Physical Properties of Jets in AGN
aa r X i v : . [ a s t r o - ph . C O ] O c t October 31, 2018 7:57 WSPC/INSTRUCTION FILE Homan˙1
International Journal of Modern Physics: Conference Seriesc (cid:13)
World Scientific Publishing Company
Physical Properties of Jets in AGN
Daniel C. Homan
Department of Physics and Astronomy, Denison UniversityGranville, OH 43023, USA [email protected]
Received Day Month YearRevised Day Month YearI review constraints on the physical properties of AGN jets revealed through Very LongBaseline Interferometry (VLBI) studies of the structure and time-evolution of parsec-scale jets, including recent results from the MOJAVE program. In particular I focus onconstraints available from very long time baseline studies which probe a wide range ofjet behavior over many outbursts. Kinematic studies of propagating jet features find anapparent speed distribution that peaks around 10 c for blazars, with speeds up to 50 c observed. These observed speeds require Lorentz factors at least as large, implying thatparsec-scale Lorentz factors up to 10 −
20 are common for blazars with a tail up to ∼ Keywords : Keyword1; keyword2; keyword3.PACS numbers: 11.25.Hf, 123.1K
1. Introduction
A key goal of Very Long Baseline Interferometry (VLBI) studies of AGN jets is to ad-dress long standing questions about jet formation, collimation, and acceleration1–4.These studies also seek to understand the distribution of intrinsic jet power andspeeds5 , , ctober 31, 2018 7:57 WSPC/INSTRUCTION FILE Homan˙1 Daniel C. Homan β obs = β sin θ − β cos θ (1)where β is the intrinsic speed of the moving feature and θ in the angle its motionmakes with our line of sight. For a given β , this motion is a maximum when β =cos θ , in which case β obs = β Γ where Γ = 1 / p − β is the Lorentz factor of themoving feature.An important question is the extent to which any single jet feature or ‘compo-nent’ is characteristic of the jet flow. It is now commonly believed that most jetfeatures represent propagating oblique or transverse shocks in the flow12–15. Con-sistent with this picture, a wide range of behavior is seen among components withina single jet, including stationary or quasi-stationary features16–21, although it isimportant to note that Ref. 21 found the distribution of apparent speeds withinindividual jets to be significantly narrower than between different jets, suggestingthat individual jets do have a characteristic flow speed. When multiple speeds areseen in a single jet, slower speeds may represent trailing shocks14, and thereforethe best estimate of the flow speed may be the fastest component observed21.The National Radio Astronomy Observatory’s a Very Long Baseline Array(VLBA) has been in continuous operation since 1994, allowing regular monitor-ing of large samples of AGN radio jets, with some individual jets having regularmonitoring observations for 16 years and counting. The large sample sizes and verylong time baselines allowed by the continuous operation of the VLBA has made itpossible to study AGN jets in entirely new ways to address these questions. In thefollowing sections I will discuss the impact on our understanding of (1) the distri-bution of apparent jet speeds and intrinsic Lorentz factors, (2) the acceleration andcollimation of jets, and (3) the morphology and opening angles of parsec-scale jets.
2. Long Time Baseline VLBI
As described above, the VLBA has made it possible to study both large sam-ples of AGN jets and to continuously monitor jets over long time intervals. Thelongest running example of this kind of program is the 2cm Survey/MOJAVE pro-gram. MOJAVE stands for Monitoring Of Jets from Active galactic nuclei withVLBA Experiments22–24, and it is a continuation of 2cm Survey which started in199425 , , ,
27. The MOJAVE program currently images the parsec scale struc-ture and polarization of more than 300 AGN jets at 15 GHz ( λ a The National Radio Astronomy Observatory is a facility of the National Science Foundationoperated under cooperative agreement by Associated Universities, Inc. ctober 31, 2018 7:57 WSPC/INSTRUCTION FILE Homan˙1
Physical Properties of Jets in AGN (22 and 43 GHz) with more closely spaced epochs28 , ,
11. While it is difficult totrack individual jet features over long periods of time at these higher frequencies,several of the AGN in their program also have data spanning more than 15 years.At lower frequencies (2 and 8 GHz) the Radio Reference Frame Image Database(RRFID) has images spanning a similar period of time29; however, only a smallwindow from 1994-1998 has been analyzed for kinematics19.Figure 1 is an example of the kind of the kinematic data that can be obtainedby observing a parsec scale jet over long periods of time. The TeV Blazar 1222+216has 15 GHz monitoring observations going back to 1996 as part of the BrandeisUniversity monitoring project16 ,
30 and has been observed as part of the 2cm Sur-vey/MOJAVE program since 1999. The MOJAVE program identifies several movingcomponents that can be tracked throughout much of this period21, and the motionof component “5” is illustrated in the figure. It has an apparent motion nearly 17 c and makes a distinct bend in its trajectory to the East, magnified by projection toappear to be a 40 ◦ change in the plane of the sky31. Distribution of Apparent Speeds
By studying the apparent speed distribution of a large, flux-density limited sampleof AGN jets we probe the underlying Lorentz factor distribution of the parentpopulation5 ,
