arXiv subjects

Zhen Pan

Publications and source records attributed to Zhen Pan.

At least 55 records · Page 3Linked to original sources

arXiv2021

Supercritical accretion of stellar-mass compact objects in active galactic nuclei

Accretion disks of active galactic nuclei (AGN) have been proposed as promising sites for producing both (stellar-mass) compact object mergers and extreme mass ratio inspirals. Along with the disk-assisted migration/evolution process, ambient gas materials inevitably accrete onto the compact objects. The description of this process is subject to significant theoretical uncertainties in previous studies. It was commonly assumed that either an Eddington accretion rate or a Bondi accretion rate (or any rate in between) takes place, although these two rates can differ from each other by several orders of magnitude. As a result, the mass and spin evolution of compact objects within AGN disks are essentially unknown. In this work, we construct a relativistic supercritical inflow-outflow model for black hole (BH) accretion. We show that the radiation efficiency of the supercritical accretion of a stellar-mass BH (sBH) is generally too low to explain the proposed electromagnetic counterpart of GW190521. Applying this model to sBHs embedded in AGN disks, we find that, although the gas inflow rates at Bondi radii of these sBHs are in general highly super-Eddington, a large fraction of inflowing gas eventually escapes as outflows so that only a small fraction accretes onto the sBH, resulting in mildly super-Eddington BH absorption in most cases. We also implement this inflow-outflow model to study the evolution of neutron stars (NS) and white dwarfs (WD) in AGN disks, taking into account corrections from star sizes and star magnetic fields. It turns out to be difficult for WDs to grow to the Chandrasekhar limit via accretion because WDs are spun up more efficiently to reach the shedding limit before the Chandrasekhar limit. For NSs the accretion-induced collapse is possible if NS magnetic fields are sufficiently strong, keeping the NS in a slow rotation state during accretion.

astro-ph.HE↗

arXiv2021

Formation Rate of Extreme Mass Ratio Inspirals in Active Galactic Nuclei

Extreme Mass Ratio Inspirals (EMRIs) are important sources for space-borne gravitational wave detectors, such as LISA (Laser Interferometer Space Antenna) and TianQin. Previous EMRI rate studies have focused on the "loss cone" scenario, where stellar-mass black holes (sBHs) are scattered into highly eccentric orbits near the central massive black hole (MBH) via multi-body interaction. In this work, we calculate the rate of EMRIs of an alternative formation channel: EMRI formation assisted by the accretion flow around accreting massive black holes. In this scenario, sBHs and stars on inclined orbits are captured by the accretion disk, and then subsequently migrate towards the MBH, under the influence of density wave generation and head wind. By solving the Fokker-Planck equation incorporating both sBH-sBH/sBH-star scatterings and sBH/star-disk interactions, we find that an accretion disk usually boosts the EMRI formation rate per individual MBH by $\mathcal O(10^1-10^3)$ compared with the canonical "loss cone" formation channel. Taking into account that the fraction of active galactic nucleus (AGNs) is $\sim \mathcal O(10^{-2}-10^{-1})$, where the MBHs are expected to be rapidly accreting, we expect EMRI formation assisted by AGN disks to be an important channel for all EMRIs observed by space-borne gravitational wave detectors. These two channels also predict distinct distributions of EMRI eccentricities and orbit inclinations with respect to the MBH spin equatorial plan, which can be tested by future gravitational wave observations.

astro-ph.HE↗

arXiv2020

Probing Crust Meltdown in Inspiraling Binary Neutron Stars

Thanks to recent measurements of tidal deformability and radius, the nuclear equation of state and structure of neutron stars are now better understood. Here, we show that through resonant tidal excitations in a binary inspiral, the neutron crust generically undergoes elastic-to-plastic transition, which leads to crust heating and eventually meltdown. This process could induce $\sim \mathcal{O}(0.1)$ phase shift in the gravitational waveform. Detecting the timing and induced phase shift of this crust meltdown will shed light on the crust structure, such as the core-crust transition density, which previous measurements are insensitive to. A direct search using GW170817 data has not found this signal, possibly due to limited signal-to-noise ratio. We predict that such signal may be observable with Advanced LIGO Plus and more likely with third-generation gravitational-wave detectors such as the Einstein Telescope and Cosmic Explorer.

astro-ph.HE↗

arXiv2020

Probing Primordial Stochastic Gravitational Wave Background with Multi-band Astrophysical Foreground Cleaning

