Andrei Lobanov: Research topics

My current research subjects include modelling and observations of parsec-scale jets; relating structural and flux density variability in compact AGN; studying spectral properties of synchrotron emission from compact AGN. Other research projects include millimeter-VLBI observations; VLBI observations of hot-spots in kiloparsec-scale jets; VLBI study of several water masers in star forming regions; space VLBI observations of high-redshift quasars.

In the coming years, I would like to concentrate on making VLBI data a better suited tool for studying the physical processes in AGN. This includes four main directions of research:

1. Spectral imaging using multi-frequency VLBI data.

This is a promising technique that allows to produce maps of the synchrotron turnover frequency distribution in VLBI jets. The turnover frequency is a sensitive indicator of pressure and velocity gradients due to relativistic shocks or plasma instabilities in the jets--this can be used for directly detecting the presence of the shocks and instabilities in the jets. Magnetic field distribution can also be reconstructed, from a combination of the turnover frequency and turnover flux density maps. A basic mapping method has been developed and tested on two compact jets in 3C273 and 3C345 (Vistas in Astronomy, 41, 253). A paper describing the method has been accepted for publication in A&A. The turnover frequency distributions in 3C120 (6 frequencies), 3C273, and 3C345 (4 frequencies each) will be obtained from available VLBA data. Further work is required for establishing this technique as a viable and accessible method of study.

The work is done in collaboration with J.A. Zensus (MPIfR) and E. Carrara (Univ. of San Paulo). A similar study has been proposed for Mk501, in collaboration with S. Aaron (JIVE-MPIfR), J. Wardle and D. Roberts (Brandeis Univ.).

2. Studying the physical conditions in the central regions of AGN using absorption and self-absorption of the synchrotron emission.

I have devised and tested a method (A&A, 330, 79) that allows to determine changes of opacity along the jet, from the frequency dependence of the position of the optically thick core of the jet. The measured opacity can be used for modelling the conditions inside the ultracompact jet and in the broad-line regions around the nucleus of the host galaxy. This brings an interesting connection to studies of the BLR based on optical data (such as spectral modelling and reverberation mapping).

The VLBA phase-referencing observations of 3C345 at 4 frequencies aimed at improving the accuracy of measurements of the core offsets have been done in January 1998 (in collaboration with M. Rioja, IRA, Medicina and L. Gurvits, JIVE)). Opacity study at 6 frequencies has been proposed for Mk501 (in collaboration with S. Aaron (JIVE-MPIfR), J. Wardle & D. Roberts (Brandeis University).

3. Bringing together the kinematic and spectral information inferred from VLBI observations and multi-band emission monitoring programs.

The work that has been done so far is concentrated on several sources (0235+164, 0836+710, 1803+784, and others) in which a detailed account on kinematic evolution in the jet can be combined with the available spectral and flux density variability data. That has allowed to better constrain the applicability of shock models to the compact jets, and improve understanding of several phenomena (such as time lags between Gamma-flares and ejections of new bright features in the jets, and energy balance between the Gamma-ray and synchrotron emission from the jets).

For 0235+164 (A&A, submitted), I have applied the precessing relativistic beam model, to reproduce the observed time delays between the peaks in the light-curves at different frequencies. For 0836+710 (A&A, 334, 489), I have used the shock-in-jet scenario for modelling the observed spectral and kinematic properties, which has helped to better understand the time lags between the high-energy outbursts and appearances of new VLBI components in this radio source.

I have imaged VSOP observations of 0836+710 (October 1997) at 1.6 and 5GHz. These observations complemented by the scheduled VSOP and VLBA observations later this year will allow to perform similar analysis for younger features in the jet, and further test the suggested explanation of the observed time-lags between outbursts in different emission bands.

The work is done in collaboration with A. Kraus, T. Krichbaum, A. Witzel, J.A. Zensus (MPIfR), S. Britzen (NFRA), K. Otterbein (ITA, Heidelberg), & A. Quirrenbach (UCSB).

