Very Long Baseline Interferometry and Radio Astronomy Group

PhD projects

Of the Very Long Baseline Interferometry and Radio Astronomy Group

Director: Prof. Dr. J. Anton Zensus                                                                                           

Group website

Code: AZ01

Direct imaging of the extreme vicinity of cosmic black holes with millimetre-wavelength Very-Long-Baseline Interferometry (mmVLBI)

Very long baseline interferometry (VLBI)  is the only method for a direct imaging of  regions in the immediate vicinity of  heavy (super-massive) cosmic black holes. This includes the attempt to image the shadow around a black hore and its surrounding photon ring (event horizon) and by this test Einstein’s Theory of General Relativity.

The angular resolution of an interferometer increases with decreasing wavelength and with increasing baseline length.  The first leads to  millimetre VLBI, the second  to space VLBI (see AZ02) -- and both methods are presently delivering images at a resolution of less than 30 microarcseconds. Since compact radio sources become more transparent with increasing frequency (lower opacity at mm-wavelength),   mm-VLBI  offers a unique possibility  to probe deeper into the self-absorbed regions of AGN, regions which are usually not observable at the longer cm-wavelengths.  Our group operates the Global Millimeter VLBI Array (GMVA), which combines up to 14 telescopes into regular 3mm/7mm VLBI observations. In parallel the group has begun to establish regular VLBI observations at 230 GHz and is active  in the pioneering VLBI observations at 230 GHz with the so called Event Horizon Telescope (EHT). This includes  the inclusion of the ALMA telescope in future  mm-VLBI observations and the further development of mm-/sub-mm VLBI.  One of the main goals of this effort is to probe the ‘shadow of the black hole’ in the Galactic Centre and in M 87, as well as to study the origin of jets in more distant radio-galaxies and quasars (AGN) with unpredecent resolution.

One of the goals of the PhD candidate is to image and study Active Galactic Nuclei (Quasars, BL Lac objects, Radio Galaxies, etc.) with highest possible  resolution in the mm-band, addressing questions related to AGN activity, outburst-ejection relations, the physical  origin of jets, the details of the jet launching and of the primary jet acceleration  processes. For this the jet kinematics, their spectral and polarimetric properties  will be studied on spatial scales which are as close as possible to the central engine  ( scales of a few 10 - 1000 gravitational radii). 

VLBI studies of the polarised fine structure of AGN probe the orientation and nature of magnetic fields at the innermost part of their relativistic jets.  mm-VLBI has the advantage that polarimetric observations at 86 GHz are only marginally affected by Faraday rotation and probe the intrinsic linearly polarised emission.  We aim to probe if and how oblique shocks  in the innermost   jet regions  are responsible  for the observed polarised emission. . The detailed study of the three-dimensional, spatially bent   complex structure of the jet close to its nozzle  will help us to answer one of the most fundamental questions in AGN physics, which is how Black Holes launch relativistic jets..  

Notice that for this Key Project one or more students can be hired.

Bibliography:

Doeleman, S., et al., 2008, Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre, Nat, 455, 78.

Krichbaum, T.P., et al., 2013, Zooming towards the Event Horizon - mm-VLBI today and tomorrow, arxiv.org/abs/1305.2811

Lobanov, A.P. et al., A&A, 364, 391 (2000)

Martí-Vidal, I. et al, On the calibration of full-polarization 86 GHz global VLBI observations, A&A 542, 107 (2012)

Tilanus, R., et al., Future mmVLBI Research with ALMA: A European Vision, 2014, arxiv.org/abs/1406.4650

GMVA web site:  www.mpifr-bonn.mpg.de/div/vlbi/globalmm

EHT web site: www.eventhorizontelescope.org


Contact:  Prof. Dr. Anton Zensus (azensus@mpifr-bonn.mpg.de), Dr. Thomas P. Krichbaum (tkrichbaum@mpifr-bonn.mpg.de),  Prof. Dr. Eduardo Ros (ros@mpifr-bonn.mpg.de), Prof. Dr. Antxon Alberdi (antxon@iaa.es).


