Some properties of synchrotron radio and inverse-Compton gamma-ray images of supernova remnants     

O. Petruk  V. Beshley  F. Bocchino  S. Orlando 《Monthly notices of the Royal Astronomical Society》2009,395(3):1467-1475

The synchrotron radio maps of supernova remnants (SNRs) in a uniform interstellar medium and interstellar magnetic field (ISMF) are analysed, allowing for different 'sensitivity' of the injection efficiency to the obliquity of the shock. The very-high-energy γ-ray maps arising from inverse Compton processes are also synthesized. The properties of images in these different wavelength bands are compared, with particular emphasis on the location of the bright limbs in bilateral SNRs. Recent High-Energy Stereoscopic System (HESS) observations of SN 1006 show that the radio and inverse Compton γ-ray limbs coincide, and we found that this may happen if (i) injection is isotropic but the variation of the maximum energy of electrons is rather fast to compensate for differences in the magnetic field, or (ii) the obliquity dependence of injection (either quasi-parallel or quasi-perpendicular) and the electron maximum energy are strong enough to dominate the magnetic field variation. In the latter case, the obliquity dependences of the injection and the maximum energy should not be opposite. We argue that the position of the limbs alone, and even their coincidence in radio, X-rays and γ-rays, as discovered by HESS in SN 1006, cannot be conclusive as regards the dependence of the electron injection efficiency, the compression/amplification of the ISMF and the electron maximum energy on the obliquity angle.  相似文献   

The gravitational wave background from star–massive black hole fly-bys     

Silvia Toonen  Clovis Hopman  Marc Freitag 《Monthly notices of the Royal Astronomical Society》2009,398(3):1228-1234

Stars on eccentric orbits around a massive black hole (MBH) emit bursts of gravitational waves (GWs) at periapse. Such events may be directly resolvable in the Galactic Centre. However, if the star does not spiral in, the emitted GWs are not resolvable for extragalactic MBHs, but constitute a source of background noise. We estimate the power spectrum of this extreme mass ratio burst background (EMBB) and compare it to the anticipated instrumental noise of the Laser Interferometer Space Antenna (LISA). To this end, we model the regions close to an MBH, accounting for mass segregation, and for processes that limit the presence of stars close to the MBH, such as GW inspiral and hydrodynamical collisions between stars. We find that the EMBB is dominated by GW bursts from stellar mass black holes, and the magnitude of the noise spectrum  ( fS _GW)^1/2  is at least a factor of ∼10 smaller than the instrumental noise. As an additional result of our analysis, we show that LISA is unlikely to detect relativistic bursts in the Galactic Centre.  相似文献   

Early evolution of newly born magnetars with a strong toroidal field     

S. Dall'Osso  S. N. Shore  L. Stella 《Monthly notices of the Royal Astronomical Society》2009,398(4):1869-1885

We present a state-of-the-art scenario for newly born magnetars as strong sources of gravitational waves (GWs) in the early days after formation. We address several aspects of the astrophysics of rapidly rotating, ultra-magnetized neutron stars (NSs), including early cooling before transition to superfluidity, the effects of the magnetic field on the equilibrium shape of NSs, the internal dynamical state of a fully degenerate, oblique rotator and the strength of the electromagnetic torque on the newly born NS. We show that our scenario is consistent with recent studies of supernova remnant surrounding Anomalous X-ray Pulsars (AXPs) and Soft Gamma-Ray Repeaters (SGRs) in the Galaxy that constrains the electromagnetic energy input from the central NS to be  ≤ 10⁵¹  erg. We further show that if this condition is met, then the GW signal from such sources is potentially detectable with the forthcoming generation of GW detectors up to Virgo cluster distances where an event rate ∼1 yr⁻¹ can be estimated. Finally, we point out that the decay of an internal magnetic field in the 10¹⁶ G range couples strongly with the NS cooling at very early stages, thus significantly slowing down both processes: the field can remain this strong for at least 10³ yr, during which the core temperature stays higher than several times 10⁸ K.  相似文献   

