arXiv subjects

En-Wei Liang

Publications and source records attributed to En-Wei Liang.

At least 199 records · Page 11Linked to original sources

arXiv2018

Examining High Energy Radiation Mechanisms of Knots and Hotspots in Active Galactic Nucleus Jets

We compile the radio-optical-X-ray spectral energy distributions (SEDs) of 65 knots and 29 hotspots in 41 active galactic nucleus jets to examine their high energy radiation mechanisms. Their SEDs can be fitted with the single-zone leptonic models, except for the hotspot of Pictor A and six knots of 3C 273. The X-ray emission of one hotspot and 22 knots is well explained as synchrotron radiations under the equipartition condition; they usually have lower X-ray and radio luminosities than the others, which may be due to a lower beaming factor. An inverse Compton (IC) process is involved for explaining the X-ray emission of the other SEDs. Without considering the equipartition condition, their X-ray emission can be attributed to the synchrotron-self-Compton (SSC) process, but the derived jet power (P_jet) are not correlated with L_k and most of them are larger than L_k with more than three orders of magnitude, where L_k is the jet kinetic power estimated with their radio emission. Under the equipartition condition, the X-ray emission is well interpreted with the IC process to the cosmic microwave background photons (IC/CMB). In this scenario, the derived P_jet of knots and hotspots are correlated with and comparable to L_k. These results suggest that the IC/CMB model may be the promising interpretation of their X-ray emission. In addition, a tentative knot-hotspot sequence in the synchrotron peak-energy--peak-luminosity plane is observed, similar to the blazar sequence, which may be attributed to their different cooling mechanisms of electrons.

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arXiv2018

First Electromagnetic Pulse Associated with a Gravitational-Wave Event: Profile, Duration, and Delay

We study the first electromagnetic pulse after the gravitational wave chirp signal, focusing on the profile and duration. It is found that the light curve, especially the steep decay (SD) phase, can be very different by adopting different viewing angle $θ_{\rm view}$ on the jet shell. For an on-axis jet with a power-law radiation spectrum, the observed flux in the SD is proportional to $t_{\rm{obs}}^{-2-β}$ with $β$ being the spectral index and $t_{\rm{obs}}$ being the observer time. Here, $t_{\rm{obs}}=0$ is set at the observed time of the jet ejected from the central engine. The SD may become steep by increasing $θ_{\rm view}$. We also study the bolometric luminosity $L$ from a jet shell with a non-power-law radiation spectrum. For an on-axis jet, $L{\propto}t_{\rm{obs}}^{-3}$ is found in the SD. However, the SD is steeper than $L{\propto}t_{\rm{obs}}^{-3}$ for the radiation from an off-axis jet. The higher value of $θ_{\rm view}$ is, the steeper of SD would be. Then, we suggest that the SD phase can be used to discriminate an off-axis jet from an on-axis jet. The reason for above behaviors is discussed. In addition, we find that the duration of first electromagnetic pulse is close to its peak time, especially for $θ_{\rm{view}}\sim20^\circ$. This result is consistent with that found in GW~170817/GRB~170817A. Thus, the jet corresponding to the prompt emission of GRB~170817A should be immediately ejected after the merger. Our results also reveal that the duration of the first electromagnetic pulse can provide the information of the time to search gravitational waves.

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arXiv2017

Flux and Spectral Variation Characteristics of 3C 454.3 at the GeV Band

We analyze the long-term lightcurve of 3C 454.3 observed with Fermi/LAT and investigate its relation to the flux in the radio, optical, and X-ray bands. By fitting the 1-day binned GeV lightcurve with multiple Gaussian function (MGF), we propose that the typical variability timescale in the GeV band is 1--10 days. The GeV flux variation is accompanied by the spectral variation characterized as flux-tracking, i.e., "harder when brighter". The GeV flux is correlated with the optical and X-ray fluxes, and a weak correlation between gamma-ray flux and radio flux is also observed. The gamma-ray flux is not correlated with the optical linear polarization degree for the global lightcurves, but they show a correlation for the lightcurves before MJD 56000. The power density spectrum of the global lightcurve shows an obvious turnover at ~7.7 days, which may indicate a typical variability timescale of 3C 454.3 in the gamma-ray band. This is also consistent with the derived timescales by fitting the global lightcurve with MGF. The spectral evolution and an increase of the optical linear polarization degree along with the increase of the gamma-ray flux may indicate that the radiation particles are accelerated and the magnetic field is ordered by the shock processes during the outbursts. In addition, the nature of 3C 454.3 may be consistent with the self-organized criticality system, similar to Sagittarius A, and thus the outbursts are from plasmoid ejections driven by magnetic reconnection. This may further support the idea that the jet radiation regions are magnetized.

