arXiv subjects

Hoa Dinh Thi

Publications and source records attributed to Hoa Dinh Thi.

10 recordsLinked to original sources

arXiv2026

Vacuum birefringence and the polarized X-ray emission from a radio magnetar

Magnetars are isolated neutron stars with exceptionally strong surface fields exceeding $10^{14}$ G. Their bright X-ray emission probes physical regimes in which quantum electrodynamic (QED) influences radiation propagation. Strong magnetic fields induce polarization-dependent refractive indices in the vacuum; such vacuum birefringence (VB) remains a long-standing but unconfirmed prediction of QED. Here, we report phase- and energy-resolved polarization measurements of the radio-emitting magnetar 1E 1547.0$-$5408 obtained by coordinating X-ray and radio observations from the Imaging X-ray Polarimetry Explorer (IXPE), the Neutron Star Interior Composition ExploreR (NICER), and the Parkes/Murriyang observatory. We detect large polarization degrees (PD) in the thermally-dominant soft X-ray band, reaching phase-averaged values of $65\%$ at 2 keV before substantially decreasing between 2$-$4 keV. At certain rotational phases, the 2$-$3 keV PD rises to nearly $80\%$ while remaining high ($\gtrsim 40\%$) throughout the radio beam crossing. The phase-dependent X-ray and radio polarization angles are both consistent with the rotating vector model, suggesting that the emission geometries track the star's large-scale magnetic field. Collectively, these characteristics challenge standard surface emission models using non-refractive propagation of light to infinity. VB-governed magnetospheric propagation can naturally explain the X-ray polarization signals. Our results represent a significant advance in probing this hallmark prediction of QED, opening a new cosmic window into superstrong-field quantum physics, thereby motivating further observational and theoretical studies concentrating on this domain.

astro-ph.HE↗

arXiv2026

Magnetar Fireballs and Short Bursts: Curved Spacetime Lensing, QED Effects, Spectra, Polarization, and Impulse Responses

Magnetar short bursts (SBs) are hard X-ray transients of durations $0.01-1$ s peaking at $\sim 10-100$ keV, and are prime targets for new high-energy missions and polarimeters. The recent association of SBs with bright radio bursts in SGR 1935+2154 has broadened interest in SB physics. We present new advanced fireball models combining general relativistic light bending, polarized transport in magnetized photospheres, magnetic photon splitting attenuation, and magnetospheric vacuum birefringence. These models also have relevance to trapped fireballs in magnetar giant flare pulsating tails. We adopt confined flux tube geometries consistent with adiabatic fireballs, and anisotropic/polarized emergent intensities to produce spectra and polarizations, and energy-time Stokes impulse responses. We predict that most fireballs are highly linearly polarized, especially when vacuum birefringence is important. There is rich potential for diagnostics: coexisting direct and lensed delayed images, gaps by occultation of the neutron star surface, and Shapiro+Rømer delay with temporal caustics. These effects can imprint spin phase dependence of the spectral and polarization character of bursts. Predicted signatures depend strongly on viewing geometry, fireball configuration, and photon splitting assumptions, yielding large variance in model high-energy spectral shapes and cutoffs, and energy-dependent polarization. The models can reproduce established double-blackbody SB spectral phenomenology, and we find that the unusual April 2020 radio-associated SB from SGR 1935+2154 is broadly consistent with a footpoint close to the magnetic pole, and possibly near pole-on viewing geometry. Our models motivate reverberation-style analyses for SBs and suggest that high-quality data might constrain source geometry, burst crustal footpoints, and, potentially, neutron star masses and radii.

astro-ph.HE↗

arXiv2026

Detailed Timing, Spectral, and Polarimetric Analysis of Magnetar 1RXS J170849.0-400910

We present a broadband timing, spectral, and polarimetric study of the magnetar 1RXS~J170849.0-400910 using XMM-Newton, NuSTAR, and IXPE. The pulse morphology evolves strongly across 0.5-70 keV. Below 3 keV, the emission is dominated by a broad soft pulse with a leading shoulder that develops into a faint interpulse near 3 keV, while the pulse fraction remains $\approx$25%. The profile becomes increasingly double-peaked between 3 and 20 keV and returns to a single peak at higher energies. The pulse fraction dips to $\sim$20% near 4 keV and rises to $\sim$42% above 25 keV. The phase-averaged spectrum is well described by an absorbed blackbody plus two power-laws, with $kT=0.468\pm0.003$ keV, $Γ_{\rm soft}=2.63\pm0.04$, and $Γ_{\rm hard}=0.5\pm0.1$. Phase-resolved spectroscopy reveals distinct soft and hard pulse components. The thermal modulation is driven primarily by a factor of $\sim$5 variation in projected emitting area, whereas the soft power-law exhibits two peaks with different phase and energy evolution, suggesting distinct emission regions or mechanisms. The 10-70 keV flux is strongly anticorrelated with the soft power-law photon index, linking spectral hardening to the hard pulse. The polarization degree also varies strongly with phase and energy. In the 2-3 keV band, it is anticorrelated with the intensity profile, consistent with magnetized-atmosphere emission, whereas in the 4-8 keV band it reaches $64\pm10$% during the nonthermal power-law-dominated peak. This high polarization can be reproduced by magnetospheric quantum pair-synchrotron emission. Together, these results reveal an intricate, phase-dependent superposition of emitting regions and radiative processes whose complexity emerges only through broadband, phase-resolved spectropolarimetry.

