1. Department of Physics, Middlebury College , Middlebury, VT 05753 , USA
2. Space Telescope Science Institute , 3700 San Martin Drive, Baltimore, MD 21218 , USA
3. Osservatorio Astronomico di Roma (INAF) , via Frascati 33, I-00040 Monte Porzio Catone, Roma , Italy
4. National Radio Astronomy Observatory , Charlottesville, VA 22903 , USA