Site: Bonn, Max-Planck-Institut für Radioastronomie, VLBI group in collaboration with the MIT/Haystack Observatory in the USA, the ESO/ALMA telescope in Chile, and the IAA/CSIC in Spain.

Code: AZ02

Extreme resolution observations with space VLBI 

VLBI with the space-borne antenna Spektr-R of the Russian-led mission "RadioAstron" provides resolutions reaching down to 10 microarcseconds of arc, holding the absolute record in resolution of astronomical observations. 

This enables studies of jet physics and black hole vicinity to be made on scales of less than 1000 gravitational  radii, which is a unique opportunity to understand the driving force behind ultrarelativistic plasma produced in active galactic nuclei.

This project will use the data from dedicated RadioAstron programs led by the VLBI group (the Key Science Project on imaging of powerful AGN and the work package on jet acceleration in the   RadioAstron AGN Survey). These data will enable unique studies of intrinsic evolution of the flow, tracing its acceleration on scales from a thousand to a million of gravitational radii, and determining the dominant physical mechanism behind it.

The addition of RadioAstron data to mm-VLBI results and comparing to available 43-GHz VLBI polarised data of prominent blazars will probe the opacity and characterise the spectral behaviour of the jet base, even transversally to it, with the sharpest images ever achieved.

Notice that for this Key Project one or more students can be hired.

Bibliography:

Kardashev, N.S. et al, RadioAstron - A telescope with a size of 300 000 km: Main parameters and first observational results, Astron. Rep. 57, 153 (2013)

Contact:  Prof. Dr. Anton Zensus (azensus@mpifr-bonn.mpg.de), Dr. Andrei Lobanov (alobanov@mpifr-bonn.mpg.de), Prof. Dr. Eduardo Ros (ros@mpifr-bonn.mpg.de)

Site: Bonn, Max-Planck-Institut für Radioastronomie, VLBI group in collaboration with the Astro Space Centre in Russia, the Metsähovi Radio Observatory of Aalto University, Finland, and the IAA/CSIC in Spain.

Code: AZ03

 AGN Jet Kinematics & Localisation of non-thermal continuum production in AGN

Very long baseline interferometry (VLBI) imaging with its unsurpassed angular resolution allows direct studies of the innermost parsecs of the jets.  About ten percent of the active galactic nuclei (AGN) eject jets of plasma at relativistic speeds from their central regions. The rapidly variable non-thermal emission and apparent superluminal motions in the jets imply that they contain highly energetic plasma moving nearly directly at us at speeds approaching that of light. Understanding the acceleration, collimation, and stability properties of these flows is one of the central topics in the modern relativistic astrophysics.

The PhD candidate will join the international MOJAVE and/or TANAMI program teams and use the survey data to analyze the properties of magnetized plasma jets in AGN.  The MOJAVE program (Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments) monitors structural changes in the parsec-scale jets of over 300 AGN at 15 GHz making it one of the largest VLBI monitoring programs to date.  TANAMI monitors the parsec-scale structures of relativistic jets in active galactic nuclei in the Southern Hemisphere.  Complementary multiwavelength observations are being done with modern X-ray telescopes (XMM-Newton, Swift, INTEGRAL) as well as in the NIR/optical regime. The PhD project will investigate the nature of active galactic nuclei, their supermassive black holes and relativistic jets, making use of unprecedented data across the whole electromagnetic spectrum.  The project includes data debugging, image analysis, spectral studies of the jets, correlation study of the morphological and brightness variation in the context of a multi-band approach are the main goals of the project.  The data being collected are unique, for its quality.  The project guarantees integration in the NASA Fermi/LAT collaboration, high-impact publications and a long time baseline for collaboration, since Fermi/LAT will operate for the next 5-10 years, and we expect to keep collecting data during this time. 

