Very long baseline interferometry Tuomas Savolainen Aalto University Metsähovi Radio Observatory Max-Planck-Institut f. Radioastronomie 10 mas - PDF

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Very long baseline interferometry Tuomas Savolainen Aalto University Metsähovi Radio Observatory Max-Planck-Institut f. Radioastronomie 10 mas Image credit: Y. Y. Kovalev Outline What is Very Long Baseline

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Very long baseline interferometry Tuomas Savolainen Aalto University Metsähovi Radio Observatory Max-Planck-Institut f. Radioastronomie 10 mas Image credit: Y. Y. Kovalev Outline What is Very Long Baseline Interferometry? Science with VLBI what can it do for you? VLBI signal path Differences to conventional radio interferometry VLBI arrays 2 Radically increase the baseline length: Very Long Baseline Interferometry There is no fundamental restriction to increasing the baseline length same interferometry principles apply Give up the centrally distributed LO signals and allow each telescope to have its own electronics can radically increase the baseline length VLBI A global array with baseline lengths up to km gives an angular resolution of 4 21cm and mm Very Long Baseline Array: a continentwide array with max. baseline of 8600 km. European VLBI Network spans over three continents with max. baseline of km. 3 Not even the sky is a limit Earth s size limits the baseline lengths below 12000km Still higher resolutions can be obtained by sending an antenna to space! Japanese HALCA satellite of VSOP project ( ) Apogee height: km Observing bands: 1.6, 5 GHz Max angular res.: 0.6 mas 4 Not even the sky is a limit Earth s size limits the baseline lengths below 12000km Still higher resolutions can be obtained by sending an antenna to space! Russian RadioAstron mission (2011-) Apogee height: km Observing bands: 0.3, 1.6, 5, 22 GHz Max nominal angular res.: 7 μas 5 but source brightness temperature is! Brightness temperature T b = λ2 S = 1.36 λ2 2k Ω cm θ 2 arcsec S mjy [K] while interferometer s HPBW is θ arcsec = Therefore, T b = S 1 mjy B max 8612 km 2 λ cm B max, km VLBA baseline sensitivity is 1 mjy at centimetre wavelengths T b,min ~ K High resolution has a price tag in terms of surface brightness sensitivity Targets (currently): compact non-thermal emitters [K] 6 Science with VLBI 7 Science targets for VLBI Pulsars Masers Supernova shock waves Magnetically active stars Jets from accreting black holes Spacecraft 8 Science motivation for VLBI: Microarcsecond astrometry Distance measurement using parallax Image credit: Adam Deller 9 Science motivation for VLBI: Microarcsecond astrometry Black hole masses and H 0 from the motion of extragalactic H 2 0 masers in accretion disks around supermassive black holes. NGC 4258 Warped Keplerian disk, diameter ~ 0.5 pc, edge-on, BH mass ~ 3.6x10 7 Solar masses (Miyoshi et al. 1995; Argon et al. 2007) 10 Science motivation for VLBI: Geodesy Use astronomical sources as test sources for modeling either the propagation effects of the signal (ISM, ionosphere, atmosphere) or the positions of the antennas. Earth Orientation Parameters Coordinate system alignment Daily observations of UT1-UTC with VLBA (U.S. Naval Observatory) Length of day variations (UT1-UTC) BKG Sonderheft Earth Rotation (1998) 11 Science motivation for VLBI: Spacecraft navigation Descent trajectory of Huygens probe on Titan Cimo et al. (2010) 12 Science motivation for VLBI: Imaging at ultra-high resolution Resolution is so high that changes in astronomical sources can be seen in time scales human life time movies Masers in young stellar objects, star-forming regions and circumstellar envelopes around Asymptotic Giant Branch stars. VLBI movie of SiO masers around Mira variable TX Cam (Gonidakis et al. 2010) 13 Science motivation for VLBI: Imaging at ultra-high resolution Resolution is so high that changes in astronomical sources can be seen in time scales human life time movies Expansion of supernova shock fronts Expansion of the shock front of supernova SN1993J in M81 (Marcaide et al.) 14 Science motivation for VLBI: Relativistic jets from AGN Outflows of magnetized plasma ejected by accreting supermassive black holes in AGN Excellent targets for VLBI: very bright, very compact, non-thermal emission + fast motion Monitoring of jets with VLBI measure kinematics in a scale of light years in sources that are billions of light years away Apparent superluminal