T. K. Fritz

A gas cloud on its way towards the supermassive black hole at the Galactic Centre

Measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* at the Galactic Centre is a black hole four million times the mass of the Sun. With the exception of modest X-ray and infrared flares, Sgr A* is surprisingly faint, suggesting that the accretion rate and radiation efficiency near the event horizon are currently very low. Here we report the presence of a dense gas cloud approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations tightly constrain the cloud’s orbit to be highly eccentric, with an innermost radius of approach of only ∼3,100 times the event horizon that will be reached in 2013. Over the past three years the cloud has begun to disrupt, probably mainly through tidal shearing arising from the black hole’s gravitational force. The cloud’s dynamic evolution and radiation in the next few years will probe the properties of the accretion flow and the feeding processes of the supermassive black hole. The kilo-electronvolt X-ray emission of Sgr A* may brighten significantly when the cloud reaches pericentre. There may also be a giant radiation flare several years from now if the cloud breaks up and its fragments feed gas into the central accretion zone.

Kinematics of the old stellar population at the Galactic centre

Aims. We aim at a detailed description of the kinematic properties of the old, (several Gyrs) late-type CO-absorption star population among the Galactic centre (GC) cluster stars. This cluster is composed of a central supermassive black hole (Sgr A*) and a selfgravitating system of stars. Understanding its kinematics thus offers the opportunity to understand the dynamical interaction between a central point mass and the surrounding stars in general, especially in view of understanding other galactic nuclei. Methods. We applied AO-assisted, near-infrared imaging and integral-field spectroscopy using the instruments NAOS/CONICA and SINFONI at the VLT. We obtained proper motions for 5445 stars, 3D velocities for 664 stars, and acceleration limits (in the sky plane) for 750 stars. Global kinematic properties were analysed using velocity and velocity dispersion distributions, phase-space maps, twopoint correlation functions, and the Jeans equation. Results. We detect for the first time significant cluster rotation in the sense of the general Galactic rotation in proper motions. Out of the 3D velocity dispersion, we derive an improved statistical parallax for the GC of R0 = 8.07 ± 0.32stat ± 0.13sys kpc. The distribution of 3D stellar speeds can be approximated by local Maxwellian distributions. Kinematic modelling provides deprojected 3D kinematic parameters, including the mass profile of the cluster. We find an upper limit of 4% for the amplitude of fluctuations in the phase-space distribution of the cluster stars compared to a uniform, spherical model cluster. Using upper limits on accelerations, we constrain the minimum line-of-sight distances from the plane of Sgr A* of five stars located within the innermost few (projected) arcsec. The stars within 0.7 �� radius from the star group IRS13E do not co-move with this group, making it unlikely that IRS13E is the core of a substantial star cluster. Overall, the GC late-type cluster is described well as a uniform, isotropic, rotating, dynamically relaxed, phase-mixed system.

Simulations of the origin and fate of the galactic center cloud G2

We investigate the origin and fate of the recently discovered gas cloud G2 close to the Galactic center. Our hydrodynamical simulations focusing on the dynamical evolution of the cloud in combination with currently available observations favor two scenarios: a Compact Cloud which started around the year 1995 and a Spherical Shell of gas, with an apocenter distance within the disk(s) of young stars and a radius of a few times the size of the Compact Cloud. The former is able to explain the detected signal of G2 in the position-velocity (PV) diagram of the Br? emission of the year 2008.5 and 2011.5 data. The latter can account for both G2s signal as well as the fainter extended tail-like structure G2t seen at larger distances from the black hole and smaller velocities. In contrast, gas stripped from a compact cloud by hydrodynamical interactions is not able to explain the location of the detected G2t emission in the observed PV diagrams. This favors the Spherical Shell Scenario and might be a severe problem for the Compact Cloud as well as the so-called Compact Source Scenario. From these first idealized simulations, we expect a roughly constant feeding of the supermassive black hole through a nozzle-like structure over a long period, starting shortly after the closest approach in 2013.51 for the Compact Cloud. If the matter accretes in the hot accretion mode, we do not expect a significant boost of the current activity of Sgr?A* for the Compact Cloud model, but a boost of the average infrared and X-ray luminosity by roughly a factor of 80 for the Spherical Shell Scenario with order of magnitude variations on a timescale of a few months. Assuming that a part of the gas is accreted in cold disk mode, even higher boost factors can be reached. The near-future evolution of the cloud will be a sensitive probe of the conditions of the gas distribution in the milli-parsec environment of the massive black hole in the Galactic center.

