Last update: February 13, 2008.
We study the energetic jets emanating from radio-loud quasars (in particular, from the subcategory known as blazars) using the radio-astronomical 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 radio telescopes. This technique can produce images with about 100 times the angular resolution of those produced by the Hubble Space Telescope. Our work has mainly used the National Radio Astronomy Observatory's Very Long Baseline Array (VLBA) to make these observations.
About sixteen 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 since 1999. Below we divide the discussion of our radio observations into three categories: the two radio-brightest TeV blazars Markarian 421 and Markarian 501; the radio-fainter but still relatively well-studied sources H 1426+428, 1ES 1959+650, and PKS 2155-304; and the recently detected sources.
We have begun VLBA monitoring of recently detected TeV sources. First epoch VLBA images of five of these sources (1ES 1101-232, 1ES 1133+704 (Markarian 180), 1ES 1218+304, PG 1553+113, and H 2356-309) are shown below.
| 1ES 1101-232 | 1ES 1133+704 | 1ES 1218+304 |
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| PG 1553+113 | H 2356-309 |
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We have completed the reduction of VLBA observations of H 1426+428, 1ES 1959+650, and PKS 2155-304 obtained over the years 2001-2004. These observations have provided the first VLBA polarization image of PKS 2155-304, the first apparent speed measurement for H 1426+428, and updated apparent speed measurements for 1ES 1959+650 and PKS 2155-304. Below we show sample VLBA images of these three sources. The image of 2155-304 is a polarization image, where the sticks indicate the magnitude of the polarized flux and the direction of the electric vector position angle, and the color shows fractional polarization. The paper on these observations is in press at The Astrophysical Journal.
| H 1426+428 | 1ES 1959+650 | PKS 2155-304 |
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Read the preprint of this Astrophysical Journal paper. This paper includes an updated table showing our latest values for apparent jet speeds for all TeV blazars that we have monitored.
In 2005, we published a paper containing an analysis of a 28-epoch VLBI dataset on Markarian 421, covering the years 1994-2002. The final four epochs were 22 GHz polarization observations taken throughout the year following the source's gamma-ray high state in early 2001. The figure below shows these four VLBA polarimetry images from that paper. The sticks indicate the magnitude of the polarized flux and the direction of the electric vector position angle, and the color shows fractional polarization.
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Read the paper on these observations.
We have also completed a series of 86 GHz VLBA observations of Markarian 421 and 501 that should show the jets of these sources with much better angular resolution than previous observations, probing the jets closer to the gamma-ray emitting regions. Reduction of this 86 GHz monitoring data is in progress.
We are currently engaged in a collaboration with the U.S. Naval Observatory to analyze the VLBA images at 8 and 2 GHz contained in the U.S. Naval Observatory's Radio Reference Frame Image Database. The database currently contains about 5,000 images of about 600 sources from 1994 through 2007. Because of the dense time sampling of the observations, this database can provide detailed measurements of jet kinematics.
We have now completed an analysis of the 8 GHz images for all sources observed at three or more epochs during the first five years of the database (1994-1998). This analysis comprised about 1,000 images of about 100 sources, for an average of about 10 epochs per source. A paper on the jet kinematics obtained from this was published in the Astronomical Journal, June 2007 issue. The complete model-fit table (comprising 2,579 Gaussian components) is available here in machine-readable form.
Download the model-fit table from Paper I.
Read the Astronomical Journal paper.
We have also completed an analysis of the misalignment angle distribution (difference between parsec and kiloparsec scale jet position angles) for all sources in the database for which VLA images were available in the literature. This analysis comprises single-epoch 8 and 2 GHz images of 223 sources.
Read the poster on the misalignment angle distribution from the Challenges of Relativistic Jets conference in Cracow, Poland, June 2006.
We are currently working as part of a collaboration that is imaging and model-fitting another five years of VLBA data from this database (1999-2003), to extend our study of jet kinematics over a ten-year time baseline. When complete, this analysis will comprise approximately 3,000 images of 75 sources, for about 40 epochs of observation per source. This will be the largest multi-epoch VLBI survey (in terms of total number of images and number of epochs per source) ever done. Collaborators include Alan Fey (USNO), Yuri Kovalev (MPIfR), Alexander Pushkarev (Pulkovo Observatory), Leonid Petrov (NASA/GSFC), and Patrick Charlot (Observatoire de Bordeaux). Some of the latest images from this collaboration can be seen here.
Links to other large VLBI surveys: the MOJAVE survey, the NRAO 2cm survey, the VIPS survey, the VCS survey, the Caltech-Jodrell Bank Survey.
For the past several years, we have been monitoring the fastest relativistic jets (those with apparent speeds exceeding 25c), in order to study the evolution of these fastest components. A monitoring program on three such sources (0235+164, 0827+243, 1406-076) was completed in 2003, and kinematic results were published in 2006. A paper on the polarimetry results is in preparation.
Read the paper on the kinematic results from these fast superluminal sources.
Below are sample VLBA images of these three blazars from that monitoring program. The 'sticks' show that magnitude of the polarized flux and the electric vector position angle.
| 0235+164 | 0827+243 | 1406-076 |
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You can also view an image mosaic of 0827+243, which shows the fast apparent motion in the feature on the left, relative to the stationary core at the right. The images have been rotated so that the jet is horizontal.
| 0827+243 Image Mosaic |
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Monitoring of five other high-speed jets, plus continued monitoring of 0827+243, was recently completed on the VLBA.
The Whittier College Physics Department is a collaborator on the NASA Space Interferometry Mission Key Science Project Binary Black Holes, Accretion Disks and Relativistic Jets: Photocenters of Nearby AGN and Quasars. The Space Interferometry Mission is an orbiting optical interferometer that will be launched sometime in the next decade. This Key Science Project will use SIM to observe the optical emission from quasars on the smallest angular scales ever achieved.
Read the New Astronomy Reviews paper on this project.
Read the PASA paper on this project.
Whittier undergraduates Suzanne Fodor, Deyan Tabakov, Mehreen Mahmud, Jeremy Milne, Kalina Gospodinova, Ghalib Bello, Dipesh Bhattarai, Niraj Pant, and Corey Nichols 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 Nos. 0305475 and 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).