6. This assumes that the apparent speeds of moving jet features aregood tracers of the underlying flow velocity, and as discussed in the introduction,the range of apparent speeds seen in individual objects suggests that we shoulduse only the largest apparent speed observed in a given jet. Thus it is necessary toobserve jets long enough to sample multiple components to increase our confidencethat we have seen a component characteristic of the flow speed. In their analysis ofthe 135 source MOJAVE-I complete sample, Ref. 21 obtained motions for 127 AGNjets and found that the distribution of the fastest component in each jet peakedaround 10 c with a tail extending up to 50 c . Analysis of the effects of beamingon flux-density limited samples show that for a large sample, like the MOJAVE-Isample, the fastest observed speed in the sample is characteristic of the maximumLorentz factor in the parent population6; thus, Ref. 21 concludes that the intrinsicLorentz factor distribution is likely a power-law distribution with a tail extending upto Γ ≃
50. It is interesting to note that a jet beamed directly at us has a Dopplerfactor δ = (1 + β )Γ, so Doppler factors of up to ∼ ∼ × higherresolution, suggesting that the much larger MOJAVE sample is indeed sensitive tothe fastest jet speeds emerging from the core region, despite the resolution difference.Ref. 18 also performed an additional analysis of the apparent fading times of the jetcomponents they followed in an attempt to obtain an independent estimate of theDoppler factor for individual components. The combination of Doppler factor andapparent speed for individual components allowed a direct estimate of the Lorentzctober 31, 2018 7:57 WSPC/INSTRUCTION FILE Homan˙1 Daniel C. Homan
Fig. 1. Plot of 12 years of MOJAVE position data for jet component “5” in the TeV Blazar1222+216 at z = 0 . c ctober 31, 2018 7:57 WSPC/INSTRUCTION FILE Homan˙1 Physical Properties of Jets in AGN factors for those components which fell in the range Γ = 5 − ≃ c . At afactor of 2-3 times lower spatial resolution, these studies sample jet emission furtherfrom the core region. So the speed difference may be due to jets already startingto slow down at these length scales, or perhaps the jet components that survive tothese distances are systematically different than those closer to the core. Acceleration and Collimation
Some models of jet formation and collimation have jets being fully accelerated andcollimated very near the super-massive black hole/accretion disk system on lengthscales much smaller than those probed by VLBI3 while other models extend thisprocess of much larger length scales2 where VLBI observations may be able to seeacceleration in action. It is also unknown where jets begin to slow down before theyreach kiloparsec scales. Very long time baseline VLBI studies have the opportunityto address these issues by tracking individual jet components over a very largenumber of epochs. We can study acceleration by looking for changes in the apparentvelocity vector ~β obs as described in Ref. 32: dβ k obs dt obs = ˙ β sin θ + β ˙ θ (cos θ − β )(1 − β cos θ ) (2) dβ ⊥ obs dt obs = β ˙ φ sin θ (1 − β cos θ ) , (3)which are the parallel and perpendicular components of the observed accelerationon the sky. ˙ β , ˙ θ , and ˙ φ are the intrinsic rates of change of the component speed,angle to the line of sight, and azimuthal angle respectively.If observed parallel accelerations are due entirely to changes in the component’sLorentz factor, Γ, there is a simple relation with the observed quantities32:˙ΓΓ = ˙ β k obs β obs β δ (4)where the Doppler factor δ = 1 / (Γ(1 − β cos θ )). If δ can be estimated, this approachallows one to measure the ratio ˙Γ / Γ which can be compared to physical models foracceleration or deceleration.A key question is the extent to which apparent accelerations in jet motions canbe assigned to changes in the Lorentz factor of the jet feature or changes in theangle to the line of sight. In their kinematic analysis of the 2cm Survey, Ref. 17ctober 31, 2018 7:57 WSPC/INSTRUCTION FILE Homan˙1 Daniel C. Homan found that 30% of the jet components they studied for non-radial motion wereindeed moving on a vector mis-aligned with the radio core, indicating that thesecomponents had changed their trajectory since being ejected from the base of theirjet. In general these “non-ballistic” jet features were misaligned toward the directionof the next structure in the jet, indicating that they were following pre-establishedchannels17. These results were confirmed by both the RRFID kinematic analysis19and the MOJAVE kinematic analysis, which extended the time baseline of the2cm Survey by several years21. When only the most well determined MOJAVEjet components were analyzed (those suitable for acceleration analysis) the fractionwith non-ballistic motion increased to nearly 50%32.The long time baseline and very large number of epochs in the MOJAVE pro-gram allowed Ref. 32 to extract the 203 best best studied jet features in their samplefor direct acceleration analysis. They measured the apparent parallel and perpendic-ular accelerations. Parallel accelerations are along the component motion, indicatingchanges in apparent speed, and perpendicular accelerations indicate changes in di-rection. By studying the ratio of these two quantities, they concluded that intrinsicchanges in the Lorentz factor of jet components were common32. They observed atendency for jet components with increasing apparent speed to be closer to the baseof their jets than components with decreasing apparent speed, suggesting that thejet flow may increase in speed near the base of the jet and decrease further out32,although this assumes the observed pattern changes are reflective of the underlyingflow. Ref. 18 also observed a tendency for positive acceleration of apparent speedsnear the base of jets in their sample of 15 blazars at 43 GHz. While the authorsinterpreted these results as evidence that the jets were bending away from the lineof sight (and closer to the optimum angle for superluminal motion)18, the MO-JAVE program results described above suggest that these changes may be betterexplained (on average) by increases in the Lorentz factors of those components.
