bluuebunny/crossref_metadata_2025_split huggingface dataset:
Heating and Cooling of Hot Accretion Flows by Non Local Radiation
Authors: Esin Ann A. 
Published: July 2016

AbstractHot optically thin accretion flow solutions are often used to model X-ray binaries and active galactic nuclei. Recently two new classes of advection-dominated models have been proposed: a two-temperature solution (Narayan & Yi 1995, ApJ, 452, 710; Abramowicz et al. 1995, ApJ, 438, L37), and a one-temperature solution (Esin et al. 1996, ApJ, 465, 312). Though detailed numerical spectra of advection-dominated flows include global radiative transfer effects (e.g. Narayan 1996, ApJ, 462, 136), analytical calculations of the physical properties of these accretion solutions generally ignore the effects of non-local radiative transfer (e.g. Narayan & Yi 1995; Esin et al. 1996). However, the optical depth for electron scattering in such flows is generally much lower than unity, and therefore, radiation emitted at one radius can potentially heat or cool the gas at other radii through Compton scattering. In this paper we investigate the importance of this effect. We discuss three situations:
1.Radiation from the inner regions of an advection-dominated flow Compton cooling gas at intermediate radii and Compton heating gas at large radii.2.Soft radiation from an outer thin accretion disk Compton cooling a hot one- or two-temperature flow on the inside.3.Soft radiation from an inner thin accretion disk Compton cooling hot gas in a surrounding one-temperature flow.We describe how previous results are modified by these non-local interactions. We find that Compton heating or cooling of the gas by the radiation emitted in the inner regions of a hot flow is not important. Likewise, Compton cooling by the soft photons from an outer thin disk is negligible when the transition from a cold to a hot flow occurs at a radius greater than some minimum Rtr,min ~ 102.8(Ṁ / ṀEdd)3.5α−7Rschw. However, if the hot flow terminates at R < Rtr,mim, non-local cooling of the hot gas near the transition radius is so strong that the flow is cooled to a thin disk configuration. As a result, the transition radius decreases and the hot gas at the new boundary experiences even stronger external cooling. The resulting runaway process continues until the entire hot flow assumes the thin disk configuration. In the case of a thin disk surrounded by a hot one-temperature flow, we find that Compton cooling by soft radiation dominates over local cooling in the hot gas for Ṁ ≳ 10−3αṀEdd. As a result, the maximum accretion rate for which an advection-dominated one-temperature solution exists, decreases by a factor of ~ 10, compared to the value computed under an assumption of local energy balance.
EK Tra: a Spectroscopic Twin of VW Hyi
Authors: Gänsicke Boris T. , Beuermann Klaus 
Published: July 2016

Observations of the inter-outburst behaviour of dwarf novae can put constraints on theoretical models for the structure of the accretion disk and the outburst mechanism. The white-dwarf properties, such as temperature, postoutburst cooling timescale, and rotational velocity play a major rôle in the understanding of the processes in the boundary layer. Alas, the white dwarf could be detected directly only in a small number of dwarf novae. We have re-analysed the available IUE observations of the southern dwarf nova EK TrA, testing the suggestion by Verbunt (1987, A&AS 71, 339) that in this system the white dwarf is a significant source of UV radiation during quiescence.We have decomposed the spectrum obtained closest to quiescence (Fig. 1) into accretion disc and white dwarf contribution following the method described by Gansicke & Beuermann (1996, A&A 309, L47): we use the observed spectrum close to outburst maximum to describe the disc contribution and a grid (log g = 8.0 and 11 000K ≤ Twd ≤ 30 000K, solar abundances) of calculated model spectra kindly provided by I. Hubeny for the white dwarf. Following our experience with VW Hyi (Gänsicke & Beuermann, 1996, A&A 309, L47), we allow for Gaussian emission lines of N V λ 1240, Si IV λ 1400 and C IV λ 1549. The observed spectrum was then fitted with the sum of these components using an evolutionary strategy algorithm. The best fit is achieved for a white dwarf of Twd = 16 000 K and the accretion disc contributing 21% and 77% of the integrated SWP flux, respectively (Fig. 1). Assuming an 0.6 M⊙ white dwarf, the scaling factor of the white dwarf model implies a distance of ~ 140 pc, roughly twice that of VW Hyi. However, EK TrA was never observed in a truly quiescent state; at the time of the IUE observations the residual accretion disc or accretion belt still dominates the UV light and the derived white-dwarf parameters remain necessarily uncertain. Comparison with VW Hyi teaches us that the decline of the spectrum towards Lyα should not, at face value, be taken as a direct measure of the white dwarf contribution to the overall flux.
UV/Optical Continuum Variability in AGN: Observational Test of Accretion Disk Models
Authors: Heisler Charlene A. , Sun Wei-Hsin , Malkan Matthew A. 
Published: July 2016

