arXiv subjects

Alexander Y. Chen

Publications and source records attributed to Alexander Y. Chen.

At least 19 recordsLinked to original sources

arXiv2026

Resonant Inverse Compton Scattering and Hard X-ray Emission in Magnetar Magnetospheres

Magnetars are a subclass of neutron stars with ultra-strong surface magnetic fields. Some magnetars exhibit persistent hard X-ray emission, characterized by power-law tails with photon indices around 1--1.5, extending from ${\sim}$10 keV to several hundred keV. The leading explanation for this hard X-ray component is resonant Compton scattering, in which the thermal seed photons are upscattered by relativistic electron-positron pairs flowing along magnetic field lines in the magnetosphere. In this work, we adopt the pair outflow framework of the magnetar magnetosphere and calculate the resonant Compton scattering opacity, as well as the spectrum and polarization of the upscattered emission. We find that resonant cooling can substantially modify the magnetospheric plasma density and impose strong optical depth constraints on the hard X-ray emission regions. Under the viewing geometry inferred from IXPE, an equatorial twist near the stellar surface provides a viable configuration for the NuSTAR hard X-ray spectrum of 4U 0142+61, while a polar-twist geometry is disfavored. Joint spectral, timing, and polarimetric modeling will be essential for distinguishing between the magnetospheric scattering geometries and understanding the physical properties of the pair plasma.

astro-ph.HE↗

arXiv2026

Combining the Mass--Radius Posteriors of J0030+0451 Allowing for Unknown Model Systematics

The NASA Neutron star Interior Composition Explorer (\emph{NICER}) mission measures the X-ray pulse profiles of select millisecond pulsars and uses sophisticated pulse profile modeling (PPM) techniques to constrain their masses ($M$) and radii ($R$), in order to probe the state of matter in their interiors. One of the most studied pulsars, PSR J0030+0451, has been analyzed by multiple groups using different choices of hotspot models. The different choices of hotspot prescriptions to fit the same observational data led to different $M$--$R$ posteriors that do not completely agree with one another, resulting in a practical bottleneck for dense-matter equation-of-state (EoS) inference. In this paper, we adapt a robust Bayesian combination framework to the published $M$--$R$ posteriors of PSR J0030+0451 while allowing for unknown systematic uncertainties that might have led to the apparently divergent results. Using this technique, we combine eight existing $M$--$R$ posteriors into a single conservative and reproducible posterior that incorporates unknown model systematics across the currently available analyses and is suitable for direct use in EoS studies. The resulting constraint is $M = 1.46^{+0.09}_{-0.08}\,M_\odot$, $R = 12.69^{+0.64}_{-0.55}\,\mathrm{km}$, and compactness $C = 0.172^{+0.006}_{-0.007}$ (68\% credible interval). Incorporating this combined J0030+0451 constraint in an EoS-agnostic joint analysis with PSR~J0437--4715 and GW170817 yields $R_{1.4} = 11.98^{+0.58}_{-0.68}\,\mathrm{km}$ and $Λ_{1.4} = 320^{+216}_{-138}$. Our results provide a combined $M$--$R$ constraint for J0030+0451 and a practical framework for incorporating cross-model uncertainty into neutron star EoS inference pipelines.

astro-ph.HE↗

arXiv2026

Collisionless Accretion of Finite-Angular-Momentum Plasma onto a Spinning Black Hole

In low-luminosity active galactic nuclei like M87* and Sgr A*, the accretion disk around the central supermassive black hole is tenuous and collisionless. As a result, the usual ideal magnetohydrodynamics (MHD) approximation may not be applicable. In this Letter, we report on the first fully kinetic simulations of the accretion process where the plasma initially has finite angular momentum. The simulated accretion flow behaves remarkably similarly to the magnetically arrested disk (MAD) regime of ideal MHD, reproducing episodes of magnetic flux saturation and eruption typical of MADs. The resemblance to fluid models owes largely to kinetic instabilities, which regulate pressure anisotropy in the disk, allowing fluid terms to dominate the angular momentum transfer. In addition, by handling vacuum regions effectively, our kinetic approach probes the matter supply to the jet funnel. We observe no efficient penetration of the accreting material into this region, which suggests that a pair discharge may be required to sustain the Blandford-Znajek process.

