arXiv subjects

Wenbin Lu

Publications and source records attributed to Wenbin Lu.

At least 145 records · Page 8Linked to original sources

arXiv2018

On the radiation mechanism of repeating fast radio bursts

Recent observations show that fast radio bursts (FRBs) are energetic but probably non-catastrophic events occurring at cosmological distances. The properties of their progenitors are largely unknown in spite of many attempts to determine them using the event rate, duration and energetics. Understanding the radiation mechanism for FRBs should provide the missing insights regarding their progenitors, which is investigated in this paper. The high brightness temperatures (>10^{35} K) of FRBs mean that the emission process must be coherent. Two general classes of coherent radiation mechanisms are considered --- maser and the antenna mechanism. We use the observed properties of the repeater FRB 121102 to constrain the plasma conditions needed for these two mechanisms. We have looked into a wide variety of maser mechanisms operating in either vacuum or plasma and find that none of them can explain the high luminosity of FRBs without invoking unrealistic or fine-tuned plasma conditions. The most favorable mechanism is antenna curvature emission by coherent charge bunches where the burst is powered by magnetic reconnection near the surface of a magnetar (B > 10^{14} G). We show that the plasma in the twisted magnetosphere of a magnetar may be clumpy due to two-stream instability. When magnetic reconnection occurs, the pre-existing density clumps may provide charge bunches for the antenna mechanism to operate. This model should be applicable to all FRBs that have multiple outbursts like FRB 121102.

astro-ph.HE↗

arXiv2018

Monte Carlo simulations of Photospheric emission in relativistic outflows

We study the spectra of photospheric emission from highly relativistic gamma-ray burst outflows using a Monte Carlo (MC) code. We consider the Comptonization of photons with a fast cooled synchrotron spectrum in a relativistic jet with photon to electron number ratio $N_γ/N_e = 10^5$. For all our simulations, we use mono-energetic protons which interact with thermalised electrons through the Coulomb interaction. The photons, electrons and protons are cooled adiabatically as the jet expands outwards. We find that the initial energy distribution of the protons and electrons do not have any appreciable effect on the photon peak energy and the power-law spectrum above the peak energy. We also find that the Coulomb interaction between the electrons and the protons does not affect the output photon spectrum significantly as the energy of the electrons is elevated only marginally. The peak energy and the spectral indices for the low and high energy power-law tails of the photon spectrum remain practically unchanged even in the presence of electron-proton coupling. Increasing the initial optical depth $τ_{in}$ results in shallower photon spectrum below the peak energy ($f_ν \propto ν^{1.1}$ for $τ_{in} = 2$ to $f_ν \propto ν^{0.3}$ for $τ_{in} = 16$) and fewer photons at the high-energy tail, although $f_ν \propto ν^{-0.5}$ above the peak energy up to $\sim 1$ MeV, independent of $τ_{in}$. The peak energy of the seed photon spectrum $E_{γ,peak}$ determines the peak energy and the shape of the output photon spectrum. Lastly, we find that our simulation results are quite sensitive to $N_γ/N_e$, for $N_{e} = 10^3$. For almost all our simulations, we obtain an output photon spectrum with power-law tail above $E_{γ,peak}$ extending up to $\sim 1$ MeV.

astro-ph.HE↗

arXiv2017

MM Algorithms for Variance Component Estimation and Selection in Logistic Linear Mixed Model

Logistic linear mixed model is widely used in experimental designs and genetic analysis with binary traits. Motivated by modern applications, we consider the case with many groups of random effects and each group corresponds to a variance component. When the number of variance components is large, fitting the logistic linear mixed model is challenging. We develop two efficient and stable minorization-maximization (MM) algorithms for the estimation of variance components based on the Laplace approximation of the logistic model. One of them leads to a simple iterative soft-thresholding algorithm for variance component selection using maximum penalized approximated likelihood. We demonstrate the variance component estimation and selection performance of our algorithms by simulation studies and a real data analysis.

