arXiv subjects

L. Zhang

Publications and source records attributed to L. Zhang.

At least 19 recordsLinked to original sources

arXiv2026

A New High-Intensity Source for Ultracold Neutrons

The TRIUMF UltraCold Advanced Neutron (TUCAN) collaboration has completed a new superthermal source for ultracold neutrons (UCNs) at TRIUMF. It uses neutrons from a spallation target driven by TRIUMF's %520-MeV main cyclotron. Heavy water and liquid deuterium serve as neutron moderators, and inelastic scattering inside superfluid $^4$He at around \qty{1.1}{\kelvin} slows the neutrons down to become ultracold. During commissioning runs with the completed source, including the deuterium moderator, up to $1.47(2)\times 10^7$ UCNs were detected in the experimental area after irradiating the target and accumulating UCNs in the source for \qty{60}{\second}. Up to \qty{6.75(3)e5} UCN/s were detected during continuous operation, more than at any other source in the world.

physics.ins-det↗

arXiv2026

Early results from the SVOM Observatory Science program

We present the organisation and early results from the Observatory Science program of the Space-based multi-band astronomical Variable Objects Monitor (SVOM), based on data collected between July 2024 and December 2025. Although primarily designed for gamma-ray burst studies, SVOM's wide-field, multi-wavelength instruments enable a broad range of high-energy astrophysical investigations. We summarize the execution and performance of the General Program and Target-of-Opportunity observations, and we describe the frameworks used for serendipitous source detection and monitoring with the ECLAIRs coded-mask instrument. Over this period, SVOM carried out more than a thousand pointed observations and detected several hundred non-GRB high-energy sources, mainly X-ray binaries, as well as blazars, stellar flares, magnetars, and unidentified events. We highlight some key results, including the monitoring of the microquasar Cygnus X-1, the detection of burst oscillations from the Low-Mass X-ray Binary 4U 0614+091, the spectral-state monitoring of Aql X-1, the first SVOM detection of an X-ray blazar flare from 1ES 1959+650, and observations of a stellar flare from HD 22468. These results demonstrate SVOM's strong capabilities for time-domain astrophysics beyond its core GRB program.

astro-ph.HE↗

arXiv2026

Scintillation of the first-known pulsar planetary system

We present a scintillation study of the first-known pulsar planetary system, PSR~B1257+12, using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). A total of 31 observations with durations greater than or equal to 30 minutes were analyzed. For 14 longer observations (greater than or equal to 120 minutes), one-dimensional autocorrelation function analyses yielded the scintillation timescale, scintillation bandwidth, and frequency-drift rate for 12 epochs. Two observations show strong periodic modulation in the frequency-domain auto-correlation function, likely caused by astronomical-unit-scale structures along the propagation path, preventing reliable measurements of the scintillation timescale and bandwidth. In three observations, secondary spectra reveal simultaneous detections of inner, middle, and outer arcs. Analysis of the annual modulation of the inner-arc curvature indicates isotropic scattering, with a screen distance of $233\pm28$~pc and transverse velocity $V_{\rm scr,\alpha}=-7.16\pm2.16$ km~s$^{-1}$, $V_{\rm scr,\delta}=-41.07\pm5.69$ km~s$^{-1}$. Delay-profile analysis for both the inner and outer arcs suggest spectral exponents consistent with, or smaller than, the Kolmogorov value. Under isotropic scattering, the screen--pulsar distances are $354\pm22$~pc and $166\pm12$~pc for the middle and outer arcs. Combining the results from long-term timing analyses with our scintillation measurements, we find that the dispersion measure (DM) variations are primarily dominated by plasma located further away from the pulsar. The low DM-change rate of the outer arc and the absence of nearby scattering screens suggest that the immediate environment of the pulsar may be relatively clean. Alternatively, scattering screens closer to the pulsar may exist but remain undetected, requiring higher-sensitivity or longer-duration observations.

astro-ph.HE↗

arXiv2026

Observation of Non-Gaussian Magnon Dynamics in a Two-Dimensional Long-Range XY Model

