arXiv subjects

Xuan Fang

Publications and source records attributed to Xuan Fang.

At least 19 recordsLinked to original sources

arXiv2026

A Two-regime Khintchine Inequality and an Improved Bound on the Degree-1 Fourier Weight for Linear Threshold Functions

The Khintchine inequality provides a lower bound on the expected absolute value of a weighted sum of independent Rademacher random variables. In the classical setting, when the weight vector has unit norm, this lower bound is a constant, with equality attained only for a simple family of extremal configurations. A refined version due to De, Diakonikolas, and Servedio (2013) -- referred to as the \emph{linear Khintchine inequality} -- strengthens this by establishing a lower bound that depends linearly on the distance of the weight vector from the extremal set. In this paper, we present a refined analysis of this dependence on the weight vector. Our results reveal a phase transition in the rate of improvement: when the dimension exceeds six, the lower bound undergoes an abrupt change as the weight vector deviates from the minimizer. Additionally, we improve the slope constant in linear Khintchine inequality. As a consequence, we establish an improved lower bound on the degree-1 Fourier weight for linear threshold functions $\mathbf{W}^{\leq 1}[\mathrm{LTF}] \geq 0.53317$, marking progress towards a conjecture of O'Donnell.

math.PR↗

arXiv2026

Early Near-Infrared Excess and Rapid Disk-Corona Evolution in the Tidal Disruption Event 2024aepd

We present multi-wavelength observations of the tidal disruption event (TDE) 2024aepd, spanning primarily the first $\sim$300 days after discovery. The X-ray spectrum is initially dominated by a thermal disk component accompanied by a hard excess. From $\sim$178 days onward, the spectrum becomes power-law dominated and subsequently hardens, indicating the rapid emergence and strengthening of a hot corona. A prominent near-infrared (NIR) excess is detected as early as $\sim40$ days. Its nearly flat power-law spectrum strongly deviates from the Rayleigh-Jeans tail of the UV-optical blackbody. Although a conventional dust-echo origin cannot be completely ruled out, free-free emission from a reprocessing photospheric envelope provides a more plausible explanation. Moreover, the UV-optical-to-NIR break shifts to higher frequencies as the density-profile index remains nearly constant, implying evolving reprocessing conditions within a broadly unchanged density structure. Together with AT2019azh and TDE 2025abcr, TDE 2024aepd is the third TDE reported to exhibit an early-time NIR excess. A larger sample with early-time NIR coverage is needed to determine whether such excesses are common among TDEs.

astro-ph.HE↗

arXiv2026

Bolometric correction factor and radiative efficiency for the super-Eddington accretion flow in tidal disruption events

The estimate of the bolometric luminosity and the radiative efficiency are two key aspects for understanding the properties of the accretion flow around a supermassive black hole (BH). In this paper, we focus on the estimate of the bolometric luminosity and the radiative efficiency of the early super-Eddington accretion flow in tidal disruption events (TDEs). Specifically, we first perform radiation hydrodynamic simulations of super-Eddington accretion flow in TDE environment, and then calculate the corresponding emergent spectra with the method of post processing for the simulation data. Based on the emergent spectra, we calculate the isotropic-equivalent X-ray bolometric correction factor $k_\mathrm{bol}$ and the radiative efficiency $\eta$ of the super-Eddington accretion flow. We find that both $k_\mathrm{bol}$ and $\eta$ are BH mass and viewing-angle dependent. $k_\mathrm{bol}$ is in the range of about a few tens to a few thousands, and $\eta$ is in the range of $\sim 10^{-3}-10^{-1}$ for BH mass in the range of $10^{6-7}M_\odot$ and the viewing angle in the range of $0^{\rm o}-90^{\rm o}$. Finally, we apply the derived $k_\mathrm{bol}$ and $\eta$ to some specific TDEs to estimate the accreted mass during an event, which can significantly alleviate the so-called missing energy problem in TDEs.

