Entity

Time filter

Source Type

München, Germany

Broderick A.E.,Perimeter Institute for Theoretical Physics | Broderick A.E.,University of Waterloo | Narayan R.,Harvard - Smithsonian Center for Astrophysics | Kormendy J.,University of Texas at Austin | And 6 more authors.
Astrophysical Journal | Year: 2015

The 6 × 109 MO supermassive black hole at the center of the giant elliptical galaxy M87 powers a relativistic jet. Observations at millimeter wavelengths with the Event Horizon Telescope have localized the emission from the base of this jet to angular scales comparable to the putative black hole horizon. The jet might be powered directly by an accretion disk or by electromagnetic extraction of the rotational energy of the black hole. However, even the latter mechanism requires a confining thick accretion disk to maintain the required magnetic flux near the black hole. Therefore, regardless of the jet mechanism, the observed jet power in M87 implies a certain minimum mass accretion rate. If the central compact object in M87 were not a black hole but had a surface, this accretion would result in considerable thermal near-infrared and optical emission from the surface. Current flux limits on the nucleus of M87 strongly constrain any such surface emission. This rules out the presence of a surface and thereby provides indirect evidence for an event horizon. © 2015. The American Astronomical Society. All rights reserved. Source


Butler K.,Universitats Sternwarte
EAS Publications Series | Year: 2012

A brief summary of some aspects of atomic physics theory useful for stellar atmospheres is given. © EAS, EDP Sciences 2010. Source


Markova N.,Institute of Astronomy with NAO | Puls J.,Universitats Sternwarte | Simon-Diaz S.,Institute of Astrophysics of Canarias | Simon-Diaz S.,University of La Laguna | And 4 more authors.
Astronomy and Astrophysics | Year: 2014

Context. Rotation is of key importance for the evolution of massive star, including their fate as supernovae or gamma-ray bursts. However, the rotational velocities of OB stars are difficult to determine. Aims. Based on our own data for 31 Galactic O stars and incorporating similar data for 86 OB supergiants from the literature, we aim at investigating the properties of rotational and extra line-broadening as a function of stellar parameters and at testing model predictions about the evolution of stellar rotation. Methods. Fundamental stellar parameters were determined by means of the code FASTWIND. Projected rotational and extra broadening velocities, v sin i and θRT, originate from a combined Fourier transform and the goodness-of-fit method. Model calculations published previously were used to estimate the initial evolutionary masses, Minit evol. Results. The sample O stars with Minit evol50 M rotate with less that 26% of their break-up velocity, and they also lack slow rotators (v sin i 50 km s -1). For the more massive stars (Minit evol ≥ 35 M) on the hotter side of the bi-stability jump, the observed and predicted rotational rates agree quite well; for those on the cooler side of the jump, the measured velocties are systematically higher than the predicted ones. In general, the derived θRT values decrease toward cooler T eff, whilst for later evolutionary phases they appear, at the same v sin i, higher for high-mass stars than for low-mass ones. None of the sample stars shows θRT ≥ 110 km s-1. For the majority of the more massive stars, extra broadening either dominates or is in strong competition with rotation. Conclusions. For OB stars of solar metallicity, extra broadening is important and has to be accounted for in the analysis. When appearing at or close to the zero-age main sequence, most of the single and more massive stars rotate slower than previously thought. Model predictions for the evolution of rotation in hot massive stars may need to be updated. © ESO 2014. Source


Umetsu K.,Academia Sinica, Taiwan | Zitrin A.,California Institute of Technology | Gruen D.,Universitats Sternwarte | Gruen D.,Max Planck Institute for Extraterrestrial Physics | And 5 more authors.
Astrophysical Journal | Year: 2016

We present a comprehensive analysis of strong-lensing, weak-lensing shear and magnification data for a sample of 16 X-ray-regular and 4 high-magnification galaxy clusters at 0.19 ≲ z ≲ 0.69 selected from Cluster Lensing And Supernova survey with Hubble (CLASH). Our analysis combines constraints from 16-band Hubble Space Telescope observations and wide-field multi-color imaging taken primarily with Suprime-Cam on the Subaru Telescope, spanning a wide range of cluster radii (10″-16′). We reconstruct surface mass density profiles of individual clusters from a joint analysis of the full lensing constraints, and determine masses and concentrations for all of the clusters. We find the internal consistency of the ensemble mass calibration to be ≤5% ± 6% in the one-halo regime (200-2000 kpc h-1) compared to the CLASH weak-lensing-only measurements of Umetsu et al. For the X-ray-selected subsample of 16 clusters, we examine the concentration-mass (c-M) relation and its intrinsic scatter using a Bayesian regression approach. Our model yields a mean concentration of c|z=0.34 = 3.95 ± 0.35 at M200c ≃14 × 1014M⊙ and an intrinsic scatter of σ (ln c200c) = 0.13 ± 0.06, which is in excellent agreement with Λ cold dark matter predictions when the CLASH selection function based on X-ray morphological regularity and the projection effects are taken into account. We also derive an ensemble-averaged surface mass density profile for the X-ray-selected subsample by stacking their individual profiles. The stacked lensing signal is detected at 33σ significance over the entire radial range ≤4000 kpc h-1, accounting for the effects of intrinsic profile variations and uncorrelated large-scale structure along the line of sight. The stacked mass profile is well described by a family of density profiles predicted for cuspy dark-matter-dominated halos in gravitational equilibrium, namely, the Navarro-Frenk-White (NFW), Einasto, and DARKexp models, whereas the single power-law, cored isothermal and Burkert density profiles are disfavored by the data. We show that cuspy halo models that include the large-scale two-halo term provide improved agreement with the data. For the NFW halo model, we measure a mean concentration of c200c= 3.79+0.30 -0.28 at M200c= 14.1+1.0 -1.0 × 1014 M⊙, demonstrating consistency between thecomplementary analysis methods. © 2016. The American Astronomical Society. All rights reserved. Source


Broz M.,Charles University | Mayer P.,Charles University | Pribulla T.,Universitats Sternwarte | Pribulla T.,Slovak Academy of Sciences | And 3 more authors.
Astronomical Journal | Year: 2010

The multiple system V505 Sagittarii is composed of at least three stars: a compact eclipsing pair and a distant component, whose orbit is measured directly using speckle interferometry. In order to explain the observed orbit of the third body in V505 Sagittarii and also other observable quantities, namely the minima timings of the eclipsing binary and three different radial velocities (RVs) detected in the spectrum, we thoroughly test a fourth-body hypothesis - a perturbation by a dim, yet-unobserved object. We use an N-body numerical integrator to simulate future and past orbital evolution of three or four components in this system. We construct a suitable χ2 metric from all available speckle-interferometry, minima-timings, and RV data and we scan a part of a parameter space to get at least some of the possible solutions. In principle, we are able to explain all observable quantities by the presence of a fourth body, but the resulting likelihood of this hypothesis is very low. We also discuss other theoretical explanations of the minima-timing variations. Further observations of the minima timings during the next decade or high-resolution spectroscopic data can significantly constrain the model. © 2010. The American Astronomical Society. All rights reserved. Source

Discover hidden collaborations