EGRET observation of the constellation Cygnus

The Compton Observatory was pointed at Cygnus twice during the full sky survey (5/30–6/8/91 & 8/8–8/15/91). The COS‐B sources 2CG 075+00 and 2CG 078+01 are detected by EGRET, and smaller error boxes have been obtained, but the sources are not yet identified at other wavelengths. An excess of counts is noted at the position of Cygnus X‐3 consistent with a point source. The corresponding flux is 8±2×10⁻⁷ cm⁻² s⁻¹ (E≳0.1 GeV)—a factor of 5 less than the flux reported by the SAS‐2 group for Cygnus X‐3. The observed intensities for the individual periods were consistent with no change in intensity between the first and second observations (the second observation began two weeks after the beginning of the July 1991 radio flare). More work is required to refine and verify the galactic diffuse model (which comprises at least 10 times more flux than Cygnus X‐3 in this direction) before a definitive Cygnus X‐ray point‐source result will be obtained. The 4.8 hr X‐ray periodicity of Cygnus X‐3 is not detected in extant EGRET data with a χ² test with 10 bins, but because of the large background, a sinusoidal periodic flux of 8×10⁻⁷ cm⁻² s⁻¹ (E≳0.1 GeV) or less cannot be excluded. The possibility of GeV emission from other objects in this direction (Cygnus X, or the Cygnus OB2 association) cannot yet be dimissed. The additional 15 weeks of Cygnus exposure planned for phase II of the mission will provide improved sensitivity for periodicity, point‐source detection, and localization.

The steep‐spectrum, radio‐galaxy Cygnus A was not detected (flux <2×10⁻⁷ cm⁻² s⁻¹, E≳0.1 Gev, with 95% confidence). An intense unidentified source (GRO J1837+59) was detected 26° off‐axis during the first observation at α(J2000)=279.4, δ=59.2 (with an error circle of radius 0.4° at 68% confidence, and 0.6° at 95% confidence). The flux is 7.9±1.3×10⁻⁷ cm⁻² s⁻¹, E≳0.1 GeV. The photon spectral index is −1.9±0.3. GRO J1837+59 was not detected during the second observation (when it was 37° off‐axis where the sensitivity is ≊0.1 of that for a source on‐axis). The flux upper limit is 6×10⁻⁷ cm⁻² s⁻¹, E≳0.1 GeV, with 95% confidence. By analogy to other high‐latitude sources seen by EGRET, this could be a blazar, although no known blazars are within 3° of this position. The 1987 Green Bank 4.85 GHz survey detected a 179 mJy source (18 39 42.7+58 50 22) within the GRO J1837+59 error circle which warrants further radio investigation.