The primordial stochastic gravitational wave background (SGWB) carries first-hand messages of early-universe physics, possibly including effects from inflation, preheating, cosmic strings, electroweak symmetry breaking, and etc. However, the astrophysical foreground from compact binaries may mask the SGWB, introducing difficulties in detecting the signal and measuring it accurately. In this paper, we propose a foreground cleaning method taking advantage of gravitational wave observations in other frequency bands. We apply this method to probing the SGWB with space-borne gravitational wave detectors, such as the laser interferometer space antenna (LISA). We find that the spectral density of the LISA-band astrophysical foreground from compact binaries (black holes and neutron stars) can be predicted with percent-level accuracy assuming 10-years' observations of third-generation GW detectors, e.g., cosmic explorer. While this multi-band method does not apply to binary white dwarfs (BWDs) which usually merger before entering the frequency band of ground-based detectors, we limit our foreground cleaning to frequency higher than $\sim5$ mHz, where all galactic BWDs can be individually resolved by LISA and the shape of the spectral density of the foreground from extragalactic BWDs can be reconstructed and/or modeled with certain uncertainties. After the foreground cleaning, LISA's sensitivity to the primordial SGWB will be substantially improved for either two LISA constellations where SGWB can be measured by cross correlating their outputs or only one constellation with three spacecrafts where SGWB can be measured by contrasting the responses of a signal channel and a null channel.

astro-ph.CO↗

arXiv2020

Probing the Growth of Massive Black Holes with Black Hole-Host Galaxy Spin Correlations

Supermassive black holes (SMBHs) are commonly found at the centers of their host galaxies, but their formation still remains an open question. In light of the tight correlation between the BH mass and the velocity dispersions of the bulge component of the host galaxy, a BH-host galaxy coevolution scenario has been established. Such description however still contains many theoretical uncertainties, including the puzzels about the formation of BH seeds at high redshifts and the growth channel fueling these seeds. In this work, we systematically analyze the signatures of different growth channels on MBH spins. We show that different growth channels can be partially distinguished with the magnitudes of MBH spins infered from extreme-mass-ratio-inspirals detected by the Laser Interferometer Space Antenna. In addition, we propose to measure the correlation between the directions of MBH spins and their host galaxy spins, which is possible for extreme mass-ratio inspirals happening in low-redshift galaxies ($z \le 0.3$). With the inclusion of spin direction correlation different formation channels shall be significantly better constrained.

astro-ph.CO↗

arXiv2020

Toward a Full MHD Jet Model of Spinning Black Holes--II: Kinematics and Application to the M87 Jet

In this paper, we investigate the magnetohydrodynamical structure of a jet powered by a spinning black hole, where electromagnetic fields and fluid motion are governed by the Grad-Shafranov equation and the Bernoulli equation, respectively. Assuming steady and axisymmetric jet structure, the global solution is uniquely determined with prescribed plasma loading into the jet and the poloidal shape of the outmost magnetic field line. We apply this model to the jet in the center of nearby radio galaxy M87, and we find it can naturally explain the slow flow acceleration and the flow velocity stratification within $10^5$ gravitational radii from the central black hole. In particular, we find the extremal black hole spin is disfavored by the flow velocity measurements, if the plasma loading to the jet is dominated by the electron/positron pair production at the jet base.

astro-ph.HE↗

arXiv2019

Variance Reduced Local SGD with Lower Communication Complexity

To accelerate the training of machine learning models, distributed stochastic gradient descent (SGD) and its variants have been widely adopted, which apply multiple workers in parallel to speed up training. Among them, Local SGD has gained much attention due to its lower communication cost. Nevertheless, when the data distribution on workers is non-identical, Local SGD requires $O(T^{\frac{3}{4}} N^{\frac{3}{4}})$ communications to maintain its \emph{linear iteration speedup} property, where $T$ is the total number of iterations and $N$ is the number of workers. In this paper, we propose Variance Reduced Local SGD (VRL-SGD) to further reduce the communication complexity. Benefiting from eliminating the dependency on the gradient variance among workers, we theoretically prove that VRL-SGD achieves a \emph{linear iteration speedup} with a lower communication complexity $O(T^{\frac{1}{2}} N^{\frac{3}{2}})$ even if workers access non-identical datasets. We conduct experiments on three machine learning tasks, and the experimental results demonstrate that VRL-SGD performs impressively better than Local SGD when the data among workers are quite diverse.

