We present current results from our ongoing project to study the parsec-scale
relativistic jet kinematics of sources in the U.S. Naval Observatory's Radio
Reference Frame Image Database (RRFID). The RRFID consists of snapshot
observations using the VLBA plus up to 9 additional antennas at 8 and 2 GHz.
The Image Database currently contains about 3000 images of 450 sources from
1994 to 2004, with some sources having images at 20 epochs or more. We have now
completed analysis of the 8 GHz images for all sources observed at 3 or more
epochs from 1994 to 1998. The completed analysis comprises 966 images of 87
sources, or an average of 11 epochs per source. Apparent jet speeds have been
measured for these sources, and the resulting speed distribution has been
compared with results obtained by other large VLBI surveys. The measured
apparent speed distribution agrees with those found by the 2 cm survey and
Caltech-Jodrell Bank (CJ) survey; however, when a source-by-source comparison
is done with the 2 cm survey results, significant disagreement is found in the
apparent speed measurements for a number of sources. This disagreement can be
traced in most cases to either an insufficient time baseline for the current
RRFID results, or to apparent component mis-identification in the 2 cm survey
results caused by insufficient time sampling. These results emphasize the need
for long time baselines and dense time sampling for multi-epoch monitoring of
relativistic jets.Comment: 4 pages, To be published in the Proceedings of the 7th European VLBI
  Network Symposiu

Fey, A. L.

Mahmud, M.

Piner, B. G.

English

arXiv

We present current results from our ongoing project to study the parsec-scale relativistic jet kinematics of sources in the U.S. Naval Observatory\u27s Radio Reference Frame Image Database (RRFID). The RRFID consists of snapshot observations using the VLBA plus up to 9 additional antennas at 8 and 2 GHz, and is intended to allow monitoring of these sources for variability or structural changes so they can be evaluated for continued suitability as radio reference frame objects. The Image Database currently contains about 3000 images of 450 sources from 1994 to 2004, with some sources having images at 20 epochs or more.
We have now completed analysis of the 8 GHz images for all sources observed at 3 or more epochs from 1994 to 1998. The completed analysis comprises 966 images of 87 sources, or an average of 11 epochs per source. Apparent jet speeds have been measured for these sources, and the resulting speed distribution has been compared with results obtained by other large VLBI surveys. The measured apparent speed distribution agrees with those found by the 2 cm survey and Caltech-Jodrell Bank (CJ) survey; however, when a source-by-source comparison is done with the 2 cm survey results, significant disagreement is found in the apparent speed measurements for a number of sources. This disagreement can be traced in most cases to either an insufficient time baseline for the current RRFID results, or to apparent component mis-identification in the 2 cm survey results caused by insufficient time sampling. These results emphasize the need for long time baselines and dense time sampling for multi-epoch monitoring of relativistic jets

