The continuum spectrum of gamma-ray burst's (GRB) afterglow at Lyα wavelength is known to be otherwise featureless except for the existence of a pair of smooth damping wings. Resonant scattering of photons with the ambient neutral hydrogen around the GRB may alter this picture. We study the formation and evolution of the spectral imprint of these resonantly scattered photons in the context of GRB's afterglow. Based on an analytic model that includes photons that are scattered only once, as well as a complete treatment of all the scatterings using Monte Carlo simulations, we are able to calculate the spectrum and luminosity of this Lyα emission from a very early moment up to a late epoch. We find that the amount, the motion, and the geometry of the neutral hydrogen around the GRB, together with the time behavior of the source are the crucial factors that affect the predicted luminosity and spectral profile. The flux of the Lyα emission is found to be mainly contributed by photons that are scattered only once. The flux is of the order 10^-4-10 ^-9 relative to the undecayed maximum flux of the transmitted continuum, making the feature negligible but potentially observable. If not obscured by the host galaxy's damped Lyα absorption systems or intergalactic neutral hydrogen, the feature may appear sometime from 1 hr to several years when the directly transmitted light has faded away. This scattered emission feature can be distinguished from Lyα photons of other origins by its luminosity evolution and by its gradual narrowing of profile with time. The typical timescale for spectral variance is that of the light crossing time of a hydrogen clump close to the GRB. If observed, the resonant peaks' time-dependent behavior is a scanning probe on the distribution of neutral hydrogen in GRB's immediate neighborhood.

Wen Xu;向平 武

Astrophysical Journal

2010

We report briefly the current status on the baryon fraction measurements in clusters of galaxies and their cosmological implications.

向平 武

Modern Physics Letters A

2001-9-7

The mass derived from gravitational leasing reflects the total mass contained in the lensing system, independent of the specific matter contents and states. A comparison of the dynamical masses from hydrostatic equilibrium with the gravitational masses from arclike images of background galaxies is made for four clusters of galaxies at intermediate redshifts. It is found that virial analysis has underestimated the total cluster masses (from lensing) by a factor of ∼3-6 within a radius of ∼0.3 Mpc h^-1₅₀ around the cluster centers, indicating that clusters of galaxies might not be regarded as the well-relaxed virialized systems. The increase of the total cluster masses obtained from lensing leads to the decrease of the baryon fractions of clusters of galaxies, which provides a clue to solving the "Ω₀ discrepancy puzzle" in cosmology.

向平 武

Astrophysical Journal

1994-12-1

We present an exact solution of the anisotropies of cosmic background radiation (CBR) from a local collapse described by a spherical overdense region embedded in a flat universe, with the emphasis on the relationship between the dipole (ΔT/T)_d and the quadrupole (ΔT/T)_q anisotropy. This result has been used to examine the kinematic quadrupole correction (ΔT/T)_q = (ΔT/T)²_d/2, which is usually applied to remove the contamination of the quadrupole produced by local density inhomogeneities when finding the cosmic amplitude of the quadrupole at the surface of last scattering. We have found that the quadrupole of local collapse origin cannot always be approximately described by the kinematic quadrupole. Our numerical result shows that the difference between the kinematic and local quadrupoles depends on the size and matter density in the peculiar field and the position of the observer. For a given dipole, the local quadrupole can be different from the kinematic quadrupole by a factor as large as 3. Therefore, the kinematic quadrupole correction remains an uncertain factor in the determination of the amplitude of a cosmic quadrupole. Nevertheless, a preliminary analysis shows that this uncertainty might not dramatically change the cosmological origin of the COBE-DMR's quadrupole, unless a huge peculiar gravitational field is assumed.

