Dark Compact Objects Research Articles

Phantom gravastar supported for the explanation of compact dark matter objects

The understanding of dark matter objects is one of the modern mysteries. Usually one interprets them as black holes, but there is no empirical evidence only a lack of alternatives. Proposing the idea of a phantom gravastar we will give an alternative explanation to these dark matter objects without supposing them to be black holes, in particular to the SgrA* and the MACHOs.

Testing strong gravity with gravitational waves and Love numbers

The LIGO observation of GW150914 has inaugurated the gravitational-wave astronomy era and the possibility of testing gravity in extreme regimes. While distorted black holes are the most convincing sources of gravitational waves, similar signals might be produced also by other compact objects. In particular, we discuss what the gravitational-wave ringdown could tell us about the nature of the emitting object, and how measurements of the tidal Love numbers could help us in understanding the internal structure of compact dark objects.

Strong deflection limit analysis and gravitational lensing of an Ellis wormhole

Observations of gravitational lenses in strong gravitational fields give us a clue to understanding dark compact objects. In this paper, we extend a method to obtain a deflection angle in a strong deflection limit provided by Bozza [Phys. Rev. D 66, 103001 (2002)] to apply to ultrastatic spacetimes. We also discuss on the order of an error term in the deflection angle. Using the improved method, we consider gravitational lensing by an Ellis wormhole, which is an ultrastatic wormhole of the Morris-Thorne class.

Probing hybrid modified gravity by stellar motion around Galactic Center

We consider possible signatures for the so called hybrid gravity within the Galactic Central Parsec. This modified theory of gravity consists of a superposition of the metric Einstein–Hilbert Lagrangian with an f(R) term constructed à la Palatiniand can be easily reduced to an equivalent scalar–tensor theory. Such an approach is introduced in order to cure the shortcomings related to f(R) gravity, in general formulated either in metric or in metric-affine frameworks. Hybrid gravity allows to disentangle the further gravitational degrees of freedom with respect to those of standard General Relativity. The present analysis is based on the S2 star orbital precession around the massive compact dark object at the Galactic Center where the simulated orbits in hybrid modified gravity are compared with astronomical observations. These simulations result with constraints on the range of hybrid gravity interaction parameter ϕ0, showing that in the case of S2 star it is between −0.0009 and −0.0002. At the same time, we are also able to obtain the constraints on the effective mass parameter mϕ, and found that it is between −0.0034 and −0.0025 AU−1 for S2 star. Furthermore, the hybrid gravity potential induces precession of S2 star orbit in the same direction as General Relativity. In previous papers, we considered other types of extended gravities, like metric power law f(R)∝Rn gravity, inducing Yukawa and Sanders-like gravitational potentials, but it seems that hybrid gravity is the best among these models to explain different gravitational phenomena at different astronomical scales.

Compact dark matter objects, asteroseismology, and gravitational waves radiated by sun

The solar surface oscillations observed by Crimean Astrophysical Observatory and Solar Helioseismic Observatory are considered to be excited by a small fraction of Dark Matter in form of Compact Dark Matter Objects (CDMO) in the solar structure. Gravitational Waves (GW) radiated by these CDMO are predicted to be the strongest at the Earth and are easily detectable by European Laser Interferometer Space Antenna or by Gravitational-Wave Observatory “Dulkyn” which can solve two the most challenging tasks in the modern physics: direct detection of GW and DM.

Are the most super-massive dark compact objects harbored at the center of dark matter halos?

We study an isothermal system of semi-degenerate self-gravitating fermions in general relativity (GR). The most general solutions present is mass density profiles with a central degenerate compact core governed by quantum statistics followed by an extended plateau, and ending in a power law behavior r-2. By fixing the fermion mass m in the keV regime, the different solutions depending on the free parameters of the model: the degeneracy and temperature parameters at the center, are systematically constructed along the one-parameter sequences of equilibrium configurations up to the critical point, which is represented by the maximum in a central density (ρ0) versus core mass (Mc) diagram. We show that for fully degenerate cores, the Oppenheimer–Volkoff (OV) mass limit [Formula: see text] is obtained, while instead for low degenerate cores, the critical core mass increases showing the temperature effects in a nonlinear way. The main result of this work is that when applying this theory to model the distribution of dark matter (DM) in big elliptical galaxies from miliparsec distance-scales up to 102Kpc, we do not find any critical core-halo configuration of self-gravitating fermions, able to explain both the most super-massive dark object at their center together with the DM halo simultaneously.