Localising the sites where non-thermal continuum is produced in AGN is pivotal for understanding their physics. Presently, this localisation is largely based on information obtained from observations of the SED and correlated variability of the broad-band continuum emission in AGN. VLBI observations enable spatial localisation of the optical, X-ray and Gamma-ray flares in AGN. Correlations observed between parsec-scale radio emission and optical and gamma-ray continuum indicate that a significant fraction of non-thermal continuum may be produced in extended regions of relativistic jet, thus requiring serious rethinking of existing models for the high-energy emission production.  Survey work will be focused on further exploration and better understanding of radio-gamma and radio-TeV relations in powerful AGN, using Fermi/LAT and HESS/MAGIC data in combination with extensive VLBI monitoring data from the MOJAVE and TANAMI surveys, geodetic VLBI data and new VLBI observations.

Another approach is to study the validity of the current paradigm of "simple" outward motion in jets, which is being questioned in a number of AGN.  A kinematical analysis of can probe this unexpected behavior in more detail and to study the physical emission processes in jets in general and the connection between the central engine and jet launching.


Bibliography:

Arshakian, Leon-Tavares, Lobanov et al., MNRAS, 401, 1231 (2010)

Leon-Tavares, Lobanov, Chavushyan et al. ApJ, 715, 355 (2010)

Lister et al. AJ 147, 143 (2014)

Ojha et al. A&A, 519, A45 (2010)

Schinzel, Lobanov, Taylor et al. A&A 637, 70 (2012)

See as well http://pulsar.sternwarte.uni-erlangen.de/tanami/ and http://www.physics.purdue.edu/astro/MOJAVE/index.html


Contact: Prof. Dr. Anton Zensus (azensus@mpifr-bonn.mpg.de) ,  Prof. Dr. Eduardo Ros (ros@mpifr-bonn.mpg.de) , Dr. Andrei Lobanov (alobanov@mpifr-bonn.mpg.de), Prof. Dr. Matthias Kadler (matthias.kadler@astro.uni-wuerzburg.de),  PD Dr. Silke Britzen (sbritzen@mpifr-bonn.mpg.de)

Site: Bonn, Max-Planck-Institut für Radioastronomie, VLBI group in collaboration with the NASA/Goddard Space Flight Centre, National Radio Astronomy Observatory and University of Purdue, USA, Metsähovi Radio Observatory of Aalto University, Finland, and the University of Würzburg, Germany

Code: AZ04

Relativistic simulations of galactic flows and feedback, binary black holes

Extragalactic jets are formed in the environments of supermassive black holes (SMBH) in active galactic nuclei (AGN). They are among the most powerful and energetic astrophysical objects. Their relevance is not only due to their role as laboratories of relativistic plasmas, but they have also an important effect in their environments, namely, in the interstellar medium of the host galaxy and the intergalactic medium. Understanding their nature and physics can thus give us key information about the progenitor SMBH and its surroundings, but also about the host galaxy and its history. A combination between detailed VLBI observations and theoretical modelling via numerical simulations has proven to be a very good approach to reach the goal of this research.

At present, we are able to perform numerical simulations in supercomputers, including all the relevant physics of these objects: relativistic gas, magnetic fields, different composition.  VLBI is addressing the innermost radio structure of these objects, and gives the observational input to theoretical studies.  Here we propose to continue an already started line of research, which consists in trying to relate the emitting, non-thermal population of particles, studied through observations, with the thermal gas in the jet and the magnetic fields, responsible for the macroscopic jet dynamics.

Another application of numerical simulations of relativistic, magnetohydrodynamic flows in supercomputing facilities as a laboratory is to use them to study the interplay between jets, host galaxy and surrounding gas and their impact in the galactic activity and evolution, with influence on the star-formation rates or the growth of the central black hole, for instance.  Given the advent of new-generation telescopes, like LOFAR, the PhD project will combine numerical simulations and observations to explain the role of galactic activity in the evolution of galaxies and clusters.

