bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Nature and Evolutionary Status of GRO J1744–28
Authors: Joss P. C. , Rappaport S. 
Published: July 2016

AbstractGRO J1744–28 is the first known X-ray source to display both bursts and periodic pulsations. This source may thus provide crucial clues that will lead to an understanding of the differences in the nature of the X-ray variability among accreting neutron stars. We deduce that the magnetic field of the neutron star is relatively weak (~ 8 × 1010G) but, nevertheless, sufficiently strong to funnel the accretion flow onto the magnetic polar caps and suppress the thermonuclear flashes that would otherwise give rise to type I X-ray bursts. We also present a series of interrelated arguments which demonstrate that the observed bursts are of type II and probably result from an instability associated with the interaction of the neutron-star magnetic field with the inner edge of the accretion disk. From these results, we infer that X-ray pulsars, GRO J1744–28, the Rapid Burster, and the type I X-ray bursters may form a sequence of possible behaviors among accreting neutron stars, with the strength of the magnetic field serving as the crucial parameter that determines the mode of X-ray variability. The companion star in the GRO J1744–28 binary system is probably a very low-mass (~ 0.2M⊙) giant that is in the final stages of losing its hydrogen-rich envelope. We have carried out binary evolution calculations which show that (1) if the mass of the giant was ~ 1M⊙when mass transfer onto the neutron star commenced, then the orbital period and the core mass of the giant have increased from ~ 1 to ~ 11.8 days and from ~ 0.15 to ~ 0.21M⊙, respectively, during the mass-transfer epoch, which has lasted for ~ 8 × 108yr, (2) the present long-term average X-ray luminosity is ~ 4 × 1036ergs s−1, which is at least two orders of magnitude lower than the luminosity at the peak of the transient outburst, and (3) the predicted long-term equilibrium rotation rate of the neutron star is remarkably close to the observed pulse rate. The transient nature of GRO J1744–28 may well be related to the final stages of dissipation of the envelope of the giant companion.
Cygnus X–1: A Case for a Magnetic Accretion Disk?
Authors: Nowak Michael A. , Vaughan B. A. , Dove J. , Wilms J. 
Published: July 2016

AbstractWith the advent of RXTE, which is capable of broad spectral coverage and fast timing, as well as other instruments which are increasingly being used in multi-wavelength campaigns (via both space-based and ground-based observations), we must demand more of our theoretical models. No current model mimics all facets of a system as complex as an x-ray binary. However, a modern theory should qualitatively reproduce — or at the very least not fundamentally disagree with – all of Cygnus X–1’s most basic average properties: energy spectrum (viewed within a broader framework of black hole candidate spectral behavior), power spectrum (PSD), and time delays and coherence between variability in different energy bands. Below we discuss each of these basic properties in turn, and we assess the health of one of the currently popular theories: Comptonization of photons from a cold disk. We find that the data pose substantial challenges for this theory, as well as all other currently discussed models.
UBV-Photometry and 35–day Cycle Model for X-ray Binary HZ Her/Her X–l
Authors: Shakura N. I. , Smirnov A. V. , Ketsaris N. A. 
Published: July 2016

AbstractBased on the detailed analysis of UBV–photometry of HZ Her obtained in 1972–1994, a model of 35–day cycle is proposed in which both forced precession of a tilted accretion disk and free precession of the neutron star are present.
HST Eclipse Studies of Magnetic CVs: Resolving the UV Hot Spot and Accretion Stream
Authors: Stockman H.S. 
Published: July 2016

AbstractTime-resolved photometry and spectroscopy of cataclysmic variables (CV) are powerful tools for clarifying the physical size, placement, and conditions of emission regions in these compact systems. We present HST time resolved UV spectroscopic data for one self-eclipsing mCV system, ST LMi, and two secondary eclipsing systems, DP Leo and UZ For. These observations clearly show the relative contributions of three emission components: the white dwarf, a hotter region surrounding the accretion shock, and the photoionized accretion stream. UZ For was observed in an unusually active state by both HST and EUVE. These data provide strong evidence that the accretion stream is “clumpy”, with overdensities 100–1000 times the average density. Using photoionization codes such as XSTAR and CLOUDY, we find that steady, supersonic, and homogenous flow in the UZ For accretion stream is unstable to compression heating. As a consequence, we propose a two-phase model for the stream and obtain density enhancements comparable to those deduced from the strengths of the UV emission lines and EUVE absorption features. Similar behavior may be expected from active, asynchronous magnetic CVs with coupling radii greater than several white dwarf radii.
Spectropolarimetry of CV’s; Signatures of Magnetic Fields in Accretion Flows
Authors: Steeghs Danny , Horne Keith 
Published: July 2016

