arXiv subjects

M. Sereno

Publications and source records attributed to M. Sereno.

At least 109 records · Page 6Linked to original sources

arXiv2006

Imprints of deviations from the gravitational inverse-square law on the power spectrum of mass fluctuations

Deviations from the gravitational inverse-square law would imprint scale-dependent features on the power spectrum of mass density fluctuations. We model such deviations as a Yukawa-like contribution to the gravitational potential and discuss the growth function in a mixed dark matter model with adiabatic initial conditions. Evolution of perturbations is considered in general non-flat cosmological models with a cosmological constant, and an analytical approximation for the growth function is provided. The coupling between baryons and cold dark matter across recombination is negligibly affected by modified gravity physics if the proper cutoff length of the long-range Yukawa-like force is > 10 h^{-1} Mpc. Enhancement of gravity affects the subsequent evolution, boosting large-scale power in a way that resembles the effect of a lower matter density. This phenomenon is almost perfectly degenerate in power-spectrum shape with the effect of a background of massive neutrinos. Back-reaction on density growth from a modified cosmic expansion rate should however also affect the normalization of the power spectrum, with a shape distortion similar to the case of a non-modified background.

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arXiv2006

The weakly perturbed Schwarzschild lens in the strong deflection limit

We investigate the strong deflection limit of gravitational lensing by a Schwarzschild black hole embedded in an external gravitational field. The study of this model, analogous to the Chang & Refsdal lens in the weak deflection limit, is important to evaluate the gravitational perturbations on the relativistic images that appear in proximity of supermassive black holes hosted in galactic centers. By a simple dimensional argument, we prove that the tidal effect on the light ray propagation mainly occurs in the weak field region far away from the black hole and that the external perturbation can be treated as a weak field quadrupole term. We provide a description of relativistic critical curves and caustics and discuss the inversion of the lens mapping. Relativistic caustics are shifted and acquire a finite diamond shape. Sources inside the caustics produce four sequences of relativistic images. On the other hand, retro-lensing caustics are only shifted while remaining point-like to the lowest order.

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arXiv2006

Solar and stellar system tests of the cosmological constant

Some tests of gravity theories - periastron shift, geodetic precession, change in mean motion and gravitational redshift - are applied in solar and stellar systems to constrain the cosmological constant. We thus consider a length scale range from 10^8 to 10^{15} km. Best bounds from the solar system come from perihelion advance and change in mean motion of Earth and Mars, Lambda < 10^{-36} km^{-2}. Such a limit falls very short to estimates from observational cosmology analyses but a future experiment performing radio ranging observations of outer planets could improve it by four orders of magnitude. Beyond the solar system, together with future measurements of periastron advance in wide binary pulsars, gravitational redshift of white dwarfs can provide bounds competitive with Mars data.

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arXiv2006

Measuring the Three-Dimensional Structure of Galaxy Clusters. II. Are clusters of galaxies oblate or prolate?

The intrinsic shape of galaxy clusters can be obtained through a combination of X-ray and Sunyaev-Zeldovich effect observations once cosmological parameters are assumed to be known. In this paper we discuss the feasibility of modelling galaxy clusters as either prolate or oblate ellipsoids. We analyze the intra-cluster medium distribution for a sample of 25 X-ray selected clusters, with measured Sunyaev-Zeldovich temperature decrements. A mixed population of prolate and oblate ellipsoids of revolution fits the data well, with prolate shapes preferred on a 60-76% basis. We observe an excess of clusters nearly aligned along the line of sight, with respect to what is expected from a randomly oriented cluster population, which might imply the presence of a selection bias in our sample. We also find signs that a more general triaxial morphology might better describe the morphology of galaxy clusters. Additional constraints from gravitational lensing could disentangle the degeneracy between an ellipsoidal and a triaxial morphology, and could also allow an unbiased determination of the Hubble constant.

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arXiv2006

Two-body problem with the cosmological constant and observational constraints

We discuss the influence of the cosmological constant on the gravitational equations of motion of bodies with arbitrary masses and eventually solve the two-body problem. Observational constraints are derived from measurements of the periastron advance in stellar systems, in particular binary pulsars and the solar system. Up to now, Earth and Mars data give the best constraint, Lambda < 10^{-36} km^{-2}; bounds from binary pulsars are potentially competitive with limits from interplanetary measurements. If properly accounting for the gravito-magnetic effect, this upper limit on $Λ$ could greatly improve in the near future thanks to new data from planned or already operating space-missions.

