A Systematic Search for Short-Term Variability of EGRET Sources

The Third EGRET Catalog of High-Energy Gamma-Ray Sources contains 170 unidentified sources, and there is great interest in the nature of these sources. One means of determining source class is the study of flux variability on timescales of days; pulsars are believed to be stable on these timescales, while blazars are known to be highly variable. In addition, previous work has demonstrated that 3EG J0241-6103 and 3EG J1837-0606 are candidates for a new gamma-ray source class. These sources near the Galactic plane display transient behavior but cannot be associated with any known blazars. Although many instances of flaring active galactic nuclei have been reported, the EGRET database has not been systematically searched for occurrences of short-timescale (~1 day) variability. These considerations have led us to conduct a systematic search for short-term variability in EGRET data, covering all viewing periods through proposal cycle 4. Six 3EG catalog sources are reported here to display variability on short timescales; four of them are unidentified. In addition, three noncatalog variable sources are discussed.