Hamburger Sternwarte

Hamburg, Germany

Hamburger Sternwarte

Hamburg, Germany
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Wittor D.,Hamburger Sternwarte | Vazza F.,Hamburger Sternwarte | Bruggen M.,Hamburger Sternwarte
Monthly Notices of the Royal Astronomical Society | Year: 2017

Weak shocks in the intracluster medium may accelerate cosmic-ray protons and cosmic-ray electrons differently depending on the angle between the upstream magnetic field and the shock normal. In this work, we investigate how shock obliquity affects the production of cosmic rays in high-resolution simulations of galaxy clusters. For this purpose, we performed a magnetohydrodynamical simulation of a galaxy cluster using the mesh refinement code ENZO. We use Lagrangian tracers to follow the properties of the thermal gas, the cosmic rays and the magnetic fields over time.We tested a number of different acceleration scenarios by varying the obliquity-dependent acceleration efficiencies of protons and electrons, and by examining the resulting hadronic γ-ray and radio emission. We find that the radio emission does not change significantly if only quasi-perpendicular shocks are able to accelerate cosmic-ray electrons. Our analysis suggests that radio-emitting electrons found in relics have been typically shocked many times before z = 0. On the other hand, the hadronic γ-ray emission from clusters is found to decrease significantly if only quasi-parallel shocks are allowed to accelerate cosmic ray protons. This might reduce the tension with the low upper limits on γ-ray emission from clusters set by the Fermi satellite. © 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Vazza F.,Hamburger Sternwarte | Vazza F.,National institute for astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2017

The emergence of cosmic structure is commonly considered one of the most complex phenomena in nature. However, this complexity has never been defined nor measured in a quantitative and objective way. In this work, we propose a method to measure the information content of cosmic structure and to quantify the complexity that emerges from it, based on Information Theory. The emergence of complex evolutionary patterns is studied with a statistical symbolic analysis of the datastream produced by state-of-the-art cosmological simulations of forming galaxy clusters. This powerful approach allows us to measure how many bits of information is necessary to predict the evolution of energy fields in a statistical way, and it offers a simple way to quantify when, where and how the cosmic gas behaves in complex ways. The most complex behaviours are found in the peripheral regions of galaxy clusters, where supersonic flows drive shocks and large energy fluctuations over a few tens of million years. Describing the evolution of magnetic energy requires at least twice as large amount of bits as required for the other energy fields. When radiative cooling and feedback from galaxy formation are considered, the cosmic gas is overall found to double its degree of complexity. In the future, Cosmic Information Theory can significantly increase our understanding of the emergence of cosmic structure as it represents an innovative framework to design and analyse complex simulations of the Universe in a simple, yet powerful way. © 2016 The Authors.

Schmitt J.H.M.M.,Hamburger Sternwarte
Astronomische Nachrichten | Year: 2017

The field of extrasolar planets has become one of the most lively and vibrant field of research in astrophysics. As is almost always the case in astrophysics, a multi-wavelength approach is required to fully explore and understand the properties of those planets. Also, X-ray astronomy plays an important role in this process. The host stars of essentially all extrasolar planets are (sometimes very vigorous) X-ray emitters, which can severely impact on the outer atmospheric layers of their planets. Furthermore, the close proximity between host stars and planets in the case of close-in “Hot Jupiters” may lead to magnetic or tidal interactions with observable consequences at X-ray wavelengths. I will address these issues and discuss how XMM-Newton can be used to advance the field. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Ruszkowski M.,University of Michigan | Bruggen M.,Hamburger Sternwarte | Lee D.,University of Chicago | Shin M.-S.,University of Oxford
Astrophysical Journal | Year: 2014