astro-ph.HE↗

arXiv2025

Pulsed, Polarized X-ray Emission from Neutron Star Surfaces: the Effects of Vacuum Birefringence in the Magnetosphere

Intense magnetic fields in the atmospheres of neutron stars render non-trivial angular dependence of intensity and polarization of soft X-ray emission originating from their surfaces. By tracking the complex electric field vector for each photon during its atmospheric transport and propagation in general relativistic and birefringent magnetospheres, our Monte Carlo simulation, named MAGTHOMSCATT, allows for capturing the complete polarization properties, including the intricate interplay between linearity and circularity. The new inclusion in MAGTHOMSCATT of quantum electrodynamical influences on polarization in the magnetosphere is presented. We simulate the pulsed and polarized X-ray emission from the outer layers of optically thick, fully ionized atmospheres of neutron stars, with a focus on the radiation emitted from extended polar caps of magnetars, which are the most highly magnetized neutron stars. Using the recent intensity pulse profile data for the magnetar 1RXS J11708-4009, we constrain the geometric parameters, namely the angles between the magnetic axis and the observer's viewing direction relative to the spin axis, as well as the sizes of emission regions. The distributions of these parameters and the best-fit configuration are provided. In addition, we discuss the important impacts of vacuum birefringence in the magnetosphere on increasing the linear polarization degree. A comparison with the case of a weakly magnetized neutron star, RX J0822.0-4300, is also discussed. Our simulation still needs further development, particularly to incorporate the vacuum resonance effect. Nevertheless, the formalism presented here can be employed to constrain geometric parameters for various types of neutron stars.

astro-ph.HE↗

arXiv2025

Monte Carlo Simulations of Polarized Radiative Transfer in Neutron Star Atmospheres

Soft X-ray emission from neutron stars affords powerful diagnostic tools for uncovering their surface and interior properties, as well as their geometric configurations. In the atmospheres of neutron stars, the presence of magnetic fields alters the photon-electron scattering cross sections, resulting in non-trivial angular dependence of intensity and polarization of the emergent signals. This paper presents recent developments of our Monte Carlo simulation, MAGTHOMSCATT, which tracks the complex electric field vector for each photon during its transport. Our analysis encompasses the anisotropy and polarization characteristics of X-ray emission for field strengths ranging from non-magnetic to extremely magnetized regimes that are germane to magnetars. In the very low field domain, we reproduced the numerical solution to the radiative transfer equation for non-magnetic Thomson scattering, and provided analytical fits for the angular dependence of the intensity and the polarization degree. These fits can be useful for studies of millisecond pulsars and magnetic white dwarfs. By implementing a refined injection protocol, we show that, in the magnetar regime, the simulated intensity and polarization pulse profiles of emission from extended surface regions becomes invariant with respect to the ratio of photon ($ω$) and electron cyclotron ($ω_{\rm B}$) frequencies once $ω/ ω_{\rm B} \lesssim 0.01$. This circumvents the need for simulations pertinent to really high magnetic field strengths, which are inherently slower. Our approach will be employed elsewhere to model observational data to constrain neutron star geometric parameters and properties of emitting hot spots on their surfaces.

astro-ph.HE↗

arXiv2025

X-ray polarization of the magnetar 1E 1841-045

We report on IXPE and NuSTAR observations beginning forty days after the 2024 outburst onset of magnetar 1E 1841-045, marking the first IXPE observation of a magnetar in an enhanced state. Our spectropolarimetric analysis indicates that both a blackbody (BB) plus double power-law (PL) and a double blackbody plus power-law spectral model fit the phase-averaged intensity data well, with a hard PL tail ($Γ$=1.19 and 1.35, respectively) dominating above $\approx 5$ keV. For the former model, we find the soft PL (the dominant component at soft energies) exhibits a polarization degree (PD) of $\approx 30\%$ while the hard PL displays a PD of $\approx 40\%$. Similarly, the cool BB of the 2BB+PL model possesses a PD of $\approx 15\%$ and a hard PL PD of $\approx 57\%$. For both models, each component has a polarization angle (PA) compatible with celestial north. Model-independent polarization analysis supports these results, wherein the PD increases from$ \approx 15\%$ to $\approx 70\%$ in the 2-3 keV and 6-8 keV ranges, respectively, while the PA remains nearly constant. We find marginal evidence for phase-dependent variability of the polarization properties, namely a higher PD at phases coinciding with the hard X-ray pulse peak. We compare the hard X-ray PL to the expectation from resonant inverse Compton scattering (RICS) and secondary pair cascade synchrotron radiation from primary high-energy RICS photons; both present reasonable spectropolarimetric agreement with the data, albeit, the latter more naturally. We suggest that the soft PL X-ray component may originate from a Comptonized corona in the inner magnetosphere.

