arXiv subjects

Xiang Li

Publications and source records attributed to Xiang Li.

At least 1,423 records · Page 79Linked to original sources

arXiv2015

The lightcurve of the macronova associated with the long-short burst GRB 060614

The {\it Swift}-detected GRB 060614 was a unique burst that straddles an imaginary divide between long- and short-duration gamma-ray bursts (GRBs), and its physical origin has been heavily debated over the years. Recently, a distinct very-soft F814W-band excess at $t\sim 13.6$ days after the burst was identified in a joint-analysis of VLT and HST optical afterglow data of GRB~060614, which has been interpreted as evidence for an accompanying Li-Paczynski macronova (also called a kilonova). Under the assumption that the afterglow data in the time interval of $1.7-3.0$ days after the burst are due to external forward shock emission, when this assumption is extrapolated to later times it is found that there is an excess of flux in several multi-band photometric observations. This component emerges at $\sim$4 days after the burst, and it may represent the first time that a multi-epoch/band lightcurve of a macronova has been obtained. The macronova associated with GRB 060614 peaked at $t\lesssim 4$ days after the burst, which is significantly earlier than that observed for a supernova associated with a long-duration GRB. Due to the limited data, no strong evidence for a temperature evolution is found. We derive a conservative estimate of the macronova rate of $\sim 16.3^{+16.3}_{-8.2}~{\rm Gpc^{-3}}{\rm yr^{-1}}$, implying a promising prospect for detecting the gravitational wave radiation from compact object mergers by upcoming Advanced LIGO/VIRGO/KAGRA detectors (i.e., the rate is ${\cal R}_{\rm GW} \sim 0.5^{+0.5}_{-0.25}(D/200~{\rm Mpc})^{3}~{\rm yr^{-1}}$).

astro-ph.HE↗

arXiv2015

The `excess' of primary cosmic ray electrons

With the accurate cosmic ray (CR) electron and positron spectra (denoted as $Φ_{\rm e^{-}}$ and $Φ_{\rm e^{+}}$, respectively) measured by AMS-02 collaboration, the difference between the electron and positron fluxes (i.e., $ΔΦ=Φ_{\rm e^{-}}-Φ_{\rm e^{+}}$), dominated by the propagated primary electrons, can be reliably inferred. In the standard model, the spectrum of propagated primary CR electrons at energies $\geq 30$ GeV softens with the increase of energy. The absence of any evidence for such a continuous spectral softening in $ΔΦ$ strongly suggests a significant `excess' of primary CR electrons and at energies of $100-400$ GeV the identified excess component has a flux comparable to that of the observed positron excess. Middle-age but `nearby' supernova remnants (e.g., Monogem and Geminga) are favored sources for such an excess.

astro-ph.HE↗

arXiv2015

Limitation of the Least Square Method in the Evaluation of Dimension of Fractal Brownian Motions

With the standard deviation for the logarithm of the re-scaled range $\langle |F(t+τ)-F(t)|\rangle$ of simulated fractal Brownian motions $F(t)$ given in a previous paper \cite{q14}, the method of least squares is adopted to determine the slope, $S$, and intercept, $I$, of the log$(\langle |F(t+τ)-F(t)|\rangle)$ vs $\rm{log}(τ)$ plot to investigate the limitation of this procedure. It is found that the reduced $χ^2$ of the fitting decreases with the increase of the Hurst index, $H$ (the expectation value of $S$), which may be attributed to the correlation among the re-scaled ranges. Similarly, it is found that the errors of the fitting parameters $S$ and $I$ are usually smaller than their corresponding standard deviations. These results show the limitation of using the simple least square method to determine the dimension of a fractal time series. Nevertheless, they may be used to reinterpret the fitting results of the least square method to determine the dimension of fractal Brownian motions more self-consistently. The currency exchange rate between Euro and Dollar is used as an example to demonstrate this procedure and a fractal dimension of 1.511 is obtained for spans greater than 30 transactions.

