These are the latest articles published in The Astrophysical Journal.

• Open/close Additional Massive Binaries in the Cygnus OB2 Association

  Daniel C. Kiminki et al. 2012 ApJ 747 41 Tag this article

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  We report the discovery and orbital solutions for two new OB binaries in the Cygnus OB2 Association, MT311 (B2V + B3V) and MT605 (B0.5V + B2.5:V). We also identify the system MT429 as a probable triple system consisting of a tight eclipsing 2.97 day B3V+B6V pair and a B0V at a projected separation of 138 AU. We further provide the first spectroscopic orbital solutions to the eclipsing, double-lined, O-star binary MT696 (O9.5V + B1:V), the double-lined, early B binary MT720 (B0-1V + B1-2V), and the double-lined, O-star binary MT771 (O7V + O9V). These systems exhibit orbital periods between 1.5 days and 12.3 days, with the majority having P <6 days. The two new binary discoveries and six spectroscopic solutions bring the total number of known massive binaries in the central region of the Cygnus OB2 Association to 20, with all but two having full orbital solutions.

• Open/close A Significant Problem with Using the Amati Relation for Cosmological Purposes

  Andrew C. Collazzi et al. 2012 ApJ 747 39 Tag this article

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  We consider the distribution of many samples of gamma-ray bursts when plotted in a diagram with their bolometric fluence ( S _bolo) versus the observed photon energy of peak spectral flux ( E _{peak, obs}). In this diagram, all bursts that obey the Amati relation (a luminosity relation where the total burst energy has a power-law relation to E _{peak, obs}) must lie above some limiting line, although observational scatter is expected to be substantial. We confirm that early bursts with spectroscopic redshifts are consistent with this Amati limit. But we find that the bursts from BATSE, Swift, Suzaku, and Konus are all greatly in violation of the Amati limit, and this is true whether or not the bursts have measured spectroscopic redshifts. That is, the Amati relation has definitely failed. In the S _bolo- E _{peak, obs} diagram, we find that every satellite has a greatly different distribution. This requires that selection effects are dominating these distributions, which we quantitatively identify. For detector selections, the trigger threshold and the threshold for the burst to obtain a measured E _{peak, obs} combine to make a diagonal cutoff with the position of this cutoff varying greatly detector to detector. For selection effects due to the intrinsic properties of the burst population, the distribution of E _{peak, obs} makes bursts with low and high values rare, while the fluence distribution makes bright bursts relatively uncommon. For a detector with a high threshold, the combination of these selection effects serves to allow only bursts within a region along the Amati limit line to be measured, and these bursts will then appear to follow an Amati relation. Therefore, the Amati relation is an artifact of selection effects within the burst population and the detector. As such, the Amati relation should not be used for cosmological tasks. This failure of the Amati relation is in no way prejudicial against the other luminosity relations.

• Open/close Topology of a Large-scale Structure as a Test of Modified Gravity

  Xin Wang et al. 2012 ApJ 747 48 Tag this article

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  The genus of the isodensity contours is a robust measure of the topology of a large-scale structure, and it is relatively insensitive to nonlinear gravitational evolution, galaxy bias, and redshift-space distortion. We show that the growth of density fluctuations is scale dependent even in the linear regime in some modified gravity theories, which opens a new possibility of testing the theories observationally. We propose to use the genus of the isodensity contours, an intrinsic measure of the topology of the large-scale structure, as a statistic to be used in such tests. In Einstein's general theory of relativity, density fluctuations grow at the same rate on all scales in the linear regime, and the genus per comoving volume is almost conserved as structures grow homologously, so we expect that the genus-smoothing-scale relation is basically time independent. However, in some modified gravity models where structures grow with different rates on different scales, the genus-smoothing-scale relation should change over time. This can be used to test the gravity models with large-scale structure observations. We study the cases of the theory, DGP braneworld theory as well as the parameterized post-Friedmann models. We also forecast how the modified gravity models can be constrained with optical/IR or redshifted 21 cm radio surveys in the near future.

• Open/close High-resolution XMM-Newton Spectroscopy of the Cooling Flow Cluster A3112

  G. Esra Bulbul et al. 2012 ApJ 747 32 Tag this article

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  We examine high signal-to-noise XMM-Newton European Photon Imaging Camera (EPIC) and Reflection Grating Spectrometer (RGS) observations to determine the physical characteristics of the gas in the cool core and outskirts of the nearby rich cluster A3112. The XMM-Newton Extended Source Analysis Software data reduction and background modeling methods were used to analyze the XMM-Newton EPIC data. From the EPIC data, we find that the iron and silicon abundance gradients show significant increase toward the center of the cluster while the oxygen abundance profile is centrally peaked but has a shallower distribution than that of iron. The X-ray mass modeling is based on the temperature and deprojected density distributions of the intracluster medium determined from EPIC observations. The total mass of A3112 obeys the M- T scaling relations found using XMM-Newton and Chandra observations of massive clusters at r ₅₀₀. The gas mass fraction f _gas = 0.149 ^+0.036 _–0.032 at r ₅₀₀ is consistent with the seven-year Wilkinson Microwave Anisotropy Probe results. The comparisons of line fluxes and flux limits on the Fe XVII and Fe XVIII lines obtained from high-resolution RGS spectra indicate that there is no spectral evidence for cooler gas associated with the cluster with temperature below 1.0 keV in the central <38'' (~52 kpc) region of A3112. High-resolution RGS spectra also yield an upper limit to the turbulent motions in the compact core of A3112 (206 km s ^–1). We find that the contribution of turbulence to total energy is less than 6%. This upper limit is consistent with the energy contribution measured in recent high-resolution simulations of relaxed galaxy clusters.

• Open/close The Evolution of Stellar Velocity Dispersion during Dissipationless Galaxy Mergers

  Nathaniel R. Stickley and Gabriela Canalizo 2012 ApJ 747 33 Tag this article

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  Using N-body simulations, we studied the detailed evolution of central stellar velocity dispersion, σ _*, during dissipationless binary mergers of galaxies. Stellar velocity dispersion was measured using the common mass-weighting method as well as a flux-weighting method designed to simulate the technique used by observers. A toy model for dust attenuation was introduced in order to study the effect of dust attenuation on measurements of σ _*. We found that there are three principal stages in the evolution of σ _* in such mergers: oscillation, phase mixing, and dynamical equilibrium. During the oscillation stage, σ _* undergoes damped oscillations of increasing frequency. The oscillation stage is followed by a phase mixing stage during which the amplitude of the variations in σ _* is smaller and more chaotic than in the oscillation stage. Upon reaching dynamical equilibrium, σ _* assumes a stable value. We used our data regarding the evolution of σ _* during mergers to characterize the scatter inherent in making measurements of σ _* in non-quiescent systems. In particular, we found that σ _* does not fall below 70% nor exceed 200% of its final, quiescent value during a merger and that a random measurement of σ _* in such a system is much more likely to fall near the equilibrium value than near an extremum. Our toy model of dust attenuation suggested that dust can systematically reduce observational measurements of σ _* and increase the scatter in σ _* measurements.