The diverse X-ray properties of four truly isolated elliptical galaxies: NGC 2954, NGC 6172, NGC 7052, and NGC 7785

Aims. We investigate the X-ray properties of four isolated elliptical galaxies, selected from the Updated Zwicky Catalog according to strict isolation criteria. Isolated galaxies are not influenced by the group/cluster environment, and their X-ray emission can be studied independently of the often overwhelming contribution of the hot intergalactic medium. They are therefore suited to studying the X-ray characteristics relative to their intrinsic properties. Methods. We analyzed our own XMM-Newton and archival Chandra data in detail for three objects, and derived, when possible, the spatial and spectral characteristics of each source. An upper limit for the fourth one was obtained from archival ASCA data. We compared their characteristics with those of other 23 isolated objects for which X-ray and optical data are available in the literature. We explored possible theoretical explanations to interpret our results. Results. In spite of our attempt to select very homogeneous objects, both in terms of optical properties and environmental characteristics, we find a wide range in X-ray luminosities and LX/LB ratios for the four objects: two of them show a hot gaseous halo, whereas no gas is detected in the other two, to a factor >10 in luminosity. In fact, we find a large spread in the LX/LB for all galaxies considered, suggesting that the presence of hot gas is not easily related to the optical luminosity or to the mass, even in isolated systems. Younger objects tend to be less luminous in X-rays than older systems. However, it appears that older objects could span a wide range in luminosities