In addition, a very special thanks to Marcus, Lucius and Eckhart for important insights not related to the subject at hand. Finally, my thanks to my family for their perpetual support. Isä, äiti, kiitos kaikesta. Turku, January 2014 Pauli Pihajoki Everything is fitting for me, my Universe, which fits thy purpose. Marcus Aurelius, Meditations, Book IV, 23 (transl. A. S. L. Farquharson) Contents Acknowledgements 3 Abstract 7 List of papers 15 1 Introduction 19 2 Overview of the OJ287 system 23 2.1 Historical perspective . 23 2.2 Basic observable characteristics . 24 2.3 Development of the precessing binary black hole model . 27 2.4 Other proposed models for OJ287 . 31 2.4.1 Jet models . 32 2.4.2 Tidal models . 32 2.4.3 Non-precessing model . 33 2.4.4 Cavity flares . 33 3 Binary black hole orbits 35 3.1 Relativistic orbits . 35 3.2 Analytic estimates . 35 3.2.1 Preliminary assumptions . 35 3.2.2 Precession of the pericenter . 40 3.2.3 Precession of the orbital plane . 41 3.2.4 Gravitational radiation . 41 3.3 Numerical methods . 44 3.3.1 Numerical relativity . 45 3.3.2 Geodesics and self gravity . 45 3.3.3 Post–Newtonian expansion . 47 3.4 Application of the numerical results to OJ287 . 49 5 3.4.1 Binary and disk orbit simulation . 50 3.4.2 Imaging . 52 3.4.3 AVP algorithm . 53 3.4.4 Testing gravitational theories . 54 4 Optical outbursts 59 4.1 Accretion disk impacts . 59 4.1.1 The model of Lehto & Valtonen . 60 4.1.2 The model of Ivanov et al. 64 4.2 Tidal flares . 66 4.3 Precursor outbursts . 67 5 Jet structure 69 5.1 Relativistic jets . 69 5.1.1 Radiative characteristics . 71 5.1.2 Relativistic effects . 77 5.2 Helical jets . 78 5.3 The case of OJ287 . 79 5.3.1 A helical jet in OJ287 . 80 5.3.2 Secondary jet and short timescale periodicity . 80 6 Summary of publications 85 6.1 Paper I . 85 6.2 Paper II . 87 6.3 Paper III . 88 6.4 Paper IV . 91 6.5 Paper V . 93 7 Conclusions and future work 95 7.1 Conclusions . 95 7.2 Future work . 96 A Appendices 99 A.1 Post–Newtonian coefficients . 99 Bibliography 103 6 Abstract This doctoral thesis concerns the active galactic nucleus (AGN) most often referred to with the catalogue number OJ287. The publications in the thesis present new discoveries of the system in the context of a supermassive binary black hole model. In addition, the introduction discusses general characteristics of the OJ287 system and the physical fundamentals behind these characteristics. The place of OJ287 in the hierarchy of known types of AGN is also discussed. The introduction presents a large selection of fundamental physics re- quired to have a basic understanding of active galactic nuclei, binary black holes, relativistic jets and accretion disks. Particularly the general rela- tivistic nature of the orbits of close binaries of supermassive black holes is explored with some detail. Analytic estimates of some of the general rela- tivistic effects in such a binary are presented, as well as numerical methods to calculate the effects more precisely. It is also shown how these results can be applied to the OJ287 system. The binary orbit model forms the basis for models of the recurring op- tical outbursts in the OJ287 system. In the introduction, two physical outburst models are presented in some detail and compared. The radiation hydrodynamics of the outbursts are discussed and optical light curve pre- dictions are derived. The precursor outbursts studied in Paper III are also presented, and tied into the model of OJ287. To complete the discussion of the observable features of OJ287, the nature of the relativistic jets in the system, and in active galactic nuclei in general, is discussed. Basic physics of relativistic jets are presented, with additional detail added in the form of helical jet models. The results of Papers II, IV and V concerning the jet of OJ287 are presented, and their relation to other facets of the binary black hole model is discussed. As a whole, the introduction serves as a guide, though terse, for the physics and numerical methods required to successfully understand and simulate a close binary of supermassive black holes. For this purpose, the 7 introduction necessarily combines a large number of both fundamental and specific results from broad disciplines like general relativity and radiation hydrodynamics. With the material included in the introduction, the publi- cations of the thesis, which present new results with a much narrower focus, can be readily understood. Of the publications, Paper I presents newly discovered optical data points for OJ287, detected on archival astronomical plates from the Har- vard College Observatory. These data points show the 1900 outburst of OJ287 for the first time. In addition, new data points covering the 1913 outburst allowed the determination of the start of the outburst with more precision than was possible before. These outbursts were then successfully numerically modelled with an N-body simulation of the OJ287 binary and accretion disc. In Paper II, mechanisms for the spin-up of the secondary black hole in OJ287 via interaction with the primary accretion disc and the magnetic fields in the system are discussed. Timescales for spin-up and alignment via both processes are estimated. It is found that the secondary black hole likely has a high spin. Paper III reports a new outburst of OJ287 in March 2013. The outburst was found to be rather similar to the ones reported in 1993 and 2004. All these outbursts happened just before the main outburst season, and are called precursor outbursts. In this paper, a mechanism was proposed for the precursor outbursts, where the secondary black hole collides with a gas cloud in the primary accretion disc corona. From this, estimates of brightness and timescales for the precursor were derived, as well as a prediction of the timing of the next precursor outburst.