Fe K emission in the ultraluminous infrared galaxy Arp 220     

K. Iwasawa  D. B. Sanders  A. S. Evans  N. Trentham  G. Miniutti  H. W. W. Spoon 《Monthly notices of the Royal Astronomical Society》2005,357(2):565-571

Prominent Fe Kα line emission is detected in the XMM–Newton spectrum of the ultraluminous infrared galaxy Arp 220. The centroid of the line is found at an energy of 6.7 keV and the equivalent width of the line is  EW ∼ 1.9 keV  (at 3.5σ significance). A few other spectral features are found at various degrees of significance in the lower energy range on a hard 2.5–10 keV continuum  (Γ∼ 1)  . The large EW of the Fe K line poses a problem with interpreting the hard X-ray emission as integrated X-ray binary emission. A thermal emission spectrum with a temperature of   kT ∼ 7 keV  modified by absorption of   N _H≃ 3 × 10²² cm⁻²  , can describe the 2.5–10 keV continuum shape and the Fe K emission. A hot bubble that is shocked internally in a starburst region would have a similar temperature and gives a good explanation for the observed X-ray properties with a high star formation rate. An ensemble of radio supernovae in a dense environment, as suggested from VLBI imaging, could be another possibility, if such powerful supernovae are produced continuously at a high rate. However, the apparent lack of emission from X-ray binaries is incompatible with the high supernova rate (∼2 SNe yr⁻¹) required by both interpretations. Highly photoionized, low-density gas illuminated by a hidden Compton-thick active galactic nucleus is a possible alternative for the hard X-ray emission, which can be tested by examining whether radiative recombination continua from highly ionized Ca and Fe are present in better quality data from a forthcoming observation.  相似文献   

Cuspy dark matter haloes and the Galaxy     

J.J. Binney  N.W. Evans 《Monthly notices of the Royal Astronomical Society》2001,327(2):L27-L31

The microlensing optical depth to Baade's Window constrains the minimum total mass in baryonic matter within the Solar circle to be greater than ∼     , assuming the inner Galaxy is barred with viewing angle ∼20°. From the kinematics of solar neighbourhood stars, the local surface density of dark matter is ∼     . We construct cuspy haloes normalized to the local dark matter density and calculate the circular-speed curve of the halo in the inner Galaxy. This is added in quadrature to the rotation curve provided by the stellar and ISM discs, together with a bar sufficiently massive so that the baryonic matter in the inner Galaxy reproduces the microlensing optical depth. Such models violate the observational constraint provided by the tangent-velocity data in the inner Galaxy (typically at radii     . The high baryonic contribution required by the microlensing is consistent with implications from hydrodynamical modelling and the pattern speed of the Galactic bar. We conclude that the cuspy haloes favoured by the cold dark matter cosmology (and its variants) are inconsistent with the observational data on the Galaxy.  相似文献   

Dark matter in dwarf spheroidals – II. Observations and modelling of Draco     

J.Kleyna  M. I.Wilkinson  N. W.Evans  G.Gilmore  C.Frayn 《Monthly notices of the Royal Astronomical Society》2002,330(4):792-806

We present stellar radial velocity data for the Draco dwarf spheroidal (dSph) galaxy obtained using the AF2/WYFFOS instrument combination on the William Herschel Telescope. Our data set consists of 186 member stars, 159 of which have good quality velocities, extending to a magnitude   V ≈19.5  with a mean velocity precision of ≈2 km s⁻¹. As this survey is based on a high-precision photometric target list, it contains many more Draco members at large radii. For the first time, this allows a robust determination of the radial behaviour of the velocity dispersion in a dSph.
We find statistically strong evidence of a rising velocity dispersion consistent with a dark matter halo that has a gently rising rotation curve. There is a <2 σ signature of rotation about the long axis, inconsistent with tidal disruption as the source of the rising dispersion. By comparing our data set with earlier velocities, we find that Draco probably has a binary distribution and fraction comparable to those in the solar neighbourhood.
We apply a novel maximum likelihood algorithm and fit the velocity data to a two parameter spherical model with an adjustable dark matter content and velocity anisotropy. Draco is best fit by a weakly tangentially anisotropic distribution of stellar orbits in a dark matter halo with a very slowly rising rotation law  ( v _circ∝ r ^0.17)  . We are able to rule out both a mass-follows-light distribution and an extended halo with a harmonic core at the 2.5 to 3 σ significance level, depending on the details of our assumptions about Draco's stellar binary population. Our modelling lends support to the idea that the dark matter in dwarf spheroidals is distributed in the form of massive, nearly isothermal haloes.  相似文献   