AbstractWe present preliminary results of spectropolarimetric observations of several cataclysmic variables using the ISIS spectropolarimeter on the 4.2m William Hershell Telescope at La Palma. Our aim is to look for Zeeman signatures of magnetic fields in cataclysmic variables. This would provide a direct measurement of magnetic fields in the accretion flows of CVs. Our current dataset allows us to set upper limits to the presence of magnetic fields and constrain the field geometry in different types of cataclysmic variables.
Unified Accretion Disk Models Around Black Holes and Neutron Stars and Their Spectral Properties
Authors: Chakrabarti S. K. 
Published: July 2016

AbstractWe review the current understanding of accretion flows around compact objects with a special emphasis on advective disks. We discuss the influence of the centrifugal pressure supported high density region around compact objects (where shocks may also form) on the emitted spectra. We show that the stationary and non-stationary spectral properties (such as, low and high states, transition of states, quasi-periodic oscillations, quiescent and rising phases of X-ray novae, etc.) of both low mass and supermassive black hole candidates could be satisfactorily explained within the framework of the analytical solution of the advective disks without invoking any ad hoc components such as Compton clouds or magnetic corona.
Evidence for Radiative Acceleration in Quasars
Authors: Arav Nahum 
Published: July 2016

AbstractI give an overview of a promising signature for radiative acceleration in quasars “the ghost of Lyα,” which is seen in the spectra of a few broad absorption line (BAL) QSOs. The ghost appears as an absorption hump in the C IV λ1549 BAL and is centered around 5,900 km s−1 to the blue of the corresponding broad emission line (BEL). This velocity separation is the same as the velocity separation between the N V λ1240 and Lyα lines. Thus, when the N V BAL and the C IV BAL are plotted as a function of velocity (where zero velocity corresponds to the peaks of these BELs), the hump in the C IV BAL appears at the same velocity as the Lyα BEL and qualitatively resembles it. The best examples and model fits are shown and the radiative dynamics explained.
The Disk Wind Model of the Broad Line Region of Active Galactic Nuclei
Authors: Chiang J. , Murray N. 
Published: July 2016

AbstractIn a fashion similar to winds in O stars, the AGN disk winds may be responsible for the broad, blue-shifted absorption of resonance lines which characterize the Broad Absorption Line QSOs. The dynamical features of the disk wind may also account for many of the properties of the broad emission lines of AGN. Although the velocity field of the material responsible for the line emission is primarily that of the accretion disk, the presence of an accelerated radial wind influences the line formation so that single-peaked profiles are generic. Further, the recent reverberation mapping response of NGC 5548 is also qualitatively explained by these same radiative transfer effects.
Radio, Infrared and X-Ray Observations of GRS 1915+105
Authors: Fender R.P. , Pooley G.G. , Robinson C.R. , Harmon B.A. , Zhang S.N. , Canosa C. 
Published: July 2016

AbstractWe present multiwavelength observations of the superluminal jet source GRS 1915+105 in 1996 April–May, over which period a variety of phenomena, including radio QPO, strong infrared emission lines and rapid X-ray flickering and outbursts were observed.
Optical and IR Observations of Aql X-1
Authors: Garcia Michael R. , Callanan Paul J. 
Published: July 2016

AbstractWe have obtained optical spectra and IR photometry in an attempt to determine the orbital parameters of the Aql X-1 system. Spectra from the MMT in October 1995 appear to detect the secondary, but are not sufficient to measure the mass function. IR photometry in and out of mini-outburst find K ~ 15.4, and may indicate that the disk contributionat K is modest during the mini-outburst.
The Spin-Resolved He ɪɪ Profile of RX J0558+53
Authors: Harlaftis E.T. , Horne K. 
Published: July 2016

AbstractWe observed the new intermediate polar RX J0558+53 withthe 4.2m WHT in March 1995. We detect 2–3%% pulsations of the He ɪɪ emission line on the 545s spin period. The spin-resolved Hell spectrogram shows a complex structure with two roughly anti-phased components moving from red to blue (double pulse). This is most likely produced by non-axisymmetric gas flow in the rotating magnetosphere of the white dwarf.
X-Ray Flares and Outflows Driven by Magnetic Interaction Between a Protostar and its Surrounding Disk
Authors: Hayashi Mitsuru , Shibata Kazunari , Matsumoto Ryoji 
Published: July 2016