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arXiv2017

External Shock in a Multi-Bursting Gamma-ray Burst: Energy Injection Phase induced by the Later Launched Ejecta

Central engine of gamma-ray bursts (GRBs) may be intermittent and launch several episodes of ejecta separated by a long quiescent interval. In this scenario, an external shock is formed due to the propagation of the first launched ejecta into the circum-burst medium and the later launched ejecta may interact with the external shock at later period. Owing to the internal dissipation, the later launched ejecta may be observed at a later time ($t_{\rm{jet}}$). In this paper, we study the relation of $t_{\rm{b}}$ and $t_{\rm{jet}}$, where $t_{\rm{b}}$ is the collision time of the later launched ejecta with the formed external shock. It is found that the relation of $t_{\rm{b}}$ and $t_{\rm{jet}}$ depends on the bulk Lorentz factor ($Γ_{\rm{jet}}$) of the later launched ejecta and the density ($ρ$) of the circum-burst medium. If the value of $Γ_{\rm{jet}}$ or $ρ$ is low, the $t_{\rm{b}}$ would be significantly larger than $t_{\rm{jet}}$. However, the $t_{\rm{b}}\sim t_{\rm{jet}}$ can be found if the value of $Γ_{\rm{jet}}$ or $ρ$ is significantly large. Our results can explain the large lag of the optical emission relative to the $γ$-ray/X-ray emission in GRBs, e.g., GRB~111209A. For GRBs with a precursor, our results suggest that the energy injection into the external shock and thus more than one external-reverse shock may appear in the main prompt emission phase. According to our model, we estimate the Lorentz factor of the second launched ejecta in GRB~160625B.

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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

Hyperaccreting Black Hole as Gamma-Ray Burst Central Engine. II. Temporal evolution of central engine parameters during Prompt and Afterglow Phases

A hyperaccreting stellar-mass black hole has been proposed as the candidate central engine of gamma-ray bursts (GRBs). The rich observations of GRBs by \textit{Fermi} and \textit{Swift} make it possible to constrain the central engine model by comparing the model predications against data. This paper is dedicated to studying the temporal evolution of central engine parameters for both prompt emission and afterglow phases. We consider two jet launching mechanisms, i.e., $ν\barν$ annihilations and the Blandford-Znajek (BZ) processe, and obtain analytical solutions to these two models. We then investigate the black hole central engine parameters, such as the jet power, the dimensionless entropy $η$, and the central engine parameter $μ_0=η(1+σ_0)$ (where $σ_0$ is the initial magnetization of the engine) at the base of the jet. The black hole may be spun up by accretion, or spun down by the BZ process, leaving imprints in GRB lightcurves. Usually, a BZ jet is more powerful and is likely responsible for the late time central engine activities. However, an initially non-spinning black hole central engine may first launch a thermal "fireball" via neutrino annihilations, and then launch a Poynting-flux-dominated jet via the BZ process. Multiple flares, giant bumps, and plateaus in GRB afterglows can be well produced as the result of late time accretion onto the black hole.

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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

Signature of a New-Born Black Hole from the Collapse of a Supra-massive Millisecond Magnetar

The X-ray plateau followed by a steep decay ("internal plateau") has been observed in both long and short gamma-ray burst (GRBs), implying a millisecond magnetar operating in some GRBs. The sharp decay at the end of plateau, marking the abrupt cessation of the magnetar central engine, has been considered as the collapse of a supra-massive magnetar to a black hole (BH) when it spins down. If "internal plateau" is indeed the evidence of a magnetar central engine, a natural expectation is a signature from the new-born BH in some candidates. In this work, we find that GRB 070110 is a particular case, which shows a small X-ray bump following its "internal plateau". We interpret the plateau with a spin-down supra-massive magnetar and the X-ray bump with a fall-back BH accretion. This indicates that the new-born BH is likely active in some GRBs. Therefore, GRB 070110-like events may provide a further support to the magnetar central engine model and enable us to investigate the properties of the magnetar as well as the new-born BH.

