P. Jovanović

VARIABILITY OF THE Hβ LINE PROFILES AS AN INDICATOR OF ORBITING BRIGHT SPOTS IN ACCRETION DISKS OF QUASARS: A CASE STUDY OF 3C 390.3

Here we show that in the case when double-peaked emission lines originate from outer parts of the accretion disk, their variability could be caused by perturbations in the disk emissivity. In order to test this hypothesis, we introduced a model of the disk perturbing region in the form of a single bright spot (or flare) by a modification of the power-law disk emissivity in an appropriate way. The disk emission was then analyzed using numerical simulations based on the ray-tracing method in the Kerr metric and the corresponding simulated line profiles were obtained. We applied this model to the observed H{beta} line profiles of 3C 390.3 (observed in the period 1995-1999) and estimated the parameters of both the accretion disk and the perturbing region. Our results show that two large amplitude outbursts of the H{beta} line observed in 3C 390.3 could be explained by successive occurrences of two bright spots on the approaching side of the disk. These bright spots are either moving, originating in the inner regions of the disk and spiralling outward by crossing small distances during the period of several years, or stationary. In both cases, their widths increase with time, indicating that they most likely decay.

Microlensing of the X‐ray, UV and optical emission regions of quasars: simulations of the time‐scales and amplitude variations of microlensing events

We consider the influence of microlensing on different spectral bands of lensed QSOs. We assumed that the emitting X-ray, ultraviolet (UV) and optical regions are different in size, but that the continuum emission in these spectral bands is originating from an accretion disc. Estimations of the time-scales for microlensing and flux amplification in different bands are given. We found that the microlensing duration should be shorter in the X-ray (several months) than in UV/optical emitting region (several years). This result indicates that monitoring of the X-ray variations in lensed QSOs that show a ‘flux anomaly’ can clarify the source of this anomaly.

Constraining the range of Yukawa gravity interaction from S2 star orbits

We consider possible signatures for Yukawa gravity within the Galactic Central Parsec, based on our analysis of the S2 star orbital precession around the massive compact dark object at the Galactic Centre, and on the comparisons between the simulated orbits in Yukawa gravity and two independent sets of observations. Our simulations resulted in strong constraints on the range of Yukawa interaction Λ and showed that its most probable value in the case of S2 star is around 5000 - 7000 AU. At the same time, we were not able to obtain reliable constrains on the universal constant δ of Yukawa gravity, because the current observations of S2 star indicated that it may be highly correlated with parameter Λ in the range (0 2 they are not correlated. However, the same universal constant which was successfully applied to clusters of galaxies and rotation curves of spiral galaxies (δ = 1/3) also gives a satisfactory agreement with the observed orbital precession of the S2 star, and in that case the most probable value for the scale parameter is Λ ≈ 3000±1500 AU. Also, the Yukawa gravity potential induces precession of S2 star orbit in the same direction as General Relativity for δ > 0 and for δ < −1, and in the opposite direction for −1 < δ < 0. The future observations with advanced facilities, such as GRAVITY or/and European Extremely Large Telescope, are needed in order to verify these claims.

On the contribution of microlensing to X-ray variability of high-redshifted QSOs

We consider a contribution of microlensing to the X-ray variability of high-redshifted QSOs. Such an effect could be caused by stellar mass objects (SMO) located in a bulge or/and in a halo of this quasar as well as at cosmological distances between an observer and a quasar. Here, we not consider microlensing caused by deflectors in our Galaxy since it is well- known from recent MACHO, EROS and OGLE observations that the corresponding optical depth for the Galactic halo and the Galactic bulge is lower than 10 −6 . 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. As a result of our analysis, we obtained that the optical depth for microlensing caused by stellar mass objects is usually small for quasar bulge and quasar halo gravitational microlens distributions (τ ∼ 10 −4 ). On the other hand, the optical depth for gravitational microlensing caused by cosmologically distributed deflectors could be significant and could reach 10 −2 −0. 1a tz ∼ 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 ∼ 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.

Photocentric variability of quasars caused by variations in their inner structure: Consequences for Gaia measurements

Context. We study the photocenter position variability caused by variations in the quasar inner structure. We consider the variability in the accretion disk emissivity and torus structure variability caused by the different illumination by the central source. We discuss the possible detection of these effects by Gaia. Observations of the photocenter variability in two AGNs, SDSS J121855+020002 and SDSS J162011+1724327 have been reported and discussed. Aims. For variations in the quasar inner structure, we explore how much this effect can affect the position determination and whether it can (or not) be detected with the Gaia mission. Methods. We use models of (a) a relativistic disk, including the perturbation that can increase the brightness of part of the disk, and consequently offset the photocenter position, and (b) a dusty torus that absorbs and re-emits the incoming radiation from the accretion disk (central continuum source). We estimate the value of the photocenter offset caused by these two effects. Results. We found that perturbations in the inner structure can cause a significant offset to the photocenter. This offset depends on the characteristics of both the perturbation and accretion disk and on the structure of the torus. In the case of the two considered QSOs, the observed photocenter offsets cannot be explained by variations in the accretion disk and other effects should be considered. We discuss the possibility of exploding stars very close to the AGN source, and also that there are two variable sources at the center of these two AGNs that may indicate a binary supermassive black hole system on a kpc (pc) scale. Conclusions. The Gaia mission seems to be very promising, not only for astrometry, but also for exploring the inner structure of AGNs. We conclude that variations in the quasar inner structure can affect the observed photocenter (by up to several mas). There is a chance to observe such an effect in the case of bright and low-redshift QSOs.

Constraints onRngravity from precession of orbits of S2-like stars

We study some possible observational signatures of

Balmer lines emission region in NGC 3516: Kinematical and physical properties

R^n

Evidence for periodicity in 43-year-long monitoring of NGC 5548

gravity at Galactic scales and how these signatures could be used for constraining this type of

Constraints on

f(R)

R^n

gravity. For that purpose, we performed two-body simulations in