Detecting mass substructure in galaxy clusters: an aperture mass statistic for gravitational flexion     

Adrienne Leonard  Lindsay J. King  Stephen M. Wilkins 《Monthly notices of the Royal Astronomical Society》2009,395(3):1438-1448

Gravitational flexion has been introduced as a technique by which one can map out and study substructure in clusters of galaxies. Previous analyses involving flexion have measured the individual galaxy–galaxy flexion signal, or used either parametric techniques or a Kaiser, Squires and Broadhurst (KSB)-type inversion to reconstruct the mass distribution in Abell 1689. In this paper, we present an aperture mass statistic for flexion, and apply it to the lensed images of background galaxies obtained by ray-tracing simulations through a simple analytic mass distribution and through a galaxy cluster from the Millennium Simulation. We show that this method is effective at detecting and accurately tracing structure within clusters of galaxies on subarcminute scales with high signal to noise even using a moderate background source number density and image resolution. In addition, the method provides much more information about both the overall shape and the small-scale structure of a cluster of galaxies than can be achieved through a weak lensing mass reconstruction using gravitational shear data. Lastly, we discuss how the zero-points of the aperture mass might be used to infer the masses of structures identified using this method.  相似文献   

Systematic Time Delay of Hemispheric Solar Activity     

K. J. Li 《Solar physics》2009,255(1):169-177

Five solar-activity indices – the monthly-mean sunspot numbers from January 1945 to March 2008, the monthly-mean sunspot areas during the period of May 1874 to March 2008, the monthly numbers of sunspot groups from May 1874 to May 2008, the monthly-mean flare indices from January 1966 to December 2006, and the numbers of solar filaments per Carrington rotation in the time interval of solar rotations 876 to 1823 – have been used to show a systematic time delay between northern and southern hemispheric solar activities in a cycle. It is found that solar activity does not occur synchronously in the northern and southern hemispheres, and there is a systematic time lag or lead (phase shift) between northern and southern hemispheric solar activity in a cycle. About an eight-cycle period is inferred to exist in such phase shifts. The activity on the Sun may be governed by two different and coupled processes, not by a single process.  相似文献   

Acoustic-Power Maps of Solar Active Regions with Direction Filters and Phase-Velocity Filters     

Dean-Yi Chou  Zhi-Chao Liang  Ming-Hsu Yang  Ming-Tsung Sun 《Solar physics》2009,255(1):39-51

We study the properties of power maps of solar acoustic waves filtered with direction filters and phase-velocity filters. A direction filter is used to isolate acoustic waves propagating in a narrow range of directions. The acoustic-power map of the waves filtered with a direction filter shows extended reduced-power features behind magnetic regions with respect to the wave direction. A phase-velocity filter is further applied to isolate waves with similar wave paths. In the power maps of the waves filtered with both a direction filter and a phase-velocity filter, a reduced-power image of a sunspot appears behind the sunspot with respect to the wave direction. The distance between the sunspot and the secondary image is consistent with the one-skip travel distance of the wave packet associated with the phase-velocity filter. The waves filtered with direction and phase-velocity filters at the location of the secondary image could be used to probe the sunspot. In the quiet Sun, spatial fluctuations exist in any acoustic-power map. These fluctuations are mainly caused by interference among modes with the same frequency. The fluctuations are random with two properties: They change rapidly with time, and their magnitude decreases with the square root of the number of frames used in computing the acoustic-power map.  相似文献   

Constraining the dark matter annihilation cross-section with Cherenkov telescope observations of dwarf galaxies     

Lidia Pieri  Massimiliano Lattanzi  Joseph Silk 《Monthly notices of the Royal Astronomical Society》2009,399(4):2033-2040

