arXiv subjects

Lin Lan

Publications and source records attributed to Lin Lan.

36 records · Page 2Linked to original sources

arXiv2020

Lorentz Invariance Violation Limits from the Spectral Lag Transition of GRB~190114C

The spectral lags of gamma-ray bursts (GRBs) have been viewed as the most promising probes of the possible violations of Lorentz invariance (LIV). However, these constraints usually depend on the assumption of the unknown intrinsic time lag in different energy bands and the use of a single highest-energy photon. A new approach to test the LIV effects has been proposed by directly fitting the spectral lag behavior of a GRB with a well-defined transition from positive lags to negative lags. This method simultaneously provides a reasonable formulation of the intrinsic time lag and robust lower limits on the quantum-gravity energy scales ($E_{\rm QG}$). In this work, we perform a global fitting to the spectral lag data of GRB~190114C by considering the possible LIV effects based on a Bayesian approach. We then derive limits on $E_{\rm QG}$ and the coefficients of the Standard Model Extension. The Bayes factors output in our analysis shows a very strong evidence for the spectral-lag transition in GRB~190114C. Our constraints on a variety of isotropic and anisotropic coefficients for LIV are somewhat weaker than existing bounds, but they can be viewed as comparatively robust and have the promise to complement existing LIV constraints. The observations of GRBs with higher-energy emissions and higher temporal resolutions will contribute to a better formulation of the intrinsic time lag and more rigorous LIV constraints in the dispersive photon sector.

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arXiv2020

Node Classification on Graphs with Few-Shot Novel Labels via Meta Transformed Network Embedding

We study the problem of node classification on graphs with few-shot novel labels, which has two distinctive properties: (1) There are novel labels to emerge in the graph; (2) The novel labels have only a few representative nodes for training a classifier. The study of this problem is instructive and corresponds to many applications such as recommendations for newly formed groups with only a few users in online social networks. To cope with this problem, we propose a novel Meta Transformed Network Embedding framework (MetaTNE), which consists of three modules: (1) A \emph{structural module} provides each node a latent representation according to the graph structure. (2) A \emph{meta-learning module} captures the relationships between the graph structure and the node labels as prior knowledge in a meta-learning manner. Additionally, we introduce an \emph{embedding transformation function} that remedies the deficiency of the straightforward use of meta-learning. Inherently, the meta-learned prior knowledge can be used to facilitate the learning of few-shot novel labels. (3) An \emph{optimization module} employs a simple yet effective scheduling strategy to train the above two modules with a balance between graph structure learning and meta-learning. Experiments on four real-world datasets show that MetaTNE brings a huge improvement over the state-of-the-art methods.

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arXiv2020

The electromagnetic and gravitational-wave radiations of X-ray transient CDF-S XT2

Binary neutron star (NS) mergers may result in remnants of supra-massive or even stable NS, which have been supported indirectly by observed X-ray plateau of some gamma-ray bursts (GRBs) afterglow. Recently, Xue et al. (2019) discovered a X-ray transient CDF-S XT2 that is powered by a magnetar from merger of double NS via X-ray plateau and following stepper phase. However, the decay slope after the plateau emission is a little bit larger than the theoretical value of spin-down in electromagnetic (EM) dominated by losing its rotation energy. In this paper, we assume that the feature of X-ray emission is caused by a supra-massive magnetar central engine for surviving thousands of seconds to collapse black hole. Within this scenario, we present the comparisons of the X-ray plateau luminosity, break time, and the parameters of magnetar between CDF-S XT2 and other short GRBs with internal plateau samples. By adopting the collapse time to constrain the equation of state (EOS), we find that three EOSs (GM1, DD2, and DDME2) are consistent with the observational data. On the other hand, if the most released rotation energy of magnetar is dominated by GW radiation, we also constrain the upper limit of ellipticity of NS for given EOS, and it is range in $[0.32-1.3]\times 10^{-3}$. Its GW signal can not be detected by aLIGO or even for more sensitive Einstein Telescope in the future.

