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 L X /L B 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 L X /L B 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.

Aims. We investigate the distribution of the luminous and the dark matter components in the isolated ellipticals NGC 7052 and NGC 7785, which are embedded in an emitting hot gas halo, by means of relevant X-ray and photometric data. Methods. To calculate the dark matter distribution in these rare objects, we performed an improved X-ray analysis of the XMM-Newton data of NGC 7785, and we used former results based on Chandra data of NGC 7052. For each object we also derived the stellar spheroid length scale from the surface photometry and the spheroid stellar mass from an analysis of the galaxy spectral energy distribution. Results. We find that a dark matter component is present in these objects. It is subdominant and mixed with the luminous matter inside the optical region half-light radius wide, while it dominates the gravitational potential at outer radii. On the whole, the dark halo structure is very similar to that found around spirals of comparable luminosity and it is well reproduced by a Burkert halo, while a Sérsic spheroid accounts well for the baryonic component.

The magnetic flux distribution is determined by the solution of the Grad-Shafranov equation. With differential rotation, i.e. the variation of the iso-rotation parameter, the shape of the light surface must be calculated in an iterative way. For the first time, we have calculated the force-free magnetic structure of truly two-dimensional, relativistic jets, anchored in a differentially rotating disk. Such an approach allows for a direct connection between parameters of the central source (mass, rotation) and the extension of the radio jet. We present an analytical estimate for the jet opening angle along the asymptotic branches of the light surface. In general, differential rotation of the iso-rotation parameter leads to an increase of the jet opening angle. Comparison to the M87 jet shows agreement in the collimation distance. We derive a light cylinder radius of the M87 jet of 50 Schwarzschild radii.Comment: 11 pags, 10 figs, Latex, accepted for Astron.Astroph., cfendt@aip.de, ememola@aip.d