The variable gamma-ray source 2CG 135+01

The EGRET instrument on board the Compton Gamma Ray Observatory has detected about 150 sources above 100 MeV. Slightly less than half of these sources have been identified either as active galactic nuclei (AGNs) associated with spectrally flat radio-loud sources of the blazar type or as isolated young/intermediate-age pulsars. Except for rotational modulation, the gamma-ray flux of pulsars does not display significant variation, whereas blazars show variability on timescales as short as similar to 7 hr. We show here that one of the most prominent unidentified gamma-ray sources near the Galactic plane, 2CG 135+01 (first discovered by the COS B satellite), does not fit into either of these two classes. By comparing different EGRET observations, we establish with 98.8% confidence that the gamma-ray flux from 2CG 135+01 is time variable. This is inconsistent with the constant flux observed from isolated gamma-ray pulsars. No potential spectrally flat radio-loud AGN counterpart exists for this source. We show that 2CG 135+01 belongs to a new class of variable gamma-ray emitters in the Galactic plane, a class that also includes the recently discovered nonblazar gamma-ray transient GRO J1838-04.