Observational Appearance of Black Holes in X-Ray Binaries and AGN

Accretion onto black holes powers most luminous compact sources in the Universe. Black holes are found with masses extending over an extraordinary broad dynamic range, from several to a few billion times the mass of the Sun. Depending on their position on the mass scale, they may manifest themselves as X-ray binaries or active galactic nuclei. X-ray binaries harbor stellar mass black holes—endpoints of the evolution of massive stars. They have been studied by X-ray astronomy since its inception in the early 60-ies, however, the enigma of the most luminous of them—ultra-luminous X-ray sources, still remains unsolved. Supermassive black holes, lurking at the centers of galaxies, are up to hundreds of millions times more massive and give rise to the wide variety of different phenomena collectively termed “Active Galactic Nuclei”. The most luminous of them reach the Eddington luminosity limit for a few billions solar masses object and are found at redshifts as high as z≥5–7. Accretion onto supermassive black holes in AGN and stellar- and (possibly) intermediate mass black holes in X-ray binaries and ultra-luminous X-ray sources in star-forming galaxies can explain most, if not all, of the observed brightness of the cosmic X-ray background radiation. Despite the vast difference in the mass scale, accretion in X-ray binaries and AGN is governed by the same physical laws, so a degree of quantitative analogy among them is expected. Indeed, all luminous black holes are successfully described by the standard Shakura-Sunyaev theory of accretion disks, while the output of low-luminosity accreting black holes in the form of mechanical and radiative power of the associated jets obeys to a unified scaling relation, termed as the “fundamental plane of black holes”. From that standpoint, in this review we discuss formation of radiation in X-ray binaries and AGN, emphasizing their main similarities and differences, and examine our current knowledge of the demographics of stellar mass and supermassive black holes.     

Time suboptimal formation flying manoeuvres through improved magnetic charged system search

The development of fast and reliable optimization algorithms is required in order to obtain real-time optimal trajectory on-board spacecraft. In addition, the wide spread of small satellites, due to their low costs, is leading to a greater number of satellite formations in space. This paper presents an Improved version of the Magnetic Charged System Search (IMCSS) metaheuristic algorithm to compute time-suboptimal manoeuvres for satellite formation flying. The proposed algorithm exploits some strategies aimed at improving the convergence to the optimum, such as the chaotic local search and the boundary handling technique, and it is able to self-tune its internal parameters and coefficients. Moreover, the inverse dynamics technique and the differential flatness approach, through the B-splines curves, are used to approximate the trajectory. The optimization procedure is applied to the circular J2 relative model developed by Schweighart and Sedwick and to the elliptical relative motion model developed by Yamanaka and Ankersen. The results of this paper show that the convergence is better achieved by using the proposed tools, thus proving the efficiency and reliability of the algorithm in solving some space engineering problems.     

The Galaxy Cluster Mass Scale and Its Impact on Cosmological Constraints from the Cluster Population

Space Science Reviews - The QB50 mission is a satellite constellation designed to carry out measurements at between 200–380 km altitude in the ionosphere. The multi-needle Langmuir probe...     

Thermo-mechanical design and testing of a microbalance for space applications

This work focuses on the thermo-mechanical design of the microbalance used for the VISTA (Volatile In Situ Thermogravimetry Analyzer) sensor. VISTA has been designed to operate in situ in different space environments (asteroids, Mars, icy satellites). In this paper we focus on its application on Mars, where the expected environmental conditions are the most challenging for the thermo-mechanical design.     

Solar-photon sail hovering orbits about single and binary asteroids

Solar-photon sails can be useful for missions towards and about asteroids. Indeed, for the interplanetary transfer phase, missions to asteroids often require a large variation in inclination and solar-photon sails perform very well for such high energy missions. In the same way, solar-photon sails are also expected to perform well in the phase about the asteroid. This paper studies single and binary asteroids’ hovering regions by using a sailcraft. In order to consider a sailcraft with its own mass and shape, the mutual polyhedral method (usually used to study asteroid dynamics) is used; therefore, the sailcraft is designed by means of tetrahedra. The procedure to obtain the hovering regions about a single asteroid is presented and an accurate analysis of the control variables is carried out. Moreover, control torques required to maintain hovering orbits are obtained by considering the gravitational torques acting on the sailcraft due to the asteroid. In the end, the theory for hovering orbits is extended to binary-asteroid systems and applied to the binary system 1999 KW4.     