physics.data-an↗

arXiv2015

Discovery of gamma-ray emission from a strongly lobe-dominated quasar 3C 275.1

We systematically analyze the 6-year {\it Fermi}/LAT data of the lobe-dominated quasars (LDQs) in the complete LDQ sample from 3CRR survey and report the discovery of high-energy $γ$-ray emission from 3C 275.1. The $γ$-ray emission of 3C 207 is confirmed and significant variability of the lightcurve is identified. We do not find statistically significant $γ$-ray emission from other LDQs. 3C 275.1 is the known $γ$-ray quasar with the lowest core dominance parameter (i.e., $R=0.11$). We also show that both the northern radio hotspot and parsec jet models can reasonably reproduce the $γ$-ray data. The parsec jet model, however, is favored by the potential $γ$-ray variability at the timescale of months. We suggest that some dimmer $γ$-ray LDQs will be detected in the future and LDQs could contribute non-negligibly to the extragalactic $γ$-ray background.

astro-ph.HE↗

arXiv2015

19 low mass hyper-velocity star candidates from the first data release of LAMOST survey

Hyper-velocity stars are believed to be ejected out from the Galactic center through dynamical interactions between (binary) stars and the central massive black hole(s). In this paper, we report 19 low mass F/G/K type hyper-velocity star candidates from over one mil- lion stars of the first data release of the LAMOST general survey. We determine the unbound probability for each candidate using a Monte-Carlo simulation by assuming a non-Gaussian proper-motion error distribution, Gaussian heliocentric distance and radial velocity error dis- tributions. The simulation results show that all the candidates have unbound possibilities over 50% as expected, and one of them may even exceed escape velocity with over 90% probabili- ty. In addition, we compare the metallicities of our candidates with the metallicity distribution functions of the Galactic bulge, disk, halo and globular cluster, and conclude that the Galactic bulge or disk is likely the birth place for our candidates.

astro-ph.GA↗

arXiv2015

GeV excess in the Milky Way: The Role of Diffuse Galactic gamma ray Emission template

Several groups have analyzed the publicly-available Fermi-LAT data and reported a spatially extended $γ-$ray excess of around $1-3$ GeV from the region surrounding the Galactic Center that might originate from annihilation of dark matter particles with a rest mass $m_χ\sim 30-40$ GeV. In this work we examine the role of the diffuse Galactic gamma ray emission (DGE) templates played in suppressing the GeV excess. For such a purpose, we adopt in total 128 background templates that have been generated by Ackermann et al. \cite{FermiLAT:2012aa} in the study of the {Fermi-LAT} observations of the diffuse gamma ray emission considering the effects of cosmic rays and the interstellar medium. The possible GeV excess, assumed to follow the spatial distribution of the prompt gamma-rays produced in the annihilation of dark matter particles taking a generalized NFW profile with an inner slope $α=1.2$, has been analyzed in some regions of interest. The introduction of such an additional component centered at the Galactic center is found to have improved the goodness of fit to the data significantly in all background template models regardless of whether the excess spectrum is fixed or not. Our results thus suggest that the presence of a statistically significant GeV excess in the inner Galaxy is robust thought its spectrum depends on the DGE model adopted in the analysis. The possible physical origin of the GeV excess component is discussed and in the dark matter model the annihilation cross section of such particles is evaluated.

astro-ph.HE↗

arXiv2015

Maximizing the Sum Rate in Cellular Networks Using Multi-Convex Optimization

In this paper, we propose a novel algorithm to maximize the sum rate in interference-limited scenarios where each user decodes its own message with the presence of unknown interferences and noise considering the signal-to-interference-plus-noise-ratio. It is known that the problem of adapting the transmit and receive filters of the users to maximize the sum rate with a sum transmit power constraint is non-convex. Our novel approach is to formulate the sum rate maximization problem as an equivalent multi-convex optimization problem by adding two sets of auxiliary variables. An iterative algorithm which alternatingly adjusts the system variables and the auxiliary variables is proposed to solve the multi-convex optimization problem. The proposed algorithm is applied to a downlink cellular scenario consisting of several cells each of which contains a base station serving several mobile stations. We examine the two cases, with or without several half-duplex amplify-and-forward relays assisting the transmission. A sum power constraint at the base stations and a sum power constraint at the relays are assumed. Finally, we show that the proposed multi-convex formulation of the sum rate maximization problem is applicable to many other wireless systems in which the estimated data symbols are multi-affine functions of the system variables.

