Probing the Lambda-DGP Braneworld model

We study cosmic dynamics in the context of the normal branch of the DGP braneworld model.Using current Planck data, we find the best fitting model and associated cosmological parameters in non-flat ΛDGP. With the transition redshift as a basic variable and statefinder parameters, our result shows that the Universe starts its accelerated expansion phase slightly earlier than expected in ΛCDM cosmology. The result also alleviates the coincidence problem of the ΛCDM model.     

Cosmological parameters for spatially flat dust filled Universe in Brans-Dicke theory

We have investigated late time acceleration for a spatially flat dust filled Universe in BransDicke theory in the presence of a positive cosmological constant Λ. Expressions for Hubble's constant,luminosity distance and apparent magnitude have been obtained for our model. The theoretical results are compared with observed values of the latest 287 high redshift(0.3 ≤ z ≤ 1.4) Type Ia supernova data taken from the Union 2.1 compilation to estimate present values of matter and dark energy parameters,(?_m)_0 and(?_Λ)_0. We have also estimated the present value of Hubble's constant H0 in light of an updated sample of Hubble parameter measurements including 19 independent data points. The results are found to be in good agreement with recent astrophysical observations. We also calculated various physical parameters such as matter and dark energy densities, present age of the Universe and deceleration parameter. The value for Brans-Dicke-coupling constant ω is set to be 40 000 based on accuracy of solar system tests and recent experimental evidence.     

The effect of f(T) gravity on an interplanetary clock and its time transfer link

As an extension of the"teleparallel"equivalent of general relativity,f(T)gravity is proposed to explain some puzzling cosmological behaviors,such as accelerating expansion of the Universe.Given the fact that modified gravity also has impacts on the Solar System,we might test it during future interplanetary missions with ultrastable clocks.In this work,we investigate the effects of f(T)gravity on the dynamics of the clock and its time transfer link.Under these influences,theΛ-term and theα-term of f(T)gravity play important roles.Here,Λis the cosmological constant andαrepresents a model parameter in f(T)gravity that determines the divergence from teleparallel gravity at the first order approximation.We find that the signal of f(T)gravity in the time transfer is much more difficult to detect with the current state of development for clocks than those effects on dynamics of an interplanetary spacecraft with a bounded orbit with parameters 0.5 au≤a≤5.5 au and 0≤e≤0.1.     

Comptonization and Reprocessing Processes in Accretion Disks： Applications to the Seyfert 1 Galaxies NGC 5548 and NGC 4051

Simultaneous multi-wavelength observations have revealed complex variability in AGNs. To explain the variability we considered a theoretical model consisting of an inner hot comptonizing corona and an outer thin accretion disk, with interactions between the two components in the form of comptonization and reprocessing. We found that the variability of AGNs is strongly affected by the parameters of the model, namely, the truncated disk radius rmin, the corona radius rs, the temperature KTe and the optical depth τ0 of the corona. We applied this model to the two best observed Seyfert 1 galaxies, NGC 5548 and NGC 4051. Our model can reproduce satisfactory the observed SEDs. Our fits indicate that NGC 5548 may have experienced dramatic changes in physical parameters between 1989-1990 and 1998, and that NGC 4051 has a much larger truncated disk radius (700 Schwarzschild radii) than NGC 5548 (several tens of Schwarzschild radii). Since we adopted a more refined treatment of the comptonization process rather than simply assuming a cut-off power law, our results should be more reasonable than the previous ones.     

Comparison of cosmological models using standard rulers and candles

In this paper, we used standard rulers and standard candles(separately and jointly) to explore five popular dark energy models under the assumption of the spatial flatness of the Universe. As standard rulers, we used a data set comprised of 118 galactic scale strong lensing systems(individual standard rulers if properly calibrated for the mass density profile) combined with BAO diagnostics(statistical standard ruler). Type Ia supernovae served as standard candles. Unlike most previous statistical studies involving strong lensing systems, we relaxed the assumption of a singular isothermal sphere(SIS) in favor of its generalization: the power-law mass density profile. Therefore, along with cosmological model parameters,we fitted the power law index and its first derivative with respect to the redshift(thus allowing for mass density profile evolution). It turned out that the best fitted γ parameters are in agreement with each other,irrespective of the cosmological model considered. This demonstrates that galactic strong lensing systems may provide a complementary probe to test the properties of dark energy. The fits for cosmological model parameters which we obtained are in agreement with alternative studies performed by other researchers.Because standard rulers and standard candles have different parameter degeneracies, a combination of standard rulers and standard candles gives much more restrictive results for cosmological parameters. Finally,we attempted an analysis based on model selection using information theoretic criteria(AIC and BIC). Our results support the claim that the cosmological constant model is still best and there is no(at least statistical)reason to prefer any other more complex model.     