AbstractWe present preliminary results of a one year monitoring campaign of a sample of active galactic nuclei. We discuss the implications of the observed variations and colour changes in terms of accretion disk models.
New Algorithms for Radiative Transfer in Accretion Disks and Surroundings
Authors: Kanschat Guido 
Published: July 2016

The simulation of astrophysical radiative transfer problems (RTP) is a hard task. On the one hand, the high dimension of the computational domain (3 space, 2 ordinate and 1 frequency variables) calls for huge amounts of memory even for coarse discretisations. On the other hand, accretion discs show very localised features, making a high resolution in some regions of the disk necessary. Furthermore, there should be an estimate of the numerical errors to avoid physical misinterpretation.Apart from the usage of parallel machines with their big memory to enable the computation of RTP’s, we developed a means to generate nearly optimal discretisation meshes. Combined with efficient error estimates, we can solve RTP’s to a very high guaranteed accuracy. These estimates, weighted residual a posteriori error estimates, can not only control global error quantities like the mean quadratic error or the error in the energy norm. They can be suited to nearly any error functional.
Eclipse Mapping of the Accretion Disk Wind in the Cataclysmic Variable UX UMa
Authors: Knigge Christian , Drew Janet E. , Mason Keith O. , Long Knox S. 
Published: July 2016

AbstractWe present the results of an effort to model recent HST eclipse observations of the wind-formed C, ɪv 1550Å resonance line in the high-inclination nova-like variable UX UMa (Mason et al. 1995, Baptista et al. 1995). Within the framework of a simple kinematic model, in which the outflow is described as a rotating, biconical accretion disk wind (Knigge et al. 1995), we are able to reproduce not only the shapes and strengths of the observed line profiles both away from and during eclipse, but also most of the detailed behaviour of different parts of the line (blue wing, line centre and red wing) as a function of orbital phase (Figure 1).The most important result of our modeling is that it strongly suggests the presence of a vertically extended (H ~ few × 10 RWD), relatively dense (ne ~ 1013 cm−3) and only slowly-outflowing (υpoloidal << υescape) transition region between the disk photosphere and the fast-moving wind. Thus the best current data already demand that the simplified picture of accretion disks and winds as quasi-independent physical phenomena should be replaced by a unified theory of accretion disk atmospheres. It may be important in this context that, barring strong contamination of the observed C ɪv wind line by an underlying disk-formed component, the evidence for rotation in the outflow is unambiguous: the observed line flux light curves clearly exhibit a rotational disturbance that is well matched by our simple model of a rotating disk wind.
A Quantitative Model for Bipolar Molecular Outflows
Authors: Li Zhi-Yun , Shu Frank H. 
Published: July 2016

AbstractSince their first discovery in 1980, nearly 200 bipolar molecular outflows have now been observed. These outflows appear to be a ubiquitous phenomenon in the star formation process, although their origin remains unclear. Here, we adopt the thin-shell model of Shu, Ruden, Lada, & Lizano (1991, ApJL, 370, 31) in which bipolar molecular outflows are the swept-up ambient material by powerful winds from central young stellar objects. Besides explaining many observed features of bipolar sources, such as shape, velocity, and the typical extent, this model also naturally accounts for the so-called “Hubble law” of velocity field. However, it was criticized by Masson & Chernin (1992, ApJ, 387, L47) on the ground that much less mass is observed at the high velocity end of some bipolar outflows than expected from this thin-shell model, unless the wind and/or the ambient medium are extremely anisotropic. We show that the extreme anisotropy is in fact inevitable both in the wind and in the ambient medium, because of the presence of dynamically important magnetic fields.The wind is anisotropic because it is magneto-centrifugally driven, and the frozen-in toroidal field tends to collimate the flow toward the rotation axis. This effect is studied using the well-known “method of characteristics” and we conclude that a MHD wind always has two components: a well-collimated jet and a wind-angle wind. It is the wide-angle wind component that will interact with the ambient medium and produce bipolar molecular outflows. The ambient core medium is anisotropic because it arises from lower-density, magnetically-supported molecular clouds through ambipolar diffusion. Using self-similarity technique, we model the “pivotal state” of star formation, which divides the core formation phase and the core collapse (accretion) phase. We find that pivotal states have toroid-like density contours in general, with lower-density regions around the magnetic axis (where matter drains more easily).When these anisotropics induced by magnetic fields are taken into account properly, the main argument against the wind-angle wind-driven, thin-shell model for bipolar molecular outflows disappears. Indeed, we show that the empirical mass-velocity relation found by Masson & Chernin and others for bipolar outflows provides the best quantitative supporting evidence for the thin-shell theory!
Magnetic Accretion Funnels in Young Stellar Objects: Thermal Structure and Emission Signatures
Authors: Martin Steven C. , Königl Arieh 
Published: July 2016

