Physical sciences: New Point γ-ray Source Lib γ-1: Evidence for Time Variation and Possible Identification with PKS 1514-24

Abstract

DURING the past 2 yr evidence has been presented for the existence of four point sources of high energy γ-radiation (E_γ > 50 MeV). Three were found by the Case-Melbourne (C-M) collaboration using high altitude balloon flights from Australia (Fig. 1) and two of these sources, G_γ 2 + 3 and G_γ 341 + 1, coincided closely with known high energy X-ray sources^1,2. A group from the Moscow Physical Engineering Institute detected a source with a counter telescope on Cosmos 251³. Three aspects of the Russian source are particularly noteworthy: (1) the source position, 54° < α < 75°, +4° < δ < + 9°, contains the unusual Seyfert galaxy 3C 120⁴ which varies at both radio^5,6 and optical⁷ frequencies; (2) the γ-ray flux is F_γ=5 × 1O⁻⁴ γs cm⁻² s⁻¹ (E_γ > 100 MeV), a factor of 30 greater than any of the C-M sources; and (3) most of the detected γ-ray flux must have been due to photons above 500 MeV, because inserted between the counters of the Cosmos counter telescope were 3.7 radiation lengths of lead in which multiple scattering and energy loss greatly reduce the efficiency between 100 and 500 MeV. The Russian group's claim for time variability of 3C 120 as a γ-ray source, because it was not observed on earlier surveys, is not valid. The surveys in this energy range which were cited (refs. 8 and 9) did not cover this region of the sky. The OSO-3 detector (ref. 10), which did cover this area, has a sensitivity to point sources which is about the same as the reported flux from 3C 120.