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Centimeter Wavelength

Ever since it was developed, VLBI science has been sensitivity-limited. Even with phase-referencing techniques, which permit long coherent integrations on target sources, recording bandwidth limitations have made VLBI the poor cousin of connected element interferometry.  VLBI is sensitive only to high brightness temperature targets, and when this is combined with low absolute sensitivity, the range of targets and scientific investigations which can be conducted is constrained.

New wideband recording techniques, whose development and deployment across the world is being pioneered by Haystack, are changing this situation in dramatic fashion.  With the advent of multi-gigabit/sec recording capabilities, the full front-end bandwidth of the world's most sensitive radio telescopes is becoming accessible to VLBI.  It is now becoming possible to coherently combine telescope collecting area from across the globe in a coherent fashion and with the widest bandwidths.  The result is the most sensitive radio array ever operated, and recent efforts have yielded  world-record radio image noise levels of order 3 microJy/beam. Instead of being the poor cousin of connected-element interferometry as regards sensitivity, VLBI has now leapfrogged into being the most sensitive radio capability we have, bar none.

The NSF-funded UVLBI program at Haystack seeks to expand usable bandwidths to 4 Gbit/sec and beyond, and to equip large centimeter-wavelength telescopes such as Arecibo and the GBT with these recording systems.  At frequencies of 1.6 GHz and above, this will yield image noise levels in the 1 microJy range, and will open up exciting new areas of science. These areas include milliarcsecond-scale investigations of stellar radio emission, searches for faint gravitational lens images associated by black holes in galactic nuclei, tracking the expansion of radio afterglows from gamma ray bursts, and performing high precision parallax and proper motion measurements of large samples of pulsars.  Many other investigations are possible, thanks to the expansion of accessible targets enabled by the bandwidth and sensitivity improvements.

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