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arXiv2017

Multiple Wavelength Variability and Quasi-Periodic Oscillation of PMN J0948+0022

We present a comprehensive analysis of multiple wavelength observational data of the first GeV-selected narrow-line Seyfert 1 galaxy PMN J0948+0022. We derive its lightcurves in the γ-ray and X-ray bands from the data observed with Fermi/LAT and Swift/XRT, and make the optical and radio lightcurves by collecting the data from the literature. These lightcurves show significant flux variations. With the LAT data we show that this source is analogue to typical flat spectrum radio quasars in the $L_γ-Γ_γ$ plane, where $L_γ$ and $Γ_γ$ are the luminosity and spectral index in the LAT energy band. The gamma-ray flux is correlated with the V-band flux with a lag of ~44 days, and a moderate quasi-periodic oscillation (QPO) with a periodicity of ~490 days observed in the LAT lightcurve. A similar QPO signature is also found in the V-band lightcurve. The gamma-ray flux is not correlated with the radio flux in 15 GHz, and no similar QPO signature is found in a confidence level of 95%. Possible mechanisms of the QPO are discussed. We propose that gravitational wave observations in the future may clarify the current plausible models for the QPO.

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arXiv2017

Hearing the birth of the lightest intermediate mass black holes with the second generation gravitational wave detectors

Intermediate mass black holes (IMBHs) with a mass between $10^{2}$ and $10^{5}$ times that of the sun, which bridges the {mass gap between the} stellar-mass black holes and the supermassive black holes, are crucial in understanding the evolution of the black holes. Although they are widely believed to exist, decisive evidence has long been absent. Motivated by the successful detection of massive stellar-mass black holes by advanced LIGO, through the gravitational wave radiation during the binary merger, in this work we investigate the prospect of detecting/identifying the lightest IMBHs (LIMBHs; the black holes $\gtrsim 100M_\odot$) with the second generation gravitational wave detectors. In general, the chance of hearing the birth of the LIMBHs is significantly higher than that to identify pre-merger IMBHs. The other formation channel of LIMBHs, where stars with huge mass/low-metallicity directly collapse, is likely "silent", so the merger-driven birth of the LIMBHs may be the only "observable" scenario in the near future. Moreover, the prospect of establishing the presence of (lightest) intermediate mass black holes in the O3 run and beyond of advanced LIGO is found quite promising, implying that such an instrument could make another breakthrough on astronomy in the near future. The joining of other detectors like advanced Virgo would only increase the detection rate.

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arXiv2017

Comparisons of Jet Properties between GeV Radio Galaxies and Blazars

We compile a sample of spectral energy distribution (SED) of 12 GeV radio galaxies (RGs), including eight FR I RGs and four FR II RGs. These SEDs can be represented with the one-zone leptonic model. No significant unification as expected in the unification model is found for the derived jet parameters between FR I RGs and BL Lacertae objects (BL Lacs) and between FR II RGs and flat spectrum radio quasars (FSRQs). However, on average FR I RGs have the larger gamma_b (break Lorentz factor of electrons) and lower B (magnetic field strength) than FR II RGs, analogous to the differences between BL Lacs and FSRQs. The derived Doppler factors (delta) of RGs are on average smaller than that of balzars, which is consistent with the unification model that RGs are the misaligned parent populations of blazars with smaller delta. On the basis of jet parameters from SED fits, we calculate their jet powers and the powers carried by each component, and compare their jet compositions and radiation efficiencies with blazars. Most of the RG jets may be dominated by particles, like BL Lacs, not FSRQs. However, the jets of RGs with higher radiation efficiencies tend to have higher jet magnetization. A strong anticorrelation between synchrotron peak frequency and jet power is observed for the GeV RGs and blazars in both the observer and co-moving frames, indicating that the "sequence" behavior among blazars, together with the GeV RGs, may be dominated by the jet power intrinsically.

