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Distances and moments of inertia of Fermi Pulsars  [PDF]
Andrei Gruzinov
Physics , 2013,
Abstract: Measurement of distances and moments of inertia of pulsars must be useful, for instance, for three-dimensional mapping of the dispersion and rotation measures, constraining the nuclear equation of state, etc. The distances and moments of inertia can be measured by fitting the gamma-ray lightcurves of pulsars, because the gamma-ray emission seems to be governed by easily calculable physics. The (first-principle) theoretical lightcurves have been computed only for weak pulsars (pair production near the light cylinder much smaller than Goldreich-Julian [1] per rotation), and at insufficient accuracy; but, since this computation has been done by a self-taught numericist, it must be possible to improve the theoretical accuracy for weak pulsars, and also to extend the computation to non-weak pulsars. To invite the computational effort of better-equipped researchers, we describe an (entirely obvious) procedure for measuring the distances and moments of inertia for weak pulsars.
Method of estimating distances to X-ray pulsars and their magnetic fields  [PDF]
S. B. Popov
Physics , 1999, DOI: 10.1080/10556790008241359
Abstract: We suggest a new method of estimating distances to X-ray pulsars and their magnetic fields. Using observations of fluxes and period variations in the model of disk accretion one can estimate the magnetic momentum of a neutron star and the distance to X-ray pulsar. As an illustration the method is applied to the system GROJ1008-57. Estimates of the distance: approximately 6 kpc, and the magnetic momentum: approximately $4\cdot10^{31} E\cdot cm^3$, are obtained.
A search for radio pulsars and fast transients in M31 using the WSRT  [PDF]
E. Rubio-Herrera,B. W. Stappers,J. W. T. Hessels,R. Braun
Physics , 2012, DOI: 10.1093/mnras/sts205
Abstract: We present the results of the most sensitive and comprehensive survey yet undertaken for radio pulsars and fast transients in the Andromeda galaxy (M31) and its satellites, using the Westerbork Synthesis Radio Telescope (WSRT) at a central frequency of 328 MHz. We used the WSRT in a special configuration called 8gr8 (eight-grate) mode, which provides a large instantaneous field-of-view, about 5 square degrees per pointing, with good sensitivity, long dwell times (up to 8 hours per pointing), and good spatial resolution (a few arc minutes) for locating sources. We have searched for both periodicities and single pulses in our data, aiming to detect bright, persistent radio pulsars and rotating radio transients (RRATs) of either Galactic or extragalactic origin. Our searches did not reveal any confirmed periodic signals or bright single bursts from (potentially) cosmological distances. However, we do report the detection of several single pulse events, some repeating at the same dispersion measure, which could potentially originate from neutron stars in M31. One in particular was seen multiple times, including a burst of six pulses in 2000 seconds, at a dispersion measure of 54.7 pc cm^-3, which potentially places the origin of this source outside of our Galaxy. Our results are compared to a range of hypothetical populations of pulsars and RRATs in M31 and allow us to constrain the luminosity function of pulsars in M31. They also show that, unless the pulsar population in M31 is much dimmer than in our Galaxy, there is no need to invoke any violation of the inverse square law of the distance for pulsar fluxes.
Fast and accurate estimation of the covariance between pairwise maximum likelihood distances  [PDF]
Manuel Gil
PeerJ , 2015, DOI: 10.7287/peerj.preprints.387v2
Abstract: Pairwise evolutionary distances are a model-based summary statistic for a set of molecular sequences. They represent the leaf-to-leaf path lengths of the underlying phylogenetic tree. Estimates of pairwise distances with overlapping paths covary because of shared mutation events. It is desirable to take these covariance structure into account in any process that compares or combines distances to increase precision. In this paper, we present a fast estimator for the covariance of two pairwise maximum likelihood distances, estimated under general Markov models. The estimator is based on a conjecture (going back to Nei and Jin, 1989) which links the covariance to path lengths. We prove it here under a simple symmetric substitution model. In a simulation, we show that our estimator outperforms previously published ones in terms of the mean squared error.
Fast and accurate estimation of the covariance between pairwise maximum likelihood distances  [PDF]
Manuel Gil
PeerJ , 2015, DOI: 10.7717/peerj.583
Abstract: Pairwise evolutionary distances are a model-based summary statistic for a set of molecular sequences. They represent the leaf-to-leaf path lengths of the underlying phylogenetic tree. Estimates of pairwise distances with overlapping paths covary because of shared mutation events. It is desirable to take these covariance structure into account to increase precision in any process that compares or combines distances. This paper introduces a fast estimator for the covariance of two pairwise maximum likelihood distances, estimated under general Markov models. The estimator is based on a conjecture (going back to Nei & Jin, 1989) which links the covariance to path lengths. It is proven here under a simple symmetric substitution model. A simulation shows that the estimator outperforms previously published ones in terms of the mean squared error.
