177 research outputs found
Microlensing towards different Galactic targets
No full textWe calculate the optical depth and the number of events due to gravitational microlensing towards the Galactic bulge, the spiral arm directions gamma Scutum, beta Scutum, gamma Normae, vartheta Muscae and some dwarf galaxies in the halo of the Galaxy. Using the events found by the MACHO collaboration during their first year of observation towards Baade's Window we estimate the mass functions for the bulge and disk populations following the mass moment method. We find that the mass function can be described by a decreasing power-law with slope alpha =~ 2.0 in both cases and a minimal mass of ~ 0.01 Msun for the bulge and ~ 0.02 Msun for the disk, respectively. Assuming that the obtained mass function for the disk is also valid in the spiral arms, we find that the expected number of events towards the spiral arms is in reasonable agreement with the observations. However, the small number of observed events does not yet constrain much the different parameters entering in the computation of the mass function. To study the influence of the Magellanic Clouds on the shape and the velocity dispersion in the halo we perform a N-body simulation. We find that their presence induces a slight flattening of the halo (qH =~ 0.8). As a result the expected number of microlensing events towards some targets in the halo, such as the LMC or the SMC, decreases by about 20%, whereas due to the modification induced on the velocity dispersion the event duration increases
Constraints from the CMB temperature and other common observational data sets on variable dark energy density models
No full textThe thermodynamic and dynamical properties of a variable dark energy model with density scaling as ρx∝(1+z)m, z being the redshift, are discussed following the outline of Jetzer et al. [ P. Jetzer, D. Puy, M. Signore and C. Tortora Gen. Relativ. Gravit. 43 1083 (2011)]. These kinds of models are proven to lead to the creation/disruption of matter and radiation, which affect the cosmic evolution of both matter and radiation components in the Universe. In particular, we have concentrated on the temperature-redshift relation of radiation, which has been constrained using a very recent collection of cosmic microwave background (CMB) temperature measurements up to z∼3. For the first time, we have combined this observational probe with a set of independent measurements (Supernovae Ia distance moduli, CMB anisotropy, large-scale structure and observational data for the Hubble parameter), which are commonly adopted to constrain dark energy models. We find that, within the uncertainties, the model is indistinguishable from a cosmological constant which does not exchange any particles with other components. Anyway, while temperature measurements and Supernovae Ia tend to predict slightly decaying models, the contrary happens if CMB data are included. Future observations, in particular, measurements of CMB temperature at large redshift, will allow to give firmer bounds on the effective equation of state parameter weff of this kind of dark energy model.
© 2011 American Physical Societ
Effects of interplanetary dust on the LISA drag-free constellation
No full textThe analysis of non-radiative sources of static or time-dependent gravitational fields in the Solar System is crucial to accurately estimate the free-fall orbits of the LISA space mission. In particular, we take into account the gravitational effects of Interplanetary Dust (ID) on the spacecraft trajectories. The perturbing gravitational field has been calculated for some ID density distributions that fit the observed zodiacal light. Then we integrated the Gauss planetary equations to get the deviations from the LISA Keplerian orbits around the Sun. This analysis can be eventually extended to Local Dark Matter (LDM), as gravitational fields are expected to be similar for ID and LDM distributions. Under some strong assumptions on the displacement noise at very low frequency, the Doppler data collected during the whole LISA mission could provide upper limits on ID and LDM densities.
