Gravitational Microlensing by Globular Clusters

Stars in globular clusters can act either as sources for MACHOs (Massive Astrophysical Compact Halo Objects) located along the line of sight or as lenses for more distant background stars. Although the expected rate of microlensing events is small, such observations can lead to very useful results. In particular, one could get information on the shape of the galactic halo along different lines of sight, allowing to better constrain its total dark matter content. Moreover, on can also infer the total dark matter content of globular clusters, which is presently not well known. To this latter purpose, we analyse the microlensing events towards the galactic bulge, which lie close to the three globular clusters NGC 6522, NGC 6528 and NGC 6540. We find evidence that some microlensing events are indeed due to MACHOs located in the globular clusters, suggesting, therefore, that these clusters contain a significant amount of dark matter.Comment: 15 pages, 10 figures included. Accepted for publication in A &

Dark matter vs. modifications of the gravitational inverse-square law. Results from planetary motion in the solar system

Dark matter or modifications of the Newtonian inverse-square law in the solar-system are studied with accurate planetary astrometric data. From extra-perihelion precession and possible changes in the third Kepler's law, we get an upper limit on the local dark matter density, rho_{DM} < 3*10^{-16} kg/m^3 at the 2-sigma confidence level. Variations in the 1/r^2 behavior are considered in the form of either a possible Yukawa-like interaction or a modification of gravity of MOND type. Up to scales of 10^{11} m, scale-dependent deviations in the gravitational acceleration are really small. We examined the MOND interpolating function mu in the regime of strong gravity. Gradually varying mu suggested by fits of rotation curves are excluded, whereas the standard form mu(x)= x/(1+x^2)^{1/2} is still compatible with data. In combination with constraints from galactic rotation curves and theoretical considerations on the external field effect, the absence of any significant deviation from inverse square attraction in the solar system makes the range of acceptable interpolating functions significantly narrow. Future radio ranging observations of outer planets with an accuracy of few tenths of a meter could either give positive evidence of dark matter or disprove modifications of gravity.Comment: 7 pages, 4 figures, accepted for publication in MNRA

Cosmological Parameter Determination in Free-Form Strong Gravitational Lens Modeling

We develop a novel statistical strong lensing approach to probe the cosmological parameters by exploiting multiple redshift image systems behind galaxies or galaxy clusters. The method relies on free-form mass inversion of strong lenses and does not need any additional information other than gravitational lensing. Since in free-form lensing the solution space is a high-dimensional convex polytope, we consider Bayesian model comparison analysis to infer the cosmological parameters. The volume of the solution space is taken as a tracer of the probability of the underlying cosmological assumption. In contrast to parametric mass inversions, our method accounts for the mass-sheet degeneracy, which implies a degeneracy between the steepness of the profile and the cosmological parameters. Parametric models typically break this degeneracy, introducing hidden priors to the analysis that contaminate the inference of the parameters. We test our method with synthetic lenses, showing that it is able to infer the assumed cosmological parameters. Applied to the CLASH clusters, the method might be competitive with other probes.Comment: 11 pages, 5 figures. Accepted for publication in MNRA

The decay of highly excited open strings

The decay rates of leading edge Regge trajectory states are calculated for very high level number in open bosonic string theories, ignoring tachyon final states. The optical theorem simplifies the analysis while enabling identification of the different mass level decay channels. The main result is that (in four dimensions) the greatest single channel is the emission of a single photon and a state of the next mass level down. A simple asymptotic formula for arbitrarily high level number is given for this process. Also calculated is the total decay rate exactly up to N=100. It shows little variation over this range but appears to decrease for larger N. The formalism is checked in examples and the decay rate of the first excited level calculated for open superstring theories. The calculation may also have implications for high spin meson resonances