Covariance of maximum likelihood evolutionary distances between sequences aligned pairwise
Christophe Dessimoz, Manuel Gil
BMC Evolutionary Biology , 2008, DOI: 10.1186/1471-2148-8-179
Abstract: In this paper, we introduce an estimator for the covariance of distances from sequences aligned pairwise. Its performance is analyzed through extensive Monte Carlo simulations, and compared to the well-known variance estimator of ML distances. Our covariance estimator can be used together with the ML variance estimator to form covariance matrices.The estimator performs similarly to the ML variance estimator. In particular, it shows no sign of bias when sequence divergence is below 150 PAM units (i.e. above ~29% expected sequence identity). Above that distance, the covariances tend to be underestimated, but then ML variances are also underestimated.The estimation of evolutionary distances between gene/protein sequences is one of the most important problems in molecular evolution. In particular, it lies at the heart of most phylogenetic tree construction methods. The estimation of such distances is a two step process: first, homologous characters are identified, then the distances are estimated from the character substitution patterns. The most accurate matching of homologous characters is obtained by multiple sequence alignments (MSAs). Indeed, by considering all sequences simultaneously, MSAs yield a consistent and in principle optimal grouping of the homologous characters. Unfortunately, MSAs are hard to compute optimally (time complexity exponential in the number of sequences), and thus are in practice computed using heuristics. Alternatively, the sequences can be analyzed exclusively on the basis of pairs of sequences, using an algorithm such as Smith-Waterman [1] that yields optimal pairwise alignments (OPAs). This approach is often taken by large-scale comparative genomics analysis such as MIPS, OMA or RoundUp [2-4], which analyze the sequences pairwise due to computational constraints.Once the homologous characters are identified, the second step of distance estimation can proceed. The method of choice is a maximum likelihood (ML) estimation based on some mode
Possible Contribution to Electron and Positron Fluxes from Pulsars and their Nebulae  [PDF]
S Della Torre,M Gervasi,PG Rancoita,D Rozza,A Treves
Physics , 2013,
Abstract: The AMS-02 experiment confirms the excess of positrons in cosmic rays (CRs) for energy above 10 GeV with respect to the secondary production of positrons in the interstellar medium. This is interpreted as evidence of the existence of a primary source of these particles. Possible candidates are dark matter or astrophysical sources. In this work we discuss the possible contribution due to pulsars and their nebulae. Our key assumption is that the primary spectrum of electrons and positrons at the source is the same of the well known photon spectrum observed from gamma-rays telescopes. Using a diffusion model in the Galaxy we propagate the source spectra up to the Solar System. We compare our results with the recent experiments and with the LIS model
Maximum likelihood algorithm for approximation of local fluctuational fluxes at the plasma periphery by fractional stable distributions  [PDF]
Viacheslav Saenko
Physics , 2012,
Abstract: Statistical properties of a local fluctuational fluxes measured at the plasma edge are investigated in the work. It's shown that the amplitudes increments of the local fluctuational fluxes decrease by power law. For approximation of experimental PDFs the fractional stable distributions are used. The new algorithm of statistical estimation of the FSD parameters based on maximum likelihood method is described. By using of the algorithm the parameters of FSD are estimated by using experimental samples. It is shown good agreement between experimental and theoretical distributions. On the basis of this results the conclusion is made about applicability of the CTRW model for description of a processes underlying of the plasma turbulence.
A Fan Beam Model for Radio Pulsars. I. Observational Evidence  [PDF]
Hong Guang Wang,Fei Peng Pi,Xiao Ping Zheng,Chun Lan Deng,Sai Qin Wen,Feng Ye,Kai Ying Guan,Yi Liu,Li Qing Xu
Physics , 2014, DOI: 10.1088/0004-637X/789/1/73
Abstract: We propose a novel beam model for radio pulsars based on the scenario that the broadband and coherent emission from secondary relativistic particles, as they move along a flux tube in a dipolar magnetic field, forms a radially extended sub-beam with unique properties. The whole radio beam may consist of several sub-beams, forming a fan-shaped pattern. When only one or a few flux tubes are active, the fan beam becomes very patchy. This model differs essentially from the conal beam models in the respects of beam structure and predictions on the relationship between pulse width and impact angle $\beta$ (the angle between line of sight and magnetic pole) and the relationship between emission intensity and beam angular radius. The evidence for this model comes from the observed patchy beams of precessional binary pulsars and three statistical relationships found for a sample of 64 pulsars, of which $\beta$ were mostly constrained by fitting polarization position angle data with the Rotation Vector Model. With appropriate assumptions, the fan beam model can reproduce the relationship between 10\% peak pulse width and $|\beta|$, the anticorrelation between the emission intensity and $|\beta|$, and the upper boundary line in the scatter plot of $|\beta|$ versus pulsar distance. An extremely patchy beam model with the assumption of narrowband emission from one or a few flux tubes is studied and found unlikely to be a general model. The implications of the fan beam model to the studies on radio and gamma-ray pulsar populations and radio polarization are discussed.
Neutron stars in X-ray pulsars and their magnetic fields  [PDF]
S. B. Popov,V. M. Lipunov
Physics , 1999,
Abstract: Estimates of the magnetic field of neutron stars in X-ray pulsars are obtained using the hypothesis of the equilibrium period for disk and wind accretion and also from the BATSE data on timing of X-ray pulsars using the observed maximum spin-down rate. Cyclotron lines at energies $\ge 100$ keV in several Be-transient are predicted for future observations. We suggest a new method of estimating distances to X-ray pulsars and their magnetic fields. Using observations of fluxes and period variations in the model of disk accretion one can estimate the magnetic momentum of a neutron star and the distance to X-ray pulsar. As an illustration the method is applied to the system GROJ1008-57.
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