The axion–photon interaction and gamma ray signals of dark matter

We explore two scenarios where the axion–photon interaction could induce additional astrophysical gamma ray signals for the dark matter. In the first scenario, dark compact objects made of axions, named axion stars, could collide with neutron stars. The whole energy of the axion star can be dissipated in the magnetized conducting medium of the neutron star generating gamma rays. The second scenario is an indirect method for observing self–annihilating dark matter trapped in stars: Gamma rays produced by the self–annihilation of neutralinos in the interior of the Sun can be transformed into axions due to photon–axion conversion. Then, the axion will travel freely in the Sun and be converted into photons again. This process is often referred as ‘shine light through walls’, in this case, the wall will be the solar interior. Hence, GeV gamma rays might pass through the Sun. We may conclude that observation of GeV photons by gamma-ray observatories like HAWC, coming from the Sun, may be a signal of annihilation of neutralinos in the interior of the Sun.

Compact dark matter objects, asteroseismology, and gravitational waves radiated by Sun

Compact dark matter objects, asteroseismology, and gravitational waves radiated by Sun

Constraining the range of Yukawa gravity interaction from S2star orbits

We consider possible signatures for Yukawa gravity within theGalactic Central Parsec, based on our analysis of the S2 star orbitalprecession around the massive compact dark object at the GalacticCentre, and on the comparisons between the simulated orbits in Yukawa gravityand two independent sets of observations. Our simulations resulted in strongconstraints on the range of Yukawa interaction Λ and showed that itsmost probable value in the case of S2 star is around 5000 - 7000 AU. Atthe same time, we were not able to obtain reliable constrains onthe universal constant δ of Yukawa gravity, because thecurrent observations of S2 star indicated that it may be highlycorrelated with parameter Λ in the range (0 < δ < 1).For δ > 2 they are not correlated. However, the same universal constant whichwas successfully applied to clusters of galaxies and rotation curvesof spiral galaxies (δ = 1/3) also gives a satisfactory agreement withthe observed orbital precession of the S2 star, and in that casethe most probable value for the scale parameter is Λ ≈ 3000±1500 AU. Also, the Yukawa gravity potential induces precession of S2 starorbit in the same direction as General Relativity for δ > 0 and forδ < −1, and in the opposite direction for −1 < δ < 0. The futureobservations with advanced facilities, such as GRAVITY or/and European ExtremelyLarge Telescope, are needed in order to verify these claims.

Properties of gravitationally bound dark compact ultra dense objects

We consider compact astrophysical objects formed from dark matter fermions of mass 250 GeV to 100 TeV or from massless fermions hidden by vacuum structure of similar energy scale. These objects have maximum stable masses of sub-planetary scale and radii of micron to centimeter scale. We describe the surface gravity and tidal forces near these compact ultra dense objects, as pertinent to signatures of their collisions with visible matter objects.

Constraining scalar fields with stellar kinematics and collisional dark matter

The existence and detection of scalar fields could provide solutions to long-standing puzzles about the nature of dark matter, the dark compact objects at the centre of most galaxies, and other phenomena. Yet, self-interacting scalar fields are very poorly constrained by astronomical observations, leading to great uncertainties in estimates of the mass mϕ and the self-interacting coupling constant λ of these fields. To counter this, we have systematically employed available astronomical observations to develop new constraints, considerably restricting this parameter space. In particular, by exploiting precise observations of stellar dynamics at the centre of our Galaxy and assuming that these dynamics can be explained by a single boson star, we determine an upper limit for the boson star compactness and impose significant limits on the values of the properties of possible scalar fields. Requiring the scalar field particle to follow a collisional dark matter model further narrows these constraints. Most importantly, we find that if a scalar dark matter particle does exist, then it cannot account for both the dark-matter halos and the existence of dark compact objects in galactic nuclei.