stat.CO↗

arXiv2017

Efficient Estimation for Dimension Reduction with Censored Data

We propose a general index model for survival data, which generalizes many commonly used semiparametric survival models and belongs to the framework of dimension reduction. Using a combination of geometric approach in semiparametrics and martingale treatment in survival data analysis, we devise estimation procedures that are feasible and do not require covariate-independent censoring as assumed in many dimension reduction methods for censored survival data. We establish the root-$n$ consistency and asymptotic normality of the proposed estimators and derive the most efficient estimator in this class for the general index model. Numerical experiments are carried out to demonstrate the empirical performance of the proposed estimators and an application to an AIDS data further illustrates the usefulness of the work.

math.ST↗

arXiv2017

Probing motion of fast radio burst sources by timing strongly lensed repeaters

Given the possible repetitive nature of fast radio bursts (FRBs), their cosmological origin, and their high occurrence, detection of strongly lensed sources due to intervening galaxy lenses is possible with forthcoming radio surveys. We show that if multiple images of a repeating source are resolved with VLBI, using a method independent of lens modeling, accurate timing could reveal non-uniform motion, either physical or apparent, of the emission spot. This can probe the physical nature of FRBs and their surrounding environments, constraining scenarios including orbital motion around a stellar companion if FRBs require a compact star in a special system, and jet-medium interactions for which the location of the emission spot may randomly vary. The high timing precision possible for FRBs ($\sim {\rm ms}$) compared to the typical time delays between images in galaxy lensing ($\gtrsim 10\, {\rm days}$) enables the measurement of tiny fractional changes in the delays ($\sim 10^{-9}$), and hence the detection of time-delay variations induced by relative motions between the source, the lens, and the Earth. We show that uniform cosmic peculiar velocities only cause the delay time to drift linearly, and that the effect from the Earth's orbital motion can be accurately subtracted, thus enabling a search for non-trivial source motion. For a timing accuracy of $\sim 1\,$ms and a repetition rate (of detected bursts) $\sim 0.05$ per day of a single FRB source, non-uniform displacement $\gtrsim 0.1 - 1\,$AU of the emission spot perpendicular to the line of sight is detectable if repetitions are seen over a period of hundreds of days.

astro-ph.HE↗

arXiv2017

Radiative interaction between the relativistic jet and optically thick envelope in tidal disruption events

Reverberation observations have uncovered an Fe Kα fluorescence line in the tidal disruption event (TDE) Swift J1644+57 (Kara et al. 2016). The discovery paper used the lag spectrum to argue that the X-ray continuum source was located very close to the blackhole (~30 gravitational radii) and moved sub-relativistically. We reanalyze the lag spectrum, pointing out that dilution effects cause it to indicate a geometric scale an order of magnitude larger than previously inferred. If the X-ray continuum is produced by a relativistic jet, as suggested by rapid variability, high luminosity and hard spectrum, this larger scale predicts an Fe ionization state consistent with efficient Kα production. Moreover, the momentum of the jet radiation impinging on the surrounding accretion flow on this larger scale accelerates a layer of gas to speeds ~0.1-0.2c, consistent with the blueshifted line profile. Implications of our results on the global picture of jetted TDEs are discussed. A power-law γ/X-ray spectrum may be produced by external UV-optical photons being repetitively inverse-Compton scattered by cold electrons in the jet, although our model for the Kα reverberation does not depend on the jet radiation mechanism (magnetic reconnection in a Poynting jet is still a viable mechanism). The non-relativistic wind driven by jet radiation may explain the late-time radio rebrightening in Swift J1644+57. This energy injection may also cause the thermal UV-optical emission from jetted TDEs to be systematically brighter than in non-jetted ones.

astro-ph.HE↗

arXiv2017

A Massive Data Framework for M-Estimators with Cubic-Rate

The divide and conquer method is a common strategy for handling massive data. In this article, we study the divide and conquer method for cubic-rate estimators under the massive data framework. We develop a general theory for establishing the asymptotic distribution of the aggregated M-estimators using a simple average. Under certain condition on the growing rate of the number of subgroups, the resulting aggregated estimators are shown to have faster convergence rate and asymptotic normal distribution, which are more tractable in both computation and inference than the original M-estimators based on pooled data. Our theory applies to a wide class of M-estimators with cube root convergence rate, including the location estimator, maximum score estimator and value search estimator. Empirical performance via simulations also validate our theoretical findings.