cs.IT↗

arXiv2014

Discovering and Predicting Temporal Patterns of WiFi-interactive Social Populations

Extensive efforts have been devoted to characterizing the rich connectivity patterns among the nodes (components) of such complex networks (systems), and in the course of development of research in this area, people have been prompted to address on a fundamental question: How does the fascinating yet complex topological features of a network affect or determine the collective behavior and performance of the networked system? While elegant attempts to address this core issue have been made, for example, from the viewpoints of synchronization, epidemics, evolutionary cooperation, and the control of complex networks, theoretically or empirically, this widely concerned key question still remains open in the newly emergent field of network science. Such fruitful advances also push the desire to understand (mobile) social networks and characterize human social populations with the interdependent collective dynamics as well as the behavioral patterns. Nowadays, a great deal of digital technologies are unobtrusively embedded into the physical world of human daily activities, which offer unparalleled opportunities to explosively digitize human physical interactions, who is contacting with whom at what time. Such powerful technologies include the Bluetooth, the active Radio Frequency Identification (RFID) technology, wireless sensors and, more close to our interest in this paper, the WiFi technology. As a snapshot of the modern society, a university is in the coverage of WiFi signals, where the WiFi system records the digital access logs of the authorized WiFi users when they access the campus wireless services. Such WiFi access records, as the indirect proxy data, work as the effective proxy of a large-scale population's social interactions.

cs.SI↗

arXiv2014

Model-dependent estimate on the connection between fast radio bursts and ultra-high energy cosmic rays

The existence of fast radio bursts (FRBs), a new type of extragalatic transients, has been established recently and quite a few models have been proposed. In this work we discuss the possible connection between the FRB sources and ultra-high energy ($>10^{18}$ eV) cosmic rays. We show that in the blitzar model and the model of merging binary neutron stars, the huge energy release of each FRB central engine together with the rather high rate of FRBs, the accelerated EeV cosmic rays may contribute significantly to the observed ones. In other FRB models including for example the merger of double white dwarfs and the energetic magnetar radio flares, no significant EeV cosmic ray is expected. We also suggest that the mergers of double neutron stars, even if they are irrelevant to FRBs, may play a non-ignorable role in producing EeV cosmic ray protons if supramassive neutron stars were formed in a good fraction of mergers and the merger rate is $\gtrsim 10^{3}~{\rm yr^{-1}~ Gpc^{-3}}$. Such a possibility will be unambiguously tested in the era of gravitational wave astronomy.

astro-ph.HE↗

arXiv2014

When reputation enforces evolutionary cooperation in unreliable MANETs

In self-organized mobile ad hoc networks (MANETs), network functions rely on cooperation of self-interested nodes, where a challenge is to enforce their mutual cooperation. In this paper, we study cooperative packet forwarding in a one-hop unreliable channel which results from loss of packets and noisy observation of transmissions. We propose an indirect reciprocity framework based on evolutionary game theory, and enforce cooperation of packet forwarding strategies in both structured and unstructured MANETs. Furthermore, we analyze the evolutionary dynamics of cooperative strategies, and derive the threshold of benefit-to-cost ratio to guarantee the convergence of cooperation. The numerical simulations verify that the proposed evolutionary game theoretic solution enforces cooperation when the benefit-to-cost ratio of the altruistic exceeds the critical condition. In addition, the network throughput performance of our proposed strategy in structured MANETs is measured, which is in close agreement with that of the full cooperative strategy.

cs.GT↗

arXiv2014

Task-group Relatedness and Generalization Bounds for Regularized Multi-task Learning

