ShantanuT01/DACTYL-Pretraining huggingface dataset:
The ordered magnetic field observed via polarized synchrotron emission in nearby disc galaxies can be explained by a mean-field dynamo operating in the diffuse interstellar medium (ISM). Additionally, vertical-flux initial conditions are potentially able to influence this dynamo via the occurrence of the magneto-rotational instability (MRI). We aim to study the influence of various initial field configurations on the saturated state of the mean-field dynamo. This is motivated by the observation that different saturation behavior was previously obtained for different supernova rates. We perform direct numerical simulations (DNS) of three-dimensional local boxes of the vertically stratified, turbulent interstellar medium, employing shearing-periodic boundary conditions horizontally. Unlike in our previous work, we also impose a vertical seed magnetic field. We run the simulations until the growth of the magnetic energy becomes negligible. We furthermore perform simulations of equivalent 1D dynamo models, with an algebraic quenching mechanism for the dynamo coefficients. We compare the saturation of the magnetic field in the DNS with the algebraic quenching of a mean-field dynamo. The final magnetic field strength found in the direct simulation is in excellent agreement with a quenched $\alpha\Omega$~dynamo. For supernova rates representative of the Milky Way, field losses via a Galactic wind are likely responsible for saturation. We conclude that the relative strength of the turbulent and regular magnetic fields in spiral galaxies may depend on the galaxy's star formation rate. We propose that a mean field approach with algebraic quenching may serve as a simple sub-grid scale model for galaxy evolution simulations including a prescribed feedback from magnetic fields.
Interstellar scintillation can be used to probe transverse sizes of radio sources on scales inaccessible to the nominal resolution of any terrestrial telescope, e.g. $\lesssim 10^{-6}$ arc sec. Methodology is presented that exploits this superresolution phenomenon for both single aperture and interferometer observations. The treatment applies to the saturated (strong-scattering) regime and holds for both thin screens and extended media. A general signal model for radio sources is presented, {\it scintillated amplitude modulated noise}, which applies to compact, incoherent synchrotron sources such as AGNs and gamma-ray burst sources and also to known, coherent sources such as masers and pulsars. The exact probability density function for measured intensities and interferometric visibilities is obtained by solving a general Fredholm problem. An approximate density function is also obtained by using the equivalent number of degrees of freedom in scintillation modulations. The scintillation modulation variance is presented, which includes the effects of source structure and time-bandwidth averaging in the signal processing. Two Bayesian methods are outlined for inferring the sizes of emission regions that use first order statistics of the intensity and visibility. Intensity cross-correlation methods for inferring source sizes are also given. Intensity interferometry in the the radio context is compared to the optical intensity interferometry of Hanbury-Brown and Twiss.
The higher Nash blowup of an algebraic variety replaces singular points with limits of certain spaces carrying higher order data associated to the variety at non-singular points. In the case of normal toric varieties we give a combinatorial description of the higher Nash blowup in terms of a Gr\"obner fan. This description will allow us to prove the analogue of Nobile's theorem on the usual Nash blowup in this context. More precisely, we prove that for a normal toric variety, the higher Nash blowup is an isomorphism if and only if the variety is non-singular.
We present the derivation and the solutions to the coupled electromagnetic and gravitational perturbations with sources in a charged black hole background. We work in the so called ghost gauge and consider as source of the perturbations the infall of radial currents. In this way, we study a system in which it is provoked a response involving both, gravitational and electromagnetic waves, which allows us to analyze the dependence between them. We solve numerically the wave equations that describe both signals, characterize the waveforms and study the relation between the input parameters of the infalling matter with those of the gravitational and electromagnetic responses.
The basic features of the complex canonical formulation of general relativity and the recent developments in the quantum gravity program based on it are reviewed. The exposition is intended to be complementary to the review articles available already and some original arguments are included. In particular the conventional treatment of the Hamiltonian constraint and quantum states in the canonical approach to quantum gravity is criticized and a new formulation is proposed.
