A search for bursts of air-shower events from Hercules X-1 at energies above 50 TeV during the calendar period 2 April 1986 to 5 July 1987 yielded two significant bursts, both occurring on UT 24 July 1986. The events during these bursts were pulsed with a period of 1.235 68 s, significantly different from estimates of the contemporaneous x-ray period. The probability that this represents random statistical fluctuations of the background is estimated to be 2 × ${10}^{-5\mathrm{}}$. The muon content of the burst events is anomalous when compared with expectations from $\gamma$-ray showers.

• Received 24 June 1988

DOI:https://doi.org/10.1103/PhysRevLett.61.1906