The star formation history of redshift z~2 galaxies:the role of the infrared prior

We build a sample of 298 spectroscopically-confirmed galaxies at redshift z~2,selected in the z850-band from the GOODS-MUSIC catalog.By utilizing the rest frame 8μm luminosity as a proxy of the star formation rate(SFR),we check the accuracy of the standard SED-fitting technique,finding it is not accurate enough to provide reliable estimates of the physical parameters of galaxies.We then develop a new SED-fitting method that includes the IR luminosity as a prior and a generalized Calzetti law with a variable RV.Then we exploit the new method to re-analyze our galaxy sample,and to robustly determine SFRs,stellar masses and ages.We find that there is a general trend of increasing attenuation with the SFR.Moreover,we find that the SFRs range between a few to 103M yr 1,the masses from 109to 4×1011M,and the ages from a few tens of Myr to more than 1 Gyr.We discuss how individual age measurements of highly attenuated objects indicate that dust must have formed within a few tens of Myr and already been copious at≤100 Myr.In addition,we find that low luminosity galaxies harbor,on average,significantly older stellar populations and are also less massive than brighter ones;we discuss how these findings and the well known‘downsizing’scenario are consistent in a framework where less massive galaxies form first,but their star formation lasts longer.Finally,we find that the near-IR attenuation is not scarce for luminous objects,contrary to what is customarily assumed;we discuss how this affects the interpretation of the observed M/L ratios.     

Stability of LRS Bianchi type-I cosmological models in f(R,T)-gravity

This paper examines the stability of the transition from the early decelerating stage of the Universe to the recent accelerating stage for the perfect fluid cosmological locally rotationally symmetric(LRS) Bianchi-I model in f(R, T) theory. To determine the solution of field equations, the idea of a timevarying deceleration parameter(DP) which yields a scale factor, for which the Universe attains a phase transition scenario and is consistent with recent cosmological observations, is used. The time-dependent DP yields a scale factor a=exp■, where β and k are respectively arbitrary and integration constants. By using the recent cons_traints(H_0 _= 73.8, and q_0 =-0.54) from Type Ia Supernova(SN Ia) data in combination with Baryonic Acoustic Oscillations(BAO) and Cosmic Microwave Background(CMB) observations(Giostri et al.), we obtain the values of β = 0.0062 and k = 0.000016 for which we have derived a cosmological model from the early decelerated phase to the present accelerating phase. By applying_ other r_ecent constraints(H_0 = 73.8, q_0 =-0.73) from SNe Ia Union data(Cunha), we obtain the values of β = 0.0036 and k = 0.000084 for which we have derived a cosmological model in the accelerating phase only. We have compared both models with experimental data. The stability of the background solution has been examined also for the metric perturbations alongside the properties of future singularities in a Universe ruled by dark energy with phantom type fluid. We demonstrate the presence of a stable fixed point with a condition of state ω <-1 and numerically affirm this is really a late-time attractor in the ghost overwhelmed Universe. Some physical and geometric properties of the model are found and examined.     

TeV cosmic-ray proton and helium spectra in the myriad model Ⅱ

Recent observations show that the spectra of cosmic ray nuclei start to harden above ～ 102 GeV, which contradicts the conventional steady-state cosmic ray model. We had suggested that this anomaly is due to the propagation effect of cosmic rays released from local young cosmic ray sources; the total flux of cosmic rays should be computed with the Myriad Model, where a contribution from sources in the local catalog is added to the background. However, although the hardening could be elegantly explained in this model, the model parameters obtained from the fit are skewed toward a region with fast diffusion and a low supernova rate in the Galaxy, in disagreement with other observations. We further explore this model in order to set up a concordant picture. Two possible improvements related to cosmic ray sources are considered. First, instead of the usual axisymmetric disk model, we examine a spiral model for the source distribution. Second, for nearby and young sources which are necessary to explain the hardening, we allow for an energy-dependent escape. We find that a major improvement comes from incorporating an energy-dependent escape time for local sources, and with both modifications not only are the cosmic ray proton and helium anomalies resolved, but also the parameters attain values in a reasonable range compatible with other analyses.     