motion is observed direct evidence for the relativistic speeds 15 Science motivation for VLBI: Relativistic jets from AGN Inner 2pc of M87 with VLBA at 43 GHz (Craig Walker et al.) Movie of 11 observations separated by three weeks Beam size 0.4x0.2 mas 16 Science motivation for VLBI: Jets from X-ray binaries 17 VLBI technicalities 18 VLBI signal path Telescopes are controlled autonomously, each executing a pre-distributed observing schedule to synchronize the array Image credit: Adam Deller 19 VLBI signal path Each site has its own frequency standard ( clock ), a hydrogen maser with phase stability of ~10-15 over 1000s H-maser with covers removed Image credit: Adam Deller 20 VLBI signal path Voltage signals are locally sampled, time-stamped, and recorded on disks (or in some cases transferred over fast internet connections) Mark 5A recorder with 2 disk modules Image credit: Adam Deller 21 VLBI signal path Signals are replayed, synchronized, and correlated offline using fast computer clusters. The correlator does not care that signals are not in real time! MPIfR-Bonn correlator Image credit: Adam Deller 22 How does VLBI differ from connectedelement interferometry? VLA VLBA 23 How does VLBI differ from connectedelement interferometry? No fundamental difference. In practice, things get a bit more difficult when the antennas are not at the same physical site: Advert consequences of the high angular resolution Independent station clocks have independent clock off-sets and rates that need to be corrected. These also fluctuate and cause phase noise. Used to be a big problem. Stations have uncorrelated atmospheric path length fluctuations that cannot be modelled. Wet troposphere is the main source of phase noise in modern VLBI (Allan deviation of while H-maser clocks have 10-14 ) Earth s curvature becomes a significant factor that has to be taken into account (as well as other geophysical effects) Recording the signals on disks (or historically, on tapes) has traditionally limited available bandwidth and, therefore, sensitivity. Not such a big problem anymore (2Gbps recording rates normal). 24 Advert consequences of the high angular resolution I Almost all sources are resolved one cannot rely on point-source gain calibrators VLBI requires sources with high T b, but these are usually variable one cannot rely on standard candles in flux calibration Therefore, calibration of gain amplitudes and setting the flux scale need to be done by radiometry of individual antennas. Works well! The longer baselines with ~10 antennas mean much more sparse sampling of the visibility function than with the VLA more difficult to image very complex sources, missing flux on large angular scales. 25 Advert consequences of the high angular resolution - II Long baselines magnify the effect of small angular quantities also small angle errors Need much more accurate source and antenna positions than with e.g., VLA Correlator model tries to predict these as well as Earth orientation parameters, clock off-sets and rates, and many geophysical effects Still, small errors in delay (dφ/dν) and rate (dφ/dt) remain Image credit: Shep Doeleman 26 Dealing with residual delays and rates Significant residual delays and rates prevent us from averaging data in frequency and time. Solution: fringe-fitting, i.e. searching for delay and rate values that maximize the signal when averaged over time and frequency. (No implementation in CASA at the moment, need to use AIPS) For weak sources: phase-referencing. Frequently observe a nearby, strong calibrator and transfer its fringe-fit solutions to the weak target. Image credit: ASC Lebedev 27 Dealing with station phase errors self-calibration Independent station clocks + atmospheric fluctuations an unknown, variable phase term at every station Prevented imaging in the early days of VLBI Solution: phase self-calibration For N 3 antennas in the array, there are N 1 N/2 N independent measurements for every integration period If we knew the source structure, we could solve for a set of N 1 N/2 equations and obtain N unknown station phases. Well, we don t. Now the trick: make an initial guess of the source structure (e.g., point), solve the station phases, correct the data, make an image and use it as the source model in the next round of iteration Hybrid mapping. 