bluuebunny/crossref_metadata_2025 huggingface dataset:
PSR B1259–63: Periastron Puzzles
Authors: Johnston Simon
Published: July 2016

I was fortunate enough to have been involved with unravelling the secrets of PSR B1259-63 right from the start. The major part of my PhD thesis was a large survey of the southern galactic plane at an observing wavelength of 20 cm. Confirmation of a further batch of good-looking pulsar candidates from the data processing took place in November 1989 at the Parkes radio telescope in central New South Wales, Australia. My fellow student Jeremy Lim and I were in the control room when the 47.7 ms periodicity of PSR, B1259–63 was confirmed. At first, the slight increase in period from the original detection led us to believe we had discovered a ‘Crab-like’ pulsar. However by the end of the following year we realised we had something much more exciting on our hands.
Tidal Interactions in Pulsar Binaries
Authors: Mardling Rosemary
Published: July 2016

The value of using pulse timing to measure orbital variations in eccentric pulsar binaries which are close enough at periastron to interact tidally is discussed. We briefly review a model for dynamical tidal evolution which shows that chaotic orbital evolution is possible, and list several applications to real systems. The system PSR J0045-7319 is then discussed, and a possible explanation for its decreasing orbital period is suggested.
The Fe Kα Line and Relativistic Effects from Accretion Disks
Authors: Ross Randy R.
Published: July 2016

One of the most exciting results from ASCA has been the discovery of relativistic line profiles for iron Kα emission from Seyfert 1 galaxies. Recent results concerning the properties of Fe Kα lines from the inner regions of black-hole accretion disks are reviewed.
Advective Disks
Authors: Narayan R.
Published: July 2016

Recent work on advection-dominated accretion flows (ADAFs) is reviewed. The article concentrates on an optically thin branch of ADAFs which is present at mass accretion rates below a critical value ~ (10−2– 10−1) the Eddington rate. Models based on this branch have been quite successful at explaining a number of low-luminosity X-ray binaries and galactic nuclei, and some brighter systems. Some progress has also been made toward understanding the various spectral states of accreting black holes. It is argued that ADAFs may provide one of the best techniques for demonstrating the reality of event horizons in black holes.
Relativistic Diskoseismology
Authors: Wagoner Robert V.
Published: July 2016

We will summarize results of relativistic calculations of the eigenfunctions and eigenfrequencies of modes of oscillation trapped within the inner region of thin accretion disks by non-Newtonian gravitational properties of a black hole (or a compact, weakly-magnetized neutron star). The focus will be on the most observable, robust, and best-studied class: the analogue of (internal) gravity modes in stars. The frequencies of the corresponding peaks in the power spectrum of the resulting luminosity modulations depend almost entirely upon only the mass and angular momentum of the black hole. The frequency width also depends upon the viscosity parameter of the disk. Searches for such features are being carried out by the RXTE satellite.
Planetary Formation in Protostellar Disks
Authors: Lin D.N.C.
Published: July 2016

Recent discoveries of planets around other stars suggest that planets are ubiquitous and their dynamical properties are diverse. We reviewed the formation mechanism for protoplanets and the post-formation planet-disk tidal interaction which may have led the short-period planets to their present configuration. We suggest that these planets may be the survivors of a populations of similar planets which have plunged into and contaminated the stellar convection zone. In the context of the solar system, the mass of the giant planets and the present distribution of the minor planets may be used to infer the structure and evolution for the primordial solar nebula. The large eccentricity of 70 Vir and HD 114762 may be due to cohesive collisions in planetary systems which become unstable during their long term orbital evolution.
Recent Observations of Flickering in CVs and AGN
Authors: Welsh W. F. , Wood Janet H. , Horne Keith
Published: July 2016

Flickering is a phenomenon associated with all known accretion powered processes. Yet despite its energetic importance and universality, our understanding of flickering remains elementary. Only recently has the flickering itself become a topic of serious investigation. This paper briefly addresses two fundamental observational questions: “Where is the flickering coming from?” and “What is the spectrum of the flickering?”.
Modeling Tidal Effects in Accretion Discs
Authors: Godon Patrick
Published: July 2016

A two-dimensional time-dependent spectral code is used for the study of tidal effects in accretion discs. A cool disc around a white dwarf (characteristic of CV systems) is modeled under the assumption of a polytropic equation of state and a standard alpha viscosity prescription. For a mass ratio q < 0.1 (considered here) and under the assumption of a reflective inner boundary, tidal effects induce an eccentric (m=l azimuthal) mode in the disc together with an elliptic (m=2 azimuthal) mode in the inner disc.
Disc Instabilities and “Soft” X-Ray Transients
Authors: Lasota J.P.
Published: July 2016