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arXiv2020

Evidence for gravitational-wave dominated emission in the central engine of short GRB 200219A

GRB 200219A is a short gamma-ray burst (GRB) with an extended emission (EE) lasting $\sim 90$s. By analyzing data observed with the {\em Swift}/BAT and {\em Fermi}/GBM, we find that a cutoff power-law model can adequately fit the spectra of the initial short pulse with $\rm E_{p}=1387^{+232}_{-134}$ keV. More interestingly, together with the EE component and early X-ray data, it exhibits plateau emission smoothly connected with a $\sim t^{-1}$ segment and followed by an extremely steep decay. The short GRB composed of those three segments is unique in the {\em Swift} era and is very difficult to explain with the standard internal/external shock model of a black hole central engine, but could be consistent with the prediction of a magnetar central engine from the merger of an NS binary. We suggest that the plateau emission followed by a $\sim t^{-1}$ decay phase is powered by the spin-down of a millisecond magnetar, which loses its rotation energy via GW quadrupole radiation. Then, the abrupt drop decay is caused by the magnetar collapsing into a black hole before switching to EM-dominated emission. This is the first short GRB for which the X-ray emission has such an intriguing feature powered by a magnetar via GW-dominated radiation. If this is the case, one can estimate the physical parameters of a magnetar, the GW signal powered by a magnetar and the merger-nova emission are also discussed.

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arXiv2020

The Second Plateau in X-ray Afterglow Providing Additional Evidence for Rapidly Spinning Magnetars as the GRB Central Engine

Evidence for the central engine of gamma-ray bursts (GRBs) has been collected in the Neil Gehrels \emph{Swift} data. For instance, some GRBs show an internal X-ray plateau followed by very steep decay, which is difficult to be interpreted within the framework of a black hole (BH) central engine, but are consistent within a rapidly spinning magnetar engine picture. The very steep decay at the end of the plateau suggests a sudden cessation of the central engine, which is explained as the collapse of a supra-massive magnetar into a black hole when it spins down. Here we propose that some additional evidence, such as a second X-ray plateau feature would show up, if the fall-back accretion could activate the newborn BH and sufficient energy could be transferred from the newborn BH to the GRB blast wave. With a systematic data analysis for all long GRBs, we find three candidates in \emph{Swift} sample, i.e., GRBs 070802, 090111, and 120213A, whose X-ray afterglow light curves contain two plateaus, with the first one being an internal plateau. We find that in a fairly loose and reasonable parameter space, the second X-ray plateau data for all 3 GRBs could be well interpreted with our proposed model. Future observations are likely to discover more similar events, which could offer more information of the properties of the magnetar as well as the newborn BH.

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arXiv2020

Constraining the nuclear equation of state via gravitational-wave radiation of short gamma-ray burst remnants

The observed internal plateau of X-ray emission in some short GRBs suggests the formation of a remnant supra-massive magnetar following a double neutron star (NS) merger. In this paper, we assume that the rotational energy is lost mainly via gravitational wave (GW) radiation instead of magnetic dipole (MD) radiation, and present further constraints on the NS nuclear equation of state (EoS) via mass quadrupole deformation and r-mode fluid oscillations of the magnetar. We present two short GRBs with measured redshifts, 101219A and 160821B, whose X-ray light curves exhibit an internal plateau. This suggests that a supra-massive NS may survive as the central engine. By considering twelve NS EoSs, within the mass quadrupole deformation scenario we find that the GM1, DD2, and DDME2 models give an $M_{\rm p}$ band falling within the 2$σ$ region of the proto-magnetar mass distribution for $\varepsilon=0.01$. This is consistent with the constraints from the MD radiation dominated model of rotational energy loss. However, for an r-mode fluid oscillation model with $α=0.1$ the data suggest that the NS EOS is close to the Shen and APR models, which is obviously different from the MD radiation dominated and mass quadrupole deformation cases.