The Two States of Sgr A* in the Near-infrared: Bright Episodic Flares on Top of Low-level Continuous Variability

In this paper we examine properties of the variable source Sgr A* in the near-infrared (NIR) using a very extensive Ks-band data set from NACO/VLT observations taken 2004 to 2009. We investigate the variability of Sgr A* with two different photometric methods and analyze its flux distribution. We find Sgr A* is continuously emitting and continuously variable in the near-infrared, with some variability occurring on timescales as long as weeks. The flux distribution can be described by a lognormal distribution at low intrinsic fluxes (. 5 mJy, dereddened with AKs = 2.5). The lognormal distribution has a median flux of �1.1 mJy, but above 5 mJy the flux distribution is significantly flatter (high flux events are more common) than expected for the extrapolation of the lognormal distribution to high fluxes. We make a general identification of the low level emission above 5 mJy as flaring emission and of the low level emission as the quiescent state. We also report here the brightest Ks-band flare ever observed (from August 5th, 2008) which reached an intrinsic Ks-band flux of 27.5 mJy (mKs = 13.5). This flare was a factor 27 increase over the median flux of Sgr A*, close to double the brightness of the star S2, and 40% brighter than the next brightest flare ever observed from Sgr A*. Subject headings: accretion, accretion disks — black hole physics — infrared: general — Galaxy: center

HYDRODYNAMICAL SIMULATIONS OF A COMPACT SOURCE SCENARIO FOR THE GALACTIC CENTER CLOUD G2

The origin of the dense gas cloud G2 discovered in the Galactic Center is still a debated puzzle. G2 might be a diffuse cloud or the result of an outflow from an invisible star embedded in it. We present hydrodynamical simulations of the evolution of different spherically symmetric winds of a stellar object embedded in G2. We find that the interaction with the ambient medium and the extreme gravitational field of the supermassive black hole in the Galactic Center must be taken into account in such a source scenario. The thermal pressure of the hot and dense atmosphere confines the wind, while its ram pressure shapes it via stripping along the orbit, with the details depending on the wind parameters. Tidal forces squeeze the wind near pericenter, reducing it to a thin and elongated filament. We also find that in this scenario most of the Brγ luminosity is expected to come from the densest part of the wind, which has a highly filamentary structure with a low filling factor. For our assumed atmosphere, the observations can be best matched by a mass outflow rate of and a wind velocity of v w = 50 km s–1. These values are comparable with those of a young T Tauri star wind, as already suggested by Scoville & Burkert.

What is limiting near‐infrared astrometry in the Galactic Centre?

We systematically investigate the error sources for high-precision astrometry from adaptive optics (AO) based near-infrared imaging data. We focus on the application in the crowded stellar field in the Galactic Centre. We show that at the level of ≲ 100 μas a number of effects are limiting the accuracy. Most important are the imperfectly subtracted seeing haloes of neighbouring stars, residual image distortions and unrecognized confusion of the target source with fainter sources in the background. Further contributors to the error budget are the uncertainty in estimating the point-spread function, the signal-to-noise ratio induced statistical uncertainty, coordinate transformation errors, the chromaticity of refraction in Earths atmosphere, the post-AO differential tilt jitter and anisoplanatism. For stars as bright as m K = 14, residual image distortions limit the astrometry, for fainter stars the limitation is set by the seeing haloes of the surrounding stars. In order to improve the astrometry substantially at the current generation of telescopes, an AO system with high performance and weak seeing haloes over a relatively small field (r ≲ 3 arcsec) is suited best. Furthermore, techniques to estimate or reconstruct the seeing halo could be promising.

