Using quasar physics to. improve the VLBI reference frame

Transcription

1 Using quasar physics to improve the VLBI reference frame Stas Shabala University of Tasmania with: Lucia Plank, Jamie McCallum, Rob Schaap (UTAS) Johannes Böhm, Hana Krásná (TU Wien) Jing Sun (Shanghai Astronomical Observatory)

2 Outline 1 What are these radio sources we look at? 2 Quantifying the effects of quasar structure 3 VieVS structure simulator Simulation strategy Effect on the reference frame (Station positions : mm level) 4 Mitigation strategies

3 Uncooperative quasars What you want them to be ² Bright point sources ² Fixed in space and Sme What they are ² Supermassive black holes ² Jets è structure ² Evolve on human Smescales Image: NASA Image: N. Reid Image: C. Mueller Lister et al. (29)

4 Inner regions of an Active Galactic Nucleus Image: BU / A. Marscher 1 z=1 One jet towards us (Doppler boosted) - > seen One jet away from us (D. deboosted) - > unseen jet origin core Stas Shabala - REFAG14 jet

5 uv plane N-E plane projected in source direction ² interferometers measure correlated amplitude and phase separation v (M ) Visibility phase u (M ) distances measured in units of wavelength λ=c/ν

6 uv plane N-E plane projected in source direction ² interferometers measure correlated amplitude and phase ² image ç FourierTransformè amplitudes/phases in uv plane separation v (M ) u (M ) 8 1 Lister et al. (29)

7 Simulated source Declination (mas) Right Ascension (mas).3.2.1

8 Visibility phase N-E plane projected in source direction different location in uv plane depending on frequency and baseline v (M ) distances measured in units of wavelength λ=c/ν u (M ) 1

9 Source structure in geodetic VLBI ² Group delay = (1 / 2π) (Δ phase / Δ frequency) ª phase depends on projected structure as seen by a given baseline ª function of: baseline observing time amount and direction of structure ª effect is different at each of 8 sub-bands at X-band (because frequencies are slightly different) ² Hence group delay (= slope across band) changes

10 Visibility phase N-E plane projected in source direction different location in uv plane depending on frequency and baseline v (M ) distances measured in units of wavelength λ=c/ν u (M ) 1

11 Structure phase on 9 km baseline 2 3 Phase (degrees) slope = structure delay Frequency (MHz)

12 Visibility phase N-E plane projected in source direction different location in uv plane depending on frequency, baseline and time v (M ) distances measured in units of wavelength λ=c/ν u (M ) 1

13 Jet baseline orientation slope = extra (structure) delay.4 Hobart - HartRAO Hobart - Katherine phase / radians.2 jet orthogonal to baseline structure delay = 11 ps (3.3 mm light travel time) -.2 structure delay = 6.9 ps (2.1 mm light travel time) frequency / GHz How a source is observed is important

14 3 effects of quasar structure 1. Measure incorrect posison PosiSon offset (X, Y, Z) from correct value 2. Measure different posisons for different schedules Scaier in stason posisons for different baseline/ schedule combinasons (even with same quasars) è increased rms for posisons derived from different schedules 3. MulSple observasons inconsistent with each other Larger formal uncertainses, within a single session slope slope changes with projected structure

15 VieVS source structure simulator Vienna VLBI Solware (VieVS) Simulate geodesc observasons Process simulated observasons è stason / source posisons, EOPs Quasar structure simulasons Quasars point- like Extra structure delay per observason (mostly) Mock source catalogues VIEVS

16 Simulated catalogues ² Structure indices è mock quasar images SI = log (τ / ps) u Choose SI (none, 1, 2, 3, 4, ICRF2 distribution) u 2-component sources u Also one real CONT11 catalogue SI 3 = 1 ps ² Simulate realistic schedules with VieVS u CONT11 o September 211 o 13 stations o 3 realizations of each day u Additional delay term due to source structure

