Finding Gamma-Ray Blazars: Preparation for GLAST
David Sowards-Emmerd
Stanford Linear Accelerator Center / Stanford University
dse@stanford.edu
Additional authors: R. W. Romani, P. F. Michelson & J. S. Ulvestad
We have implemented a new method of identifying candidate blazar counterparts for gamma-ray sources from the Third EGRET (3EG) catalog, selecting flat spectrum radio sources, many with X-ray counterparts. This method was originally applied to all of the northern 3EG sources (|b| > 10 degrees). Subsequently, we extended the survey south to decl. = -40 degrees, creating a small 8.4 GHz VLA A-array survey targeting flat spectrum radio sources in the area covered by 3EG error boxes. Optical spectroscopy of candidates was obtained using the Hobby-Eberly Telescope. This survey has found dozens of new counterparts, increasing the fraction of 3EG sources (|b| > 10 degrees) with blazar identifications to nearly 70%. These new counterparts include sources extending to nearly double the redshift of the most distant 3EG IDs. Of equal interest, we find a subset of the 3EG catalog without any plausible blazar counterparts according to our selection criteria, which may represent a new population of gamma-ray emitters. In preparation for the GLAST mission, we have also created a radio/X-ray selected sample with properties similar to the 3EG countparts, but outside the 3EG error contours. Further, in an effort to extend detection of gamma-ray sources below the 3EG threshold, we have developed a new technique to quantify the significance of variable gamma-ray flux at candidate positions. For example, the non-3EG selected radio candidates have a small but significant excess of flux over the random background.
