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We present results of 3 mm observations of SiO maser sources in the Galactic Centre (GC) from observations with the Australia Telescope Compact Array between $2010-2014$, along the transitions of the SiO molecule at $v = 1, J = 2-1$ at 86.243 GHz and $v = 2, J = 2-1$ at 85.640 GHz. We also present the results of the 3 mm observations with Atacama Large Millimeter/Submillimeter Array (ALMA). We detected 5 maser sources from the ATCA data, IRS 7, IRS 9, IRS 10EE, IRS 12N, and IRS 28; and 20 sources from the ALMA data including 4 new sources. These sources are predominantly late-type giants or emission line stars with strong circumstellar maser emission. We analyse these sources and calculate their proper motions. We also study the variability of the maser emission. IRS 7, IRS 12N and IRS 28 exhibit long period variability of the order of $1 - 2$ years, while other sources show steady increase or decrease in flux density and irregular variability over observation timescales. This behaviour is consistent with the previous observations., Comment: 22 pages, 5 figures, to be published in the proceedings of the XIII workshop on "Multifrequency Behaviour of High Energy Cosmic Sources - XIII - MULTIF2019", held on 3-8 June, 2019, in Palermo, Italy

Accretion onto black holes often proceeds via an accretion disk or a temporary disk-like pattern. Variability features observed in light curves as well as theoretical models of accretion flows suggest that accretion disks tend to be inhomogeneous -- variety of substructures (clumps) emerge within the flow. Rapid orbital motion of individual clumps then modulates the observed signal in X-rays. Furthermore, changes of spectral lines and polarization properties of the observed signal (or the absence of changes) constrain the models and reveal information about general relativity (GR) effects. In this write-up we summarize the basic equations that have been employed to study light propagation near black holes and to derive the radiation signal that can be expected at a detector within the framework of geometrical optics approximation., Comment: 10 pages, 5 figures; to appear in Proceedings of the 10th International Workshop on Astronomical X-Ray Optics in Prague (Czech Republic, 4-7 December 2017)

Magnetic null points can develop near the ergosphere boundary of a rotating black hole by the combined effects of strong gravitational field and the frame-dragging mechanism. The induced electric component does not vanish an efficient process of particle acceleration can occur. Furthermore, the effect of imposed (weak) magnetic field can trigger an onset of chaos. The model set-up appears to be relevant for low-accretion-rate nuclei of some galaxies which exhibit episodic accretion events (such as the Milky Way's supermassive black hole) embedded in a large-scale magnetic field of external origin. We review our recent results and we give additional context for future work with the focus on the role of gravito-magnetic effects caused by rotation of the black hole. While the test motion is strictly regular in the classical black hole space-time, with and without effects of rotation or electric charge, gravitational perturbations and imposed external electromagnetic fields may lead to chaos., write-up of a talk at 11th INTEGRAL/BART Workshop in Karlovy Vary (Czech Republic, 22-25 April 2014); 13 pages, 11 figures (resolution downgraded to reduce the file size; for full resolution see the journal paper)

eXTP is a science mission designed to study the state of matter under extreme conditions of density, gravity and magnetism. Primary targets include isolated and binary neutron stars, strong magnetic field systems like magnetars, and stellar-mass and supermassive black holes. The mission carries a unique and unprecedented suite of state-of-the-art scientific instruments enabling for the first time ever the simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range from 0.5-30 keV (and beyond). Key elements of the payload are: the Spectroscopic Focusing Array (SFA) - a set of 11 X-ray optics for a total effective area of about 0.9 m^2 and 0.6 m^2 at 2 keV and 6 keV respectively, equipped with Silicon Drift Detectors offering, 16 pages, 16 figures. Oral talk presented at SPIE Astronomical Telescopes and Instrumentation, June 26 to July 1, 2016, Edingurgh, UK

A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ~15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from September to late November 2014, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ~350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy., 11 pages, 7 figures, 2 tables; accepted for publication in the Astrophysical Journal Letters; this version with small changes to affiliations