Jet Opening Angles and Morphology
Ref 39 used the most recent MOJAVE epochs available at the time to study thecorrelation between apparent jet opening angle and Gamma-ray brightness of AGNjets. They found that Gamma-ray blazars had significantly larger apparent openingangles than non-detected AGN, indicating that these Gamma-ray bright AGN aremore closely aligned with the line of sight and therefore more highly beamed39.They also combined their apparent opening angle results with Doppler factor mea-surements from Ref. 40 and apparent speeds from the MOJAVE program21 toextract intrinsic opening angles, finding average jet opening angles on parsec scalesof 1 . ± . . ± . Physical Properties of Jets in AGN of ejections angles, the structure at any single epoch may only show those areasof the jet that have been recently illuminated by a passing component. This cangive the false impression of bent or twisted jet trajectories when the componentmotion is actually largely ballistic. An example is shown in figure 2, where a singleepoch image of the quasar 1308+326 is compared to a ’stack’ of 58 epochs fromthe MOJAVE program24. The single epoch image gives the impression of a sharplybent trajectory, whereas the reality is that the features are moving outward b in abroad cone on the sky which is revealed in the stacked image. Images of this kindsuggest a possible new way to study jet opening angles and collimation throughtime-averaged morphology over the course of many outbursts.
3. Conclusions
Very Long Baseline Interferometry studies of parsec-scale jets with structural andpolarization information gathered over very long time baselines are producing inter-esting new results. By observing multiple jet features over long periods of time, weare able to obtain better estimates of maximum apparent jet speeds in individual jetsand use these when studying the Lorentz factor distribution of AGN jets as a whole.Lorentz factors up to 10-20 appear to be relatively common in blazar jets with a tailextending up to a maximum Lorentz factor of ∼
50 for the blazar population18 , b Although note that component 5 in figure 2 is non-ballistic, indicating that it has changed itstrajectory since ejection to be about 8 degrees to the south, relative to a ballistic trajectory fromthe base of the jet. ctober 31, 2018 7:57 WSPC/INSTRUCTION FILE Homan˙1 Daniel C. Homan
Fig. 2. 15 GHz VLBA Images of the quasar 1308+326 at z = 0 . ctober 31, 2018 7:57 WSPC/INSTRUCTION FILE Homan˙1 Physical Properties of Jets in AGN Acknowledgments
I would like to thank all the members of the MOJAVE program including MattLister, Yuri Kovalev, Ken Kellermann, Hugh Aller, Margo Aller, Tigran Arshakian,Andrei Lobanov, Tuomas Savolainen, Anton Zensus, Eduardo Ros, Matthias Kadler,Neil Gehrels, Julie McEnery, Marshall Cohen, Alexander Pushkarev, and TalvikkiHovatta. The MOJAVE program is funded at Purdue University through NationalScience Foundation grant AST-0807860 and NASA-Fermi grant NNX08AV67G. D.Homan was funded by National Science Foundation grant AST-0707693.