math.ST↗

arXiv2017

Fast radio burst source properties and curvature radiation model

We use the observed properties of fast radio bursts (FRBs) and a number of general physical considerations to provide a broad-brush model for the physical properties of FRB sources and the radiation mechanism. We show that the magnetic field in the source region should be at least 10^{14} Gauss. This strong field is required to ensure that the electrons have sufficiently high ground state Landau energy so that particle collisions, instabilities, and strong electric and magnetic fields associated with the FRB radiation do not perturb electrons' motion in the direction transverse to the magnetic field and destroy their coherent motion; coherence is required by the high observed brightness temperature of FRB radiation. The electric field in the source region required to sustain particle motion for a wave period is estimated to be of order 10^{11} esu. These requirements suggest that FRBs are produced near the surface of magnetars perhaps via forced reconnection of magnetic fields to produce episodic, repeated, outbursts. The beaming-corrected energy release in these bursts is estimated to be ~10^{36} ergs, whereas the total energy in the magnetic field is at least ~10^{45} ergs. We provide a number of predictions for this model which can be tested by future observations. One of which is that short duration FRB-like bursts should exist at much higher frequencies, possibly up to optical.

astro-ph.HE↗

arXiv2017

Stellar disruption events support the existence of the black hole event horizon

Many black hole (BH) candidates have been discovered in X-ray binaries and in the nuclei of galaxies. The prediction of Einstein's general relativity is that BHs have an event horizon --- a one-way membrane through which particles fall into the BH but cannot exit. However, except for the very few nearby supermassive BH candidates, our telescopes are unable to resolve and provide a direct proof of the event horizon. Here, we propose a novel observation that supports the existence of event horizons around supermassive BH candidates heavier than 10^{7.5} solar masses. Instead of an event horizon, if the BH candidate has a hard surface, when a star falls onto the surface, the shocked baryonic gas will form a radiation pressure supported envelope that shines at the Eddington luminosity for an extended period of time from months to years. We show that such emission has already been ruled out by the Pan-STARRS1 3pi survey if supermassive BH candidates have a hard surface at radius larger than 1 + 10^{-4.4} times the Schwarzschild radius. Future observations by LSST should be able to improve the limit to 1 + 10^{-6}.

astro-ph.HE↗

arXiv2016

A universal energy distribution function for repeating fast radio bursts?

Assuming: fast radio bursts (FRBs) are produced by neutron stars at cosmological distances; FRB rate tracks core-collapse supernova rate; and all FRBs repeat with a universal energy distribution function (EDF) dN/dE ~ E^(-beta) with a high-end cutoff at burst energy E_max. We find that observations so far are consistent with a universal EDF with a power-law index 1.5 < beta < 2.2, high-end cutoff E_max/E_0 > 30 and normalization N_0 < 2 per day; where N_0 is the integrated rate above the reference burst energy E_0 = 1.2e39 f_r^(-1) erg (f_r is the radio emission efficiency). Implications of such an EDF are discussed.

astro-ph.HE↗

arXiv2016

Swift J1644+57: an Ideal Test Bed of Radiation Mechanisms in a Relativistic Super-Eddington Jet

Within the first 10 days after Swift discovered the jetted tidal disruption event (TDE) Sw J1644+57, simultaneous observations in the radio, near-infrared, optical, X-ray and gamma-ray bands were carried out. These multiwavelength data provide a unique opportunity to constrain the emission mechanism and make-up of a relativistic super-Eddington jet. We consider an exhaustive variety of radiation mechanisms for the generation of X-rays in this TDE, and rule out many processes such as SSC, photospheric and proton synchrotron. The infrared to gamma-ray data for Sw J1644+57 are consistent with synchrotron and external-inverse-Compton (EIC) processes provided that electrons in the jet are continuously accelerated on a time scale shorter than ~1% of the dynamical time to maintain a power-law distribution. The requirement of continuous electron acceleration points to magnetic reconnection in a Poynting flux dominated jet. The EIC process may require fine tuning to explain the observed temporal decay of the X-ray lightcurve, whereas the synchrotron process in a magnetic jet needs no fine tuning for this TDE.

