arXiv subjects

Y. L. Wang

Publications and source records attributed to Y. L. Wang.

At least 19 recordsLinked to original sources

arXiv2026

The X-ray emission of the long-period transient and accreting cataclysmic variable ASKAP J174508.9-505149

Long-period transients (LPTs) challenge our knowledge of the mechanism producing radio periodic pulsations in compact objects. Some LPTs have been associated with systems hosting a white dwarf and a low-mass star in a detached binary. Recently, a new LPT (ASKAP\,J174508.9-505149) has been classified as an accreting cataclysmic variable (CV). In the present letter, we report on the detailed study of the X-ray variability of ASKAP\,J174508.9-505149 as observed by \emph{XMM-Newton} and \emph{Einstein Probe} between September 2025 and May 2026. Simultaneous optical and radio observations are also presented. We studied the timing variability of the source, and estimated an X-ray periodicity of $P=4868(22)$\,s, consistent with radio and optical periods. We also observe the same periodicity in the hardness ratio extracted from the \emph{XMM-Newton} observation, peaking at the minimum of the modulation. A long-term modulation is also present in the X-rays and in the B-band photometry, but it is poorly constrained by the current dataset. Spectral X-ray analysis shows the presence of a black-body component ($\sim$0.1\,keV), a collisionally ionised plasma ($\sim$15\,keV), and an absorption feature at 0.77 keV (possibly due to Oxygen-VII). This is the third LPT detected in the X-ray band, the second with a detected X-ray periodicity and variable X-ray emission, and the first conclusively recognised as an accreting magnetic CV.

astro-ph.HE↗

arXiv2026

Periodic Radio and X-ray Emission from an Accreting White Dwarf Binary

Long period radio transients (LPTs) are coherent bursts of polarised radio emission that repeat periodically on timescales of minutes to hours. Little is known about the physical origins of these systems. Astronomers have proposed magnetars that rotate slowly and white dwarfs that rapidly orbit with a companion star as potential explanations. While several recent examples appear to support the latter hypothesis, the mechanism generating these bright radio pulses remains poorly understood. Here we report our discovery and classification of the LPT ASKAP J174508.9-505149 as an accreting white dwarf binary. This object has a ~1.3h spectroscopic orbital period and exhibits orbitally-modulated X-ray emission and radio bursts. These elliptically polarised radio bursts drift in emission frequency, potentially due to a longer beat period, and turn off for several hours at a time. Some long period radio transients have been associated with non-interacting white dwarf binaries. We have spectroscopically confirmed this system as an accreting cataclysmic variable, identified through characteristic optical emission lines and an ongoing X-ray outburst. Our results strengthen the link between at least some long period radio transients and white dwarf binaries.

astro-ph.HE↗

arXiv2026

Multi-wavelength outburst activity from EP J174942.2-384834: a very faint X-ray transient discovered by Einstein Probe

We report the discovery and multi-wavelength characterization of the Galactic transient EP J174942.2$-$384834, first detected by the Einstein Probe during a faint X-ray outburst in March 2025. Coordinated follow-up observations revealed two major outbursts and a rebrightening over a seven-month period. Broadband X-ray spectral modeling shows that the outburst emission was dominated by thermal Comptonization of very soft seed photons. The absence of a detected thermal disk component, together with the low inferred seed-photon temperature, is consistent with a cool and possibly truncated accretion disk. The X-ray spectrum remained consistently hard throughout the outburst activity, with a power-law photon index of $Γ\approx 1$-2, gradually softening as the flux declined. The optical/UV counterpart brightened in tandem with the X-ray emission and exhibited a blue continuum with broad Balmer absorption features. Together with the optical/UV - X-ray luminosity correlation, this supports a disk-dominated origin of the optical/UV outburst emission, with viscous heating likely playing a major role and irradiation possibly contributing, especially in the UV. No radio counterpart was detected, implying at most very faint jet activity. Taken together, the observed properties support the classification of EP J174942.2$-$384834 as a very faint X-ray transient black hole candidate. This study demonstrates the ability of Einstein Probe to uncover and characterize the faintest accreting compact objects in the Galaxy.