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arXiv2020

The properties of prompt emission in short GRBs with extended emission observed by {\em Fermi}/GBM

Short GRBs with extended emission (EE) that are composed initially of a short-hard spike and followed by a long-lasting EE, are thought to be classified as a subsection of short GRBs. The narrow energy band available during the {\em Swift} era combined with a lack of spectral information prevented discovery of the intrinsic properties of those events. In this paper, we performed a systematic search of short GRBs with EE by using all available {\em Fermi}/GBM data. The search identified 26 GBM-detected short GRBs with EE that are similar to GRB 060614 observed by {\em Swift}/BAT. We focus on investigating the spectral and temporal properties for both the hard spike and the EE components of all 26 GRBs, and explore differences and possible correlations between them. We find that while the peak energy ($E_{\rm p}$) of the hard spikes is a little bit harder than that of the EE, but their fluences are comparable. The harder $E_{\rm p}$ seems to correspond to a larger fluence and peak flux with a large scatter for both the hard spike and EE components. Moreover, the $E_{\rm p}$ of both the hard spikes and EE are compared to other short GRBs. Finally, we also compare the properties of GRB 170817A with those short GRBs with EE and find no significant statistical differences between them. We find that GRB 170817A has the lowest $E_{\rm p}$, likely because it was off-axis.

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arXiv2019

Precursors in Short Gamma-ray Bursts as a Possible Probe of Progenitors

We extract 18 candidate short gamma-ray bursts (SGRBs) with precursors from 660 SGRBs observed by {\em Fermi} and {\em Swift} satellites, and carry out a comprehensive analysis on their temporal and spectral features. We obtain the following results: (1) For a large fraction of candidates, the main burst durations are longer than their precursor durations, comparable to their quiescent times from the end of precursors to the beginning of their main bursts. (2) The average flux of precursors tends to increase as their main bursts brighten. (3) As seen from the distributions of hardness ratio and spectral fitting, the precursors are slightly spectrally softer with respect to the main bursts. Moreover, a significant portion of precursors and all main bursts favor a non-thermal spectrum. (4) The precursors might be a probe of the progenitor properties of SGRBs such as the magnetic field strength and the crustal equation of state if they arise from some processes before mergers of binary compact objects rather than post-merger processes.

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arXiv2019

The shallow decay segment of GRB X-ray afterglow revisited

Based on the early-year observations from Neil Gehrels \emph{Swift} Observatory, \cite{liang07} performed a systematic analysis for the shallow decay component of gamma-ray bursts (GRBs) X-ray afterglow, in order to explore its physical origin. Here we revisit the analysis with an updated sample (with Swift/XRT GRBs between February 2004 and July 2017). We find that with a larger sample, 1) the distributions of the characteristic properties of the shallow decay phase (e.g. $t_{b}$ , $S_{X}$, $Γ_{X,1}$, and $α_{X,1}$) still accords with normal or lognormal distribution; 2) $Γ_{X,1}$ and $Γ_γ$ still show no correlation, but the tentative correlations of durations, energy fluences, and isotropic energies between the gamma-ray and X-ray phases still exist; 3) for most GRBs, there is no significant spectral evolution between the shallow decay segment and its follow-up segment, and the latter is usually consistent with the external-shock models; 4) assuming that the central engine has a power-law luminosity release history as $L(t)=L_{0}(\frac{t}{t_{0}})^{-q}$, we find that the value $q$ is mainly distributed between -0.5 and 0.5, with an average value of 0.16$\pm$ 0.12; 5) the tentative correlation between $E_{\rm{iso},X}$ and $t'_{b}$ disappears, so that the global 3-parameter correlation ($E_{\rm{iso},X}-E'_{p}-t'_{b}$) becomes less significant; 6) the anti-correlation between $L_{X}$ and $t'_{b}$ and the three-parameter correlation ($E_{\rm{iso},γ}-L_{X}-t_{b}$) indeed exist with a high confidence level. Overall, our results are generally consistent with \cite{liang07}, confirming their suggestion that the shallow decay segment in most bursts is consistent with an external forward shock origin, probably due to a continuous energy injection from a long-lived central engine.

