Search

Multi-messenger observations of the transient sky to detect cosmic explosions and counterparts of gravitational wave mergers critically rely on orbiting wide-FoV telescopes to cover the wide range of wavelengths where atmospheric absorption and emission limit the use of ground facilities. Thanks to continuing technological improvements, miniaturised space instruments operating as distributed-aperture constellations are offering new capabilities for the study of high energy transients to complement ageing existing satellites. In this paper we characterise the performance of the upcoming joint SpIRIT + HERMES-TP/SP nano-satellite constellation for the localisation of high-energy transients through triangulation of signal arrival times. SpIRIT is an Australian technology and science demonstrator satellite designed to operate in a low-Earth Sun-synchronous Polar orbit that will augment the science operations for the equatorial HERMES-TP/SP. In this work we simulate the improvement to the localisation capabilities of the HERMES-TP/SP when SpIRIT is included in an orbital plane nearly perpendicular (inclination = 97.6$^\circ$) to the HERMES orbits. For the fraction of GRBs detected by three of the HERMES satellites plus SpIRIT, the combined constellation is capable of localising 60% of long GRBs to within ~ 30 deg$^2$ on the sky, and 60% of short GRBs within ~ 1850 deg$^2$. Based purely on statistical GRB localisation capabilities (i.e., excluding systematic uncertainties and sky coverage), these figures for long GRBs are comparable to those reported by the Fermi GBM. Further improvements by a factor of 2 (or 4) can be achieved by launching an additional 4 (or 6) SpIRIT-like satellites into a Polar orbit, which would both increase the fraction of sky covered by multiple satellite elements, and enable $\geq$ 60% of long GRBs to be localised within a radius of ~ 1.5$^\circ$ on the sky., Comment: 17 pages, 10 figures, 1 table. Accepted for publication in PASA

HERMES is a scientific mission composed of 3U nano-satellites dedicated to the detection and localization of high-energy astrophysical transients, with a distributed space architecture to form a constellation in Earth orbits. The space segment hosts novel miniaturized detectors to probe the X-ray temporal emission of bright events, such as Gamma-Ray Bursts (GRBs), and the electromagnetic counterparts of Gravitational Wave Events (GWEs), playing a crucial role in future multi-messenger astrophysics. During operations, at least three instruments, separated by a minimum distance shall observe a common area of the sky to perform a triangulation of the observed event. An effective detection by the nano-satellite payload is achieved by guaranteeing a beneficial orbital and pointing configuration of the constellation. The design has to cope with the limitations imposed by small space systems, such as the lack of on-board propulsion and the reduced systems budgets. The paper describes the methodologies and the proposed strategies to overcome the mission limitations, while achieving a satisfactory constellation visibility of the sky throughout the mission duration. The mission design makes use of a high-fidelity orbit propagator, combined with an innovative mission analysis tool that estimates the scientific performances of the constellation. The influence of the natural relative motion, which is crucial to achieve an effective constellation configuration without on-board orbit control, is assessed. The presented methodology can be easily extended to any kind of distributed scientific space applications, as well as to constellations dedicated to Earth and planetary observation. In addition, the visibility tool is applicable in the context of the constellation flight dynamics operations, yielding optimized results and pointing plans based on actual satellite orbital positions., Comment: 31 pages, 18 figures, 3 tables. Published in Journal of Astronomical Telescopes, Instruments, and Systems

A dynamical system involving a driven pendulum filled with liquid, is analyzed in the present paper series. The study of such a system is conducted in order to understand energy dissipation resulting from the shallow water sloshing and induced wave breaking. This analysis is relevant for the design of Tuned Liquid Damper devices. The complexity and violence of the flow generated by the roll motion results in the impossibility of using an analytical approach, requiring in turn the use of a suitable numerical solver. In Part I, the coupled dynamical system is thoroughly described, revealing its nonlinear features associated with the large amplitude of the forcing, both in terms of mechanical and fluid dynamical aspects. A smoothed particle hydrodynamics (SPH) model, largely validated in literature, is used to calculate the frequency behavior of the whole system. For small rotation angles, a semi-analytical model of the energy dissipated by the fluid, based on a hydraulic jump solution, is developed; the energy transfer is numerically calculated in order to extend the analysis to large oscillation angles. The experimental part of the investigation is carried out in Part II of this work., Comment: 20 pages, The following article has been published in Physics of Fluids http://dx.doi.org/10.1063/1.4869233

In Part I of this paper series, a theoretical and numerical model for a a driven pendulum filled with liquid was developed. The system was analyzed in the framework of tuned liquid dampers (TLD) and hybrid mass liquid dampers (HMLD) theory. In this Part II, in order to measure the energy dissipation resulting from shallow water sloshing, an experimental investigation is conducted. Accurate evaluations of energy transfers are obtained through the recorded kinematics of the system. A set of experiments is conducted with three different liquids: water, sunflower oil and glycerine. Coherently with the results of Part I, the energy dissipation obtained when the tank is filled with water can mainly be explained by the breaking waves. For all three liquids, the effects of varying the external excitation amplitude are discussed., Comment: 22 pages, The following article has been published in Physics of Fluids http://dx.doi.org/10.1063/1.4869234