I review several problems which arise when one tries to apply the standard dwarf nova instability model to outbursts of soft X-ray transients
Breaking the 100 MG Barrier: The First High Field Magnetic CV
Authors: Szkody Paula , Schmidt Gary D. , Smith Paul S. , Silber Andrew , Hoard D.W. , Tovmassian Gaghik , Gänsicke B.T. , de Martino D.
Published: July 2016

We present results from IUE and optical spectra, optical photometry and circular polarimetry during high and low states of the highly luminous soft X-ray cataclysmic variable AR UMa that identifies the primary in this system as a white dwarf with a magnetic field of 230 MG. The high magnetic field likely threads accretion blobs all the way from the secondary to below the surface of the white dwarf, resulting in a lack of polarised cyclotron emission and an extreme soft-X-ray luminosity during the high state.
Discless Accretion in Intermediate Polars?
Authors: Buckley David A.H.
Published: July 2016

I review the evidence for disc and discless accretion in the asynchronous magnetic CVs, the intermediate polars (IPs). The presence and relative amplitudes of the spin and synodic (beat) X-ray periods, and related orbital sidebands, are very model dependent, and support disc accretion dominating in all the well-studied IPs. A component of non-disc accretion is sometimes observed, a result of stream overflow penetrating to the magnetosphere, and is responsible for the synodic period. The recently discovered polarized IP, RX J1712.6–2414, is a prime candidate (the first) for purely discless, stream-fed accretion in such a system. While the polarisation varies at the spin period, the X-ray variations occur at the synodic period, a result of the modulation in the mass transfer rate for a discless accretor.
Emission Lines of Dwarf Novae Accretion Disks
Authors: Hummel W. , Horne K. , Marsh T. , Wood J. H.
Published: July 2016

We derive the temperature and density structure of the accretion disk of the dwarf nova U Gem in quiescence from 3D radiative line and continuum transport calculations of a differentially rotating disk.
Young Binary Star/Disk Interactions
Authors: Lubow Stephen H. , Artymowicz Pawel
Published: July 2016

Young binary star systems have associated gaseous disks that affect their evolution. Strong tidal forces of a binary give rise to gaps that separate circumstellar (CS) disks from the circumbinary (CB) disk. We discuss two important processes that involve binary/disk interactions. The first is eccentricity evolution of a binary embedded in a disk. The second is accretion of gas across the gap that separates CS and CB disks. These results may also have implications for the recently discovered planets.
Theory and Simulation of Asymmetrically Perturbed Radiatively Cooled Jets
Authors: Hardee P. E. , Stone J. M. , Xu J.
Published: July 2016

Results of a spatial stability analysis and of numerical simulations of a “slab” jet in which optically thin radiative cooling is dynamically important are presented. Two different cooling curves are used. Unstable Kelvin-Helmholtz modes are significantly different from the adiabatic limit, and the form of the cooling function strongly affects the results. The numerical simulations are in excellent agreement with the linear stability analysis. In the non-linear regime growth of the surface wave at low frequencies results in sinusoidal oscillation which can disrupt the jet, while non-linear body waves produce low amplitude wiggles within the jet that can result in shocks within the jet. In cooling jets, these shocks can produce dense knots and filaments of cooling gas within the jet, and weak shock “spurs” in the ambient gas. Acceleration of ambient gas can be produced by these “spurs”, or by rapid entrainment if the jet is disrupted. For parameters typical of protostellar jets perturbations with a period of < 100 yrs should excite body waves which produce internal shocks and small amplitude wiggles. The lack of large amplitude wiggles in most observed systems is consistent with the suggestion that jets arise from the inner regions (r < 1 AU) of accretion disks.
Parameters of Disks from VLBI Observations
Authors: Walker R. C.
Published: July 2016

VLBI provides the only direct imaging technique currently available that is able to resolve even the outer parts of accretion disks. Recent observations of water masers in NGC 4258 and of free-free absorption of counterjet radiation in NGC 1275 are the best examples of where VLBI has provided important data on the parameters of disks. Other observations are now being made of similar sources. In addition, recent VLBI observations of HI absorption promise to add yet more constraints on the outer disk regions.
Order and Chaos in Accretion Disks of Active Galactic Nuclei
Authors: Chian Abraham C.-L.
Published: July 2016

