Galactic Center Minispiral: Interaction Modes of Neutron Stars

Streams of gas and dust in the inner parsec of the Galactic center form a distinct feature known as the Minispiral, which has been studied in radio waveband as well as in the infrared wavebands. A large fraction of the Minispiral gas is ionized by radiation of OB stars present in the Nuclear Star Cluster (NSC). Based on the inferred mass in the innermost parsec ($\sim 10^6$ solar masses), over $\sim 10^3$ -- $10^4$ neutron stars should move in the sphere of gravitational influence of the SMBH. We estimate that a fraction of them propagate through the denser, ionized medium concentrated mainly along the three arms of the Minispiral. Based on the properties of the gaseous medium, we discuss different interaction regimes of magnetised neutron stars passing through this region. Moreover, we sketch expected observational effects of these regimes. The simulation results may be applied to other galactic nuclei hosting NSC, where the expected distribution of the interaction regimes is different across different galaxy types.Comment: 12 pages, 17 figures, published in Acta Polytechnic

Dust-enshrouded star near supermassive black hole: predictions for high-eccentricity passages near low-luminosity galactic nuclei

Supermassive black holes reside in cores of galaxies, where they are often surrounded by a nuclear cluster and a clumpy torus of gas and dust. Mutual interactions can set some stars on a plunging trajectory towards the black hole. We model the pericentre passage of a dust-enshrouded star during which the dusty envelope becomes stretched by tidal forces and is affected by the interaction with the surrounding medium. In particular, we explore under which conditions these encounters can lead to periods of enhanced accretion activity. We discuss different scenarios for such a dusty source. To this end, we employed a modification of the Swift integration package. Elements of the cloud were modelled as numerical particles that represent the dust component that interacts with the optically thin gaseous environment. We determine the fraction of the total mass of the dust component that is diverted from the original path during the passages through the pericentre at $\sim 10^3$ Schwarzschild radii and find that the main part of the dust ($\gtrsim 90\%$ of its mass) is significantly affected upon the first crossing. The fraction of mass captured at the second passage generally decreases to very low values. As an example, we show predictions for the dusty source evolution assuming the current orbital parameters of the G2 cloud (also known as Dusty S-Cluster Object, DSO) in our Galactic centre. Encounter of a core-less cloud with a supermassive black hole is, most likely, a non-repeating event: the cloud is destroyed. However, in the case of a dust-enshrouded star, part of the envelope survives the pericentre passage. We discuss an offset of $\lesssim 0.3$ arcsec between the centre of mass of the diverted part and the star along the eccentric orbit. Finally, we examine an interesting possibility of a binary star embedded within a common wind envelope that becomes dispersed at the pericentre passage.Comment: 18 pages, 15 figures, Astronomy and Astrophysics accepte

Interaction between interstellar medium and black hole environment

Studying the interaction between the interstellar medium and the black hole environment on the parsec scale is of crucial importance in the full understanding of galaxy evolution. Since the Galactic Centre is the closest galactic nucleus, it offers us the unique possibility to observationally study the dynamics of individual stars as well as the properties of the Nuclear Star Cluster as a whole. This thesis deals with the transition region where the complex interstellar medium in the Galactic Centre meets a rather simple object at the very centre -- most probably a black hole of $4\times 10^6$ Solar masses characterized by only three classical parameters: mass, spin, and electric charge. Recently, a NIR-excess object named DSO/G2 was detected that is moving on a highly eccentric orbit, with the pericentre reached in 2014 at $\sim 2000$ Schwarzschild radii. The monitoring, the analysis of NIR data, and the comparison with models have provided an unprecedented opportunity to constrain the properties of previously unexplored region around Sgr~A* as well as to determine the nature of this enigmatic source. In a series of papers, we explored the dynamics of different scenarios for DSO/G2, its interaction with the ambient medium close to the Galactic centre, and the radiative properties of its NIR continuum emission. The main findings include the asymmetry of the stellar bow-shock evolution along the orbit when the outflow from the Galactic centre is present. Subsequently, using polarimetry measurements and 3D Monte Carlo radiative transfer, we were able to set up a model of a young star with a non-spherical dusty envelope that can explain its compactness, NIR-excess as well as its linearly polarized emission. Finally, we explore a possibility that the DSO and objects with similar characteristics could be candidates for young neutron stars that should be observable in NIR bands with current and future facilities, which can help to resolve the ``missing pulsar paradox''. Approaching the innermost region of the Galactic Centre, we explore the problem of an electric charge associated with Sgr~A*, which is assumed to be zero in most studies. We found that a stable charge can be maintained by several mechanisms. One of the most promising ones is the charging due to the rotating black hole that is immersed in a uniform magnetic field. Realistic values of the charge that we calculated do not influence space-time metric, but can significantly influence the dynamics of plasma in the vicinity of the Galactic centre. Furthermore, we also propose a novel observational test for detecting the signature of the charge using a bremsstrahlung brightness distribution