astro-ph.HE↗

arXiv2025

Global Kinetic Simulations of Monster Shocks and Their Emission

Fast magnetosonic waves are one of the two low-frequency plasma modes that can exist in a neutron star magnetosphere. It was recently realized that these waves may become nonlinear within the magnetosphere and steepen into some of the strongest shocks in the universe. These shocks, when in the appropriate parameter regime, may emit GHz radiation in the form of precursor waves. We present the first global Particle-in-Cell simulations of the nonlinear steepening of fast magnetosonic waves in a dipolar magnetosphere, and quantitatively demonstrate the strong plasma acceleration in the upstream of these shocks. In these simulations, we observe the production of precursor waves in a finite angular range. Using analytic scaling relations, we predict the expected frequency, power, and duration of this emission. Within a reasonable range of progenitor wave parameters, these precursor waves can reproduce many aspects of FRB observations.

astro-ph.HE↗

arXiv2025

Stable Collisionless Tori Around Kerr Black Holes

In low-luminosity active galactic nuclei like M87$^\ast$ and Sgr A$^\ast$, the accretion flow in the vicinity of the black hole is in the collisionless regime, meaning that the collisional mean free path of charged particles is much larger than the dynamical length scales. To properly model the particle energization and emission from the collisionless accretion flow, a promising approach is to employ the global general-relativistic particle-in-cell simulations$\unicode{x2014}$a newly developed, fully kinetic, first-principles method. However, it has been challenging to set up an initial condition that involves collisionless gas with finite angular momentum. We present, for the first time, a class of analytic kinetic equilibria of collisionless tori around a Kerr black hole. We have successfully implemented the collisionless tori in our GPU-based GRPIC code framework Aperture, and found them to be stable for hundreds to thousands of dynamical times in 2D axisymmetric simulations when there is no initial seed magnetic field. These kinetic equilibria serve as ideal starting points for future studies of the physics of collisionless accretion and jet launching.

astro-ph.HE↗

arXiv2025

Physics motivated models of pulsar X-ray hotspots: off-center dipole configurations

Recently, it was proposed that an off-center dipole magnetic configuration, together with a non-trivial temperature profile, may be the best model to explain the X-ray light curve of PSR J0030+0451 observed by the Neutron Star Interior Composition Explorer (\emph{NICER}). Using a theoretical model for the electric current density in a force-free pulsar magnetosphere, we compute from first principles the distribution of electric current over the polar cap associated with an off-center magnetic dipole. We then use a simple prescription to compute the resulting temperature distribution, which allows us to derive the observed X-ray light curve. We investigate the role of the volumetric return current region in the polar cap and find that although it does not make a big difference in an aligned dipole case, the difference can be bigger in the case of an off-center dipole. Finally, we apply Markov Chain Monte Carlo (MCMC) fitting to the X-ray light curves of pulsars PSR J0030+0451 and PSR J0437--4715 with and without the volumetric return current, and find that our model can reasonably recover the observed X-ray light curves.

astro-ph.HE↗

arXiv2025

1D Vlasov Simulations of QED Cascades Over Pulsar Polar Caps

Recent developments in the study of pulsar radio emission revealed that the microphysics of quantum electrodynamic (QED) pair cascades at pulsar polar caps may be responsible for generating the observed coherent radio waves. However, modeling the pair cascades in the polar cap region poses significant challenges, particularly under conditions of high plasma multiplicity. Traditional Particle-in-Cell (PIC) methods often face rapidly increasing computational costs as the multiplicity grows exponentially. To address this issue, we present a new simulation code using the Vlasov method, which efficiently simulates the evolution of charged particle distribution functions in phase space without a proportional increase in computational expense at high multiplicities. We apply this code to study $e^\pm$ pair cascades in 1D, incorporating key physical processes such as curvature radiation, radiative cooling, and magnetic pair production. We study both the Ruderman-Sutherland (RS) and the Space-charge-limited Flow (SCLF) regimes, and find quasiperiodic gap formation and pair production bursts in both cases. These features produce strong electric field oscillations, potentially enabling coherent low-frequency radio emission. We construct a unified analytic model that describes the key features of the polar cap cascade, which can be used to estimate the return current heating rate that can be used to inform X-ray hotspot models. Spectral analysis shows that a significant amount of energy is carried in superluminal modes -- collective excitations that could connect to observed radio features. Our results align with previous PIC studies while offering enhanced fidelity in both dense and rarefied regions.

