Search for TeV gamma-ray emission from Hercules X-1

Six years of observations of Hercules X-1 with the Whipple Observatory gamma-ray telescope have been subjected to a Fourier analysis to search for emission at the 0.8079 Hz neutron star frequency. Evidence for a signal is found at the 99.5% confidence level for data taken with the medium-resolution imaging camera (spanning the first 4 years, 1984-1987) with some indications of emission at frequencies blueshifted from the fundamental frequency. However, analysis of the high-resolution camera data base (the subsequent 2 years of data, 1988-1989) have failed to substantiate this effect. Selection of events on the basis of gamma-ray-like image parameters did not enhance the signal from the medium-resolution data nor produce any indication of a signal from the high-resolution data. The overall conclusion is that no statistically significant evidence for TeV gamma-ray emission was found in the Whipple Observatory data base when the 6 years of data are taken as a whole.