Cerdonio, Massimo; de Marchi, Fabrizio; de Pietri, Roberto; Jetzer, Philippe; Marzari, Francesco; Mazzolo, Giulio; Ortolan, Antonello; Sereno, Maur
Effective-one-body Hamiltonian with next-to-leading order spin-spin coupling
No full textWe propose a way of including the next-to-leading (NLO) order spin-spin coupling in an effective-one-body (EOB) Hamiltonian. This work extends by S. Balmelli and P. Jetzer, [Phys. Rev. D 87, 124036 (2013)], which is restricted to the case of equatorial orbits and aligned spins, to general orbits with arbitrary spin orientations. This is done by applying appropriate canonical phase-space transformations to the NLO spin-spin Hamiltonian in Arnowitt-Deser-Misner (ADM) coordinates, and systematically adding effective quantities at NLO to all spin-squared terms appearing in the EOB Hamiltonian. As required by consistency, the introduced quantities reduce to zero in the test-mass limit. We expose the result both in a general gauge and in a gauge-fixed form. The last is chosen such as to minimize the number of new coefficients that have to be inserted into the effective spin squared. As a result, the 25 parameters that describe the ADM NLO spin-spin dynamics get condensed into only 12 EOB terms
Limits on compact halo objects as dark matter from gravitational microlensing
No full textMicrolensing started with the seminal paper by Paczyński in 1986 [1], first with observations towards the Large Magellanic Cloud and the galactic bulge. Since then many other targets have been observed and new applications have been found. In particular, it turned out to be a powerful method to detect planets in our galaxy and even in the nearby M31. Here, we will present some results obtained so far by microlensing without being, however, exhaustive
Measuring polarization in microlensing events
No full textWe re-consider the polarization of the star light that may arise during microlensing events due to the high gradient of magnification across the atmosphere of the source star, by exploring the full range of microlensing and stellar physical parameters. Since it is already known that only cool evolved giant stars give rise to the highest polarization signals, we follow the model by Simmons et al. (2002) to compute the polarization as due to the photon scattering on dust grains in the stellar wind. Motivated by the possibility to perform a polarization measurement during an ongoing microlensing event, we consider the recently reported event catalog by the OGLE collaboration covering the 2001-2009 campaigns (OGLE-III events), that makes available the largest and more comprehensive set of single lens microlensing events towards the Galactic bulge. The study of these events, integrated by a Monte Carlo analysis, allows us to estimate the expected polarization profiles and to predict for which source stars and at which time is most convenient to perform a polarization measurement in an ongoing event. We find that about two dozens of OGLE-III events (about 1 percent of the total) have maximum polarization degree in the range 0.1 < P_{\rm max} <1 percent, corresponding to source stars with apparent magnitude I < 14.5, being very cool red giants.This signal is measurable by using the FORS2 polarimeter at VLT telescope with about 1 hour integration time
General relativity tests with space clocks in highly elliptic orbits
No full text The test of the Einstein Equivalence Principle (EEP) is of crucial importance as a deviation from it could hint to quantum effects in gravity or to unification with the other fundamental forces. One aspect of EEP is the local position invariance (LPI), which can be tested by measuring the gravitational red-shift. As an example of a possible space mission which could test the EEP, we will discuss a recently proposed satellite experiment, Einstein Gravitational RedShift Probe (E-GRIP), with the aim to test LPI using an hydrogen maser atomic clock on a highly elliptic orbit around Earth and compare the on-board clock to clocks located on Earth via a microwave link. </jats:p
Gravitational wave detection from space
No full textWe review the expected science performance of the satellites eLISA (evolved Laser Interferometer Space Antenna) for the detection of gravitational waves
An analysis of the M31 microlensing events
No full textWe discuss the results obtained so far by microlensing towards the Andromeda galaxy. In particular, we focus on the events detected by the POINT-AGAPE and MEGA collaborations. The POINT-AGAPE collaboration concluded that at least 20% of the halo mass in the direction of
M31 should be in the form of MACHOs, whereas the MEGA collaboration, although finding more events, claimed that their result is compatible with all events being due to self-lensing. We consider the spatial and time-duration distributions of the microlensing events for several mass
distribution models of the M31 bulge. We find that only for extreme models of theM31 luminous components it is possible to reconcile the total observed MEGA events with the expected selflensing contribution. Moreover, the expected spatial distribution of self-lensing events is more
concentrated and hardly agrees with the distribution observed by the MEGA collaboration. Nevertheless, the small number of events at disposal so far does not allow to draw firm conclusions on the halo dark matter fraction in the form of MACHOs
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