We discuss two dimensional Yang -- Mills theories with massless fermions in arbitrary representations of a gauge group $G$. It is shown that the physics (spectrum and interactions) of the massive states in such models is independent of the detailed structure of the model, and only depends on the gauge group $G$ and an integer $k$ measuring the total anomaly. The massless physics, which does depend on the details of the model, decouples (almost) completely from that of the massive one. As an example, we discuss the equivalence of QCD$_2$ coupled to fermions in the adjoint, and fundamental representations.
An explanation for the glass-like anomaly observed in the low-temperature specific heat of incommensurate phases is proposed. The key point of this explanation is the proper account for the phason damping when computing the thermodynamic magnitudes. The low-temperature specific heat of the incommensurate phases is discussed within three possible scenarios for the phason dynamics: no phason gap, static phason gap and a phason gap of dynamical origin. Existing NMR and inelastic scattering data indicate that these scenarios are possible in biphenyl, blue bronze $\rm K_{0.30}MoO_3$ and BCPS respectively. Estimates of the corresponding low-temperature specific heat are in reasonable agreement with the experiments.
We investigate the 1/8 BPS geometries with SU(2) x U(1) x SO(4) x R symmetry in IIB supergravity which were classified by Gava et al, (hep-th/0611065). It is desirable to have a complete set of differential equations imposed on the controlling functions such that they are not only necessary but also sufficient to produce supergravity solutions with those symmetries. We work on this issue and find a new differential equation for the controlling functions. For a special case, we exhaust all the remaining constraints and show that they reduce to one Liouville equation. The solutions of this equation produce geometries which are locally equivalent to the near horizon geometries of intersecting D3-branes.
Despite its potential for device application, the non-magnetic Zeeman effect has only been predicted and observed in two-dimensional compounds. We demonstrate that non-centrosymmetric three-dimensional compounds can also exhibit a Zeeman-type spin splitting, allowing the splitting control by changing the growth direction of slabs formed by these compounds. We determine the required conditions for this effect: $i$) non-centrosymmetric including polar and non-polar point groups, $ii$) valence band maximum or conduction band minimum in a generic $k$-point, i.e., non-time-reversal-invariant momentum, and $iii$) zero magnetic moment. Using these conditions as filters, we perform a material screening to systematically search for these systems in the aflow-ICSD database. We find 20 material candidates featuring the Zeeman-type effect. We also found that the spin-splitting in confined systems can be controlled by an external electric field, which in turns can induce a metal-insulator transition. We believe that the Zeeman-type effect in three-dimensional compounds can potentially be used for spin-filtering devices.
A concise discussion of a 3+1-dimensional derivative coupling model, in which a massive Dirac field couples to the four-gradient of a massless scalar field, is given in order to elucidate the role of different concepts in quantum field theory like the regularization of quantum fields as operator valued distributions, correlation distributions, locality, causality, and field operator gauge transformations.
We show that Ramsey theory, a domain presently conceived to guarantee the existence of large homogeneous sets for partitions on k-tuples of words (for every natural number k) over a finite alphabet, can be extended to one for partitions on Schreier-type sets of words (of every countable ordinal). Indeed, we establish an extension of the partition theorem of Carlson about words and of the (more general) partition theorem of Furstenberg-Katznelson about combinatorial subspaces of the set of words (generating from k-tuples of words for any fixed natural number k) into a partition theorem about combinatorial subspaces (generating from Schreier-type sets of words of order any fixed countable ordinal). Furthermore, as a result we obtain a strengthening of Carlson's infinitary Nash-Williams type (and Ellentuck type) partition theorem about infinite sequences of variable words into a theorem, in which an infinite sequence of variable words and a binary partition of all the finite sequences of words, one of whose components is, in addition, a tree, are assumed, concluding that all the Schreier-type finite reductions of an infinite reduction of the given sequence have a behavior determined by the Cantor-Bendixson ordinal index of the tree-component of the partition, falling in the tree-component above that index and in its complement below it.
