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Delayed onset and fast rise of prompt optical-UV emission from gamma-ray bursts in molecular clouds

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Research in astronomy and astrophysics 2013年第1期13卷 57-70页

作者： Xiao-Hong Cui Zhuo Li Li-Ping Xin National Astronomical Observatories Chinese Academy of Sciences department of astronomy Peking University kavli institute for astronomy and astrophysics Peking University

Observations imply that long ＂y-ray bursts （GRBs） originate from the explo- sions of massive stars, therefore they may occur in the molecular clouds where their progenitors were born. We show that the prompt optical-UV emission from GRBs may be delayed due to dust extinction, which can explain the observed optical delayed on- set and fast rise in GRB 080319B well. The density and the size of the molecular cloud around GRB 080319B are roughly constrained to be ~ 103 cm-z and ~ 8 pc, respectively. We also investigate other GRBs with prompt optical-UV data, and find similar values of the densities and sizes of the local molecular clouds. Future obser- vations of prompt optical-UV emission from GRBs on a timescale of subseconds, e.g. by UFFO-Pathfinder and SVOM-GWAC, will provide more evidence and probes of the local environments of GRBs.

关键词： radiation mechanisms: non-thermal -- gamma-rays: bursts -- dust: ex-tinction

A new unified theory of electromagnetic and gravitational interactions

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Frontiers of physics 2016年第6期11卷 35-66页

作者： Li-Xin Li kavli institute for astronomy and astrophysics Peking University Beijing 100871 China

In this paper we present a new unified theory of electromagnetic and gravitational interactions. By considering a four-dimensional spacetime as a hypersurface embedded in a five-dimensional bulk spacetime, we derive the complete set of field equations in the four-dimensional spacetime from the five-dimensional Einstein field equation. Besides the Einstein field equation in the four-dimensional spacetime, an electromagnetic field equation is obtained: del F-a(ab) - xi R-b (a)A(a) = -4 pi J(b) with xi = -2, where F-ab is the antisymmetric electromagnetic field tensor defined by the potential vector A(a), R-ab is the Ricci curvature tensor of the hypersurface, and J(a) is the electric current density vector. The electromagnetic field equation differs from the Einstein-Maxwell equation by a curvature-coupled term xi R-b (a)A(a), whose presence addresses the problem of incompatibility of the Einstein-Maxwell equation with a universe containing a uniformly distributed net charge, as discussed in a previous paper by the author [L.-X. Li, Gen. Relativ. Gravit. 48, 28 (2016)]. Hence, the new unified theory is physically different from Kaluza-Klein theory and its variants in which the Einstein-Maxwell equation is derived. In the four-dimensional Einstein field equation derived in the new theory, the source term includes the stress-energy tensor of electromagnetic fields as well as the stress-energy tensor of other unidentified matter. Under certain conditions the unidentified matter can be interpreted as a cosmological constant in the four-dimensional spacetime. We argue that, the electromagnetic field equation and hence the unified theory presented in this paper can be tested in an environment with a high mass density, e.g., inside a neutron star or a white dwarf, and in the early epoch of the universe.

关键词： general relativity Maxwell's equations unified field theory Kaluza-Klein theory brane world theory

Astrometric calibration of the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anti-center(XSTPS-GAC)

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Research in astronomy and astrophysics 2014年第4期14卷 456-470页

作者： Hui-Hua Zhang Xiao-Wei Liu Hai-Bo Yuan Hai-Bin Zhao Jin-Sheng Yao Hua-Wei Zhang Mao-Sheng Xiang Yang Huang department of astronomy Peking University kavli institute for astronomy and astrophysics Peking University Purple Mountain Observatory Chinese Academy of Sciences

Abstract We present astrometric calibration of the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anti-center （XSTPS-GAC）. XSTPS-GAC is the photometric part of the Digital Sky Survey of the Galactic Anti-center （DSS-GAC）, which is a photometric and spectroscopic sky survey, in combination with LAMOST. In order to select an astrometric reference catalog, we made comparisons between the four widely used astrometric catalogs, GSC2.3, USNO-B1.0, UCAC3 and PPMXL. PPMXL shows relatively small systematic errors in positions and more homogeneous proper motion distributions toward the Galactic Anti-center （GAC）, and was selected as the reference catalog. Based on the high quality and bright reference stars that were picked out from PPMXL, we performed a 4th-order polynomial fitting in image units, to construct the transformation relation between coordinates used by XSTPS-GAC and standard coordinates, and to simultaneously correct the image distortions in the CCD. Then we applied the derived relation to all sources to obtain their mean celestial coordinates based on the Intemational Celestial Reference System. For bright point sources with r 〈 17.0 mag, the accuracy of astrometric calibration could reach about 80 mas for each of the g, r, i bands, with systematic errors being less than 10 mas. But for the faint sources at the brightness limit of the survey, which was r - 19.0 mag, the accuracy can still reach 200 mas. After combining all observations, the final weighted average coordinates could reach an accuracy of less than 70 mas for bright stars. For faint stars, the rms residuals of weighted coordinates decrease to -110 mas. The final combined XSTPS-GAC coordinates show a good consistency with the Sloan Digital Sky Survey.

