7,733 research outputs found

    An Imaging and Spectral Study of Ten X-Ray Filaments around the Galactic Center

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    We report the detection of 10 new X-ray filaments using the data from the {\sl Chandra} X-ray satellite for the inner 66^{\prime} (15\sim 15 parsec) around the Galactic center (GC). All these X-ray filaments are characterized by non-thermal energy spectra, and most of them have point-like features at their heads that point inward. Fitted with the simple absorbed power-law model, the measured X-ray flux from an individual filament in the 2-10 keV band is 2.8×1014\sim 2.8\times10^{-14} to 101310^{-13} ergs cm2^{-2} s1^{-1} and the absorption-corrected X-ray luminosity is 10321033\sim 10^{32}-10^{33} ergs s1^{-1} at a presumed distance of 8 kpc to the GC. We speculate the origin(s) of these filaments by morphologies and by comparing their X-ray images with the corresponding radio and infrared images. On the basis of combined information available, we suspect that these X-ray filaments might be pulsar wind nebulae (PWNe) associated with pulsars of age 1033×10510^3 \sim 3\times 10^5 yr. The fact that most of the filament tails point outward may further suggest a high velocity wind blowing away form the GC.Comment: 29 pages with 7 figures and 3 pages included. Accepted to Ap

    Axisymmetric stability criterion for two gravitationally coupled singular isothermal discs

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    Using the two-fluid formalism with the polytropic approximation, we examine the axisymmetric stability criterion for a composite system of gravitationally coupled stellar and gaseous singular isothermal discs (SIDs).Comment: 12 pages, 11 figures, accepted by MNRA

    Envelope Expansion with Core Collapse. III. Similarity Isothermal Shocks in a Magnetofluid

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    We explore MHD solutions for envelope expansions with core collapse (EECC) with isothermal MHD shocks in a quasi-spherical symmetry and outline potential astrophysical applications of such magnetized shock flows. MHD shock solutions are classified into three classes according to the downstream characteristics near the core. Class I solutions are those characterized by free-fall collapses towards the core downstream of an MHD shock, while Class II solutions are those characterized by Larson-Penston (LP) type near the core downstream of an MHD shock. Class III solutions are novel, sharing both features of Class I and II solutions with the presence of a sufficiently strong magnetic field as a prerequisite. Various MHD processes may occur within the regime of these isothermal MHD shock similarity solutions, such as sub-magnetosonic oscillations, free-fall core collapses, radial contractions and expansions. We can also construct families of twin MHD shock solutions as well as an `isothermal MHD shock' separating two magnetofluid regions of two different yet constant temperatures. The versatile behaviours of such MHD shock solutions may be utilized to model a wide range of astrophysical problems, including star formation in magnetized molecular clouds, MHD link between the asymptotic giant branch phase to the proto-planetary nebula phase with a hot central magnetized white dwarf, relativistic MHD pulsar winds in supernova remnants, radio afterglows of soft gamma-ray repeaters and so forth.Comment: 21 pages, 33 figures, accepted by MNRA

    Global axisymmetric stability analysis for a composite system of two gravitationally coupled scale-free discs

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    In a composite system of gravitationally coupled stellar and gaseous discs, we perform linear stability analysis for axisymmetric coplanar perturbations using the two-fluid formalism. The background stellar and gaseous discs are taken to be scale-free with all physical variables varying as powers of cylindrical radius rr with compatible exponents. The unstable modes set in as neutral modes or stationary perturbation configurations with angular frequency ω=0\omega=0.Comment: 7 pages using AAS styl

    Periodicities in Solar Coronal Mass Ejections

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    Mid-term quasi-periodicities in solar coronal mass ejections (CMEs) during the most recent solar maximum cycle 23 are reported here for the first time using the four-year data (February 5, 1999 to February 10, 2003) of the Large Angle Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO). In parallel, mid-term quasi-periodicities in solar X-ray flares (class >M5.0) from the Geosynchronous Operational Environment Satellites (GOES) and in daily averages of Ap index for geomagnetic disturbances from the World Data Center (WDC) at the International Association for Geomagnetism and Aeronomy (IAGA) are also examined for the same four-year time span. Several conceptual aspects of possible equatorially trapped Rossby-type waves at and beneath the solar photosphere are discussed.Comment: Accepted by MNRAS, 6 figure

    Distribution of Spectral Lags in Gamma Ray Bursts

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    Using the data acquired in the Time To Spill (TTS) mode for long gamma-ray bursts (GRBs) collected by the Burst and Transient Source Experiment on board the Compton Gamma Ray Observatory (BATSE/CGRO), we have carefully measured spectral lags in time between the low (25-55 keV) and high (110-320 keV) energy bands of individual pulses contained in 64 multi-peak GRBs. We find that the temporal lead by higher-energy gamma-ray photons (i.e., positive lags) is the norm in this selected sample set of long GRBs. While relatively few in number, some pulses of several long GRBs do show negative lags. This distribution of spectral lags in long GRBs is in contrast to that in short GRBs. This apparent difference poses challenges and constraints on the physical mechanism(s) of producing long and short GRBs. The relation between the pulse peak count rates and the spectral lags is also examined. Observationally, there seems to be no clear evidence for systematic spectral lag-luminosity connection for pulses within a given long GRB.Comment: 20 pages, 4 figure

