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The effects of frequency-dependent quasar variability on the celestial reference frame

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Abstract

We examine the relationship between source position stability and astrophysical properties of radio-loud quasars making up the International Celestial Reference Frame (ICRF2). Understanding this relationship is important for improving quasar selection and analysis strategies, and therefore reference frame stability. We construct flux density time series, known as light curves, for 95 of the most frequently observed ICRF2 quasars at both the 2.3 and 8.4 GHz geodetic very long baseline interferometry (VLBI) observing bands. Because the appearance of new quasar components corresponds to an increase in quasar flux density, these light curves alert us about potential changes in source structure before they appear in VLBI images. We test how source position stability depends on three astrophysical parameters: (1) flux density variability at X band; (2) time lag between flares in S and X bands; (3) spectral index root-mean-square (rms), defined as the variability in the ratio between S and X band flux densities. We find that the time lag between S and X band light curves provides a good indicator of position stability: sources with time lags \(<\)0.06 years are significantly more stable (\(>\)20 % improvement in weighted rms) than sources with larger time lags. A similar improvement is obtained by observing sources with low \((<\)0.12) spectral index variability. On the other hand, there is no strong dependence of source position stability on flux density variability in a single frequency band. These findings can be understood by interpreting the time lag between S and X band light curves as a measure of the size of the source structure. Monitoring of source flux density at multiple frequencies therefore appears to provide a useful probe of quasar structure on scales important to geodesy. The observed astrometric position of the brightest quasar component (the core) is known to depend on observing frequency. We show how multi-frequency flux density monitoring may allow the dependence on frequency of the relative core positions along the jet to be elucidated. Knowledge of the position–frequency relation has important implications for current and future geodetic VLBI programs, as well as the alignment between the radio and optical celestial reference frames.

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Notes

  1. This sub-resolution structure will be reflected in visibility amplitudes and phases of VLBI measurements, suggesting the presence of substructure on scales smaller than the synthesised beam.

  2. http://www.physics.purdue.edu/astro/mojave.

  3. http://lupus.gsfc.nasa.gov/sess/sessions/cumulative/source_perf/.

  4. http://rorf.usno.navy.mil/rrfid.shtml.

  5. For typical flat-spectrum quasar proper motion of 0.2 mas year\(^{-1}\) (Lister et al. 2009), VLBI imaging at X band will only resolve the new jet component 6 months after the beginning of the flare.

  6. We note that Monte Carlo simulations estimate that typical uncertainties on the derived time lags are \(0.05\) year, and thus the \(\tau <0.06\) year group is consistent with no measured time lag between S and X band light curves.

  7. The position referred to here is the astrometric position of the core as obtained from phase delays, for example via phase-referenced VLBI imaging. This is different to the geodetic source position derived from the group delay, which may or may not be affected by the core shift (see Sect. 5.3).

  8. Equation 7 can also be used to validate the use in Sect. 2.1 of 6.7 GHz data as a proxy for 8.4 GHz variability: for \(k_r=0.6\), the time lag between 6.7 and 8.4 GHz is \(<\)15 % of the time lag between 2.3 and 8.4 GHz; and this value is even smaller for larger \(k_r\).

References

  • Altamimi Z, Collilieux X, Legrand J, Garayt B, Boucher C (2007) ITRF2005: a new release of the International Terrestrial Reference Frame based on time series of station positions and Earth Orientation Parameters. J Geophys Res (Solid Earth) 112:9401

    Article  Google Scholar 

  • Blandford RD, Königl A (1979) Relativistic jets as compact radio sources. ApJ 232:34

    Article  Google Scholar 

  • Boehm J, Werl B, Schuh H (2006) Troposphere mapping functions for GPS and very long baseline interferometry from European Centre for Medium-Range Weather Forecasts operational analysis data. J Geophys Res (Solid Earth) 111:2406

    Article  Google Scholar 

  • Bourda G, Charlot P, Porcas RW, Garrington ST (2010) VLBI observations of optically-bright extragalactic radio sources for the alignment of the radio frame with the future Gaia frame. I. Source detection. A&A 520:A113

    Article  Google Scholar 

  • Bourda G, Collioud A, Charlot P, Porcas R, Garrington S (2011) VLBI observations of optically-bright extragalactic radio sources for the alignment of the radio frame with the future Gaia frame. II. Imaging candidate sources. A&A 526:A102

    Article  Google Scholar 

  • Carter SJB, Ellingsen SP, Macquart J-P, Lovell JEJ (2009) Annual cycles in the interstellar scintillation time-scales of PKSB1519-273 and PKSB1622-253. MNRAS 396:1222

    Article  Google Scholar 

  • Charlot P (1990) Radio-source structure in astrometric and geodetic very long baseline interferometry. AJ 99:1309

    Article  Google Scholar 

  • Charlot P, Boboltz DA, Fey AL, Fomalont EB, Geldzahler BJ, Gordon D, Jacobs CS, Lanyi GE, Ma C, Naudet CJ, Romney JD, Sovers OJ, Zhang LD (2010) The Celestial Reference Frame at 24 and 43 GHz. II. Imaging. AJ 139:1713