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arXiv2017

Steep Decay Phase Shaped by the Curvature Effect. II. Spectral Evolution

We derive a simple analytical formula to describe the evolution of spectral index $β$ in the steep decay phase shaped by the curvature effect with assumption that the spectral parameters and Lorentz factor of jet shell is the same for different latitude. Here, the value of $β$ is estimated in 0.3$-$10keV energy band. For a spherical thin shell with a cutoff power law (CPL) intrinsic radiation spectrum, the spectral evolution can be read as a linear function of observer time. For the situation with Band function intrinsic radiation spectrum, the spectral evolution may be complex. If the observed break energy of radiation spectrum is larger than 10keV, the spectral evolution is the same as that shaped by jet shells with a CPL spectrum. If the observed break energy is less than 0.3keV, the value of $β$ would be a constant. Others, the spectral evolution can be approximated as a logarithmal function of the observer time in generally.

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arXiv2017

Steep Decay Phase Shaped by the Curvature Effect. I. Flux Evolution

The curvature effect may be responsible for the steep decay phase observed in gamma-ray bursts. For testing the curvature effect with observations, the zero time point $t_0$ adopted to plot observer time and flux on a logarithmic scale should be appropriately selected. In practice, however, the true $t_0$ cannot be directly constrained from the data. Then, we move $t_0$ to a certain time in the steep decay phase, which can be easily identified. In this situation, we derive an analytical formula to describe the flux evolution of the steep decay phase. The analytical formula is read as $F_ν\propto (1+\tilde t_{\rm obs}/{\tilde t_c})^{-α}$ with $α(\tilde{t}_{\rm obs})=2+{\int_{0}^{\log (1+\tilde{t}_{\rm obs}/{\tilde{t}_c})} {β(τ)d[\log(1+τ/{\tilde{t}_c})]}}/{\log (1 + {\tilde t}_{\rm obs}/{{\tilde t}_c})}$, where $F_ν$ is the flux observed at frequency $ν$, $\tilde t_{\rm obs}$ is the observer time by setting zero time point $t_0$ at a certain time in the steep decay phase, $β$ is the spectral index estimated around $ν$, and ${\tilde t}_c$ is the decay timescale of the phase with $\tilde{t}_{\rm obs}{\geqslant}0$. We test the analytical formula with the data from numerical calculations. It is found that the analytical formula presents a well estimation about the evolution of flux shaped by the curvature effect. Our analytical formula can be used to confront the curvature effect with observations and estimate the decay timescale of the steep decay phase.

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arXiv2017

Evaluating Optical Classification for {\em Fermi} Blazar Candidates with a Statistical method using Broadband Spectral Indices

We aim to test if a blazar candidate of uncertain-type (BCU) in the third Fermi active galactic nuclei catalog (3LAC) can be potentially classified as a BL Lac object or a flat spectrum radio quasar (FSRQ) by performing a statistical analysis of its broadband spectral properties. We find that 34% of the radio-selected BCUs (583 BCUs) are BL Lac-like and 20% of them are FSRQ-like, which maybe within 90% level of confidence. Similarly, 77.3% of the X-ray selected BCUs (176 BCUs) are evaluated as BL Lac-like and 6.8% of them may be FSRQ-like sources. And 88.7% of the BL Lac-like BCUs that have synchrotron peak frequencies available are high synchrotron peaked BL Lacs in the X-ray selected BCUs. The percentages are accordingly 62% and 7.3% in the sample of 124 optical-selected BCUs. The high ratio of source numbers of the BL Lac-like to the FSRQ-like BCUs in the X-ray and optically selected BCU samples is due to the selection effect. Examining the consistency between our evaluation and spectroscopic identification case by case with a sample of 78 radio-selected BCUs, it is found that the statistical analysis and its resulting classifications agree with the results of the optical follow-up spectroscopic observations. Our observation campaign for high-|ρ_{s}| BCUs , i.e., |ρ_{s}|>0.8, selected with our method is ongoing.

