A framework for the study of physical conditions in astrophysical plasmas through radio and optical polarization: Application to extragalactic jets

This work presents a framework for the study of the physical conditions in astrophysical plasma elements through linear and circular, radio and optical polarization monitoring. The term "framework" is meant to describe the self-consistent character of the work which contains all necessary elements to: 1. Design and conduct high-cadence, multi-frequency polarimetric observations. 2. Reconstruct the Stokes 4-vector of the observed radiation with high accuracy. 3. Interpret polarimetric observations based on the theoretical predictions of several emission, absorption and propagation effects which can generate, modify, or eliminate the Stokes parameters of the radiation. 4. Reproduce the observed characteristics in the complete Stokes parameters set using a radiative transfer code we developed on the basis of the model of Hughes et al. (1989, ApJ, 341, 54). The development of the machinery of this framework was based on data obtained within the F-GAMMA monitoring program. Our polarimetric methodology eliminates a number of sources of uncertainty and is directly applicable to radio telescopes equipped with circularly polarized feeds. Using this methodology we recovered the polarization characteristics for a sample of 87 AGNs at 4 bands: 2.64, 4.85, 8.35 and 10.45 GHz. Our analysis focuses on observations conducted with the 100-m Effelsberg telescope between July 2010 and January 2015 with a mean cadence of 1.6 months. We used these datasets to characterize the observed sample in terms of linear and circular radio polarization. The computed polarization parameters were subsequently used as the basis for the computation of the magnetic field strength of those jets and the rotation measure which is attributed to the low energy magnetized plasma located in regions where the radiation is emitted or propagated through. The theoretical predictions for the emission, absorption and propagation of radiation were used to perform a thorough correlation analysis between several observed characteristics in order to investigate the physical conditions of the emitting plasma elements. Multi-frequency, high cadence observations are essential in the study of the pronounced variability these sources usually show. This variability was found to follow repeating patterns in the Fν–ν domain for many sources, a prototype of which is the blazar 3C454.3. In many cases, these patterns agree with the predictions of the "shock-in-jet" model (Marscher and Gear, 1985, ApJ, 298, 114) which attributes them to the evolution of physical conditions at shocked regions as they propagate downstream the jet. Our results showed coordinated changes of the polarization characteristics which mark the transitions between the optically thick and thin regimes of synchrotron emission. Assuming that these transitions are due to the optical depth evolution of the propagated shocks, we used our radiative transfer code to emulate them and reproduce the variability observed in all Stokes parameters in the case of the prototype source 3C 454.3. We followed the strict requirement to reproduce this variability just by evolving the physical characteristics of the emitting region according to the predictions of the "shock-in-jet" model. This approach resulted in a number of estimates for the physical conditions of this jet, like its particle number density, magnetic field coherence length and Doppler factor. Finally, we characterized our sample in terms of optical linear polarization using the data obtained with the RoboPol monitoring program between May 2013 and July 2015. The comparison of those partially simultaneous, radio and optical polarization datasets did not show any correlation between the two bands, suggesting that the physical conditions at the two emission sites are different. We detected 60 EVPA rotation events in the examined radio bands, that occurred in 22 sources of our sample, 6 of which have also shown such events in the optical wavelengths. Assuming that these rotations are caused by the helical motion of emission elements propagating downstream the jet, we used the rotation rates and the linear polarization degree measurements in both the radio and optical bands to compare the (tangential) velocities and hence the kinetic energies of those emission elements while they are propagating through the radio and optical emission sites in the jet. This thesis is structured as follows; In Chapter 1 we give a brief overview of the AGN structure and the theoretical predictions for the polarization properties necessary throughout the rest of the thesis. In Chapters 2 and 3 we describe the machinery of our framework used to extract and calibrate all the Stokes parameters for radio telescopes equipped with circularly polarized feeds. In Chapter 4 we characterize the linear and circular radio polarization properties of our sample of AGNs. Those are later used to calculate other physical parameters at the emission or propagation regions and to perform a correlation analysis investigating the underlying physical mechanisms. In Chapter 5 we describe our radiative transfer code and its application on the radio polarimetric data of 3C 454.3 to reproduce the observed variability and extract a number of its physical parameters. In Chapter 6 we characterize the optical polarization properties of our sample, correlate them with the ones found in the radio bands and use the combined information to compare the physical conditions in the two emission sites in the jet. Finally, in Chapter 7, we summarize the results and conclusions reached throughout this thesis

Physical Conditions and Variability Processes in AGN Jets through Multi-Frequency Linear and Circular Radio Polarization Monitoring

