Long baselines. Long baselines. VLBI techniques: other lecture. Very Long Baseline Interferometry: definition. Connected VLBI : more details

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1 Long baselines GLOW Interferometry School II Long baselines Olaf Wucknitz Bielefeld, 15 February 2012 full VLBI lecture desirable several available online (see next slide) ask your VLBI colleagues try own VLBI experiments! this lecture discuss some long-baseline issues (general and LOFAR) explain current procedures show some results future development 1 VLBI techniques: other lecture Very Long Baseline Interferometry: definition Need for long baselines What defines VLBI? Techniques VLBI science Practical issues VLBI arrays, how to observe calibration New developments space VLBI e-vlbi very long baselines other definitions no direct connection between stations record signals on tapes, disks, etc. play back simultaneously and correlate later none of them true for e-vlbi (or LOFAR) synchronisation: also record time-stamps observe at exactly the same frequency our definition: no common clock or local oscillator 2 3 Connected interferometer VLBI Connected VLBI : more details [ Thompson (1999) ] [ Napier (1999) ] connected interferometer mix down to IF mix down to baseband sample, correlate VLBI mix down to IF amplify at IF mix down to baseband sample, record, fly, play back LOFAR: send via internet correlate need accurate LOs / clocks! LOFAR: no mixing 4 5 keep the time The role of the local clock need to play back signals synchronised required accuracy: coherence time coherence time 1/ bandwidth keep synchronisation over observation The correlation direct correlation of signals V 1 and V 2 signals V 1 (t)=a 1 e 2πiνt V 2 (t)=a 2 e 2πiνt correlation:c 12 := V 1 (t)v2(t) = A 1 A 2e 2πi(ν ν)t =A 1 A 2 define the observing frequency observing frequency ν shifted to baseband (mixing with LO) recorded frequency ν : ν =ν ν 0 error in ν 0 translates to error in ν,ν (LOFAR: direct sampling) [ Thompson (1999) ] correlation of down-mixed signals V 1 and V 2 frequencies of local oscillators: ν 1 and ν 2 signals V 1 (t)=a 1e 2πi(ν ν 1)t V 2 (t)=a 2e 2πi(ν ν 2)t correlation: C 12:= V 1(t)V 2 (t) = A 1 A 2e 2πi(ν 1 ν 2 )t 6 7

2 Stability of local oscillators / clocks Geometric delays atomic clocks (rubidium or hydrogen masers) long-term synchronisation with GPS receiver τ 1000km km/s 3ms 1 ν 10ns τν Delays, phases, rates Delays: connected vs. VLBI effect of a delay τ telescope signal V j (t)=a j e 2πiν(t τ j) correlation V 1 V2 =A 1A 2 e2πiν(τ 2 τ 1 ) phase φ=2πν(τ 2 τ 1 ) frequency dependence φ ν =2πτ delay is frequency-derivative of phase phase rate and delay rate φ t =2πν τ t equiv. Doppler effect, frequency error Delay model Calibration of VLBI data [ Walker (1999) ] predictable delays are corrected by the correlator geometric delay earth rotation aberration (dry atmosphere) unpredictable delays have to be calibrated later wet atmosphere ionosphere station clocks very similar to connected interferometers additional steps due to long baselines high resolution need accurate source positions no amplitude calibrators available (use T sys to calibrate) limited field long baselines unstable phases need bright fringe-finder source phase-referencing stop phase-winding: fringe-fitting The need for fringe-fitting Linear approach for residual phases large time-varying delays phases change rapidly phase changes frequency-dependent standard calibration techniques determine phases regularly constant between the measurements had to do this every few seconds! fit delays and rates instead of phases allows for rapid changes rate of changes and delays vary more slowly φ(t,ν)=φ 0 + φ ν ν+ φ t [+dispersive delay] t have to determine phase φ 0 delay φ ν rate φ t delays and rates are stable over a longer time and wider band than φ(t,ν) the process to find phase, delay, rate is called fringe-fitting need AIPS 14 15

3 VLBI science Some pictures... jets from AGN, microquasars superluminal motion gravitational lenses extragalactic supernovae pulsars masers circumstellar megamasers in AGN astrometry geodesy 16 Wide-field VLBI at 90 cm 17 Challenges in wide-field VLBI higher resolution in time and frequency needed to avoid time-averaging smearing bandwidth smearing this means less compression huge uv data sets [ Lenc et al. (2008) ] high resolution with wide fields implies very many pixels huge computational challenge some clever ideas available this is relevant for long-baseline LOFAR! 18 Geodesy 19 VLBI arrays Very Long Baseline Array (VLBA) 10 identical telescopes of 25 m (USA) full-time VLBI array European VLBI Network (EVN) 20 telescopes (Europe, Asia, South Africa, Arecibo) 3 sessions each year (+ e-vlbi) VLBI Exploration of Radio Astrometry (VERA) 4 stations (Japan) High Sensitivity Array (HSA) VLBA + VLA + Arecibo + Green Bank + Effelsberg Long Baseline Array (LBA) 8 telescopes in Australia global VLBI VLBA + EVN + anything 20 VLBA 21 EVN 22 23