astro-ph.HE↗

arXiv2026

High-Fidelity Reconstruction of Charge Boundary Layers and Sharp Interfaces in Electro-Thermal-Convective Flows via Residual-Attention PINNs

Accurate reconstruction of localized extreme structures remains a critical bottleneck in the physics-informed modeling of electro-thermal-convective flows. Although conventional physics-informed neural networks effectively capture smooth global dynamics, they frequently suffer from numerical diffusion and distortion when attempting to resolve sharp charge boundary layers or abrupt multiphase interfaces. To address these limitations, we propose a Residual-Attention Physics-Informed Neural Network (RA-PINN) that embeds gated attention modulation within a residual feature framework to adaptively enhance local sensitivity to steep physical gradients. The proposed architecture is rigorously evaluated against standard and recurrent network baselines using canonical electrohydrodynamic scenarios, encompassing near-electrode exponential boundary layers and sharply concentrated charge fields. Quantitative analyses demonstrate that the RA-PINN significantly reduces localized errors and faithfully preserves critical interface topologies without compromising the global consistency dictated by the coupled governing equations. Ultimately, this methodology establishes a highly robust predictive framework for resolving complex interfacial and boundary layer phenomena in advanced fluid dynamics applications.

physics.flu-dyn↗

arXiv2026

A Residual-Attention Physics-Informed Neural Network for Irregular Interfaces and Multi-Peak Transport Fields

In complex engineering systems such as electro-thermal-fluid coupling, rapid and accurate prediction of multi-physics fields is essential for advanced applications like digital twins and real-time condition monitoring. Traditional numerical methods often suffer from high computational latency, whereas standard Physics-Informed Neural Networks (PINNs) frequently fail to capture critical local features, such as irregular interfaces, localized high-gradient regions, and multi-peak transport structures. To address these limitations and provide high-fidelity intelligent predictions for engineering decision-making, this paper proposes a Residual-Attention Physics-Informed Neural Network (RA-PINN) as a powerful surrogate modeling engine. The proposed method incorporates residual learning and attention enhancement into the network backbone to improve the representation of oblique transition structures, narrow charge layers, and distributed hotspots while strictly preserving global field consistency. To evaluate its effectiveness as an intelligent prediction framework, three representative benchmark cases are constructed, including an oblique asymmetric interface, a bipolar high-gradient charge layer, and a multi-peak Gaussian charge migration field. Under unified training settings, the proposed RA-PINN is systematically compared with a standard pure PINN and an LSTM-PINN in terms of average error, local maximum error, structural similarity, and convergence behavior. The results show that RA-PINN consistently achieves the best overall performance across all benchmark cases, demonstrating its tremendous potential as a highly reliable core inference engine for the condition monitoring and digital twin modeling of complex multi-physics engineering systems.

physics.comp-ph↗

arXiv2025

A Morpho-kinematic Study of Galactic High-ADF PNe Based on the VLT/UVES Deep Spectroscopy

We report detailed analyses of deep, high-resolution spectra of three Galactic planetary nebulae (PNe) with high abundance discrepancy factors (ADFs), Hf2-2, M1-42 and NGC6153, obtained with the Ultraviolet and Visual Echelle Spectrograph (UVES) on the 8.2m Very Large Telescope (VLT). These spectra were carefully reduced, including rigorous absolute flux calibration, yielding detections of ~410-800 emission lines in each PN. Plasma diagnostics and abundance calculations were critically performed using nebular lines. In all three PNe, the electron temperatures derived using the collisionally excited lines (CELs) are higher than those yielded by the HI Balmer and Paschen jumps, while the temperatures yielded by the OII and NII optical recombination lines (ORLs) are very low, <2000 K, indicating that the heavy-element ORLs probe cold nebular regions. The ORL abundances of N, O and Ne are systematically higher than the corresponding CEL values, confirming high ADFs in the three objects. Position-velocity (PV) diagrams were created, and spatio-kinematical studies show that CELs come from the outer nebular regions, while the ORL-emitting regions are close to nebular center. Additionally, the velocity indicated by CEL line-splitting decreases with ionization potential, which was not obvious in ORLs. These spatial and kinematic differences support two distinct components of ionized gas: a cold, metal-rich component and a warmer component with normal metallicity. Heavy elements are strongly enriched in the cold gas, while its H^+ fraction is low but still produces significant HI emission, affecting CEL abundance estimates.

