arXiv subjects

S. Zhang

Publications and source records attributed to S. Zhang.

At least 73 records · Page 4Linked to original sources

arXiv2024

Study of magnetic reconnection at low-$β$ using laser-powered capacitor coils

Magnetic reconnection is a ubiquitous fundamental process in space and astrophysical plasmas that rapidly converts magnetic energy into some combination of flow energy, thermal energy, and non-thermal energetic particles. Over the past decade, a new experimental platform has been developed to study magnetic reconnection using strong coil currents powered by high power lasers at low plasma beta, typical conditions under which reconnection is energetically important in astrophysics. KJ-class lasers were used to drive parallel currents to reconnect MG-level magnetic fields in a quasi-axisymmetric geometry, similar to the Magnetic Reconnection Experiment or MRX, and thus this platform is named micro-MRX. This presentation summarizes two major findings from micro-MRX: direct measurement of accelerated electrons and observation of ion acoustic waves during anti-parallel reconnection. The angular dependence of the measured electron energy spectrum and the resulting accelerated energies, supported by particle-in-cell simulations, indicate that direct acceleration by the out-of-plane reconnection electric field is at work. Furthermore, a sudden onset of ion acoustic bursts has been measured by collective Thomson scattering in the exhaust of magnetic reconnection, followed by electron acoustic bursts with electron heating and bulk acceleration. These results demonstrate that the micro-MRX platform offers a novel and unique approach to study magnetic reconnection in the laboratory in addition to the capabilities provided by traditional magnetized plasma experiments such as MRX and the upcoming FLARE (Facility for Laboratory Reconnection experiments). Future approaches to study other particle acceleration mechanisms and ion acoustic waves from magnetic reconnection are also discussed.

physics.plasm-ph↗

arXiv2024

estimating the spin of MAXI J1348--630 from intermediate and soft states using \textit{Insight}-HXMT data

We present a broadband spectral analysis of the soft-intermediate and soft states using \textit{Insight}-HXMT observations of the black hole binary MAXI J1348--630 during its outburst in 2019. We employ a combination of reflection and continuum fitting methods to measure the spin of the black hole and determine a spin of $a=0.79\pm0.13$, which is consistent with most of the previous results. Additionally, we investigate some sources of systematic uncertainty that could bias the measurement. The valid spectral hardening factor can partially account for the spin evolution observed during the outburst. Other potential factors, such as assumptions about the corona's structure and their interaction with reflected photons, can also affect the accuracy of spin measurements. Furthermore, our analysis reveals that MAXI J1348--630 exhibits a significantly high disc density ($\sim10^{22}\,\rm{cm}^{-3}$), with a moderate iron abundance of approximately 2.5 times solar.

astro-ph.HE↗

arXiv2024

A Revised Spin of the Black Hole in GRS 1716-249 with a New Distance

GRS 1716-249 is a stellar-mass black hole in a low-mass X-ray binary that underwent a gaint outburst in 2016/17. In this paper we use simultaneous observations of Insight-HXMT and NuSTAR to determine its basic parameters. The observations were performed during the softest part of the outburst, and the spectra show clear thermal disk emission and reflection features. We have fitted the X-ray energy spectra using the joint fitting method of the continuum and reflection components with the kerrbb2+ relxill model. Since there is a possibility that the distance to this source was previously underestimated, we use the latest distance parameter of 6.9 kpc in our study, in contrast to previous work in which the distance was set at 2.4 kpc. Through spectral fitting of fixing black hole mass at 6.4 $M_{\rm \odot}$, we observe a strong dependence of the derived spin on the distance: $a_{*}=0.972_{-0.005}^{+0.004}$ at an assumed distance of 2.4 kpc and $a_{*}=0.464_{-0.007}^{+0.016}$ at an assumed distance of 6.9 kpc, at a confidence level of 90%. If considering the uncertainties in the distance and black hole mass, there will be a wider range of spin with $a_{*}$ < 0.78. The fitting results with the new distance indicate that GRS 1716-249 harbors a moderate spin black hole with an inclined ($i\sim 40-50^{\circ}$) accretion disk around it. Additionally, we have also found that solely using the method of the reflection component fitting but ignoring the constraints on the spin from the accretion disk component will result in an extremely high spin.

astro-ph.HE↗

arXiv2024

Characterization of the RD50-MPW4 HV-CMOS pixel sensor

The RD50-MPW4 is the latest HV-CMOS pixel sensor from the CERN-RD50-CMOS working group, designed to evaluate the HV-CMOS technology in terms of spatial resolution, radiation hardness and timing performance. Fabricated by LFoundry using a 150nm process, it features an improved architecture to mitigate crosstalk, which has been an issue with the predecessor RD50-MPW3, allowing more sensitive threshold settings and full matrix operation. Enhancements include separated power domains for peripheral and in-pixel digital readout, a new backside-biasing step, and an improved guard ring structure supporting biasing up to 500V, significantly boosting radiation hardness. Laboratory measurements and test beam results presented in this paper show significant improvements over its predecessor regarding noise behavior, spatial resolution, and efficiency.

