Survey of ultra-high-energy gamma-ray emission in the magellanic clouds

Various objects in the Magellanic Clouds were monitored for ultra-high-energy (UHE) gamma-ray emission during 1988 and 1990 using Cerenkov technique at large zenith angles with the JANZOS facility. During 1990 the equipment was modified to allow a sky coverage of 7° × 23° at large zenith angles. This enabled most of the extent of the Large Magellanic Cloud to be surveyed at ultra-high energies. An examination of the data base yielded no clear eivdence for persistent UHE emission from SN 1987A and selected X-ray pulsars in the Magellanic Clouds. Upper limits on the UHE fluxes (in cm^-2 s^-1) above the given threshold energies were obtained as follows: SMC X-1, 2.6 × 10^-13 (>30 TeV); SN 1987A, 3.8 × 10^-13 (>65 TeV); PSR 0540-693, 3.7 × 10^-13 (>65 TeV); A0538-66, 1.1 × 10^-13 (>130 TeV); LMC X-4, 3.5 × 10^-13 (> 140 TeV). We then examined the data base for pulsed UHE emission from PSR 0540 - 693 at the well-determined X-ray period (50 ms); and for the X-ray binary systems SMC X-1 and LMC X-4, we carried out pulsar periodicity searches on a night-by-night basis near their respective X-ray periods. No statistically significant evidence for pulsed UHE emission was found for these objects. The data base was further examined for evidence of previously undetected UHE sources in the Magellanic Clouds, and no significant DC excesses were found. Finally, we obtained upper limits on the UHE gamma-ray emission from these galaxies as a whole of 1.9 × 10^-12 cm^-2 s^-1 msr^-1 (>30 TeV) for the SMC and 1.8 × 10^-13 cm^-2 s^-1 msr^-1 (>65 TeV) for the LMC.