Energy spectra of very high energy γ-rays from the crab and vela pulsars

Using the atmospheric Cerenkov radiation technique we have observed pulsed emission of very high energy, Eγ≳1013 eV, γ‐rays from the Crab and Vela pulsars. Ooty Cerenkov telescope composed of 18 reflectors has been used to the ‘Distributed Mode’ for these observations during early 1981. In this mode the arrival direction of individual showers has been measured to an accuracy ∼0.3° which has made possible a more reliable estimation of the γ‐ray energy thresholds in the direction of these two pulsars. Observations on the Crab pulsar show evidence of sporadic emission with significant variability of the pulsed flux over periods as short as tens of minutes. The measured pulsed flux of 2.2±0.5×10−11 ph. cm−2 s−1 at energies ≳9×1011 eV during the active phase agrees well with the extrapolation of the Cos‐B spectra 2.9×10−11 E−1.17 ph.cm−2 s−1, both in amplitude and spectralslope. The time averaged flux is however considerably smaller. On the other hand the time averaged pulsed flux of 1.0±0.4×10−11 ph.cm−2 s−1 at energies ≳2.3×1012 eV for the Vela pulsar is about two orders of magnitude smaller than the extrapolated value from the Cos‐B spectrum, 1.8×10−7 EGeV−1,11 ph.cm−2 s−1. Also the energy spectrum for the Vela pulsar at energies ∼2−5×1012 eV with a slope of −2.2±0.2 is much steeper than at lower energies. These observations suggest significant differences between γ‐ray emission from these two pulsars.