Beyond Bell's theorem: correlation scenarios

Bells theorem witnesses that the predictions of quantum theory cannot be reproduced by theories of local hidden variables in which observers can choose their measurements independently of the source. Working out an idea of Branciard, Rosset, Gisin and Pironio, we consider scenarios which feature several sources, but no choice of measurement for the observers. Every Bell scenario can be mapped into such a correlation scenario, and Bells theorem then discards those local hidden variable theories in which the sources are independent. However, most correlation scenarios do not arise from Bell scenarios, and we describe examples of (quantum) non-locality in some of these scenarios, while posing many open problems along the way. Some of our scenarios have been considered before by mathematicians in the context of causal inference.

High-precision astrometry with MICADO at the European Extremely Large Telescope

In this article, we identify and discuss various statistical and systematic effects influencing the astrometric accuracy achievable with Multi-adaptive optics Imaging CAmera for Deep Observations, the near-infrared (NIR) imaging camera proposed for the 42-m European Extremely Large Telescope. These effects are instrumental (e.g. geometric distortion), atmospheric (e.g. chromatic differential refraction) and astronomical (reference source selection). We find that there are several phenomena having impact on ∼100 μas scales, meaning they can be substantially larger than the theoretical statistical astrometric accuracy of an optical/NIR 42-m telescope. Depending on type, these effects need to be controlled via dedicated instrumental design properties or via dedicated calibration procedures. We conclude that if this is done properly, astrometric accuracies of 40 μas or better - with 40 μas yr ―1 in proper motions corresponding to ≈20 km s ―1 at 100 kpc distance - can be achieved in one epoch of actual observations.

The shape of the inner Milky Way halo from observations of the Pal 5 and GD-1 stellar streams

We constrain the shape of the Milky Ways halo by dynamical modeling of the observed phase-space tracks of the Pal 5 and GD-1 tidal streams. We find that the only information about the potential gleaned from the tracks of these streams are precise measurements of the shape of the gravitational potential---the ratio of vertical to radial acceleration---at the location of the streams, with weaker constraints on the radial and vertical accelerations separately. The latter will improve significantly with precise proper-motion measurements from Gaia. We measure that the overall potential flattening is 0.95 +/- 0.04 at the location of GD-1 ([R,z] ~ [12.5,6.7] kpc) and 0.94 +/- 0.05 at the position of Pal 5 ([R,z] ~ [8.4,16.8] kpc). Combined with constraints on the force field near the Galactic disk, we determine that the axis ratio of the dark-matter halos density distribution is 1.05 +/- 0.14 within the inner 20 kpc, with a hint that the halo becomes more flattened near the edge of this volume. The halo mass within 20 kpc is 1.1 +/- 0.1 x 10^{11} M_sun. A dark-matter halo this close to spherical is in tension with the predictions from numerical simulations of the formation of dark-matter halos.

Quantum correlations in the temporal Clauser–Horne–Shimony–Holt (CHSH) scenario

We consider a temporal version of the Clauser–Horne–Shimony–Holt (CHSH) scenario using projective measurements on a single quantum system. It is known that quantum correlations in this scenario are fundamentally more general than correlations obtainable with the assumptions of macroscopic realism and non-invasive measurements. In this paper, we also educe some fundamental limitations of these quantum correlations. One result is that a set of correlators can appear in the temporal CHSH scenario if and only if it can appear in the usual spatial CHSH scenario. In particular, we derive the validity of the Tsirelson bound and the impossibility of Popescu–Rohrlich (PR)-box behavior. The strength of possible signaling also turns out to be surprisingly limited, giving a maximal communication capacity of approximately 0.32 bit. We also find a temporal version of Hardys non-locality paradox with a maximal quantum value of 1/4.We consider a temporal version of the CHSH scenario using projective measurements on a single quantum system. It is known that quantum correlations in this scenario are fundamentally more general than correlations obtainable with the assumptions of macroscopic realism and non-invasive measurements. In this work, we also educe some fundamental limitations of these quantum correlations. One result is that a set of correlators can appear in the temporal CHSH scenario if and only if it can appear in the usual spatial CHSH scenario. In particular, we derive the validity of the Tsirelson bound and the impossibility of PR-box behavior. The strength of possible signaling also turns out to be surprisingly limited, giving a maximal communication capacity of approximately 0.32 bits. We also find a temporal version of Hardy’s nonlocality paradox with a maximal quantum value of 1/4.