The hadronic quark structure is investigated in the frame of high energy electron proton scattering. A phenomenological model based on the Born approximation is used to calculate the transition matrix element for the quark system forming the proton target. A potential of electromagnetic nature is assumed for the calculation of the multiple scattering of electron with the constituent valance quarks of the proton. It is found that the first two terms of the Born approximation are sufficient to describe the experimental data of differential cross section for the electron proton system only at low momentum transfer square. On the other hand, a two body scattering amplitude with a relevant form factor may give proper agreement extended to high momentum transfer square region. A harmonic oscillator wave function is used to reproduce the data of the experiment at low momentum transfer. This satisfies quietly the confinement condition of the quarks in the hadron system. However the data of the experiment at relatively high momentum transfer, show that hadronic quarks behave more freely and may be described by just a Coulomb wave function.   PACS: 13.40.Fn , 25.40.Cm
We present the current results of a survey for OH megamasers (OHMs) underway at the Arecibo Observatory. The survey is 2/3 complete and has produced a high OHM detection rate (1 in 6) from a redshift-selected sample of IRAS galaxies. The survey will relate the OHM luminosity function to the galaxy merger rate, allowing subsequent blind OHM surveys to measure the galaxy merger rate as a function of cosmic time. The survey has also made the first detection of strong variability in OHMs. Variability will provide a powerful tool for understanding the small-scale physical settings and mechanisms of OHMs.
We report new polarimetric and photometric maps of the massive star-forming region OMC-1 using the HAWC+ instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA). We present continuum polarimetric and photometric measurements of this region at 53, 89, 154, and 214 microns at angular resolutions of 5.1, 7.9, 14.0, and 18.7 arcseconds for the four bands, respectively. The photometric maps enable the computation of improved SEDs for the region. We find that at the longer wavelengths, the inferred magnetic field configuration matches the `hourglass' configuration seen in previous studies, indicating magnetically-regulated star formation. The field morphology differs at the shorter wavelengths. The magnetic field inferred at these wavelengths traces the bipolar structure of the explosive Becklin-Neugebauer (BN)/Kleinman-Low (KL) outflow emerging from OMC-1 behind the Orion Nebula. Using statistical methods to estimate the field strength in the region, we find that the explosion dominates the magnetic field near the center of the feature. Farther out, the magnetic field is close to energetic equilibrium with the ejecta and may be providing confinement to the explosion. The correlation between polarization fraction and the local polarization angle dispersion indicates that the depolarization as a function of unpolarized intensity is a result of intrinsic field geometry as opposed to decreases in grain alignment efficiency in denser regions.
We show that if a homogeneous polynomial $f$ in $n$ variables has Waring rank $n+1$, then the corresponding projective hypersurface $f=0$ has at most isolated singularities, and the type of these singularities is completely determined by the combinatorics of a hyperplane arrangement naturally associated with the Waring decomposition of $f$. We also discuss the relation between the Waring rank and the type of singularities on a plane curve, when this curve is defined by the suspension of a binary form, or when the Waring rank is 5.
We study certain kind of polynomials associated with Lissajous curves, called Chebyshev-Lissajous polynomials. We investigate their irreducibilities over the real numbers and complex numbers, thus comfirming two conjectures proposed by Merino.