In this paper, we study the generalization performance of regularized multi-task learning (RMTL) in a vector-valued framework, where MTL is considered as a learning process for vector-valued functions. We are mainly concerned with two theoretical questions: 1) under what conditions does RMTL perform better with a smaller task sample size than STL? 2) under what conditions is RMTL generalizable and can guarantee the consistency of each task during simultaneous learning? In particular, we investigate two types of task-group relatedness: the observed discrepancy-dependence measure (ODDM) and the empirical discrepancy-dependence measure (EDDM), both of which detect the dependence between two groups of multiple related tasks (MRTs). We then introduce the Cartesian product-based uniform entropy number (CPUEN) to measure the complexities of vector-valued function classes. By applying the specific deviation and the symmetrization inequalities to the vector-valued framework, we obtain the generalization bound for RMTL, which is the upper bound of the joint probability of the event that there is at least one task with a large empirical discrepancy between the expected and empirical risks. Finally, we present a sufficient condition to guarantee the consistency of each task in the simultaneous learning process, and we discuss how task relatedness affects the generalization performance of RMTL. Our theoretical findings answer the aforementioned two questions.

cs.LG↗

arXiv2014

Fast radio bursts as a cosmic probe?

We discuss the possibility of using fast radio bursts (FRBs), if cosmological, as a viable cosmic probe. We find out that the contribution of the host galaxies to the detected dispersion measures can be inapparent for the FRBs not from galaxy centers or star forming regions. The inhomogeneity of the intergalactic medium (IGM), however, causes significant deviation of the dispersion measure from that predicted in the simplified homogeneous IGM model for individual event. Fortunately, with sufficient FRBs along different sightlines but within a very narrow redshift interval (e.g., $Δz \sim 0.05$ or $Δz \sim 0.05(1+z)$), the mean from averaging observed dispersion measures does not suffer such a problem and hence may be used as a cosmic probe. We show that in the optimistic case (e.g., tens FRBs in each $Δz$ have been measured; the most distant FRBs were at redshift $\geq 3$; the host galaxies and the FRB sources contribute little to the detected dispersion measures) and with all the uncertainties (i.e. the inhomogeneity of the IGM, the contribution and uncertainty of host galaxies as well as the evolution and error of $f_{\rm IGM}$) considered, FRBs could help constrain the equation of state of dark energy.

astro-ph.CO↗

arXiv2014

A Search for Strongly Mg-enhanced Stars from the Sloan Digital Sky Survey

Strongly Mg-enhanced stars with [Mg/Fe] $>$ 1.0 show peculiar abundance patterns and hence are of great interest for our understanding of stellar formation and chemical evolution of the Galaxy. A systematical search for strongly Mg-enhanced stars based on the low-resolution $(R\simeq2000)$ spectra of the Sloan Digital Sky Survey (SDSS) is carried out by finding the best matched synthetic spectrum to the observed one in the region of Mg I b lines around $λ$5170Å$~$via a profile matching method. The advantage of our method is that fitting parameters are refined by reproducing the [Mg/Fe] ratios of 47 stars from very precise high-resolution spectroscopic (HRS) analysis by Nissen and Schuster (2010); and these parameters are crucial to the precision and validity of the derived Mg abundances. As a further check of our method, Mg abundances are estimated with our method for member stars in four Galactic globular clusters (M92, M13, M3, M71) which cover the same metallicity range as our sample, and the results are in good agreement with those of HRS analysis in the literature. The validation of our method is also proved by the agreement of [Mg/Fe] between our values and those of HRS analysis by Aoki et al.(2013). Finally, 33 candidates of strongly Mg-enhanced stars with [Mg/Fe]$>$1.0 are selected from 14850 F and G stars. Follow-up observations will be carried out on these candidates with high-resolution spectroscopy by large telescopes in the near future, so as to check our selection procedure and to perform a precise and detailed abundance analysis and to explore the origins of these stars.

astro-ph.SR↗

arXiv2014

Object Selection under Team Context

Context-aware database has drawn increasing attention from both industry and academia recently by taking users' current situation and environment into consideration. However, most of the literature focus on individual context, overlooking the team users. In this paper, we investigate how to integrate team context into database query process to help the users' get top-ranked database tuples and make the team more competitive. We introduce naive and optimized query algorithm to select the suitable records and show that they output the same results while the latter is more computational efficient. Extensive empirical studies are conducted to evaluate the query approaches and demonstrate their effectiveness and efficiency.

