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Non-baryonic, or "dark", matter is believed to be a major component of the total mass budget of the Universe. We review the candidates for particle dark matter and discuss the prospects for direct detection (via interaction of dark matter particles with laboratory detectors) and indirect detection (via observations of the products of dark matter self-annihilations), focusing in particular on the Galactic center, which is among the most promising targets for indirect detection studies. The gravitational potential at the Galactic center is dominated by stars and by the supermassive black hole, and the dark matter distribution is expected to evolve on sub-parsec scales due to interaction with these components. We discuss the dominant interaction mechanisms and show how they can be used to rule out certain extreme models for the dark matter distribution, thus increasing the information that can be gleaned from indirect detection searches.

It is shown that the DAMA data indicate two dark matter components, one that circulates around the galactic center (GC) and another that is emitted from the GC. From the location of the maximum yearly variation, one can compute the ratio of the two components.

This review outlines the observations that now provide an overwhelming scientific case that the center of the Milky Way harbors a supermassive black hole. Observations at infrared wavelength trace stars that orbit about a common focal position and require a central mass (M) of 4 × 10⁶ M_⊙ within a radius of 100 AU. Orbital speeds have been observed to exceed 5,000 km s^-1. At the focal position there is an extremely compact radio source (Sgr A*), whose apparent size is near the Schwarzschild radius (2GM/c²). This radio source is motionless at the ~ 1 km s^-1 level at the dynamical center of the Galaxy. The mass density required by these observations is now approaching the ultimate limit of a supermassive black hole within the last stable orbit for matter near the event horizon.

The center of our Galaxy provides a uniquely accessible laboratory. It is a rich environment of extreme density, velocity and tidal fields of stars. It is the closest example of a galactic nucleus and could give the opportunity to understand the role that massive black-holes play in the formation and evolution of galaxies. It could be used to test the effects of relativity and dark matter in the Galactic Center. If the central object is a black-hole such observation would be a milstone: the first direct proof that an event horizon, and therefore a black-hole exists. The next decade will be decisive in new discoveries.

Black holes have the ability to generate infinite number of images of any given source. These relativistic images are formed by light rays winding around the black hole several times. The phenomenology associated with these images is very rich, since these features are very sensitive to the metric structure of the black hole. Here, we review some aspects of gravitational lensing by black holes and consider some fundamental aspects related to alternative solutions, which do not reproduce Schwarzschild in the asymptotic limit and are supported by exotic matter.

While the event horizon of a black hole could cast a shadow that was observed recently, a central singularity without horizon could also give rise to such a feature. This leaves us with a question on the nature of the supermassive black holes at the galactic centers, and if they admit an event horizon necessarily. We point out that observations of motion of stars around the galactic center should give a clear idea of the nature of this central supermassive object. We examine and discuss here recent developments that indicate intriguing behavior of the star motions that could possibly distinguish the existence or otherwise of an event horizon at the galactic center. We compare the motion of the S2 star with these theoretical results, fitting the observational data with theory, and it is seen that the star motions and precession of their orbits around the galactic center provide important clues on the nature of this central compact object.

Stars that evolve near the Galactic massive black hole show strange behaviors. The spectroscopic features of these stars show that they must be old. But their luminosities are much higher than the amounts that are predicted by the current stellar evolutionary models, which means that they must be active and young stars. In fact, this group of stars shows signatures of old and young stars, simultaneously. This is a paradox known as the “paradox of youth problem” (PYP). Some people tried to solve the PYP without supposing dark matter (DM) effects on stars. But, in this work, we implemented Weakly Interacting Massive Particles (WIMPs) annihilation as a new source of energy inside such stars. This implementation is logical for stars that evolve at high DM density environments. The new source of energy causes stars to follow different evolutionary paths on the H-R diagram in comparison with classical stellar evolutionary models. Increasing DM density in stellar evolutionary simulations causes the deviations from the standard H-R diagrams becomes more pronounced. By investigating the effects of WIMPs density on stellar structures and evolutions, we concluded that by considering DM effects on stars at the Galactic center, it is possible to solve the PYP. In addition to DM effect, complete solutions to PYP must consider all extreme and unique physical conditions that are present near the Galactic massive black hole.

Dynamical chaos is a fundamental manifestation of gravity in astrophysical, many-body systems. The spectrum of Lyapunov exponents quantifies the associated exponential response to small perturbations. Analytical derivations of these exponents are critical for understanding the stability and predictability of observed systems. This paper presents a new model for chaos in systems with eccentric and crossing orbits. Here, exponential divergence is not a continuous process but rather the cumulative effect of an ever-increasing linear response driven by discrete events at regular intervals, i.e. punctuated chaos. We show that long-lived systems with punctuated chaos can magnify Planck length perturbations to astronomical scales within their lifetime, rendering them fundamentally indeterministic.

We consider the periapsis shifts of bound orbits of stars on static clouds around a black hole. The background spacetime is constructed from a Schwarzschild black hole surrounded by a static and spherically symmetric self-gravitating system of massive particles, which satisfies all the standard energy conditions and physically models the gravitational effect of dark matter distribution around a nonrotating black hole. Using nearly circular bound orbits of stars, we obtain a simple formula for the precession rate. This formula explicitly shows that the precession rate is determined by a positive contribution (i.e. a prograde shift) from the conventional general-relativistic effect and a negative contribution (i.e. a retrograde shift) from the local matter density. The four quantities for such an orbit (i.e. the orbital shift angle, the radial oscillation period, the redshift and the star position mapped onto the celestial sphere) determine the local values of the background model functions. Furthermore, we not only evaluate the precession rate of nearly circular bound orbits in several specific models but also simulate several bound orbits with large eccentricity and their periapsis shifts. The present exact model demonstrates that the retrograde precession does not mean any exotic central objects such as naked singularities or wormholes but simply the existence of significant energy density of matters on the star orbit around the black hole.

