A. A. Nucita

Solar system constraints on R n gravity

Recently, gravitational microlensing has been investigated in the framework of the weak field limit of fourth order gravity theory. However, solar system data (i.e. planetary periods and light bending) can be used to put strong constraints on the parameters of this class of gravity theories. We find that these parameters must be very close to those corresponding to the Newtonian limit of the theory.

Measuring the black hole parameters in the galactic center with RADIOASTRON

Recently, Zakharov et al. (2005) considered the possibility of evaluating the spin parameter and the inclination angle for Kerr black holes in nearby galactic centers by using future advanced astrometrical instruments. A similar approach which uses the characteristic properties of gravitational retro-lensing images can be followed to measure the charge of Reissner-Nordstrom black hole. Indeed, in spite of the fact that their formation might be problematic, charged black holes are objects of intensive investigations. From the theoretical point of view it is well-known that a black hole is described by only three parameters, namely, its mass M, angular momentum J and charge Q. Therefore, it would be important to have a method for measuring all these parameters, preferably by model independent way. In this paper, we propose a procedure to measure the black hole charge by using the size of the retro-lensing images that can be revealed by future astrometrical missions. A discussion of the Kerr-Newmann black hole case is also offered.

Pixel lensing as a way to detect extrasolar planets in M31

We study the possibility to detect extrasolar planets in M31 through pixel-lensing observations. Using a Monte Carlo approach, we select the physical parameters of the binary lens system, a star hosting a planet, and we calculate the pixel-lensing light curve taking into account the finite source effects. Indeed, their inclusion is crucial since the sources in M31 microlensing events are mainly giant stars. Light curves with detectable planetary features are selected by looking for significant deviations from the corresponding Paczy´ nski shapes. We find that the timescale of planetary deviations in light curves increase (up to 3–4 d) as the source size increases. This means that only few exposures per day, depending also on the required accuracy, may be sufficient to reveal in the light curve a planetary companion. Although the mean planet mass for the selected events is about 2 MJupiter, even small mass planets (M P < 20 M⊕) can cause significant deviations, at least in the observations with large telescopes. However, even in the former case, the probability to find detectable planetary features in pixel-lensing light curves is at most a few per cent of the detectable events, and therefore many events have to be collected in order to detect an extrasolar planet in M31. Our analysis also supports the claim that the anomaly found in the candidate event PA-99-N2 towards M31 can be explained by a companion object orbiting the lens star.

Apoastron shift constraints on dark matter distribution at the Galactic Center

The existence of dark matter (DM) at scales of a few parsecs down to

The Seyfert 1 Galaxy Mrk 335 at a very low flux state: mapping the soft X-ray photoionised gas

\ensuremath{\simeq}{10}^{\ensuremath{-}5}\text{ }\text{ }\mathrm{pc}

Sgr A*: A Laboratory to Measure the Central Black Hole and Stellar Cluster Parameters

around the centers of galaxies and, in particular, in the Galactic Center region has been considered in the literature. Under the assumption that such a DM clump, principally constituted by nonbaryonic matter (like weakly interacting massive particles) does exist at the center of our galaxy, the study of the

M31 PIXEL LENSING EVENT OAB-N2: A STUDY OF THE LENS PROPER MOTION

\ensuremath{\gamma}

CANDIDATE MICROLENSING EVENTS FROM M31 OBSERVATIONS WITH THE LOIANO TELESCOPE

-ray emission from the Galactic Center region allows us to constrain both the mass and the size of this DM sphere. Further constraints on the DM distribution parameters may be derived by observations of bright infrared stars around the Galactic Center. Hall and Gondolo [J. Hall and P. Gondolo, Phys. Rev. D 74, 063511 (2006)] used estimates of the enclosed mass obtained in various ways and tabulated by Ghez et al. [A. M. Ghez et al., Astron. Nachr. 324, 527 (2003); A. M. Ghez et al., Astrophys. J. 620, 744 (2005)]. Moreover, if a DM cusp does exist around the Galactic Center it could modify the trajectories of stars moving around it in a sensible way depending on the DM mass distribution. Here, we discuss the constraints that can be obtained with the orbit analysis of stars (as S2 and S16) moving inside the DM concentration with the present and next generations of large telescopes. In particular, consideration of the S2 star apoastron shift may allow improving limits on the DM mass and size.

A new analysis of the MEGA M31 microlensing events

Context. This paper reports on an XMM-Newton observation of the Seyfert 1 Galaxy Mrk 335 performed as a target of opportunity when the source was in an unusually low flux state. Aims. The low level of continuum emission unveiled an underlying line-rich soft X-ray spectrum, which can be studied with the Reflection Grating Spectrometer. Methods. The emission features were analysed at high resolution with unprecedented detail for this source. Line ratio diagnostics from H-like and He-like ions indicate that the line emission arises in X-ray photoionised plasma. Extensive simulations were performed with the CLOUDY photoionisation code. The physical properties of the line emitting material were derived from the comparison of the expected and observed line intensities. Results. Because of the degeneracy in the ionisation parameter, a number of different solutions for the electron density and column density of the gas are consistent with the spectral diagnostics. This prevents us from uniquely determining the properties of the plasma; however, the location(s) of the X-ray photoionised gas can be constrained to within 0.06 pc (outer boundary). This limit places the X-ray line emitting gas at the inner edge of the material where the broad optical lines are produced (broad line region).

A characterization of the NGC 4051 soft X-ray spectrum as observed by XMM-Newton

Several stars orbit around a black hole candidate of mass 3.7 × 106 M⊙ in the region of the Galactic center. Looking for general relativistic (GR) periastron shifts is limited by the existence of a stellar cluster around the black hole, which modifies the cluster orbits due to classical effects that might mask the GR effect. Only if one knows the cluster parameters (its mass and core radius) it is possible to unequivocally deduce the expected GR effects and then test them. In this paper, we show that the observation of the proper motion of Sgr A* vSgr A* = 0.4 ± 0.9 km s–1; Reid & Bruthaler) could help us to constrain the cluster parameters significantly, and that future measurements of the periastron shifts for at least three stars may adequately determine the cluster parameters and the mass of the black hole.