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As galactic halos are not directly visible, there are many ambiguities regarding their composition and rotational velocity. Though most of the dark matter is non-baryonic, some fraction is, and it can be used to trace the halo rotation. Asymmetries in the CMB towards M31 had been seen in the Planck data and ascribed to the rotational Doppler shift of the M31 halo. Subsequently, the same methods were used in the direction of five other galaxies belonging to the Local Group, namely M33, M81, M82, NGC 5128, and NGC 4594. It had been proved that there could be stable clouds of gas and dust in thermal equilibrium with the CMB at 2.7 K, which had been called "virial clouds". In this paper, adopting this scenerio, an attempt is made to constrain the fraction of dust grains and gas molecules in the clouds. [ABSTRACT FROM AUTHOR]

The standard technique for very low-frequency gravitational wave detection is mainly based on searching for a specific spatial correlation in the variation of the times of arrival of the radio pulses emitted by millisecond pulsars with respect to a timing model. This spatial correlation, which in the case of the gravitational wave background must have the form described by the Hellings and Downs function, has not yet been observed. Therefore, despite the numerous hints of a common red noise in the timing residuals of many millisecond pulsars compatible with that expected for the gravitational wave background, its detection has not yet been achieved. By now, the reason is not completely clear and, from some recent works, the urgency to adopt new detection techniques, possibly complementary to the standard one, is emerging clearly. Of course, this demand also applies to the detection of continuous gravitational waves emitted by supermassive black hole binaries populating the Universe. In the latter case, important information could, in principle, emerge from the millisecond pulsars considered individually in a single-pulsar search of continuous GWs. In this context, the surfing effect can then be exploited, helping to select the best pulsars to carry out such analysis. This paper aims to clarify when the surfing effect occurs and describe it exhaustively. A possible application to the case of the supermassive black hole binary candidate PKS 2131–021 and millisecond pulsar J2145–0750 is also analyzed. [ABSTRACT FROM AUTHOR]

The rotation of galactic halos is a particularly difficult subject to be dealt with. It has been shown that CMB data toward nearby galaxies can be used to probe the galactic halo rotation and can be ascribed to cold molecular clouds populating the halos. We present some methods to study the physical properties and distribution of such molecular gas clouds in the M31 galaxy halo. [ABSTRACT FROM AUTHOR]

Timing analysis is a powerful tool used to determine periodic features of physical phenomena. Here we review two applications of timing analysis to gravitational microlensing events. The first one, in particular cases, allows the estimation of the orbital period of binary lenses, which in turn enables the breaking of degeneracies. The second one is a method to measure the rotation period of the lensed star by observing signatures due to stellar spots on its surface. [ABSTRACT FROM AUTHOR]

The emission of gravitational waves from a system of massive objects interacting on hyperbolic orbits is studied in the quadrupole approximation. Analytic expressions are derived for the gravitational radiation luminosity, the total energy output and the gravitational radiation amplitude. An estimation of the expected number of events towards different targets (i.e. globular clusters and the center of the Galaxy) is also given. In particular, for a dense stellar cluster at the galactic center, a rate up to one event per year is obtained. [ABSTRACT FROM AUTHOR]

Pulsar timing uses the highly stable pulsar spin period to investigate many astrophysical topics. In particular, pulsar timing arrays make use of a set of extremely well-timed pulsars and their time correlations as a challenging detector of gravitational waves. It turns out that pulsar timing arrays are particularly sensitive to ultra-low-frequency gravitational waves, which makes them complementary to other gravitational-wave detectors. Here, we summarize the basics, focusing especially on supermassive black-hole binaries and cosmic strings, which have the potential to form a stochastic gravitational-wave background in the pulsar timing array detection band, and the scientific goals on this challenging topic. We also briefly outline the recent interesting results of the main pulsar timing array collaborations, which have found strong evidence of a common-spectrum process compatible with a stochastic gravitational-wave background and mention some new perspectives that are particularly interesting in view of the forthcoming radio observatories such as the Five hundred-meter Aperture Spherical Telescope, the MeerKAT telescope, and the Square Kilometer Array. [ABSTRACT FROM AUTHOR]

THESEUS is an ESA space based project, which aims to explore the early universe by unveiling a complete census of Gamma-ray Burst (GRB) population in the first billion years. This goal is expected to be realized by the combined observations of its three instruments on board: the Soft X-ray Imager (SXI), the X and Gamma Imaging Spectrometer (XGIS), and the InfraRed Telescope (IRT). This last one will identify, localise, and study the afterglow of the GRBs detected by SXI and XGIS, and about 40% of its time will be devoted to an all-sky photometric survey, which will certainly detect a relevant number of extragalactic sources, including Quasars. In this paper, we focus on the capability of IRT-THESEUS Telescope to observe Quasars and, in particular, Quasars lensed by foreground galaxies. In our analysis we consider the recent results for the Quasar Luminosity Function (QLF) in the infrared band based on the Spitzer Space Telescope imaging survey. In order to estimate the number of lensed Quasars, we develop Monte Carlo simulations using the mass-luminosity distribution function of galaxies and the galaxy and Quasar redshift distributions. We predict about 2.14 × 105 Quasars to be observed during IRT-Theseus sky survey, and approximately 140 of them lensed by foreground galaxies. Detailed studies of these events would provide a powerful probe of the physical properties of Quasars and the mass distribution models of the galaxies. [ABSTRACT FROM AUTHOR]

There are different methods for finding exoplanets such as radial spectral shifts, astrometrical measurements, transits, timing, etc. Gravitational microlensing (including pixel-lensing) is among the most promising techniques with the potential of detecting Earth-like planets at distances about a few astronomical units from their host stars. Here we emphasize the importance of polarization measurements which can help to resolve degeneracies in theoretical models. In particular, the polarization angle could give additional information about the relative position of the lens with respect to the source. [ABSTRACT FROM PUBLISHER]

Abstract: General relativity (GR) can be probed by several tests in the weak gravitational field limit. On the contrary, very poor information exists about GR tests in strong gravitational fields. Here, we focus on the interaction of light rays with the strong gravitational field of a massive black hole and show that relativistic images may form. Hence, we calculate the shapes of shadows (mirages) forming just near BH horizons and discuss the possibility to estimate the black hole parameters (mass, spin and charge) by future astrometric missions. In 2007, the Radioastron space telescope will be launched and it will allow to evaluate those parameters for the black hole hosted at the center of our Galaxy. [Copyright &y& Elsevier]