arXiv subjects

Alexander F. Zakharov

Publications and source records attributed to Alexander F. Zakharov.

28 records · Page 2Linked to original sources

arXiv2014

Are signatures of anti-de-Sitter black hole at the Galactic Center?

Using Schwarzschild -- de-Sitter (Kottler) metric we derive a simple analytical relation between a shadow size and $Λ$-term. Current observations of the smallest spot to evaluate shadow size at the Galactic Center do not reach an accuracy comparable with cosmological $Λ$-term $\sim 10^{-52}{\rm m}^{-2}$, however, if in reality we have dark energy instead of a constant $Λ$-term then dark energy may be a function depending on time and space and it could be approximated with a local constant $Λ$-term near the Galactic Center and it is important to introduce a procedure to evaluate the $Λ$-term. We suggest such a procedure based on a black hole shadow evaluation. Surprisingly, current observational estimates of shadows are in agreement with anti-de-Sitter spacetimes corresponding to a negative $Λ$-term which is about $-0.4\times 10^{-20}{\rm m}^{-2}$. A negative $Λ$-term has been predicted in the framework of a some class of multidimensional string models.

astro-ph.GA↗

arXiv2014

Constraints on $R^n$ gravity from precession of orbits of S2-like stars: a case of a bulk distribution of mass

Here we investigate possible applications of observed stellar orbits around Galactic Center for constraining the R$^n$ gravity at Galactic scales. For that purpose, we simulated orbits of S2-like stars around the massive black hole at Galactic Center, and study the constraints on the R$^n$ gravity which could be obtained by the present and next generations of large telescopes. Our results show that R$^n$ gravity affects the simulated orbits in the qualitatively similar way as a bulk distribution of matter (including a stellar cluster and dark matter distributions) in Newton's gravity. In the cases where the density of extended mass is higher, the maximum allowed value of parameter $β$ in R$^n$ gravity is noticeably smaller, due to the fact that the both extended mass and $R^n$ gravity cause the retrograde orbital precession.

astro-ph.GA↗

arXiv2011

Astrodynamical Space Test of Relativity using Optical Devices I (ASTROD I) - A class-M fundamental physics mission proposal for Cosmic Vision 2015-2025: 2010 Update

This paper on ASTROD I is based on our 2010 proposal submitted for the ESA call for class-M mission proposals, and is a sequel and an update to our previous paper [Experimental Astronomy 23 (2009) 491-527; designated as Paper I] which was based on our last proposal submitted for the 2007 ESA call. In this paper, we present our orbit selection with one Venus swing-by together with orbit simulation. In Paper I, our orbit choice is with two Venus swing-bys. The present choice takes shorter time (about 250 days) to reach the opposite side of the Sun. We also present a preliminary design of the optical bench, and elaborate on the solar physics goals with the radiation monitor payload. We discuss telescope size, trade-offs of drag-free sensitivities, thermal issues and present an outlook. ASTROD I is a planned interplanetary space mission with multiple goals. The primary aims are: to test General Relativity with an improvement in sensitivity of over 3 orders of magnitude, improving our understanding of gravity and aiding the development of a new quantum gravity theory; to measure key solar system parameters with increased accuracy, advancing solar physics and our knowledge of the solar system; and to measure the time rate of change of the gravitational constant with an order of magnitude improvement and the anomalous Pioneer acceleration, thereby probing dark matter and dark energy gravitationally. It is envisaged as the first in a series of ASTROD missions. ASTROD I will consist of one spacecraft carrying a telescope, four lasers, two event timers and a clock. Two-way, two-wavelength laser pulse ranging will be used between the spacecraft in a solar orbit and deep space laser stations on Earth, to achieve the ASTROD I goals.

gr-qc↗

arXiv2010

Conformal Cosmological Model Parameters with Distant SNe Ia Data: "gold" and "silver"

Assuming that supernovae type Ia (SNe Ia) are standard candles one could use them to test cosmological theories. The Hubble Space Telescope team analyzed 186 SNe Ia\cite{Riess_04} to test the Standard Cosmological model (SC) associated with expanded lengths in the Universe and evaluate its parameters. We use the same sample to determine parameters of Conformal Cosmological model (CC) with relative reference units of intervals, so that conformal quantities of General Relativity are interpreted as observables. We concluded, that really the test is extremely useful and allows to evaluate parameters of the model. From a formal statistical point of view the best fit of the CC model is almost the same quality approximation as the best fit of SC model with $Ω_Λ=0.72, Ω_m=0.28$. As it was noted earlier, for CC models, a rigid matter component could substitute the $Λ$-term (or quintessence) existing in the SC model. We note that a free massless scalar field can generate such a rigid matter. We describe results of our analysis for more recent "gold" data (for 192 SNe Ia).

gr-qc↗

arXiv2006

Gravitational microlensing: results and perspectives in brief

Basics of standard theory of microlensing are introduced. Results of microlensing observations toward Magellanic Clouds and relations with dark matter (DM) problem in our Galaxy are described. Pixel microlensing observations and recent discoveries of planets with microlensing observations are listed.