astro-ph.HE↗

arXiv2026

Aql X-1 from dawn 'til dusk: the early rise, fast state transition and decay of its 2024 outburst

Transient Low-Mass X-ray Binaries (LMXBs) are usually first detected by all-sky X-ray monitors when they enter new outbursts, typically at X-ray luminosities above $\sim$10$^{36}$ erg/s. Observations of these sources during the early rise of the outbursts have so far been very limited. However, the launch of the Einstein Probe (EP) has greatly improved our ability to detect fainter X-ray activity, unlocking access to the outburst early rise. In September 2024, EP detected the early onset of a new outburst from the neutron star LMXB Aql X-1, catching the source at a luminosity below 10$^{35}$ erg/s. In this paper we present results from a comprehensive, multi-wavelength campaign of this event, combining data from EP, NICER, NuSTAR, Swift and Las Cumbres Observatory covering the full outburst from its early rise through its decay. By comparing X-ray and optical light curves obtained with Las Cumbres Observatory during the initial rise, we show that the start of the X-ray emission lagged the optical rise by, at most, 13 days. Time-resolved X-ray spectroscopy revealed how the geometry and the physical properties of the accretion flow evolve during this early stage of the outburst, as well as at higher luminosities as the source transitioned through the canonical X-ray spectral states - hard, intermediate and soft. These data show that the source underwent a very rapid, about 12-h long, transition from the hard to the soft state about two weeks after the optical onset of the outburst. The evolution of the temperature and physical sizes of both the inner region of the disk and a black body near the NS surface suggest that at the state transition, a boundary and spreading layer likely formed. We discuss these results in the context of time-scales for outburst evolution and state transitions in accreting neutron stars and black holes.

astro-ph.HE↗

arXiv2026

Einstein Probe discovery of EP J171159.4-333253: an eclipsing neutron star low-mass X-ray binary with clocked bursts

EP J171159.4-333253 is a new neutron-star low-mass X-ray binary discovered in outburst by the Einstein Probe (EP) on 2025 June 23, exhibiting clocked type-I X-ray bursts, eclipses and dips. In this paper, we report on the results of the X-ray spectral and timing analyses for EP J171159.4-333253 using data collected by EP and NuSTAR during the first 21 days of the outburst. The X-ray burst recurrence time can be characterized over a subset of nine bursts spanning 1.6 days around the NuSTAR observation, and the result is $t_{\rm rec}=8196 \pm 177\,$s with indications of a possible decreasing trend. From the X-ray eclipse events, the binary orbital period and the eclipse duration are estimated to be $P_{\rm orb}=6.48301 \pm 0.00003\,$hr and $D_{\star,X} = 1245.5^{+6.9}_{-6.5}\,$s, respectively. These enable an estimate of the mass and radius of the companion star and the binary inclination, which are $M_2\approx0.6-0.8\,M_\odot$, $R_2\approx0.7-0.8\,R_\odot$ and $i\approx73-75^\circ$, respectively. We also report on joint ULTRACAM and EP observations on 2025 July 21--22, detecting the source optical counterpart and covering an eclipse in both X-ray and optical bands. The optical eclipse is wavelength-dependent and broader than in X-rays, indicating that part of the optical emission arises from an extended region in the accretion flow. Despite a moderate variation in the source flux, the properties of the persistent X-ray emission are typical of a hard spectral state. We further evaluated the ratio of the accretion energy to the thermonuclear energy to be 120--130, implying helium bursts with the accreted hydrogen being depleted in-between bursts.

astro-ph.HE↗

arXiv2026

Charmed $Λ_c^+$ baryon decays into light scalar mesons in the topological $SU(3)_f$ framework