    Article  Google Scholar 

  • Edelson RA, Krolik JH (1988) The discrete correlation function—a new method for analyzing unevenly sampled variability data. ApJ 333:646

    Article  Google Scholar 

  • Fey AL, Charlot P (1997) VLBA observations of radio reference frame sources. II. Astrometric suitability based on observed structure. ApJ Suppl Ser 111:95

    Article  Google Scholar 

  • Fey AL, Charlot P (2000) VLBA observations of radio reference frame sources. III. Astrometric suitability of an additional 225 sources. ApJ Suppl. Ser 128:17

    Article  Google Scholar 

  • Fey AL, Clegg AW, Fomalont EB (1996) VLBA observations of radio reference frame sources. I. ApJ Suppl. Ser. 105:299

    Article  Google Scholar 

  • Hirotani K (2005) Kinetic luminosity and composition of active galactic nuclei jets. ApJ 619:73

    Article  Google Scholar 

  • Kellermann KI, Lister ML, Homan DC, Vermeulen RC, Cohen MH, Ros E, Kadler M, Zensus JA, Kovalev YY (2004) Sub-milliarcsecond imaging of quasars and active galactic nuclei. III. Kinematics of parsec-scale radio jets. ApJ 609:539

    Article  Google Scholar 

  • Königl A (1981) Relativistic jets as X-ray and gamma-ray sources. ApJ 243:700

    Article  Google Scholar 

  • Kovalev YY, Lobanov AP, Pushkarev AB, Zensus JA (2008) Opacity in compact extragalactic radio sources and its effect on astrophysical and astrometric studies. A&A 483:759

    Article  Google Scholar 

  • Kudryavtseva NA, Gabuzda DC, Aller MF, Aller HD (2011) A new method for estimating frequency-dependent core shifts in active galactic nucleus jets. MNRAS 415:1631

    Article  Google Scholar 

  • Lanyi GE, Boboltz DA, Charlot P, Fey AL, Fomalont EB, Geldzahler BJ, Gordon D, Jacobs CS, Ma C, Naudet CJ, Romney JD, Sovers OJ, Zhang LD (2010) The Celestial Reference Frame at 24 and 43 GHz. I. Astrometry. AJ 139:1695

    Article  Google Scholar 

  • Lister ML, Cohen MH, Homan DC, Kadler M, Kellermann KI, Kovalev YY, Ros E, Savolainen T, Zensus JA (2009) MOJAVE: monitoring of jets in active galactic nuclei with VLBA experiments. VI. Kinematics analysis of a complete sample of blazar jets. AJ 138:1874

    Article  Google Scholar 

  • Lobanov AP (1998) Ultracompact jets in active galactic nuclei. A&A 330:79

    Google Scholar 

  • Lovell JEJ, McCallum JN, Reid PB, McCulloch PM, Baynes BE, Dickey JM, Shabala SS, Watson CS, Titov O, Ruddick R, Twilley R, Reynolds C, Tingay SJ, Shield P, Adada R, Ellingsen SP, Morgan JS, Bignall HE (2013) The AuScope geodetic VLBI array. ArXiv e-prints

  • Lovell JEJ, Rickett BJ, Macquart J-P, Jauncey DL, Bignall HE, Kedziora-Chudczer L, Ojha R, Pursimo T, Dutka M, Senkbeil C, Shabala S (2008) The micro-arcsecond scintillation-induced variability (MASIV) survey. II. The first four epochs. ApJ 689:108

    Article  Google Scholar 

  • Ma C, Arias EF, Bianco G, Boboltz DA, Bolotin SL, Charlot P, Engelhardt G, Fey AL, Gaume RA, Gontier A-M, Heinkelmann R, Jacobs CS, Kurdubov S, Lambert SB, Malkin ZM, Nothnagel A, Petrov L, Skurikhina E, Sokolova JR, Souchay J, Sovers OJ, Tesmer V, Titov OA, Wang G, Zharov VE, Barache C, Boeckmann S, Collioud A, Gipson JM, Gordon D, Lytvyn SO, MacMillan DS, Ojha R (2009) The second realization of the International Celestial Reference Frame by very long baseline interferometry. IERS Tech. Note 35:1

    Google Scholar 

  • Macquart J-P, Godfrey LEH, Bignall HE, Hodgson JA (2013) The microarcsecond structure of an active galactic nucleus jet via interstellar scintillation. ApJ 765:142

    Article  Google Scholar 

  • Malkin Z, Titov O (2008) Measuring the future. In: Finkelstein A, Behrend D (eds) Proceedings of the fifth IVS, p 183

  • Marscher AP, Gear WK (1985) Models for high-frequency radio outbursts in extragalactic sources, with application to the early 1983 millimeter-to-infrared flare of 3C 273. ApJ 298:114

    Article  Google Scholar 

  • McCulloch PM, Ellingsen SP, Jauncey DL, Carter SJB, Cimò G, Lovell JEJ, Dodson RG (2005) COSMIC: microarcsecond resolution with a 30 meter radio telescope. AJ 129:2034

    Article  Google Scholar 

  • Niell AE, Whitney B, Petrachenk B, Schlüter W, Vandenberg N, Hase H, Koyama Y, Ma C, Schuh H, Tuccari (2006) VLBI2010: current and future requirements for geodetic VLBI systems. Tech. Rep.

