bluuebunny/crossref_metadata_2025_split huggingface dataset:
Localization of flow structures using -norm optimization
Authors: Foures D. P. G. , Caulfield C. P. , Schmid P. J. 
Published: June 2019

AbstractStability theory based on a variational principle and finite-time direct-adjoint optimization commonly relies on the kinetic perturbation energy density ${E}_{1} (t)= (1/ {V}_{\Omega } )\int \nolimits _{\Omega } e(\boldsymbol{x}, t)\hspace{0.167em} \mathrm{d} \Omega $ (where $e(\boldsymbol{x}, t)= \vert \boldsymbol{u}{\vert }^{2} / 2$) as a measure of disturbance size. This type of optimization typically yields optimal perturbations that are global in the fluid domain $\Omega $ of volume ${V}_{\Omega } $. This paper explores the use of $p$-norms in determining optimal perturbations for ‘energy’ growth over prescribed time intervals of length $T$. For $p= 1$ the traditional energy-based stability analysis is recovered, while for large $p\gg 1$, localization of the optimal perturbations is observed which identifies confined regions, or ‘hotspots’, in the domain where significant energy growth can be expected. In addition, the $p$-norm optimization yields insight into the role and significance of various regions of the flow regarding the overall energy dynamics. As a canonical example, we choose to solve the $\infty $-norm optimal perturbation problem for the simple case of two-dimensional channel flow. For such a configuration, several solutions branches emerge, each of them identifying a different energy production zone in the flow: either the centre or the walls of the domain. We study several scenarios (involving centre or wall perturbations) leading to localized energy production for different optimization time intervals. Our investigation reveals that even for this simple two-dimensional channel flow, the mechanism for the production of a highly energetic and localized perturbation is not unique in time. We show that wall perturbations are optimal (with respect to the $\infty $-norm) for relatively short and long times, while the centre perturbations are preferred for very short and intermediate times. The developed $p$-norm framework is intended to facilitate worst-case analysis of shear flows and to identify localized regions supporting dominant energy growth.
Meridional Compression of Radio Pulsar Beams
Authors: Biggs James D. 
Published: June 2019

AbstractWe have studied the radio pulsar emission beam assuming a) a magnetic dipole field geometry and b) that the beam geometry is defined by the field lines that are not contained within the light cylinder. In general, the beam is compressed in the meridional direction. When the magnetic and rotation axes are aligned the beam is circular and as, the angle between these axes increases, the ratio of meridional to longitudinal dimension decreases monotonically to the minimum value 0.62 when the axes are orthogonal. This beam shape is thus consistent with that inferred from the study of circular polarization in average pulse profiles by Radhakrishnan and Rankin. Evidence for meridional compression is also found in the extensive observational study of Lyne and Manchester (1988).The beam evolution was determined using this data set, the beamwidth being found proportional to P–1/2, where P is the pulsar period. This relation implies that the more rapidly rotating pulsars should have larger beams, and this should aid in their detection. The more numerous, slower pulsars should have somewhat smaller beams than previously determined. This implies that the pulsar birthrate is probably close to the highest current estimates (1 in 25yr).
The Vela Pulsar Glitch of Christmas 1988
Authors: McCulloch P. M. , Hamilton P. A. 
Published: June 2019

AbstractWe report the results of detailed observations of the Vela pulsar glitch which occurred on 24 December 1988. The period decrease occurred without warning and took place in much less than 2 minutes. The recovery, which commenced immediately, requires a complex function to model it, including three exponential terms and a damped sinusoid. An offset between data at two observing frequencies commenced at about the glitch epoch and continued for about 40 days; this is consistent with a small increase in dispersion measure or a change in the pulsar magnetic field configuration.
Results of Pulsar Timing Observations at 102.5 MHz
Authors: Shabanova T. V. 
Published: June 2019

AbstractA program of pulse arrival time observations made at Pushchino during 1978-1986 is described. The period, period derivative and position derived from the full timing solutions were obtained for 16 pulsars. Comparison of our results with others obtained during 1970-1980 by different groups has shown that the parameters for a number of pulsars have suffered a significant change.
Pulsars, their Evolution, Winds and Radiation
Authors: Kundt W. 
Published: June 2019

