Search

The Crab pulsar (PSR0531 + 21) was one of the earliest identified sources of high-energy γ radiation1–6. As such it was selected as the first object of detailed study by ESA's γ-ray astronomy satellite COS B, launched in August 1975. The experiment and mission have been described elsewhere7. In the first 6 yr of operation COS B has made five observations (each of 30–40 days duration) in which PSR0531 + 21 was within 15° of the centre of the field of view. The first of these confirmed the result of the SAS 2 experiment that the γ-ray light curve for energies above a few tens of MeV consisted essentially of two equally strong peaks but the later measurements found a decrease in the strength of the second peak relative to the first. No change was observed in the 2–12 keV X-ray light curve simultaneously measured by COS B. The total data from the five observations contain the first evidence of interpulse emission between the two peaks in the energy range 50–3,000 MeV.

Bignami et al.1 have reported evidence for an increasing 59-s period in the X-ray emission from the high-energy (E ≳ 50 MeV) γ-ray source Geminga. Their claim is based on results of a search for periodicities at around 59s in the emission from the X-ray source 1EO630 + 178, which has been proposed as a possible counterpart of Geminga2. We show here that the primitive indication by SAS-2 (ref. 3) of a 59-s periodicity in the γ-ray flux from Geminga is not statistically significant, and because it has also not been confirmed by COS-B (ref. 4), must be disregarded; the statistical significance of the results obtained from periodicity searches in the X-ray data is significantly lower than that reported by Bignami et al.1. As a consequence, the identification of 1E0630 + 178 with the γ-ray source 2CG195 + 04 (Geminga) through a 59-s pulsation cannot be claimed.

THE discovery of a high-energy γ-ray source at α(1950) = 12h 29 min±6 min, δ(1950)= +3°±1.5° is reported here. Arguments are given for the identification with 3C273. If this identification is correct, the γ-ray luminosity of 3C273 in the energy range 50–500 MeV is 2×1046 erg s−1 for H0 = 60 km s−1 Mpc−1.