Using the topological-diagram approach based on $SU(3)$ flavor symmetry, we investigate two-body $Λ_c^+\to {\bf B}S$ decays, where ${\bf B}$ denotes the final-state baryon and $S$ refers to a light scalar meson, such as $f_0/f_0(980)$, $a_0/a_0(980)$, $σ_0/f_0(500)$, or $κ/K_0^*(700)$. Our analysis indicates that interpreting the light scalar mesons as tetraquark states provides a more consistent description of the currently available experimental data. In particular, this framework naturally accommodates the experimentally observed branching fraction ${\cal B}(Λ_c^+ \to Λa_0^+)$, which exceeds predictions based solely on long-distance effects by an order of magnitude. Within the tetraquark scenario, we predict ${\cal B}(Λ_c^+\to Σ^+ f_0)=(4.9\pm 1.9)\times 10^{-2}$ and ${\cal B}(Λ_c^+\to p f_0)=(3.6\pm 1.4)\times 10^{-3}$. Owing to $f_0-σ_0$ mixing, ${\cal B}(Λ_c^+\to Σ^+ σ_0)$ and ${\cal B}(Λ_c^+\to pσ_0)$ are suppressed to the levels of $1\times 10^{-3}$ and $5\times 10^{-5}$, respectively. These modes therefore provide sensitive probes of the internal structure of the light scalar mesons. More generally, the remaining branching ratios of $Λ_c^+ \to {\bf B} S$ are found to be comparable to those of $Λ_c^+ \to {\bf B}M$, where $M$ denotes a pseudoscalar meson. Their predicted sizes suggest that these decay modes should be accessible to ongoing and near-future experimental studies at BESIII, Belle II, and LHCb.

hep-ph↗

arXiv2025

Dichotomy in Low- and High-energy Band Renormalizations in Trilayer Nickelate $La_{4}Ni_{3}O_{10}$: a Comparison with Cuprates

Band renormalizations comprise crucial insights for understanding the intricate roles of electron-boson coupling and electron correlation in emergent phenomena such as superconductivity. In this study, by combining high-resolution angle-resolved photoemission spectroscopy and theoretical calculations, we systematically investigate the electronic structure of the trilayer nickelate superconductor $La_{4}Ni_{3}O_{10}$ at ambient pressure. We reveal a dichotomy in the electronic band renormalizations of $La_{4}Ni_{3}O_{10}$ in comparison to cuprate superconductors. At a high energy scale of hundreds of meV, its band structure is strongly renormalized by electron correlation effect enhanced by Hund coupling. The resultant waterfall-like dispersions resemble the high-energy kinks in cuprate superconductors. However, at low energy scales of tens of meV, the dispersive bands are nearly featureless and devoid of any resolvable electron-boson interactions, in drastic contrast to the low-energy kinks observed in cuprates and other correlated 3d transition-metal compounds. The dichotomic band renormalizations highlight the disparity between nickelate and cuprate superconductors and emphasize the importance of strong electron-correlation in the superconductivity of Ruddlesden-Popper phase nickelates.

cond-mat.str-el↗

arXiv2025

Topological $SU(3)_f$ approach for two-body $Ω_c$ weak decays

We explore the two-body non-leptonic weak decays of $Ω_c^0$ into final states ${\bf B}^{(*)}M$ and ${\bf B}^{(*)}V$, where ${\bf B}^{(*)}$ denotes an octet (a decuplet) baryon and $M(V)$ represents a pseudoscalar (vector) meson. We employ the topological $SU(3)_f$ approach to depict and parameterize the $W$-emission and $W$-exchange processes. We find that the topological parameters can be associated and combined, making them extractable for calculation. Consequently, we explain the partially measured branching fractions relative to ${\cal B}(Ω_c^0\to Ω^-π^+)$, recombined or kept as the following ratios: ${\cal B}(Ω_c^0\toΞ^{*0}\bar K^{*0})/{\cal B}(Ω_c^0\toΩ^-ρ^+)=0.28\pm 0.11$, ${\cal B}(Ω_c^0\toΞ^-π^+)/{\cal B}(Ω_c^0\toΞ^{0}\bar K^{0})=0.10\pm 0.02$, and ${\cal B}(Ω_c^0 \to Ω^- K^+)/{\cal B}(Ω_c^0 \to Ω^- π^+)=0.06\pm 0.01$. In particular, we present ${\cal B}(Ω_c^0 \to Ξ^0 π^0)=(2.3\pm0.2)\times 10^{-4}$ as half the value of ${\cal B}(Ω_c^0 \to Ξ^- π^+)$ in the approximate isospin relation, and highlight potential candidates for testing $SU(3)_f$ symmetry breaking.