The reionization of He II and the temperature evolution of the intergalactic medium

A number of observations suggest that He II in the intergalactic medium(IGM) was fully ionized at z ～ 3, probably by quasi-stellar objects(QSOs). Here we construct a simple model of a QSO to study the reionization of He II and the corresponding thermal evolution of the IGM. We assume that QSOs are triggered by major mergers of dark matter halos, and the luminosity evolution of individual QSOs is described by an initial accretion stage with a constant Eddington ratio and then a powerlaw decay driven by long term disk evolution or fueling. Once a QSO is triggered, it immediately ionizes its surrounding area as an ionized bubble. The resulting changes in size and volume of the bubble are determined by the luminosity evolution of the central QSO. With the emergence of more and more bubbles, they eventually overlap each other and finally permeate the whole universe. During the He II reionization,the IGM temperature increases due to the photoheating by the ionization processes.Applying the bubble model and considering various heating and cooling mechanisms,we trace the thermal evolution of the IGM and obtain the average IGM temperature as a function of redshift, which is very consistent with observations. The increase in IGM temperature due to the reionization of He II may be determined more accurately in the future, which may put robust constraints on the QSO model and the physics of He II reionization.     

On the torque exerted by a warped,magnetically threaded accretion disk

Most astrophysical accretion disks are likely to be warped.In X-ray binaries,the spin evolution of an accreting neutron star is critically dependent on the interaction between the neutron star magnetic field and the accretion disk.There have been extensive investigations on the accretion torque exerted by a coplanar disk that is magnetically threaded by the magnetic field lines from the neutron stars,but relevant works on warped/tilted accretion disks are still lacking.In this paper we develop a simplified twocomponent model,in which the disk is comprised of an inner coplanar part and an outer,tilted part.Based on standard assumption on the formation and evolution of the toroidal magnetic field component,we derive the dimensionless torque and show that a warped/titled disk is more likely to spin up the neutron star compared with a coplanar disk.We also discuss the possible influence of various initial parameters on the torque.     

Shallow Decay of X-ray Afterglows in Short GRBs： Energy Injection from a Millisecond Magnetar？

With the successful launch of Swift satellite,more and more data of early X-ray afterglows from short gamma-ray bursts have been collected.Some interesting features such as unusual afterglow light curves and unexpected X-ray flares are revealed.Especially,in some cases,there is a fiat segment in the X-ray afterglow light curve.Here we present a simplified model in which we believe that the flattening part is due to energy injection from the central engine.We assume that this energy injection arises from the magnetic dipole radiation of a millisecond pulsar formed after the merger of two neutron stars.We check this model with the short GRB 060313.Our numerical results suggest that energy injection from a millisecond magnetar could make part of the X-ray afterglow light curve flat.     

An empirical model to form and evolve galaxies in dark matter halos

Based on the star formation histories of galaxies in halos with different masses, we develop an empirical model to grow galaxies in dark matter halos. This model has very few ingredients, any of which can be associated with observational data and thus be efficiently assessed. By applying this model to a very high resolution cosmological N-body simulation, we predict a number of galaxy properties that are a very good match to relevant observational data. Namely, for both centrals and satellites, the galaxy stellar mass functions up to redshift z ≈ 4 and the conditional stellar mass functions in the local universe are in good agreement with observations. In addition, the two point correlation function is well predicted in the different stellar mass ranges explored by our model. Furthermore, after applying stellar population synthesis models to our stellar composition as a function of redshift, we find that the luminosity functions in the 0.1_u,0.1_g,0.1_r,0.1_i and 0.1_z bands agree quite well with the SDSS observational results down to an absolute magnitude at about –17.0. The SDSS conditional luminosity function itself is predicted well. Finally, the cold gas is derived from the star formation rate to predict the HI gas mass within each mock galaxy. We find a remarkably good match to observed HI-to-stellar mass ratios. These features ensure that such galaxy/gas catalogs can be used to generate reliable mock redshift surveys.     

