The status of ultra-high-energy gamma-ray observations

Abstract

In 1983 Samorski and Stamm found evidence of ultra-high-energy (UHE) gamma-ray eniission from Cygnus X-3 in archival data from the Kiel air shower array. The emission appeared to be emitted only at one phase in the binary orbit. The Kiel array was sensitive to air showers generated by particles with energies above 3 X 10 15 eV. Their discovery led to searches in archival data from other arrays for evidence of UHE emission from Cygnus X-3 and other potential sources in the Galaxy, and to the construction of new air shower arrays dedicated to UHE astronomy.In the mid 1980s many groups around the world claimed detections of Cygnus X-3, Hercules X-l, Vela X-l, and other objects. Cygnus X-3 is strongly variable at all wavelengths, and so it would be surprising if this variability did not extend to UHE gamma-rays also. This indeed appeared to be the case, and in the mid to late 1980s the UHE gamma-ray output showed a steady decline apart from transient UHE emission following giant radio flares. With the exception of possible detections above 10 17 eV by the Fly's Eye and Akeno Array, I am unaware of any statistically compefling claimed detections of this source reported over the past 4 years. The new arrays-CASA-MIA and SPASE-have not detected any of the claimed sources, and it seems likely that at least some of these early claims may have been premature. However, although none of the observations taken alone was of extremely high statistical significance, the observations of Cygnus X-3 by different groups showed a consistency which I feel cannot be ascribed to chance alone. The observations of Cygnus X-3 and the Crab Nebula/Pulsar are reviewed with particular reference to the results published during the last 5 years. Subject headings: gamma rays: observations -pulsars: individual (Crab Nebula)stars: individual (Cygnus X-3)

Cygnus X-3 in outburst: quenched radio emission, radiation losses and variable local opacity

“…Cyg X-3 is also a strong source of γ-rays, with claimed detections at up to PeV energies (see e.g. Bonnet-Bidaud & Chardin 1988;Protheroe 1994 for reviews). Radio mapping observations have revealed the presence of a milliarcsecond-scale bipolar jet with an expansion velocity of ∼ 0.35 c (at 10 kpc) following both giant-and small-flaring periods (Geldzahler et al 1983;Spencer et al 1986;Molnar, Reid & Grindlay 1988;Schalinkski et al 1995) as well as more extended structures on arcsecond- (Strom, van Paradijs & van der Klis 1989) and arcminutescales (Wendker, Higgs & Landecker 1991).…”

High statistics search for ultrahigh energy γ-ray emission from Cygnus X-3 and Hercules X-1

“…The apparent disappearance of Cygnus X-3 and Hercules X-1 from the ultra-high energy gamma-ray sky can be interpreted in two ways. An optimistic view [3] is that the earlier results indicated the presence of ultra-high energy gamma-rays (or particles) from Cygnus X-3 and Hercules X-1, and that the sources, which are episodic on long times scales, are now dormant. A more pessimistic view is that the earlier reported detections were largely, if not entirely, statistical fluctuations, and that no compelling evidence exists for ultra-high energy gamma-rays from any astrophysical source.…”

“…Often a statistically significant signal could only be extracted as a result of a periodicity analysis, where the data were phase-locked to a known source X-ray periodicity. In spite of these difficulties, the sheer number of reports made it difficult to dismiss the detections as being entirely due to statistical fluctuation [3]. In fact, by the late 1980's, it was generally established that Cygnus X-3 and Hercules X-1 were powerful emitters of high energy gamma-rays (although contrary interpretations of the data existed [4]).…”

High Energy Emission from Microquasars

“…On the other hand, these pulse periods are based on rather small numbers of photons. Furthermore, the statistical singificance of the pulse detection has been debated on the ground that the calculations of the signficance level have ignored the DC component of the emission (Protheroe 1994).…”