Emmanouil Angelakis | Max Planck Institute fur Radioastronomie (original) (raw)

Papers by Emmanouil Angelakis

arXiv (Cornell University), Nov 18, 2019

There are several methods to calculate the radiative and kinetic power of relativistic jets, but ... more There are several methods to calculate the radiative and kinetic power of relativistic jets, but their results can differ by one or two orders of magnitude. Therefore, it is necessary to perform a calibration of the jet power, to understand the reasons for these differences (whether wrong hypotheses or intrinsic source variability), and if it is possible to converge to a reliable measurement of this physical quantity. We present preliminary results of a project aimed at calibrating the power of relativistic jets in active galactic nuclei (AGN) and X-ray binaries (XRB). We started by selecting all the AGN associations with known redshift in the Fourth Fermi LAT Gamma-Ray Catalog (4FGL). We then calculated the radiative and/or kinetic powers from available data or we extracted this information from literature. We compare the values obtained for overlapping samples and highlight preliminary conclusions.

The astronomer's telegram, Dec 1, 2015

VizieR Online Data Catalog, Jul 1, 2018

Astronomy and Astrophysics, Nov 1, 2008

Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71 III.... more Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71 III. Rapid and correlated flux density variability from radio to sub-mm bands

arXiv (Cornell University), Feb 7, 2019

Context. The advent of the Fermi gamma-ray space telescope with its superb sensitivity, energy ra... more Context. The advent of the Fermi gamma-ray space telescope with its superb sensitivity, energy range, and unprecedented capability to monitor the entire 4π sky within less than 2-3 h, introduced a new standard in time domain gamma-ray astronomy. Among several breakthroughs, Fermi has-for the first time-made it possible to investigate, with high cadence, the variability of the broadband spectral energy distribution (SED), especially for active galactic nuclei (AGN). This is necessary for understanding the emission and variability mechanisms in such systems. To explore this new avenue of extragalactic physics the Fermi-GST AGN Multi-frequency Monitoring Alliance (F-GAMMA) programme undertook the task of conducting nearly monthly, broadband radio monitoring of selected blazars, which is the dominant population of the extragalactic gamma-ray sky, from January 2007 to January 2015. In this work we release all the multi-frequency light curves from 2.64 to 43 GHz and first order derivative data products after all necessary post-measurement corrections and quality checks. Aims. Along with the demanding task to provide the radio part of the broadband SED in monthly intervals, the F-GAMMA programme was also driven by a series of well-defined fundamental questions immediately relevant to blazar physics. On the basis of the monthly sampled radio SEDs, the F-GAMMA aimed at quantifying and understanding the possible multiband correlation and multi-frequency radio variability, spectral evolution and the associated emission, absorption and variability mechanisms. The location of the gammaray production site and the correspondence of structural evolution to radio variability have been among the fundamental aims of the programme. Finally, the programme sought to explore the characteristics and dynamics of the multi-frequency radio linear and circular polarisation. Methods. The F-GAMMA ran two main and tightly coordinated observing programmes.