Radio polarimetry is an invaluable tool to investigate the physical conditions and variability processes in active galactic nuclei (AGN) jets. However, detecting their linear and circular polarization properties is a challenging endeavor due to their low levels and possible depolarization effects. We have developed an end-to-end data analysis methodology to recover the polarization properties of unresolved sources with high accuracy. It has been applied to recover the linear and circular polarization of 87 AGNs measured by the F-GAMMA program from July 2010 to January 2015 with a mean cadence of 1.3 months. Their linear polarization was recovered at four frequencies between 2.64 and 10.45 GHz and the circular polarization at 4.85 and 8.35 GHz. The physical conditions required to reproduce the observed polarization properties and the processes which induce their variability were investigated with a full-Stokes radiative transfer code which emulates the synchrotron emission of modeled jets. The model was used to investigate the conditions needed to reproduce the observed polarization behavior for the blazar 3C 454.3, assuming that the observed variability is attributed to evolving internal shocks propagating downstream.Comment: 6 pages, 2 figure

Radio QPO in the γ{\gamma}-ray-loud X-ray binary LS I +61∘{^\circ}303

LS I +61${^\circ}$303 is a $\gamma$-ray emitting X-ray binary with periodic radio outbursts with time scales of one month. Previous observations have revealed microflares superimposed on these large outbursts with periods ranging from a few minutes to hours. This makes LS I +61${^\circ}$303, along with Cyg X-1, the only TeV emitting X-ray binary exhibiting radio microflares. To further investigate these microflaring activity in LS I +61${^\circ}$303 we observed the source with the 100-m Effelsberg radio telescope at 4.85, 8.35, and 10.45 GHz and performed timing analysis on the obtained data. Radio oscillations of 15 hours time scales are detected at all three frequencies. We also compare the spectral index evolution of radio data to that of the photon index of GeV data observed by Fermi-LAT. We conclude that the observed QPO could result from multiple shocks in a jet.Comment: 5 pages, 4 figures, accepted for publication in MNRA

The dependence of optical polarisation of blazars on the synchrotron peak frequency

The RoboPol instrument and the relevant program was developed in order to conduct a systematic study of the optical polarisation variability of blazars. Driven by the discovery that long smooth rotations of the optical polarisation plane can be associated with the activity in other bands and especially in gamma rays, the program was meant to investigate the physical mechanisms causing them and quantify the optical polarisation behaviour in blazars. Over the first three nominal observing seasons (2013, 2014 and 2015) RoboPol detected 40 rotations in 24 blazars by observing a gamma–ray-loud and gamma–ray-quite unbiassed sample of blazars, providing a reliable set of events for exploring the phenomenon. The obtain datasets provided the ground for a systematic quantification of the variability of the optical polarisation in such systems. In the following after a brief review of the discoveries that relate to the gamma-ray loudness of the sources we move on to discuss a simple jet model that explains the observed dichotomy in terms of polarisation between gamma–ray-loud and quite sources and the dependence of polarisation and the stability of the polarisation angle on the synchrotron peak frequency

An Exceptional Radio Flare in Markarian 421

In September 2012, the high-synchrotron-peaked (HSP) blazar Markarian 421 underwent a rapid wideband radio flare, reaching nearly twice the brightest level observed in the centimeter band in over three decades of monitoring. In response to this event we carried out a five epoch centimeter- to millimeter-band multifrequency Very Long Baseline Array (VLBA) campaign to investigate the aftermath of this emission event. Rapid radio variations are unprecedented in this object and are surprising in an HSP BL Lac object. In this flare, the 15 GHz flux density increased with an exponential doubling time of about 9 days, then faded to its prior level at a similar rate. This is comparable with the fastest large-amplitude centimeter-band radio variability observed in any blazar. Similar flux density increases were detected up to millimeter bands. This radio flare followed about two months after a similarly unprecedented GeV gamma-ray flare (reaching a daily E>100 MeV flux of (1.2 +/- 0.7)x10^(-6) ph cm^(-2) s^(-1)) reported by the Fermi Large Area Telescope (LAT) collaboration, with a simultaneous tentative TeV detection by ARGO-YBJ. A cross-correlation analysis of long-term 15 GHz and LAT gamma-ray light curves finds a statistically significant correlation with the radio lagging ~40 days behind, suggesting that the gamma-ray emission originates upstream of the radio emission. Preliminary results from our VLBA observations show brightening in the unresolved core region and no evidence for apparent superluminal motions or substantial flux variations downstream.Comment: 5 pages, 8 figures. Contributed talk at the meeting "The Innermost Regions of Relativistic Jets and Their Magnetic Fields", Granada, Spain. Updated to correct author list and reference

The dependence of optical polarisation of blazars on the synchrotron peak frequency

