Search for Eγ⩾5×1013 eV γ-ray transients through the BAKSAN and EAS-TOP correlated data

Abstract A search for transient point sources of ultra-high-energy (UHE) γ-rays has been performed, based on the correlation of two extensive air shower arrays, BAKSAN (North Caucasus, 1700 m a.s.l., BAKSAN Neutrino Observatory, Russia) and EAS-TOP (Campo Imperatore, 2005 a.s.l., Laboratori Nazionali del Gran Sasso, Italy), which are located at very similar latitudes ( φ ≈43° N), and separated in longitude by Δλ ≈33.7°. The search has been conducted at primary energy E γ ⩾5×10 13 eV on the time scale of a daily source transit over the sky, observable in the northern hemisphere (19° δ The stability of the single detectors has been studied and verified up to high significance level. No coincident excess, not compatible with the expectations from statistical fluctuations, has been observed. The obtained upper limits to the rate of transient events, n γ , at 90% c.l., are • concerning the sky survey, in the declination band corresponding to the zenith, n γ /(Ωt) /(yr sr) with Φ γ ( E γ >5×10 13 eV)>2.0×10 −11 cm −2 s −1 and duration Δt • concerning the candidate sources, e.g., for Markarian 421: n γ / t Φ γ ( E γ >5×10 13 eV)>1.3×10 −11 cm −2 s −1 and Δt A coincident episode from Markarian 421, observed on 15 January, 1994, with expected chance imitation rate n ch = 0.01 is discussed.