arXiv (Cornell University), Nov 13, 2006

arXiv (Cornell University), Sep 5, 2018

Aims. We present a multifrequency, dense radio monitoring program of the blazar OJ 287 using the ... more Aims. We present a multifrequency, dense radio monitoring program of the blazar OJ 287 using the 100-m Effelsberg radio telescope. The program aims to test different binary supermassive black hole (SMBH) scenarios and studying the physical conditions in the central region of this bright blazar. Here, we analyze the evolution in total flux density, linear and circular polarization as a means to study the OJ 287 jet structure and its magnetic field geometry. Methods. We used a recently developed, high-precision data analysis methodology to recover all four Stokes parameters. We measured the total flux density of OJ 287 at nine bands from 2.64 GHz to 43 GHz, the linear polarization parameters at four bands between 2.64 GHz and 10.45 GHz, and the circular polarization at two bands, 4.85 GHz and 8.35 GHz. The mean cadence of our measurements is ten days. Results. Between December 2015 and January 2017 (MJD 57370-57785), OJ 287 showed flaring activity and complex linear and circular polarization behavior. The radio electric vector position angle (EVPA) showed a large clockwise (CW) rotation by ∼340 • with a mean rate of −1.04 • /day. Based on concurrent very long baseline interferometric (VLBI) polarization data at 15 GHz and 43 GHz, the rotation seems to originate within the jet core at 43 GHz (projected angular size ≤0.15 mas or 0.67 pc at the redshift of the source). Moreover, optical polarization data show a similar monotonic CW rotation with a rate of about −1.1 • /day which is superposed with shorter and faster rotations that exhibit rates of about 7.8 • /day, mainly in the CW sense. Conclusions. The flux density and polarization variability of the single dish, VLBI and optical data is consistent with a polarized emission component propagating on a helical trajectory within a bent jet. We constrained the helix arc length to 0.26 pc and radius to ≤0.04 pc as well as the jet bending arc length projected on the plane of the sky to ≤1.9-7.6 pc. A similar bending has been observed also in high angular resolution VLBI images of the OJ 287 jet at its innermost regions. The helical trajectory covers only a part of the jet width, possibly its spine. In addition, our results indicate the presence of a stable polarized emission component. Its EVPA (−10 •) is oriented perpendicular to the large scale jet, suggesting dominance of the poloidal magnetic field component. Finally, the EVPA rotation begins simultaneously with an optical flare and hence the two might be physically connected. That optical flare has been suggested to be linked to the interaction of a secondary SMBH with the inner accretion disk or originating in the jet of the primary.

We have continued our monitoring program of ∼ 1200 sources with the Owens Valley Radio Observator... more We have continued our monitoring program of ∼ 1200 sources with the Owens Valley Radio Observatory 40 m Telescope. Most of the sources are part of the Candidate Gamma-Ray Blazar Survey (CGRaBS). We have also been observing Targets Of Opportunity triggered by gamma-ray detections and participating in multiwalength campaigns lead by Fermi-GST. Regular program sources are observed twice a week and have been observed for ∼ 24 months at 15 GHz. We use these light curves to study the radio variability and its relation with optical properties. A significant fraction of the bright AGNs detected by Fermi-GST are in our program, which will allow for correlations between the radio and gamma-ray behavior for a number of sources. Here we present some example light curves, describe the tools used to study the variability and present some preliminary variability statistics for the sample.

arXiv (Cornell University), Jun 2, 2006

Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71 II. ... more Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71 II. A search for intra-day variability at millimetre wavelengths with the IRAM 30 m telescope

arXiv (Cornell University), Mar 14, 2016

Context. The origin of blazar variability, as seen from radio up to γ rays, is still a heavily de... more Context. The origin of blazar variability, as seen from radio up to γ rays, is still a heavily debated matter, and broadband flares offer a unique testbed for developing a better understanding of these extreme objects. One of these energetic outbursts was detected by Fermi/LAT in 2008 from the blazar PKS 1502+106. The outburst was observed from γ rays down to radio frequencies. Aims. Through the delay between flare maxima at different radio frequencies, we study the frequency-dependent position of the unitopacity surface and infer its absolute position with respect to the jet base. This nuclear opacity profile enables the jet's magnetic field tomography. We also localize the γ-ray emission region and explore the flare production mechanism. Methods. The PKS 1502+106 radio flare is studied through single-dish flux density measurements at 12 frequencies in the range 2.64 to 226.5 GHz. To quantify the flare, we employ both a Gaussian process regression and a discrete cross-correlation function analysis. Results. We find that the light curve parameters (flare amplitude and cross-band delays) show a power-law dependence on frequency. Delays decrease with frequency, and the flare amplitudes increase up to about 43 GHz, and then decay. This behavior is consistent with the propagation of a shock downstream in the jet. The self-absorbed radio cores are located between approximately ten and four pc from the jet base, and their magnetic field strengths range between 14 and 176 mG, at the frequencies 2.64 to 86.24 GHz. Finally, the γ-ray active region is located at (1.9 ± 1.1) pc away from the jet base.

