Physics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.........4d&link_type=abstract
Thesis (PH.D.)--MASSACHUSETTS INSTITUTE OF TECHNOLOGY, 1995.Source: Dissertation Abstracts International, Volume: 56-06, Secti
Physics
2
Scientific paper
The technique of very long baseline interferometry (VLBI) allows data recorded simultaneously at distant telescopes to be combined to provide information on the smallest angular scales. To increase resolution we have continued an ongoing effort to raise frequencies. At a frequency of 86GHz, the coherence limitation of the atmosphere becomes severe and attention to accurate recovery of the complex visibility on each baseline is crucial. Traditional detection methods that do not take into account the coherence losses at high frequencies are inadequate to detect weak sources. The first goal of this project was to develop mm-VLBI capabilities in areas of both hardware and software. As a starting point, Chapter 2 reviews basic relations of interferometry to aid in later discussion. Chapter 3 details the design, construction, and testing of a polarization window for the Haystack antenna. Chapter 4 begins with a review of VLBI detection methods and then describes the new incoherent methods developed to lower detection thresholds for low coherence data. Application to mm-VLBI data is explained and methods of recovering phase information are discussed. The second part of the thesis was the scheduling and observation of a global 3mm-VLBI experiment. Chapter 5 gives the particulars of antennas, sources, scheduling, calibration and data reduction. Chapter 6 focuses on the source 3C111 which was included in the observation only as a test source. It is a nearby radio galaxy and with its widely space radio lobes is atypical of the core dominated sources that make up most of the source list. We examine the possibility that 3mm observations may be able to set constraints on its orientation, investigate physical parameters of the core, and find an improved position using VLBI observables. Chapter 7 describes observations of the galactic center source SgrA^*, thought to be an AGN in our own galaxy. A reanalysis of data taken in 1994 is performed and a new size limit for this compact object is found which is consistent with the scattering size for a point source in the ISM. Two different models of emission are discussed and one is shown to be inconsistent with observations. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.).
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