cs.LG↗

arXiv2019

Crowdfunding Dynamics Tracking: A Reinforcement Learning Approach

Recent years have witnessed the increasing interests in research of crowdfunding mechanism. In this area, dynamics tracking is a significant issue but is still under exploration. Existing studies either fit the fluctuations of time-series or employ regularization terms to constrain learned tendencies. However, few of them take into account the inherent decision-making process between investors and crowdfunding dynamics. To address the problem, in this paper, we propose a Trajectory-based Continuous Control for Crowdfunding (TC3) algorithm to predict the funding progress in crowdfunding. Specifically, actor-critic frameworks are employed to model the relationship between investors and campaigns, where all of the investors are viewed as an agent that could interact with the environment derived from the real dynamics of campaigns. Then, to further explore the in-depth implications of patterns (i.e., typical characters) in funding series, we propose to subdivide them into $\textit{fast-growing}$ and $\textit{slow-growing}$ ones. Moreover, for the purpose of switching from different kinds of patterns, the actor component of TC3 is extended with a structure of options, which comes to the TC3-Options. Finally, extensive experiments on the Indiegogo dataset not only demonstrate the effectiveness of our methods, but also validate our assumption that the entire pattern learned by TC3-Options is indeed the U-shaped one.

cs.LG↗

arXiv2019

A Dynamic and Cooperative Tracking System for Crowdfunding

Crowdfunding is an emerging finance platform for creators to fund their efforts by soliciting relatively small contributions from a large number of individuals using the Internet. Due to the unique rules, a campaign succeeds in trading only when it collects adequate funds in a given time. To prevent creators and backers from wasting time and efforts on failing campaigns, dynamically estimating the success probability of a campaign is very important. However, existing crowdfunding systems neither have the mechanism of dynamic predictive tracking, nor provide the real-time campaign status for creators and backers on the platform. To address these issues, we develop a novel system, which contains a dynamic data-driven approach to tracking the success probability and status. We demonstrate the following scenarios using our system. First, users can utilize our system to analyze the emotion of incremental reviews so as to understand backers' perspectives of the campaign in time. Meanwhile, our system visualizes the statistic number of positive and negative reviews. On this basis, our system can dynamically track the success probability of each campaign.

cs.SI↗

arXiv2019

Estimating Early Fundraising Performance of Innovations via Graph-based Market Environment Model

Well begun is half done. In the crowdfunding market, the early fundraising performance of the project is a concerned issue for both creators and platforms. However, estimating the early fundraising performance before the project published is very challenging and still under-explored. To that end, in this paper, we present a focused study on this important problem in a market modeling view. Specifically, we propose a Graph-based Market Environment model (GME) for estimating the early fundraising performance of the target project by exploiting the market environment. In addition, we discriminatively model the market competition and market evolution by designing two graph-based neural network architectures and incorporating them into the joint optimization stage. Finally, we conduct extensive experiments on the real-world crowdfunding data collected from Indiegogo.com. The experimental results clearly demonstrate the effectiveness of our proposed model for modeling and estimating the early fundraising performance of the target project.

cs.LG↗

arXiv2019

Black Hole Discharge: very-high-energy gamma rays from black hole-neutron star mergers

With mass ratio larger than $\sim 5$ (which depends on the black hole spin and the star radius), star disruption is not expected for a black hole merging with a neutron star during the final plunge phase. In the late inspiral stage, the black hole is likely charged as it cuts through the magnetic field carried by the neutron star, leaving a temporarily charged black hole after merger. The unstable charged state of the remnant black hole rapidly neutralizes by interacting with the surrounding plasma and photons, which we investigate in first principle by numerically solving a coupled set of Boltzmann equations of 1+1 form for non-spinning BH background. The resulting basic picture is as follows. Electrons and positrons are accelerated in the BH electric field, which then lose energy to surrounding soft photons via Compton scattering; more electrons and positrons will be created from pair production as the hard photons colliding with soft photons, or through the Schwinger process in strong electromagnetic fields. The cascade stops when the charged black hole accretes enough opposite charges and becomes neutralized. We find that $\sim 10\%$ (which depends on the soft photon energy and number density) of the total electric energy is carried away to infinity in a time interval $\sim 1$ ms by very-high-energy ($>50$ GeV, the low energy detection threshold of the MAGIC telescope) gamma rays whose spectrum is approximately a power law with spectral index $\sim -2.3$. We expect the discharge picture to be true for spinning charged BHs as well.