AbstractA self-consistent procedure is outlined for determining the thermal structure of gas inflowing along magnetic field lines of a young stellar object. A young pre-main-sequence star (e.g., a classical T Tauri star) is assumed to possess a dipole magnetic field that disrupts a geometrically thin accretion disk and channels the incoming gas toward the stellar surface, leading to the formation of a pair of accretion funnels that terminate in shocks at high stellar latitudes. The heat equation is solved together with the rate equations for hydrogen, and the main physical processes that heat and cool the gas are identified. In particular, in the case of T Tauri stars, it is found that adiabatic compression is the principal heat source and that the Ca II and Mg II ions act as a powerful thermostat that regulates the gas temperature. The ionization state of the gas in the radiation field of the stellar photosphere and of the accretion shocks is found in this case to be controlled by Balmer continuum photons. The implications of these calculations to the observational signatures of accreting YSOs (e.g., their near-infrared hydrogen and CO overtone line emission) are discussed.
Axisymmetric MHD Simulations of Stellar Magnetosphere/Accretion Disk Interaction
Authors: Miller K. , Stone J. 
Published: July 2016

AbstractThe evolution of the inner disk region of magnetized accreting systems is studied by means of numerical simulations. Two different initial magnetic field topologies are studied: a pure dipole field which threads the disk continuously and a dipole field excluded from the disk by surface currents. When present, the Balbus-Hawley (BH) instability provides efficient angular momentum transport, and accretion is equatorial. Otherwise, magnetic coupling between the disk and star surfaces as the dominant mechanism for transport and nonequatorial accretion results. In all of the simulations, low density, collimated disk-driven winds lead to mass loss.
Photometric Evidence for the Presence of An Accretion Disk and for a Superhump - Like Phenomenon in the Classical Nova V1974 Cygni 1992
Authors: Retter A. , Leibowitz E.M. , Ofek E.O. 
Published: July 2016

AbstractThe light curve of V1974 Cyg shows two distinct periodicities. The shorter periodicity is clearly the orbital period of the binary system. We show that the longer variation has similar features to permanent superhumps. This result indicates the existence of an accretion disk in the system no later than 30 months after the nova outburst. We used the precessing disk model of the superhump phenomenon and previous results in order to estimate that the white dwarf mass is at the range 0.75 – 1.07 M⊙.
Accretion Disk Instabilities and the Quasar Luminosity Function
Authors: Siemiginowska Aneta 
Published: July 2016

AbstractWe predict the quasar luminosity function (LF) from a population of sources with the activity driven by instabilities in an accretion disk. The ligth curve of a single source convolved with the mass distribution gives the quasar luminosity function at a given redshift. We model the observed LF at four different redshifts.
Optical Photometry of the Eclipsing LMC Supersoft Source CAL 87
Authors: Southwell K.A. , Charles P.A. , O’Donoghue D. , Alcock C. , Allsman R.A. , Alves D. , Axelrod T.S. , Bennett D.P. , Cook K.H. , Freeman K.C. , Griest K. , Guern J. , Lehner M.J. , Marshall S.L. , Peterson B.A. , Pratt M.R. , Quinn P.J. , Rodgers A.W. , Stubbs C.W. , Sutherland W. , Welch D.L. 
Published: July 2016

AbstractWe present optical photometry of the eclipsing supersoft source, CAL 87. We find the eclipse structure to be stable over ~ 4 y, derive an improved ephemeris of To = HJD 2450111.5144(3)+0.442674(7)E, and see new structure in the light curve morphology.
Soft X-rays of AGN: Emission from Accretion Disks?
Authors: Staubert R. , Dörrer T. , Müller C. , Friedrich P. , Brunner H. 
Published: July 2016

Soft X-ray spectra of many Active Galactic Nuclei (AGN) show structure which suggests excess emission at low energies, mostly below 1 keV. This was confirmed by the ROSAT spectra (0.1–2.4 keV) AGN in our samples which generally have steeper power law spectra than the canonical index of 0.7. The soft excess component may be the high energy tail of the big blue bump which in turn may be due to the integrated emission from an accretion disk around the central black hole.We discuss results of our spectral analysis of two different samples of AGN: 1) QSO/Seyfert-I from the ROSAT All Sky Survey (RASS) and 2) radio-quiet QSO from ROSAT Pointed Observations. The ROSAT data are combined with UV Data from IUE and hard X-ray data from various hard X-ray missions.
Mapping Quasi-Periodic Oscillations from the Outbursting Intermediate Polar GK Persei
Authors: Still M. D. , Morales-Rueda L. , Roche P. 
Published: July 2016