AbstractControversial interpretations of the functioning and evolution of (radio) pulsars are discussed and confronted with the constraints. It is argued that (i) polarized e± bunches are ejected from near the polar caps at high Lorentz factors γ ≳ 102, that (ii) coherent radio pulses are emitted by them during transit to the outer magnetosphere via curvature radiation, (iii) in the shape of (transverse) fan beams, that (iv) there is no indication of magnetic field decay (or buildup), and that (v) pulsars die either by alignment or by interstellar quenching.The so-called “millisecond” pulsars (outside globular clusters) are likely to be an abundant, young population of fast-born pulsars.
On the Shape of the Pulsar Emission Diagram
Authors: Yusifov I. M. 
Published: June 2019

AbstractOn the basis of pulse profiles in a limited interval of wavelength for ~260 pulsars, the dependence of the shape and width of the pulsar radiation pattern on pulsar period is investigated. The relative pulse duration is compared with the ratio of numbers of single and multicomponent pulsars. It is shown that the simple circular hollow cone model does not agree with observational data. Preliminary estimates show that the young pulsars (with short periods) have an elliptical pattern which at long periods becomes circular.
Pulsar Observations at Decameter Wavelengths using the Gauribidanur Telescope
Authors: Deshpande A. A. , Radhakrishnan V. 
Published: June 2019

AbstractWe report measurement of pulse profiles at 34.5 MHz of 8 pulsars using the Decameter-wave Radio Telescope at Gauribidanur, India. The dispersion measures of these pulsars range from 3 to 35 pc cm–3. None of the pulsars show any significant interpulse emission at this frequency which conflicts with earlier claims from 25-MHz observations (Bruck and Ustimenko 1976, Bruck and Ustimenko 1977a, Bruck and Ustimenko 1979). The intrinsic pulse width of PSR 1919+21 at 34.5 MHz appears to be smaller than at higher frequencies. These observations were made during the period 1984-86 and were reported by Deshpande (1987).
On the Primary Importance of Pulsars with Five-Component Profiles
Authors: Rankin Joanna M. 
Published: June 2019

AbstractSome 15 members of the class of profiles with five components have been identified. These profiles have a central core component and two pairs of conal outriders, thus exhibiting the highest degree of formal complexity observed. No good examples of pulsars with six or more profile components are known.Members of the five-component class have very similar physical parameters and emission-region geometries. An analysis of their emission geometry indicates both that the core and conal beams scale as P–1/2 and that the 1-GHz conal emission comes from a height of some 100-200 km.
Observations of the Millisecond Pulsar PSR 1855+09 At 102 MHz
Authors: Kuz'min A. D. , Alekseev Yu. I. , Lapaev K. A. , Losovsky B. Ya. , Salnikov A. A. 
Published: June 2019

AbstractThe study of millisecond pulsars is of great astrophysical interest. One may expect that the rotation effect on the structure of the magnetosphere should be very significant. In view of the short duration of the pulses they are very suitable for investigations of the interstellar medium; at least they hold the promise for the pulsar time scale.Millisecond pulsars were discovered and have been studied on the basis of their radio-emission at decimeter wavelengths. At longer wavelengths scattering of the radio emission in the interstellar medium is the principal limitation of millisecond pulsar observations.
Profile Changes in the Binary Pulsar PSR 1913+16
Authors: Weisberg J. M. , Taylor J. H. 
Published: June 2019

AbstractAccording to general relativity, the spin axis of binary pulsar PSR 1913+16 should precess at a rate of 1.21 degrees per year. This precession will cause the pulse profile to change as our line of sight samples different pulsar latitudes. In order to search for this phenomenon, we have carefully monitored the pulse profile at 1408 MHz for 8.5 years. The ratio of flux density of the first to second pulse component has declined at a rate of approximately 1.65% per year, with some evidence of a steeper decrease over the past three years. We have detected no evidence for a change in the separation of the two components. We discuss the nature of the pulsar emission region in light of these results.
The Maximum Moment of Inertia of Neutron Stars and its Implications for Pulsar Observations
Authors: Haensel Pawel 
Published: June 2019