astro-ph.HE↗

arXiv2019

Forecasted constraints on modified gravity from Sunyaev Zel'dovich tomography

Observational cosmology has become an important laboratory for testing General Relativity, with searches for modified gravity forming a significant portion of the science case for existing and future surveys. In this paper, we illustrate how future measurements of the Cosmic Microwave Background (CMB) temperature and polarization anisotropies can be combined with large galaxy surveys to improve constraints on modified gravity using the technique of Sunyaev Zel'dovich (SZ) tomography. SZ tomography uses the correlations between the kinetic/polarized SZ contributions to the small-angular scale CMB and the distribution of structure measured in a galaxy redshift survey to reconstruct the remote dipole and quadrupole fields, e.g. the CMB dipole and quadrupole observed throughout the Universe. We compute the effect of a class of modifications of gravity on the remote dipole and quadrupole fields, illustrating that these observables combine a number of the desirable features of existing probes. We then perform a fisher forecast of constraints on a two-parameter class of modifications of gravity for next-generation CMB experiments and galaxy surveys. By incorporating information from the reconstructed remote dipole and quadrupole fields, we find that it is possible to improve the constraints on this model by a factor of $\sim 2$ beyond what is possible with a galaxy survey alone. We conclude that SZ tomography is a promising method for testing gravity with future cosmological datasets.

astro-ph.CO↗

arXiv2019

Towards a Full MHD Jet Model of Spinning Black Holes--I: Framework and a split monopole example

In this paper, we construct a framework for investigating magnetohydrodynamical jet structure of spinning black holes (BHs), where electromagnetic fields and fluid motion are governed by the Grad-Shafranov equation and the Bernoulli equation, respectively. Assuming steady and axisymmetric jet structure, we can self-consistently obtain electromagnetic fields, fluid energy density and velocity within the jet, given proper plasma loading and boundary conditions. Specifically, we structure the two coupled governing equations as two eigenvalue problems, and develop full numerical techniques for solving them. As an example, we explicitly solve the governing equations for the split monopole magnetic field configuration and simplified plasma loading on the stagnation surface where the poloidal fluid velocity vanishes. As expected, we find the rotation of magnetic field lines is dragged down by fluid inertia, and the fluid as a whole does not contribute to energy extraction from the central BH, i.e., the magnetic Penrose process is not working. However, if we decompose the charged fluid as two oppositely charged components, we find the magnetic Penrose process does work for one of the two components when the plasma loading is low enough.

astro-ph.HE↗

arXiv2018

Magnetosphere structure of a Kerr black hole: marginally force-free equatorial boundary condition

The role of equatorial boundary condition in the structure of a force-free black hole magnetosphere was rarely discussed, since previous studies have been focused on the field lines entering the horizon. However, recent high-accuracy force-free electrodynamics (FFE) simulations \cite{East2018} show that there are both field lines entering the horizon and field lines ending up on the equatorial current sheet within the ergosphere for asymptotic uniform field configuration. For the latter field lines, the equatorial boundary condition is well approximated being marginally force-free, i.e., $B^2-E^2\approx 0$, where $B$ and $E$ are the magnetic and electric field strength, respectively. In this paper, we revisit the uniform field solution to the Kerr BH magnetosphere structure and investigate the role of the marginally force-free equatorial boundary condition. We find this boundary condition plays an important role in shaping the BH magnetosphere in various aspects, including the shape of the light surface, the near-horizon field line configuration and the source of the Poynting flux. We also propose an algorithm for numerically solving the Grad-Shafranov equation and self-consistently imposing the marginally force-free equatorial condition. As a result, we find a good agreement between our numerical solutions and the high-accuracy FFE simulations. We also discuss the applicability of the marginally force-free boundary condition and the numerical algorithm proposed in this paper for general magnetic field configurations.

astro-ph.HE↗

arXiv2018

Signatures of Self-Interacting Dark Matter in the Matter Power Spectrum and the CMB

We consider a self-interacting dark matter model in which the massive dark photon mediating the self-interaction decays to light dark fermions to avoid over-closing the universe. We find that if the model is constrained to explain the dark matter halos inferred for spiral galaxies and galaxy clusters simultaneously, there is a strong indication that dark matter is produced asymmetrically in the early universe. It also implies the presence of dark radiation, late kinetic decoupling for dark matter, and a suppressed linear power spectrum due to dark acoustic damping. The Lyman-$α$ forest power spectrum measurements put a strong upper limit on the damping scale and the model has little room to reduce the abundances of satellite galaxies. Future observations in the matter power spectrum and the CMB, in tandem with the impact of self-interactions in galactic halos, makes it possible to measure the gauge coupling and masses of the dark sector particles even when signals in conventional dark matter searches are absent.