References
1. Meier, D. L., Koide, S., & Uchida, Y., Science, , 84 (2001).2. Vlahakis, N., K¨onigl, A.,
The Astrophysical Journal , , 656 (2004).3. Sikora, M., Begelman, M. C., Madejski, G. M., & Lasota, J.-P., The AstrophysicalJournal , , 72 (2005).4. Marscher, A. P., Jorstad, S. G., D’Arcangelo, F. D., et al., Nature , , 966 (2008).5. Vermeulen, R. C., & Cohen, M. H., The Astrophysical Journal , , 467 (1994).6. Lister, M. L. & Marscher, A. P., The Astrophysical Journal , , 572 (1997).7. Cohen, M. H., Lister, M. L., Homan, D. C., Kadler, M., Kellermann, K. I., Kovalev,Y. Y., & Vermeulen, R. C., The Astrophysical Journal , , 232 (2007).8. Kovalev, Y. Y., et al., The Astrophysical Journal, Letters , , L17 (2009).9. Lister, M. L., Homan, D. C., Kadler, M., Kellermann, K. I., Kovalev, Y. Y., Ros, E.,Savolainen, T., & Zensus, J. A., The Astrophysical Journal, Letters , , L22 (2009).10. Piner, B. G., Pant, N., & Edwards, P. G., The Astrophysical Journal , , 1150 (2010).11. Marscher, A. P., Jorstad, S. G., Larionov, V. M., et al., The Astrophysical Journal,Letters , , L126 (2010).12. Marscher, A. P., & Gear, W. K., The Astrophysical Journal , 298, 114 (1985).13. Hughes, P. A., Aller, H. D., & Aller, M. F.,
The Astrophysical Journal , , 54 (1989).14. G´omez, J. 2005, Real versus Simulated Relativistic Jets in Future Directions in HighResolution Astronomy eds. J. Romney and M. Reid, ASP Conference ProceedingsVol. , 1315. Hughes, P. A., Aller, M. F., & Aller, H. D.,
The Astrophysical Journal , , 81 (2011).16. Homan, D. C., Ojha, R., Wardle, J. F. C., Roberts, D. H., Aller, M. F., Aller, H. D.,& Hughes, P. A., The Astrophysical Journal , , 840 (2001).17. Kellermann et al., The Astrophysical Journal , , 539 (2004).18. Jorstad, S. G., et al., The Astronomical Journal , , 1418 (2005).19. Piner, B. G., Mahmud, M., Fey, A. L., & Gospodinova, K., The Astronomical Journal , , 2357 (2007).20. Britzen, S., et al., Astronomy and Astrophysics , , 119 (2008).21. Lister, M. L., Cohen, M. H., Homan, D. C., et al., The Astronomical Journal , ,1874 (2009).22. Lister, M. L., & Homan, D. C., The Astronomical Journal , , 1389 (2005).23. Homan, D. C., & Lister, M. L., The Astronomical Journal , , 1262 (2006).24. Lister, M. L., et al., The Astronomical Journal , , 3718 (2009).25. Kellermann, K. I., Vermeulen, R. C., Zensus, J. A., & Cohen, M. H., The AstronomicalJournal , , 1295 (1998).26. Zensus, A., Ros, E., Kellermann, K. I., Cohen, M. H., and Vermeulen, R. C., TheAstronomical Journal , , 662 (2003).27. Kovalev, Y. Y., et al., The Astronomical Journal , , 2473 (2005). ctober 31, 2018 7:57 WSPC/INSTRUCTION FILE Homan˙1 Daniel C. Homan
28. Jorstad, S. G., Marscher, A. P., Mattox, J. R., Aller, M. F., Aller, H. D., Wehrle,A. E., & Bloom, S. D.,
The Astrophysical Journal , , 738 (2001).29. Fey, A. L., Clegg, A. W., & Fomalont, E. B., The Astrophysical Journal, Supplement , , 299 (1996).30. Ojha, R., Homan, D. C., Roberts, D. H., et al., The Astrophysical Journal, Supplement , , 187 (2004).31. Lister, M. L. et al. in preparation.32. Homan, D. C., Kadler, M., Kellermann, K. I., et al., The Astrophysical Journal , ,1253 (2009).33. Abraham, Z., & Carrara, E. A., The Astrophysical Journal , , 172 (1998).34. Wehrle, A. E., Piner, B. G., Unwin, S. C., Zook, A. C., Xu, W., Marscher, A. P.,Ter¨asranta, H., and Valtaoja, E., The Astrophysical Journal, Supplement , , 297(2001)35. Jorstad, S. G., Marscher, A. P., Lister, M. L., Stirling, A. M., Cawthorne, T. V.,G´omez, J.-L., & Gear, W. K., The Astronomical Journal , , 3115 (2004).36. Caproni, A., & Abraham, Z., The Astrophysical Journal , , 625 (2004).37. Lobanov, A. P., & Roland, J., Astronomy and Astrophysics , , 831 (2005).38. Stirling, A. M., et al., Monthly Notices of the Royal Astronomical Society , , 405(2003).39. Pushkarev, A. B., Kovalev, Y. Y., Lister, M. L., & Savolainen, T., Astronomy andAstrophysics , , L33 (2009).40. Hovatta, T., Valtaoja, E., Tornikoski, M., L¨ahteenm¨aki, A., Astronomy and Astro-physics ,494