AbstractWe provide a kinematically-resolved analysis of 5000s QPOs found in the optical emission lines of GK Per during a dwarf nova outburst. These are consistent with models of reprocessing off blobs of gas orbiting within the inner accretion disc.
Hα Spectroscopy of WW Cet
Authors: Tappert C. , Wargau W. , Hanuschik R.W. 
Published: July 2016

AbstractTime-resolved spectroscopic data of the cataclysmic variable WW Cet were taken during quiescence, outburst, and decline from outburst. The profile of the Hα emission line is studied by measuring radial velocities of different parts of the line.The complete poster is available on the World Wide Web at http://www. astro, ruhr-uni-bochum.de/cta/paper.html.
Ejection Phenomena in Galaxies and QSOs: Are We Going Far Enough?
Authors: Burbidge G. 
Published: July 2016

AbstractI summarize the observational evidence which suggests: (1) that many of the compact radio sources associated with QSOs and related objects have been ejected from active galaxies so that their redshifts are not measures of their distances, and (2), that the energy emitted by compact sources is not gravitational in origin, but arises through creation processes in regions of galaxies with very strong gravitational fields.
Sub-mas Jets in Gamma-Active Blazars: Results from High Frequency VLBI
Authors: Krichbaum T.P. , Kraus A. , Otterbein K. , Britzen S. , Witzel A. , Zensus J.A. 
Published: July 2016

AbstractWe report new results from high frequency (22-86 GHz) VLBI monitoring observations of selected blazars. These Gamma-bright sources show pronounced correlated flux density variations over the full electromagnetic spectrum (radio to Gamma-ray bands). From our high-angular resolution images (0.1-0.2 mas), we find increasing evidence for a tight correlation between this activity and the production of new jet components. Here we present results for the 3 sources PKS 0528+134, 3C 273, & 0836+710.
Monthly VLBI Monitoring of Selected Gamma-Bright Quasars
Authors: Britzen S. , Witzel A. , Krichbaum T.P. , Roland J. , Wagner S.J. 
Published: July 2016

AbstractIn the past few years EGRET observations of extragalactic radio sources demonstrated the presence of blazars at high energies and supplied informations about their enormous luminosities. Questions arose whether outbursts at these high frequencies can be linked to structural changes in the radio regime and the need for VLBI monitoring became obvious. In the past two decades geodetic VLBI measurements gathered this kind of data.We analyzed λ =3.6 cm observations with high time-resolution (up to once a month) from the geodetic IRIS campaigns for the γ-ray active quasars PKS 0420-014 and PKS 0528+134. In both sources superluminally moving jet components could be traced over time periods of several years on curved trajectories. Both sources exhibit component ejections preceding the time of an observed gamma-ray flaring.
Markarian 421: Te V Gamma-ray Flares and VLBI Structure
Authors: Edwards P.G. , Moellenbrock G.A. , Unwin S.C. , Wehrle A.E. , Weekes T.C. 
Published: July 2016

AbstractPhotons spanning over 17 decades in energy have been detected from Markarian 421. The discovery of >100 MeV photons by the EGRET instrument on board the Compton Gamma Ray Observatory led to the discovery of >300 GeV photons by the Whipple group using the air Čerenkov method. In May 1996 the source underwent an unprecedented outburst at these energies, inspiring observations with the VLBA to determine whether such high energy flares are associated with changes in the milliarcsecond structure of this BL Lac object.
Superluminal Motion in the Gamma-Ray Blazar 3C 279
Authors: Unwin S.C. , Wehrle A.E. , Xu W. , Zook A.C. , Marscher A.P. 
Published: July 2016

AbstractThe blazar 3C 279 is one of the strongest extragalactic sources of γ-rays, and is also one of the best studied superluminal radio sources. Definitive testing of models of the broad-band spectral energy distribution, especially in the X-ray and γ-ray regions requires knowledge of the evolution of the spectrum with time. Within the context of the relativistic jet model, multi-wavelength monitoring of the parsec-scale radio jet is also required. We present here the first steps toward such a test, using extensive VLBI monitoring over a 13 year interval at 22 GHz, and γ-ray observations between 1991 and 1996.
Unification of Radio Galaxies
Authors: Cohen M.H. , Ogle P.M. , Iran H.D. , Goodrich R.W. 
Published: July 2016