hep-ph↗

arXiv2024

Cuprate-like Electronic Structures in Infinite-Layer Nickelates with Substantial Hole Dopings

The superconducting infinite-layer (IL) nickelates offer a new platform for investigating the long-standing problem of high-temperature superconductivity. Many models were proposed to understand its superconducting mechanisms based on the calculated electronic structure, and the multiple Fermi surfaces and multiple orbitals involved create complications and controversial conclusions. Over the past 5 years, the lack of direct measurements of the electronic structure has hindered the understanding of nickelate superconductors. Here we fill this gap by directly resolving the electronic structures of the parent compound LaNiO$_2$ and superconducting La$_{0.8}$Ca$_{0.2}$NiO$_2$ using angle-resolved photoemission spectroscopy (ARPES). We find that their Fermi surfaces consist of a quasi-two-dimensional (quasi-2D) hole pocket and a three-dimensional (3D) electron pocket at the Brillouin zone corner, whose volumes change upon Ca doping. The Fermi surface topology and band dispersion of the hole pocket closely resemble those observed in hole-doped cuprates. However, the cuprate-like band exhibits significantly higher hole doping in superconducting La$_{0.8}$Ca$_{0.2}$NiO$_2$ compared to superconducting cuprates, highlighting the disparities in the electronic states of the superconducting phase. Our observations highlight the novel aspects of the IL nickelates, and pave the way toward the microscopic understanding of the IL nickelate family and its superconductivity.

cond-mat.supr-con↗

arXiv2023

The Lobster Eye Imager for Astronomy Onboard the SATech-01 Satellite

The Lobster Eye Imager for Astronomy (LEIA), a pathfinder of the Wide-field X-ray Telescope of the Einstein Probe (EP) mission, was successfully launched onboard the SATech-01 satellite of the Chinese Academy of Sciences on 27 July 2022. In this paper, we introduce the design and on-ground test results of the LEIA instrument. Using state-of-the-art Micro-Pore Optics (MPO), a wide field-of-view (FoV) of 346 square degrees (18.6 degrees * 18.6 degrees) of the X-ray imager is realized. An optical assembly composed of 36 MPO chips is used to focus incident X-ray photons, and four large-format complementary metal-oxide semiconductor (CMOS) sensors, each of 6 cm * 6 cm, are used as the focal plane detectors. The instrument has an angular resolution of 4 - 8 arcmin (in FWHM) for the central focal spot of the point spread function, and an effective area of 2 - 3 cm2 at 1 keV in essentially all the directions within the field of view. The detection passband is 0.5 - 4 keV in the soft X-rays and the sensitivity is 2 - 3 * 10-11 erg s-1 cm-2 (about 1 mini-Crab) at 1,000 second observation. The total weight of LEIA is 56 kg and the power is 85 W. The satellite, with a design lifetime of 2 years, operates in a Sun-synchronous orbit of 500 km with an orbital period of 95 minutes. LEIA is paving the way for future missions by verifying in flight the technologies of both novel focusing imaging optics and CMOS sensors for X-ray observation, and by optimizing the working setups of the instrumental parameters. In addition, LEIA is able to carry out scientific observations to find new transients and to monitor known sources in the soft X-ray band, albeit limited useful observing time available.

astro-ph.IM↗

arXiv2023

Ultra-high-resolution Observations of Persistent Null-point Reconnection in the Solar Corona

