Photon Propagation Around Compact Objects and the Inferred Properties of Thermally Emitting Neutron Stars

Anomalous X-ray pulsars, compact non-pulsing X-ray sources in supernova remnants, and X-ray bursters are three distinct types of sources for which there are viable models that attribute their X-ray emission to thermal emission from the surface of a neutron star. Inferring the surface area of the emitting regions in such systems is crucial in assessing the viability of different models and in providing bounds on the radii of neutron stars. We show that the inferred areas of the emitting regions may be over- or under-estimated by a factor of <=2, because of the geometry of the system and general relativistic light deflection, combined with the effects of phase averaging. Such effects make the determination of neutron-star radii uncertain, especially when compared to the ~5% level required for constraining the equation of state of neutron-star matter. We also note that, for a given spectral shape, the inferred source luminosities and pulse fractions are anticorrelated because they depend on the same properties of the emitting regions, namely their sizes and orientations, i.e., brighter sources have on average weaker pulsation amplitudes than fainter sources. We argue that this property can be used as a diagnostic tool in distinguishing between different spectral models. As an example, we show that the high inferred pulse fraction and brightness of the pulsar RXS J1708-40 are inconsistent with isotropic thermal emission from a neutron-star surface. Finally, we discuss the implication of our results for surveys in the soft X-rays for young, cooling neutron stars in supernova remnants and show that the absence of detectable pulsations from the compact source at the center of Cas A (at a level of >=30%) is not a strong argument againts its identification with a spinning neutron star.Comment: 6 pages, 6 figures, to appear in the Astrophysical Journal; minor change

General Relativistic Constraints on Emission Models of Anomalous X-ray Pulsars

Most models of anomalous X-ray pulsars (AXPs) account for the observed X-ray spectra and pulsations by means of radiation processes that occur on the surfaces of neutron stars. For any such model, general relativistic deflection of light severely suppresses the amplitude of the observed pulsations. We calculate the expected pulsation amplitudes of AXPs according to various models and compare the results with observations. We show that the high (<= 70%) pulse amplitudes observed in some AXPs can be accounted for only if the surface emission is localized (spot radius <40 degrees) and strongly beamed (cos^n[theta'] with n>2, where theta' is the angle to the normal). These constraints are incompatible with those cooling and magnetar models in which the observed X-rays originate as thermal emission from the neutron-star surface. Accretion models, on the other hand, are compatible with observations for a wide range of parameters. Finally, definitive conclusions cannot be reached on magnetospheric models, since their localization and beaming properties are not well understood.Comment: 7 pages, 9 figures, submitted to The Astrophysical Journa

Computational fact checking from knowledge networks

Traditional fact checking by expert journalists cannot keep up with the enormous volume of information that is now generated online. Computational fact checking may significantly enhance our ability to evaluate the veracity of dubious information. Here we show that the complexities of human fact checking can be approximated quite well by finding the shortest path between concept nodes under properly defined semantic proximity metrics on knowledge graphs. Framed as a network problem this approach is feasible with efficient computational techniques. We evaluate this approach by examining tens of thousands of claims related to history, entertainment, geography, and biographical information using a public knowledge graph extracted from Wikipedia. Statements independently known to be true consistently receive higher support via our method than do false ones. These findings represent a significant step toward scalable computational fact-checking methods that may one day mitigate the spread of harmful misinformation

Impact of Systematic Errors in Sunyaev-Zel'dovich Surveys of Galaxy Clusters

Future high-resolution microwave background measurements hold the promise of detecting galaxy clusters throughout our Hubble volume through their Sunyaev-Zel'dovich (SZ) signature, down to a given limiting flux. The number density of galaxy clusters is highly sensitive to cluster mass through fluctuations in the matter power spectrum, as well as redshift through the comoving volume and the growth factor. This sensitivity in principle allows tight constraints on such quantities as the equation of state of dark energy and the neutrino mass. We evaluate the ability of future cluster surveys to measure these quantities simultaneously when combined with PLANCK-like CMB data. Using a simple effective model for uncertainties in the cluster mass-SZ flux relation, we evaluate systematic shifts in cosmological constraints from cluster SZ surveys. We find that a systematic bias of 10% in cluster mass measurements can give rise to shifts in cosmological parameter estimates at levels larger than the $1\sigma$ statistical errors. Systematic errors are unlikely to be detected from the mass and redshift dependence of cluster number counts alone; increasing survey size has only a marginal effect. Implications for upcoming experiments are discussed.Comment: 12 pages, 6 figures; accepted to JCAP; revised to match submitted versio

