Periodic Reporting for period 1 - RASPO (non-thermal Radiation from AStrophysical jets: from theory of Plasma turbulence to Observations)
Période du rapport: 2023-09-01 au 2025-08-31
The centers of galaxies host supermassive black holes that can reach a million or even a billion solar masses. Supermassive black holes can eject narrow beams of material that travel close to the speed of light. These jets shine through the entire spectrum of the electromagnetic radiation, and are observed from low frequency radio waves to very high energy gamma-rays. Even if jets have now been studied for more than a century, we do not fully understand how they are launched, and we do not fully understand how the observed radiation is produced. An interesting possibility (originally proposed by Blandford and Znajek) is that magnetic field lines are anchored to the supermassive black hole. Since the black hole rotates, electrons and ions are accelerated as they move away from the black hole along the field lines. The accelerated electrons can emit the observed radiation via two channels: by gyrating about the magnetic field lines (synchrotron radiation), and by scattering to higher energies the ambient photons produced near the black hole or by the jet itself (inverse Compton radiation). The aforementioned theoretical paradigm leads to a testable prediction: jets should be magnetically dominated, i.e. most of their energy should be stored in their electromagnetic fields. Equivalently, the kinetic energy of the particles within the jet should be much smaller than the electromagnetic energy.
The main goal of this project is testing whether this paradigm for the launching of jets by rotating magnetized black holes is consistent with observational constraints. The most important observational constraints are the following. Theoretical models of jets should be able to reproduce the observed spectral energy distribution, i.e. the radiation power emitted by the jet per unit photon wavelength. Theoretical models should also reproduce the polarization degree of the observed radiation (i.e. the fraction of radiation that is polarized) and the polarization angle (i.e. the direction of the electric field of the wave). Radio, optical and X-ray polarization measurements are currently available. In this project, we develop new models of the jet emission where the emission zone (where the observed radiation is produced) is magnetically dominated. We test these models against spectral and polarimetric observations of jets launched by supermassive black holes. We focus on blazars, a particular class of jets that point nearly along our line of sight. Focusing on blazars has the main advantage that the radiation from the jet is strongly beamed due to Doppler effect. Then, blazars can be observed more easily than misaligned jets.
The main goal of this project is testing whether this paradigm for the launching of jets by rotating magnetized black holes is consistent with observational constraints. The most important observational constraints are the following. Theoretical models of jets should be able to reproduce the observed spectral energy distribution, i.e. the radiation power emitted by the jet per unit photon wavelength. Theoretical models should also reproduce the polarization degree of the observed radiation (i.e. the fraction of radiation that is polarized) and the polarization angle (i.e. the direction of the electric field of the wave). Radio, optical and X-ray polarization measurements are currently available. In this project, we develop new models of the jet emission where the emission zone (where the observed radiation is produced) is magnetically dominated. We test these models against spectral and polarimetric observations of jets launched by supermassive black holes. We focus on blazars, a particular class of jets that point nearly along our line of sight. Focusing on blazars has the main advantage that the radiation from the jet is strongly beamed due to Doppler effect. Then, blazars can be observed more easily than misaligned jets.
We developed a model of the radiative emission from magnetically dominated jets, and test the model against polarimetric observations of blazars. Multifrequency polarimetric observations are now available due to the advent of the Imaging X-Ray Polarimetry Explorer (IXPE) space observatory. The most important observational results are the following. The polarization degree is strongly chromatic, i.e. the X-rays are significantly more polarized than the optical radiation. The polarization angle is typically aligned with the projection of the jet axis on the plane of the sky. The IXPE collaboration interpreted these observations as an evidence that jets are matter dominated (and particles are accelerated by shocks), in tension with the leading theoretical paradigm for the launching of jets by rotating magnetized black holes, which requires that jets are magnetically dominated.
We wanted to test whether multifrequency polarimetric observations of blazars could be instead consistent with the hypothesis that jets are magnetically dominated. As a first step, we identified a suitable jet model. We assumed that the jet is magnetically dominated, axisymmetric, stationary. We determined analytical expressions for the jet electromagnetic fields, which depend on the jet shape. For cylindrical jets, the magnetic field is dominated by the poloidal component along the jet axis, whereas for parabolic and conical jets the magnetic field is dominated by the toroidal component perpendicular to the jet axis. As a second step, we populated the jet with accelerated electrons. We assumed that the distribution of the X-ray emitting electrons is softer than the distribution of the optical emitting electrons (our choice is motivated by the fact that the observed spectral energy distribution of the blazar synchrotron radiation softens for high photon energies). Finally, we calculated the polarization degree and the polarization angle of the synchrotron radiation, and compared results of our model with multifrequency polarimetric observations of blazars.
We wanted to test whether multifrequency polarimetric observations of blazars could be instead consistent with the hypothesis that jets are magnetically dominated. As a first step, we identified a suitable jet model. We assumed that the jet is magnetically dominated, axisymmetric, stationary. We determined analytical expressions for the jet electromagnetic fields, which depend on the jet shape. For cylindrical jets, the magnetic field is dominated by the poloidal component along the jet axis, whereas for parabolic and conical jets the magnetic field is dominated by the toroidal component perpendicular to the jet axis. As a second step, we populated the jet with accelerated electrons. We assumed that the distribution of the X-ray emitting electrons is softer than the distribution of the optical emitting electrons (our choice is motivated by the fact that the observed spectral energy distribution of the blazar synchrotron radiation softens for high photon energies). Finally, we calculated the polarization degree and the polarization angle of the synchrotron radiation, and compared results of our model with multifrequency polarimetric observations of blazars.
Our main results are the following. For cylindrical jets, the polarization degree is weakly chromatic (i.e. the X-rays and the optical radiation have the same polarization degree), and the polarization angle is nearly perpendicular to the projection of the jet axis on the plane of the sky. These results are inconsistent with multifrequency polarimetric observations of blazars, which show that the polarization degree is strongly chromatic, and the polarization angle is nearly parallel to the jet axis. For parabolic and conical jets, X-rays are significantly more polarized than the optical radiation, and the polarization angle is nearly parallel to the jet axis, consistent with multifrequency polarimetric observations of blazars. Then, multifrequency polarimetric observations imply that the jet should be parabolic or conical. The jet can be magnetically dominated, consistent with the predictions of the leading theoretical paradigm for the launching of the jet. In our model, the strong chromaticity of the polarization degree is due to the softening of the electron distribution at high energies.
Since the project was terminated early (after 8 months), we could not complete the original work plan. Further research is needed to test the predictions of the theoretical paradigm for the launching of the jet. In particular, one should model in detail the spectral energy distribution of blazars, including both the synchrotron radiation and the inverse Compton radiation. We will develop a more complete model of the blazar emission in the near future.
Since the project was terminated early (after 8 months), we could not complete the original work plan. Further research is needed to test the predictions of the theoretical paradigm for the launching of the jet. In particular, one should model in detail the spectral energy distribution of blazars, including both the synchrotron radiation and the inverse Compton radiation. We will develop a more complete model of the blazar emission in the near future.