The astroclimatological comparison of the Paranal Observatory and El Roque de Los Muchachos Observatory     

G. Lombardi  V. Zitelli  S. Ortolani 《Monthly notices of the Royal Astronomical Society》2009,399(2):783-793

The new extremely large telescope projects need accurate evaluation of the candidate sites. In this paper, we present the astroclimatological comparison between the Paranal Observatory, located on the coast of the Atacama Desert (Chile), and the Observatorio del Roque de Los Muchachos (ORM), located in La Palma (Canary Islands). We apply a statistical analysis using long-term data bases from Paranal and Carlsberg Automatic Meridian Circle (CAMC) weather stations. The monthly, seasonal and annual averages of the main synoptical parameters in the two sites are computed. We compare the long-term trends in order to understand the main differences between the two sites. Significant differences between the two analysed sites have been found. Temperature has increasing trends in both observatories with somewhat higher evidence at the ORM. Seasonal variations of pressure at Paranal have been highly decreasing since 1989, and we do not see the same phenomenon at the ORM. The two sites are dominated by high pressure. In cold seasons, relative humidity (RH) is lower than 60 per cent at CAMC and 15 per cent at Paranal. In warm seasons, RH is lower than 40 per cent at CAMC and 20 per cent at Paranal. The analysis of the dew point has shown better conditions at Paranal with respect to CAMC in winter, autumn and spring before 2001, while the two sites are becoming similar afterwards. Winds at the ORM are subject to pronounced local variations.  相似文献   

The Durham/ESO SLODAR optical turbulence profiler     

R. W. Wilson  T. Butterley  M. Sarazin 《Monthly notices of the Royal Astronomical Society》2009,399(4):2129-2138

An instrument for monitoring of the vertical profile of atmospheric optical turbulence strength, employing the Slope Detection and Ranging (SLODAR) double star technique applied to a small telescope, has been developed by Durham University and the European South Observatory. The system has been deployed at the Cerro Paranal observatory in Chile for statistical characterization of the site. The instrument is configured to sample the turbulence at altitudes below 1.5 km with a vertical resolution of approximately 170 m. The system also functions as a general-purpose seeing monitor, measuring the integrated optical turbulence strength for the whole atmosphere, and hence the seeing width. We give technical details of the prototype and present data to characterize its performance. Comparisons with contemporaneous measurements from a differential image motion monitor (DIMM) and a multi-aperture scintillation sensor (MASS) are discussed. Statistical results for the optical turbulence profile at the Paranal site are presented. We find that, in the median case, 49 per cent of the total optical turbulence strength is associated with the surface layer (below 100 m), 35 per cent with the 'free atmosphere' (above 1500 m) and 16 per cent with the intermediate altitudes (100–1500 m).  相似文献   

Propagation and Damping of a Localized Impulsive Longitudinal Perturbation in Coronal Loops     

L. Di G. Sigalotti  J. A. Guerra  C. A. Mendoza-Briceño 《Solar physics》2009,254(1):127-144

We use linear analysis to simulate the evolution of a coronal loop in response to a localized impulsive event. The disturbance is modeled by injecting a narrow Gaussian velocity pulse near one footpoint of a loop in equilibrium. Three different damping mechanisms, namely viscosity, thermal conduction, and optically thin radiation, are included in the loop calculations. We consider homogeneous and gravitationally stratified, isothermal loops of varying length (50≤L≤400 Mm) and temperature (2≤T≤10 MK). We find that a localized pulse can effectively excite slow magnetoacoustic waves that propagate up along the loop. The amplitudes of the oscillations increase with decreasing loop temperature and increasing loop length and size of the pulse width. At T≥4 MK, the waves are dissipated by the combined effects of viscosity and thermal conduction, whereas at temperatures of 2 MK, or lower, wave dissipation is governed by radiative cooling. We predict periods in the range of 4.6?–?41.6 minutes. The wave periods remain unaltered by variations of the pulse size, decrease with the loop temperature, and increase almost linearly with the loop length. In addition, gravitational stratification results in a small reduction of the periods and amplification of the waves as they propagate up along the loop.  相似文献   