Solar sail optimal control with solar irradiance fluctuations

The optimization of a solar sail-based orbital transfer amounts to searching for the control law that minimizes the flight time. In this context, the optimal trajectory is usually determined assuming constant solar properties. However, the total solar irradiance undergoes both long-term (solar cycles) and short-term variations, and recent analyses have shown that this may have an impact on solar sailing for missions requiring an accurate thrust modulation. In this regard, the paper discusses a strategy to overcome such an issue by suitably adjusting the thrust vector in order to track a reference, optimal, transfer trajectory. In particular, the sail propulsive acceleration magnitude is modified by means of a set of electrochromic material panels, which change their optical properties on application of a suitable electric voltage. The proposed control law is validated with a set of numerical simulations that involve a classical Earth-Mars, orbit-to-orbit, heliocentric transfer.     

Optimal steering law of refractive sail

The interaction between electromagnetic waves and matter is the working principle of a photon-propelled spacecraft, which extracts momentum from the solar radiation to obtain a propulsive acceleration. An example is offered by solar sails, which use a thin membrane to reflect the impinging photons. The solar radiation momentum may actually be transferred to matter by means of various optical phenomena, such as absorption, emission, or refraction. This paper deals with the novel concept of a refractive sail, through which the Sun’s light is refracted by crossing a film made of polymeric micro-prisms. The main feature of a refractive sail is to give a large transverse component of thrust even when the sail nominal plane is orthogonal to the Sun-spacecraft line. Starting from the recent literature results, this paper proposes a semi-analytical thrust model that estimates the characteristics of the propulsive acceleration vector as a function of the sail attitude angles. Such a mathematical model is then used to analyze a simplified Earth-Mars and Earth-Venus interplanetary transfer within an optimal framework.     

A Detailed Analysis of the Operational Orbit of the Hipparcos Satellite

An analysis of the orbital evolution of the ESA's Hipparcos satellite is presented. Hipparcos operated between August 1989 and March 1993 in a highly elliptical orbit: a geostationary transfer orbit with increased perigee height. The requirements of the scientific mission included high accuracy knowledge of the position and velocity vectors of the spacecraft as a function of time. Through a study of the variations in the total orbital energy, the loss of energy during the mission as a result of non-conservative forces is recovered. These are explained as largely due to atmospheric drag during perigee passages. Apparent variations in the drag coefficient are in agreement with orientation variations of the satellite during those perigee passages. Two different models used for calculating the atmospheric drag give significantly different results, confirming earlier findings by other users of those models. This revised version was published online in August 2006 with corrections to the Cover Date.     

Optimal solar sail trajectory approximation with finite Fourier series

The heliocentric transfer of a solar sail-based spacecraft is usually studied from an optimal perspective, by looking for the control law that minimizes the total flight time. The optimal control problem can be solved either with an indirect approach, whose solution is difficult to obtain due to its sensitivity to an initial guess of the costates, or with a direct method, which requires a good estimate of a feasible (guess) trajectory. This work presents a procedure to generate an approximate optimal trajectory through a finite Fourier series. The minimum time problem is solved using a nonlinear programming solver, in which the optimization parameters are the coefficients of the Fourier series and the positions of the spacecraft along the initial and target orbits. Suitable constraints are enforced on the direction and magnitude of the sail propulsive acceleration vector in order to obtain feasible solutions. A comparison with the numerical results from an indirect approach shows that the proposed method provides a good approximation of the optimal trajectory with a small computational effort.     

Relation algebras over containers and surfaces: An ontological study of a room space

Recent research in geographic information systems hasbeen concerned with the construction of algebras tomake inferences about spatial relations by embeddingspatial relations within a space in which decisionsabout compositions are derived geometrically. Wepursue an alternative approach by studying spatialrelations and their inferences in a concrete spatialscenario, a room space that contains such manipulableobjects as a box, a ball, a table, a sheet of paper,and a pen. We derive from the observed spatialproperties an algebra related to the fundamentalspatial concepts of containers and surfaces and showthat this container-surface algebra holds allproperties of Tarski's relation algebra, except forthe associativity. The crispness of the compositionscan be refined by considering the relative size of theobjects) and their roles (i.e., whether it isexplicitly known that the objects are containers orsurfaces).