Non-Gaussian evolution of high-order spin correlations characterizes important properties of quantum many-body systems. In practice, decoherence, statistical fluctuation and miscalibration of experimental parameters all hinder the witness of non-Gaussian dynamics. Here we demonstrate the crossover between Gaussian and non-Gaussian dynamics on a two-dimensional XY model with long-range and spatially structured interaction using a trapped ion quantum simulator. We prepare different initial densities of magnon excitations and verify the dynamics of single-spin observables for the engineered Hamiltonian. Then we compare the high-order spin correlations with the mean-field solution and the Holstein-Primakoff approximation, and demonstrate the non-Gaussian behavior in a way independent of the calibration errors. Our work provides a verifiable path from classically simulatable dynamics to regimes where quantum advantage may emerge.

quant-ph↗

arXiv2026

Discovery of EP J175257.3-351923 as a Candidate Black Hole Low-Mass X-ray Binary

We report the discovery of a new X-ray transient, EP J175257.3-351923 (EP250916a), by the Einstein Probe (EP) near the Galactic plane. The outburst lasted for at least 250\,days, reached a peak 2--10 keV flux of $\sim 4 \times 10^{-10}$~erg~cm$^{-2}$~s$^{-1}$, and exhibited a fast-rise exponential-decay (FRED) profile typical of X-ray binary outbursts. The source remained in the hard state throughout the outburst, with only modest variations in the photon index ($\sim 1.6$--$2.2$) and no evidence for a spectral state transition. Broadband spectral modeling suggests a truncated disk, a weak reflection component, and a high-energy cutoff at $\sim 217$~keV, consistent with hard-state accretion in black-hole systems. No reliable optical or radio counterpart is detected within the Swift/XRT error circle. The inferred X-ray-to-optical and X-ray-to-radio flux ratios are consistent with a low-mass X-ray binary classification. Neither pulsations nor significant aperiodic variability are detected. Although the compact object cannot yet be firmly identified, the timing, spectral, and multiwavelength evidence favors EP~J175257.3--351923 as a black-hole low-mass X-ray binary candidate, highlighting EP's potential to uncover a faint, previously hidden population of X-ray binaries.

astro-ph.HE↗

arXiv2026

The strength of Type-C quasi-periodic oscillations in black hole X-ray binaries correlates with the jet inclination

X-ray quasi-periodic oscillations (QPOs) are a characteristic feature of low-mass X-ray binaries (LMXBs). These oscillations have been studied for decades and revealed a rich and complex phenomenology that is still not fully understood. RXTE archival studies have shown that the amplitude of these oscillations differs significantly between black holes (BH) with high or low inclination. Yet, the actual dependence on inclination has never been adequately estimated. Thanks to the improvement of inclination measurements through radio observations and the recent observations by the HXMT satellite, we quantified for the first time the dependence of Type-C QPO amplitudes on the jet inclination of individual BH LMXBs. Our analysis reveals the presence of a significant linear correlation up to 8 Hz, strengthening the case for a ''geometrical'' origin of the QPOs. In addition, for a given QPO frequency, we observe systematically lower amplitudes during the decay of outbursts compared to the rise. This data collection represents a key benchmark for any QPO model. Our comparison with the predictions from a precessing hot flow shows that the amplitude of the QPOs can be reproduced by this scenario if the spin-orbit misalignment is at least $\approx$10-15$^\circ$.

astro-ph.HE↗

arXiv2026

Partition Tree Search Acceleration for VVC: Survey and Evaluation with VTM Evolution

The Versatile Video Coding (VVC) standard, introduced in 2020, offers 40-50% bitrate savings for equivalent visual quality of reconstructed videos over its predecessor, High Efficiency Video Coding (HEVC), at the cost of significantly increased encoding complexity. This growth in encoding complexity is mainly due to the addition of the Quad Tree Multi Type Tree (QTMTT) partitioning structure, which increases the split combinatorial complexity. This paper presents a critical evaluation of state-of-the-art (SOTA) partitioning acceleration techniques designed to reduce the complexity of the partitioning search in VVC. Particular attention is given to how these methods have evolved alongside successive versions of the VVC Test Model (VTM), which serves as the reference software for benchmarking coding tools. These techniques are analyzed in the context of their adaptation to internal changes in VTM, such as updated heuristics for fast partitioning decisions. The study also highlights the challenges involved in improving the trade-off between encoding complexity and compression efficiency. This challenge becomes more pronounced when evaluating methods across diverse VTM configurations and multiple software versions.

eess.IV↗

arXiv2026

The GRB joint scientific analysis pipeline of the ECLAIRs and GRM instruments on board SVOM

