We describe a new search for diffuse ultrahigh energy gamma-ray emission associated with molecular clouds in the galactic disk. The Chicago Air Shower Array (CASA), operating in coincidence with the Michigan muon array (MIA), has recorded over 2.2 × 10 9 air showers from April 4, 1990 to October 7, 1995. We search for gamma rays based upon the muon content of air showers arriving from the direction of the galactic plane. We find no significant evidence for diffuse gamma-ray emission, and we set an upper limit on the ratio of gamma rays to normal hadronic cosmic rays at less than 2.4×10 −5 at 310 TeV (90% confidence limit) from the galactic plane region: (50 • < ℓ II < 200 • ; −5 • < b II < 5 • ). This limit places a strong constraint on models for emission from molecular clouds in the galaxy. We rule out significant spectral hardening in the outer galaxy, and conclude that emission from the plane at these energies is likely to be dominated by the decay of neutral pions resulting from cosmic rays interactions with passive target gas molecules.

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We have carried out a high statistics (2ϫ10 9 events͒ search for ultrahigh energy ␥-ray emission from the x-ray binary sources Cygnus X-3 and Hercules X-1. Using data taken with the CASA-MIA detector over a five year period ͑1990-1995͒, we find no evidence for steady emission from either source. The derived 90 % C.L. upper limit to the steady integral flux of ␥ rays from Cygnus X-3 is ⌽(EϾ115 TeV͒Ͻ6.3ϫ10 Ϫ15 photons cm Ϫ2 sec Ϫ1 , and from Hercules X-1 it is ⌽(EϾ115 TeV)Ͻ8.5ϫ10 Ϫ15 photons cm Ϫ2 sec Ϫ1 . These limits are more than two orders of magnitude lower than earlier claimed detections and are better than recent experiments operating in the same energy range. We have also searched for transient emission on time periods of one day and 0.5 h and find no evidence for such emission from either source. The typical daily limit on the integral ␥-ray flux from Cygnus X-3 or Hercules X-1 is ⌽ daily (EϾ115 TeV)Ͻ2.0ϫ10 Ϫ13 photons cm Ϫ2 sec Ϫ1 . For Cygnus X-3, we see no evidence for emission correlated with the 4.8 h x-ray periodicity or with the occurrence of large radio flares. Unless one postulates that these sources were very active earlier and are now dormant, the limits presented here put into question the earlier results, and highlight the difficulties that possible future experiments will have in detecting ␥-ray signals at ultrahigh energies. ͓S0556-2821͑97͒07004-5͔ PACS number͑s͒: 98.70. Rz, 95.85.Pw, 96.40.Pq, 98.70.Sa