Results on candidate UHE gamma-ray sources by the EAS-TOP array (1989–1993)

Abstract

Abstract A search for UHE gamma-rays from 13 candidate point sources observable in the northern hemisphere (the Crab Nebula and Pulsar, Cygnus X-3, Hercules X-1, Geminga and others) has been performed by the EAS-TOP array during four years of operation since January 1989 to December 1993, at different energy thresholds ( E 0 = 30–300 TeV). DC, periodic and sporadic emissions have been studied, and no evidence for significant excesses has been found from any of these searches. The derived 90% c.l. upper limits to the d.c. flux, for a source culminating at the zenith such as Cygnus X-3, are Φ ( E > 230 TeV) −14 cm −2 s −1 , Φ ( E > 90 TeV) −14 cm −2 s −1 and Φ ( E > 25 TeV) −13 cm −2 s −1 . The excess from the Crab Nebula of February 23, 1989, as reported by the EAS-TOP, Baksan and KGF arrays, remains the only sporadic excess detected with statistically significant confidence level (probability of background imitation ≈ 10 −5 ).