Patterns of variability in γ-ray blazars

Preliminary results of a systematic study on the simultaneous optical-to-X-rays variability in blazars are presented. Data from Swift observations of four bright γ-ray blazars (3C 279, ON +231, S5 0716+71, PKS 2155−304) have been analyzed, compared, and discussed. Specifically, 3C 279 shows a variable flattening in the low energy part of the X-ray spectrum that appears to be confined in a specific X-ray vs optical/UV fluxes region. Some implications are shortly analyzed.     

Lense-Thirring Precession in the Astrophysical Context

This paper surveys some of the astrophysical environments in which the effects of Lense-Thirring precession and, more generally, frame dragging are expected to be important. We concentrate on phenomena that can probe in situ the very strong gravitational field and single out Lense-Thirring precession in the close vicinity of accreting neutron stars and black holes: these are the fast quasi periodic oscillations in the X-ray flux of accreting compact objects. We emphasise that the expected magnitude of Lense-Thirring/frame dragging effects in the regions where these signals originate are large and thus their detection does not pose a challenge; rather it is the interpretation of these phenomena that needs to be corroborated through deeper studies. Relativistic precession in the spin axis of radio pulsars hosted in binary systems hosting another neutron star has also been measured. The remarkable properties of the double pulsar PSR J0737–3039 has opened a new perspective for testing the predictions of general relativity also in relation to the precession of spinning bodies.     

A genetic algorithm for Initial Orbit Determination from a too short arc optical observation

Optical observations constitute a source of angular measurements of a satellite pass. Commonly, these observations have short durations with respect to the satellite orbit period. As a consequence, the use of classical orbit determination algorithms, as Laplace, Gauss or Escobal methods, give very poor results. The present work faces with the problem of estimating the orbital parameters of an orbiting object using its optical streak acquired by a telescope or a high accuracy camera. In the paper a new technique is developed for the Initial Orbit Determination from optical data by exploiting the genetic algorithms. The algorithm works without restrictions on the observer location. A recent challenging problem is the Initial Orbit Determination with space-based observations. This work focuses on the problem of determinating the orbital parameters of a satellite from an orbiting observer in LEO, using short time observations. We present the results based on a simulation with the observer on a sun-synchronous orbit with a single observation of just 60 s. Monte Carlo simulations are presented with different levels of sensor accuracy to show the reliability of the algorithm. The algorithm is able to yield a candidate solution for each observation. The coplanar case is analyzed and discussed as well.     

Global validation of IRI TEC for high and medium solar activity conditions

The IRI model offers a choice of options for the computation of the electron density profile and electron content (TEC). Recently new options for the topside electron density profile have been developed, which have a strong impact on TEC. Therefore it is important to test massively the IRI and the new options with experimental data. A large number of permanent stations record dual frequency GPS data from which it is possible to obtain TEC values. Thirty-one worldwide distributed stations have been selected to investigate the capabilities of the IRI to reproduce experimental TEC. Data for years 2000 (high solar activity) and 2004 (medium solar activity) have been analyzed computing modeled values with the IRI-2001 and the IRI-2007-NeQuick topside options. It is found that IRI-2007-NeQuick option generally improves the estimate of the slant TEC, especially in the case of high latitudes stations during high solar activity.