PHASE COHERENCE FOR TEV/PEV BINARY SOURCES

There are reported detections of energetic gamma-rays from binary X-ray pulsars using the atmospheric Cerenkov technique (E almost-equal-to 10(12) eV, 1 TeV) or arrays of particle detectors (E almost-equal-to 10(15) eV, 1 PeV). Because the observations are ground-based, the data consist of time segments up to approximately 4 hr in length, separated by 1 or more days. For several sources, detection is based upon analyses in which the signal coherence time was assumed to be at least a few days to several months. In all cases where multiday coherence is claimed, a search was performed over a range of possible neutron-star spin frequencies. It is shown here that a frequency search with gapped data forces phase alignment of marginal signals (or noise) in the segments, allowing weak, incoherent signals (or excess noise) to appear as relatively strong coherent effects in a standard analysis. We argue that long-term phase coherence has not been established for binary TeV/PeV sources and propose a simple method by which it could be established.