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Whittier College Poet Commons Physics Faculty Publications & Research 2004 Current Results from the RRFID Kinematic Survey: Apparent Speeds from the First Five Years of Data B. G. Piner Whittier College, gpiner@whittier.edu A. L. Fey M. Mahmud Follow this and additional works at: https://poetcommons.whittier.edu/phys Recommended Citation Piner, B. G., Fey, A. L., & Mahmud, M. (2004). Current Results from the RRFID Kinematic Survey: Apparent Speeds from the First Five Years of Data. Proceedings of the 7th Symposium of the European VLBI Network on New Developments in VLBI Science and Technology, 99. Retrieved from https://poetcommons.whittier.edu/phys/54 This Conference Paper is brought to you for free and open access by the Faculty Publications & Research at Poet Commons. It has been accepted for inclusion in Physics by an authorized administrator of Poet Commons. For more information, please contact library@whittier.edu. Proceedings of the 7th European VLBI Network SymposiumBachiller, R. Colomer, F., Desmurs, J.F., de Vicente, P. (eds.)October 12th-15th 2004, Toledo, SpainCurrent Results from the RRFID Kinematic Survey: ApparentSpeeds from the First Five Years of DataB. G. Piner1, A. L. Fey2, and M. Mahmud11 Department of Physics and Astronomy, Whittier College, 13406 E. Philadelphia Street, Whittier, CA 90608, U.S.A.2 U.S. Naval Observatory, 3450 Massachusetts Ave., Washington D.C. 20392, U.S.A.Abstract. We present current results from our ongoing project to study the parsec-scale relativistic jet kinematics of sources inthe U.S. Naval Observatory’s Radio Reference Frame Image Database (RRFID). The RRFID consists of snapshot observationsusing the VLBA plus up to 9 additional antennas at 8 and 2 GHz, and is intended to allow monitoring of these sources forvariability or structural changes so they can be evaluated for continued suitability as radio reference frame objects. The ImageDatabase currently contains about 3000 images of 450 sources from 1994 to 2004, with some sources having images at 20 epochsor more.We have now completed analysis of the 8 GHz images for all sources observed at 3 or more epochs from 1994 to 1998. Thecompleted analysis comprises 966 images of 87 sources, or an average of 11 epochs per source. Apparent jet speeds have beenmeasured for these sources, and the resulting speed distribution has been compared with results obtained by other large VLBIsurveys. The measured apparent speed distribution agrees with those found by the 2 cm survey and Caltech-Jodrell Bank (CJ)survey; however, when a source-by-source comparison is done with the 2 cm survey results, significant disagreement is found inthe apparent speed measurements for a number of sources. This disagreement can be traced in most cases to either an insufficienttime baseline for the current RRFID results, or to apparent component mis-identification in the 2 cm survey results caused byinsufficient time sampling. These results emphasize the need for long time baselines and dense time sampling for multi-epochmonitoring of relativistic jets.1. The Radio Reference Frame Image DatabaseThe U.S. Naval Observatory (USNO) maintains a multi-epoch database of VLBI images of objects that comprise theRadio Reference Frame. This Radio Reference Frame ImageDatabase (RRFID) consists of Very Long Baseline Array(VLBA) snapshot observations at 8 and 2 GHz (with the ad-dition of up to 9 geodetic antennas — including European an-tennas at Medicina, Ny Alesund, Onsala, and Wettzell), and isintended to allow monitoring of these sources for variability orstructural changes so they can be evaluated for continued suit-ability as radio reference frame objects. The Image Databasecurrently contains about 3000 images of 450 sources from1994 to 2004. (although the number of fully reduced epochsafter 1998 is currently sparse). The web-based interface to theRRFID is available at rorf.usno.navy.mil/RRFID/. The major-ity of objects in this database are radio-loud quasars or BL Lacobjects. A sample image of the well-known object BL Lac atepoch 1998 December 21 (the final epoch analyzed for this pa-per) is shown in Figure 1, to show the typical noise levels andbeam sizes of these snapshot images. The RRFID has recentlybegun adding high-frequency images at 24 and 43 GHz as well.Some of the RRFID epochs are described by Fey, Clegg, &Fomalont (1996), Fey and Charlot (1997), and Fey and Charlot(2000).2. The RRFID Kinematic SurveyLarge multi-epoch VLBI surveys containing many sources areimportant for characterizing the physics of the relativistic jets.Fig. 1. A sample image from the RRFID: BL Lac on 1998 Dec 21.Measurement of the apparent speed distribution can shed lighton the true distribution of jet Lorentz factors, and the maximumspeeds present in the distribution can reveal the maximum ki-netic energy that must be able to be supplied by the central en-100 B. G. Piner, A. L. Fey, and M. Mahmud: Current Results from the RRFID Kinematic Surveygine. The apparent speed distribution can also reveal whetherthe motions seen in VLBI images are true bulk motions of theplasma, or only a pattern speed superimposed on the underlyingjet. Large VLBI surveys should also yield interesting results instudies of jet bending. Some individual jets have been reportedto follow helical trajectories (e.g., Hong et al. 2004), possiblydue to precession of the jet nozzle induced by torques in binarysupermassive black hole systems. A large survey of hundredsof parsec-scale jets enables the question of helical jets to bestudied statistically.We are currently engaged in a project to extract the infor-mation on jet kinematics and dynamics that is contained in theRRFID. Our project consists of two major parts:1. A multi-epoch kinematic survey of the 8 GHz images of allsources that have been observed at 3 or more epochs, forthe purpose