astro-ph.HE↗

arXiv2025

Alfvén Wave Conversion to Low Frequency Fast Magnetosonic Waves in Magnetar Magnetospheres

Rapid shear motion of magnetar crust can launch Alfvén waves into the magnetosphere. The dissipation of the Alfvén waves has been theorized to power the X-ray bursts characteristic of magnetars. However, the process by which Alfvén waves convert their energy to X-rays is unclear. Recent work has suggested that energetic fast magnetosonic (fast) waves can be produced as a byproduct of Alfvén waves propagating on curved magnetic field lines; their subsequent dissipation may power X-ray bursts. In this work, we investigate the production of fast waves by performing axisymmetric force-free simulations of Alfvén waves propagating in a dipolar magnetosphere. For Alfvén wave trains that do not completely fill the flux tube confining them, we find a fast wave dominated by a low frequency component with a wavelength defined by the bouncing time of the Alfvén waves. In contrast, when the wave train is long enough to completely fill the flux tube, and the Alfvén waves overlap significantly, the energy is quickly converted into a fast wave with a higher frequency that corresponds to twice the Alfvén wave frequency. We investigate how the energy, duration, and wavelength of the initial Alfvén wave train affect the conversion efficiency to fast waves. For modestly energetic star quakes, we see that the fast waves that are produced will become non-linear well within the magnetosphere, and we comment on the X-ray emission that one may expect from such events.

astro-ph.HE↗

arXiv2025

Physics of Pair Producing Gaps in Black Hole Magnetospheres: Two Dimensional General Relativistic Particle-in-cell Simulations

Black holes can launch powerful jets through the Blandford-Znajek process. This relies on enough plasma in the jet funnel to conduct the necessary current. However, in some low luminosity active galactic nuclei, the plasma supply near the jet base may be an issue. It has been proposed that spark gaps -- local regions with unscreened electric field -- can form in the magnetosphere, accelerating particles to initiate pair cascades, thus filling the jet funnel with plasma. In this paper, we carry out 2D general relativistic particle-in-cell (GRPIC) simulations of the gap, including self-consistent treatment of inverse Compton scattering and pair production. We observe gap dynamics that is fully consistent with our earlier 1D GRPIC simulations. We find strong dependence of the gap power on the soft photon spectrum and energy density, as well as the strength of the horizon magnetic field. We derive physically motivated scaling relations, and applying to M87, we find that the gap may be energetically viable for the observed TeV flares. For Sgr A$^*$, the energy dissipated in the gap may also be sufficient to power the X-ray flares.

astro-ph.HE↗

arXiv2025

Introducing APERTURE: A GPU-based General Relativistic Particle-in-Cell Simulation Framework

Low-luminosity Active Galactic Nuclei (AGN) are believed to be surrounded by a collisionless, highly magnetized accretion flow. As a result, Particle-in-Cell simulations are the best tools to study the immediate vicinity of the event horizons of these supermassive black holes. We present a GPU-based general relativistic particle-in-cell (GRPIC) code framework called Aperture. Aperture is developed in C++, with compute kernels written in CUDA and HIP to take advantage of the massive acceleration modern GPUs enable. The code is organized in a fully modular way, allowing easy extensions to new physics problems. In this paper, we describe in detail the particle pusher, field solver, and charge-conserving current deposition algorithms employed in Aperture, and present test cases to validate their correctness. Then, we apply the code to study spark gaps and plasma injection in black hole magnetospheres. We find that the apparent location and time-evolution of the gap depend on the observer. Our results reconcile the previous conflicting findings from 1D and 2D simulations in the literature.