AbstractMany FR 2 narrow-line radio galaxies also display polarized broad lines. The broad-line region is hidden from direct view and is seen by reflection (ie scattering). In these objects the spectral classification is controlled by the aspect at which they are viewed.
The Sub-Parsec-Scale Structure and Evolution of Centaurus A at 8.4 and 22.2 GHz
Authors: Tingay S.J. , Jauncey D.L. , Reynolds J.E. , Tzioumis A.K. , Preston R.A. , Jones D.L. , Murphy D.W. , Meier D.L. , Lovell J.E.J. , McCulloch P.M. , Costa M.E. , Nicolson G. , Quick J.F.H. 
Published: July 2016

AbstractWe have been observing the sub-parsec-scale radio jet in the nucleus of Centaurus A, the closest active radio galaxy, at 8.4 GHz over the last 6 years with the SHEVE array and most recently with the VLBA at 8.4 and 22.2 GHz. In this paper we will review the results of these observations and give a brief summary of our interpretation, concentrating on the evolution of the sub-parsec-scale jet structure with time; subluminal component motions and rapid short timescale evolution. A full description of this work is soon to be published in a major journal article.
Sub-arcsecond Radio Imaging of Jets in Low-luminosity Radio Galaxies
Authors: Bondi M. , Parma P. , Fanti R. , de Ruiter H. , Laing R.A. 
Published: July 2016

AbstractWe used VLA and MERLIN observations of 4 FRI radio galaxies to study the radio jet properties in the inner kiloparsecs from the core. We present evidence for relativistic motion in the inner regions.
The Counterjet in the Nucleus of Centaurus A
Authors: Preston R.A. , Tingay S.J. , Jones D.L. , Murphy D.W. , Meier D.L. , Jauncey D.L. , Reynolds J.E. , Tzioumis A.K. , Lovell J.E.J. , McCulloch P.M. , Costa M.E. , Nicolson G. 
Published: July 2016

AbstractWe have observed the nucleus of Centaurus A with the VLBA and the SHEVE arrays, individually and in combination. In sensitive experiments at 8.4 GHz which use both arrays together we have detected components in a sub-parsec-scale counterjet. The counterjet is also seen in 2.3 and 22 GHz images. The nuclear region appears to be partially obscured by a free-free absorbing structure of about 1 pc in extent. The jet axis appears to lie at a 50°–70° angle to our line of sight.
Some Theoretical Studies of Two Gamma-Ray Blazars: PKS 0528+134 and Mrk 421
Authors: Qian S. J. , Zhang X. Z. , Witzel A. , Krichbaum T. P. , Britzen S. , Kraus A. 
Published: July 2016

AbstractThe high energy gamma-ray flares observed in PKS 0528+134 are interpreted in terms of the external inverse Compton scattering (EICS) mechanism. The evolutional relationship between the gamma-ray flares and the associated mm-radio outbursts is investigated. The TeV/X-ray flare detected in May of 1994 from Mrk 421 is interpreted in terms of the SSC mechanism and it is shown that it may be due to the acceleration of relativistic electrons with an initially flat energy spectrum (N(E)∝E−s with s~1.5), rather than just a flattening of the high energy tail in the electron energy distribution of the source in the quiescent state.
X-Band VLBA Polarization of 3C 287
Authors: Dallacasa D. , Schilizzi R. T. , Rendong Nan 
Published: July 2016

AbstractVLBA polarimetric observations at 8.15 and 8.54 GHz of the quasar 3C 287 are presented and discussed. The source structure is complex, and the magnetic field geometry does not help in the interpretation of the source.
A Polarization Study of Quasar 4C 71.07
Authors: Hutchison J.M. , Cawthorne T.V. 
Published: July 2016

AbstractParsec-scale polarization observations of the quasar 4C 71.07 show a misalignment between the inferred magnetic field direction and the structural direction of the jet. Dual frequency VLBI polarimetry was used to to determine whether Faraday rotation was the origin of the misalignment. We find insufficient rotation to realign the field and jet. Other possible mechanisms which may result in a misalignment are discussed.
Sub-milliarcsec Structure of 3C 111 at 0.7 and 3.6 cm
Authors: Alef W. , Preuss E. , Kellermann K. I. , Gabuzda D. 
Published: July 2016

AbstractVLBA+Effelsberg observations of the radio galaxy 3C 111 at 7 mm show remarkable rapid changes in structure which cannot be described in terms of simple component motions or changes in the structure of fixed components.