Accreting X-ray millisecond pulsars observed with INTEGRAL

I review the properties of three X-ray accreting millisecond pulsars observed with INTEGRAL. Out of seven recently discovered accretion-powered pulsars (one discovered by INTEGRAL), three were observed with the INTEGRAL satellite up to 300 keV. Detailed timing and spectral results will be presented, including data obtained during the most recent outburst of the pulsar HETE J1900.1-2455. Accreting X-ray millisecond pulsars are key systems to understand the spin and accretion history of neutron stars. They are also a good laboratory in which to study the source spectra, pulse profile, and phase shift between X-ray pulses in different energy ranges which give additional information of the X-ray production processes and emission environment.Comment: 8 pages, 7 figures, 1 table, accepted for publication in ESA SP-622. Proceeding of the 6th INTEGRAL Workshop, July 3-7, 2006 Moscow, Russi

Influence of tyre pressure on vehicle steady-state lateral dynamics

This M.Sc. thesis is the result of experience made at the Department of Engineering Design and Production at Aalto University School of Engineering. It examines the steady-state turning of a real vehicle using the handling diagram theory. A test vehicle, equipped with a number of sensors, is used. This was first modelled with the simulation software CarMaker® and then the results obtained were compared to experimental data acquired during proving ground tests. The understeering-oversteering behavior of the vehicle was analysed. Moreover, the influence that tyre inflation pressure has on the modification of the handling diagram is shown. Thanks to this approach, the inflation pressure is considered as a new variable for the handling diagram, and so a three-dimensional graphic was built. Two sets of tyres were used with different inflation pressures; front and rear axle tyre pressure influences were considered separately. This leads to several combinations which generate different handling surfaces. These results are presented to illustrate the characteristic of this analysis. Il presente lavoro di tesi e’ il risultato dell’esperienza condotta presso il Department of Engineering Design and Production presso la Aalto University School of Engineering. Va ad esaminare il comportamento in curva in condizioni stazionarie di un veicolo reale facendo uso della teoria del diagramma di maneggevolezza. Si e’ utilizzato un veicolo strumentato, equipaggiato con un certo numero di sensori. E’ stata effettutata prima una modellazione di questo per mezzo del software di simulazione CarMaker®; i dati sperimentali ottenuti durante le prove effettuate in pista sono stati simulati all’interno dell’ambiente di modellazione e i risultati successivamente paragonati. E’ stato quindi analizzato il comportamento sovra-sottosterzante del veicolo. Inoltre, e’ stata data particolare attenzione all’influenza che la pressione di gonfiaggio dei pneumatici ha sulle modifiche del diagramma di maneggevolezza. Grazie a questo approccio, la pressione di gonfiaggio viene considerata come una nuova variabile per il tracciamento del diagramma, in maniera tale da costruire un diagramma tridimensionale. Sono stati usati due set di pneumatici con diverse pressioni di gonfiaggio ognuno; inoltre, sono considerate separatamente le influenze esercitate dalla pressione dei pneumatici anteriori e quelli posteriori. Cio’ porta alla nascita di varie combinazioni che a loro volta generano diverse superfici di maneggevolezza. Tali risultati sono quindi mostrati per meglio illustrare le caratteristiche di questa analisi

Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17498-2921

We analyze the spectral and timing properties of IGR J17498-2921 and the characteristics of X-ray bursts to constrain the physical processes responsible for the X-ray production in this class of sources. The broad-band average spectrum is well-described by thermal Comptonization with an electron temperature of kT_e ~ 50 keV, soft seed photons of kT_bb ~ 1 keV, and Thomson optical depth \taut ~ 1 in a slab geometry. The slab area corresponds to a black body radius of R_bb ~9 km. During the outburst, the spectrum stays remarkably stable with plasma and soft seed photon temperatures and scattering optical depth that are constant within the errors. This behavior has been interpreted as indicating that the X-ray emission originates above the neutron star (NS) surface in a hot slab (either the heated NS surface or the accretion shock). The INTEGRAL, RXTE, and Swift data reveal the X-ray pulsation at a period of 2.5 milliseconds up to ~65 keV. The pulsed fraction is consistent with being constant, i.e. energy independent and has a typical value of 6-7%. The nearly sinusoidal pulses show soft lags that seem to saturate near 10 keV at a rather small value of ~ -60\mu s with those observed in other accreting pulsars. The short burst profiles indicate that there is a hydrogen-poor material at ignition, which suggests either that the accreted material is hydrogen-deficient, or that the CNO metallicity is up to a factor of about two times solar. However, the variation in the burst recurrence time as a function of \dot{m} (inferred from the X-ray flux) is much smaller than predicted by helium-ignition models.Comment: 9 pages, 8 figures, accepted for publication in A&A. arXiv admin note: text overlap with arXiv:1012.022

Approximate analytical calculations of photon geodesics in the Schwarzschild metric

We develop a method for deriving approximate analytical formulae to integrate photon geodesics in a Schwarzschild spacetime. Based on this, we derive the approximate equations for light bending and propagation delay that have been introduced empirically. We then derive for the first time an approximate analytical equation for the solid angle. We discuss the accuracy and range of applicability of the new equations and present a few simple applications of them to known astrophysical problems.Comment: 8 pages, 10 Figures; Received: 08 June 2016 / Accepted: 04 August 2016and accepted from A&

Aggressive Quadrotor Flight through Narrow Gaps with Onboard Sensing and Computing using Active Vision

We address one of the main challenges towards autonomous quadrotor flight in complex environments, which is flight through narrow gaps. While previous works relied on off-board localization systems or on accurate prior knowledge of the gap position and orientation, we rely solely on onboard sensing and computing and estimate the full state by fusing gap detection from a single onboard camera with an IMU. This problem is challenging for two reasons: (i) the quadrotor pose uncertainty with respect to the gap increases quadratically with the distance from the gap; (ii) the quadrotor has to actively control its orientation towards the gap to enable state estimation (i.e., active vision). We solve this problem by generating a trajectory that considers geometric, dynamic, and perception constraints: during the approach maneuver, the quadrotor always faces the gap to allow state estimation, while respecting the vehicle dynamics; during the traverse through the gap, the distance of the quadrotor to the edges of the gap is maximized. Furthermore, we replan the trajectory during its execution to cope with the varying uncertainty of the state estimate. We successfully evaluate and demonstrate the proposed approach in many real experiments. To the best of our knowledge, this is the first work that addresses and achieves autonomous, aggressive flight through narrow gaps using only onboard sensing and computing and without prior knowledge of the pose of the gap

PAMPC: Perception-Aware Model Predictive Control for Quadrotors

We present the first perception-aware model predictive control framework for quadrotors that unifies control and planning with respect to action and perception objectives. Our framework leverages numerical optimization to compute trajectories that satisfy the system dynamics and require control inputs within the limits of the platform. Simultaneously, it optimizes perception objectives for robust and reliable sens- ing by maximizing the visibility of a point of interest and minimizing its velocity in the image plane. Considering both perception and action objectives for motion planning and control is challenging due to the possible conflicts arising from their respective requirements. For example, for a quadrotor to track a reference trajectory, it needs to rotate to align its thrust with the direction of the desired acceleration. However, the perception objective might require to minimize such rotation to maximize the visibility of a point of interest. A model-based optimization framework, able to consider both perception and action objectives and couple them through the system dynamics, is therefore necessary. Our perception-aware model predictive control framework works in a receding-horizon fashion by iteratively solving a non-linear optimization problem. It is capable of running in real-time, fully onboard our lightweight, small-scale quadrotor using a low-power ARM computer, to- gether with a visual-inertial odometry pipeline. We validate our approach in experiments demonstrating (I) the contradiction between perception and action objectives, and (II) improved behavior in extremely challenging lighting conditions