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

A search for transient point sources of ultra-high-energy (UHE) gamma-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 (phi approximate to 43 degrees N), and separated in longitude by Delta lambda approximate to 33.7 degrees. The search has been conducted at primary energy E (gamma) greater than or equal to 5 x 10(13) eV on the time scale of a daily source transit over the sky, observable in the northern hemisphere (19 degrees < delta < 69 degrees), and for three sources, namely the Crab Nebula, Markarian 421 and Markarian 501, observed at TeV energies by atmospheric Cherenkov detectors. 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 gamma, at 90% c.l., are concerning the sky survey, in the declination band corresponding to the zenith, n gamma/(Omega t)< 12/(yr sr) with Phi(gamma),(E-gamma > 5 x 10(13) eV)> 2.0 x 10(-11) cm(-2)s(-1) and duration Delta t < 8 h; concerning the candidate sources, e.g., for Markarian 421: n gamma/t < 7.7/yr with Phi(gamma),(E-gamma, > 5 x 10(13) eV) > 1.3 x 10(11) cm(-2) s(-1) and Delta t < 7.5 h. cm A coincident episode from Markarian 421. observed on 15 January, 1994, with expected chance imitation rate n(ch) = 0.01 is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.