4 e-vlbi (and LOFAR) International LOFAR stations classical VLBI record on tape/disk ship tapes/disks to correlator correlate later e-vlbi send data directly to correlator high-bandwidth data links ( internet ) but no common clock! advantages of e-vlbi immediate feedback, quick turnaround disadvantages of e-vlbi cannot repeat correlation no multiple passes VLBI aspects of LOFAR sources resolved weak signal less calibrators have to (coherently) combine in time and frequency have to correct for delays ( φ/ ν) have to correct for rates ( φ/ t) need fringe fitting non-dispersive: clocks, positions (τ =const) dispersive: ionosphere (τ ν 2 ) larger ionospheric delays (and rates) larger clock offsets differential Faraday rotation (don t use XX and YY) limited uv coverage 26 Fringe-fitting for LOFAR either for single subbands ( τ 5µsec) or coherent multi-band ( τ 0.02µsec) beware of multiple peaks in delay/rate produce 2-d delay/rate spectra simultaneously fit for four parameters dispersive/nondispersive delays/rates development software, no full calibration yet! 27 Single-band, Ef-NL, 3C196, 20 August 2009 first long-baseline-fringes! 5 subbands continuous mod Delays and phases phase [full turns] freq [MHz] do not fit phases directly Delay fitting only know phase modulo 2π data are noisy equivalent (but better!): maximise the corrected signal measured and original visibility hope that V 0 (ν)=const and correct for delay find maximum of this is Fourier transform if τ =const V(ν)=e 2πiντ(ν) V 0 (ν) dνe 2πiντ(ν) V(ν) 30 2 Multi-band delay fitting delay almost constant within subbands apply FFT for all subbands combine the results incoherently combine the results coherently f i (τ i )= i dνe 2πi(ν ν i)τ i V(ν) with coarse FFT ( F[τ]= e 2πiν iτ(ν i ) f i τ(νi ) ) on fine grid, interpolation i τ(ν) arbitrary function of frequency (non-/dispersive) 31

5 Include fringe rates Interfering sources (observations of 3C48) have to integrate in time to increase S/N take into account rates (time-derivatives) do not use phase rates but delay rates r = τ dispersive/non-dispersive t f i (τ i,r i )= dνe 2πi(ν ν i)τ i dte 2πi(t t 0)r i V(ν,t) i ( F[τ,r]= e 2πiν iτ(ν i ) f i τ(νi ),r(ν i ) ) i all phase rates are frequency-dependent Delay/rate map of field around 3C196 Clock offsets / delays station clocks (Rubidium standard + GPS) should be accurate to 20ns VLSS (74 MHz): A 19Jy B 6Jy C 17Jy 3C Jy some Dutch stations had offsets 90ns Effelsberg typically had 1µs Tautenburg once had 18µs makes fringe-fitting harder reasons were unclear, now mostly fixed Differential Faraday rotation (+ swapped polarisations) Converting XX/XY/YX/YY to RR/RL/LR/LL Long-baseline LOFAR imaging procedure First LBA long-baseline map: some details fundamental problem sparse uv coverage practical problems (not implemented in pipeline) fringe-fitting correct clock-offsets Faraday rotation (and swapped polarisations) current solution : own ad hoc software do all this + flag + calculate weights convert to circular, uvfits this is no user software! but soon! then continue with AIPS, difmap, CASA, BBS,... 3C196, LBA, 31 / 160 subbands, / MHz (ripple!) bandwidth 6 MHz / 48 MHz D h on 12/13 Feb NL + 3 DE stations (Effelsberg, Unterweilenbach, Tautenburg) corrected for 1µsec and 17µsec constant delays RR and LL from XX/XY/YX/YY using geometric model (self-)calibrated and imaged LL/RR in difmap MFS with/without spectral index correction 38 39