The Influence of a New Model of Progenitor Star on Supernova Explosion     

Miao-Jing Zhang  Xiu-Qiang QiuJiang-Hong Pan  Xu-Ning Zhang 《Chinese Astronomy and Astrophysics》2009

By adopting the progenitor model proposed by Woosley in 2002 and using the program WZYW89, the process of explosion of type II supernovae with masses of 11−40M_⊙ is numerically simulated. The results of computation reveal that this new model can in different degrees affect the collapse and explosion of type II supernovae as well as the propagation of shock waves. Besides, the definitions of the primary energy of shock waves are also discussed.  相似文献   

Analytical solutions for the equations of motion of a space vehicle during the atmospheric re-entry phase on a 2-D trajectory     

Gabriel Mititelu 《Celestial Mechanics and Dynamical Astronomy》2009,103(4):327-342

A practical and important problem encountered during the atmospheric re-entry phase is to determine analytical solutions for the space vehicle dynamical equations of motion. The author proposes new solutions for the equations of trajectory and flight-path angle of the space vehicle during the re-entry phase in Earth’s atmosphere. Explicit analytical solutions for the aerodynamic equations of motion can be effectively applied to investigate and control the rocket flight characteristics. Setting the initial conditions for the speed, re-entering flight-path angle, altitude, atmosphere density, lift and drag coefficients, the nonlinear differential equations of motion are linearized by a proper choice of the re-entry range angles. After integration, the solutions are expressed with the Exponential Integral, and Generalized Exponential Integral functions. Theoretical frameworks for proposed solutions as well as, several numerical examples, are presented.  相似文献   

Dependence of the acceleration of the outer edge of a stellar envelope on supernova explosion energy     

D. K. Nadyozhin  A. V. Karamyan  E. K. Grasberg 《Astronomy Letters》2009,35(6):388-395

Based on numerical simulations of a supernova explosion, we investigate the shock-wave breakout through the stellar surface. The computations have been performed in a wide range of explosion energies and presupernova masses. The results are compared with the classical Gandelman-Frank-Kamenetsky self-similar solution. We have determined the dependence of an arbitrary coefficient in the self-similar solution on the explosion energy and presupernova structure. The derived analytical approximation formula for this coefficient can be used to estimate the supernova explosion energy from such parameters of the ejected envelope determined from astronomical observations as its maximum expansion velocity and the density distribution along its outer edge. The formula may prove to be also useful in studying the X-ray and gamma-ray bursts that accompany the shock-wave breakout through the surface of compact presupernovae.  相似文献   

Acceleration of a spherical neutral shell produced by an ionization-shock front in an inhomogeneous interstellar medium     

G. Yu. Kotova  K. V. Krasnobaev 《Astronomy Letters》2009,35(3):167-174

We numerically model the formation and acceleration of a neutral gas shell as an ionization-shock front propagates in a spherical cloud by taking into account the photoionization and radiative heating of the gas, the spectral radiative transfer. We suggest and implement an approximation of the cooling function that allows calculations to be performed in a wide range of gas ionization fractions and temperatures. The total mass, average velocity, and thickness of the shell have been determined. The results are compared with approximate formulas known in the literature. Based on the parameters of the shell found, we estimate its acceleration, characteristic scales, and the growth times of unstable perturbations. We analyze the influence of the cloud particle density, cloud radius, stellar temperature, and radiation spectrum on the integrated characteristics of the neutral gas in the layer between the ionization and shock fronts. The distribution of matter in the shell and its thickness are shown to differ significantly from those used in approximate models.  相似文献