The study of the prompt high-energy emission of Gamma-Ray Bursts (GRBs) with SVOM relies on the observations performed by ECLAIRs (4-150 keV) and the Gamma-Ray Monitor (GRM, 0.015-5 MeV), the two wide field-of-view instruments on board the satellite. In this article, we introduce the eclgrm pipelines running at the French Science Center of SVOM, which combine the ECLAIRs and GRM data to generate scientific data products describing the GRB broad-band temporal and spectral properties. The architecture of the pipelines is presented, as well as their activation following each onboard trigger, and their workflow. The statistical data analysis methods employed by the pipelines are described, along with the scientific data products that are created in real time or from the full event data. We also present the eclgrm-ui user interface which allows the scientists on shift to monitor the automated data processings in the pipelines, and to optimize the analysis results interactively.

astro-ph.HE↗

arXiv2026

Quantum simulation of thermalization dynamics of a nonuniform Dicke model

Previous experimental realizations of Dicke model in atomic or ionic systems are based on global observables assuming uniform spin-boson coupling, while inevitable experimental nonuniformity on the one hand requires site-resolved measurement of spin states, and on the other hand provides potential quantum advantage on the simulation of multi-spin distributions. Here we report the quantum simulation of a nonuniform Dicke-like model in a two-dimensional (2D) crystal of up to 200 ions. We explicitly demonstrate the sensitivity of few-spin observables and multi-spin distributions to the spatial inhomogeneity of the model, and examine the thermalization dynamics of the nonuniform model by measuring the subsystem entropies of selected ion groups. Our work enables the study of Dicke-like models beyond the symmetric subspace, paving the way toward understanding the role of disorder in its thermalization and quantum chaos behavior.

quant-ph↗

arXiv2026

Long-time storage of entangled logical states in decoherence-free subspaces

The maintenance of quantum entanglement lays the elementary building block of quantum information processing, requiring an integration of long coherence time, sufficient storage capacity, and high-fidelity entangling gates. Here we encode two-qubit entangled states into the decoherence-free subspaces (DFS) of four ions in a cryogenic trap. By crosstalk-free sympathetic cooling under dual-type encoding and multi-state detection which discards the collision-induced leakage error, we achieve a storage lifetime of about one hour for the entangled logical states. We further study the second-order DFS and show its advantage in suppressing the spatially nonuniform noise over the first-order DFS. Our work paves the way for applications of DFS quantum memories in quantum computing, quantum network and precision measurement.

quant-ph↗

arXiv2026

PSR J0024$-$7204ai: a massive, eccentric binary system in the globular cluster 47 Tucanae

In this paper we present PSR J0024$-$7204ai, a 13.026-ms binary pulsar recently discovered in the globular cluster 47 Tucanae by the MeerKAT radio telescope. This is the slowest spinning pulsar known in this globular cluster, and has a $\sim1.67$-day orbit with an eccentricity of $e\approx0.18$. Although it was not yet possible to derive an unambiguous phase-connected timing solution, by combining detections obtained from MeerKAT and archival Parkes data we were able to measure the rate of advance of periastron to high significance, $\dot{\omega}$ = 0.1601 $\pm 0.0046$ deg yr$^{-1}$. This value implies a total system mass of $2.41 \pm 0.11\, \mathrm{M}_\odot$ (68.3\% C. L.), which, when combined with the binary mass function, gives a maximum pulsar mass of $\sim 1.7 \, \mathrm{M}_\odot$ and a minimum companion mass of $\sim 0.7\, \mathrm{M}_\odot$. Apart from being the slowest pulsar in 47~Tucanae, its orbit is by far the most eccentric and its companion is the most massive among all known binary pulsars in this globular cluster. One possibility is that system is an old MSP - Carbon-Oxygen White Dwarf binary, whose orbit was perturbed by stellar dynamical interactions in the cluster core. Further follow-up observations of this system will be essential for a more detailed characterisation of this system and its evolution.

astro-ph.HE↗

arXiv2026

Multi-wavelength Study of A Superflare on RS CVn-type Star HD22468 Triggered at Hard X-ray by SVOM

