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Things to do with Single Dish: VLBI

Learn VLBI concepts, interferometry basics, and scientific results. Explore single-dish applications and antenna correlators in radio astronomy.

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Things to do with Single Dish: VLBI

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  1. Things to do with Single Dish:VLBI • Outline: • Interferometry 101 - Basic Concepts • Very Long Baseline Interferometry (VLBI) • Some Scientific Results Tapasi Ghosh NAIC/Arecibo Observatory

  2. Incoming radiation tg=b.s/c s b.s b Amplifier Correlator Basic Concepts Two element interferometer: Young’s double slit experiment: Wavefront splitting at the two slits generates Huygens wavelets that produces interference fringes. Constructive interference occurs when path difference is an integer number of wavelengths. • b: baseline vector. • s: unit vector pointing to the source. • tg: geometric delay. • The correlator Antenna

  3. If the source is very small compared to the fringe half-spacing /(2B), we say it is unresolved. The output signal is then just the fringe pattern. (2B) radians output signal Basic Concepts By analogy to the double slit experiment, regions which would cause constructive and destructive interference can be considered “stripes” in the sky. As the source moves through it, it produces oscillating output signal. The angular resolution is now given by the fringe half-spacing 2B (in radian). west meridian east

  4. Basic Concept-contd. : Delay tracking • If the source size is comparable to, or greater than, the fringe spacing, there will be little change In the output signal -- the source is then said to be “resolved out”. • Since the fringe spacing is proportional to wavelength, different frequencies in the observing band will have slightly different fringe patterns. To counter this, a variable delay is added to the signal from one dish, causing the white light fringe to follow the source across the sky.

  5. However, by having the fringes move with the source, less information is available at a time about the source structure. So a phase shift of /2 is inserted in parallel to effectively shift the fringe pattern.This is done automatically by using a complex correlator, and we measure a quantity called the complex fringe visibility of the source, V(b). Basic Concept-contd. : Delay tracking Phase center

  6. The changing baseline geometry causes the fringes to change orientation with time! Basic Concept-contd. : Fringe rotation 03:00

  7. As the source moves across the sky due to Earth’s rotation, the baseline vector traces part of an ellipse in the (u,v) plane. Basic Concept - contd. : Visibility (u-v) plane Tracking a source across the sky provides more information than a “snapshot” observation, because the source is sampled with a variety of fringe spacings orientated at different angles to the source. One way to formalize this is to adopt the “view” of the baseline from the source. |B| = (u2 + v2)1/2 v (kl)  T2 u (kl) T1 T2 We obtain data for both (u,v) and (-u,-v) simultaneously, since the two antennas are interchangeable. So the ellipse completed in 12h, not 24!

  8. Basic Concept - contd. : Visibility  Brightness distribution For an extended source, each point in the source contributes to the fringe visibility I() = F -1 V(b)

  9. From a “Dirty” to “CLEAN” image, Dirty Beam Dirty Image Deconvolve Clean Image

  10. Radio interferometry with distant, physically unconnected, antennas High resolution – milliarcsecond (mas) or better Baselines up to an Earth diameter for ground-based VLBI Can extend to space (HALCA) Traditionally uses no IF/LO link between antennas Atomic clocks for time and frequency – usually H-masers Disc-based recorders for temporary data storage & transport Delayed correlation after shipment of disc packs Real time over fiber (eVLBI) is a developing option Can use available single-dish antennas No fundamental difference from linked interferometry What is VLBI?

  11. At 1.4 GHz, VLBA’s maximum resolution is 5 mas. At 86 GHz, VLBA’s resolution is ~0.1 mas. So VLBI allows us to study very compact sources VLBI: Resolution I12≈√[SEFD1 . SEFD2 ⁄t] VLBI: Image Sensitivity For 10 hr observing, dual polarization, = 16 MHz, the rms noise of the VLBA is ~40 m Jy/beam. VLBI: Baseline Sensitivity Ar-VLBA baseline is 10 times more sensitive than an intra-VLBA baseline. VLBI: Nominal Field of View Determined by the bandwidth, the primary beam of the biggest dish in the array, and the basic integration time. Ignoring the last two factors, f ≈ b (/). For 104 km baseline, and 50-MHz bandwidth at 6cm, (with a resolution of 1mas), the FOV is 120 mas.

  12. VLBA Global VLBI Global Single Dishes 10  25-m antennas across US. NRAO Antennas Baselines: 236 to 8611 km. Frequencies: 0.3 to 86 GHz. Resolution: ~0.1 mas at 86 GHz. EVN Antennas & Associates

  13. The High Sensitivity Array(HSA) VLBA+Y27+GBT+AR+EB

  14. Arecibo added to the VLBA => 4.5 times better sensitivity. VLBI @ Arecibo  March 2005: A successful 64-Mbps e-VLBI run with EVN of SN2001em at 18 cm.

  15. The ULIRG Arp 220 • VLBI observations reveal a population of luminous radio supernovae in each of Arp 220’s nuclei. • Array: Global VLBI • Frequency: 1.6 GHz Lonsdale et al. 2006

  16. NE C E The Seyfert-2 NGC 7674 NE W C J E SW 79.5 pc Beam=129 x 108 mas

  17. VLBI provides extremely high angular resolution. Recent trends are to move to ultra-wide bandwidth data recording and wide-field mapping. Use of large single-dishes (e.g. GBT and Arecibo) with VLBA provides significant boost in sensitivity and u-v coverage. Large single-dish telescopes with VLBA are invaluable for spectral line and transient source studies where bandwidths and integration times have natural limits. Concluding Remarks

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