4. Cosmological applications of VLBI observations of AGN.

I am also interested in working on other possible applications of VLBI observations, such as, for instance, using compact AGN as probes for measuring the cosmological parameters. Apart from the conventional techniques (studies of dependence of measured angular sizes or proper motions on redshift), one other interesting prospect in this field would be to use relativistic distortions of a brightness profile of compact VLBI objects as a kind of standard rode--for which very high resolution space VLBI data can be used.

I am participating in a joint VSOP, VLBI, ASCA campaign aimed at observing a sample of extremely high-redshift quasars. We are using these observations to study the jet physics in the young Universe, investigate possible effects of cosmological evolution in AGN, and search for a suitable tool to measure the cosmological parameters. Recently made 1.6GHz VSOP image of PKS2215+020 (z=3.55) is the first VLBI image of the source, and it has revealed a substantially extended (  mas  pc) jet pointing in the direction of possible X-ray hotspots detected in ROSAT HRI observations. This makes the source a unique laboratory for studying the jet physics at high-redshifts. We are now planning an extensive observing campaign including follow-up VLBI observations, X-ray observations with ASCA, and mapping the large-scale structure of PKS2215+020 with ATCA.

This work is done in collaboration with L.I. Gurvits, R.T. Schilizzi (JIVE), K.I. Kellermann (NRAO), S. Frey (FÖMI, Hungary), N. Kawaguchi (ISAS), & I.I.K. Pauliny-Toth (MPIfR).

Other past and present research topics include:

Physics of the parsec-scale regions in 3C345.

A long-term monitoring of the parsec-scale jet of 3C345. Kinematic and flux density evolution of enhanced emission regions embedded in the jet have been followed now for over 14 years. The kinematics of the enhanced emission regions is found to be non-linear, requiring both apparent and intrinsic accelerations. The luminosity, flux density and kinematic changes observed along the jet indicate possible transition from a shock-dominated to plasma-interaction dominated emission mechanism. The observed variations of the turnover frequency have been used for testing the shock-in-jet model. All results are summarized in the thesis. A paper on spectral evolution has been submitted to ApJ; a paper discussing the jet kinematic evolution is in preparation. The long-term kinematic changes are also being modeled using the two-fluid scenario; the corresponding paper should be submitted in about 6 months.

The work is being done in collaboration with J.A. Zensus (MPIfR), J. Roland (IAP, Paris), S. Unwin (JPL) & A.Wehrle (IPAC, JPL).

High-frequency VLBI.

The first epoch (1993.25) of an 86-GHz VLBI survey of 26 AGN has been processed: images and model fits of the sources have been made, allowing to measure the brightness temperature and compactness of the observed VLBI cores, as well as to study the most compact structures of the jets. A summary paper is in preparation, and will be submitted within 1-2 months. Several most prominent objects will be discussed later in separate papers. The collaboration on this subject includes D. Graham, T. Krichbaum, A. Witzel, J.A. Zensus (MPIfR), and mm-VLBI teams at the participating observatories.

Mapping the hot-spot emission in double-lobed quasars.

A study aimed at detection and investigating the compact, VLBI-scale emission in the hot-spots of several prominent double-lobed quasars. Current status: VLBA observations - data reduction stage; EVN - observations in February '97 had failed; will be re-observed. The work is being done in collaboration with R. Porcas (MPIfR), M. Swain, & R. Ojha (Cornell University).

Maser emission from star-forming regions.

A search for proper motions of HO maser spots in star-forming regions, and further study of the correlation between spectral and structural variability of HO masers. MERLIN observations - data reduction stage. Current status: EVN - 3 epochs have been observed; correlation is planned to be done in July 1998. The work is done in collaboration with L. Moscadelli (MPIfR), V.Samodourov, I. Berulis, E. Lekht (Moscow), M. Felli (Arcetri), R. Cohen, & A. Richards (NRAL, UK)