关键词： astrometry -- methods: data analysis -- surveys

Strong matter: Rethinking philosophically

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Science China(Physics,Mechanics & astronomy) 2018年第10期61卷 108-112页

作者： RenXin Xu School of Physics and kavli institute for astronomy and astrophysics Peking University

As we continue to comprehend the underlying nature of matter in different forms, human civilization develops further. Normal matter in everyday life, the basic stuff of which was posited by speculators even before Socrates, is dominated by electromagnetic interaction, and it could hence be simply termed electric matter. Analogously, gravitationally controlled systems （e.g., galaxies and galaxy clusters） belong to gravitational matter, and atomic nuclei to strong matter. What if normal baryonic matter is compressed intensely by grav- ity so that a huge number of nuclei would gradually merge to form a gigantic nucleus？ This is certainly of strong matter, and will be discussed succinctly here. We argue that 2- flavored （u and d quarks） nucleons constitute a microscopic nucleus, but 3-flavored （u, d and s quarks） strangeons constitute a gigantic nucleus. This has profound implications in astrophysics and cosmology, as explained in this essay.

关键词：物质哲学原子核天体物理学相互作用显微镜宇宙论重力

The evolution of stellar metallicity gradients of the Milky Way disk from LSS-GAC main sequence turn-off stars: a two-phase disk formation history?

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Research in astronomy and astrophysics 2015年第8期15卷 1209-1239页

作者： Mao-Sheng Xiang Xiao-Wei Liu Hai-Bo Yuan Yang Huang Chun Wang Juan-Juan Ren Bing-Qiu Chen Ning-Chen Sun Hua-Wei Zhang Zhi-Ying Huo Alberto Rebassa-Mansergas department of astronomy Peking University kavli institute for astronomy and astrophysics Peking University National Astronomical Observatories Chinese Academy of Sciences

Accurate measurements of stellar metallicity gradients in the radial and vertical directions of the disk and their temporal variations provide important constraints on the formation and evolution of the Milky Way disk. We use 297 042 main sequence turn-off stars selected from the LAMOST Spectroscopic Survey of the Galactic Anticenter（LSS-GAC） to determine the radial and vertical gradients of stellar metallicity,△[Fe/H]/△R and △[Fe/H]/△｜Z ｜ of the Milky Way disk in the direction of the anticenter. We determine ages of those turn-off stars by isochrone fitting and measure the temporal variations of metallicity gradients. We have carried out a detailed analysis of the selection effects resulting from the selection, observation and data reduction of LSS-GAC targets and the potential biases of a magnitude limited sample on the determinations of metallicity gradients. Our results show that the gradients, both in the radial and vertical directions, exhibit significant spatial and temporal variations. The radial gradients yielded by stars with the oldest ages（ 11 Gyr） are essentially zero at all heights from the disk midplane, while those given by younger stars are always negative. The vertical gradients deduced from stars with the oldest ages（ 11 Gyr）are negative and only show very weak variations with Galactocentric distance in the disk plane, R, while those yielded by younger stars show strong variations with *** being essentially flat at the earliest epochs of disk formation, the radial gradients steepen as age decreases, reaching a maximum（steepest） at age 7–8 Gyr, and then they flatten again. Similar temporal trends are also found for the vertical gradients. We infer that the assembly of the Milky Way disk may have experienced at least two distinct phases. The earlier phase is probably related to a slow, pressure-supported collapse of gas, when the gas settles down to the disk mainly in the vertical direction. In the later phase, there are significant radial flows

关键词： Galaxy: abundances—Galaxy: disk—Galaxy: evolution – Galaxy: formation—techniques: spectroscopic

Prompt optical emission from gamma-ray bursts with multiple timescale variability of central engine activities

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Research in astronomy and astrophysics 2014年第4期14卷 411-422页

作者： Si-Yao Xu Zhuo Li department of astronomy and kavli institute for astronomy and astrophysics Peking University Key Laboratory for the Structure and Evolution of Celestial Objects Chinese Academy of Sciences

Complete high-resolution light curves of GRB 080319B observed by Swift present an opportunity for detailed temporal analysis of prompt optical emission. With a two-component distribution of initial Lorentz factors, we simulate the dynamical process of shells being ejected from the central engine in the framework of the in- ternal shock model. The emitted radiations are decomposed into different frequency ranges for a temporal correlation analysis between the light curves in different energy bands. The resulting prompt optical and gamma-ray emissions show similar tempo- ral profiles, with both showing a superposition of a component with slow variability and a component with fast variability, except that the gamma-ray light curve is much more variable than its optical counterpart. The variability in the simulated light curves and the strong correlation with a time lag between the optical and gamma-ray emis- sions are in good agreement with observations of GRB 080319B. Our simulations suggest that the variations seen in the light curves stem from the temporal structure of the shells injected from the central engine of gamma-ray bursts. Future observations with high temporal resolution of prompt optical emission from GRBs, e.g., by UFFO- Pathfinder and SVOM-GWAC, will provide a useful tool for investigating the central engine activity.