The actual Rees--Sciama effect from the Local Universe

Observations of the Cosmic Microwave Background (CMB) have revealed an unexpected quadrupole-octopole alignment along a preferred axis pointing toward the Virgo cluster. We here investigate whether this feature can be explained in the framework of the concordance model by secondary anisotropies produced by the non-linear evolution of the gravitational potential, the so-called Rees-Sciama (RS) effect. We focus on the effect caused by the local superclusters, which we calculate using a constrained high-resolution hydrodynamical simulation, based on the IRAS 1.2-Jy all-sky galaxy redshift survey, which reproduces the main structures of our Universe out to a distance of 110 Mpc from our Galaxy. The resulting RS effect peaks at low multipoles and has a minimum/maximum amplitude of -6.6\mu K 1.9\mu K. Even though its quadrupole is well aligned with the one measured for the CMB, its amplitude is not sufficient to explain the observed magnitude of the quadrupole/octopole alignment. In addition, we analyze the WMAP-3 data with a linear matched filter in an attempt to determine an upper limit for the RS signal amplitude on large scales. We found that it is possible to infer a weak upper limit of 30\mu K for its maximum amplitude.Comment: 7 pages, 4 figures, submitted to A&

Semi-Analytic Stellar Structure in Scalar-Tensor Gravity

Precision tests of gravity can be used to constrain the properties of hypothetical very light scalar fields, but these tests depend crucially on how macroscopic astrophysical objects couple to the new scalar field. We develop quasi-analytic methods for solving the equations of stellar structure using scalar-tensor gravity, with the goal of seeing how stellar properties depend on assumptions made about the scalar coupling at a microscopic level. We illustrate these methods by applying them to Brans-Dicke scalars, and their generalization in which the scalar-matter coupling is a weak function of the scalar field. The four observable parameters that characterize the fields external to a spherically symmetric star (the stellar radius, R, mass, M, scalar `charge', Q, and the scalar's asymptotic value, phi_infty) are subject to two relations because of the matching to the interior solution, generalizing the usual mass-radius, M(R), relation of General Relativity. We identify how these relations depend on the microscopic scalar couplings, agreeing with earlier workers when comparisons are possible. Explicit analytical solutions are obtained for the instructive toy model of constant-density stars, whose properties we compare to more realistic equations of state for neutron star models.Comment: 39 pages, 9 figure

Two approaches to testing general relativity in the strong-field regime

Observations of compact objects in the electromagnetic spectrum and the detection of gravitational waves from them can lead to quantitative tests of the theory of general relativity in the strong-field regime following two very different approaches. In the first approach, the general relativistic field equations are modified at a fundamental level and the magnitudes of the potential deviations are constrained by comparison with observations. In the second approach, the exterior spacetimes of compact objects are parametrized in a phenomenological way, the various parameters are measured observationally, and the results are finally compared against the general relativistic predictions. In this article, I discuss the current status of both approaches, focusing on the lessons learned from a large number of recent investigations.Comment: To appear in the proceedings of the conference New Developments in Gravit

The Tensor-Vector-Scalar theory and its cosmology

Over the last few decades, astronomers and cosmologists have accumulated vast amounts of data clearly demonstrating that our current theories of fundamental particles and of gravity are inadequate to explain the observed discrepancy between the dynamics and the distribution of the visible matter in the Universe. The Modified Newtonian Dynamics (MOND) proposal aims at solving the problem by postulating that Newton's second law of motion is modified for accelerations smaller than ~10^{-10}m/s^2. This simple amendment, has had tremendous success in explaining galactic rotation curves. However, being non-relativistic, it cannot make firm predictions for cosmology. A relativistic theory called Tensor-Vector-Scalar (TeVeS) has been proposed by Bekenstein building on earlier work of Sanders which has a MOND limit for non-relativistic systems. In this article I give a short introduction to TeVeS theory and focus on its predictions for cosmology as well as some non-cosmological studies.Comment: 44 pages, topical review for Classical and Quantum Gravit

Suppressing CMB Quadrupole with a Bounce from Contracting Phase to Inflation

Recent released WMAP data show a low value of quadrupole in the CMB temperature fluctuations, which confirms the early observations by COBE. In this paper, a scenario, in which a contracting phase is followed by an inflationary phase, is constructed. We calculate the perturbation spectrum and show that this scenario can provide a reasonable explanation for lower CMB anisotropies on large angular scales.Comment: 5 pages, 3 figure

Born-Infeld-type phantom on the brane world

We study the evolution of Born-Infeld-type phantom in the second Randall-Sundrum brane scenario, and find that there exists attractor solution for the potential with a maximum, which implies a cosmological constant at the late time. Especially, we discuss the BI model of constant potential without and with dust matter. In the weak tension limit of the brane, we obtain an exact solution for the BI phantom and scale factor and show that there is no big rip during the evolution of the brane.Comment: 5 pages, 2 figures, Reference added, Phys. Rev. D in pres