astro-ph.HE↗

arXiv2016

Robust regression for optimal individualized treatment rules

Because different patients may response quite differently to the same drug or treatment, there is increasing interest in discovering individualized treatment rule. In particular, people are eager to find the optimal individualized treatment rules, which if followed by the whole patient population would lead to the "best" outcome. In this paper, we propose new estimators based on robust regression with general loss functions to estimate the optimal individualized treatment rules. The new estimators possess the following nice properties: first, they are robust against skewed, heterogeneous, heavy-tailed errors or outliers; second, they are robust against misspecification of the baseline function; third, under certain situations, the new estimator coupled with pinball loss approximately maximizes the outcome's conditional quantile instead of conditional mean, which leads to a different optimal individualized treatment rule comparing with traditional Q- and A-learning. Consistency and asymptotic normality of the proposed estimators are established. Their empirical performance is demonstrated via extensive simulation studies and an analysis of an AIDS data.

stat.ME↗

arXiv2016

External inverse-Compton emission from jetted tidal disruption events

The recent discoveries of Swift J1644+57 and J2058+05 show that tidal disruption events (TDEs) can launch relativistic jets. Super-Eddington accretion produces a strong radiation field of order Eddington luminosity. In a jetted TDE, electrons in the jet will inverse-Compton scatter the external radiation field from the accretion disk and wind. Motivated by observations of thermal optical-UV spectra in Swift J2058+05 and several other TDEs, we assume the spectrum of the external radiation field intercepted by the relativistic jet to be blackbody. Hot electrons in the jet scatter this thermal radiation and produce luminosities 10^45-10^48 erg/s in the X/gamma-ray band. This model of thermal plus inverse-Compton radiation is applied to Swift J2058+05. First, we show that the blackbody component in the optical-UV spectrum most likely has its origin in the super-Eddington wind from the disk. Then, using the observed blackbody component as the external radiation field, we show that the X-ray luminosity and spectrum are consistent with the inverse-Compton emission, under the following conditions: (1) the jet Lorentz factor is ~5-10; (2) electrons in the jet have a powerlaw distribution with minimum Lorentz factor ~1 and powerlaw index p = 2.4; (3) the wind is mildly relativistic (Lorentz factor >~1.5) and has isotropic-equivalent mass-loss rate ~5 M_sun/yr. We describe the implications for jet composition and the radius where jet energy is converted to radiation.

astro-ph.HE↗

arXiv2015

Infrared emission from tidal disruption events --- probing the pc-scale dust content around galactic nuclei

Recent UV-optical surveys have been successful in finding tidal disruption events (TDEs), in which a star is tidally disrupted by a supermassive black hole (BH). These TDEs release a huge amount of radiation energy ~ 10^51-52 erg into the circum-nuclear medium. If the medium is dusty, most of the radiation energy will be absorbed by dust grains within ~ 1 pc from the BH and re-radiated in the infrared. We calculate the dust emission lightcurve from a 1-D radiative transfer model, taking into account the time-dependent heating, cooling and sublimation of dust grains. We show that the dust emission peaks at 3-10 microns and has typical luminosities ~ 10^42-43 erg/s (with sky covering factor of dusty clouds ranging from 0.1-1). This is detectable by current generation of telescopes. In the near future, James Webb Space Telescope will be able to perform photometric and spectroscopic measurements, in which silicate or polycyclic aromatic hydrocarbon (PAH) features may be found. Observations at rest-frame wavelength > 2 microns have only been reported from two TDE candidates, SDSS J0952+2143 and Swift J1644+57. Although consistent with the dust emission from TDEs, the mid-infrared fluxes of the two events may be from other sources. Long-term monitoring is needed to draw a firm conclusion. We also point out two nearby TDE candidates (ASSASN-14ae and -14li) where the dust emission may be currently detectable. The dust infrared emission can give a snapshot of the pc-scale dust content around weakly- or non-active galactic nuclei, which is hard to probe otherwise.