28 Geophysics of long baselines The Earth is round Source has different elevations at different antennas Parallactic angles are different for alt/az -mounted antennas at different sites need to use circularly polarized signal to get correlation The Earth is surrounded with a layer of neutral and ionized gas Tropospheric path lengths are very different at different antennas (due to different elevations). Produces a relative delay, which has to be modelled The Earth is not entirely solid Effects taken into account in the VLBA correlator model 29 VLBI networks 30 Choose an array that fits your needs Freq. ALMA 100 GHz ATCA PdB JVLA VLBA emerlin mm-vlbi 1 GHz Westerbork LBA EVN Space-VLBI 100 MHz LOFAR GMRT arcmin arcsec milliarcsec! Angular 31 resolution VLBI networks - astronomical European VLBI Network Coordinated VLBI operation of independent radio observatories Correlator in Dwingeloo, Netherlands Astronomical VLBI ~25 telescopes Max. baseline ~10000 km Freq GHz 3 sessions / year + evlbi 32 VLBI networks - astronomical Very Long Baseline Array (USA) A dedicated VLBI array works all year round Correlator in Socorro, New Mexico Astronomical and geodetic VLBI 10 x 25m telescopes Max. baseline 8600 km Freq GHz Add Arecibo, VLA, GBT and Effelsberg for High Sensitivity Array 33 VLBI networks - astronomical East Asian VLBI Network (China, Japan, Korea) Long Baseline Array (Australia) 34 VLBI networks - geodetic International VLBI Service for Geodesy & Astrometry Global network for geodetic VLBI Several correlators (e.g., MPIfR, Haystack and Washington) 35 Summary: VLBI Very Long Baseline Interferometry pushes the limits of (radio) astronomy in terms of angular resolution Only slightly more complicated than conventional radio interferometry Wide area of application (astrometry, geodesy, spacecraft navigation, astrophysics of black holes, neutron stars, supernovae ) Requires high brightness temperature targets (non-thermal emission) 36 Extra: space-vlbi and mm-vlbi 37 Space-VLBI Earth s size puts on upper limit on the baseline length. Further increase the resolution of VLBI possible by putting an antenna in space! First technology demonstration in the 1980s (TDRSS) First dedicated mission: Japanese HALCA satellite (1.6 and 5 GHz; max baseline ~30000km) Currently operating: Russian Spektr-R satellite of the RadioAstron project Future: Two Chinese orbiting antennas? 38 Problems of space-vlbi Uncertainties in the orbit residual acceleration term in the spacecraft clock rate Small antenna poor baseline sensitivity target sources must be very bright and compact Baseline changes quickly close to Earth Source structure can also limit the integration time 39 Space-VLBI with RadioAstron Project originates from the 1980s and was significantly delayed due to the collapse of the Soviet Union Finally launched in m antenna Highly elliptical orbit: baselines from ~1000 km to ~ km Max. angular resolution: 7µas Two hydrogen masers onboard 40 Space-VLBI with RadioAstron Receivers onboard: P (330 MHz) L (1.6 GHz) C (5 GHz) K (18-25 GHz) 32 MHz bandwidth, dual-pol. 128 Mbps data rate Two-tracking stations (Puschino, Russia and Green Bank, USA) for receiving data 41 Space-VLBI with RadioAstron First fringes (RadioAstron Effelsberg; 18cm; 8000km baseline) Angular resolution record: 27µas (RadioAstron GBT; 1.3cm; km baseline) 42 Nearby radio galaxy 3C84 observed with RadioAstron 3C84 Ground-only Image at 5 GHz Savolainen et al. in prep. 3C84 Space-VLBI image at 5GHz 43 Nearby radio galaxy 3C84 observed with RadioAstron Core 1 mas 0.35 pc 0.3 mas 0.1 pc Hot spot Mach disk / reverse shock? ~2000r g Beam: 0.05x0.15 mas Savolainen et al. in prep. 44 mm-vlbi and Event Horizon Telescope One can also increase the angular resolution by decreasing the observing wavelength mm-vlbi Global mm-vlbi Array operates at 3.5 mm VLBA + IRAM telescopes + Effelsberg + Yebes + GBT + Onsala + Metsähovi Correlator in Bonn 45 mm-vlbi and Event Horizon Telescope How does a black hole look like? Simulations by Psaltis & Broderick Dark shadow inside a photon ring. Shadow size and shape encodes GR. 46 mm-vlbi and Event Horizon Telescope Event Horizon Telescope Project develops VLBI capability at 1.3mm and aims to image the shadow of the black hole at the center of our Galaxy. It s predicted size is ~40 microarcseconds. Radio astronomy lecture Images: S. Doeleman mm-vlbi and Event Horizon Telescope Two key development areas in the Event Horizon Telescope Project Phasing up ALMA array being commissioned Mark 6 recorder (16 Gbps) Data per session ~7 PetaBytes 48
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