17 CONT11

18 Real sources X coord.3.2 real structure no structure median X position offset / cm Bd Hh Hb Zc Wf Wz On Ts Ny Ys Kk Tc Ft Shabala+14, J. Geod. submitted

19 Real sources Y coord.3.2 real structure no structure median Y position offset / cm Bd Hh Hb Zc Wf Wz On Ts Ny Ys Kk Tc Ft Shabala+14, J. Geod. submitted

20 Real sources Z coord.3.2 real structure no structure median Z position offset / cm Bd Hh Hb Zc Wf Wz On Ts Ny Ys Kk Tc Ft Shabala+14, J. Geod. submitted

21 Real sources 3D coord offset.5 real structure no structure median position offset / cm Bd Hh Hb Zc Wf Wz On Ts Ny Ys Kk Tc Ft Shabala+14, J. Geod. submitted

22 Real sources 3D coord offset median position offset / cm real structure no structure Offset > 2 mm Bd Hh Hb Zc Wf Wz On Ts Ny Ys Kk Tc Ft Shabala+14, J. Geod. submitted

23 How important is quasar structure?.3.25 median offset rms uncertainty.2 position / cm none ICRF2 real structure index Shabala+14, J. Geod. submitted SI = log (τ / ps)

24 How important is quasar structure?.3.25 median offset rms uncertainty position / cm Higher SIs give worse positions.5 none ICRF2 real structure index Shabala+14, J. Geod. submitted SI = log (τ / ps)

25 How important is quasar structure?.3.25 median offset rms uncertainty theory.2 position / cm none ICRF2 real structure index Shabala+14, J. Geod. submitted SI = log (τ / ps)

26 How important is quasar structure?.3.25 median offset rms uncertainty position / cm mm error.5 none ICRF2 real structure index Shabala+14, J. Geod. submitted SI = log (τ / ps)

27 How important is quasar structure?.3.25 median offset rms uncertainty position / cm Rms and σ underes1mate how wrong the soluson really is.5 none ICRF2 real structure index Shabala+14, J. Geod. submitted SI = log (τ / ps)

28 EOP rms.3 x pol y pol (UT1-UTC).2 pol rms / mas (UT1-UTC) rms / ms none ICRF2 real structure index Shabala+14, J. Geod. submitted

29 What happens if we include troposphere?

30 Station positions (with trop.).8.7 median offset rms uncertainty.6 position / cm Troposphere dominates none ICRF2 real structure index Shabala+14, J. Geod. submitted SI = log (τ / ps)

31 EOP rms (with troposphere).12.1 x pol y pol (UT1-UTC) pol rms / mas (UT1-UTC) rms / ms.2.1 none ICRF2 real structure index Shabala+14, J. Geod. submitted

32 Source structure : effects on CRF Simulation (Cont11), with troposphere & measurement noise NO STRUCTURE STRUCTURE, SI=3 SIMULATED

33 Source structure : effects on CRF Simulation (Cont11), with troposphere & measurement noise NO STRUCTURE STRUCTURE, SI=3 MEDIAN ESTIMATED POSITION JET DIRECTION SIMULATED Systematic displacement ~ 1 µas

34 Source structure : effects on CRF Schaap et al. 213, MNRAS, 434, More structure = worse source posisons less stable posisons log µas (,1.5) (1.5,1.75) (1.75,2) (2,2.5) (2.5,3) (3,3.5) Structure Index (3.5,4) (4,4.5) (4.5,5) (5,6) OBSERVED more structure

35 Problem summary ² Quasar structure simulations with VieVS ² mock + real quasar catalogues ² Station positions u offset from true values u larger formal uncertainties u increased rms u important at mm level ² EOPs and source positions also get worse with increasing structure

36 How do we improve the Reference Frames? Good news 1 We can image quasar structure (and make corrections) (with quasar physics) Lister et al. (29)

