Testing the nature of the black hole candidate in GRO J1655-40 with the relativistic precession model

Quasi-periodic oscillations (QPOs) are a common feature in the X-ray flux of stellar-mass black hole candidates, but their exact origin is not yet known. Recently, some authors have pointed out that data of GRO J1655-40 simultaneously show three QPOs that nicely fit in the relativistic precession model. However, they find an estimate of the spin parameter that disagrees with the measurement of the disk's thermal spectrum. In the present work, I explore the possibility of using the relativistic precession model to test the nature of the black hole candidate in GRO J1655-40. If properly understood, QPOs may become a quite powerful tool to probe the spacetime geometry around black hole candidates, especially if used in combination with other techniques. It turns out that the measurements of the relativistic precession model and of the disk's thermal spectrum may be consistent if we admit that the black hole candidate in GRO J1655-40 is not of the Kerr type.Comment: 9 pages, 4 figures. v2: added new material and found different conclusion

Testing the Kerr-nature of stellar-mass black hole candidates by combining the continuum-fitting method and the power estimate of transient ballistic jets

Astrophysical black hole candidates are thought to be the Kerr black holes predicted by General Relativity, as these objects cannot be explained otherwise without introducing new physics. However, there is no observational evidence that the space-time around them is really described by the Kerr solution. The Kerr black hole hypothesis can be tested with the already available X-ray data by extending the continuum-fitting method, a technique currently used by astronomers to estimate the spins of stellar-mass black hole candidates. In general, we cannot put a constraint on possible deviations from the Kerr geometry, but only on some combination between these deviations and the spin. The measurement of the radio power of transient jets in black hole binaries can potentially break this degeneracy, thus allowing for testing the Kerr-nature of these objects.Comment: 8 pages, 6 figures. v2: some typos correcte

Can the supermassive objects at the centers of galaxies be traversable wormholes? The first test of strong gravity for mm/sub-mm VLBI facilities

The near future mm/sub-mm VLBI experiments are ambitious projects aiming at imaging the "shadow" of the supermassive black hole candidate at the center of the Milky Way and of the ones in nearby galaxies. An accurate observation of the shape of the shadow can potentially test the nature of these objects and verify if they are Kerr black holes, as predicted by general relativity. However, previous work on the subject has shown that the shadows produced in other spacetimes are very similar to the one of the Kerr background, suggesting that tests of strong gravity are not really possible with these facilities in the near future. In this work, I instead point out that it will be relatively easy to distinguish black holes from wormholes, topologically non-trivial structures of the spacetime that might have been formed in the early Universe and might connect our Universe with other universes.Comment: 5 pages, 2 figures. v2: some typos correcte

A code to compute the emission of thin accretion disks in non-Kerr space-times and test the nature of black hole candidates

Astrophysical black hole candidates are thought to be the Kerr black holes predicted by General Relativity, but the actual nature of these objects has still to be proven. The analysis of the electromagnetic radiation emitted by a geometrically thin and optically thick accretion disk around a black hole candidate can provide information about the geometry of the space-time around the compact object and it can thus test the Kerr black hole hypothesis. In this paper, I present a code based on a ray-tracing approach and capable of computing some basic properties of thin accretion disks in space-times with deviations from the Kerr background. The code can be used to fit current and future X-ray data of stellar-mass black hole candidates and constrain possible deviations from the Kerr geometry in the spin parameter-deformation parameter plane.Comment: 22 pages, 7 figures. v2: some typos correcte

Wagner's law and augmented Wagner's law in EU-27. A time-series analysis on stationarity, cointegration and causality

The relationship between public expenditure and aggregate income has long been debated in economic literature. According to Wagner, expenditure is an endogenous factor or an outcome. On the other hand, Keynes considered public expenditure as an exogenous factor to be used as a policy instrument to in-fluence growth. “Augmented” version of Wagner’s Law, where public deficit appears as further explanatory variable, is also investigated. The aim of this paper is to assess empirical evidence of these hypotheses in EU-27, for the period 1970-2009. After a brief introduction, a survey of the economic literature on this issue is offered, before evaluating some specifications of “Wagner’s Law” due to several researchers. Few notes on the empirical evidence’ comparisons conclude the paper.Wagner’s law; public expenditure; EU-27; correlation; unit root tests; cointegration analysis; causality