astro-ph.HE↗

arXiv2024

CompactObject: An open-source Python package for full-scope neutron star equation of state inference

The CompactObject package is an open-source software framework developed to constrain the neutron star equation of state (EOS) through Bayesian statistical inference. It integrates astrophysical observational constraints from X-ray timing, gravitational wave events, and radio measurements, as well as nuclear experimental constraints derived from perturbative Quantum Chromodynamics (pQCD) and Chiral Effective Field Theory ($χ$EFT). The package supports a diverse range of EOS models, including meta-model like and several physics-motivated EOS models. It comprises three independent components: an EOS generator module that currently provides seven EOS choices, a Tolman-Oppenheimer-Volkoff (TOV) equation solver, that allows the determination of the Mass Radius and Tidal deformability as observables, and a comprehensive Bayesian inference workflow module, including a complete pipeline for implementing EOS Bayesian inference. Each component can be used independently in different scientific research contexts, such as nuclear physics and astrophysics. In addition, CompactObject is designed to work in synergy with existing software such as CompOSE, allowing the use of the CompOSE EOS database to extend the EOS options available.

astro-ph.HE↗

arXiv2024

Evaluation of several explanations of the strong X-ray polarization of the black hole X-ray binary 4U 1630-47

The Imaging X-ray Polarimetry Explorer (IXPE) observations of the X-ray binary 4U 1630-47 in the high soft state revealed high linear polarization degrees (PDs) rising from 6% at 2 keV to 10% at 8 keV. We discuss in this letter three different mechanisms that impact the polarization of the observed X-rays: the reflection of gravitationally lensed emission by the accretion disk, reprocessing of the emission in outflowing plasma, and electron and ion anisotropies in the accretion disk atmosphere. We conducted detailed raytracing studies to evaluate the impact of the reflection of strongly gravitationally lensed emission on the PDs. Although the reflected emission can produce high PDs in the high-energy tail of the thermal emission component, we do not find models that describe the PDs and are consistent with independent estimates of the source distance. We discuss the energetics of another proposed mechanism: the emission or scattering of the X-rays in mildly relativistically moving plasma outflows. We argue that these models are disfavored as they require large mechanical luminosities on the order of, or even exceeding the Eddington Luminosity. We investigated the impact of electron and ion anisotropies, but find that their impact on the observed PDs are likely negligible. We conclude with a discussion of all three effects and avenues for future research.

astro-ph.HE↗

arXiv2024

A Machine Learning Method for Monte Carlo Calculations of Radiative Processes

Radiative processes such as synchrotron radiation and Compton scattering play an important role in astrophysics. Radiative processes are fundamentally stochastic in nature, and the best tools currently used for resolving these processes computationally are Monte Carlo (MC) methods. These methods typically draw a large number of samples from a complex distribution such as the differential cross section for electron-photon scattering, and then use these samples to compute the radiation properties such as angular distribution, spectrum, and polarization. In this work we propose a machine learning (ML) technique for efficient sampling from arbitrary known probability distributions that can be used to accelerate Monte Carlo calculation of radiative processes in astrophysical scenarios. In particular, we apply our technique to inverse Compton radiation and find that our ML method can be up to an order of magnitude faster than traditional methods currently in use.

astro-ph.HE↗

arXiv2024

Alfven Wave Mode Conversion in Neutron Star Magnetospheres: A Semi-analytic Approach

We write down the force-free electrodynamics (FFE) equations in dipole coordinates, and solve for normal modes corresponding to Alfvénic perturbations in the magnetosphere of a neutron star. We show that a single Alfvén wave propagating on dipole field lines spontaneously sources a fast magnetosonic (fms) wave at the next order in the perturbation expansion, without needing 3-wave interaction. The frequency of the sourced fms wave is twice the original Alfvén wave frequency, and the wave propagates spherically outwards. The properties of the outgoing fms wave can be computed exactly using the usual devices of classical electrodynamics. We extend the calculation to the closed zone of a rotating neutron star magnetosphere, and show that the Alfvén wave also sources a spherical fms wave but at the same frequency as the primary Alfvén wave.