physics.ins-det↗

arXiv2024

Development of the 220/270 GHz Receiver of BICEP Array

Measurements of B-mode polarization in the CMB sourced from primordial gravitational waves would provide information on the energy scale of inflation and its potential form. To achieve these goals, one must carefully characterize the Galactic foregrounds, which can be distinguished from the CMB by conducting measurements at multiple frequencies. BICEP Array is the latest-generation multi-frequency instrument of the BICEP/Keck program, which specifically targets degree-scale primordial B-modes in the CMB. In its final configuration, this telescope will consist of four small-aperture receivers, spanning frequency bands from 30 to 270 GHz. The 220/270 GHz receiver designed to characterize Galactic dust is currently undergoing commissioning at Stanford University and is scheduled to deploy to the South Pole during the 2024--2025 austral summer. Here, we will provide an overview of this high-frequency receiver and discuss the integration status and test results as it is being commissioned.

astro-ph.IM↗

arXiv2024

The HERMES (High Energy Rapid Modular Ensemble of Satellites) Pathfinder mission

HERMES (High Energy Rapid Modular Ensemble of Satellites) Pathfinder is a space-borne mission based on a constellation of six nano-satellites flying in a low-Earth orbit (LEO). The 3U CubeSats, to be launched in early 2025, host miniaturized instruments with a hybrid Silicon Drift Detector/GAGG:Ce scintillator photodetector system, sensitive to X-rays and gamma-rays in a large energy band. HERMES will operate in conjunction with Australian Space Industry Responsive Intelligent Thermal (SpIRIT) 6U CubeSat, launched in December 2023. HERMES will probe the temporal emission of bright high-energy transients such as Gamma-Ray Bursts (GRBs), ensuring a fast transient localization in a field of view of several steradians exploiting the triangulation technique. HERMES intrinsically modular transient monitoring experiment represents a keystone capability to complement the next generation of gravitational wave experiments. In this paper we outline the scientific case, development and programmatic status of the mission

astro-ph.IM↗

arXiv2024

Filamentary mass accretion towards the high-mass protobinary system G11.92-0.61 MM2

We present deep, sub-arcsecond ($\sim$2000 AU) resolution ALMA 0.82 mm observations of the former high-mass prestellar core candidate G11.92-0.61 MM2, recently shown to be an $\sim$500 AU-separation protobinary. Our observations show that G11.92-0.61 MM2, located in the G11.92-0.61 protocluster, lies on a filamentary structure traced by 0.82 mm continuum and N$_2$H$^+$(4-3) emission. The N$_2$H$^+$(4-3) spectra are multi-peaked, indicative of multiple velocity components along the line of sight. To analyse the gas kinematics, we performed pixel-by-pixel Gaussian decomposition of the N$_2$H$^+$ spectra using SCOUSEPY and hierarchical clustering of the extracted velocity components using ACORNS. Seventy velocity- and position-coherent clusters (called "trees") are identified in the N$_2$H$^+$-emitting gas, with the 8 largest trees accounting for >60% of the fitted velocity components. The primary tree, with $\sim$20% of the fitted velocity components, displays a roughly north-south velocity gradient along the filamentary structure traced by the 0.82 mm continuum. Analysing a $\sim$0.17 pc-long substructure, we interpret its velocity gradient of $\sim$10.5 km s$^{-1}$pc$^{-1}$ as tracing filamentary accretion towards MM2 and estimate a mass inflow rate of $\sim$1.8$\times10^{-4}$ to 1.2$\times10^{-3}$ M$_\odot$ yr$^{-1}$. Based on the recent detection of a bipolar molecular outflow associated with MM2, accretion onto the protobinary is ongoing, likely fed by the larger-scale filamentary accretion flows. If 50% of the filamentary inflow reaches the protostars, each member of the protobinary would attain a mass of 8 M$_\odot$ within $\sim1.6\times$10$^5$ yr, comparable to the combined timescale of the 70 $μ$m- and MIR-weak phases derived for ATLASGAL-TOP100 massive clumps using chemical clocks.

astro-ph.GA↗

arXiv2024

Constraining Inflation with the BICEP/Keck CMB Polarization Experiments

The BICEP/$\textit{Keck}$ (BK) series of cosmic microwave background (CMB) polarization experiments has, over the past decade and a half, produced a series of field-leading constraints on cosmic inflation via measurements of the "B-mode" polarization of the CMB. Primordial B modes are directly tied to the amplitude of primordial gravitational waves (PGW), their strength parameterized by the tensor-to-scalar ratio, $r$, and thus the energy scale of inflation. Having set the most sensitive constraints to-date on $r$, $σ(r)=0.009$ ($r_{0.05}<0.036, 95\%$ C.L.) using data through the 2018 observing season ("BK18"), the BICEP/$\textit{Keck}$ program has continued to improve its dataset in the years since. We give a brief overview of the BK program and the "BK18" result before discussing the program's ongoing efforts, including the deployment and performance of the $\textit{Keck Array}$'s successor instrument, BICEP Array, improvements to data processing and internal consistency testing, new techniques such as delensing, and how those will ultimately serve to allow BK reach $σ(r) \lesssim 0.003$ using data through the 2027 observing season.