Millisecond pulsars (MSPs) represent nearly half of the more than 160 currently known $\gamma$-ray pulsars detected by the Large Area Telescope on the \textit{Fermi} satellite, and a third of all known MSPs are seen in $\gamma$ rays. The least energetic $\gamma$-ray MSPs enable us to probe the so-called deathline for high-energy emission, i.e., the spin-down luminosity limit under which pulsars (PSRs) cease to produce detectable high-energy radiation. Characterizing the MSP luminosity distribution helps to determine their contribution to the Galactic diffuse $\gamma$-ray emission. We made use of the high-quality pulsar timing data recorded at the Nan\c{c}ay Radio Telescope over several years to characterize the properties of a selection of MSPs. For one of the pulsars, the dataset was complemented with Westerbork Synthesis Radio Telescope observations. The rotation ephemerides derived from this analysis were also used to search the LAT data for new $\gamma$-ray MSPs. For the MSPs considered in this study, we obtained new transverse proper motion measurements or updated the existing ones, and placed new distance constraints for some of them, with four new timing parallax measurements. We discovered significant GeV $\gamma$-ray signals from four MSPs, i.e., PSRs J0740+6620, J0931$-$1902, J1455$-$3330, and J1730$-$2304. The latter is now the least energetic $\gamma$-ray pulsar found to date. Despite the improved $\dot E$ and $L_\gamma$ estimates, the relationship between these two quantities remains unclear, especially at low $\dot E$ values.
Under Newtonian gravity total masses for dSph galaxies will scale as $M_{T} \propto R_{e} \sigma^{2}$, with $R_{e}$ the effective radius and $\sigma$ their velocity dispersion. When both of the above quantities are available, the resulting masses are compared to observed stellar luminosities to derive Newtonian mass to light ratios, given a physically motivated proportionality constant in the above expression. For local dSphs and the growing sample of ultrafaint such systems, the above results in the largest mass to light ratios of any galactic systems known, with values in the hundreds and even thousands being common. The standard interpretation is for a dominant presence of an as yet undetected dark matter component. If however, reality is closer to a MONDian theory at the extremely low accelerations relevant to such systems, $\sigma$ will scale with { stellar mass} $M_{*}^{1/4}$. This yields an expression for the mass to light ratio which will be obtained under Newtonian assumptions of $(M/L)_{N}=120 R_{e}(\Upsilon_{*}/L)^{1/2}$. Here we compare $(M/L)_{N}$ values from this expression to Newtonian inferences for this ratios for the actual $(R_{e}, \sigma, L)$ observed values for a sample of recently observed ultrafaint dSphs, obtaining good agreement. Then, for systems where no $\sigma$ values have been reported, we give predictions for the $(M/L)_{N}$ values which under a MONDian scheme are expected once kinematical observations become available. For the recently studied Dragonfly 44 { and Crater II systems}, reported $(M/L)_{N}$ values are also in good agreement with MONDian expectations.
The method of correlation mapping on the full sphere is used to study the properties of the ILC map, as well as the dust and synchrotron background components. An anomalous correlation of the components with the ILC map in the main plane and in the poles of the ecliptic and equatorial coordinate systems was discovered. Apart from the bias, a dominant quadrupole contribution in the power spectrum of the mosaic correlation maps was found in the pixel correlation histogram. Various causes of the anomalous signal are discussed.
Using the plane wave pseudopotential method we performed density functional theory calculations on the stability of steps and self-diffusion processes on Ag(100). Our calculated step formation energies show that the {111}-faceted step is more stable than the {110}-faceted step. In accordance with experimental observations we find that the equilibrium island shape should be octagonal very close to a square with predominately {111}-faceted steps. For the (100) surface of fcc metals atomic migration proceeds by a hopping or an exchange process. For Ag(100) we find that adatoms diffuse across flat surfaces preferentially by hopping. Adatoms approaching the close-packed {111}-faceted step edges descend from the upper terrace to the lower level by an atomic exchange with an energy barrier almost identical to the diffusion barrier on flat surface regions. Thus, within our numerical accuracy (approx +- 0.05 eV) there is no additional step-edge barrier to descent. This provides a natural explanation for the experimental observations of the smooth two-dimensional growth in homoepitaxy of Ag(100). Inspection of experimental results of other fcc crystal surfaces indicates that our result holds quite generally.
