Final Report Summary - JETPOP (Unveiling the Physics of the Most Active of Galaxies: Using Blazars as Laboratories to Study Supermassive Black Holes and Relativistic Jets) Blazars are the most active of galaxies. They are powered by relativistic jets of matter speeding towards us almost head-on at the speed of light, radiating exclusively through extreme, non-thermal particle interactions, energized by accretion onto supermassive black holes. Despite intensive observational and theoretical efforts over the last four decades, the details of blazar astrophysics remain elusive to this day. The primary reason is that relativistic effects cloud the intrinsic properties of blazar jets. Small variations in the angle between blazar jet and line of sight produce a vast range of observed properties and associated blazar “phenotypes.” The unifying features that would illuminate the basic blazar physics are thus obscured. To overcome this difficulty, relativistic effects have to be properly accounted for and corrected. However, this is best done on a population rather than on an object-by-object basis. The aim of the JetPop project was to implement exactly this scientific strategy: develop population models that are based on high-quality data that have recently become available through multi-wavelength, large-sample monitoring programs. These models then can be used as the necessary phenomenological bridge between theory and observations, which will allow us to best use the multitude of observational data now available to us in a manner that will most effectively test theoretical models, and uncover the properties of blazar jets in their rest frame. Current observational efforts, including gamma-ray observations with Fermi, radio observations OVRO at Caltech and FGAMMA at the Max-Planck Institute for Radioastronomy, as well as VLBI measurements by the MOJAVE program, and optical observations (both photometric and polarimetric) by the RoboPol collaboration using instrumentation hosted by FORTH's Skinakas Observatory, offer an unprecedented coverage in the frequency and time domains, as well as additional, independently measured blazar properties (such as superluminal motions, black hole masses, host redshifts, and jet magnetic field properties accessible through polarization observations). For this reason, the timing is especially opportune for progress in the field of jet astrophysics, using blazars as a laboratory. To take full advantage of the current auspicious observational focus on blazars, the JetPop program’s aim was to combine, in a robust way, data from a large variety of observational programs accounting for the differences in sample sizes, sample selection, biases, and uncertainties; to identify appropriate measures quantifying different aspects of blazar behaviour, in a way which directly connects to blazar physical properties; and to develop a formalism connecting physical blazar models to the statistical description of the selected observables. The JetPop project resulted in a total of 21 publications in refereed journals, and the training of six young scientists (students and postdoctoral researchers) in blazar physics. At the conclusion of the program, this strategy has come to fruition, leading to important strides in our understanding of the physics of blazar jets: 1. A novel population model of blazars was developed, based on the simplest possible assumptions for the properties of blazars (single power-law distributions for their rest-frame luminosity and Lorentz fractor), and optimized on flux-limited samples and observables that are the least affected by variability (jet apparent speed, redshift). For more information, see https://arxiv.org/abs/1412.2634 2. The model was used to extract important information about blazar jets, including the distribution of the amount of relativistic boosting (quantified by the Doppler factor) in flux-limited samples, which allowed us to evaluate different Doppler factor estimation techniques that are used in the literature. These tests in turn revealed that equipartition between electrons and magnetic field holds in blazar jets, and that the two major types of blazars, BL Lac objects and flat-spectrum radio quasars, appear to be comparably beamed, so that any statistical difference in their properties must correspond to actual intrinsic differences in their respective rest frames. For more information, http://arxiv.org/abs/1412.2634 and http://arxiv.org/abs/1412.2638 3. Blazar jets are believed to be ejected symmetrically from the host black hole. However, due to relativistic boosting of the jet that is moving towards the observer and relativistic de-boosting of the jet moving away from the observer, typically only the approaching jet is observable for sources with jets closely aligned with the line of sight. Using our population models, we have estimated that with next-generation interferometers, many counter-jets could be resolvable and observable, and we have demonstrated how new information regarding the black hole – jet system can be extracted from observations of both the jet and the counter-jet in each source. More information can be found in https://arxiv.org/abs/1610.06561 4. Based on our population models, we have developed a novel methodology for recovering intrinsic (rest-frame) timescales of blazar jets from their observed values. We applied the new method to the timescales of the largest optical polarization rotations in blazar jets, and we found that these span only a narrow range of rest-frame timescales. More information can be found in https://arxiv.org/abs/1511.00434 5. Using monitoring data from the RoboPol program which was in part supported by JetPop, we found that optical polarization rotations occur in only a fraction of blazars, which are gamma-ray loud and more variable in gamma rays; that gamma-ray—loud blazars are more polarized that gamma-ray—quiet blazars; and that the polarization fraction can either increase or decrease during polarization rotations. More information can be found in https://arxiv.org/abs/1601.03392 ; https://arxiv.org/abs/1607.04292 ; and https://arxiv.org/abs/1609.00640 6. We found that optical and radio variability is connected with gamma-ray loudness in blazars, with gamma-ray—loud blazars being more variable both in radio and in optical frequencies than their gamma-ray—loud counterparts that have otherwise similar properties. More information can be found in http://arxiv.org/abs/1401.0538 and http://arxiv.org/abs/1312.3634 Additional synergies were sought with the high-energy and cosmology and X-ray astrophysics community, for the joint study of the cosmological evolution of jets, and of jets in other frequencies. Such a new avenue of investigation with very high potential impact concern searches for an inflationary signal in the polarization of the cosmic microwave background. JetPop researchers have identified a new effect that has been subsequently shown to be the dominant foreground in microwave polarization experiments: the three-dimensional structure of interstellar dust that emits partially polarized thermal radiation at microwave frequencies. A way to beat this background has also been proposed: polarization surveys of stars at known distances. This new type of research is, as a result, now aggressively pursued by JetPop host institution FORTH, with partners at the University of Crete, Caltech in the US, the Inter-University Centre for Astronomy and Astrophysics in India, and the South African Astronomical Observatory in South Africa. Project JetPop has been supported by the European Union's Seventh Framework Programme, through a Career Integration Grant (CIG) Marie Curie Action, under grant agreement PCIG-GA-2011-304001.