cs.DB↗

arXiv2014

A 2D based Partition Strategy for Solving Ranking under Team Context (RTP)

In this paper, we propose a 2D based partition method for solving the problem of Ranking under Team Context(RTC) on datasets without a priori. We first map the data into 2D space using its minimum and maximum value among all dimensions. Then we construct window queries with consideration of current team context. Besides, during the query mapping procedure, we can pre-prune some tuples which are not top ranked ones. This pre-classified step will defer processing those tuples and can save cost while providing solutions for the problem. Experiments show that our algorithm performs well especially on large datasets with correctness.

cs.DB↗

arXiv2014

Power of individuals -- Controlling centrality of temporal networks

Temporal networks are such networks where nodes and interactions may appear and disappear at various time scales. With the evidence of ubiquity of temporal networks in our economy, nature and society, it's urgent and significant to focus on structural controllability of temporal networks, which nowadays is still an untouched topic. We develop graphic tools to study the structural controllability of temporal networks, identifying the intrinsic mechanism of the ability of individuals in controlling a dynamic and large-scale temporal network. Classifying temporal trees of a temporal network into different types, we give (both upper and lower) analytical bounds of the controlling centrality, which are verified by numerical simulations of both artificial and empirical temporal networks. We find that the scale-free distribution of node's controlling centrality is virtually independent of the time scale and types of datasets, meaning the inherent heterogeneity and robustness of the controlling centrality of temporal networks.

cs.SI↗

arXiv2013

Knowledge-Aided STAP Using Low Rank and Geometry Properties

This paper presents knowledge-aided space-time adaptive processing (KA-STAP) algorithms that exploit the low-rank dominant clutter and the array geometry properties (LRGP) for airborne radar applications. The core idea is to exploit the fact that the clutter subspace is only determined by the space-time steering vectors, {red}{where the Gram-Schmidt orthogonalization approach is employed to compute the clutter subspace. Specifically, for a side-looking uniformly spaced linear array, the} algorithm firstly selects a group of linearly independent space-time steering vectors using LRGP that can represent the clutter subspace. By performing the Gram-Schmidt orthogonalization procedure, the orthogonal bases of the clutter subspace are obtained, followed by two approaches to compute the STAP filter weights. To overcome the performance degradation caused by the non-ideal effects, a KA-STAP algorithm that combines the covariance matrix taper (CMT) is proposed. For practical applications, a reduced-dimension version of the proposed KA-STAP algorithm is also developed. The simulation results illustrate the effectiveness of our proposed algorithms, and show that the proposed algorithms converge rapidly and provide a SINR improvement over existing methods when using a very small number of snapshots.

cs.IT↗

arXiv2013

Constraining the distribution of dark matter in inner galaxy with indirect detection signal: The case of tentative 130 gev γ-ray line

The dark matter distribution in the very inner region of our Galaxy is still in debate. In the N-body simulations a cuspy dark matter halo density profile is favored. Several dissipative baryonic processes however are found to be able to significantly flatten dark matter distribution and a cored dark matter halo density profile is possible. The baryons dominate the gravitational potential in the inner Galaxy, hence a direct constrain on the abundance of the dark matter particles is rather challenging. Recently, a few groups have identified a tentative 130 GeV line signal in the Galactic center, which could be interpreted as the signal of the dark matter annihilation. With current 130 GeV line data and adopting the generalized Navarro-Frenk-White profile of the dark matter halo, for local dark matter density ρ_0=0.4 GeV cm^{-3} and r_s=20 kpc we obtain a 95% confidence level lower (upper) limit on the inner slope of dark matter density distribution α= 1.06 (the cross section of dark matter annihilation into gamma-rays <σv >_{χχ-> γγ}= 1.3\times 10^{-27} cm^3 s^{-1}). Such a slope is consistent with the results of some N-body simulations, and if the signal is due to dark matter, suggests that baryonic processes may be unimportant.

astro-ph.HE↗