The super-massive 4 million solar mass black hole Sagittarius~A* (SgrA*) shows flare emission from the millimeter to the X-ray domain. A detailed analysis of the infrared light curves allows us to address the accretion phenomenon in a statistical way. The analysis shows that the near-infrared flare amplitudes are dominated by a single state power law, with the low states in SgrA* limited by confusion through the unresolved stellar background. There are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO is one of them. Its nature is unclear. It may be comparable to similar stellar dusty sources in the region or may consist predominantly of gas and dust. In this case a particularly enhanced accretion activity onto SgrA* may be expected in the near future. Here the interpretation of recent data and ongoing observations are discussed., Comment: 30 pages - 7 figures - accepted for publication by Springer's "Fundamental Theories of Physics" series; summarizing GC contributions of 2 conferences: 'Equations of Motion in Relativistic Gravity' at the Physikzentrum Bad Honnef, Bad Honnef, Germany, (Feb. 17-23, 2013) and the COST MP0905 'The Galactic Center Black Hole Laboratory' Granada, Spain (Nov. 19 - 22, 2013)

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of active galactic nuclei. For a summary, we refer to the paper., Comment: White Paper in Support of the Mission Concept of the Large Observatory for X-ray Timing

The origin of irradiation and fluorescence of the 6.4 keV bright giant molecular clouds surrounding Sgr A*, the central supermassive black hole of our Galaxy, remains enigmatic. Testing the theory of a past active period of Sgr A* requires X-ray polarimetry. In this paper, we show how modern imaging polarimeters could revolutionize our understanding of the Galactic Center. Through Monte Carlo modeling, we produce a 4-8 keV polarization map of the Galactic Center, focusing on the polarimetric signature produced by Sgr B1, Sgr B2, G0.11-0.11, Bridge E, Bridge D, Bridge B2, MC2, MC1, Sgr C3, Sgr C2, and Sgr C1. We estimate the resulting polarization, include polarized flux dilution by the diffuse plasma emission detected toward the GC, and simulate the polarization map that modern polarimetric detectors would obtain assuming the performances of a mission prototype. The eleven reflection nebulae investigated in this paper present a variety of polarization signatures, ranging from nearly unpolarized to highly polarized (about 77%) fluxes. A major improvement in our simulation is the addition of a diffuse, unpolarized plasma emission that strongly impacts soft X-ray polarized fluxes. The dilution factor is in the range 50% - 70%, making the observation of the Bridge structure unlikely even in the context of modern polarimetry. The best targets are the Sgr B and Sgr C complexes, and the G0.11-0.11 cloud. An exploratory observation of a few hundred kilo-seconds of the Sgr B complex would allow a significant detection of the polarization and be sufficient to derive hints on the primary source of radiation. A more ambitious program (few Ms) of mapping the giant molecular clouds could then be carried out to probe with great precision the turbulent history of Sgr A*, and place important constraints on the composition and three-dimensional position of the surrounding gas., Comment: 7 pages, 3 figures, 2 tables, accepted for publication in A&A

The Galactic center (GC) holds the closest-to-Earth supermassive black hole (SMBH), which makes it the best laboratory to study the close environment of extremely massive compact objects. Polarimetry is sensitive to geometry of the source, which makes it a particularly suitable technique to probe the medium surrounding the GC SMBH. The detection of hard X-ray spectra and prominent iron K$\alpha$ fluorescence features coincident with localized gas clouds (e.g. Sgr~B2, Sgr~C) is known for nearly twenty years now and is commonly associated with a past outburst of the SMBH whose radiation is reprocessing onto the so-called "reflection nebulae". Since scattering leads to polarization, the re-emitted signal from the giant molecular clouds in the first 100~pc of the GC is expected to be polarized. X-ray polarization measurement is thus particularly adapted to probe the origin of the diffuse X-ray emission from the GC reflection nebulae and reveal the past activity of the central SMBH. In this research note, we summarize the results from past and current polarimetric simulations in order to show how a future X-ray polarimeter equipped with imaging detectors could improve our understanding of high-energy astrophysics., Comment: 4 pages, 2 figures, to be published in the Proceedings of the annual meeting of the French Astronomical Society