Ram pressure stripping can remove significant amounts of gas from galaxies in clusters and massive groups and thus has a large impact on the evolution of cluster galaxies. Recent observations have shown that key properties of ram-pressure-stripped tails of galaxies, such as their width and structure, are in conflict with predictions by simulations. To increase the realism of existing simulations, we simulated for the first time a disk galaxy exposed to a uniformly magnetized wind including radiative cooling and self-gravity of the gas. We find that magnetic fields have a strong effect on the morphology of the gas in the tail of the galaxy. While in the purely hydrodynamical case the tail is very clumpy, the magnetohydrodynamical case shows very filamentary structures in the tail. The filaments can be strongly supported by magnetic pressure and, wherever this is the case, the magnetic fields vectors tend to be aligned with the filaments. The ram pressure stripping process may lead to the formation of magnetized density tails that appear as bifurcated in the plane of the sky and resemble the double tails observed in ESO 137-001 and ESO 137-002. Such tails can be formed under a variety of situations, both for the disks oriented face-on with respect to the intracluster medium (ICM) wind and for the tilted ones. While this bifurcation is the consequence of the generic tendency for the magnetic fields to produce very filamentary tail morphology, the tail properties are further shaped by the combination of the magnetic field orientation and the sliding of the field past the disk surface exposed to the wind. Despite the fact that the effect of the magnetic field on the morphology of the tail is strong, magnetic draping does not strongly change the rate of gas stripping. For a face-on galaxy, the field tends to reduce the amount of gas stripping compared to the pure hydrodynamical case, and is associated with the formation of a stable magnetic draping layer on the side of the galaxy exposed to the incoming ICM wind. For significantly tilted disks, the situation may be reversed and the stripping rate may be enhanced by the "scraping" of the disk surface by the magnetic fields sliding past the ISM/ICM interface. Instabilities, such as gravitational instabilities, undo the protective effect of this layer and allow the gas to leak out of the galaxy. © 2014. The American Astronomical Society. All rights reserved..

Claret A.,Institute Astrofisica Of Andalucia | Hauschildt P.H.,Hamburger Sternwarte | Witte S.,Hamburger Sternwarte
Astronomy and Astrophysics | Year: 2012

Aims. The knowledge of how the specific intensity is distributed over the stellar disk is crucial for interpreting the light curves of extrasolar transiting planets, double-lined eclipsing binaries, and other astrophysical phenomena. To provide theoretical inputs for light curve modelling codes, we present new calculations of limb-darkening coefficients for the spherically symmetric phoenix models. Methods. The limb-darkening coefficients were computed by covering the transmission curves of Kepler, CoRoT, and Spitzer space missions, as well as the passbands of the Strömgren, Johnson-Cousins, Sloan, and 2MASS. These computations adopted the least-square method. In addition, we also calculated the linear and bi-parametric approximations by adopting the flux conservation method as an additional tool for estimating the theoretical error bars in the limb-darkening coefficients. Results. Six laws were used to describe the specific intensity distribution: linear, quadratic, square root, logarithmic, exponential, and a more general one with 4 terms. The computations are presented for the solar chemical composition, with log g varying between 2.5 and 5.5 and effective temperatures between 1500-4800 K. The adopted microturbulent velocity and the mixing-length parameters are 2.0 kmâ ‰ s -1 and 2.0, respectively. ©2012 ESO.

Vazza F.,Hamburger Sternwarte | Vazza F.,National institute for astrophysics | Bruggen M.,Hamburger Sternwarte
Monthly Notices of the Royal Astronomical Society | Year: 2014

Radio relics in galaxy clusters are thought to be associated with powerful shock waves that accelerate particles via diffusive shock acceleration (DSA). Among the particles accelerated by DSA, relativistic protons should outnumber electrons by a large factor.While the relativistic electrons emit synchrotron emission detectable in the radio band, the protons interact with the thermal gas to produce gamma-rays in hadronic interactions. Using simple models for the propagation of shock waves through clusters, the distribution of thermal gas and the efficiency of DSA, we find that the resulting hadronic gamma-ray emission lies very close or above the upper limits from the Fermi data on nearby clusters. This suggests that the relative acceleration efficiency of electrons and protons is at odds with predictions from DSA. The inclusion of re-accelerated 'fossil' particles does not seem to solve the problem. Our study highlights a possible tension of the commonly assumed scenario for the formation of radio relics and we discuss possible solutions to the problem. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Poppenhaeger K.,Hamburger Sternwarte | Schmitt J.H.M.M.,Hamburger Sternwarte
Astrophysical Journal | Year: 2011