Langmuir turbulence plays an important role in electron heating and generation of plasma emission in accretion disks of active galactic nuclei. The nonlinear dynamical behavior of Langmuir turbulence and its relevance in the interpretation of AGN variability is discussed. In particular, we study nonlinear saturation of the Langmuir stimulated modulational instability, for which the low-frequency mode is a resonant ion-acoustic wave. The nonlinear system of coupled wave equations is shown to undergo transition from order to chaos via the route of quasiperiodicity. The periodic, quasiperiodic and chaotic variabilities in AGN emissions may be the electromagnetic signatures of the ordered and chaotic states of Langmuir turbulence in accretion disks or jets of AGN.
Interaction of Jets with the Interstellar Medium in GPS, CSS and CSO Sources
Authors: Bicknell Geoffrey V. , Dopita Michael A. , O’Dea Christopher P.
Published: July 2016

In the past, the interaction of AGN jets with the surrounding medium has mainly been considered important only insofar as it gave us some understanding of the properties of jets. The emphasis of the work summarised here is different. In a recent paper (Bicknell, Dopita and O’Dea, 1996) we have proposed that the key optical and radio properties of Gigahertz Peak Spectrum (GPS) and Compact Steep Spectrum (CSS) sources, both treated as examples of Compact Symmetric Objects (CSOs) can be explained in terms of the interaction of a jet with a dense ISM - presumably the result of a merger. We therefore suggest that these sources represent an important evolutionary phase in Active Galactic Nuclei in which the interaction with the ISM is dominant.
X-Ray and Optical Observations of a New X-Ray Soft Intermediate Polar: RX J0512.2–3241
Authors: Burwitz V. , Reinsch K. , Beuermann K. , Thomas H.-C.
Published: July 2016

The V~17.6 mag optical counterpart of the bright, soft, high-galactic latitude X-ray source RX J0512.2–3241 detected during the ROSAT All-Sky Survey, has been identified as a new, asynchronously rotating, magnetic cataclysmic variable (intermediate polar). The X-ray spectrum of RXJ0512–32 is similar to that of polars, it shows a soft component with no intrinsic absorption and a blackbody temperature kTbb~38 eV. From our optical follow-up B and V CCD photometry (cf. Fig. 1) we derive most probable spin and orbital periods of (863.5 ± 0.7) s and (3.45 ± 0.03) h respectively. A lower limit for the distance to the system is d > 740 pc. From this evidence we suggest that RXJ0512-32 is a further member of the ROSAT discovered class of soft X-ray intermediate polars (for details see Burwitz et al., 1996, A&A 310, L25). This still small class of systems (see Haberl and Motch 1995, A&A 297, L37) has X-ray characteristics similar to those of low magnetic field polars and may be their long sought evolutionary progenitors.
Heating and Cooling of Hot Accretion Flows by Non Local Radiation
Authors: Esin Ann A.
Published: July 2016

Hot 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.
X-Ray Observations of GRS 1915+105
Authors: Greiner J. , Harmon B.A. , Paciesas W.S. , Morgan E.H. , Remillard R.A.
Published: July 2016

After the discovery of GRS 1915+105 (Castro-Tirado et al. 1992) we obtained pointed ROSAT observations every six months (12 until now). The flux in the ROSAT (0.1–2.4 keV) band is strikingly different from the simultaneous BATSE (25–50 keV) flux which was obtained by integrating the best fit power law (Fig. 1). Motivated by the different intensity evolution in the soft and hard X-ray band we have selected BATSE monitoring data collected simultaneously to ROSATdata and performed joint spectral fitting with XSPEC. As a result, we never got an acceptable fit (see Fig. 2): The BATSE power law (upper dotted line) is too steep to match the ROSAT band, and even allowing for an increased absorbing column (lower dotted line) does not solve the problem. Alternatively, neither a thermal bremsstrahlung fit (solid line) nor a power law fit (lower dash-dot line) to the ROSAT data match the BATSE flux. The upper dash-dot line is a -2.5 powerlaw which would match the BATSE data while giving too much 1–2 keV emission. A similar, but less stringent result is obtained when folding the best fit BATSE power law models with the HRI detector response to compare the expected count rate with the observed one. We therefore conclude that the spectrum during all simultaneous ROSAT/BATSE observations seemingly consists of two different spectral components.
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

We 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

We 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

Since 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

A 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.
On the Difference Between Radio Quiet and Radio Loud AGN
Authors: Marziani P. , Calvani M. , Sulentic J. W.
Published: July 2016

A systematic difference in alignment between the central black hole spin and the angular momentum of the accreting gas may help to explain several differences found in the optical and UV HST spectra of radio loud and radio quiet AGN.
Axisymmetric MHD Simulations of Stellar Magnetosphere/Accretion Disk Interaction
Authors: Miller K. , Stone J.
Published: July 2016