Detection of stellar flares at hard X-ray is still rare at the current stage. A transient was recently detected by the hard X-ray camera, ECLAIRs onboard the SVOM mission at 11:39:01.2UT on 2025, January 09. Simultaneous monitor in the optical band on the ground by SVOM/GWAC and follow-up spectroscopy enable us to confirm that the transient is caused by a superflare on HD~22468, a RS CVn-type star. The bolometric energy released in the flare is estimated to be $\sim7.2\times10^{37}-1.7\times10^{38}\ \mathrm{erg}$. The hard X-ray spectra of the event at the peak can be reproduced by the ``apec'' model of a hot plasma with a temperature of $106^{+27}_{-22}$~MK. In the optical range, the H$\alpha$ emission-line profile obtained at $\sim1.7$ hrs after the trigger shows a bulk blueshift of $-96\pm20\ \mathrm{km\ s^{-1}}$, which can be explained by either a chromospheric evaporation or a prominence eruption. The ejected mass is estimated to be $3.9\times10^{20}$ g for the evaporating plasma, and to be $3.2\times10^{21}\ \mathrm{g}<M_{\mathrm{p}}<8.8\times10^{21}\ \mathrm{g}$ for the erupted prominence.

astro-ph.SR↗

arXiv2026

SVOM discovery of a strong X-ray outburst of the blazar 1ES~1959+650 and multi-wavelength follow-up with the Neil Gehrels Swift observatory

On December 6, 2024, 1ES 1959+650, one of the X-ray brightest blazars known, underwent a high-amplitude X-ray outburst detected by SVOM, the first such discovery with this mission. The source was subsequently monitored with SVOM and Swift from December 2024 to March 2025. We report the detection and multi-wavelength follow-up of this event, and describe the temporal and spectral evolution observed during the campaign. Data from SVOM/MXT, SVOM/ECLAIRs, and Swift/XRT were analyzed with log-parabola models to track flux and spectral variability. The source was detected in a bright state over the 0.3-50 keV range. During the three months of monitoring, the X-ray flux varied significantly, showing episodes of spectral hardening at high flux levels. The spectral curvature evolved more irregularly and did not show a clear trend with flux. A shift of the Spectral Energy Distribution (SED) synchrotron peak to higher energies is seen when the flux increases. This constitutes the first blazar outburst discovered in X-rays by SVOM. The coordinated follow-up with Swift provided continuous coverage of the flare and highlights the strong complementarity of the two missions for time-domain studies of blazars. The flare shows no clear signatures of either Fermi I or Fermi II acceleration, suggesting a mixed Fermi I/II scenario.

astro-ph.HE↗

arXiv2026

Detailed lens modeling and kinematics of the submillimeter galaxy G09v1.97. An analysis of CO, H2O, H2O+, and dust continuum emission

The formation mechanisms of intensely starbursting galaxies at high redshift remain unknown. One possible mechanism for triggering these starbursts is mergers and interactions, but detecting these at high redshift remains a challenge. Observations of high-redshift gravitationally lensed galaxies enable studies of the interstellar medium and environment of these extreme starbursts in detail. We used high angular resolution observations of dust continuum, CO(6-5), H2O(211-202), and H2O+(202-111) emission to constrain the ongoing processes in the z = 3.63 gravitationally lensed submillimeter galaxy H-ATLAS J083051.0+013224 (G09v1.97). We used PyAutoLens to create a de-magnified source plane CO(6-5) emission line cube and performed kinematic modeling using 3DBarolo. Additionally, we investigated the properties of the continuum and molecular line emission in the source plane. We find that the regions of CO(6-5) and H2O(211-202) emission match closely in the source plane but that the dust continuum emission is more compact. We find that our lens modeling results do not require more than one source, contrary to what has been found in previous studies. Instead, we find that G09v1.97 resembles a rotating disk with Vmax/sigma = 2.8 +/- 0.4 with evidence for residual emission indicative of non-circular motions such as outflows, tidal tails, or an additional background galaxy. We suggest that the origin of the non-circular motions may be associated with a bi-conical outflow, a tidal tail from an interaction, or indicate the possible presence of an additional galaxy. We calculate the dynamical mass, gas mass, star-formation rate, and depletion time for G09v1.97 and find a high star-formation rate and low gas depletion time. In combination, this suggests that G09v1.97 has recently undergone an interaction, triggering intense star formation, and is in the process of settling into a disk.