Size Evolution of Early-Type Galaxies and Massive Compact Objects as Dark Matter

Abstract The dramatic size evolution of early-type galaxies from $z\sim$ 2 to 0 poses a new challenge in the theory of galaxy formation, which may not be explained by the standard picture. It is shown here that the size evolution can be explained if the non-baryonic cold dark matter is composed of compact objects having a mass scale of $\sim$10$^{5}M_\odot$. This form of dark matter is consistent with, or only weakly constrained by the currently available observations. The kinetic energy of the dark compact objects is transferred to stars by dynamical friction, and stars around the effective radius are pushed out to larger radii, resulting in a pure size evolution. This scenario has several good properties to explain the observations, including the ubiquitous nature of size evolution and faster disappearance of higher density galaxies.

The OGLE view of microlensing towards the Magellanic Clouds - I. A trickle of events in the OGLE-II LMC data

We present the results from the OGLE-II survey (1996-2000) towards the Large Magellanic Cloud (LMC), which has the aim of detecting the microlensing phenomena caused by dark matter compact objects in the Galactic Halo (Machos). We use high resolution HST images of the OGLE fields and derive the correction for the number of monitored stars in each field. This also yield blending distributions which we use in 'catalogue level' Monte Carlo simulations of the microlensing events in order to calculate the detection efficiency of the events. We detect two candidates for microlensing events in the All Stars Sample, which translates into an optical depth of 0.43+-0.33x 10e-7. If both events were due to Macho the fraction of mass of compact dark matter objects in the Galactic halo would be 8+-6 per cent. This optical depth, however, along with the characteristics of the events, seems to be consistent with the self-lensing scenario, i.e., self-lensing alone is sufficient to explain the observed microlensing signal. Our results indicate a non-detection of Machos lensing towards the LMC with an upper limit on their abundance in the Galactic halo of 19 per cent for M=0.4 Msun and 10 per cent for masses between 0.01 and 0.2 Msun.

Black hole shadow image and visibility analysis of Sagittarius A

The compact dark objects with very large masses residing at the centres of galaxies are believed to be black holes. Due to the gravitational lensing effect, they would cast a shadow larger than their horizon size over the background, whose shape and size can be calculated. For the supermassive black hole candidate Sgr A*, this shadow spans an angular size of about 50 micro arc second, which is under the resolution attainable with the current astronomical instruments. Such a shadow image of Sgr A* will be observable at about 1 mm wavelength, considering the scatter broadening by the interstellar medium. By simulating the black hole shadow image of Sgr A* with the radiatively inefficient accretion flow model, we demonstrate that analyzing the properties of the visibility function can help us determine some parameters of the black hole configuration, which is instructive to the sub-millimeter VLBI observations of Sgr A* in the near future.

Bounds on the Mass and Abundance of Dark Compact Objects and Black Holes in Dwarf Spheroidal Galaxy Halos

We establish new dynamical constraints on the mass and abundance of compact objects in the halo of dwarf spheroidal (dSph) galaxies. In order to preserve kinematically cold the second peak of the Ursa Minor (UMi) dSph against gravitational scattering, we place upper limits on the density of compact objects as a function of their assumed mass. The mass of the dark matter constituents cannot be larger than 103 M? at a halo density in UMi's core of 0.35 M? pc-3. This constraint rules out a scenario in which dark halo cores are formed by two-body relaxation processes. Our bounds on the fraction of dark matter in compact objects with masses 3 ? 103 M? improve those based on dynamical arguments in the Galactic halo. In particular, objects with masses ~105 M? can comprise no more than a halo mass fraction ~0.01. Better determinations of the velocity dispersion of old overdense regions in dSph galaxies may result in more stringent constraints on the mass of halo objects. For illustration, if the preliminary value of 0.5 km s-1 for the secondary peak of UMi is confirmed, compact objects with masses above ~100 M? could be excluded from comprising all its dark matter halo.

QSO-Galaxy Association and Gravitational Lensing

The amplification caused by gravitational light bending by compact objects in a foreground galaxy can affect the apparent number density of background QSOs, as well as their distribution in the fields of galactic halos. In this work we investigate the number enhancement of QSOs in the fields of galactic halos caused by point mass lensing effect and singular isothermal lensing effect, and apply the micro-lensing effect due to dark compact objects in the halo to NGC 3628. NGC 3628 is a well-studied nearby edge-on Sbc peculiar galaxy, where QSOs are shown to be concentrated around the galaxy with a density much higher than background. We show that if present understanding of the luminosity function of QSOs is right, such concentration could not be caused by gravitational lensing.