Analysis of the X-ray emission of OB stars: O stars

We investigate the global properties of X-ray emission from O stars, analyzing the X-ray spectra of 32 O stars from archival data of the XMM-Newton space observatory. We examine two hypotheses about of the origin of X-ray emission from O stars. The first is a paradigm proposed by Pollock, that was revealed from an analysis of the ζ Ori X-ray observation. The second is the magnetically confined wind-shock(MCWS) model. For checking Pollock's hypothesis, we determine the distribution of the ratio of half width at half maximum(HWHM) to the wind terminal velocity for lines in spectra of all examined stars. In addition, we check three probable consequences from the MCWS model. We analyze if a correlation exists between the spectral hardness and such stellar parameters as the wind terminal velocity, stellar magnetic field and mass loss rate. The result showed that Pollock's hypothesis is not correct. We also established that not all consequences of the MCWS model considered by us are confirmed. In addition, our spectral analysis method indicated that O stars probably have clumped stellar winds with spherical clumps.     

On the reality of broad iron L lines from the narrow line Seyfert 1 galaxies 1H0707–495 and IRAS 13224–3809

We performed time resolved spectroscopy of 1H0707–495 and IRAS 13224–3809 using long XMM-Newton observations. These are strongly variable narrow line Seyfert 1 galaxies and show broad features around 1 ke V that have been interpreted as relativistically broad Fe Lα lines. Such features are not clearly observed in other active galactic nuclei despite sometimes having high iron abundance required by the best fitted blurred reflection models. Given the importance of these lines, we explore whether the rapid variability of spectral parameters may introduce broad bumps/dips artificially in the time averaged spectrum, which may then be mistaken as broadened lines. We tested this hypothesis by performing time resolved spectroscopy using long(100 ks) XMM-Newton observations and by dividing them into segments with typical exposures of a few ks. We extracted spectra from each such segment and modeled them using a two component phenomenological model consisting of a power law to represent the hard component and a black body to represent the soft emission. As expected, both the sources showed variations in the spectral parameters. Using these variation trends, we simulated model spectra for each segment and then co-added to get a combined simulated spectrum. In the simulated spectra, we found no broad features below 1 ke V and in particular no deviation near 0.9 ke V as seen in the real averaged spectra. This implies that the broad Fe Lα line that is seen in the spectra of these sources is not an artifact of the variation of spectral components and, hence, provides evidence that the line is indeed genuine.     

The metallicity of K giant stars along the Sagittarius streams

The Large Sky-Area Multi-Object Fiber Spectroscopic Telescope(LAMOST) Data Release3 provided 341 691 K giant stars with stellar parameters. Based on the models of Law Majewski, we identified 252 K giant stars in the leading stream associated with the Sagittarius(Sgr) dwarf galaxy. We obtained 132 K giant stars belonging to the trailing arm of Sgr using the model of Belokurov et al. We studied the metallicity distribution of member stars along the streams and found a flat gradient in the first wrap of the leading stream, –(0.88±0.3) × 10~(-3)dex/(~?) in the second wrap of the leading stream and–(1.2±0.3) × 10~(-3)dex/(~?) in the first wrap of the trailing stream. Moreover, we obtained a combined metallicity gradient with our sample and data from the literature. We also analyzed the properties of an overdensity, which is located in the leading stream of the Sgr.     

Computation of the atmosphere-less light intensity curve during a total solar eclipse by using Lunar Reconnaissance Orbiter topography data and the DE430 astronomical ephemeris

Observations of the sky irradiation intensity in the visible wavelengths during a solar eclipse permit to model the Sun diameter,a key number to constrain the internal structure of our star.In this paper,we present an algorithm that takes advantage of the precise Moon topography from Lunar Reconnaissance Orbiter to compute,with a high resolution in time,the geometrical part(i.e.top-of-atmosphere,and for a given wavelength)of the sky irradiation at any given location on the Earth during these events.The algorithm is also able to model the Baily’s beads.We give as an application the theoretical computation of the light curve corresponding to the solar eclipse observed at Lakeland(Queensland,North Australia)on 2012 November 13.The application to real data,with the introduction of atmospheric and instrumental passbands,will be considered in a forthcoming paper.     