astro-ph.GA↗

arXiv2026

Hardware-Economic Manipulation of Dual-Type ${}^{171}$Yb$^+$ Qubits

The dual-type qubit scheme is an emerging method to suppress crosstalk errors in scalable trapped-ion quantum computation and quantum network. Here we report a hardware-economic way to control dual-type $^{171}\mathrm{Yb}^+$ qubits using a single $355\,$nm mode-locked pulsed laser. Utilizing its broad frequency comb structure, we drive the Raman transitions of both qubit types encoded in the $S_{1/2}$ and the $F_{7/2}$ hyperfine levels, and probe their carrier transitions and the motional sidebands. We further demonstrate a direct entangling gate between the two qubit types. Our work can simplify the manipulation of the $^{171}\mathrm{Yb}^+$ qubits both at the hardware and the software level.

quant-ph↗

arXiv2025

Discovery of the redback millisecond pulsar PSR J1728-4608 with ASKAP

We present the discovery of PSR J1728-4608, a new redback spider pulsar identified in images from the Australian SKA Pathfinder telescope. PSR J1728-4608 is a millisecond pulsar with a spin period of 2.86 ms, in a 5.05 hr orbit with a companion star. The pulsar exhibits a radio spectrum of the form $S_\nu \propto \nu^\alpha$, with a measured spectral index of $\alpha = -1.8(3)$. It is eclipsed for 42% of its orbit at 888 MHz, and multi--frequency image--domain observations show that the egress duration scales with frequency as a power law with index $n = -1.74$, where longer duration eclipses are seen at lower frequencies. An optical counterpart is detected in archival Gaia data within $0.5''$ of the radio position. It has a mean G-band magnitude of 18.8 mag and its light curve displays characteristics consistent with a combination of ellipsoidal modulation and irradiation effects. We also report the nearest Fermi $\gamma$-ray source, located 2$'$ away from our source, as a possible association. A radio timing study constrains the intrinsic and orbital properties of the system, revealing orbital period variations that we attribute to changes in the gravitational quadrupole moment of the companion star. At the eclipse boundary, we measure a maximum dispersion measure excess of $2.0 \pm 1.2 \ \mathrm{pc\ cm^{-3}}$, corresponding to an electron column density of $5.9 \pm 3.6 \times10^{18} \ \mathrm{cm^{-2}}$. Modelling of the eclipse mechanism suggests that synchrotron absorption is the dominant cause of the eclipses observed at radio wavelengths. The discovery and characterisation of systems like \psr\ provide valuable insights into pulsar recycling, binary evolution, the nature of companion-driven eclipses, and the interplay between compact objects and their plasma environments.

astro-ph.HE↗

arXiv2025

Rational regulation strategies of interstitial localized electrons in electride: A density functional theory study

As a class of electron-rich materials, electrides demonstrate promising applications in many fields. However, the required high pressure restricts the practical applications to some extent. This study reveals that the unique feature of electride, i.e., the localization of interstitial electrons, can be greatly enhanced and tuned by self-defective doping, applying tensile/compressive stress, or shear stress. Moreover, the requirement of orbital orthogonality between the valence and core electron wave functions, as well as the Pauli exclusion principle, should be the driven force for the electron interstitial localization; and the exertion of external pressure modifies the available space to accommodate the electronic wave functions, thus enhances the interstitial localization. These discoveries lay down the ground for searching for promising electrides that are practicable at ambient conditions.

cond-mat.mtrl-sci↗

arXiv2025

Complexity Reduction Study Based on RD Costs Approximation for VVC Intra Partitioning

In this paper, a complexity study is conducted for Versatile Video Codec (VVC) intra partitioning to accelerate the exhaustive search involved in Rate-Distortion Optimization (RDO) process. To address this problem, two main machine learning techniques are proposed and compared. Unlike existing methods, the proposed approaches are size independent and incorporate the Rate-Distortion (RD) costs of neighboring blocks as input features. The first method is a regression based technique that predicts normalized RD costs of a given Coding Unit (CU). As partitioning possesses the Markov property, the associated decision-making problem can be modeled as a Markov Decision Process (MDP) and solved by Reinforcement Learning (RL). The second approach is a RL agent learned from trajectories of CU decision across two depths with Deep Q-Network (DQN) algorithm. Then a pre-determined thresholds are applied for both methods to select a suitable split for the current CU.

cs.LG↗