The presence of dark matter in the halo of our Galaxy could be revealed through indirect detection of its annihilation products. Dark matter annihilation is one possible interpretation of the recently measured excesses in positron and electron fluxes, provided that boost factors of the order of 10³ or more are taken into account. Such boost factors are actually achievable through the velocity-dependent Sommerfeld enhancement of the annihilation cross-section. Here, we study the expected γ-ray flux from two local dwarf galaxies for which Cherenkov telescope measurements are available, namely Draco and Sagittarius. We use recent stellar kinematical measurements to model the dark matter haloes of the dwarfs and the results of numerical simulations to model the presence of an associated population of subhaloes. We incorporate the Sommerfeld enhancement of the annihilation cross-section. We compare our predictions with the observations of Draco and Sagittarius performed by MAGIC and HESS, respectively, and derive exclusion limits on the effective annihilation cross-section. We also study the sensitivities of Fermi and of the future Cherenkov telescope array to cross-section enhancements. We find that the boost factor due to the Sommerfeld enhancement is already constrained by the MAGIC and HESS data, with enhancements greater than ∼10⁴ being excluded.  相似文献   

Cyclical Behavior of Coronal Mass Ejections     

K. J. Li  P. X. Gao  Q. X. Li  J. Mu  T. W. Su 《Solar physics》2009,257(1):149-154

With the use of coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO) from January 1996 through December 2005, it is found that, for the cyclical activity of CMEs, there is surprisingly no equatorward drift at low latitudes (thus, no “butterfly diagram”) and no poleward drift at high latitudes, and no antiphase relationship between CME activity at low and high latitudes. The cyclical behaviors of CMEs differ in a significant way from that of the small-scale solar photospherical and chromospherical phenomena. Thus, our analysis leads to results that are inconsistent with a close, physical relationship with small-scale aspects of solar activity, and it is suggested that there is possibly a single so-called large-scale activity cycle in CMEs.  相似文献   

The Long-term Behavior of the North – South Asymmetry of Sunspot Activity     

K. J. Li  P. X. Gao  L. S. Zhan 《Solar physics》2009,254(1):145-154

A new index, the cumulative difference of sunspot activity in the northern and southern hemispheres, respectively, is proposed to describe the long-term behavior of the North – South asymmetry of sunspot activity and to show the balance (or bias) of sunspot activity in the two solar hemispheres on a long-term scale. Sunspot groups and sunspot areas from June 1874 to January 2007 are used to show the advantage of the index. The index clearly shows a long-term characteristic time scale of about 12 cycles in the North – South asymmetry of sunspot activity. Sunspot activity is found to dominate in the southern hemisphere in cycle 23, and in cycle 24 it is predicted to dominate still in the southern hemisphere. A comparison of the new index with other similar indexes is also given.  相似文献   

3D Coronal Structures and Magnetic Field During the Total Solar Eclipse of 29 March 2006     

P. Ambrož  M. Druckmüller  A. A. Galal  R. H. Hamid 《Solar physics》2009,258(2):243-265

The good quality of the observing sequence of about 60 photographs of the white-light corona taken during the total solar eclipse observations on 29 March 2006, in Al Sallum, Egypt, enable us to use a new method of image processing for enhancement of the fine structure of coronal phenomena. We present selected magnetic-field lines derived for different parameters of the extrapolation model. The coincidence of the observed coronal white-light fine structures and the computed field-line positions provides a 3D causal relationship between coronal structures and the coronal magnetic field.  相似文献   

Consistently large cosmic flows on scales of  100 h⁻¹ Mpc : a challenge for the standard ΛCDM cosmology     

Richard Watkins  Hume A. Feldman  Michael J. Hudson 《Monthly notices of the Royal Astronomical Society》2009,392(2):743-756

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The effect of large-scale power on simulated spectra of the Lyα forest     

David Tytler  Pascal Paschos  David Kirkman  Michael L. Norman  Tridivesh Jena 《Monthly notices of the Royal Astronomical Society》2009,393(3):723-758

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Analysis of non-Gaussian cosmic microwave background maps based on the N-pdf. Application to Wilkinson Microwave Anisotropy Probe data     

P. Vielva  J. L. Sanz 《Monthly notices of the Royal Astronomical Society》2009,397(2):837-848

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