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arXiv2005

Analytic Kerr black hole lensing for equatorial observers in the strong deflection limit

In this paper we present an analytical treatment of gravitational lensing by Kerr black holes in the limit of very large deflection angles, restricting to observers in the equatorial plane. We accomplish our objective starting from the Schwarzschild black hole and adding corrections up to second order in the black hole spin. This is sufficient to provide a full description of all caustics and the inversion of lens mapping for sources near them. On the basis of these formulae we argue that relativistic images of Low Mass X-ray Binaries around Sgr A* are very likely to be seen by future X-ray interferometry missions.

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arXiv2005

Abell 370: A Cluster with a Pronounced Triaxial Morphology

We here combine Sunyaev-Zel'dovich effect, X-ray observations and spectroscopic redshifts of member galaxies, to constrain the intrinsic three-dimensional shape of the galaxy cluster: Abell 370. The cluster turns out to be strongly elongated along the l.o.s., with two (or more) substructures in the process of merging. Spectroscopy further suggests that the process must be taking place at a small angle respect to the l.o.s.

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arXiv2005

Measuring the Three-Dimensional Structure of Galaxy Clusters. I. Application to a Sample of 25 Clusters

We discuss a method to constrain the intrinsic three-dimensionale shapes of galaxy clusters by combining X-Ray and Sunyaev-Zeldovich observations. The method is applied to a sample of 25 X-Ray selected clusters, with measured Sunyaev-Zeldovich temperature decrements. The sample turns out to be slightly biased, with strongly elongated clusters preferentially aligned along the line of sight. This result demonstrates that X-Ray selected cluster samples may be affected by morphological and orientation effects even if a relatively high threshold signal-to-noise ratio is used to select the sample. A large majority of the clusters in our sample exhibit a marked triaxial structure, with prolate-like shapes being slightly more likely than oblate-like ones; the spherical hypothesis is strongly rejected for most sample members. Cooling flow clusters do not show preferentially regular morphologies.

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arXiv2005

On gravitational lensing by deflectors in motion

Gravitational lensing by a spinning deflector in translational motion relative to the observer is discussed in the weak field, slow motion approximation. The effect of rotation, which generates an intrinsic gravito-magnetic field, separates from that due to radial motion. Corrections to the lens equation, deflection angle and time delay are derived.

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arXiv2005

Modelling the Milky Way through adiabatic compression of cold dark matter halo

We use the adiabatic compression theory to build a physically well - motivated Milky Way mass model in agreement with the observational data. The visible mass of the Galaxy is distributed in a spheroidal bulge and a multi - components disc parametrized by three galactic parameters, the Sun distance to the galactic centre, R_0, the total bulge mass, M_{bulge}, and the local disc surface density, Σ_{\odot}. To model the dark matter component, we adiabatically compress a Navarro, Frenk and White (NFW) halo (with concentration $c$ and total mass M_{vir}) for fixed values of the spin parameter, λ, the fraction of the mass in baryons, $m_b$, and the thin disc contribution to total angular momentum, j_d. An iterative selection procedure is used to explore in very detail the wide space of parameters only selecting those combinations of {R_0, M_{bulge}, Σ_{\odot}, λ, m_b, j_b, c, M_{vir}} that give rise to a Milky Way model in agreement with the observational constraints. This analysis leads us to conclude that only models with R_0 = 8.5 kpc, $0.8 x 10^{10} M_{\odot} < M_{bulge} < 1.6 x 10^{10} M_{\odot}$ and $49 M_{\odot} pc^{-2} \le Σ_{\odot} \le 56 M_{\odot} pc^{-2}$ can be reconciled with the set of observational constraints. As regard the parameters entering the adiabatic compression, we find $0.03 \le λ\le 0.10$ and $0.04 \le m_b \le 0.10$, while the final estimates of the parameters describing the initial halo profile turn out to be $5 \stackrel{<}{\sim} c \stackrel{<}{\sim} 12$ and $7 x 10^{11} M_{\odot} \stackrel{<}{\sim} M_\mathrm{vir} \stackrel{<}{\sim} 17 x 10^{11} M_{\odot}$ (all at 95.7% CL).

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arXiv2005

Statistics of gravitational lenses in the clumpy Universe

We evaluate the effect of small scale inhomogeneities on large scale observations within the statistics of gravitationally lensed quasars. At this aim, we consider a cosmological model whose large scale properties (dynamics, matter distribution) are the same as in Friedmann-Lemaitre models, but whose matter distribution is locally inhomogeneous. We use the well known Dyer-Roder distances to allow a simple analytical expression of the optical depth, and pay particular attention on the different role played by the different notions of distances (filled beam angular diameter distance and Dyer-Roder distances) when calculating this quantity, following the prescription from Ehlers & Schneider for a coherent formalism. We find that the expected number of gravitationally lensed quasars is a decreasing function of the clumpiness parameter.