of measuring apparent jet velocities from themotions of VLBI components.2. An analysis of single-epoch images at 8 and 2 GHz for all450 sources for the purposes of obtaining those measure-ments that can be made from single-epoch images, such as:bending of the parsec-scale jet ridgeline, misalignment withkiloparsec-scale structure, apparent jet opening angles, andcore and component spectral indices.Some RRFID sources are observed quite frequently, the best-observed sources included in this paper have been observed at19 epochs through the end of 1998. We are currently workingon the multi-epoch portion of this project (the kinematic sur-vey), and have completed the analysis of all sources observedat 3 or more epochs from 1994 through 1998. This list com-prises 87 sources, with the average source being observed at 11epochs, so that a total of 966 images and model fits have beenanalyzed for this paper. Note that over 30 epochs that will even-tually be added to the RRFID have also been observed since theend of 1998, but most of this more recent data has yet to be re-duced.The current results are comparable in terms of the numberof images to the other two large VLBI surveys: the NRAO 2 cmsurvey (Kellermann et al. 2004), and the Caltech-Jodrell Bank(CJ) survey (Britzen et al. 2001; Vermeulen 1995). However (inparticular when compared to the CJ survey), we have a smallernumber of sources with a much higher number of epochs persource, and therefore much denser time sampling. It is also im-portant to note that the RRFID is not a flux-limited sample.3. Apparent SpeedsThe apparent speed measurements were made as follows. Theself-calibrated visibility data corresponding to the CLEAN im-ages on the RRFID web site were loaded into the DIFMAPsoftware package, and circular Gaussian components were fit tothe visibilities. Occasionally an elliptical Gaussian componentwas used to represent the core or an especially bright or well-resolved jet component. The distances of the Gaussian compo-nents from the presumed core were recorded and componentswere identified from epoch to epoch. Positional error bars wereassigned as a fraction of the beam size that decreased with in-creasing component surface brightness and the presence or ab-Fig. 2. Fits to component separations vs. time for 0552+398 and1308+326.sence of other confusing components. A linear least-squaresfit was then done to the separation from the core versus timeto yield a proper motion. The proper motion was converted toan apparent speed using the following cosmological parame-ters: Ωm = 0.27, ΩΛ = 0.73, and h = 0.71. Two samplefits are shown in Figure 2, for the two sources 0552+398 and1308+326. The component separations for these two sourcesshow a very small scatter about the linear fits (only a few per-cent of the beam width), demonstrating the ability to make pre-cise positional measurements with this snapshot data. The plotfor 0552+398 shows the common phenomenon of a stationarycomponent, presumably a stationary pattern on the underlyingrelativistic jet. The plot for 1308+326 shows one of our highestconfidence detections of a fast apparent speed; the measuredapparent speed is 18 c.The histogram of apparent speeds for all components withapparent speed errors < 10 c is shown in Figure 3 (140 compo-nents, some components had large errors associated with theirapparent speed measurement because they were not added tothe observing sessions until the last year of the data consid-B. G. Piner, A. L. Fey, and M. Mahmud: Current Results from the RRFID Kinematic Survey 101Fig. 3. Histogram of measured apparent speeds of jet components.ered in this paper). The histogram has a peak at slow apparentspeeds, and a tail extending out to 30 c. The mean apparentspeed for all 140 components is 4.5 c. The high-speed tail of thedistribution indicates that Lorentz factors up to ≈ 30 (and truespeeds up to 0.999 c) exist in some sources, and must be ableto be produced by theories for jet acceleration. If the quasars inFigure 3 are separated from the BL Lac objects, then a KS testdetects a difference in the apparent speeds of the two groups atlow significance (92%). The mean apparent speed of the quasarcomponents is 5.3 c, that of the BL Lac components is 2.3 c.A KS test comparing the histogram in Figure 3 to the equiv-alent histograms produced by the 2 cm survey and the CJ sur-vey shows with high significance that the 2 cm survey, the CJsurvey, and the RRFID survey are measuring the same speeddistribution (but since many sources overlap between these sur-veys, the KS test would only be expected to show a differenceif speeds were changing in a systematic way with observingfrequency, i.e. with distance from the core). However, a KS testdoes show a difference at the 99.95% confidence level with thehistogram of apparent speeds of EGRET blazars by Jorstad etal. (2001), as expected if their gamma-ray selected sample ismore highly beamed than our radio selected sample. It has beennoted by the 2 cm survey authors (Kellermann et al. 2004) thata speed distribution like that in Figure 3 cannot be explained bythe simplest model in which all sources have the same Lorentzfactor. Either a power law distribution of bulk Lorentz factors,or separate bulk and pattern velocities, needs to be invoked toexplain such a distribution (Kellermann et al. 2004).4. Detailed Comparison with 2 cm Survey ResultsOf the 87 sources included in this paper, 42 are also membersof the 2 cm survey. In the previous section, we showed that theoverall apparent speed distributions as measured by these twosurveys agree with each other with high significance. In thissection, we investigate whether this agreement also extends tomeasurements of individual components, rather than just theoverall distribution. From the 42 sources in common betweenFig. 