SDSS-FIRST-selected interacting galaxies: Optical long-slit spectroscopy study using MODS at the LBT

Context. In the hierarchical model of evolution of the Universe, galaxy mergers play an important role, especially at high redshifts. Interactions among galaxies appear to be associated with incidences of radio-loudness in quasars and it is of interest to study the galaxies that are in the process of interacting with each other, where there is at least one nucleus that is active in the radio regime. Aims. In order to understand the various processes taking place within colliding galaxies, it is important to study the radio and optical properties of these sources, as well as any possible correlations that might exist. Methods. To this end, we present optical long-slit spectroscopy data for ten pairs of interacting galaxies selected from SDSS-FIRST at redshifts of ~0.05, observed using the multi-object double spectrographs at the Large Binocular Telescope. Results. We used line fluxes extracted from the spectra of the nuclear regions of galaxies to plot optical diagnostic diagrams and estimate the masses of the central supermassive black holes, as well as their Eddington ratios. Additionally, we used previously published Effelsberg radio telescope data at 4.85 GHz and FIRST survey data at 1.4 GHz to estimate radio spectral slopes and the radio-loudness parameters for all of the radio-detected sources. We also used WISE data to plot a mid-infrared colour-colour diagram. Conclusions. We see that while the sample of galaxies covers all of the classes on the optical diagnostic diagrams, the sources that are radio-detected fall in the composite or transition region of the diagram. Additionally, we notice a trend of the highest radio-loudness parameter in a pair of interacting galaxies being associated with the galaxy that hosts the more massive central supermassive black hole. We do not see any obvious trends with respect to the radio spectral slope, radio-loudness parameter, and Eddington ratio. With respect to the mid-infrared data of the galaxies detected by WISE, we see that most of them have some type of contribution from star formation, however, two of them seem to have a significant contribution from an AGN as well

Polarimetry and strong gravity effects from spots orbiting near a black hole

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.Comment: 5 pages, 2 figures; to appear in Proceedings of the 15th Marcel Grossman Meeting on General Relativity - the session AC1 on "Spectral and Temporal properties of Black Holes and Neutron Stars and the Theoretical Models" by Sandip Chakrabarti (Rome, 1-7 July 2018), edited by Elia Battistelli, Robert T. Jantzen, and Remo Ruffini, in preparatio

Expectations for time-delay measurements in active galactic nuclei with the Vera Rubin Observatory

The Vera Rubin Observatory will provide an unprecedented set of time-dependent observations of the sky. The planned Legacy Survey of Space and Time (LSST) operating for 10 years will provide dense lightcurves for thousands of active galactic nuclei (AGN) in Deep Drilling Fields (DDFs) and less dense lightcurves for millions of AGN. We model the prospects for measuring time delays for emission lines with respect to the continuum, using these data. We model the artificial lightcurves using Timmer-Koenig algorithm, we use the exemplary cadence to sample them, we supplement lightcurves with the expected contamination by the strong emission lines (Hbeta, Mg II and CIV as well as with Fe II pseudo-continuum and the starlight). We choose the suitable photometric bands appropriate for the redshift and compare the assumed line time delay with the recovered time delay for 100 statistical realizations of the light curves. We show that time delays for emission lines can be well measured from the Main Survey for the bright tail of the quasar distribution (about 15% of all sources) with the accuracy within 1 sigma error, for DDFs results for fainter quasars are also reliable when all 10 years of data are used. There are also some prospects to measure the time delays for the faintest quasars at the smallest redshifts from the first two years of data, and eventually even from the first season. The entire quasar population will allow obtaining results of apparently high accuracy but in our simulations, we see a systematic offset between the assumed and recovered time delay depending on the redshift and source luminosity which will not disappear even in the case of large statistics. Such a problem might affect the slope of the radius-luminosity relation and cosmological applications of quasars if simulations correcting for such effects are not performed.Comment: Submitted to Astronomy & Astrophysics, comments wellcom

Time delay measurement of Mg II line in CTS C30.10 with SALT

We report 6 yr monitoring of a distant bright quasar CTS C30.10 (z = 0.90052) with the Southern African Large Telescope (SALT). We measured the rest-frame time-lag of $562\pm 2$ days between the continuum variations and the response of the Mg II emission line, using the Javelin approach. More conservative approach, based on five different methods, imply the time delay of $564^{+109}_{-71}$ days. This time delay, combined with other available measurements of Mg II line delay, mostly for lower redshift sources, shows that the Mg II line reverberation implies a radius-luminosity relation very similar to the one based on a more frequently studied H$\beta$ line.Comment: submitted to ApJ; comments welcom