We discuss the possibility that the detection of gravitational waves emitted by compact stars may allow to constrain the MIT bag model of quark matter equation of state. Our results show that the combined knowledge of the frequency of the emitted gravitational wave and of the mass, or the radiation radius, of the source allows one to discriminate between strange stars and neutron stars and set ...

Context. The absolute calibration of space-borne instruments in the soft X-ray regime rests strongly on model spectra of hot white dwarfs. Aims. We analyze the Chandra LETG+HRC observations of the white dwarfs HZ43 A and Sirius B and of the neutron star RX J185635−3754 with the aim of resolving current uncertainties in the soft X-ray spectral fluxes and photospheric parameters of the three star...

We discuss efforts to determine the mass, radius, and surface composition of the nearby compact object RX J185635-3754 from its multi-wavelength spectral energy distribution. We compute non-magnetized model atmospheres and emergent spectra for selected compositions and gravities, and discuss efforts to fit existing and new observational data from ROSAT, EUVE and the HST. The spectral energy dis...

We present spectroscopy and imaging with the Very Large Telescope (VLT) of the isolated neutron star RX J1856.5−3754. Little is known about the nature of this source other than that it is a nearby hot neutron star. Our VLT spectrum does not show any strong emission or absorption features. With considerable care to photometric calibration, we obtain photometric measurements over the optical and ...

X-ray emission from the surface of isolated neutron stars (NSs) has been now observed in a variety sources. The ubiquitous presence pulsations clearly indicates that thermal photons either come limited area, possibly heated by some external mechanism, or entire (cooling) but with an inhomogeneous temperature distribution. In NS map is shaped magnetic field topology since heat flows crust mostly...

We present results of the analysis of data collected in 57-ks XMM-Newton and 505-ks Chandra observations of the nearby (≃ 120 pc) isolated neutron star RXJ1856.5–3754. We confirm most of the statements made by Burwitz et al. (2001) who discussed the original 55-ks Chandra data. Detailed spectral analysis of the combined X-ray and optical data rules out the currently available nonmagnetic light ...

A graph Γ is said to be locally (G, 2)-arc transitive for G a subgroup of Aut(Γ) if, for any vertex α of Γ, G is transitive on the 2-arcs of Γ starting at α. In this talk, we will discuss general results involving locally (G, 2)-arc transitive graphs and recent progress toward the classification of the locally (G, 2)-arc transitive graphs, where Sz(q) ≤ G ≤ Aut(Sz(q)), q = 2 for some k ∈ N. In ...

The cooling history of a quark star in the color superconductive phase is investigated. Here we specifically focus on the 2-flavour color (2SC) phase where novel process of photon generation via glueball (GLB) decay have been already investigated (Ouyed & Sannino 2001). The picture we present here can in principle be generalized to quark stars entering a superconductive phase where similar phot...

RX J0720.4−3125 is the third brightest neutron star in the soft X-ray sky and has been a source of mystery since its discovery, as its long 8-s period separates it from the population of typical radio pulsars. Three models were proposed for this source: a neutron star accreting from the interstellar medium, an off-beam radio pulsar, or an old, cooling magnetar. Using data from Chandra, ROSAT, a...