astro-ph↗

arXiv2005

Microlensing with the space interferometer Radioastron

For microlensing case angular distances between images or typical astrometric shifts due to microlensing are about $10^{-5}-10^{-6} μas$. Such an angular resolution will be reached with the space space--ground interferometer Radioastron. The basic targets for microlensing searches should be bright point-like radio sources at cosmological distances. In this case, an analysis of their variability and a solid determination of microlensing could lead to an estimation of their cosmological mass density, moreover, in this case one could not exclude a possibility that non-baryonic dark matter also form microlenses if the corresponding optical depth will be high enough. To search for microlensing the most perspective objects are gravitational lensed systems as usually, like CLASS gravitational lens B1600+434, for instance. However, for direct resolving these images and direct detection of apparent motion of the knots, a Radioastron sensitivity have to be improved, since an estimated flux density is too low and to observe the phenomena one should improve sensitivity in 10 times at 6 cm wavelength, for instance, otherwise, it is necessary to increase an integration time (assuming that a radio source has a typical core -- jet structure and microlensing phenomenon is caused superluminal apparent motion of knots). Therefore, a launch of space interferometer Radioastron will give new excellent facilities to investigate microlensing in radio band, since in this case there is a possibility not only to resolve microimages but also observe astrometric microlensing.

astro-ph↗

arXiv2005

Different Types of Fe K_αLines from Radiating Annuli near Black Holes

Recent X-ray observations of microquasars and Seyfert galaxies reveal the broad emission lines in their spectra, which can arise in the innermost parts of accretion disks. A theoretical analysis of observations and their interpretations were discussed in a number of papers. We consider a radiating annulus model to simulate spectral line shapes. That is a natural approximation for narrow emitting circular rings without extra astrophysical assumptions about emissivity laws. Recently Muller & Camenzind (2004) presented results of their calculations and classified different types of spectral line shapes and described their origin. We clarified their hypothesis about an origin of doubled peaked and double horned line shapes. Based on results of numerical simulations we showed that double peaked spectral lines arise almost for any location of narrow emission rings (annuli) although Muller & Camenzind (2004) suggested that such profiles arise for relatively flat space-times and typical radii for emission region about 25 r_g. We showed that triangular spectral lines could arise for nearest annuli and high inclination angles. We discuss a possibility of appearance of narrow spectral line shapes as a result of spiralling evolution of matter along quasi-circular orbits which could be approximated by narrow annuli.

astro-ph↗

arXiv2004

Contribution of microlensing to X-ray variability of distant QSOs

We consider a contribution of microlensing to the X-ray variability of high-redshifted QSOs. Cosmologically distributed gravitational microlenses could be localized in galaxies (or even in bulge or halo of gravitational macrolenses) or could be distributed in a uniform way. We have analyzed both cases of such distributions. We found that the optical depth for gravitational microlensing caused by cosmologically distributed deflectors could be significant and could reach $10^{-2} - 0.1$ at $z\sim 2$. This means that cosmologically distributed deflectors may contribute significantlly to the X-ray variability of high-redshifted QSOs ($z>2$). Considering that the upper limit of the optical depth ($τ\sim 0.1$) corresponds to the case where dark matter forms cosmologically distributed deflectors, observations of the X-ray variations of unlensed QSOs can be used for the estimation of the dark matter fraction of microlenses.

astro-ph↗

arXiv2004

Gravitational microlensing and dark matter problem in our Galaxy: 10 years later

Foundations of standard theory of microlensing are described, namely we consider microlensing stars in Galactic bulge, the Magellanic Clouds or other nearby galaxies. We suppose that gravitational microlenses lie between an Earth observer and these stars. Criteria of an identification of microlensing events are discussed. We also consider such microlensing events which do not satisfy these criteria (non-symmetrical light curves, chromatic effects, polarization effects). We describe results of MACHO collaboration observations towards the Large Magellanic Cloud (LMC) and the Galactic bulge. Results of EROS observations towards the LMC and OGLE observations towards the Galactic bulge are also presented. Future microlensing searches are discussed.

astro-ph↗

arXiv2003

The iron $K_α$-line diagnostics of a rotational black hole metric

The original idea to show the spacetime geometry using few geodesics was developed by Johnson and Ruffini (1974). We used this idea to interpret the observational data for rotating BH's. We developed the imitation approach to simulate a propagation of radiation near BH's. An important problem for this approach is the diagnostics of a black hole metric using X-ray observational data of the iron $K_α$-line. Observations of Seyfert galaxies in X-ray region reveal the broad emissiion lines in their spectra, which can arise in inner parts of accretion disks, where the effects of General Relativity (GR) must be counted. A spectrum of a solitary emission line (the $K_α$-line of iron, for example) of a hot spot in Kerr accretion disk is simulated, depending on the radial coordinate $r$ and the angular momentum $a=J/M$ of a black hole, under the assumption of an equatorial circular motion of a hot spot. Using results of numerical simulations it is shown that the characteristic two-peak line profile with the sharp edges arises at a large distance, (about $r \approx (3-10)r_g$). The inner regions emit the line, which is observed with one maximum and extremely broad red wing. High accuracy future spectral observations, being carried out, could detect the angular momentum $a$ of the black hole. We analyzed the different parameters of problems on the observable shape of this line and discussed some possible kinds of these shapes. The total number of geodesics is about $10^9$ (to simulate possible shapes of the $K_α$-line), so the number is great enough, especially in comparison with few geodesics in the original paper by Johnson and Ruffini (1974).

astro-ph↗