astro-ph.HE↗

arXiv2024

A Model for Pair Production Limit Cycles in Pulsar Magnetospheres

It was recently proposed that the electric field oscillation as a result of self-consistent $e^{\pm}$ pair production may be the source of coherent radio emission from pulsars. Direct Particle-in-Cell (PIC) simulations of this process have shown that the screening of the parallel electric field by this pair cascade manifests as a limit cycle, as the parallel electric field is recurrently induced when pairs produced in the cascade escape from the gap region. In this work, we develop a simplified time-dependent kinetic model of $e^{\pm}$ pair cascades in pulsar magnetospheres that can reproduce the limit-cycle behavior of pair production and electric field screening. This model includes the effects of a magnetospheric current, the escape of $e^{\pm}$, as well as the dynamic dependence of pair production rate on the plasma density and energy. Using this simple theoretical model, we show that the power spectrum of electric field oscillations averaged over many limit cycles is compatible with the observed pulsar radio spectrum.

astro-ph.HE↗

arXiv2023

Particle-in-cell simulations of pulsar magnetospheres: transition between electrosphere and force-free regimes

Global particle-in-cell (PIC) simulations of pulsar magnetospheres are performed with a volume, surface and pair production-based plasma injection schemes to systematically investigate the transition between electrosphere and force-free pulsar magnetospheric regimes. A new extension of the PIC code OSIRIS to model pulsar magnetospheres using a two-dimensional axisymmetric spherical grid is presented. The sub-algorithms of the code and thorough benchmarks are presented in detail, including a new first-order current deposition scheme that conserves charge to machine precision. It is shown that all plasma injection schemes produce a range of magnetospheric regimes. Active solutions can be obtained with surface and volume injection schemes when using artificially large plasma injection rates, and with pair production-based plasma injection for sufficiently large separation between kinematic and pair production energy scales.

astro-ph.HE↗

arXiv2023

Novel Constraints on Axions Produced in Pulsar Polar-Cap Cascades

Axions can be copiously produced in localized regions of neutron star magnetospheres where the ambient plasma is unable to efficiently screen the induced electric field. As these axions stream away from the neutron star they can resonantly transition into photons, generating a large broadband contribution to the neutron star's intrinsic radio flux. In this work, we develop a comprehensive end-to-end framework to model this process from the initial production of axions to the final detection of radio photons, and derive constraints on the axion-photon coupling, $g_{aγγ}$, using observations of 27 nearby pulsars. We study the modeling uncertainty in the sourced axion spectrum by comparing predictions from 2.5 dimensional particle-in-cell simulations with those derived using a semi-analytic model; these results show remarkable agreement, leading to constraints on the axion-photon coupling that typically differ by a factor of no more than $\sim 2$. The limits presented here are the strongest to date for axion masses $10^{-8} \, {\rm eV} \lesssim m_a \lesssim 10^{-5} \, {\rm eV}$, and crucially do not rely on the assumption that axions are dark matter.

hep-ph↗

arXiv2022

Propagation of a Strong Fast Magnetosonic Wave in the Magnetosphere of a Neutron Star

We study the propagation of a strong, low frequency, linearly polarized fast magnetosonic wave inside the magnetosphere of a neutron star. The relative strength $δB/B$ of the wave grows as a function of radius before it reaches the light cylinder, and what starts as a small perturbation can grow to become nonlinear before it escapes the magnetosphere. Using first-principles Particle-in-Cell (PIC) simulations, we study in detail the evolution of the wave as it becomes nonlinear. We find that an initially sinusoidal wave becomes strongly distorted as $δB/B$ approaches order unity. The wave steepens into a shock in each wavelength. The plasma particles drift into the shock and undergo coherent gyration in the rest of the wave, and subsequently become thermalized. This process quickly dissipates the energy of an FRB emitted deep within the magnetosphere of magnetar, effectively preventing GHz waves produced in the closed field line zone from escaping. This mechanism may also provide an effective way to launch shocks in the magnetosphere from kHz fast magnetosonic waves without requiring a relativistic ejecta. The resulting shock can propagate to large distances and may produce FRBs as a synchrotron maser.

astro-ph.HE↗