Last update: January 14, 2013.
We currently study the energetic jets emanating from radio-loud quasars (in particular, from the subcategory known as blazars) using the radio-astronomy technique of Very Long Baseline Interferometry (VLBI). This technique combines data recorded by widely separated radio telescopes to obtain the same angular resolution as a single radio telescope with a size equal to the maximum separation between individual telescopes. This technique produces the highest angular resolution images in astronomy, allowing us to see closer to the supermassive black hole at the quasar's center. Our work has mainly used the National Radio Astronomy Observatory's Very Long Baseline Array (VLBA) to make these observations.
About forty high-frequency peaked blazars have been observed to emit very high-energy gamma-rays (> 1 TeV) by ground-based TeV gamma-ray telescopes. We have been studying the parsec-scale structures of these TeV blazars through VLBA monitoring for a number of years. Below we divide our VLBA observations into three categories: 1) observations of the first five TeV blazars to be discovered (which are brighter in the radio, and for which we have a longer time baseline of monitoring) , 2) observations of TeV blazars discovered circa 2006 (which tend to be fainter in the radio, and for which we have a shorter time baseline of monitoring), and 3) the most recent TeV blazar discoveries, which are often quite faint in the radio, and for which we have a single epoch of VLBA data. Many of these TeV blazars have also been detected by the Fermi Gamma-Ray Space Telescope.
We have observed these five blazars with the VLBA at frequencies ranging up to 43 GHz (and up to 86 GHz for Markarian 501), making some of the highest angular resolution images of these five blazars available, and probing the jets close to the gamma-ray emitting regions. The 43 GHz images show a clear limb-brightened structure in the two brightest sources (Markarian 421 and Markarian 501), suggesting that the jet is divided into a fast spine and a slower outer layer. Sample images from our observations of these five sources from VLBA experiments BP143 and BB281 are shown below (click for full-scale images).
| Markarian 421 43 GHz | Markarian 501 43 GHz | 1959+650 43 GHz | 2155-304 43 GHz | 2344+514 43 GHz |
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Read the 2010 Astrophysical Journal paper about the 43 GHz VLBA observations of these five blazars.
Read the 2009 Astrophysical Journal Letter about earlier observations showing limb-brightening in Markarian 501.
Read the 2008 Astrophysical Journal paper on earlier observations of H 1426+428, 1ES 1959+650, and PKS 2155-304.
We are also engaged in VLBA monitoring of more recently discovered TeV blazars, and have completed a multi-epoch series of observations of the five sources 1ES 1101-232, Markarian 180, 1ES 1218+304, PG 1553+113, and H 2356-309 as VLBA experiment BP146. Sample VLBA images of each of these sources from this series of observations are shown below (click for full-scale images).
| 1ES 1101-232 | Markarian 180 | 1ES 1218+304 | PG 1553+113 | H 2356-309 |
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Read the paper from the 2012 Fermi & Jansky conference proceedings on the multi-epoch observations of the five blazars shown above.
| RGB J0152+017 | 1ES 0229+200 | RBS 0413 | 1ES 0347-121 | 1ES 0414+009 |
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| 1ES 0502+675 | PKS 0548-322 | RGB J0710+591 | 1ES 0806+524 | 1ES 1011+496 |
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This is a large project designed to study the jet structure and kinematics of sources observed in the RDV series of astrometric and geodetic VLBA experiments. It is being done in collaboration with Alan Fey (U.S. Naval Observatory), Patrick Charlot (Observatoire de Bordeaux), Alexander Pushkarev (MPIfR), Yuri Kovalev (Astro Space Center), and others.
The RDV series of experiments observes about 100 sources each at 8 and 2 GHz every two months for the purposes of astrometry and geodesy. Because of the dense time sampling of the observations, this database can also provide detailed measurements of jet kinematics. Many of the VLBI images from these experiments are available at The Radio Reference Frame Image Database. Others are available at The Bordeaux VLBI Image Database and at Astrogeo.org.
Our most recent project involved studying the kinematics of all sources observed at 20 or more epochs over the first 50 experiments in the database (from 1994-2003), for the purpose of measuring jet accelerations. It included the production and analysis of 2753 global VLBI images of 68 sources. Results from this project were published in 2012.
Read the 2012 Astrophysical Journal paper on this project
Download the model-fit table (Table 3) from this paper.
Download the accelerations table (Table 6) from this paper.
A past project investigated the kinematics of all sources observed at 3 or more epochs over the first five years of the database (from 1994-1998), and included the analysis of 966 images of 87 sources. The results of this phase of the project were published in 2007.
Read the 2007 Astronomical Journal paper on that project.
Links to other large VLBI surveys: the MOJAVE survey, the NRAO 2cm survey, the VIPS survey, the VCS survey, the Caltech-Jodrell Bank Survey.
Whittier undergraduates Suzanne Fodor, Deyan Tabakov, Mehreen Mahmud, Jeremy Milne, Kalina Gospodinova, Ghalib Bello, Dipesh Bhattarai, Niraj Pant, Corey Nichols, Christopher Marvin, Joshua Arenson, Vivian Tiet, Faraz Zaerpoor, and Jon-Paul Cook have all participated in the Quasar Research Program at Whittier College.
This material is based upon work supported by the National Science Foundation under Grant No 0707523. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation (NSF).