HEAO 1 high energy X-ray observations of the Virgo cluster and A2142

We report observations of the Virgo cluster and Abell 2142 taken with the UCSD/MIT hard X-ray and low energy γ-ray instrument (A-4) on board the HEAO 1 spacecraft, during 1977 and 1978. We observe a mean flux from the Virgo cluster of (1.3 +- 0.3) x 10-3 photons cm-2 s-1 between 20 and 100 keV. We also present the complete spectrum of this cluster from 0.2 to 100 keV, using data from both the A-2 and A-4 detectors on HEAO 1. Our data are best fitted by a two component model, a thermal with kT = 2 keV and a power law. We derive a limit of EM< or =1.2 x 1065 cm-3 for the emission measure of any moderate temperature (kTapprox.6 keV) gas. For A2142 we obtain a marginally significant (15--40) keV flux of (2 +- 0.8) x 10-3 photons cm-2 s-1 and a 2 sigma upper limit to the 40--150 keV flux of <2 x 10-4 photons cm-2 s-1. We use our results to derive limits to the intracluster magnetic field of B> or approx. =5 x 10-7 gauss and B> or approx. = x 10-8 gauss in the Virgo cluster and A2142, respectively. We also discuss our results in terms of possible variability of the hard X-ray emission in these two clusters