  • Ojha R, Fey AL, Jauncey DL, Lovell JEJ, Johnston KJ (2004) Milliarcsecond structure of microarcsecond sources: comparison of scintillating and nonscintillating extragalactic radio sources. ApJ 614:607

    Google Scholar 

  • Ojha R, Fey AL, Lazio TJW, Jauncey DL, Lovell JEJ, Kedziora-Chudczer L (2006) Scatter broadening of scintillating and nonscintillating AGNS. I. A multifrequency VLBA survey. ApJS 166:37

    Article  Google Scholar 

  • Petit G, Luzum B (2010) IERS technical note no. 36, pp 179. verlag des Bundesamts für Kartographie und Geodäsie Frankfurt am Main

  • Petrachenko B, Niell A, Behrend D, Corey B, Boehm J, Charlot P, Collioud A, Gipson J, Haas R, Hobiger T, Koyama Y, MacMillan D, Malkin Z, Nilsson T, Pany A, Tuccari G, Whitney A, Wresnik J (2009) Design aspects of the VLBI2010 system. Progress Report of the IVS VLBI2010 Committee. Tech. Rep

  • Porcas RW (2009) Radio astrometry with chromatic AGN core positions. A&A 505:L1

    Article  Google Scholar 

  • Pushkarev AB, Kovalev YY, Lister ML, Savolainen T (2009) Jet opening angles and gamma-ray brightness of AGN. A&A 507:L33

    Article  Google Scholar 

  • Rees MJ (1966) Appearance of relativistically expanding radio sources. Nature 211:468

    Article  Google Scholar 

  • Schaap RG, Shabala SS, Ellingsen SP, Titov OA, Lovell JEJ (2013) Scintillation is an indicator of astrometric stability. MNRAS 434:585

    Article  Google Scholar 

  • Schuh H, Behrend D (2012) VLBI: a fascinating technique for geodesy and astrometry. J Geodyn 61:68

    Article  Google Scholar 

  • Shabala SS, McCallum JN, Plank L, Böhm J (2014) Simulating the effects of quasar structure on parameters from geodetic VLBI. J Geod (submitted)

  • Shabala SS, Santoso JS, Godfrey LEH (2012) Measuring the jet power of flat-spectrum radio quasars. ApJ 756:161

    Article  Google Scholar 

  • Sokolovsky KV, Kovalev YY, Pushkarev AB, Lobanov AP (2011) A VLBA survey of the core shift effect in AGN jets. I. Evidence of dominating synchrotron opacity. A&A 532:A38

    Article  Google Scholar 

  • Teräsranta H, Wiren S, Koivisto P, Saarinen V, Hovatta T (2005) 24 year monitoring of extragalactic sources at 22 and 37 GHz. A&A 440:409

    Google Scholar 

  • Titov O, Tesmer V, Böhm J (2004) In: International VLBI service for geodesy and astrometry: general meeting proceedings, p 267

  • Turner RJ, Ellingsen SP, Shabala SS, Blanchard J, Lovell JEJ, McCallum JN, Cimò G (2012) BL Lac object PKS B1144-379: an extreme scintillator. ApJ Lett 754:L19

    Article  Google Scholar 

  • Wall JV, Jenkins CR (2012) Practical statistics for astronomers. Cambridge University Press

  • Weston S, Takiguchi H, Natusch T, Gulyaev S (2013) Warkworth 12-m VLBI station: WARK12M—2012. ArXiv e-prints

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Acknowledgments

SS and JM thank the Australian Research Council for Super Science Fellowships (FS100100037 and FS110200045). We are grateful to Harald Schuh, Simon Ellingsen and John Dickey for useful discussions, and Chris Jacobs, Richard Porcas and two other anonymous referees for thorough and constructive comments that have significantly improved the manuscript. This research has made use of the United States Naval Observatory (USNO) Radio Reference Frame Image Database (RRFID).

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Authors and Affiliations

  1. School of Physical Sciences, University of Tasmania, Private Bag 37, Hobart, TAS, 7001, Australia

    Stanislav S. Shabala, Jonathan G. Rogers, Jamie N. McCallum, Jay Blanchard & James E. J. Lovell

  2. Departamento de Astronomía, Universidad de Concepción, Casilla 160C, Concepción, Chile

    Jay Blanchard

  3. Geoscience Australia, PO Box 378, Canberra, ACT, 2615, Australia

    Oleg A. Titov

  4. School of Land and Food, University of Tasmania, Private Bag 78, Hobart , 7001, Australia

    Christopher S. Watson

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  1. Stanislav S. Shabala
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Correspondence to Stanislav S. Shabala.

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Shabala, S.S., Rogers, J.G., McCallum, J.N. et al. The effects of frequency-dependent quasar variability on the celestial reference frame. J Geod 88, 575–586 (2014). https://doi.org/10.1007/s00190-014-0706-z

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  • DOI: https://doi.org/10.1007/s00190-014-0706-z

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