关键词： gamma rays: bursts- GRB 080319B

The role of FAST in pulsar timing arrays

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Research in astronomy and astrophysics 2019年第2期19卷 39-54页

作者： George Hobbs Shi Dai Richard N.Manchester Ryan M.Shannon Matthew Kerr Ke-Jia Lee Ren-Xin Xu CSIRO astronomy and Space Science department of astronomy School of Physics Peking University kavli institute for astronomy and astrophysics Peking University Max-Planck-Institut für Radioastronomie

The Five-hundred-meter Aperture Spherical radio Telescope(FAST) will become one of the world-leading telescopes for pulsar timing array(PTA) research. The primary goals for PTAs are to detect(and subsequently study) ultra-low-frequency gravitational waves, to develop a pulsar-based time standard and to improve solar system planetary ephemerides. FAST will have the sensitivity to observe known pulsars with significantly improved signal-to-noise ratios and will discover a large number of currently unknown pulsars. We describe how FAST will contribute to PTA research and show that jitter-and timing-noise will be the limiting noise processes for FAST data sets. Jitter noise will limit the timing precision achievable over data spans of a few years while timing noise will limit the precision achievable over many years.

关键词： stars:pulsars gravitational waves

Galactic halo age estimated from LAMOST DR4 and Gaia DR1

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Research in astronomy and astrophysics 2019年第1期19卷 87-94页

作者： Jin-Cheng Guo Hua-Wei Zhang Yang Huang Xiao-Wei Liu Ji-Feng Liu Mao-Sheng Xiang Bing-Qiu Chen Hai-Bo Yuan Zhi-Jia Tian Zhi-Ying Huo Chun Wang department of astronomy Peking University kavli institute for astronomy and astrophysics Peking University Key Laboratory of Optical astronomy National Astronomical ObservatoriesChinese Academy of Sciences South-Western institute for astronomy Research Yunnan University department of astronomy Beijing Normal University

The stellar halo is one of the major components in the Milky Way. Research on its age can provide critical constraints on the origin of the stellar halo and further on the formation of our *** far, different approaches and samples have been used to estimate the age of the Galactic halo. In our previous paper, we carefully selected 63 field halo turn-off stars within 1 kpc from the literature using a kinematic approach, then estimated the age of the halo. In this following work, we not only update the data from LAMOST DR4 and Gaia DR1, but also try a different method to select a clean halo sample by combining the metallicity and orbital parameters. Then we compare this halo turn-off sample with the GARSTEC model in the B-V vs. metallicity plane. After Monte Carlo simulations are performed, the age is estimated to be 10.5±1.4 Gyr, highly consistent with our previous result and other studies. However,due to the limited common sources between LAMOST DR4 and Gaia DR1, the final sample in this paper is still quite small. The estimated age will be more robust with the much larger Gaia DR2.

关键词： galaxies:halos Galaxy:evolution stars:kinematics and dynamics

Mapping the emission line strengths and kinematics of supernova remnant S147 with extensive LAMOST spectroscopic observations

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Research in astronomy and astrophysics 2018年第9期18卷 63-78页

作者： Juan-Juan Ren Xiao-Wei Liu Bing-Qiu Chen Mao-Sheng Xiang Hai-Bo Yuan Yang Huang Hua-Wei Zhang Chun Wang Zhi-Jia Tian Gao-Chao Liu Hong Wu department of astronomy Peking University Key Laboratory of Optical astronomy National Astronomy Observatories Chinese Academy of Sciences kavli institute for astronomy and astrophysics Peking University South-Western institute for astronomy Research Yunnan University department of astronomy Beijing Normal University College of Science China Three Gorges University

We present extensive spectroscopic observations of supernova remnant （SNR） S 147 collected with the Large sky Area Multi-Object fiber Spectroscopic Telescope （LAMOST）. The spectra were care- fully sky-subtracted taking into account the complex filamentary structure of S 147. We have utilized all available LAMOST spectra toward S 147, including sky and stellar spectra. By measuring the prominent optical emission lines including Ha, [NII] ）λ 6584 and [S n] λλ6717, 6731, we present maps of radial velocity and line intensity ratio covering the whole nebula of S 147 with unprecedented detail. The maps spatially correlate well with the complex filamentary structure of S147. For the central 2° of S147, the radial velocity varies from - 100 to 100 krn s^-1 and has peaks between - 0 and 10 km s^-1. The intensity ratios of Hα/[S n）λλ6717,6731, [Sn] λ 6717/λ 6731 and Ha/IN Hα/λ 6584 peak at about 0.77, 1.35 and 1.48, respectively, with a scatter of 0.17, 0.19 and 0.37, respectively. The intensity ratios are consistent with the literature values. However, the range of variations of line intensity ratios estimated here, which are representative of the whole nebula, is larger than previously estimated.

关键词： ISM supernova remnants - ISM kinematics and dynamics - ISM general