astro-ph.SR↗

arXiv2025

A Proof of Talagrand's Creating Large Sets Conjecture

Talagrand conjectured that if a family of sets $\mathcal{F}$ over $X = \{ 1,2,\cdots, N \}$ is of large measure, then constant times of unions of sets in $\mathcal{F}$ will cover a large portion of the power set of $X$. This conjecture is a central open problem at the intersection of combinatorics and probability theory, and was described by Talagrand as a personal favorite. This paper provides a proof confirming this conjecture.

math.CO↗

arXiv2025

PyEMILI: A New Generation Computer-aided Spectral Line Identifier -- II. Emission-line Identification and Plasma Diagnostics of a Sample of Gaseous Nebulae

In order to test the robustness and reliability of the new generation spectral-line identifier PyEMILI, as initially introduced in Paper I, in line identification and establish a reference/benchmark dataset for future spectroscopic studies, we run the code on the line lists of a selected sample of emission-line nebulae, including planetary nebulae (PNe), HII regions, and Herbig-Haro (HH) objects with deep high-dispersion spectroscopic observations published over the past two decades. The automated line identifications by PyEMILI demonstrate significant improvements in both completeness and accuracy compared to the previous manual identifications in the literature. Since our last report of PyEMILI, the atomic transition database used by the code has been further expanded by cross-matching the Kurucz Line Lists. Moreover, to aid the PyEMILI identification of numerous faint optical recombination lines (ORLs) of CII, NII, OII and NeII, we compiled a new dataset of effective recombination coefficients for these nebular lines, and created a new subroutine in the code to generate theoretical spectra of heavy-element ORLs at various electron temperature and density cases; these theoretical spectra can be used to fit the observed recombination spectrum of a PN to obtain the electron temperature, density and ionic abundances using the Markov-Chain Monte Carlo (MCMC) method. We present MCMC-derived parameters for a sample of PNe. This work establishes PyEMILI as a robust and versatile tool for both line identification and plasma diagnostics in deep spectroscopy of gaseous nebulae.

astro-ph.SR↗

arXiv2025

LAMOST Medium-resolution Spectroscopic Survey of the Rosette Nebula

We report multi-fiber, medium-resolution spectroscopy of the Rosette Nebula with full spatial coverages, and present a table of the nebular parameters based on the spatially-resolved measurements of emission lines. These new observations were conducted through the Medium-Resolution Spectroscopic Survey of Nebulae (MRS-N) on the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Comprehensive analyses were performed on a total of 3854 high-quality nebular spectra, so far the most extensive spectral dataset available for this nebula that encompasses an area of 4.52 square degrees. Various physical parameters, including relative line intensities, radial velocities (RVs), and full widths at half maximum (FWHMs), were derived through measurements of the H-alpha, [N II] 6548,6584 and [S II] 6716,6731 emission lines detected in the LAMOST MRS-N spectra. For the first time, we found a bow-shaped feature in the spatial distribution of RVs of the Rosette Nebula. Moreover, the spatial distributions of RVs and FWHMs, as well as additional parameters such as gas temperature and turbulent velocity in the vicinity of the nebula, indicate possible interaction between Rosette and the nearby supernova remnant (SNR), Monoceros Loop. Our new observations provide indispensable measurements of the Rosette Nebula. The parameter table in particular can be used as valuable constraint on the chemo-dynamical modeling of the nebula, which will enable deeper understanding of the characteristics of this H II region.

astro-ph.GA↗

arXiv2025

GTC/OSIRIS Deep Spectroscopy of Galactic Compact Planetary Nebulae: PNG048.5+04.2 and PNG068.7+14.8