The Astrophysical Journal, 2022

We present the first polarimetric space VLBI observations of OJ 287, observed with RadioAstron at... more We present the first polarimetric space VLBI observations of OJ 287, observed with RadioAstron at 22 GHz during a perigee session on 2014 April 4 and five near-in-time snapshots, together with contemporaneous ground VLBI observations at 15, 43, and 86 GHz. Ground-space fringes were obtained up to a projected baseline of 3.9 Earth diameters during the perigee session, and at a record 15.1 Earth diameters during the snapshot sessions, allowing us to image the innermost jet at an angular resolution of ∼ 50µas, the highest ever achieved at 22 GHz for OJ 287. Comparison with ground-based VLBI observations reveals a progressive jet bending with increasing angular resolution that agrees with predictions from a supermassive binary black hole model, although other models cannot be ruled out. Spectral analyses suggest that the VLBI core is dominated by the internal energy of the emitting particles during the onset of a multi-wavelength flare, while the parsec-scale jet is consistent with being in equipartition between the particles and magnetic field. Estimated minimum brightness temperatures from the visibility amplitudes show a continued rising trend with projected baseline length up to 10 13 K, reconciled with the inverse Compton limit through Doppler boosting for a jet closely oriented to the line of sight. The observed electric vector position angle suggests that the innermost jet has a predominantly toroidal magnetic field, which together with marginal evidence of a gradient in rotation

arXiv (Cornell University), Jun 17, 2013

Narrow-Line Seyfert 1 galaxies have been established as a new class of gamma-ray emitting AGN wit... more Narrow-Line Seyfert 1 galaxies have been established as a new class of gamma-ray emitting AGN with relatively low black hole masses, but near-Eddington accretion rates. Other extragalactic gamma-ray sources observed so far such as Flat Spectrum Radio Quasars, Radio Galaxies, and BL Lacertae Objects generally exhibit much higher black hole masses and, in the case of BL Lac objects and FRI Radio Galaxies, much lower accretions rates. The multifrequency campaign of 2013 centered on the bright source 1H 0323+342 will provide further insights into the nature of the jets and their gamma ray production mechanisms in a largely unexplored corner of AGN parameter space. Here, we show preliminary results of this campaign and discuss them.

VizieR Online Data Catalog, Jul 1, 2012

VizieR Online Data Catalog, Nov 1, 2014

The F-GAMMA program is a coordinated effort to investigate the physics of Active Galactic Nuclei ... more The F-GAMMA program is a coordinated effort to investigate the physics of Active Galactic Nuclei (AGNs) via multi-frequency monitoring of Fermi blazars. In the current study we show and discuss the evolution of broad-band radio spectra, which are measured at ten frequencies between 2.64 and 142 GHz using the Effelsberg 100-m and the IRAM 30-m telescopes. It is shown that any of the 78 sources studied can be classified in terms of their variability characteristics in merely 5 types of variability. It is argued that these can be attributed to only two classes of variability mechanisms. The first four types are dominated by spectral evolution and can be described by a simple two-component system composed of: (a) a steep quiescent spectral component from a large scale jet and (b) a time evolving flare component following the "Shock-in-Jet" evolutionary path. The fifth type is characterised by an achromatic change of the broad band spectrum, which could be attributed to a different mechanism, likely involving differential Doppler boosting caused by geometrical effects. Here we present the classification, the assumed physical scenario and the results of calculations that have been performed for the spectral evolution of flares.

Proceedings of 7th International Fermi Symposium — PoS(IFS2017), 2017

The compact radio source TXS 2013+370 is a γ-ray blazar which is located at a redshift of z = 0.8... more The compact radio source TXS 2013+370 is a γ-ray blazar which is located at a redshift of z = 0.859 at a galactic latitude b = 1.2 o. We observed the source with Very Long Baseline Interferometry (VLBI) at 15, 43 and 86 GHz and studied the morphology and the kinematic properties of the jet. The VLBI data were then combined with flux density variability measurements at 15 and 235 GHz and with the available γ-ray light curve in the period 2008-2017. A cross-correlation analysis was performed to investigate the existence of a correlation between the variability observed in the different bands. The preliminary results of our study showed that the most prominent flares and maxima stem from the central VLBI region and most likely are associated with the nuclear region, namely the core, indicating that the activity is caused by the passage of traveling shocks through the core region. In the course of our analysis, we present for the first time a 86 GHz Global Millimeter VLBI Array (GMVA) image of the innermost jet region.

Astronomy & Astrophysics, 2018

Astronomy & Astrophysics, 2016