Discovery of a Nonblazar Gamma-Ray Transient Source Near the Galactic Plane: GRO J1838−04

We report the discovery of a remarkable γ-ray transient source near the Galactic plane, GRO J1838-04. This source was serendipitously discovered by EGRET in 1995 June with a peak intensity of ~(4 ± 1) × 10 -6 photons cm -2 s -1 (for photon energies larger than 100 MeV) and a 5.9 σ significance. At that time, GRO J1838-04 was the second brightest γ-ray source in the sky. A subsequent EGRET pointing in 1995 late September detected the source at a flux smaller than its peak value by a factor of ~7. We determine that no radio-loud spectrally flat blazar is within the error box of GRO J1838-04. We discuss the origin of the γ-ray transient source and show that interpretations in terms of active galactic nuclei or isolated pulsars are highly problematic. GRO J1838-04 provides strong evidence for the existence of a new class of variable γ-ray sources.