AbstractA simple approximate formula, expressing the maximum moment of inertia of a neutron star as a function of the mass and radius of the configuration with a maximum allowable mass, is shown to be a quite precise representation of the results obtained for a broad set of equations of state of dense matter. The resulting possible observational constraints in the mass-radius plane for neutron star models are discussed.
The Effect of Light Bending and Redshift on Pulsar Beaming: The Case of Shorter Rotation Periods
Authors: Kapoor R. C. 
Published: June 2019

AbstractAn estimate of the effect of light bending and redshift on pulsar beam characteristics has been made using a weak Kerr metric for the case of a 1.4 M⊙ neutron star with a radius in the range 6-10 km and rotation periods of 1.56 ms and 33 ms, respectively. Assuming that the pulsar emission has the form of a narrow conical beam directed away from the surface and is located within two stellar radii, the beam is found to be widened by a factor of ≤ 2 and to suffer a reduction in the intensity (flattening of the profile) by an order of magnitude or less. The effect is largest for the most rapidly rotating the neutron stars. For an emission region located beyond 20 km, the flattening is generally insignificant. The pulse profile is slightly asymmetrical due to dragging of the inertial frames. For millisecond periods, aberration tends to reverse the flattening effect of space-time curvature by narrowing the pulse and can completely overcome it for emission from a location beyond ≃30km. Although the pulse must slightly brighten up, a large redshift factor overcomes this effect to keep the pulse flattened for all neutron star radii considered here.
Probing Magnetospheres of Rotation-Driven Neutron Stars
Authors: Cordes James M. 
Published: June 2019

AbstractMagnetospheric issues are discussed as they relate to radio intensity variations, spindown, rotational noise, and emission altitudes. Radio fluctuations trace the overall state(s) of the magnetosphere in spite of the fact that Lradio ≪ IΩ. Five methods for estimating emission altitudes re are discussed. Four measure either the transverse width or radial depth of the emission region; assumption of a dipolar field and, in some cases, a radius to frequency mapping, yields estimates for the absolute radius of the emission region. The fifth method (v/c effects in Stokes parameter waveforms) provides direct altitude estimates. Most results suggest that re ≲ 0.02 RLC at v = 0.4 GHz. A radius to frequency mapping appears viable for some but not necessarily all objects. The mapping need not be one-to-one. Prospects are discussed for using simultaneous radio and high energy observations to make further progress in our understanding of magnetospheres.
On the Subpulse Drift in PSR 2303+30
Authors: Gil J. A. , Hankins T. H. , Nowakowski L. 
Published: June 2019

AbstractWe present an analysis of high signal-to-noise ratio, 430-MHz total intensity single pulse observations from PSR 2303+30 recorded at the Arecibo Observatory in August 1986. We believe that the regular subpulse modulation observed in this pulsar can be explained only in terras of subpulse-associated emission beams rotating around the axis of the pulsar beam. This idea was first introduced by Ruderman and Sutherland (1975) in their spark model. We use the quantitative prediction of this model to calculate the pulsar magnetic field. For this reason we need to estimate the rotation period of a spark around the magnetic pole. We estimate this period by reproducing the observed pulse sequences within an angular beaming model of subpulse emission and fitting this model to the data.
Causal Relationship between Drifting Subpulse Parameters and Some Pulsar Quantities
Authors: Shapirovskaya Natasha 
Published: June 2019

AbstractCorrelation analysis of subpulse spacing P2, subpulse bands P3, subpulse drift rate with some quantities such as pulsar period P, profile width We, characteristic age r and others indicates some new casual dependencies not previously confirmed. Relations obtained in this paper possibly point out that interaction between pulsar radio emission and the interstellar environment may exert some influence on the drift appearance.
Oscillations of Rotating Neutron Stars
Authors: Strohmayer T. E. 
Published: June 2019