The activity levels of stars are influenced by several stellar properties, such as stellar rotation, spectral type, and the presence of stellar companions. Analogous to binaries, planetary companions are also thought to be able to cause higher activity levels in their host stars, although at lower levels. Especially in X-rays, such influences are hard to detect because coronae of cool stars exhibit a considerable amount of intrinsic variability. Recently, a correlation between the mass of close-in exoplanets and their host star's X-ray luminosity has been detected, based on archival X-ray data from the ROSAT All-Sky Survey. This finding has been interpreted as evidence for star-planet interactions. We show in our analysis that this correlation is caused by selection effects due to the flux limit of the X-ray data used and due to the intrinsic planet detectability of the radial velocity method, and thus does not trace possible planet-induced effects. We also show that the correlation is not present in a corresponding complete sample derived from combined XMM-Newton and ROSAT data. © 2011. The American Astronomical Society. All rights reserved..

Kohl S.,Hamburger Sternwarte
Contributions of the Astronomical Observatory Skalnate Pleso | Year: 2014

The performance of a near infrared camera for photometry is investigated by observations of multiple samples of spatially close non-variable stars. Objects with a J-band magnitude less than 9.2 have been observed by the application of differential photometry. For the faint objects a statistical scatter of 7.9 mmag has been determined. Possible future applications include the examination of planets orbiting late type stars by an eclipse or a timing analysis.

Poppenhaeger K.,Harvard - Smithsonian Center for Astrophysics | Schmitt J.H.M.M.,Hamburger Sternwarte | Wolk S.J.,Harvard - Smithsonian Center for Astrophysics
Astrophysical Journal | Year: 2013

We present new X-ray observations obtained with Chandra ACIS-S of the HD 189733 system, consisting of a K-type star orbited by a transiting Hot Jupiter and an M-type stellar companion. We report a detection of the planetary transit in soft X-rays with a significantly deeper transit depth than observed in the optical. The X-ray data favor a transit depth of 6%-8%, versus a broadband optical transit depth of 2.41%. While we are able to exclude several possible stellar origins for this deep transit, additional observations will be necessary to fully exclude the possibility that coronal inhomogeneities influence the result. From the available data, we interpret the deep X-ray transit to be caused by a thin outer planetary atmosphere which is transparent at optical wavelengths, but dense enough to be opaque to X-rays. The X-ray radius appears to be larger than the radius observed at far-UV wavelengths, most likely due to high temperatures in the outer atmosphere at which hydrogen is mostly ionized. We furthermore detect the stellar companion HD 189733B in X-rays for the first time with an X-ray luminosity of log LX = 26.67 erg s-1. We show that the magnetic activity level of the companion is at odds with the activity level observed for the planet-hosting primary. The discrepancy may be caused by tidal interaction between the Hot Jupiter and its host star. © 2013. The American Astronomical Society. All rights reserved.

Ogrean G.A.,Hamburger Sternwarte | Bruggen M.,Hamburger Sternwarte
Monthly Notices of the Royal Astronomical Society | Year: 2013

The Coma cluster is one of the nearest galaxy clusters, and the first one in which a peripheral radio relic was discovered. However, X-ray observations of the plasma near the relic have been scarce. Here, we present results from a re-analysis of a 22 ks archival XMM-Newton observation. Across the relic, we detect a temperature discontinuity indicative of a shock of Mach number ~2. This challenges the previously suggested hypothesis that the relic was formed by turbulence. Furthermore, multiwavelength observations and numerical models do not support the scenario in which the shock is an outgoing cluster-merger shock. Instead, our results lend support to the idea that the relic coincides with an infall shock front formed just as a 'wall' of galaxies, possibly associated with NGC 4839, falls on to the cluster along a cosmic filament. © 2013.

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