6 UV coverage with long and short baselines MTRLI (MERLIN) observations of 3C196 at 408 MHz [ Lonsdale & Morison (1980) ] LOFAR maps: short and long baselines LOFAR LBA vs. MERLIN 408MHz NL only, beam NL+DE, beam [ Wucknitz + Lonsdale & Morison (1980) ] Sub-arcsec: HBA Images of 3C196 (12h, L ) Early HBA image of 3C147 (12 h, L ) international Dutch resolution: This is with only 20% of the subbands, RR pol. Dynamic range already 3000:1 in first attempt. 45 Long-baseline observations of the Crab (TauA/M1/3C144) Image of the pulsar PLot file version 1 created 26-JUN :14:37 BOTH: IPOL MHZ ICL h on 13/14 Feb 2011 L MHz international stations: DE601, DE603, FR606 NL stations: CS032, RS106,205,208,306,307,406,503 DECLINATION (J2000) problems DE601 failed 50% of subbands bad in blocks of 30/31 significant ionospheric delays, differential Faraday rotation RIGHT ASCENSION (J2000) Grey scale flux range= JY/BEAM Cont peak flux = E+00 JY/BEAM Levs = 8.566E-03 * (-2, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024)

7 Imaging the nebula (HBA) Crab with VLA at 5GHz [ Bietenholz (2004) ] Direct comparison Crab with LOFAR (HBA) LOFAR (red) and 5 GHz VLA (green) Crab with VLA at 5GHz (smoothed) Direct comparison (smoothed) LOFAR (red) and 5 GHz VLA (green) New observations of the Crab: preliminary LBA image calibrated RR and LL, averaged over 30min POSSM plots: IQUV (set I to 10Jy) Plot file version 6 created 04-MAR :49: MULTI.1 Freq = GHz, Bw = MH Calibrated with CL # 3 and BP # 1 (BP mode 1) FR60 - RS IF 1(I) IF 1(U) IF 2(I) FREQ MHz IF 2(U) I U IF 3(I) FR60 - RS FREQ MHz IF 3(U) FR60 - RS IF 1(Q) IF 1(V) IF 2(Q) FREQ MHz Lower frame: Real Jy Top frame: Imag Jy Vector averaged cross-power spectrum Baseline: FR606HBA(04) - RS306HBA(08) Timerange: 00/20:30:17 to 00/21:00:17 FR606 RS306 IF 2(V) Q V FREQ MHz IF 3(Q) FR60 - RS IF 3(V) imag real imag real 54 55

8 Faraday dispersion function from Q and U (30 subbands) After subtraction of leakage peak (one CLEAN iteration) with LOFAR: VERY crude maps Jupiter: sources of decametric emission Io aurora VLA [ Harvanek et al ] 58 [ Zarka (2004) ] 59 Jupiter observations 8th July 2011: DE604-DE602 RR Jupiter observations 8th July 2011: DE604-DE602 LL Jupiter observations 6th August: RS503-DE604 RR Jupiter observations 6th August: RS503-DE604 LL 62 63

9 First full-resolution Jupiter observations Entire observation DE602-FR606, RR amplitudes L35624, 24/25 November :30 01:45 UTC 61 subbands MHz, 256 channels/sb (763 Hz) correlations 6 stations: DE602, FR606, RS106, RS307, SE607, UK608 1 sec integration time fly s eye raw voltages integration time 1.3 msec only 3 stations: DE602, FR606, RS106 nice bursts, analysis in progress 64 Entire observation DE602-FR606, LL amplitudes Zooming in (3) DE602-FR606, RR amplitudes 68 Zooming in (4) DE602-FR606, RR amplitudes 65 Zooming in (1) DE602-FR606, RR amplitudes Zooming in (2) DE602-FR606, RR amplitudes 69 Zooming in (5) DE602-FR606, RR amplitudes 70 71

10 Elsewhere: Zooming in (1) DE602-FR606, RR amp Elsewhere: Zooming in (2) DE602-FR606, RR amp Elsewhere: Zooming in (3) DE602-FR606, RR amp In progress: 3C123 (HBA) NL only [ Olaf Wucknitz ] international [ Adam Deller ] (Near) future development Summary long-baseline issues short term standard tool for conversion to circular polarisation basis (Tobia Carozzi) first-order solution for differential Faraday rotation (DFR) let BBS correct for DFR has problems finding the solution long term implement full fringe-fitting in BBS including proper DFR calibration more difficult higher resolution required (t,ν) data volume fringe-fitting required (much) less signal larger clock offsets stronger ionospheric effects sparse uv coverage less stable imaging and calibration differential Faraday rotation less difficult RFI and interfering sources weaker (in the field and A team) large structures resolved out, often simpler source structure RFI spikes strong spikes (< 1sec) left after initial flagging had consistent phases and sometimes small delays seen on all Dutch baselines Mapping the signal can measure delays on Dutch baselines 3 baselines sufficient to determine position (in 3d) add all baselines incoherently, scan in 2d or 3d 2d scan at 10km height find clear moving peak, scans with 8 60 subbands wide: <115MHz 141MHz beyond airband detectable for about 30min 78 79

11 Altitude and nearby trajectory (3d scans) Full trajectory focused at m (2d scans) Trajectory Identification AFL 231 Moscow to Brussels aircraft VP-BUO