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arXiv2019

Meta Reinforcement Learning with Task Embedding and Shared Policy

Despite significant progress, deep reinforcement learning (RL) suffers from data-inefficiency and limited generalization. Recent efforts apply meta-learning to learn a meta-learner from a set of RL tasks such that a novel but related task could be solved quickly. Though specific in some ways, different tasks in meta-RL are generally similar at a high level. However, most meta-RL methods do not explicitly and adequately model the specific and shared information among different tasks, which limits their ability to learn training tasks and to generalize to novel tasks. In this paper, we propose to capture the shared information on the one hand and meta-learn how to quickly abstract the specific information about a task on the other hand. Methodologically, we train an SGD meta-learner to quickly optimize a task encoder for each task, which generates a task embedding based on past experience. Meanwhile, we learn a policy which is shared across all tasks and conditioned on task embeddings. Empirical results on four simulated tasks demonstrate that our method has better learning capacity on both training and novel tasks and attains up to 3 to 4 times higher returns compared to baselines.

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arXiv2019

Diagnosing the remnants of binary neutron star merger from GW170817/GRB170817A event

The event GW170817/GRB 170817A, discovered via the successful joint observation of its gravitational wave radiation and its multi-wavelength electromagnetic counterparts, was the first definite "smoking-gun" from the merger of two neutron stars (NSs). However, the remnant of the merger remains unknown. Piro et al. recently claimed that a low-significance X-ray variability in GRB 170817A. By systematically comparing the properties of variability in the afterglow of GRB 170817A and X-ray flares in GRB afterglows, we find that this X-ray variability seems to share similar statistical correlations with X-ray flares in GRB afterglows. We further investigate several possible merger product scenarios to see whether they can produce the observed X-ray variability in GRB 170817A. The first scenario invokes a stable magnetar as the central engine producing the later X-ray variability via differential rotation or fall-back accretion onto the NS. The second scenario invokes a black hole as the central engine with a fall-back accretion process. The final scenario is a central engine with a long-lived supra-massive NS. We find that the first two scenarios have difficulty producing the later X-ray variability, which requires either an impractical NS magnetic field or an extraordinarily large stellar envelope and an extremely long accretion timescale. However, the third scenario seems to be consistent with observations, and the later X-ray variability can be produced by the magnetosphere which is expelled following the collapse of the NS with a $B_p\in(3.6, 13.5)\times10^{13}$ G.

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arXiv2019

Constraining the Braking Index and Energy Partition of Magnetar spindown with {\em Swift}/XRT data

The long-lasting X-ray plateau emission in long gamma-ray bursts (GRBs) shows observational evidence for ongoing energy injection, which may be from magnetar spindown due to energy released via either magnetic dipole (MD) or gravitational wave (GW) radiation. In this paper, by systematically analyzing the {\em Swift}/XRT light curves detected before 2018 July, we find 45 light curves with a measured redshift that monotonically decay as a smooth broken power law. By assuming that the central engines of these GRBs are newly born magnetars, we measure the braking index $n$ of putative millisecond magnetars, due to MD and GW radiations. The inferred braking indices are not close to 3 or 5, but range between them with a normal distribution ($n_{\rm c}=4.02\pm 0.11$). We define a dimensionless parameter $\Re$, which is the ratio between the MD and GW components, and find that the energy released via magnetar spindown in most GRBs of our sample is dominated by GW radiation for $P_0=3$ ms and $ε=0.005$ and 0.01. On the other hand, we find that $\Re$ and the braking index $n$ seem to be anticorrelated within a large systematic error at $t=0$, but depend on the values of the parameters $P_0$ and $ε$. These results suggest that the contribution of GW radiation cannot be ignored, and that a larger braking index leads to GWs dominating the energy released during magnetar spindown if indeed magnetars are operating in some long GRBs.

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arXiv2018

Electromagnetic Emission from newly-born Magnetar Spin-Down by Gravitational-Wave and Magnetic Dipole Radiations