We report deep long-slit spectroscopy of two Galactic compact planetary nebulae (PNe), PNG048.5+04.2 and PNG068.7+14.8, obtained using the 10.4m Gran Telescopio Canarias (GTC). These spectra cover a broad wavelength range of ~3630-10370 A, enabling detection of numerous emission lines critical for nebular analysis, including [O II] 3727 and [O III] 4363 in the blue and [S III] 6312,9069 in the red. Also detected in the spectrum of PNG068.7+14.8 are broad C III and C IV lines probably due to stellar emission, indicating that the central star of this PN is [WC]-type. These GTC optical-IR spectra were analyzed in details in conjunction with the archival Spitzer/IRS mid-IR spectra, and elemental abundances of the two PNe determined. Photoionization models were established for the two PNe using CLOUDY, based on the ratios of line fluxes measured from the GTC optical-NIR spectra. Our best photoionization modeling, in combination with state-of-the-art post-AGB evolutionary model tracks, shows that both PNe evolved from low-mass progenitors (<2 M_sun) with relatively young ages (<3 Gyr), although their central cores are probably in different evolutionary stages - the central star of PNG068.7+14.8 is still in the process of heating up, while that of PNG048.5+04.2 has already entered the white dwarf cooling track. A comparison with AGB model predictions also constrains the progenitors of both PNe to be of low masses. Moreover, the two PNe are in line with the radial oxygen gradients exhibited by the Galactic PN populations, using the most up-to-date distances.

astro-ph.GA↗

arXiv2025

A Massive Yellow Supergiant in the Far Outer Disk of M31: Evidence for In Situ Massive Star Formation Beyond the Optical Radius

While massive stars are known to shape galactic ecosystems, their formation has long been assumed to require the high-density environments of inner galactic disks. This paradigm is challenged by mounting evidence of young massive stars in extended galaxy outskirts, yet direct confirmation of in situ massive star formation in such extreme low-density environments remains scarce. Here, we present the discovery of LAMOST J0048+4154, a massive yellow supergiant situated at a deprojected galactocentric distance of ~34 kpc in M31, making it the most distant massive star confirmed in this galaxy. Through spectroscopic and photometric analyses, we classify J0048+4154 as an F5-F8I supergiant with an effective temperature of $6357^{+121}_{-118}$ K and a luminosity of $\log L/L_{\odot} = 5.00^{+0.06}_{-0.06}$, corresponding to an ~18 $M_{\odot}$ progenitor and an age of ~10 Myr. FAST H I observations reveal close spatial and kinematic alignment between the star and a faint H I external arm, suggesting in situ formation in a region of low gas density. The presence of other UV-bright, early-type stars in the vicinity further supports low-level recent star formation in M31's very outer disk. These findings challenge the prevailing assumption that massive star formation is confined to inner disks or classical star-forming regions and underscore the need to re-examine the role of spiral galaxy outskirts in fueling and sustaining star formation. J0048+4154 thereby expands our understanding of the extent of M31's young stellar component and exemplifies how outer disks may harbor conditions conducive to forming massive stars, despite low-density environments.

astro-ph.GA↗

arXiv2025

PyEMILI: A New Generation Computer-aided Spectral Line Identifier

Deep high-dispersion spectroscopy of Galactic photoionized gaseous nebulae, mainly planetary nebulae and HII regions, has revealed numerous emission lines. As a key step of spectral analysis, identification of emission lines hitherto has mostly been done manually, which is a tedious task, given that each line needs to be carefully checked against huge volumes of atomic transition/spectroscopic database to reach a reliable assignment of identity. Using Python, we have developed a line-identification code PyEMILI, which is a significant improvement over the Fortran-based package EMILI introduced ~20 years ago. In our new code PyEMILI, the major shortcomings in EMILI's line-identification technique have been amended. Moreover, the atomic transition database utilized by PyEMILI was adopted from Atomic Line List v3.00b4 but greatly supplemented with theoretical transition data from the literature. The effective recombination coefficients of the CII, OII, NII and NeII nebular lines are collected from the literature to form a subset of the atomic transition database to aid identification of faint optical recombination lines in the spectra of PNe and HII regions. PyEMILI is tested using the deep, high-dispersion spectra of two Galactic PNe, Hf2-2 and IC418, and gives better results of line identification than EMILI does. We also ran PyEMILI on the optical spectrum of a late-type [WC11] star UVQS J060819.93-715737.4 recently discovered in the Large Magellanic Cloud, and our results agree well with the previous manual identifications. The new identifier PyEMILI is applicable to not only emission-line nebulae but also emission stars, such as Wolf-Rayet stars.