AbstractWe use a perturbation technique to compute the rotational corrections to the non-radial oscillation spectrum of a realistic neutron-star model. We compute, to first order in the rotation rate, the corrections to the normal mode eigenfrequencies and eigenfunctions. We find that l = l0 oscillations are coupled to l = l0 ± 1 oscillations by the Coriolis force. For the toroidal modes, this coupling introduces a non-zero radial component to the velocity field. We have used this result to compute the neutrino damping rates for several corrected toroidal modes. This damping mechanism is inoperative for toroidal modes in a non-rotating star because these modes produce no density nor temperature perturbations. The neutrino damping time can approach the gravitational radiation damping time in rotating neutron stars if the central temperature is high enough, (Tc ≥ 108 K). The rotationally induced coupling of spheroidal oscillations to toroidal modes can also produce significant displacements at the stellar surface. This may have interesting implications for channeling energy, e.g., that associated with a glitch, to the surface of the star. Perhaps this might produce observable effects in the pulsar emission process or a γ-ray burst event.
Observation of the Linear Polarization of Pulsar Integrated Pulses and Sub-Pulses at Meter Wavelengths
Authors: Suleymanova S. A. , Pugachev V. D. 
Published: June 2019

AbstractA brief review of polarization observations at 103, 60, and 40 MHz is given. Our peculiar measurement technique allows us to obtain average polarization profiles as well as statistical distributions of the polarization parameters of individual pulses and subpulses. Some examples are given in this paper.
Radiation Damping and the Two Mode Behavior in Pulsars
Authors: Björnsson C. -I. 
Published: June 2019

AbstractIt is argued that both observations and theory indicate that radiation damping plays an important rôle in pulsar emission. The two-mode behavior as well as the observed value of the brightness temperature can both be understood as a result of radiation damping. In this context, a possible cause for the enhanced depolarization in the wings of pulsar profiles is discussed.
The Vibrations of a Particle about a Position of Equilibrium—Part III
Authors: Baker Bevan B. 
Published: June 2019

In the two parts of this investigation previously published it has been shown that the solution in terms of elliptic functions represents the motion of the particular dynamical system under consideration throughout the whole range of values of s and g for which a real solution exists, except for those values for which s = 2g and k = 1, but that, on the other hand, the series solution is convergent and represents the motion only so long asfor values of s and g for which the sign of this inequality is reversed the trigonometric series representing the solution are divergent. It is of importance to investigate what discontinuities, if any, of the system correspond to values of s and g which lie on the boundary of the region of convergence; the present part is concerned primarily with showing that under such circumstances no discontinuity of the system exists, thus confirming the suggestions made in Part I., § 12.
The Vibration of a Particle about a Position of Equilibrium—Part IV
Authors: Baker Bevan B. 
Published: June 2019

It has been shown in the previous three parts of this work, that the whole question of the convergence of the series solution, for the particular dynamical system under consideration, has turned upon the cubic equationBefore proceeding to generalize the results it will be shown how this cubic equation may be derived in a slightly different fashion.
Palaeomagnetic dating of hydrothermal alteration in the Woodford Shale, Oklahoma, USA
Authors: Roberts Jennifer , Heij Gerhard , Elmore R. Douglas 
Published: June 2019

AbstractAn oriented Woodford Shale core from the eastern Ardmore Basin was sampled to test if the shale was an open or closed system to hydrothermal fluids, and to determine the timing of alteration. Mineralized fractures are ubiquitous in the core, and the shale exhibits a complex paragenesis with multiple hydrothermal minerals, including biotite, magnesite, norsethite, gorceixite and potassium feldspar present in and around the fractures. These minerals suggest that the Woodford Shale was an open system during part of its diagenetic history. Vitrinite reflectance (Ro) measurements indicate values of ∼1.81 % (∼230 °C). Palaeomagnetic analysis reveals a characteristic remanent magnetization (ChRM) with south-southeasterly declinations and shallow inclinations that resides in magnetite. This ChRM is interpreted to be either a chemical remanent magnetization (CRM) or a thermochemical remanent magnetization (TCRM) that was acquired at 245 ± 10 Ma during Late Permian time based on the pole position (51.0° N, 115.6° E). Because the palaeomagnetic specimens show evidence of extensive hydrothermal alteration, the CRM/TCRM is interpreted to date the migration of hydrothermal fluids through the shale. The agreement in timing with other studies that report hydrothermal alteration in southern Oklahoma and the Ouachita Mountains in Late Permian time, suggest that there were post-collisional fluid-flow events which accessed reservoirs of warm fluids.
Siliceous-sulphate rock coatings at Zhenzhu Spring, Tengchong, China: the integrated product of acid-fog deposition, spring water capillary action, and dissolution
Authors: Wen Huaguo , Xu Wenli , Li Yi , You Yaxian , Luo Xiaotong 
Published: June 2019