An alternative study would focus on a study of mergers of galaxies as the origin of quasar is a different approach to be explored.  A number of phenomena were attributed to the presence of binary systems, including X-shaped radio galaxies and double-double radio galaxies, precession of a jet emitted by one of the binary components, wiggling of a jet due to the orbital motion, periodic variations in the luminosity due to perturbation of an accretion disk around one of the holes, binary galaxies with radio-jet cores, and binary quasars, etc.  For this, the PhD candidate should study the connection between galaxy interactions and active galactic nuclei with particular emphasis on the proof of the existence and analysis of supermassive binary systems in the centers of AGN. 

Bibliography:

Fromm et al., Catching the radio flare in CTA 102. II. VLBI kinematic analysis, A&A 551, A32 (2013)

Fromm et al., Core-shift and spectral analysis of the 2006 radio flare in CTA102, 11th EVN Symp, arXiv:1301.7674 (2013)

Contact: Prof. Dr. Anton Zensus (azensus@mpifr-bonn.mpg.de), Prof. Dr. Eduardo Ros (ros@mpifr-bonn.mpg.de), Dr. Manel Perucho (Univ. Valencia, Spain, perucho@uv.es), PD Dr. Silke Britzen (sbritzen@mpifr-bonn.mpg.de)

Site: Bonn, Max-Planck-Institut für Radioastronomie, VLBI Group in collaboration with the Universitat de València, Spain

Code: AZ05

Magnetic fields in Blazars: polarimetry studies

Blazars are the most active galaxies known. 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. The launch of NASA’s Fermi Gamma-ray Space Telescope in 2008 has provided an unprecedented opportunity for the systematic study of blazar jets and has prompted large-scale blazar monitoring efforts across wavelengths.

In such a multi-wavelength campaign, a novel effect was discovered: fast changes in the optical polarization during gamma-ray flares. Such events probe the magnetic field structure in the jet and the evolution of disturbances responsible for blazar flares. Their systematic study can answer long-standing questions in our theoretical understanding of jets; however, current optical polarimetry programs are not adequate to find and follow similar events with the efficiency and time-resolution needed.

In the context of this program we aim at regular flux density and polarization observations of selected blazars at short cm- and mm-wavelength, using the Effelsberg and IRAM telescopes.

The radio/mm measurements will be complemented by optical polarimetry.

RoboPol is a massive program of optical polarimetric monitoring of over 100 blazars, using a specially-built, innovative polarimeter mounted on the 1.3 m telescope of the University of Crete’s Skinakas Observatory, a dynamical observing schedule, and a large amount of dedicated telescope time. The program is a collaboration between the Max-Planck Institute for Radioastronomy, the University of Crete and the Foundation for Research and Technology - Hellas in Greece, Caltech in the US, the Nicolaus Copernicus University in Poland, and the Inter-University Centre for Astronomy and Astrophysics in India.

The PhD student will collaborate closely withl team members  at the MPIfR (coordinated by E. Angelakis) and the U. of Crete (coordinated by V. Pavlidou), as well as the other international RoboPol collaborators. The student will have the opportunity to combine polarization observations with datasets across wavelengths, and participate in a complementary theory and phenomenology program, aiming to constrain the underlying physics of the blazar phenomenon.

Bibliography

The RoboPol optical polarization survey of gamma-ray - loud blazars, V. Pavlidou et al, 2014MNRAS.442.1693P

Fuhrmann, L. et al. 2014, Detection of significant cm- to sub-mm band radio and gamma-ray correlated variability in Fermi bright blazars, MN 441, 1899

Myserlis, I., et al., 2014, Multi-frequency linear and circular radio polarization monitoring of jet emission elelements in FERMI blazars, arXiv:1401.2072.

Marscher, A.P., et al., 2005, Probing the inner jet of the quasar PKS 1510-089  with multi-waveband monitoring during strong gamma-ray activity, ApJ 710, 126

Contact:  Prof. Dr. J. Anton Zensus (azensus@mpifr-bonn.mpg.de), Dr. Emmanouil Angelakis (eangelakis@mpifr-bonn.mpg.de), Dr. Thomas P. Krichbaum (tkrichbaum@mpifr-bonn.mpg.de)


Site: Bonn, Max-Planck-Institut für Radioastronomie, VLBI Group in collaboration with the University of Crete, Greece

 
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