AbstractHere we present a model of hard X-ray flares and hot plasma outflows (optical jets) observed in protostars. Assuming that the dipole magnetic field of a protostar threads the protostellar disk, we carried out 2.5-dimensional magnetohydrodynamic (MHD) simulations of the diskstar interaction. The closed magnetic loops connecting the central star and the disk are twisted by the rotation of the disk. In the presence of resistivity, magnetic reconnection takes place in the current sheet formed inside the expanding loops. Hot, outgoing plasmoid and post flare loops are formed as a result of the reconnection. Numerical results are consistent with the observed plasma temperature (107 – 108K), the length of the flaring loop (1011 – 1012cm), and the speed of optical jets (200 – 400 km s−1 ).
Dust Evolution in Protoplanetary Accretion Disks
Authors: Henning Th. , Schmitt W. , Mucha R. 
Published: July 2016

AbstractWe have numerically investigated the dust evolution in a protoplanetary accretion disk which is described by a time-dependent one-dimensional (radial) α-model. The coagulation of particles due to cohesive collisions is calculated by solving the non-linear Smoluchowski equation. The feedback of the dust evolution and the related opacity changes to the disk evolution is explicitly treated. Three different regimes during the evolution of the grain population can be identified.
Clues and Modelling for Missing Boundary Layer in Cataclysmic Variables
Authors: Huang M. , Sparks W.M. , Sion E.M. 
Published: July 2016

AbstractRecently, it has become observationally evident that during a dwarf nova (DN) outburst, a significant portion of the gravitational energy accreted onto the white dwarf (WD) may not be radiated away instantly from a narrow boundary layer as predicted by the standard disk theory. Instead, it may be stored in the WD through various mechanisms and the radiative area may be much larger; thus the long-puzzling missing boundary layer may be accounted for when the response of the WD to the accretion is considered. The results from our group and collaborators on this aspect are outlined in the first part. A progress report on the development of a new numerical model forms the second.
A Self-Similar Model for Extragalactic Jets and the FRI/FRII Divide
Authors: Kaiser C.R. , Alexander P. 
Published: July 2016

AbstractWe present a model for the large scale structure of FRIItype extragalactic radio sources. The sources are shown to grow selfsimilar and the dependence of the length of the source and the pressure inside the cocoon on the life time are calculated. The stability of the jets in such sources is investigated and the jet power at which FRII sources turn into FRIs is found to be in good agreement with observations.
Water Maser Emission from Dusty Clouds in AGNs
Authors: Kartje John F. , Königl Arieh , Elitzur Moshe 
Published: July 2016

AbstractA natural site for water maser emission in AGNs is provided by dusty gas with properties characteristic of broad line region (BLR) clouds. Radiation shielding by dust in the clouds is critical for allowing molecular gas to exist ≤ 1 pc from the central engine. Thus, the innermost radius at which such masers appear should correspond to the grain sublimation radius rsub. We suggest a dynamical model in which the masing clouds are embedded within a magnetized accretion disk wind.
Spiral Shocks in Accretion Disks with SPH
Authors: Murray James Rhys 
Published: July 2016

AbstractSmoothed Particle Hydrodynamics (SPH) is now seen as a numerical scheme well suited to the study of accretion disks. SPH simulations have been conducted of cataclysmic variable disks (Lubow 1991, Murray 1996, Armitage and Livio 1996), galactic disks (Artymowicz and Lubow 1989), and protostellar disks (Artymowicz and Lubow 1994). It is therefore important to test the technique against theory and other numerical results to obtain an estimate of the accuracy and reliability of SPH in this context. Previously SPH has been tested against standard stationary and time-dependent results of viscous thin disk theory (Murray 1996). Strictly these tests relate to disks where ‘viscous’ terms dominate pressure terms in the equations of motion.In this paper we describe tests of the code more appropriate for hot disks where pressure forces are relatively more important than viscosity. Specifically we consider the form of the spiral density waves that can be excited in a disk by a perturbing gravitational potential. Very low mass perturbing bodies excite linear spiral waves which redistribute angular momentum in the disk. For increasingly massive perturbers, the disk response becomes nonlinear and eventually shocks form. In the standard formulation of SPH, an artificial viscosity term is added to the SPH equations to improve shock capture. This is equivalent to introducing a fixed ratio of shear to bulk viscosity into the equations of motion. In Eulerian schemes, artificial viscosity has been discarded in favour of other more accurate, less dissipative schemes for resolving shocks. The continued use of artificial viscosity in SPH has become a source of ‘friction’ between numericists. The simulations described here demonstrate the scheme’s ability to resolve spiral shocks, and show that SPH is a valuable tool for probing the structure of tidally perturbed accretion disks.
Stellar Winds of Classical T Tauri Stars
Authors: Paatz Gemot , Camenzind Max 
Published: July 2016