37 How do we improve the Reference Frames? (with quasar physics) Good news 1 We can image quasar structure (and make corrections) Bad news Quasars evolve (need to to this often) Lister et al. (29)

38 How do we improve the Reference Frames? (with quasar physics) Good news 1 We can image quasar structure Bad news Quasars evolve 2 Jet direction remains constant (avoid unfavourable baseline jet orientation)

39 How do we improve the Reference Frames? (with quasar physics) Good news 1 We can image quasar structure Bad news Quasars evolve 2 Jet direction remains constant 3 Quasars evolve! (observe quasars when they are well behaved )

40 How do we improve the Reference Frames? (with quasar physics) Good news 1 We can image quasar structure Bad news Quasars evolve 2 Jet direction remains constant 3 Quasars evolve! Strategies Use real sources to correct observations Minimize projected structure through clever scheduling Include / exclude sources at appropriate times

41 1. Quasar structure corrections Corrected using available images Raw observasons rms CONT11 OBSERVATIONS Improvement OBSERVED baseline length [1 3 km]

42 1. Quasar structure corrections Corrected using available images Raw observasons rms Improvement = raw - corrected difference in mm! Improvement OBSERVED baseline length [1 3 km]

43 1. Quasar structure corrections Small improvement 45 bl beier: +.22 mm 33 bl worse: -.12 mm < 1 mm... but promising... Problems Dominated by troposphere Many quasars don t have images Quasars evolve Future Imaging from geodesc VLBI data OBSERVED difference in mm! baseline length [1 3 km] Improvement

44 1. Quasar structure corrections Small improvement 45 bl beier: +.22 mm 33 bl worse: -.12 mm < 1 mm... but promising... Long baselines most improved Problems Dominated by troposphere Many quasars don t have images Quasars evolve Future Imaging from geodesc VLBI data OBSERVED difference in mm! baseline length [1 3 km] Improvement

45 2. Re-analysis using source structure Jet direcson is constant Worst case: parallel, long baselines R1/R4 of Oct 213, SI=3 Structure delay in ps %... median value & % of observations Structure delay in ps Angle between jet & baseline [ ] Angle + length of baseline

46 2. Re-analysis using source structure Jet direcson is constant Worst case: perpendicular, long baselines R1/R4 of Oct 213, SI=3 Structure delay in ps %... median value & % of observations Structure delay in ps ¼ of data downweighted Angle between jet & baseline [ ] Half the data is ok Angle + length of baseline

47 2. Re-analysis using source structure Simula1on test Downweight 23% of observasons Structure only simulasons Improvement StaSon posisons by 22% Formal uncertainses by 3% Problem Need these observasons to resolve troposphere! Structure delay in ps Half the data is ok ¼ of data downweighted Angle + length of baseline SIMULATED

48 2. Re-scheduling w.r.t source structure Scheduling Complex task OpSmizing for max. number of scans, sky coverage, etc. Strategy Schedule w.r.t. relasve orientason and baseline length Simulate First results appear promising R1/R4 (re-scheduled) of Oct 213 Improvement SIMULATED

49 3. Quasar evolution! NEW! 3.5 Source SI X-band Structure ans- correlates with flux density è Observe sources when flux density is high (and so structure is small) 1.5 structure index flux density / Jy Year

50 3. Quasar evolution structure index SI X-band flux density / Jy! NEW! Structure ans- correlates with flux density è Observe sources when flux density is high (and so structure is small) PosiSon scaier decreases for sources with high flux density Year Source Dec Deviation (mas) Low flux density Medium flux density High flux density Dec Deviation (mas) Dec Deviation (mas) RA Deviation (mas) RA Deviation (mas) RA Deviation (mas)

51 Summary q Quasars are not point sources o extra group delay ² Baseline-, time-, frequency- dependent o systematic error è simply more observations won t help q Quasar structure simulations with VieVS 2.2 ² new source structure simulator module ² mock + real quasar catalogues ² station positions affected at mm level ² troposphere dominates (for now) u Mitigation strategies v corrections using source images in analysis v not scheduling unfavourable jet / baseline combinations avoid long baselines parallel to jet v source selection radio sources vary in structure on year timescales more compact when flaring (bright)