The RoboPol instrument and the relevant program was developed in order to conduct a systematic study of the optical polarisation variability of blazars. Driven by the discovery that long smooth rotations of the optical polarisation plane can be associated with the activity in other bands and especially in gamma rays, the program was meant to investigate the physical mechanisms causing them and quantify the optical polarisation behaviour in blazars. Over the first three nominal observing seasons (2013, 2014 and 2015) RoboPol detected 40 rotations in 24 blazars by observing a gamma–ray-loud and gamma–ray-quite unbiassed sample of blazars, providing a reliable set of events for exploring the phenomenon. The obtain datasets provided the ground for a systematic quantification of the variability of the optical polarisation in such systems. In the following after a brief review of the discoveries that relate to the gamma-ray loudness of the sources we move on to discuss a simple jet model that explains the observed dichotomy in terms of polarisation between gamma–ray-loud and quite sources and the dependence of polarisation and the stability of the polarisation angle on the synchrotron peak frequency

A Near Magnetic-to-kinetic Energy Equipartition Flare from the Relativistic Jet in AO 0235+164 during 2013-2019

We present the multiwavelength flaring activity of the blazar AO 0235+164 during its recent active period from 2013 to 2019. From a discrete correlation function (DCF) analysis, we find a significant (>95%) correlation between radio and $\gamma$-ray light curves with flares at longer wavelengths following flares at shorter wavelengths. We identify a new jet component in 43 GHz VLBA data that was ejected from the radio core on MJD $57246^{+26}_{-30}$ (2015 August 12), during the peak of the 2015 radio flare. From the analysis of the jet component, we derived a Doppler factor of $\delta_{\rm var}=28.5\pm8.4$, a bulk Lorentz factor of $\Gamma=16.8^{+3.6}_{-3.1}$, and an intrinsic viewing angle of $\theta_{\rm v}=1.42^{+1.07}_{-0.52}\textrm{ degrees}$. Investigation of the quasi-simultaneous radio data revealed a partially absorbed spectrum with the turnover frequency varying in the range of $10-70$ GHz and the peak flux density varying in the range of $0.7-4$ Jy. We find the synchrotron self-absorption magnetic field strength to be $B_{\rm SSA}=15.3^{+12.6}_{-14.0}$ mG at the peak of the 2015 radio flare, which is comparable to the equipartition magnetic field strength of $B_{\rm EQ}=43.6^{+10.6}_{-10.4}$ mG calculated for the same epoch. Additional analysis of the radio emission region in the relativistic jet of AO 0235+164 suggests that it did not significantly deviate from equipartition during its recent flaring activity.Comment: 13 pages, 11 figures, 4 tables; Accepted for publication in MNRA

The Repeating Flaring Activity of Blazar AO 0235+164

Context. Blazar AO 0235+164, located at redshift z = 0.94, has undergone several sharp multi-spectral-range flaring episodes during the last decades. In particular, the episodes peaking in 2008 and 2015, that received extensive multi-wavelength coverage, exhibited interesting behavior. Aims. We study the actual origin of these two observed flares by constraining the properties of the observed photo-polarimetric variability, those of the broad-band spectral energy-distribution and the observed time-evolution behavior of the source as seen by ultra-high resolution total-flux and polarimetric Very-long-baseline interferometry (VLBI) imaging. Methods. The analysis of VLBI images allows us to constrain kinematic and geometrical parameters of the 7 mm jet. We use the Discrete Correlation Function to compute the statistical correlation and the delays between emission at different spectral ranges. Multi-epoch modeling of the spectral energy distributions allows us to propose specific models of emission; in particular for the unusual spectral features observed in this source in the X-ray region of the spectrum during strong multi spectral-range flares. Results. We find that these X-ray spectral features can be explained by an emission component originating in a separate particle distribution than the one responsible for the two standard blazar bumps. This is in agreement with the results of our correlation analysis that do not find a strong correlation between the X-rays and the remaining spectral ranges. We find that both external Compton dominated and synchrotron self-Compton dominated models can explain the observed spectral energy distributions. However, synchrotron self-Compton models are strongly favored by the delays and geometrical parameters inferred from the observations

Multiwavelength Picture of the Blazar S5 0716+714 during Its Brightest Outburst

S5 0716+714 is a well known BL Lac object, and one of the brightest and most active blazars. The discovery in the Very High Energy band (VHE, E > 100 GeV) by MAGIC happened in 2008. In January 2015, the source went through the brightest optical state ever observed, triggering MAGIC follow-up and a VHE detection with ∼ 13σ significance (ATel ♯6999 ). Rich multiwavelength coverage of the flare allowed us to construct the broad-band spectral energy distribution of S5 0716+714 during its brightest outburst. In this work, we will present the preliminary analysis of MAGIC and Fermi-LAT data of the flaring activity in January and February 2015 for the HE (0.1 < HE < 300 GeV) and VHE band, together with radio (Metsähovi, OVRO, VLBA, Effelsberg), sub-millimeter (SMA), optical (Tuorla, Perkins, Steward, AZT-8+ST7, LX-200, Kanata), X-ray and UV (Swift-XRT and UVOT), in the same time-window and discuss the time variability of the multiwavelength light curves during this impressive outburst.</p