High-energy γ-ray light curve of PSR0531 + 21

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.