astro-ph.SR↗

arXiv2025

Periodic Variability of the Central Stars of Planetary Nebulae Surveyed through the Zwicky Transient Facility

A consensus has been reached in recent years that binarity plays an important role in the formation and evolution of a significant fraction of planetary nebulae (PNe). Utilizing the archived photometric data from the Zwicky Transient Facility survey, we conducted a comprehensive data mining in search for brightness variations in a large sample of Galactic PNe. This effort leads to identification of 39 PNe, whose central stars exhibit periodic variation in light curves. Among these objects, 20 are known binary central stars of PNe, while the remaining 19 are new discoveries. Additionally, we identified 14 PNe with central stars displaying anomalous variation in light curves, as well as eight variables based on the high-cadence photometric data. Among the new discoveries of periodicity, we found compelling evidence of binary systems at the centres of two archetypal quadrupolar PNe. We also report on very peculiar brightness variation observed in the central core of the compact PN NGC6833. Several PNe in our sample deserve follow-up observations, both high-dispersion spectroscopy and high-precision photometry, to reveal the true nature of their central binarity or even multiplicity.

astro-ph.SR↗

arXiv2025

A Comprehensive Catalog of Emission-line Nebulae, Star Clusters, and Supergiants in M31 from the LAMOST Spectroscopic Survey

Spectroscopic observations of various tracers in nearby galaxies, such as Andromeda (M31), play a crucial role in identifying and classifying individual stellar populations and nebular objects, thereby enhancing our understanding of galactic composition, environment, and dynamics as well as stellar evolution. While the LAMOST (Large Sky Area Multi-Object Fibre Spectroscopic Telescope) survey of M31 has produced extensive datasets, a comprehensive catalog of emission-line nebulae, star clusters, and supergiants is yet to be completed. In this paper, we present a final catalog of 384 emission-line nebulae, 380 star clusters, and 375 supergiants and candidates in M31, as carefully selected and identified from the LAMOST spectroscopic database. These objects were classified using a random forest algorithm, followed by thorough visual examinations of their spectral characteristics as well as morphologies revealed by archive images. For emission-line nebulae, we measured radial velocities and relative fluxes of emission lines, enabling further classification of planetary nebulae and HII regions. Additionally, we identified 245 emission-line nebulae in M33. This work lays the data foundation for the study of M31, and offers valuable tracers to investigate M31's structure and evolution.

astro-ph.GA↗

arXiv2024

Ultra-compact topological photonic crystal rainbow nanolasers operating in the 1550 nm telecom band with wavelength-scale mode volumes

Density-integrated, multi-wavelength nanoscale lasers with ultra-low power consumption and ultra-compact footprints are essential for energy-efficient, fast and high-throughput data processing. Currently, on-chip multi-wavelength lasers predominantly rely on arrays of discrete large-scale conventional semiconductor lasers that are susceptible to the fabrication imperfections. Topological rainbow nanolasers, which spatially confine and emit specific topologically protected light frequencies, offer a prospective approach for achieving ultra-compact integrated multi-wavelength light sources with enhanced robustness against perturbations and defects. However, it remains a significant challenge to achieve highly localized topological rainbow trapping in nanocavities for laser emission with both high quality factors and ultra-small mode volumes. Here, we experimentally report ultra-compact topological photonic crystal rainbow nanolasers operating in the 1550 nm telecom band. Specifically, we present rainbow-like emission with uniform wavelength spacing and wavelength-scale mode volume $\sim 0.7 \left(\frac{\lambda}{n}\right)^3$ in a one-dimensional topological rainbow nanolaser, exhibiting robust lasing operation across a wide temperature range and a spectral tuning capability of approximately 70 nm. Additionally, we demonstrate an ultra-compact two-dimensional topological rainbow nanolaser in an exceptionally compact footprint of nearly 0.002 $\text{mm}^2$, featuring a broad rainbow spectra with 64 continuously tuned lasing peaks. Our work provides a promising method for realizing robust and nanoscale multi-wavelength tunable laser sources, paving the way for numerous potential applications in ultra-compact photonic chips.