向平 武;Li Zhi Fang

Astrophysical Journal

1994-4-1

The expected microlensing events of the Large Magellanic Cloud (LMC) by the MACHOs of the LMC itself are calculated and compared with analog events by objects in the Galactic halo. The LMC matter distribution is modeled by a spherical halo and an exponential disk, while a face-on exponential disk is used for the stellar distribution of the LMC. Among the microlensing events discovered by the MACHO and EROS projects, a fraction of 22% could be caused by the lenses near the center of the LMC or 13% from lenses at 5° from the LMC center, provided that the halo of the Galaxy and of the LMC are composed of MACHOs. The LMC disk produces a relatively lower optical depth to microlensing of ∼3 × 10^-8 but contributes a fraction of ∼40% of the observed microlensing events toward the LMC center, if the actual detection rate is taken to be (7-9) × 10^-8. It is then likely that the presently detected microlensing events near the center of the LMC are due to some combination of the MACHOs of the Galactic halo, the Galactic spheroid, the LMC halo, and particularly the LMC disk. Therefore, any statistical study of these microlensing events must take the LMC lenses into account.

向平 武

Astrophysical Journal

1994-11-1

We present the Chandra study of the intermediate-redshift (z = 0.194) cluster of galaxies MS 0839.9+2938. By performing both projected and deprojected spectral analyses, we find that the gas temperature is approximately constant at about 4 keV within 130-444 h₇₀^-1 kpc. In the inner regions, the gas temperature decreases toward the center, reaching ≲3 keV in the central 37 h₇₀^-1 kpc. This implies that the lower and upper limits of the mass deposit rate are 9-34 and 96-126 M⊙ yr ^-1, respectively, within 74 h₇₀^-1 kpc, where the gas is significantly colder. Along with the temperature drop, we detect a significant inward iron abundance increase from about 0.4 Z⊙ in the outer regions to ≃1 Z⊙ within the central 37 h₇₀^-1 kpc. Thus, MS 0839.9+2938 is the cluster showing the most significant central iron excess at z ≳ 0.2. We argue that most of the excess iron should have been contributed by SNe Ia. Using the observed SN Ia rate and stellar mass loss rate, we estimate that the time needed to enrich the central region with excess iron is 6.4-7.9 Gyr, which is similar to those found for nearby clusters. Coinciding with the optical extension of the cD galaxy (up to about 30 h ₇₀^-1 kpc), the observed X-ray surface brightness profile exhibits an excess beyond the distribution expected by either the β model or the Navarro-Frenk-White (NFW) model and can be well fitted with an empirical two-β model that leads to a relatively natter mass profile in the innermost region.

Yu Wang;Haiguang Xu;Zhongli Zhang;Yueheng Xu;向平 武;Sui Jian Xue;Zongwei Li

Astrophysical Journal

2005-9-20

Based on a 21.5 ks Chandra observation of A2556, we identify an edge on the surface brightness profile at about 160 h ^-1 ₇₁ kpc northeast of the cluster center, and it corresponds to a shock front whose Mach number is calculated to be 1.25^+0.02_-0.03. No prominent substructure, such as sub-cluster, is found in either the optical or X-ray band that can be associated with the edge, suggesting that the conventional supersonic motion mechanism may not work in this case. As an alternative solution, we propose that the nonlinear steepening of an acoustic wave, which is induced by the turbulence of the intracluster medium at the core of the cluster, can be used to explain the origin of the shock front. Although nonlinear steepening weak shock is expected to occur frequently in clusters, why it is rarely observed still remains a question that requires further investigation, including both deeper X-ray observation and extensive theoretical studies.

Zhenzhen Qin;Haiguang Xu;Jingying Wang;Yu Wang;Junhua Gu;向平 武

Astrophysical Journal

2013-1-1

In this work, we simulate the 50-200 MHz radio sky that is constrained in the field of view (5° radius) of the 21 Centimeter Array (21CMA), a low-frequency radio interferometric array constructed in the remote area of Xinjiang, China, by carrying out Monte Carlo simulations to model the strong contaminating foreground of the redshifted cosmological reionization signals, including emissions from our Galaxy, galaxy clusters, and extragalactic discrete sources (i.e., star-forming galaxies, radio-quiet active galactic nuclei (AGNs), and radio-loud AGNs). As an improvement over previous works, we consider in detail not only random variations of morphological and spectroscopic parameters within the ranges allowed by multi-band observations, but also the evolution of radio halos in galaxy clusters, assuming that relativistic electrons are re-accelerated in the intracluster medium (ICM) in merger events and lose energy via both synchrotron emission and inverse Compton scattering with cosmic microwave background photons. By introducing a new approach designed on the basis of independent component analysis and wavelet detection algorithm, we prove that, with a cumulative observation of one month with the 21CMA array, about 80% of galaxy clusters (37 out of 48 clusters assuming a mean magnetic field of B = 2μG in the ICM, or 15 out of 18 clusters assuming B = 0.2μG) with central brightness temperatures of >10K at 65 MHz can be safely identified and separated from the overwhelmingly bright foreground. By examining the brightness temperature images and spectra extracted from these identified clusters, we find that the morphological and spectroscopic distortions are extremely small compared to the input simulated clusters, and the reduced χ² of brightness temperature profiles and spectra are controlled to be ≲0.5 and ≲1.3, respectively. These results robustly indicate that in the near future a sample of dozens of bright galaxy clusters will be disentangled from the foreground in 21CMA observations, the study of which will greatly improve our knowledge about cluster merger rates, electron acceleration mechanisms in cluster radio halos, and magnetic field in the ICM.