AbstractWe present calculations of the structure of magnetized winds of classical T Tauri stars (TTSs) in an axisymmetric, stationary, polytropic approximation in a given magnetosphere (i.e. we do not selfconsistently take into account the influence of the flow dynamics on the structure of the underlying magnetosphere). According to the widely accepted model of CTTSs the magnetosphere is dominated by a strong stellar magnetic field (B* ≃ 1 kG) which may be of dipolar type. The behaviour of the magnetic flux-tube function yields valuable constraints on the wind solutions.
ASCA Observations of the Eclipsing IP XY Ari
Authors: Schlegel Eric M. , Mukai Koji , Ishida M. 
Published: July 2016

AbstractWe describe the ASCA observation of the eclipsing intermediate polar (IP) XY Ari (=H0253+19). This system was first discussed by Patterson & Halpern (1990, ApJ, 361, 173) as a possible IP based upon the HEAO-1 data. This system is impossible to study in the optical or ultraviolet because of the 12 mag of optical extinction associated with the molecular cloud Lynds 1457=MBM 12. The system has an orbital period of 0.2526 day and a pulse period of 208 sec. Our preliminary analysis of the ASCA data shows that the eclipse is nearly total (the precision of the background subtraction sets the limit). The eclipse ingress and egress occur very quickly, although we have not yet derived accurate numbers. There are sufficient photons that we can perform crude phase-resolved spectroscopy. The resulting spectra, each covering about 1/4 of the orbit, show that the Fe Kα line is not constant as a function of phase. A sample spectrum is reproduced below. We expect to submit a paper describing the results to the ApJ before the end of 1996.
Magnetic Torque Induced High/Low Accretion States in AM Herculis Binaries
Authors: Sparks Warren M. , Howell Steve B. , Huang Min 
Published: July 2016

AbstractThe AM Herculis binaries (AM Hers) have high and low accretion states. A model is proposed in which the white dwarf’s rotation acts like an angular momentum storage battery for the binary orbit. The accreted material from the Roche lobe-overflowing secondary charges up this battery, whereas the magnetic torque of the white dwarf on the secondary discharges it back into the binary’s orbit. The Roche lobe’s volume becomes alternately larger and smaller than the secondary’s volume. This alternation causes the mass flow to vary between a high and low rate.
The Formation of Astrophysical Jets
Authors: Livio Mario 
Published: July 2016

AbstractIt is assumed that the acceleration and collimation mechanisms of jets are the same in all the classes of astrophysical objects which are observed to produce jets. These classes now include: active galactic nuclei, young stellar objects, massive x-ray binaries, low mass x-ray binaries, black hole x-ray transients, symbiotic systems, planetary nebulae, and supersoft x-ray sources.On the basis of this assumption, an attempt is made, to identify the necessary ingredients for the acceleration and collimation mechanism. It is argued that: (i) jets are produced at the center of accretion disks which are threaded by a vertical magnetic field, (ii) the production of powerful jets requires, in addition, an energy/wind source associated with the central object. Tentative explanations for the presence of jets in some classes of objects and absence in others are given. Some critical observation that can test the ideas presented in this paper are suggested.
Search for Short Timescale Structural Variation of 3C 273 at 22 GHz and 43 GHz
Authors: Mantovani F. , Valerio C. , Junor W. , McHardy I.M. 
Published: July 2016

AbstractThe results of VLBI observations of the quasar 3C 273, obtained during a multi-frequency campaign in late 1992 in the radio, millimeter, and X-ray bands are presented. The aim of the campaign was to test the application of the SSC (Synchrotron Self-Compton) model to 3C 273. Independent estimates are obtained through the assumption of the energy equipartition between particles and magnetic field.
Multi-Epoch VLBA Observations of γ-Ray Bright Blazars
Authors: Marchenko S.G. , Marscher A.P. , Mattox J.R. , Wehrle A.E. , Xu W. 
Published: July 2016