The 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

The 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

We 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.
A Magnetohydrodynamic Model of Galaxy Formation
Authors: Sleath J. P. , Nelson A. H.
Published: July 2016

We present a full 3D model of galaxy formation, incorporating for the first time magnetohydrodynamical effects. By this means we hope to simulate the creation of active galactic nuclei, including jets. Preliminary results are encouraging.
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

We 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

We 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

Time-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

I 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

We 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

In 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

Photons 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

The 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

Many 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

We 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

We 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

We 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

The 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

VLBA 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

Parsec-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

VLBA+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.
Limiting Brightness Temperature for Synchrotron Sources
Authors: Lähteenmäki A. , Valtaoja E.
Published: July 2016

The maximum brightness temperature limit for synchrotron sources derived from inverse Compton catastrophe is usually taken to be 1012 K. Readhead has argued in 1994 that the equipartition brightness temperature may be a better estimate for Tb,lim. It provides a value of 5 × 1010 K. We suggest that a reasonable estimate of the value of Tb,lim can also be achieved by comparing Doppler boosting factors from total flux density variations at 22 and 37 GHz with traditional Doppler factors from the SSC X-ray flux. We also compare our variability brightness temperatures with values calculated for individual sources observed with VLBI, obtaining other independent estimates of Tb,lim.
Low Frequency (327 MHz) VLBI Survey: First Results
Authors: Altunin V. , Beasley A. , Lűdke E. , Alekseev V. , Lipatov B. , Dagkesamansky R. , Kardashev N. , Kovalenko A. , Molotov I. , Slysh V. , Migenes V.
Published: July 2016

In this paper we report the first results of the 327 MHz VLBI survey which has been carried out in the last two years to select potential candidate sources for the future SVLBI missions and to search for directions of small scattering-“holes” in the interstellar scattering medium. During the three VLBI sessions conducted so far, we observed about 80 sources. Preliminary data analyzed from the first part of the survey shows that at least 17 out of 50 sources have compact components with a size smaller than 30 mas.
The Submilliarcsecond Structure of Quasars and AGN
Authors: Kellermann K. I. , Zensus J. A. , Vermeulen R. C. , Cohen M.H.
Published: July 2016

VLBA observations of 130 quasars and AGN made at 15 GHz with submilliarcsecond resolution show a variety of structures, most of which appear linear.
A VLBA Calibrator Survey
Authors: Peck A. B. , Beasley A. J.
Published: July 2016

The Very Long Baseline Array (VLBA) Calibrator Survey consists of snapshot observations at 2.7 and 8.4 GHz of nearly 2000 flat-spectrum extragalactic sources selected from the Jodrell Bank–VLA Astrometric Survey (JVAS). When complete, this survey will assist in defining the extragalactic radio frame in the northern sky by providing positions for these sources which will be accurate on the milliarcsecond scale. This survey is accessible on the WWW at http://magnolia.nrao.edu/vlba_calib/index.html.
Polarization VLBI Observations of a Complete Sample of Northern BL Lacertae Objects
Authors: Pushkarev A. B. , Gabuzda D. C.
Published: July 2016

The polarization electric vectors in the VLBI jets of BL Lacertae objects are typically aligned with the jet structure. If the jet radio emission is optically thin synchrotron emission, this implies that the magnetic field is perpendicular to the jet, usually interpreted as a signature of shocks. The distribution of polarization position angles in the VLBI core components appears to be bimodal, with the polarization angles either aligned with or perpendicular to the jet direction. In order to study the origin of this characteristic polarization structure, we have made VLBI polarization observations of all 34 sources in the Kühr and Schmidt sample of BL Lacertae objects.
The Recent Highly-Polarized Centimeter Burst in BL Lacertae
Authors: Aller H. D. , Hughes P. A. , Aller M. F.
Published: July 2016

Observations with the University of Michigan 26-meter paraboloid at 4.8, 8.0, and 14.5 GHz have been used to test generic shock-based models of the emitting region of BL Lacertae. Large changes in the polarization position angle since 1980 are not consistent with a jet with a simple conical flow.
Radio Emission from Low-Luminosity Active Galactic Nuclei
Authors: Van Dyk Schuyler D. , Ho Luis C.
Published: July 2016

Many nearby galaxies show optical evidence for low-luminosity AGNs that are far less luminous than classical Seyfert nuclei and QSOs. LINERs, the most common variety of such emission-line objects, comprise ~ 1/3 of nearby galaxies, and may serve as an important “missing link“ between normal and Seyfert galaxies. To shed light on the physical origin of LINERs, which remains controversial, we are conducting a high-resolution continuum survey of a representative sample of galaxies using the VLA to search for compact radio cores. We additionally discuss the dramatic radio variability in the LINER nucleus of M81.
Proper Motions and Jets in NGC 4258: Further Evidence for a Sub-Parsec Nuclear Disk
Authors: Herrnstein J. R.
Published: July 2016