We present a statistical analysis of polarized near-infrared (NIR) light from Sgr A*, the radio source associated with the supermassive black hole at the center of the Milky Way. The observations have been carried out using the adaptive optics instrument NACO at the VLT UT4 in the infrared $K_\mathrm{s}$-band from 2004 to 2012. Several polarized flux excursions were observed during these years. Linear polarization at 2.2 $\mu m$, its statistics and time variation, can be used constrain the physical conditions of the accretion process onto this supermassive black hole. With an exponent of about 4 for the number density histogram of fluxes above 5~mJy, the distribution of polarized flux density is closely linked to the single state power-law distribution of the total $K_\mathrm{s}$-band flux densities reported earlier. We find typical polarization degrees of the order of 20\%$\pm$10\% and a preferred polarization angle of 13$^o$$\pm$15$^o$. Simulations show the uncertainties under a total flux density of $\sim 2\,{\rm mJy}$ are probably dominated by observational effects. At higher flux densities there are intrinsic variations of polarization degree and angle within rather well constrained ranges. Since the emission is most likely due to optically thin synchrotron radiation, this preferred polarization angle we find is very likely coupled to the intrinsic orientation of the Sgr A* system i.e. a disk or jet/wind scenario associated with the super massive black hole. If they are indeed linked to structural features of the source the data imply a rather stable geometry and accretion process for the Sgr A* system., Comment: 19 pages, 18 figures, 4 tables, submitted to Astronomy & Astrophysics

We discuss a possibility that a tidal disruption event near a dormant supermassive black hole (SMBH) can give rise to spectral features of iron in 6-7 keV X-ray signal: a relativistic line profile emerges from debris illuminated and ionised by an intense flash produced from the destroyed star. This could provide a unique way to determine parameters of the system. We consider a model where the nuclear stellar population acquires an oblate shape (i.e., a flattened distribution) in the inner region near a supermassive black hole, and also the primary irradiation flare is expected to occur more likely near the equatorial plane, co-planar with the infalling material. This suggests that the reprocessing of primary X-rays results in a transient profile that should be relevant for tidal-disruption events (TDE) in otherwise under-luminous (inactive) galactic nuclei, i.e. with no prior accretion disc. Resonance mechanisms of the stellar motion can increase the orbital eccentricity for some stars in the nuclear cluster and help to bring them close to the tidal radius, where they can give rise to TDEs. The proposed scenario appears to be distinguishably different from the standard scheme of lamp-post model for the origin of the relativistic line in active galaxies, where the source is thought to be located predominantly near the symmetry axis., Comment: 8 pages, 2 figures, submitted to the Proceedings of RAGtime: Workshops on black holes and neutron stars

We explore the conditions for the thermal instability to operate in the mini-spiral region of the Galactic centre (Sgr A*), where both the hot and cold media are known to coexist. The photoionisation Cloudy calculations are performed for different physical states of plasma. We neglect the dynamics of the material and concentrate on the study of the parameter ranges where the thermal instability may operate, taking into account the past history of Sgr A* bolometric luminosity. We show that the thermal instability does not operate at the present very low level of the Sgr A* activity. However, Sgr A* was much more luminous in the past. For the highest luminosity states the two-phase medium can be created up to 1.4 pc from the centre. The presence of dust grains tends to suppress the instability, but the dust is destroyed in the presence of strong radiation field and hot plasma. The clumpiness is thus induced in the high activity period, and the cooling/heating timescales are long enough to preserve later the past multi-phase structure. The instability enhances the clumpiness of the mini-spiral medium and creates a possibility of episodes of enhanced accretion of cold clumps towards Sgr A*. The mechanism determines the range of masses and sizes of clouds; under the conditions of Sgr A*, the likely values come out $1$ - $10^2M_{\oplus}$ for the cloud typical mass., Comment: Accepted for publication in MNRAS, 10 pages, 7 figures