astro-ph.HE↗

arXiv2015

Robust Learning for Optimal Treatment Decision with NP-Dimensionality

In order to identify important variables that are involved in making optimal treatment decision, Lu et al. (2013) proposed a penalized least squared regression framework for a fixed number of predictors, which is robust against the misspecification of the conditional mean model. Two problems arise: (i) in a world of explosively big data, effective methods are needed to handle ultra-high dimensional data set, for example, with the dimension of predictors is of the non-polynomial (NP) order of the sample size; (ii) both the propensity score and conditional mean models need to be estimated from data under NP dimensionality. In this paper, we propose a two-step estimation procedure for deriving the optimal treatment regime under NP dimensionality. In both steps, penalized regressions are employed with the non-concave penalty function, where the conditional mean model of the response given predictors may be misspecified. The asymptotic properties, such as weak oracle properties, selection consistency and oracle distributions, of the proposed estimators are investigated. In addition, we study the limiting distribution of the estimated value function for the obtained optimal treatment regime. The empirical performance of the proposed estimation method is evaluated by simulations and an application to a depression dataset from the STAR*D study.

stat.ML↗

arXiv2015

Probing Massive Stars Around Gamma-Ray Burst Progenitors

Long Gamma-Ray Bursts (GRBs) are produced by ultra-relativistic jets launched from core collapse of massive stars. Most massive stars form in binaries and/or in star clusters, which means that there may be a significant external photon field (EPF) around the GRB progenitor. We calculate the inverse-Compton scattering of EPF by the hot electrons in the GRB jet. Three possible cases of EPFs are considered: the progenitor is (I) in a massive binary system, (II) surrounded by a Wolf-Rayet-star wind, and (III) in a dense star cluster. Typical luminosities of 10^46 - 10^50 erg/s in the 1 - 100 GeV band are expected, depending on the stellar luminosity, binary separation (I), wind mass loss rate (II), stellar number density (III), etc. We calculate the lightcurve and spectrum in each case, taking fully into account the equal-arrival time surfaces and possible pair-production absorption with the prompt gamma-rays. Observations can put constraints on the existence of such EPFs (and hence on the nature of GRB progenitors) and on the radius where the jet internal dissipation process accelerates electrons.

astro-ph.HE↗

arXiv2014

Gene-level pharmacogenetic analysis on survival outcomes using gene-trait similarity regression

Gene/pathway-based methods are drawing significant attention due to their usefulness in detecting rare and common variants that affect disease susceptibility. The biological mechanism of drug responses indicates that a gene-based analysis has even greater potential in pharmacogenetics. Motivated by a study from the Vitamin Intervention for Stroke Prevention (VISP) trial, we develop a gene-trait similarity regression for survival analysis to assess the effect of a gene or pathway on time-to-event outcomes. The similarity regression has a general framework that covers a range of survival models, such as the proportional hazards model and the proportional odds model. The inference procedure developed under the proportional hazards model is robust against model misspecification. We derive the equivalence between the similarity survival regression and a random effects model, which further unifies the current variance component-based methods. We demonstrate the effectiveness of the proposed method through simulation studies. In addition, we apply the method to the VISP trial data to identify the genes that exhibit an association with the risk of a recurrent stroke. The TCN2 gene was found to be associated with the recurrent stroke risk in the low-dose arm. This gene may impact recurrent stroke risk in response to cofactor therapy.

stat.AP↗

arXiv2014

On Estimation of Optimal Treatment Regimes For Maximizing t-Year Survival Probability

A treatment regime is a deterministic function that dictates personalized treatment based on patients' individual prognostic information. There is a fast-growing interest in finding optimal treatment regimes to maximize expected long-term clinical outcomes of patients for complex diseases, such as cancer and AIDS. For many clinical studies with survival time as a primary endpoint, a main goal is to maximize patients' survival probabilities given treatments. In this article, we first propose two nonparametric estimators for survival function of patients following a given treatment regime. Then, we derive the estimation of the optimal treatment regime based on a value-based searching algorithm within a set of treatment regimes indexed by parameters. The asymptotic properties of the proposed estimators for survival probabilities under derived optimal treatment regimes are established under suitable regularity conditions. Simulations are conducted to evaluate the numerical performance of the proposed estimators under various scenarios. An application to an AIDS clinical trial data is also given to illustrate the methods.

math.ST↗