The evidence for a subparsec-scale disk in the nucleus of NGC 4258 is reviewed in the context of multi-epoch VLBA observations. The detection of jet continuum emission and of maser proper motions in these data bolster the case for a nuclear disk.
VLBI Observations of a Megamaser in a Seyfert Galaxy IC 2560
Authors: Nakai N. , Inoue M. , Hagiwara Y. , Miyoshi M. , Diamond P. J.
Published: July 2016

VLBA observations of water-vapor maser emission in the active nucleus of a Seyfert 2, IC 2560, show linear velocity gradient along the north-south elongation, suggesting a compact rotating disk. The binding mass density within the disk is 7.7 × 106 M⊙ pc−3. We are also monitoring the velocity variations of the maser features with single dish telescopes. By combining the velocity drift with the linear velocity gradient, we would be able to determine the rotation speed and radius of the nuclear disk, and hence the central mass.
VLA Observations of H2O Masers in the Class 0 Protostar S106FIR
Authors: Furuya R. , Kawabe R. , Saito M. , Umemoto T. , Kitamura Y.
Published: July 2016

We measured relative positions of the 22 GHz H2O maser components toward the class 0 protostar S106FIR with the VLA. Two clusters of H2O masers were found. The separation between these clusters is approximately 50 AU at position angle = 73° and each cluster is spread over about 10–20 AU The size of the S106FIR maser clusters is unusually compact compared with other known H2O masers associated with outflows of forming stars. The maser emission appears to originate from MHD driven outflow very close to the star.
The H2O Supermaser Emission Region in Orion KL
Authors: Matveenko L. I. , Diamond P. J. , Graham D. A.
Published: July 2016

We compare spatial structure of the H20 maser during an active period with that during a later quiescent period, received with VLBA in 1993.
Shock Structure of the High-Velocity Water Masers in W 49 N
Authors: Liljeström T.
Published: July 2016

Magnetic field parameters and other characteristics of the postshocked gas in W 49 N are derived from a linewidth comparison between the multi-epoch VLBI data of Gwinn and contemporaneous long-term single-dish observations. The observations support predictions of dissociative shock models.
Intraday Variability of Flat-Spectrum Radio Sources
Authors: Wagner S. J.
Published: July 2016

The characteristics of rapid variability of flat-spectrum radio sources are reviewed. A large fraction of the blazar population is found to show variability on timescales shorter than one day throughout the entire electromagnetic spectrum. The spectral indices and polarization characteristics change equally fast.

Structure functions of the well-monitored sources show pronounced breaks on scales of about 10 to 50 hours, with flatter slopes towards the fast end. This illustrates that Intraday Variability (IDV), i.e. the high frequency end of the power spectrum is qualitatively different and requires different mechanisms than slower variations.

While intrinsic IDV provides direct clues on small-scale structure over fifteen decades in frequency, extrinsic contributions from interstellar scattering contributes at the lowest frequencies, and remains difficult to disentangle from the intrinsic effects.
VLBA and MERLIN Polarization Observations of Compact Steep-Spectrum Source
Authors: Mantovani F. , Bondi M. , Junor W. , Salter C. J. , Saikia D. J.
Published: July 2016

A few compact steep-spectrum sources (CSSs) have been observed with the VLBA at 8.4 GHz and MERLIN at 5 GHz to study the possible dynamical interaction between jets and the ambient media suggested by previous measurements of the rotation measures.
VLBI Observations of the Gravitational Lens B1422+231
Authors: Patnaik A. R. , Porcas R. W.
Published: July 2016

We present results from VLBI observations of the gravitational lens B1422+231. All the four images of this system have been detected and appear unresolved at 18cm. Images A and B show elongated structures at 6cm with 1 mas resolution. With a resolution of 0.75 mas at 2cm, three images are elongated with axial ratios of about 10:1 for A and B, and 2:1 for C.
Astrometry of the Planetary-System Millisecond Pulsar B1257+12
Authors: Nunes N. V. , Bartel N.
Published: July 2016

We present the VLBI determined position of the millisecond pulsar PSR B1257+12 which has been shown to have a planetary system (Wolszczan & Frail 1992). The position determination for epoch June 20, 1992 is: αJ2000=13h0m3s.05005(5),δJ2000=+12°40’56″.7043(34). We indicate how the combination of such pulsar observations along with timing observations can be used to directly tie the solar-system dynamic reference frames and the extragalactic reference frame.