Evidence For Time Variability Of Pev Energy Flux From Cygnus-X-3 - Ooty Observations During The Period 1984-1987

An enhancement in the flux of radiation from Cyg X-3 at energies > 10(14) eV during 1986 April-May has been suggested by several observations. We have studied the observed rate of showers arriving from the direction of Cyg X-3 as a function of time from data collected with the EAS array at Ooty during 1984 June-1987 May. A total of 83 showers were observed in the source bin during the period 1986 March 2-April 21 against an expected number of 56.5. The chance probability for observing such an excess during a 13 week time inter val, which covers completely all observations of similar enhancements, has been estimated to be 1%. Ooty observations therefore support earlier suggestions on variability in the PeV flux from Cyg X-3 over time scale of few weeks.A study of the observed rate of "doubles" in Ooty data has shown that during the last 2 weeks, 1986 April 5-21, of the period of enhanced flux, the number of "doubles" observed from the direction of Cyg X-3 was 10 compared to the expected number of 1.5. Simulations carried out using the observed shower rate and its zenith angle dependence give the chance probability for this excess to be 8 x 10(-3). Ooty observations therefore suggest that a part of the emission at PeV energies was in the form of short duration bursts which lasted only few tens of minutes. Further evidence for short-duration activity was provided by the observation of a burst on 1985 June 19 when five showers were observed from the direction of Cyg X-3 within a time interval of 12 minutes. The chance probability for detection of such a burst in Ooty data on Cyg X-3 is 1.4 x 10(-3). The flux during this intense burst is estimated to be 2.4 x 10(-10) cm-2 s-1 at energies > 2.5 x 10(14) eV, which is more than two orders of magnitude larger than the time-averaged flux observed at Ooty.