    Scale-Free Thin Discs with an Isopedic Magnetic Field

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    Global stationary configurations of both aligned and logarithmic spiral MHD perturbations are constructed analytically within an axisymmetric background ofrazor-thin scale-free gas disc, which is embedded in an axisymmetric gravitational potential of a dark matter halo and involves an isopedic magnetic field almost vertically threaded through the disc plane. The scale-free index β\beta of the disc rotation speed vθRβv_{\theta}\propto R^{-\beta} falls in the rangeof (1/2,1/2)(-1/2 , 1/2) where RR is the cylindrical radius. With the holding-back of a deep background dark matter halo potential, the isopedic magnetic field may be strong enough to allow for the magnetic tension force overtaking the disc self-gravity, which can significantly influence global stationary MHD perturbation configurations and stability properties of the scale-free disc system. Only for stationary logarithmic spiral MHD perturbations with a perturbation scale-free index β1=1/4\beta_1=1/4 or for aligned stationary MHD perturbations, can the MHD disc maintain a constant radial flux of angularmomentum. The variable radial flux of angular momentum in the radial direction corresponds to a complex dispersion relation. The marginal instabilities foraxisymmetric MHD disturbances are also examined for a special case as an example. When the magnetic tension force overtakes the disc self-gravity, the scale-free disc can be completely stable against axisymmetric MHD disturbances of all wavelengths. We predict the possible existence of an isopedicallymagnetized gas disc system in rotation primarily confined by a massive dark matter halo potential.Comment: 31 pages, 7 figures, Accepted for publication in MNRA

    Coupled KdV equations derived from atmospherical dynamics

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    Some types of coupled Korteweg de-Vries (KdV) equations are derived from an atmospheric dynamical system. In the derivation procedure, an unreasonable yy-average trick (which is usually adopted in literature) is removed. The derived models are classified via Painlev\'e test. Three types of τ\tau-function solutions and multiple soliton solutions of the models are explicitly given by means of the exact solutions of the usual KdV equation. It is also interesting that for a non-Painlev\'e integrable coupled KdV system there may be multiple soliton solutions.Comment: 19 pages, 2 figure

    Global structures in a composite system of two scale-free discs with a coplanar magnetic field

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    We investigate a theoretical MHD disc problem involving a composite disc system of gravitationally coupled stellar and gaseous discs with a coplanar magnetic field in the presence of an axisymmetric dark matter halo. The two discs are expediently approximated as razor-thin, a ring-like magnetic field, and a power-law rotation curve in radius . By imposing the scale-free condition, we construct analytically stationary global MHD perturbation configurations for both aligned and logarithmic spiral patterns. MHD perturbation configurations in a composite system of partial discs in the presence of an axisymmetric dark matter halo are also considered. We derive analytically the stationary MHD dispersion relations for both aligned and unaligned perturbation structures and analyze the corresponding phase relationships between surface mass densities and the magnetic field. Compared with earlier results, we obtain three solution branches corresponding to super fast MHD density waves, fast MHD density waves and slow MHD density waves, respectively. By evaluating the unaligned m=0m=0 case, we determine the marginal stability curves where the two unstable regimes corresponding to Jeans collapse instability and ring fragmentation instability are identified. We find that the aligned m=0m=0 case is simply the limit of the unaligned m=0m=0 case with the radial wavenumber ξ0\xi\to0. We further show that a composite system of partial discs behaves much differently from a composite system of full discs in certain aspects. Our formalism provides a useful theoretical framework in the study of stationary global perturbation configurations for MHD disc galaxies with bars, spirals and barred spirals.Comment: 35 pages, 24 figures, Accepted for publication in MNRA

    Dynamic Evolution of a Quasi-Spherical General Polytropic Magnetofluid with Self-Gravity

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    In various astrophysical contexts, we analyze self-similar behaviours of magnetohydrodynamic (MHD) evolution of a quasi-spherical polytropic magnetized gas under self-gravity with the specific entropy conserved along streamlines. In particular, this MHD model analysis frees the scaling parameter nn in the conventional polytropic self-similar transformation from the constraint of n+γ=2n+\gamma=2 with γ\gamma being the polytropic index and therefore substantially generalizes earlier analysis results on polytropic gas dynamics that has a constant specific entropy everywhere in space at all time. On the basis of the self-similar nonlinear MHD ordinary differential equations, we examine behaviours of the magnetosonic critical curves, the MHD shock conditions, and various asymptotic solutions. We then construct global semi-complete self-similar MHD solutions using a combination of analytical and numerical means and indicate plausible astrophysical applications of these magnetized flow solutions with or without MHD shocks.Comment: 21 pages, 7 figures, accepted for publication in APS
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