AbstractWe have investigated the sub-milliarcsecond scale structure of 41 γ-ray bright blazars with the VLBA at frequencies of 22, 43, and occasionally 8.4 GHz. All of the blazars have dominant, unresolved (less than 0.1 to 0.2 milliarcsec) cores, with the exception of 0954+556, which has no structure apparent on scales less than about 1 milliarcsec. Some have prominent jets, others show only faint secondary components or no non-core structure at all. Apparent velocities have been measured for 13 sources. These range from no detected motion to > 25c.
A Polarimetric Survey of CSS Sources at 22 GHz with the VLA
Authors: Lüdke Everton , Adornes Rosane B. 
Published: July 2016

AbstractWe discuss the preliminary results from radio polarization observations of 22 compact steep spectrum sources made with the VLA at 1.3 cm wavelength. We confirm that 1) the detection of a flat-spectrum synchrotron halo and a radio bridge in two CSSs, suggesting that they may be young sources, 2) CSS galaxies with two lobes of emission asymmetric to the core position show strong polarization asymmetry with the highest polarized emission arising from the component farther away from the core and 3) Source components which are nearest to the cores are generally unpolarized even at such short wavelength, suggesting that the spread in the λ2 polarization spectra is much narrower than in extended sources.
Faraday Rotation Measure Observations on the Pc-Scale: Probing the AGN Environment
Authors: Taylor G. B. , Venturi T. , Udomprasert P. S. 
Published: July 2016

AbstractWe present multi-frequency VLBA polarimetric observations of a small sample of quasars revealing their Faraday rotation measure (RM) structure at ~ 1 mas resolution. Two of the sources (OQ 172 and 3C 216) were chosen on the basis of high RMs seen by the VLA and single dish telescopes. A few more typical quasars (3C 273, 3C 279, 3C 345 and 3C 380) were also included in the study. In OQ 172, 3C 273, 3C 279 and 3C 380 we find high RMs (>3000 rad m−2) associated with the nuclear region. Only 1-10 mas from the nucleus the jet |RM|s in all 4 of these sources fall to less than 100 rad m−2. This RM structure may be produced by strong magnetic fields in the the narrow line region of the quasar.
Evolution of the Parsec-Scale Structures of OJ 287 and 3C 273
Authors: Ojha R. , Homan D. C. , Roberts D. H. , Wardle J. F.C. 
Published: July 2016

AbstractWe have obtained 15 and 22 GHz linear polarization VLBA images of a sample of strong and rapidly variable polarized sources at six epochs over one year. We observe rapid changes, in both cores and jet components, in total intensity, both with and without associated changes in polarization. Changes in the cores are probably associated with the births of new components. Images of one quasar (3C 273) and one BL Lacertae object (OJ 287) from two epochs are shown here. No significant Faraday rotation is observed at any epoch in any position in either source.
A MERLIN and VLBI Survey of Faint Compact Radio Sources
Authors: Garrett M. A. , Garrington S. T. 
Published: July 2016

AbstractWe have selected a field from the VLA FIRST Survey which is typical in all aspects except one: it contains a bright, but extremely compact VLBI calibrator - J1159+291. 127 unresolved FIRST sources with SFIRST ≥ 10 mJy lie within 2.5° of this calibrator. λ6 cm MERLIN observations with a resolution ~ 60 mas detect around half the sources. These detections form the basis of a sample of faint but compact radio sources which are ideally suited to follow-up VLBI observations.
A VLBI Survey of 461 Southern Flat Spectrum Sources
Authors: King E. A. , Lovell J. E. J. , Ellingsen S. P. , Costa M. E. , McCulloch P. M. , Reynolds J. E. , Jauncey D. L. 
Published: July 2016

AbstractWe are undertaking a 1-baseline “cuts” VLBI survey of 461 flat spectrum radio sources selected from the Parkes Catalogue.
3C 216: Is It the CSS Source with the Highest Rotation Measure?
Authors: Lüdke E. , Cotton W. D. , Sanghera H. S. , Dallacasa D. 
Published: July 2016

AbstractWe discuss our VLBA observations at 5 and 2.7 GHz and our MERLIN observations at 1.6 and 5 GHz. A MERLIN+VLBA image provided good starting model for self-calibration and we obtained an unprecedent image of the bent jet of 3C 216. Our observations suggest the detection of strong polarization position angle variation across the observing band. If this is due to Faraday rotation then 3C216 may have a four-figure observed rotation measure, which is unlikely to be due to errors in the polarization position angle calibration.