Magnetic reconnection is a key mechanism involved in solar eruptions and is also a prime possibility to heat the low corona to millions of degrees. Here, we present ultra-high-resolution extreme ultraviolet observations of persistent null-point reconnection in the corona at a scale of about 390 km over one hour observations of the Extreme-Ultraviolet Imager on board Solar Orbiter spacecraft. The observations show formation of a null-point configuration above a minor positive polarity embedded within a region of dominant negative polarity near a sunspot. The gentle phase of the persistent null-point reconnection is evidenced by sustained point-like high-temperature plasma (about 10 MK) near the null-point and constant outflow blobs not only along the outer spine but also along the fan surface. The blobs appear at a higher frequency than previously observed with an average velocity of about 80 km/s and life-times of about 40 s. The null-point reconnection also occurs explosively but only for 4 minutes, its coupling with a mini-filament eruption generates a spiral jet. These results suggest that magnetic reconnection, at previously unresolved scales, proceeds continually in a gentle and/or explosive way to persistently transfer mass and energy to the overlying corona.

astro-ph.SR↗

arXiv2022

Spin-phonon coupling driven Charge density wave in a Kagome Magnet

The intertwining between spin, charge, and lattice degrees of freedom can give rise to unusual macroscopic quantum states, including high-temperature superconductivity and quantum anomalous Hall effects. Recently, a charge density wave (CDW) is observed in the kagome antiferromagnet FeGe, indicative of possible intertwining physics. An outstanding question is that whether magnetic correlation is fundamental for the spontaneous spatial symmetry breaking orders. Here, utilizing elastic and high-resolution inelastic x-ray scattering, we discover a charge dimerization superlattice that coexists with the 2$\times$2$\times$1 CDW in the kagome sublattice. Most interestingly, between the magnetic and CDW transition temperature, the phonon dynamical structure factor shows a giant phonon-energy hardening and a substantial phonon linewidth broadening near the charge-dimerization wavevectors, both signaling the spin-phonon coupling. By first principles calculations, we show that both the static and dynamic spin excitations intertwine with the phonon to drive the spatial symmetry breaking.

cond-mat.str-el↗

arXiv2022

Equivalent $SU(3)_f$ approaches for two-body anti-triplet charmed baryon decays

For the two-body ${\bf B}_c\to{\bf B}M$ decays, where ${\bf B}_c$ denotes the anti-triplet charm baryon and ${\bf B}(M)$ the octet baryon (meson), there exist two theoretical studies based on the $SU(3)$ flavor [$SU(3)_f$] symmetry. One is the irreducible $SU(3)_f$ approach (IRA). In the irreducible $SU(3)_f$ representation, the effective Hamiltonian related to the initial and final states forms the amplitudes for ${\bf B}_c\to{\bf B}M$. The other is the topological-diagram approach (TDA), where the $W$-boson emission and $W$-boson exchange topologies are drawn and parameterized for the decays. As required by the group theoretical consideration, we present the same number of the IRA and TDA amplitudes. We can hence relate the two kinds of the amplitudes, and demonstrate the equivalence of the two $SU(3)_f$ approaches. We find a specific $W$-boson exchange topology only contributing to $Ξ_c^0\to{\bf B}M$. Denoted by $E_M$, it plays a key role in explaining ${\cal B}(Ξ_c^0\to Λ^0 K_S^0,Σ^0 K_S^0,Σ^+ K^-)$. We consider that $Λ_c^+ \to n π^+$ and $Λ_c^+ \to pπ^0$ proceed through the constructive and destructive interfering effects, respectively, which leads to ${\cal B}(Λ_c^+ \to n π^+)\gg{\cal B}(Λ_c^+ \to pπ^0)$ in agreement with the data. With the exact and broken $SU(3)_f$ symmetries, we predict the branching fractions of ${\bf B}_c\to{\bf B}M$ to be tested by future measurements.

