Microquasars are compact binary stars (a normal very massive star and a compact object), which have an accretion disk around the compact object and an intense and variable radio emission, normally as bipolar jets (symmetric jets of matter in opposite directions). The unusual characteristic of the discovered microquasar in M81 is that the speed of the ejected material is close to the speed of light (that is known as relativistic jets), with a measured velocity of 17% that of light. The main properties of this microquasar all point to a black hole accreting at rates far exceeding the critical

We provide a complete characterization of the astrophysical properties of the σ Ori Aa, Ab, B hierarchical triple system and an improved set of orbital parameters for the highly eccentric σ Ori Aa, Ab spectroscopic binary. We compiled a spectroscopic data set comprising 90 high-resolution spectra covering a total time span of 1963 days. We applied the Lehman-Filhés method for a detailed orbital analysis of the radial velocity curves and performed a combined quantitative spectroscopic analysis of the σ Ori Aa, Ab, B system by means of the stellar atmosphere code FASTWIND. We used our own plus

The two pyramids built during the Old Kingdom by the Fourth Dynasty King Sneferu at Dahshur are usually considered as two consecutive projects, the second – that of the Red Pyramid – being generated by a presumably failure of the first, the Bent Pyramid. In the present paper, we show that the archaeological proofs of such a scenario are far from obvious and that, on the contrary, a series of architectural, topographical, epigraphic, and astronomical hints point to a unitary project probably conceived from the very beginning in terms of the two pyramids and their annexes. Altogether the two

The one-way frontier around a black hole is the event horizon from which nothing can escape.  Close to it, strong effects of gravity and exotic physics are expected. The most massive black holes are at the centre of distant quasars, which cannot be resolved by telescopes. Fortunately, in some quasars aligned with an intervening galaxy, gravitational microlensing can magnify very small regions within the quasar allowing horizon scale resolution. We have used three events of gravitational microlensing that scan the source of a lensed quasar (Einstein Cross) to resolve a structure, affected by

Proving the magnetic configuration of solar spicules has hitherto been difficult due to the lack of spatial resolution and image stability during off-limb ground-based observations.  We report spectropolarimetric observations of spicules taken in the He I 1083 nm spectral region with the Tenerife Infrared Polarimeter II at the German Vacuum Tower Telescope at the Teide Observatory (Tenerife; Canary Islands; Spain). The data provide the variation with geometrical height of the Stokes I, Q, U, and V profiles whose encoded information allows the determination of the magnetic field vector by

The hypothesis of a universal initial mass function (IMF) - motivated by observations in nearby stellar systems - has been recently challenged by the discovery of a systematic variation of the IMF with the centralvelocity dispersion, σ, of early-type galaxies (ETGs), towards an excessof low-mass stars in high-σ galaxies. This trend has been derived so farfrom integrated spectra, and remains unexplained at present. To testwhether such trend depends on the local properties within a galaxy, we have obtained new, extremely deep, spectroscopic data, for three nearby ETGs, two galaxies with high σ