Hyperstrong Radio‐Wave Scattering in the Galactic Center. I. A Survey for Extragalactic Sources Seen through the Galactic Center

The scattering diameters of Sgr A* and several nearby OH masers (~ 1" at 1GHz) indicate that a region of enhanced scattering is along the line of sightto the Galactic center. The scattering diameter of an extragalactic source seenthrough this scattering region is larger by the ratio (Sun-GCdistance)/(GC-scattering region separation). This ratio could be a factor of afew, if the scattering region is far from the GC and only a randomsuperposition with it, to more than 100, if the scattering region is within theGC. We have used the VLA to survey 10 (11) fields at 20 cm (6 cm) ranging from7' to 137' from Sgr A*. Our objective was to identify extragalactic sources andmeasure their scattering diameters so as to constrain this ratio. In order tofind sources within these fields, we have employed pdfCLEAN, an algorithm inwhich sources are identified in an image by comparing the intensity histogramof the image to that expected from a noise-only image. We found over 100sources, with the faintest sources being 3 mJy. The average number of sourcesper field is approximately 10, though fields close to Sgr A* tend to containfewer sources. In a companion paper we use this survey in assessing thelikelihood that the scattering region is within the GC. A number of Galactic sources are included in our source catalog. We discuss adouble-lobed source, potentially an X-ray quiet version of 1E1740.7-2942, ashell-like structure with a central point source, and a possible radiotransient.Comment: 36 pages with 6 figures in 7 PostScript files; LaTeX2e with AASTeX macro aaspp4, to be published in ApJS; two tables in separate files which use apjpt4 and should be printed in landscape mod