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arXiv2016

The History of GRB Outflows: Ejection Lorentz Factor and Radiation Location of X-Ray Flares

We present time-resolved spectral analysis of the steep decay segments of 29 bright X-ray flares of gamma-ray bursts (GRBs) observed with the Swift/X-ray telescope, and model their lightcurves and spectral index evolution behaviors with the curvature effect model. Our results show that the observed rapid flux decay and strong spectral index evolution with time can be well fit with this model, and the derived characteristic timescales ($t_c$) are in the range of $33\sim 264$ seconds. Using an empirical relation between the peak luminosity and the Lorentz factor derived from the prompt gamma-rays, we estimate the Lorentz factors of the flares ($Γ_{\rm X}$). We obtain $Γ_{\rm X}=17\sim 87$ with a median value of $52$, which is smaller than the initial Lorentz factors of prompt gamma-ray fireballs. With the derived $t_c$ and $Γ_{\rm X}$, we constrain the radiating regions of 13 X-ray flares, yielding $R_{\rm X}=(0.2\sim 1.1)\times 10^{16}$ cm, which are smaller than the radii of the afterglow fireballs at the peak times of the flares. A long evolution feature from prompt gamma-ray phase to the X-ray epoch is found by incorporating our results with a sample of GRBs whose initial Lorentz factors are available in literatures, i.e., $Γ\propto [t_{p}/(1+z)]^{-0.69\pm 0.06}$. These results may shed lights on the long term evolution of GRB central engines.

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arXiv2016

Lorentz factor - Beaming Corrected Energy/Luminosity Correlations and GRB Central Engine Models

We work on a GRB sample whose initial Lorentz factors ($Γ_0$) are constrained with the afterglow onset method and the jet opening angles ($θ_{\rm j}$) are determined by the jet break time. We confirm the $Γ_0$ - $E_{γ,\rm iso}$ correlation by Liang et al. (2010), and the $Γ_0$ - $L_{γ,\rm iso}$ correlation by L{ü} et al. (2012). Furthermore, we find correlations between $Γ_0$ and the beaming corrected $γ$-ray energy ($E_γ$) and mean $γ$-ray luminosity ($L_γ$). By also including the kinetic energy of the afterglow, we find rough correlations (with larger scatter) between $Γ_0$ and the total ($γ$-ray plus kinetic) energy and the total mean luminosity, both for isotropic values and beaming corrected values: these correlations allow us to test the data with GRB central engine models. Limiting our sample to the GRBs that likely have a black hole central engine, we compare the data with theoretical predictions of two types of jet launching mechanisms from BHs, i.e. the non-magnetized $ν\barν$ - annihilation mechanism, and the strongly magnetized Blandford-Znajek (BZ) mechanism. We find that the data are more consistent with the latter mechanism, and discuss the implications of our findings for GRB jet composition.

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arXiv2016

Magnetar central engine and possible gravitational wave emission of nearby short GRB 160821B

GRB 160821B is a short gamma-ray burst (GRB) at redshift $z=0.16$, with a duration less than 1 second and without detection of any "extended emission" up to more than 100 seconds in both {\em Swift}/BAT and {\em Fermi}/GBM bands. An X-ray plateau with a sharp drop 180 seconds after the BAT trigger was observed with {\em Swift}/XRT. No supernova or kilo-nova signature was detected. Assuming the central engine of this SGRB is a recently born supra-massive magnetar, we can explain the SGRB as jet radiation and its X-ray plateau as the internal energy dissipation of the pulsar wind as it spins down. We constrain its surface magnetic field as $B_{\rm p}<3.12\times 10^{16}$ G and initial spin period as $P_0< 8.5\times 10^{-3}$ seconds. Its equation of state is consistent with the GM1 model with $M_{\rm TOV} \sim 2.37 M_\odot$ and ellipticity $ε<0.07$. Its gravitational wave (GW) radiation may be detectable with the future Einstein Telescope, but is much weaker than the current detectability limit of advanced-LIGO. The GW radiation of such an event would be detectable by advanced-LIGO if it occurred at a distance of 100 Mpc ($z=0.023$).

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arXiv2016

GRB Observational Properties

We summarize basic observational properties of gamma-ray bursts (GRBs), including prompt emission properties, afterglow properties, and classification schemes. We also briefly comment on the current physical understanding of these properties.

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