Degeneracy and discreteness in cosmological model fitting

We explore the problems of degeneracy and discreteness in the standard cosmological model(ΛCDM). We use the Observational Hubble Data(OHD) and the type Ia supernovae(SNe Ia) data to study this issue. In order to describe the discreteness in fitting of data, we define a factor G to test the influence from each single data point and analyze the goodness of G. Our results indicate that a higher absolute value of G shows a better capability of distinguishing models, which means the parameters are restricted into smaller confidence intervals with a larger figure of merit evaluation. Consequently, we claim that the factor G is an effective way of model differentiation when using different models to fit the observational data.     

The Historical Re-Brightening and Distance Recheck of SN 1006

We presume the re-brightening of SN 1006 in AD 1016 as recorded in the ancient Chinese literature to be true and the re-brightening was caused by the encounter either of photons or the shock wave from the SN outburst with the circumstellar thin envelope mate- rials. Based on these considerations, and combining the observational results on the optical proper motion of the N-W limb and the radio observations of the other parts of the supernova remnant G327.6 14.5, we re-determine the distance to SN 1006. For the photon-encounter model, the average radius of the envelope material would be 10ly; and for the shock wave- encounter model, the radius would be about 1 ly. We then set up four equations to solve for the distance of the SN, the initial shock speed, the expansion index for two different parts of the supernova remnant, and the real original radius of the thin envelope nebula. It is indicated that only the case of photon-encounter will lead to a reasonable result. We derived a distance of 5074ly (1.56kpc), an original shock expansion velocity of 0.071c, an expansion index of 0.72 for the N-W limb of the SNR, and 0.76 for the other parts . We deem that the SNR evolution is still in the stage of reverse shock.     

The orbital phase resolved spectroscopy of X-ray binary 4U 1822–371 with Suzaku

4U 1822–371 is a typical edge-on eclipsing low mass X-ray binary and the prototype of accretion disk coronal sources. We report on the results of a spectral analysis over the energy range 0.5–45 ke V observed by Suzaku in 2006. We extract spectra from five orbital phases. The spectra can be equally well described by various previously proposed models: an optically thick model described by a partially covered cutoff power law and an optically thin model described by a blackbody plus a cutoff power law. The optically thick model requires a covering fraction of about 55%, while the optically thin model requires a temperature of the central source of about 0.16 ke V. The spectrum in the optically thick model also shows the previously detected cyclotron line feature at ~30 ke V with the same Suzaku observation. This feature confirms the presence of a strong magnetic field. The Fe Kα fluorescent line strengths as well as the detected Fe XXVI strengths are similar to previous Chandra and XMM-Newton detections in our phased spectral analysis; however, we also observe strong Fe XXVI during the eclipse, which indicates a slightly larger central corona.     

Light curve inversion of asteroid(585) Bilkis with Lommel-Seeliger ellipsoid method

The basic physical parameters of asteroids, such as spin parameters, shape and scattering parameters, can provide us with information on the formation and evolution of both the asteroids themselves and the entire solar system. In a majority of asteroids, the disk-integrated photometry measurement constitutes the primary source of the above knowledge. In the present paper, newly observed photometric data and existing data on(585) Bilkis are analyzed based on a Lommel-Seeliger ellipsoid model. With a Markov chain Monte Carlo(MCMC) method, we have determined the spin parameters(period, pole orientation)and shape(b/a, c/a) of(585) Bilkis and their uncertainties. As a result, we obtained a rotational period of 8.5738209 h with an uncertainty of 9×10-7h, and derived a pole of(136.46?, 29.0?) in the ecliptic frame of J2000.0 with uncertainties of 0.67?and 1.1?in longitude and latitude respectively. We also derived triaxial ratios b/a and c/a of(585) Bilkis as 0.736 and 0.70 with uncertainties of 0.003 and 0.03 respectively.