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

Abstract

The scattering diameters of Sgr A* and several nearby OH masers (~ 1" at 1 GHz) indicate that a region of enhanced scattering is along the line of sight to the Galactic center. The scattering diameter of an extragalactic source seen through this scattering region is larger by the ratio (Sun-GC distance)/(GC-scattering region separation). This ratio could be a factor of a few, if the scattering region is far from the GC and only a random superposition with it, to more than 100, if the scattering region is within the GC. We have used the VLA to survey 10 (11) fields at 20 cm (6 cm) ranging from 7' to 137' from Sgr A*. Our objective was to identify extragalactic sources and measure their scattering diameters so as to constrain this ratio. In order to find sources within these fields, we have employed pdfCLEAN, an algorithm in which sources are identified in an image by comparing the intensity histogram of the image to that expected from a noise-only image. We found over 100 sources, with the faintest sources being 3 mJy. The average number of sources per field is approximately 10, though fields close to Sgr A* tend to contain fewer sources. In a companion paper we use this survey in assessing the likelihood that the scattering region is within the GC. A number of Galactic sources are included in our source catalog. We discuss a double-lobed source, potentially an X-ray quiet version of 1E1740.7-2942, a shell-like structure with a central point source, and a possible radio transient.