AbstractSiliceous-sulphate rock coatings were observed at Zhenzhu Spring, an acid sulphate hot spring in the Tengchong volcanic field, China. These rock coatings are mainly formed of gypsum and amorphous silica. Some alum-(K), voltaite, α-quartz and muscovite were also found. Four different laminae are developed in the rock coatings: gypsum layer, tight siliceous layer, tabular siliceous layer and siliceous debris layer. The gypsum layer is located at the top of the rock coatings, while other siliceous layers appear below the gypsum layer. Geochemical modelling of the fluids was performed to identify the mechanisms responsible for the formation of gypsum and amorphous silica. The results indicated that the occurrence of gypsum is related to the acid-fog deposition and amorphous silica mainly originates from spring water. Fog deposition provided the rock coatings with abundant SO42−and Ca, and the subsequent complete evaporation of the condensed fluids produced gypsum. Seasonal climate change (especially variation in rainfall) determines the fluctuations of capillary action and dissolution. Rainfall events in the wet season led to periods of non-precipitating gypsum and promoted the capillary rise of the spring water. Slightly diluted capillary water (a small amount of rainwater) covered the rock coatings, formed a tight siliceous layer on the rock-coating surface and/or filled the pores among the gypsum crystals forming many tabular siliceous aggregates. Heavy rainfall (high dilution), however, resulted in non-precipitating amorphous silica and accelerated the gypsum dissolution, leaving tabular pores around tabular siliceous aggregates and forming a tabular siliceous layer.
A Possible Egyptian Dating for the End of the Third Late Minoan Period
Authors: Peet T. E. 
Published: June 2019

In the last number of the Zeitschrift für Aegyptische Sprache Max Burchardt publishes two swords lately acquired from Egypt by the Berlin Museum. The first (Inventory Number 20447), (Fig. 1), almost perfectly preserved, is a fine example of Naue's Type II. It is a cutting sword with edges almost parallel and the end bluntly pointed. The handle is made in one piece with the blade. Its edges are raised on either side and it is fitted with eight rivet holes. Down the blade, whose faces are slightly convex, run on either face four fine furrows parallel to the edges. The total length is 715 mm. The sword was found at Zagazig (Bubastis) in the Delta.The second sword (Inventory Number 20305) is unfortunately badly damaged but probably belongs to the same type. It certainly has the same section and the same almost parallel edges. There is, too, the same thickening of the handle at the point where it joins the blade and the disposition of the rivets is similar in the case of such as remain. I have not yet had the opportunity of examining the original and so must be content with quoting Burchardt's opinion on this point.
The bronzes
Authors: Sandars Nancy 
Published: June 2019

The Knives. Three complete one-edged knives, and one fragmentary, were found in the tombs. The bronze knive (I. 5) belongs to a small group in which the whole of the handle including a T-shaped pommel extension was cast in one piece with the blade, while inset panels of perishable material were secured by rivets and deep flanges in the metal. Apart from this pommel-flange it is similar to a large contemporary group of bronze knives to be found throughout the Aegean.One-edged knives with flanged handles are at least as old as the Shaft-grave period at Mycenae. In Crete a riveted handle without flanges was preferred from Middle Minoan times, but the flanged handle appears at the end of Late Minoan II or the beginning of IIIA. The pommel-flange is rare, though found occasionally on both riveted and unriveted knives. The earliest dated example is probably that from the Palace of Mycenae found with other objects of Late Helladic II or early III date; one comes from Dendra, Chamber Tomb 7, and one unriveted from Chamber Tomb 2, dated L.H. IIIB. The Mycenae knife has not the true T-projection but only a broadening at the end of the handle which is in shape very like handles of Levantine daggers and two-edged knives found in the so-called ‘Hyksos tombs’ of Ras Shamra and at Tell el Ajjul, Tell Fara, and elsewhere in Palestine.