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arXiv2004

On gravito-magnetic time delay by extended lenses

Gravitational lensing by rotating extended deflectors is discussed. Due to spin, corrections on image positions, caustics and critical curves can be significant. In order to obtain realistic quantitative estimates, the lens is modeled as a singular isothermal sphere. Gravito-magnetic time delays of nearly 0.2 days between different images of background sources can occur.

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arXiv2004

Constraints on a quintessence model from gravitational lensing statistics

Constraints on an exact quintessence scalar-field model with an exponential potential are derived from gravitational lens statistics. An exponential potential can account for data from both optical quasar surveys and radio selected sources. Based on the Cosmic Lens All-Sky Survey (CLASS) sample, lensing statistics provides, for the pressureless matter density parameter, an estimate of Omega_{M0} = 0.31(-0.14)(+0.12).

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arXiv2004

Detecting gravito-magnetism with rotation of polarization by a gravitational lens

We discuss the effects of an isolated gravitational lens on the rotation of the plane of polarization of linearly polarized light rays, the so called gravitational Faraday rotation, in metric theories of gravity. By applying the thin lens approximation, we derive simple expressions for extended, rotating astrophysical systems deflecting electromagnetic radiation from background sources. Higher order corrections and the case of a Kerr black hole are also considered to show how the rotation of polarization is a purely gravito-magnetic effect. Prospects of a future detection of a gravitational Faraday rotation are examined.

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arXiv2004

Determining cosmological parameters from X-ray measurements of strong lensing clusters

We discuss a new method which is potentially capable of constraining cosmological parameters using observations of giant luminous arcs in rich X-ray clusters of galaxies. The mass profile and the mass normalization of the lenses are determined from X-ray measurements. The method also allows to probe the amount and equation of state of the dark energy in the universe. The analysis of a preliminary sample of 6 luminous, relatively relaxed clusters of galaxies strongly favours an accelerating expansion of the universe. Under the assumption that the dark energy is in the form of a cosmological constant, the data provide an estimate of $ Ω_Λ= 1.1$ with a statistical error of $\pm 0.2$. Including the constraint of a flat universe and an equation of state for the dark energy $w_X \geq -1$, we obtain $Ω_M = 0.10 \pm 0.10$ and $w_X = -0.84 \pm 0.14$. Relaxing the prior on $w_X$, we find that the null energy condition is satisfied at the 3-$σ$ confidence level.

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arXiv2004

AC 114: a cluster with a soft X-ray tail

We present Chandra observations of the galaxy cluster AC114, which shows a strongly irregular morphology, with signs of multiple merging activity. We report the discovery of a soft X-ray filament originating close to the core of the cluster. We confirm that X-ray emission is associated with two of three mass concentrations identified in previous gravitational lensing studies of this object. These two mass concentrations are located at opposite ends of the soft filament, evidence for interaction between them. In the northern part, the cluster shows two sharp discontinuities, both in surface brightness and in temperature, evincing another, more recent merger event which took place in, or close, to the cluster core. In spite of the merger activity, a combined mass and lensing analysis shows remarkably good agreement between lensing and X-ray masses. We therefore advocate for the robustness of the X-ray mass estimates, and conclude that the assumption of hydrostatic equilibrium can yield accurate mass estimates even in clusters as dynamically active as AC 114, once the gas density distribution is properly mapped.

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arXiv2003

Weak field limit of Reissner-Nordstrom black hole lensing

We study gravitational lensing by a Reissner-Nordstrom (RN) black hole in the weak field limit. We obtain the basic equations for the deflection angle and time delay and find analytical expressions for the positions and amplifications of the primary and secondary images. Due to a net positive charge, the separation between images increases, but no change in the total magnification occurs.

gr-qc↗

arXiv2003

Simultaneous determination of $Ω_{\rm M0}$ and $H_0$ from joint Sunyaev-Zeldovich effect and X-ray observations with median statistics

Clusters of galaxies contain a fair sample of the universal baryonic mass fraction. A combined analysis of the intracluster medium (ICM) within their hydrostatic regions, as derived from both Sunyaev-Zeldovich effect (SZE) measurements and X-ray images, makes it possible to constrain the cosmological parameters. We consider both gas fraction estimates and angular diameter distance measurements. Adopting median statistics, we find, at the 2-$σ$ level, the pressureless matter density, $Ω_{\rm M0}$, to be between 0.30 and 0.40 and the Hubble constant, $H_0$, between 44 and 66 Km s$^{-1}$ Mpc$^{-1}$.

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