The gravitational interplay between a supermassive black hole and dark matter has set up an exotic environment at the center of a galaxy. In this paper, I present an analytic framework modeling the gravitational influence on the stars orbiting a galactic supermassive black hole. In particular, we discuss two intriguing features, stellar orbital precession and orbital shrinking, which demonstrate an extraordinary gravitational environment near a galactic supermassive black hole. Moreover, I show that these features can be analytically determined by the supermassive black hole mass.

The possibility to describe in a consistent way both the supermassive compact object at the center and the halo of galaxies is explored by assuming the existence of a self-gravitating semi-degenerate system of fermions in thermodynamical equilibrium.

We propose a unified model for dark matter haloes and central galactic objects as a self-gravitating system of semidegenerated fermions in thermal equilibrium. We consider spherical symmetry and then we solve the equations of gravitational equilibrium using the Fermi integrals in a dimensionless manner, obtaining the density profile and velocity curve. We also obtain scaling laws for the observables of the system and show that, for a wide range of our parameters, our model is consistent with the so called universality of the surface density of dark matter.

The central dense stellar cluster in the Galactic Center (GC) contains the mass of ~4 times larger than that of the central black hole. It is expected to be formed as a result of a merging process of several massive globular clusters which provided a large number of millisecond pulsars (MSPs). We propose that the GeV-TeV γ-ray emission observed from GC is in fact a cumulative effect of the emission from several globular clusters captured by the GC black hole. We calculate the expected TeV γ-ray emission produced by leptons, injected by pulsars, by the Inverse Compton Scattering process in the diffusive radiation field. It is shown that this emission can be responsible for the multi-TeV γ-rays observed by the Cherenkov telescopes from GC if about a thousand of MSPs are present in the central cluster in GC.

We study the modulation of the observed radiation flux and the associated changes in the polarization degree and angle that are predicted by the orbiting spot model for flares from accreting black holes. The geometric shape of the emission region influences the resulting model lightcurves, namely, the emission region of a spiral shape can be distinguished from a simpler geometry of a small orbiting spot.

We further explore this scheme for the observed flares from the supermassive black hole in the context of Galactic center (Sgr A^*). Our code simulates the lightcurves for a wide range of parameters. The energy dependence of the changing degree and angle of polarization should allow us to discriminate between the cases of a rotating and a non-rotating black hole.

Super-Massive Black Holes reside in galactic nuclei, where they exhibit episodic bright flares due to accretion events. Taking into account relativistic effects, namely, the boosting and lensing of X-ray flares, we further examine the possibility to constraint the mass of the SMBH from the predicted profiles of the observed light curves. To this end, we have studied four bright flares from Sagittarius A*, which exhibit an asymmetric shape consistent with a combination of two intrinsically separate peaks that occur with a specific time delay with respect to each other. We have thus proposed (Karssen et al. 2017, Mon. Not. R. Astron. Soc. 472, 4422) that an interplay of relativistic effects could be responsible for the shape of the observed light curves and we tested the reliability of the method.

The dynamic center of our galaxy is known to host a source of TeV gamma rays since the very beginning of the 21st century and a link to the supermassive black hole at the Galactic Center has been speculated on ever since. But not only the point-like source, spatially coincident with SgrA, can be observed from the ground using the Imaging Air Cherenkov Telescope technique, but also diffuse emission from the vicinity, spanning more than one degree along the Galactic plane and emitting a remarkably hard energy spectrum, reaching energies well beyond 10 TeV.

Recent observations by the H.E.S.S., MAGIC and VERITAS facilities have enabled detailed studies of the dynamics of high-energy particles in Galactic Center region that indicate a link between the diffuse component and central point-like gamma-ray source. These studies suggest the presence of a powerful cosmic-ray accelerator in close proximity to Sgr^A*. This could potentially even be one of the long-sought-after Galactic PeVatrons, needed in order to explain the cosmic-ray spectrum up to the the feature called ‘knee’ at around 10¹⁵ eV.

Suzaku is the fifth in the series of Japanese astronomy satellites devoted to observations of celestial X-ray sources launched on a Japanese M-V rocket on July 10, 2005. Suzaku features the excellent X-ray sensitivity, with high throughput over a broad-band energy range of 0.2 to 600 keV. Suzaku's broad bandpass, low background, and good CCD resolution makes it a unique tool capable of addressing a variety of outstanding problems in astrophysics.

INTEGRAL is an orbital observatory covering a broad energy range from keVs to MeVs. Its strongest features are sensitive imaging in hard X-rays (15–100 keV) and ultra-fine spectroscopy of gamma-ray lines. We present selected results of INTEGRAL observations in 2003–2006 on such subjects as positron annihilation in the Milky Way, activity of the central Galactic black hole in the recent past, Galactic absorbed X-ray sources, statistics of nearby AGN and the cosmic X-ray background.

In this paper we show the possibility of constraining Dark Matter properties through multiwavelenght observations of the Sgr A* region. We consider Kaluza-Klein Dark Matter annihilation and study the resulting X-ray synchrotron and gamma-ray emission. We show how the combination of these observations puts severe constraints on the shape of a Kaluza-Klein Dark Matter halo.

During its first cycle of observations, the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescope has observed very high energy γ-rays from five galactic objects: the Crab Nebula, the SNRs HESS J1813-178 and HESS J1834-087, the Galactic Center and the γ-ray binary LS I +61 303. After a short introduction to the MAGIC telescope and the data analysis procedure, the results of these five sources are reviewed.