Coded mask X-ray images of the Virgo cluster – I. Hard X-rays from the Seyfert galaxy NGC 4388

Spectrally resolved hard X-ray (2–32 keV) images of the Virgo cluster of galaxies were obtained with the University of Birmingham coded mask X-ray telescope on the Spacelab-2 mission. The images demonstrate that much of the hard X-ray emission previously reported from the cluster originates in NGC 4388 which has been described on the basis of optical observations as a type 2 Seyfert galaxy. Flux was detected from NGC 4388 in the energy range ≈ 2–18 keV and has a spectrum consistent with that expected from a type 1 Seyfert, but with a very high hydrogen column density. Its luminosity in the 2–10 keV energy-band is abut $$\approx {10}^{42}\,\text{erg}\,{\text{s}}^{-1}$$ (assuming a distance of 20 Mpc). When combined with recent optical detections of faint broad wings to the H α line, our results suggest that NGC 4388 should be reclassified as a narrow emission-line galaxy (NELG), a class of objects widely held to contain heavily obscured, low-luminosity type 1 Seyfert nuclei.