52

53 Inner regions of an Active Galactic Nucleus Image: BU / A. Marscher One jet towards us (Doppler boosted) - > seen One jet away from us (D. deboosted) - > unseen 1 z=1 jet origin core jet Synchrotron self- absorpson è core locason depends on frequency è Core ShiN Image: Kovalev et al. (28) Stas Shabala - REFAG14

54 Quasar structure in VGOS phase (arbitary units) total ionosphere non-dispersive Expected delay accuracy 2-4 ps.4 max structure delay = +/- 26 ps (7.6 mm light travel time) frequency / GHz Quasar structure > 2 ps phase / radians max structure delay = +/- 8.2 ps (2.4 mm light travel time) frequency / GHz How do we connect phases across 1 GHz?

55 Quasars What you want them to be ² Bright point sources ² Fixed in space and Sme What they are ² Supermassive black holes ² 1 6 Smes more distant than stars Quasars are really far away Image: N. (z Reid >=1 ) (z = 1 is half the age of Image: the NASA Universe) Use quasars because Image: C. Mueller D(z = 1 quasar) = 6.7 x 1 9 pc D(stars at GalacSc centre) = 8 x 1 3 pc è quasi- iner1al reference frame

56 Structure delay (ps)

57 EOP formal error (structure only).3 x pol y pol (UT1-UTC).2 pol formal error / mas (UT1-UTC) formal error / ms none ICRF2 real structure index

58 EOP formal error (with trop.).12.1 x pol y pol (UT1-UTC).8.7 pol formal error / mas (UT1-UTC) formal error / ms none ICRF2 real structure index

59 Median position offset 1 no structure median of 15 x 3 realizations median position offset / cm OnWz YsNyZc BdWf FtTs Hb Kk Tc Hh average distance to other stations / km Shabala+14, J. Geod. submitted

60 Median position offset 1 SI=1 no structure median position offset / cm OnWz YsNyZc BdWf FtTs Hb Kk Tc Hh average distance to other stations / km Shabala+14, J. Geod. submitted

61 Median position offset 1 SI=1 SI=2 no structure median position offset / cm OnWz YsNyZc BdWf FtTs Hb Kk Tc Hh average distance to other stations / km Shabala+14, J. Geod. submitted

62 Median position offset median position offset / cm SI=1 SI=2 SI=3 no structure OnWz YsNyZc BdWf FtTs Hb Kk Tc Hh average distance to other stations / km Shabala+14, J. Geod. submitted

63 Median position offset median position offset / cm SI=1 SI=2 SI=3 SI=4 no structure OnWz YsNyZc BdWf FtTs Hb Kk Tc Hh average distance to other stations / km Shabala+14, J. Geod. submitted

64 Median position offset median position offset / cm SI=1 SI=2 SI=3 SI=4 ICRF2 no structure OnWz YsNyZc BdWf FtTs Hb Kk Tc Hh average distance to other stations / km Shabala+14, J. Geod. submitted

65 Median position offset median position offset / cm SI=1 SI=2 SI=3 SI=4 ICRF2 real structure no structure OnWz YsNyZc BdWf FtTs Hb Kk Tc Hh average distance to other stations / km Shabala+14, J. Geod. submitted

66 Median position offset median position offset / cm SI=1 SI=2 SI=3 SI=4 ICRF2 real structure no structure OnWz YsNyZc BdWf FtTs Hb Kk Tc Hh average distance to other stations / km Shabala+14, J. Geod. submitted

67 Median position offset median position offset / cm SI=1 SI=2 SI=3 SI=4 ICRF2 real structure no structure OnWz YsNyZc Bd Wf FtTs Hb Kk Tc Hh average distance to other stations / km Shabala+14, J. Geod. submitted