astro-ph.HE↗

arXiv2025

Magnetic Thomson Transport in High Opacity Domains

X-ray radiation from neutron stars manifests itself in a variety of settings. Isolated pulsars, and magnetars both exhibit quasi-thermal persistent soft X-ray emission from their surfaces. Transient magnetospheric bursts from magnetars and pulsed signals from accreting neutron stars mostly appear in harder X rays. The emission zones pertinent to these signals are all highly Thomson optically thick so that their radiation anisotropy and polarization can be modeled using sophisticated simulations of scattering transport from extended emission regions. Validation of such codes and their efficient construction is enhanced by a deep understanding of scattering transport in high opacity domains. This paper presents a new analysis of the polarized magnetic Thomson radiative transfer in the asymptotic limit of high opacity. The integro-differential equations for photon scattering transport that result from a phase matrix construction are reduced to a compact pair of equations. This pair is then solved numerically for two key parameters that describe the photon anisotropy and polarization configuration of high Thomson opacity environs. Empirical approximations for these parameters as functions of the ratio of the photon and cyclotron frequencies are presented. Implementation of these semi-analytic transport solutions as interior boundary conditions is shown to speed up scattering simulations. The solutions also enable the specification of the anisotropic radiation pressure. The analysis is directly applicable to the atmospheres of magnetars and moderate-field pulsars, and to the accretion columns of magnetized X-ray binaries, and can be adapted to address other neutron star settings.

astro-ph.HE↗

arXiv2024

Pulsed and Polarized X-ray Emission from Neutron Star Surfaces

The intense magnetic fields of neutron stars naturally lead to strong anisotropy and polarization of radiation emanating from their surfaces, both being sensitive to the hot spot position on the surface. Accordingly, pulse phase-resolved intensities and polarizations depend on the angle between the magnetic and spin axes and the observer's viewing direction. In this paper, results are presented from a Monte Carlo simulation of neutron star atmospheres that uses a complex electric field vector formalism to treat polarized radiative transfer due to magnetic Thomson scattering. General relativistic influences on the propagation of light from the stellar surface to a distant observer are taken into account. The paper outlines a range of theoretical predictions for pulse profiles at different X-ray energies, focusing on magnetars and also neutron stars of lower magnetization. By comparing these models with observed intensity and polarization pulse profiles for the magnetar 1RXS J1708-40, and the light curve for the pulsar PSR J0821-4300, constraints on the stellar geometry angles and the size of putative polar cap hot spots are obtained.

astro-ph.HE↗

arXiv2023

Confronting the nucleonic hypothesis with current neutron star observations from GW170817 and PSR J0740+6620

The nuclear matter equation of state is relatively well constrained at sub-saturation densities thanks to the knowledge from nuclear physics. However, studying its behavior at supra-saturation densities is a challenging task. Fortunately, the extraordinary progress recently made in observations of neutron stars and neutron star mergers has provided us with unique opportunities to unfold the properties of dense matter. Under the assumption that nucleons are the only constituents of neutron star cores, we perform a Bayesian inference using the so-called meta-modeling technique with a nuclear-physics-informed prior. The latest information from the GW170817 event by the LIGO-Virgo Collaboration (LVC) and from the radius measurement of the heaviest known neutron star PSR J0740+6620 by the Neutron Star Interior Composition Explorer (NICER) telescope and X-ray Multi-Mirror (XMM-Newton) are taken into account as likelihoods in the analysis. The impacts of different constraints on the equation of state as well as on the predictions of neutron star properties are discussed. The obtained posterior reveals that all the current observations are fully compatible with the nucleonic hypothesis. Strong disagreements between our results with future data can be identified as a signal for the existence of exotic degrees of freedom.

nucl-th↗

arXiv2021

The nuclear matter density functional under the nucleonic hypothesis

A Bayesian analysis of the possible behaviors of the dense matter equation of state informed through recent LIGO-Virgo as well as NICER measurements reveals that all the present observations are compatible with a fully nucleonic hypothesis for the composition of dense matter, even in the core of the most massive pulsar PSR J0740+6620. Under the hypothesis of a nucleonic composition, we extract the most general behavior of the energy per particle of symmetric matter and density dependence of the symmetry energy, compatible with the astrophysical observations as well as our present knowledge of low energy nuclear physics from effective field theory predictions and experimental nuclear mass data. These results can be used as a null hypothesis to be confronted with future constraints on dense matter to search for possible exotic degrees of freedom.

astro-ph.HE↗