hep-ph↗

arXiv2018

An Improved Algorithm for Crossing Curved Light Surfaces: rapidly rotating pulsar magnetospheres in curved spacetime

The structure of force-free, steady and axisymmetric magnetosphere of a neutron star (NS) is governed by the Grad-Shafranov (GS) equation, which is a second-order differential equation but degrades to first-order on the light surface (LS). The key to numerically solving the GS equation is to enable magnetic field lines smoothly cross the LS, and crossing a straight LS in flat spacetime has been a well-studied problem. But the numerical algorithm implementation becomes complicate in the presence of a bent LS, e.g. in curved spacetime, since there is no suitable computation grid adapted to it. We propose to circumvent this grid-LS mismatch problem by introducing a new coordinate frame designed such that the LS in it is a straight line. As an application, we investigate the general relativistic (GR) effect in magnetosphere structure of rapidly rotating pulsars in detail, where the LS is bent towards the central NS. We split the GR effect into two parts, curvature and frame-dragging; measure each of them and examine their dependence on the NS mass and the angular velocity for pulsars embedded in aligned dipole and multipole magnetic fields. Qualitatively speaking, we find that the curvature effect compactifies the magnetic field lines near the NS, therefore reduces the open magnetic flux and the Poynting luminosity, while the frame-dragging effect contributes a minor part in shaping the magnetosphere structure but plays a role in enhancing the spacelike current generation.

astro-ph.HE↗

arXiv2018

Searching for Signatures of Dark matter-Dark Radiation Interaction in Observations of Large-scale Structure

In this paper, we conduct a search in the latest large-scale structure measurements for signatures of the dark matter-dark radiation interaction proposed by Buen-Abad et al. (2015). We show that prior claims of a detection of this interaction rely on a use of the SZ cluster mass function that ignores uncertainty in the mass-observable relationship. Including this uncertainty we find that the inferred level of interaction remains consistent with the data, but so does zero interaction; i.e., there is no longer a detection. We also point out that inference of the shape and amplitude of the matter power spectrum from Ly$α$ forest measurements is highly inconsistent with the predictions of the $Λ$CDM model conditioned on Planck CMB temperature, polarization, and lensing power spectra, and that the dark matter-dark radiation model can restore that consistency. We also phenomenologically generalize the model of Buen-Abad et al. (2015) to allow for interaction rates with different scalings with temperature, and find that the original scaling is preferred by the data.

astro-ph.CO↗

arXiv2018

A tale of two modes: Neutrino free-streaming in the early universe

We present updated constraints on the free-streaming nature of cosmological neutrinos from cosmic microwave background (CMB) power spectra, baryonic acoustic oscillation data, and local measurements of the Hubble constant. Specifically, we consider a Fermi-like four-fermion interaction between massless neutrinos, characterized by an effective coupling constant $ G_{\rm eff}$, and resulting in a neutrino opacity $\dotτ_ν\propto G_{\rm eff}^2 T_ν^5$. Using a conservative prior on the parameter $\log_{10}\left(G_{\rm eff} {\rm MeV}^2\right)$, we find a bimodal posterior distribution. The first of these modes is consistent with the standard $Λ$CDM cosmology and corresponds to neutrinos decoupling at redshift $z_{ν,{\rm dec}} > 1.3\times10^5$. The other mode of the posterior, dubbed the "interacting neutrino mode", corresponds to neutrino decoupling occurring within a narrow redshift window centered around $z_{ν,{\rm dec}}\sim8300$. This mode is characterized by a high value of the effective neutrino coupling constant, together with a lower value of the scalar spectral index and amplitude of fluctuations, and a higher value of the Hubble parameter. Using both a maximum likelihood analysis and the ratio of the two mode's Bayesian evidence, we find the interacting neutrino mode to be statistically disfavored compared to the standard $Λ$CDM cosmology. Interestingly, the addition of CMB polarization and direct Hubble constant measurements significantly raises the statistical significance of this secondary mode, indicating that new physics in the neutrino sector could help explain the difference between local measurements of $H_0$, and those inferred from CMB data. A robust consequence of our results is that neutrinos must be free streaming long before the epoch of matter-radiation equality.

astro-ph.CO↗