Spectropolarimetry of O supergiants     

Tim J. Harries  Ian D. Howarth  Christopher J. Evans 《Monthly notices of the Royal Astronomical Society》2002,337(1):341-355

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Special theme: Modern analogues in Quaternary palaeoglaciological reconstruction     

David J.A. Evans 《Quaternary Science Reviews》2009,28(3-4):181-182

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Algorithmically scalable block preconditioner for fully implicit shallow-water equations in CAM-SE     

P. Aaron Lott  Carol S. Woodward  Katherine J. Evans 《Computational Geosciences》2015,19(1):49-61

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Correcting lateral boundary biases in regional climate modelling: the effect of the relaxation zone     

Rocheta  Eytan  Evans  Jason P.  Sharma  Ashish 《Climate Dynamics》2020,55(9-10):2511-2521

Climate Dynamics - Regional climate models (RCM) are an important tool for simulating atmospheric information at finer resolutions often of greater relevance to local scale climate change impact...  相似文献   

Metoz: Total ozone from meteorological parameters     

L. Poulin  W.F.J. Evans 《大气与海洋》2013,51(2):285-297

Abstract

On the basis of two well‐known ozone‐weather relationships, an algorithm is proposed that estimates total ozone amounts using readily available meteorological information. The technique has been labelled METOZ from Meteorological Total Ozone. A rationale and derivation of the technique is presented.

METOZ total ozone amounts were generated for Toronto and Edmonton locations and compared with ground‐based Brewer spectrophotometer measures for these two stations in the Canadian ozone monitoring network for data from January to April 1989. Per cent differences between METOZ and Brewer total ozone measurements were calculated and are presented.

This mid‐latitude wînter metoz algorithm was also used to produce a hemispherical total ozone field for 16 March 1986, which was compared with the corresponding TOMS data to demonstrate other potential applications of the technique.  相似文献   

Effects of climate variability on the distribution and fishing conditions of yellowfin tuna (Thunnus albacares) in the western Indian Ocean     

Kuo-Wei Lan  Karen Evans  Ming-An Lee 《Climatic change》2013,119(1):63-77

Variations in the abundance and distribution of pelagic tuna populations have been associated with large-scale climate indices such as the Southern Oscillation Index in the Pacific Ocean and the North Atlantic Oscillation in the Atlantic Ocean. Similarly to the Pacific and Atlantic, variability in the distribution and catch rates of tuna species have also been observed in association with the Indian Ocean Dipole (IOD), a basin-scale pattern of sea surface and subsurface temperatures that affect climate in the Indian Ocean. The environmental processes associated with the IOD that drive variability in tuna populations, however, are largely unexplored. To better understand these processes, we investigated longline catch rates of yellowfin tuna and their distributions in the western Indian Ocean in relation to IOD events, sea surface water temperatures (SST) and estimates of net primary productivity (NPP). Catch per unit effort (CPUE) was observed to be negatively correlated to the IOD with a periodicity centred around 4 years. During positive IOD events, SSTs were relatively higher, NPP was lower, CPUE decreased and catch distributions were restricted to the northern and western margins of the western Indian Ocean. During negative IOD events, lower SSTs and higher NPP were associated with increasing CPUE, particularly in the Arabian Sea and seas surrounding Madagascar, and catches expanded into central regions of the western Indian Ocean. These findings provide preliminary insights into some of the key environmental features driving the distribution of yellowfin tuna in the western Indian Ocean and associated variability in fisheries catches.  相似文献   

HT Camelopardalis: the simplest intermediate polar spin pulse     

P. A. Evans  Coel Hellier 《Monthly notices of the Royal Astronomical Society》2005,359(4):1531-1536

The intermediate polar (IP) HT Cam is unusual in that it shows no evidence for dense absorption in its spectrum. We analyse an XMM–Newton observation of this star, which confirms the absence of absorption and shows that the X-ray spin pulse is energy independent. The modulation arises solely from occultation effects and can be reproduced by a simple geometrical model in which the lower accretion footprint is fainter than the upper one.
We suggest that the lack of opacity in the accretion columns of HT Cam, and also of EX Hya and V1025 Cen, results from a low accretion rate owing to their being below the cataclysmic variable period gap.  相似文献