A newly-born magnetar is thought to be central engine of some long gamma-ray bursts (GRBs). We investigate the evolution of the electromagnetic (EM) emission from the magnetic dipole (MD) radiation wind injected by spin-down of a newly-born magnetar via both quadrupole gravitational-wave (GW) and MD radiations. We show that the EM luminosity evolves as $L_{\rm em}\propto (1+t/τ_c)^α$, and $α$ is $-1$ and $-2$ in the GW and MD radiation dominated scenarios, respectively. Transition from the GW to MD radiation dominated epoch may show up as a smooth break with slope changing from $-1$ to $-2$. If the magnetar collapses to a black hole before $τ_c$, the MD radiation should be shut down, then the EM light curve should be a plateau followed by a sharp drop. The expected generic light curve in this paradigm is consistent with the canonical X-ray light curve of {\em Swift} long GRBs. The X-ray emission of several long GRBs are identified and interpreted as magnetar spin-down via GW or MD, as well as constrain the physical parameters of magnetar. The combination of MD emission and GRB afterglows may make the diversity of the observed X-ray light curves. This may interpret the observed chromatic behaviors of the X-ray and optical afterglow light curves and the extremely low detection rate of a jet-like break in the X-ray afterglow light curves of long GRBs.

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arXiv2018

Characteristics of Two-episode Emission Patterns in {\em Fermi} Long Gamma-Ray Bursts

Two-episode emission components separated by quiescent gaps in the prompt emission of gamma-ray bursts (GRBs) have been observed in the {\em Swift} era, but there is a lack of spectral information due to the narrow energy band of the {\em Swift}/Burst Alert Telescope. In this paper, a systematic analysis of the spectral and temporal properties of the prompt emission of 101 {\em Fermi}/Gamma-ray Burst Monitor detected long GRBs show the existence of two-episode emission components in the light curves, with quiescent times of up to hundreds of seconds. We focus on investigating the differences of those two emission episodes. We find that the light curves of the two emission components exhibit different behavior, e.g., a soft emission component that either precedes or follows the main prompt emission or that the intensity of the two emission episodes are comparable with each other. No statistically significant correlation in the duration of the two emission episodes can be claimed. We define a new parameter $\varepsilon$ as the ratio of the peak flux of the first and second emission episodes and find that a higher $\varepsilon$ corresponds to a larger fluence. The preferred spectral model in our analysis is a cutoff power-law model for most GRBs. The distribution of $E_p$ for episodes I and II range from tens of keV to 1000 keV with a lognormal fit and there are no significant differences between them. Moreover, we do not find significant relationships between $\varepsilon$ and $E_p$ for the two emission episodes. Those results suggest that these two-episode emission components likely share the same physical origin.

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arXiv2018

Gamma-Ray Burst/Supernova Associations: Energy partition and the case of a magnetar central engine

The favored progenitor model for Gamma-ray Bursts (GRBs) with Supernova (SN) association is the core collapse of massive stars. One possible outcome of such a collapse is a rapidly spinning, strongly magnetized neutron star ("magnetar"). We systematically analyze the multi-wavelength data of GRB/SN associations detected by several instruments before 2017 June. Twenty GRB/SN systems have been confirmed via direct spectroscopic evidence or a clear light curve bump, as well as some spectroscopic evidence resembling a GRB-SN. We derive/collect the basic physical parameters of the GRBs and the SNe, and look for correlations among these parameters. We find that the peak brightness, $\rm ^{56}Ni$ mass, and explosion energy of SNe associated with GRBs are statistically higher than other Type Ib/c SNe. A statistically significant relation between the peak energy of GRBs and the peak brightness of their associated SNe is confirmed. No significant correlations are found between the GRB energies (either isotropic or beaming-corrected) and the supernova energy. We investigate the energy partition within these systems and find that the beaming-corrected GRB energy of most systems is smaller than the SN energy, with less than 30\% of the total energy distributed in the relativistic jet. The total energy of the systems is typically smaller than the maximum available energy of a millisecond magnetar ($2\times 10^{52}$ erg), especially if aspherical SN explosions are considered. The data are consistent with-though not a proof of-the hypothesis that most, but not all, GRB/SN systems are powered by millisecond magnetars.