4. Comparison of proper motions measured in the 2 cm surveyand RRFID survey for common components.this paper and the 2 cm survey, we identify 64 components thatcan (at least at a single epoch) be identified with a correspond-ing component from the other survey. The proper motions mea-sured for these 64 components from both the 2 cm survey andthe RRFID survey are compared in Figure 4. The Pearson cor-relation coefficient shows significant correlation between themeasured proper motions; however, this is due entirely to thefour points in the upper right of the figure, all of which are com-ponents of BL Lac. If the components of BL Lac are removedfrom the comparison, then there is no significant correlation.For many of the common components there is apparentlysignificant disagreement in the measured proper motion be-tween these two surveys, and this is what causes the scatter plotappearance of Figure 4. This is a cause for significant concernfor studies that rely on the robustness of VLBI model-fittingresults on specific sources. In order to determine the causesof the disagreements in the proper motion measurements, wehave made a detailed comparison of the proper motion resultsfrom these two surveys by over plotting the 2 cm survey com-ponent separations and linear fits on top of our results. Forapproximately half of the common components, the measuredproper motions from the two surveys agree within their statederrors, and for a number of these the fitted motions lie virtu-ally on top of each other. For the other half, we have madea component-by-component examination of the cause of thedifferences in the proper motion measurements. In the major-ity of cases where the two surveys have observations at nearlysimultaneous epochs, the fitted separations of model compo-nents between the two surveys agree, despite the differences inobserving frequency and fitting algorithm used. The disagree-ment in the proper motions in some cases can be traced to thedifferences in the identification of components from epoch toepoch, in others to an insufficient time baseline for the RRFIDobservations, which are currently reduced only through 1998.Examples of these two sources of disagreement are shownin Figure 5. Points from the RRFID are shown with error barsand with solid-line fits, points from the 2 cm survey are shown102 B. G. Piner, A. L. Fey, and M. Mahmud: Current Results from the RRFID Kinematic Surveywithout error bars and with dotted-line fits. In the top panelnote that the disagreement in the proper motion of the innercomponent is due to the extra 2 cm survey point in 2001; dis-agreements of this type should be resolved once the unreducedepochs in the RRFID are analyzed. The bottom panel showsa case where the discrepancy could be resolved by re-labelingthe components from the 2 cm survey to match our identifi-cations, i.e., a fewer number of slower components instead ofa greater number of faster components. In particular, note thatif one follows along the solid line representing the fit to thesecond component in the RRFID results (the diamonds), youencounter successively what has been identified as three fastercomponents in the 2 cm survey: represented by diamonds, tri-angles, and squares. In this case the dense time sampling of theRRFID results in 1997 and 1998 seems to preclude the fasterspeeds measured by the 2 cm survey. It seems that for apparentjet speeds derived from VLBI model fitting to be truly robustrequires both a long time baseline and dense time sampling.5. ConclusionsAlthough about 50 epochs have been observed for the RRFIDprogram over the last ten years, only about the first 20 epochsfrom 1994-1998 have been fully reduced. If the additional 30epochs obtained from 1999-2004 are eventually added to thosealready reduced for this paper, then the RRFID KinematicSurvey will be by far the largest survey of jets (in terms of num-ber of images) yet made, and will have the advantages of botha long time baseline (10 years), and dense time sampling (≈ 5epochs/year). This survey also has the obvious advantage thatthe VLBI observations comprising it have already been made,so it remains only to reduce the data.Meanwhile, work is continuing on the current data set, in-cluding searches for non-linear motions of jet components, andanalysis of parsec to kiloparsec misalignment angles.Acknowledgements. The National Radio Astronomy Observatory isa facility of the National Science Foundation operated under co-operative agreement by Associated Universities, Inc. This researchhas made use of the United States Naval Observatory (USNO)Radio Reference Frame Image Database (RRFID). We thank KenKellermann for providing model component data from the 2 cm sur-vey. This work was supported by the National Science Foundationunder Grant No. 0305475, and by a Cottrell College Science Awardfrom Research Corporation.ReferencesBritzen, S., et al. 2001 in Galaxies and their Constituents at theHighest Angular Resolutions, ed. R. T. Schilizzi, 106Fey, A. L., & Charlot, P. 1997, ApJS, 111, 95Fey, A. L., & Charlot, P. 2000, ApJS, 128, 17Fey, A. L., Clegg, A. W., & Fomalont, E. B. 1996, ApJS, 105, 299Hong, X. Y., et al. 2004, A&A, 417, 887Jorstad, S. G., Marscher, A. P., Mattox, J. R., Wehrle, A. E., Bloom,S. D., & Yurchenko, A. V. 2001, ApJS, 134, 181Kellermann, K. I., et al. 2004, ApJ, 609, 539Vermeulen, R. C. 1995, Proc. Natl. Acad. Sci., 92, 11385Fig. 5. Fits to component separations vs. time for 0202+149 and1606+106. The points with associated error bars and the solid-line fitsrepresent data from the RRFID survey. The points with no error barsand the dotted-line fits are the results of the 2 cm survey (Kellermannet al. 2004). Components are identified by asterisks, diamonds, tri-angles, squares, and circles from the outermost identified componentinward.

Current Results from the RRFID Kinematic Survey: Apparent Speeds from the First Five Years of Data

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