physics.optics↗

arXiv2024

GTC Spectroscopic Surveys of Planetary Nebulae in the Milky Way and M31

We report spectroscopic surveys of planetary nebulae (PNe) in the Milky Way and Andromeda (M31), using the 10.4-m Gran Telescopio Canarias (GTC). The spectra are of high quality and cover the whole optical range, mostly from 3650 \r{A} to beyond 1 micron, enabling detection of nebular emission lines critical for spectral analysis as well as photoionization modeling. We obtained GTC spectra of 24 compact (angular diameter <5 arcsec) PNe located in the Galactic disk, ~3-20 kpc from the Galactic centre, and can be used to constrain stellar evolution models and derive radial abundance gradients of the Milky Way. We have observed 30 PNe in the outer halo of M31 using the GTC. These halo PNe are uniformly metal-rich and probably all evolved from low-mass stars, consistent with the conjecture that they all formed from the metal-rich gas in M31 disk but displaced to their present locations due to galaxy interactions.

astro-ph.GA↗

arXiv2024

A $\lambda$ 3 mm line survey towards the circumstellar envelope of the carbon-rich AGB star IRC+10216 (CW Leo)

We present an unbiased $\lambda$ 3 mm spectral line survey (between 84.5 and 115.8 GHz), conducted by the Purple Mountain Observatory 13.7 meter radio telescope, together with updated modeling results, towards the carbon-rich Asymptotic Giant Branch star, IRC+10216 (CW Leo). A total of 75 spectral lines (96 transitions) are detected, and identified to arise from 19 molecules: C$_2$H, $l$-C$_3$H, C$_4$H, CN, C$_3$N, HC$_3$N, HC$_5$N, HCN, HNC, CH$_3$CN, MgNC, CO, $c$-C$_3$H$_2$, SiC$_2$, SiO, SiS, CS, C$_2$S, C$_3$S, and their isotopologues. Among them, one molecular emission line (H$^{13}$CCCN $J=13-12$) is discovered in IRC+10216 for the first time. The excitation temperature, column density, and fractional abundance of the detected species are deduced by assuming they are in local thermodynamic equilibrium. In addition, the isotopic ratios of [$^{12}$C]/[$^{13}$C], [$^{32}$S]/[$^{34}$S], [$^{28}$Si]/[$^{29}$Si], and [$^{12}$C$^{34}$S]/[$^{13}$C$^{32}$S] are obtained and found to be consistent with previous studies. Finally, we summarize all of the 106 species detected in IRC+10216 to date with their observed and modeled column densities for the convenience of future studies.

astro-ph.SR↗

arXiv2023

Photochemical origin of SiC$_2$ in the circumstellar envelope of carbon-rich AGB stars revealed by ALMA

Whether SiC$_2$ is a parent species, that is formed in the photosphere or as a by-product of high-temperature dust formation, or a daughter species, formed in a chemistry driven by the photodestruction of parent species in the outer envelope, has been debated for a long time. Here, we analyze the ALMA observations of four SiC$_2$ transitions in the CSEs of three C-rich AGB stars (AI Vol, II Lup, and RAFGL 4211), and found that SiC$_2$ exhibits an annular, shell-like distribution in these targets, suggesting that SiC$_2$ can be a daughter species in the CSEs of carbon-rich AGB stars. The results can provide important references for future chemical models.

astro-ph.SR↗