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arXiv2017

{\em Fermi}/GBM Short Gamma-ray Burst Catalog and Case Study for GRB 170817A/GW 170817

Motivating by the discovery of association between GW 170817 and sGRB 170817A, we present a comprehensive analysis for sGRBs observed with Fermi/GBM in 9 operation years and study the properties of sGRB 170817A -like events. We derive a catalog of 275 typical sGRBs and 48 sGRB 170817A-like weak events from the GBM data of 2217 GRBs. We visibly identify two patterns of their light curve, single episode (Pattern I, 61\% of the SGRBs) and multiple episodes (Pattern II, 39\% of the SGRBs). Their duration distribution shows a tentative bimodal feature. Their spectra can be fitted with a cutoff power-law model, except for 4 sGRBs, and the spectral indices normally distribute at $Γ=0.69\pm 0.40$. Their $E_p$ values show a tentative bimodal distribution with peaks at 145 keV and 500 keV. No correlation among $T_{90}$, $E_p$, and $Γ$ is found. GRB 170817A is a soft, weak sGRB with $ E_{p}=124\pm 106$ keV, $L_{\rm iso}=(5.67\pm4.65)\times10^{46}\rm ~erg~s^{-1}$, and $E_{\rm iso}=(3.23\pm2.65)\times10^{46}\rm ~erg$. It follows the $E_{\rm iso}-E_{\rm p}$ relation of typical short GRBs. Its lightcurve is of Pattern II. Two lightcurve patterns, together with the potential two components in the $E_{\rm p}$ and $T_{90}$ distributions, we suspect that the current sample may include two distinct types of sGRBs from different progenitors. sGRB 170817A-like events may be from NS-NS mergers and those sGRBs with a Pattern I lightcurve may be from another distinct type of compact binary.

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arXiv2017

Extremely bright GRB 160625B with multi-episodes emission: Evidences for long-term ejecta evolution

GRB 160625B is an extremely bright GRB with three distinct emission episodes. By analyzing its data observed with the GBM and LAT on board the {\em Fermi} mission, we find that a multi-color black body (mBB) model can be used to fit the spectra of initial short episode (Episode I) very well within the hypothesis of photosphere emission of a fireball model. The time-resolved spectra of its main episode (Episode II), which was detected with both GBM and LAT after a long quiet stage ($\sim 180$ seconds) of the initial episode, can be fitted with a model composing of an mBB component plus a cutoff power-law (CPL) component. This GRB was detected again in the GBM and LAT bands with a long extended emission (Episode III) after a quiet period of $\sim 300$ seconds. The spectrum of Episode III is adequately fitted with a CPL plus a single power-law models, and no mBB component is required. These features may imply that the emission of three episodes are dominated by distinct physics process, i.e., Episode I is possible from cocoon emission surrounding the relativistic jet, Episode II may be from photosphere emission and internal shock of relativistic jet, and Episode III is contributed by internal and external shocks of relativistic jet. On the other hand, both X-ray and optical afterglows are consistent with standard external shocks model.

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arXiv2017

The peculiar GRB 110731A: Lorentz factor, jet composition, central engine, and progenitor

The jet compositions, central engines, and progenitors of gamma-ray bursts (GRBs) remain open questions in GRB physics. Applying broadband observations, including GRB prompt emission and afterglow properties derived from {\em Fermi} and {\em Swift} data, as well as from Keck host-galaxy observations, we address these questions for the peculiar, bright GRB 110731A. By using the pair-opacity method, we derive $Γ_{0}>190$ during the prompt emission phase. Alternatively, we derive $Γ_{0} \approx 580$ and $Γ_{0} \approx 154$ by invoking the early-afterglow phase within the homogeneous density and wind cases, respectively. On the other hand, nondetection of a thermal component in the spectra suggests that the prompt emission is likely powered by dissipation of a Poynting-flux-dominated jet leading to synchrotron radiation in an optically thin region. The nondetection of a jet break in the X-ray and optical bands allows us to place a lower limit on the jet opening angle $θ_{\rm j}>5.5^{\circ}$. Within a millisecond magnetar central engine scenario, we derive the period $P_{0}$ and polar magnetic field strength $B_{\rm p}$, which have extreme (but still allowed) values. The moderately short observed duration (7.3\,s) and relatively large redshift ($z=2.83$) places the burst as a "rest-frame short" GRB, so the progenitor of the burst is subject to debate. Its relatively large $f_{{\rm eff}, z}$ parameter (ratio of the 1\,s peak flux of a pseudo-GRB and the background flux) and a large physical offset from a potential host galaxy suggest that the progenitor of GRB 110731A may be a compact-star merger.

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