Jingying Wang;Haiguang Xu;Junhua Gu;涛 安;Haijuan Cui;Jianxun Li;Zhongli Zhang;Qian Zheng;向平 武

Astrophysical Journal

2010-11-1

The statistics of multiple gravitational lensing in a locally inhomogeneous universe is taken into account, based on the expansion of the mean amplification into the summation of various lenses and the pointlike models for both background sources and lensing objects. By comparing the deviation of the effects of various lenses from mean amplification, the redshift ranges over which different lenses dominate are investigated. In particular, the accuracy of a single lensing approximation, which has been widely used in previous statistical lensing, is demonstrated. As the application of a single lensing approximation, the lowest correction to the Mattig's relation due to the inhomogeneities in universe and the lowest effect on the number counts of background sources due to amplification bias are reconsidered. It is emphasized the fact that the effect of statistical lensing, even in a single lensing approximation, is of significance for modern observational cosmology.

向平 武

Astrophysics and Space Science

1992-1

We present the relation of X-ray luminosity versus dynamical mass for 63 nearby clusters of galaxies in a flux-limited sample, the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS, consisting of 64 clusters). The luminosity measurements are obtained based on ~1:3 Ms of clean XMM-Newton data and ROSAT pointed observations. The masses are estimated using optical spectroscopic redshifts of 13647 cluster galaxies in total. We classify clusters into disturbed and undisturbed based on a combination of the X-ray luminosity concentration and the o set between the brightest cluster galaxy and X-ray flux-weighted center. Given suffcient numbers (i.e., ≥45) of member galaxies when the dynamical masses are computed, the luminosity versus mass relations agree between the disturbed and undisturbed clusters. The cool-core clusters still dominate the scatter in the luminosity versus mass relation even when a core-corrected X-ray luminosity is used, which indicates that the scatter of this scaling relation mainly reflects the structure formation history of the clusters. As shown by the clusters with only few spectroscopically confirmed members, the dynamical masses can be underestimated and thus lead to a biased scaling relation. To investigate the potential of spectroscopic surveys to follow up highredshift galaxy clusters or groups observed in X-ray surveys for the identifications and mass calibrations, we carried out Monte Carlo resampling of the cluster galaxy redshifts and calibrated the uncertainties of the redshift and dynamical mass estimates when only reduced numbers of galaxy redshifts per cluster are available. The resampling considers the SPIDERS and 4MOST configurations, designed for the follow-up of the eROSITA clusters, and was carried out for each cluster in the sample at the actual cluster redshift as well as at the assigned input cluster redshifts of 0.2, 0.4, 0.6, and 0.8. To follow up very distant clusters or groups, we also carried out the mass calibration based on the resampling with only ten redshifts per cluster, and redshift calibration based on the resampling with only five and ten redshifts per cluster, respectively. Our results demonstrate the power of combining upcoming X-ray and optical spectroscopic surveys for mass calibration of clusters. The scatter in the dynamical mass estimates for the clusters with at least ten members is within 50%.

Yu Ying Zhang;Thomas H. Reiprich;Peter Schneider;Nicolas Clerc;Andrea Merloni;Axel Schwope;Katharina Borm;Heinz Andernach;César A. Caretta;向平 武

Astronomy and Astrophysics

2017-3-1