hep-ph↗

arXiv2021

Flat Band Induced Negative Magnetoresistance in Multi-Orbital Kagome Metal

Electronic flat band systems are a fertile platform to host correlation-induced quantum phenomena such as unconventional superconductivity, magnetism and topological orders. While flat band has been established in geometrically frustrated structures, such as the kagome lattice, flat band-induced correlation effects especially in those multi-orbital bulk systems are rarely seen. Here we report negative magnetoresistance and signature of ferromagnetic fluctuations in a prototypical kagome metal CoSn, which features a flat band in proximity to the Fermi level. We find that the magnetoresistance is dictated by electronic correlations via Fermi level tuning. Combining with first principles and model calculations, we establish flat band-induced correlation effects in a multi-orbital electronic system, which opens new routes to realize unconventional superconducting and topological states in geometrically frustrated metals.

cond-mat.str-el↗

arXiv2020

The Hund's Superconductor Li(Fe,Co)As

We combine transport, angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy to investigate several low energy manifestations of the Hund coupling in a canonical FeSC family Li(Fe,Co)As. We determine the doping dependence of the coherent-incoherent crossover temperature and the quasi-particle effective mass enhancement in the normal state. Our tunneling spectroscopy result in the superconducting state supports the idea that superconductivity emerging from Hund's metal state displays a universal maximal superconducting gap vs transition temperature (2$Δ_{max}/k_{B}T_{c}$) value, which is independent of doping level and $T_{c}$.

cond-mat.supr-con↗

arXiv2019

Schwinger pair production correction in thermal system

In this paper, we give formal results of Schwinger pair production correction in thermal systems with external background field by using the evolution operator method of thermo field dynamics, where especially tree level correction of thermal photons is considered with linear response approaches by an effective mass shift. We consider initial systems in two types of vacuums as zero temperature and thermal vacuum, respectively, with correction of thermal photons is or not included. As an example we give results of these corrections to pair production for a constant external background electric field.

hep-ph↗

arXiv2018

EDRIXS: An open source toolkit for simulating spectra of resonant inelastic x-ray scattering

Resonant inelastic x-ray scattering (RIXS) has become a very powerful experimental technique to probe a broad range of intrinsic elementary excitations, for example, from low energy phonons and (bi-)magnons to high energy $d$-$d$, charge-transfer and plasmons excitations in strongly correlated electronic systems. Due to the complexity of the RIXS cross-section and strong core-hole effects, theoretical simulation of the experimental RIXS spectra is still a difficult task which hampers the understanding of RIXS spectra and the development of the RIXS technique. In this paper, we present an open source toolkit (dubbed EDRIXS) to facilitate the simulations of RIXS spectra of strongly correlated materials based on exact diagonalization (ED) of certain model Hamiltonians. The model Hamiltonian can be from a single atom, small cluster or Anderson impurity model, with model parameters from density functional theory plus Wannier90 or dynamical mean-field theory calculations. The spectra of x-ray absorption spectroscopy (XAS) and RIXS are then calculated using Krylov subspace techniques. This toolkit contains highly efficient ED, XAS and RIXS solvers written in modern Fortran 90 language and a convenient Python library used to prepare inputs and set up calculations. We first give a short introduction to RIXS spectroscopy, and then we discuss the implementation details of this toolkit. Finally, we show three examples to demonstrate its usage.

cond-mat.str-el↗

arXiv2018

Observation of Double Weyl Phonons in Parity-Breaking FeSi

Condensed matter systems have now become a fertile ground to discover emerging topological quasi-particles with symmetry protected modes. While many studies have focused on Fermionic excitations, the same conceptual framework can also be applied to bosons yielding new types of topological states. Motivated by the recent theoretical prediction of double-Weyl phonons in transition metal monosilicides [Phys. Rev. Lett. 120, 016401 (2018)], we directly measured the phonon dispersion in parity-breaking FeSi using inelastic x-ray scattering. By comparing the experimental data with theoretical calculations, we make the first observation of double-Weyl points in FeSi, which will be an ideal material to explore emerging Bosonic excitations and its topologically non-trivial properties.

cond-mat.mtrl-sci↗