From targeted observations of ground-state OH masers towards 702 Multibeam (MMB) survey 6.7-GHz methanol masers, between Galactic longitudes 186$^{\circ}$ through the Galactic centre to 20$^{\circ}$, made as part of the `MAGMO' project, we present the physical and polarisation properties of the 1720-MHz OH maser transition, including the identification of Zeeman pairs. We present 10 new and 23 previously catalogued 1720-MHz OH maser sources detected towards star formation regions. In addition, we also detected 16 1720-MHz OH masers associated with supernova remnants and two sites of diffuse OH emission. Towards the 33 star formation masers, we identify 44 Zeeman pairs, implying magnetic field strengths ranging from $-$11.4 to $+$13.2 mG, and a median magnetic field strength of $|B_{LOS}|$ $\sim$ 6 mG. With limited statistics, we present the in-situ magnetic field orientation of the masers and the Galactic magnetic field distribution revealed by the 1720-MHz transition. We also examine the association statistics of 1720-MHz OH SFR masers with other ground-state OH masers, excited-state OH masers, class I and class II methanol masers and water masers, and compare maser positions with mid-infrared images of the parent star forming regions. Of the 33 1720-MHz star formation masers, ten are offset from their central exciting sources, and appear to be associated with outflow activity.
The multiple Doppler readouts available on the Laser Interferometer Space Antenna (LISA) permit simultaneous formation of several interferometric observables. All these observables are independent of laser frequency fluctuations and have different couplings to gravitational waves and to the various LISA instrumental noises. Within the functional space of interferometric combinations LISA will be able to synthesize, we have identified a triplet of interferometric combinations that show optimally combined sensitivity. As an application of the method, we computed the sensitivity improvement for sinusoidal sources in the nominal, equal-arm LISA configuration. In the part of the Fourier band where the period of the wave is longer than the typical light travel-time across LISA, the sensitivity gain over a single Michelson interferometer is equal to $\sqrt{2}$. In the mid-band region, where the LISA Michelson combination has its best sensitivity, the improvement over the Michelson sensitivity is slightly better than $\sqrt{2}$, and the frequency band of best sensitivity is broadened. For frequencies greater than the reciprocal of the light travel-time, the sensitivity improvement is oscillatory and $\sim \sqrt{3}$, but can be greater than $\sqrt{3}$ near frequencies that are integer multiples of the inverse of the one-way light travel-time in the LISA arm.
This manuscript reviews recent results of large scale shell model calculations on N=Z nuclei. The topics discussed include the role proton-neutron pairing on the binding energies of pf-shell nuclei and his influence on backbending rotors; coulomb energy differences and their relationship with alignment. Proton-neutron correlations are found responsible for the characteristic features of the isotopic shifts in calcium, the parabolic dependence on A and the prominent odd-even staggering.
Using first-principles calculations based on density-functional theory (DFT) we investigated the effects of the van der Waals (vdW) interactions on the structural and electronic properties of anthracene and pentacene adsorbed on the Ag(111) surface. We found that the inclusion of vdW corrections strongly affects the binding of both anthracene/Ag(111) and pentacene/Ag(111), yielding adsorption heights and energies more consistent with the experimental results than standard DFT calculations with generalized gradient approximation (GGA). For anthracene/Ag(111) the effect of the vdW interactions is even more dramatic: we found that pure DFT-GGA calculations (without including vdW corrections) result in preference for a tilted configuration, in contrast to experimental observations of flat-lying adsorption; including vdW corrections, on the other hand, alters the binding geometry of anthracene/Ag(111), favoring the flat configuration. The electronic structure obtained using a self-consistent vdW scheme was found to be nearly indistinguishable from the conventional DFT electronic structure once the correct vdW geometry is employed for these physisorbed systems. Moreover, we show that a vdW correction scheme based on a hybrid functional DFT calculation (HSE) results in an improved description of the highest occupied molecular level of the adsorbed molecules.