Several massive black holes exhibit flux variability on time scales that correspond to source sizes of the order of few Schwarzschild radii. We survey the potential of near-infrared and X-ray polarimetry to constrain physical properties of such black hole systems, namely their spin and inclination. We have focused on a model where an orbiting hot spot is embedded in an accretion disk. A new method of searching for the time-lag between orthogonal polarization channels is developed and applied to an ensemble of hot spot models that samples a wide range of parameter space. We found that the hot spot model predicts signatures in polarized light which are in the range to be measured directly in the near future. However, our estimations are predicted upon the assumption of a Keplerian velocity distribution inside the flow where the dominant part of the magnetic field is toroidal. We also found that if the right model of the accretion flow can be chosen for each source (e.g. on the basis of magnetohydrodynamics simulations) then the black hole spin and inclination can be constrained to a small two-dimensional area in the spin-inclination space. The results of the application of the method to the available near-infrared polarimetric data of Sagittarius A* (Sgr A*) is presented. It is shown that even with the currently available data the spin and inclination of Sgr A* can be constrained. Next generations of near-infrared and X-ray polarimeters should be able to exploit this tool., Comment: 12 pages, 13 figures, accepted for publication in MNRAS

We report on new modeling results based on the mm- to X-ray emission of the SgrA* counterpart associated with the massive black hole at the Galactic Center. Our modeling is based on simultaneous observations carried out on 07 July, 2004, using the ESO NACO adaptive optics instrument and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the SMA and the VLA. The observations revealed several flare events in all wavelength domains. Here we show that a combined synchrotron self-Compton (SSC) model followed by an adiabatic expansion of the source components can fully account for the observed flare flux densities and delay times covering the spectral range from the X-ray to the mm-radio domain. The derived physical quantities that describe the flare emission give a blob expansion speed of v{exp}=0.005c, magnetic field of < 60G and spectral indices of 0.8 to 1.4. The derived model parameters suggest that the adiabatic expansion takes place in source components that have a bulk motion larger than v{exp} or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*., 13 pages, 10 figures, A&A in press

12 pages, 9 figures, 3 tables.-- In press with Astronomy & Astrophysics., We report on a successful, simultaneous observation and modeling of the sub-millimeter to near-infrared flare emission of the Sgr A* counterpart associated with the super-massive black hole at the Galactic center. Our modeling is based on simultaneous observations that have been carried out on 03 June, 2008 using the NACO adaptive optics (AO) instrument at the ESO VLT and the LABOCA bolometer at the APEX telescope. Inspection and modeling of the light curves show that the sub-mm follows the NIR emission with a delay of 1.5+/-0.5 hours. We explain the flare emission delay by an adiabatic expansion of the source components., We are grateful to all the ESO PARANAL and Sequitor staff, and especially to the members of the NAOS/CONICA, VLTI, and APEX team. The observations were made possible through a special effort by the APEX/ONSALA staff to have the LABOCA bolometer ready for triggering. Leo Meyer is supported by the DAAD exchange program. Macarena García-Marín is supported by the German federal department for education and research (BMBF) under the project numbers: 50OS0502 & 50OS0801. The X-ray work was supported by NASA through Chandra award G05-6093X. M. Zamaninasab, D. Kunneriath, and R.-S. Lu, are members of the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the MPIfR and the Universities of Bonn and Cologne. R. Schödel acknowledges support by the Ramón y Cajal programme by the Ministerio de Ciencia e Innovación of the government of Spain.

Black holes cannot support their own internal magnetic field like, for example, compact stars can. Despite this fact observations indicate that event horizons of supermassive black holes (SMBH) are threaded by field lines along which plasma streams flow. Various magnetohydrodynamical mechanisms have been suggested to generate turbulent magnetic fields on small scales, however, the origin of the large-scale component is unclear. In this write-up we describe our progress in an on-going work and discuss the possibility of dipole-type magnetic fields being brought onto SMBH by magnetized neutron stars, which are expected to drift inward from a hidden population in the Nuclear Star Cluster. This can contribute to an organised component of the magnetic field on the characteristic length-scale of the stellar size, which thread the horizon during the final stages of the magnetized star plunge into or its close flyby around SMBH. Because of mass--size scaling relations for black holes, the effect is more important for lower-mass SMBH., Comment: 9 pages, 2 figures; to appear in Proceedings of the 14th INTEGRAL/BART Workshop in Karlovy Vary (Czech Republic, 3-7 April 2017)