The Anomaly in the Candidate Microlensing Event PA‐99‐N2

The lightcurve of PA-99-N2, one of the recently announced microlensing candidates towards M31, shows small deviations from the standard Paczynski form. We explore a number of possible explanations, including correlations with the seeing, the parallax effect and a binary lens. We find that the observations are consistent with an unresolved RGB or AGB star in M31 being microlensed by a binary lens. We find that the best fit binary lens mass ratio is about one hundredth, which is one of most extreme values found for a binary lens so far. If both the source and lens lie in the M31 disk, then the standard M31 model predicts the probable mass range of the system to be 0.02-3.6 solar masses (95 % confidence limit). In this scenario, the mass of the secondary component is therefore likely to be below the hydrogen-burning limit. On the other hand, if a compact halo object in M31 is lensing a disk or spheroid source, then the total lens mass is likely to lie between 0.09-32 solar masses, which is consistent with the primary being a stellar remnant and the secondary a low mass star or brown dwarf. The optical depth (or alternatively the differential rate) along the line of sight toward the event indicates that a halo lens is more likely than a stellar lens provided that dark compact objects comprise no less than 15 per cent (or 5 per cent) of haloes.

Study of a strategy for parallax microlensing detection towards the Magellanic Clouds

In this article, we have investigated the possibility to distinguish between different galactic models through the microlensing parallax studies. We show that a systematic search for parallax effects can be done using the currently running alert systems and complementary photometric telescopes, to distinguish between different lens distance distributions. We have considered two galactic dark compact objects distributions, with total optical depths corresponding to the EROS current upper limits. These models correspond to two extreme hypothesis on a three component galactic structure made of a thin disc, a thick disc, and a spherically symmetric halo. Our study shows that for sub-solar mass lenses, an exposure of $8\times 10^7$ star$\times$yr toward LMC should allow to distinguish between these two extreme models. In addition the self-lensing hypothesis (lensing by LMC objects) can efficiently be tested through the method proposed here.

Supermassive Black Holes in BL Lacertae Objects: Estimated Masses and Their Relation to Nuclear Luminosity

We compile a sample of 13 BL Lac objects for which rapid optical variabilities are reliably established and luminosities of the nuclei and the host galaxies have been well observed by the Hubble Space Telescope (HST). We present a new method for estimating the masses of the supermassive black holes (SMBHs) at the centers of these objects using the rapid optical variability, and find that the masses of the SMBHs range from 10(7.42) to 10(9.19) M(.). This mass range is quite similar to that of dark compact objects in galaxies. To further investigate the reliability of our method, we also compile another sample with nine high-energy gamma-ray emission sources of BL Lac objects and obtain the masses for the SMBHs in these objects using the Dermer & Gehrels method. We find that the masses derived from these two methods are quite similar. These results strongly imply that the estimation of masses of SMBHs by rapid optical variability is reliable. We examine the correlations of the masses with the luminosities of the nuclei and the host galaxies observed by HST, and find that the masses are significantly correlated with the luminosities of the nuclei, but not correlated with the luminosities of host galaxies at all. In addition, we examine the relation between the observed minimal time-scale of variation and the bolometric luminosities of the objects. We find that they obey the Abromowicz & Nobili relation. These results possibly imply that the luminosities of the nuclei are produced by accretion of the central SMBHs.

The Motion of Stars near the Galactic Center: A Comparison of the Black Hole and Fermion Ball Scenarios

After a discussion of the properties of degenerate fermion balls, we analyze the orbits of the stars S0-1 and S0-2, which have the smallest projected distances to Sgr A*, in the supermassive black hole as well as in the fermion ball scenarios of the Galactic center. It is shown that both scenarios are consistent with the data, as measured during the last six years by Genzel et al. and Ghez et al. The free parameters of the projected orbit of a star are the unknown components of its velocity v_z and distance z to Sgr A* in 1995.4, with the z-axis being in the line of sight. We show, in the case of S0-1 and S0-2, that the z-v_z phase-space which fits the data, is much larger for the fermion ball than for the black hole scenario. Future measurements of the positions or radial velocities of S0-1 and S0-2 could reduce this allowed phase-space and eventually rule out one of the currently acceptable scenarios. This may shed some light into the nature of the supermassive compact dark object, or dark matter in general at the center of our Galaxy.