astro-ph.CO↗

arXiv2024

Ab initio calculations with a new local chiral N3LO nucleon-nucleon force

Ab initio calculations have achieved remarkable success in nuclear structure studies. Numerous works highlight the pivotal role of three-body forces in nuclear ab initio calculations. Concurrently, efforts have been made to replicate these calculations using only realistic nucleon-nucleon (NN) interactions. A novel local chiral next-to-next-to-next-to-leading order (N3LO) NN interaction, distinct due to its weaker tensor force, has recently been established. This paper applies this local NN interaction in ab initio frameworks to calculate the low-lying spectra of p-shell light nuclei, particularly 10B, ground-state energies and shell evolution in oxygen isotopes. Results are compared with calculations utilizing nonlocal chiral N3LO NN and chiral NN +3N interactions. The ab initio calculations with the local N N potential accurately describe the spectra of p-shell nuclei, notably the 10B. Additionally, the neutron drip line for oxygen isotopes, with 24O as the drip line nucleus, is accurately reproduced in ab initio calculations with the local NN interaction. Calculations with the local NN interaction also reproduce the subshell closure at N = 14 and 16, albeit with a stronger shell gap compared to experimental data. However, the calculated charge radii based on the local NN interaction are underestimated compared with experimental data, which is similar to results from the nonlocal NN interaction. Consequently, the present ab initio calculations further indicate significant spin-orbit splitting effects with the new local NN potential, suggesting that 3N forces remain an important consideration.

nucl-th↗

arXiv2024

Score-CDM: Score-Weighted Convolutional Diffusion Model for Multivariate Time Series Imputation

Multivariant time series (MTS) data are usually incomplete in real scenarios, and imputing the incomplete MTS is practically important to facilitate various time series mining tasks. Recently, diffusion model-based MTS imputation methods have achieved promising results by utilizing CNN or attention mechanisms for temporal feature learning. However, it is hard to adaptively trade off the diverse effects of local and global temporal features by simply combining CNN and attention. To address this issue, we propose a Score-weighted Convolutional Diffusion Model (Score-CDM for short), whose backbone consists of a Score-weighted Convolution Module (SCM) and an Adaptive Reception Module (ARM). SCM adopts a score map to capture the global temporal features in the time domain, while ARM uses a Spectral2Time Window Block (S2TWB) to convolve the local time series data in the spectral domain. Benefiting from the time convolution properties of Fast Fourier Transformation, ARM can adaptively change the receptive field of the score map, and thus effectively balance the local and global temporal features. We conduct extensive evaluations on three real MTS datasets of different domains, and the result verifies the effectiveness of the proposed Score-CDM.

cs.LG↗

arXiv2024

Broadband noise and quasi-periodic oscillation characteristics of the X-ray pulsar RX J0440.9+4431

We present a comprehensive timing analysis on the Be/X-ray binary pulsar RX J0440.9+4431 using observations from \textit{NICER} and \textit{Insight}-HXMT during the 2022--2023 outburst. The power density spectrum (PDS) of RX J0440.9+4431 exhibits typical aperiodic variability in X-ray flux across a wide frequency range. During a super-critical accretion state, we detect quasi-periodic oscillations (QPOs) at 0.2--0.5\,Hz in the light curves of five pulses for RX J0440.9+4431. The observed QPOs manifest during flares, while the flares appear at the peaks of the pulse profiles on a timescale of seconds and are primarily caused by an increase in hard photons. These flares can be explained by increased material ingestion in the accretion column at a fixed phase, primarily generating hard photons. Alternatively, an increase in accretion rate, independent of phase, may result in highly beamed hard photons within the accretion column, causing the flares. We argue the origin of QPOs to instabilities within the accretion flow. Additionally, we find that the break frequencies in the noise power spectra align well with $\propto L_{\mathrm{x}}^{3 / 7}$ across three orders of magnitude in the luminosity, which points to a relatively strong magnetic field in RX J0440.9+4431, estimated to be \textasciitilde$10^{13}$\,G.

astro-ph.HE↗

arXiv2024

Spontaneous Chirality Flipping in an Orthogonal Spin-Charge Ordered Topological Magnet

The asymmetric distribution of chiral objects with opposite chirality is of great fundamental interests ranging from molecular biology to particle physics. In quantum materials, chiral states can build on inversion-symmetry-breaking lattice structures or emerge from spontaneous magnetic ordering induced by competing interactions. Although the handedness of a chiral state can be changed through external fields, a spontaneous chirality flipping has yet to be discovered. In this letter, we present experimental evidence of chirality flipping via changing temperature in a topological magnet EuAl$_4$, which features orthogonal spin and charge density waves (SDW/CDW). Using circular dichroism of Bragg peaks in the resonant magnetic x-ray scattering, we find that the chirality of the helical SDW flips through a first order phase transition with modified SDW wavelength. Intriguingly, we observe that the CDW couples strongly with the SDW and displays a rare commensurate-to-incommensurate transition at the chirality flipping temperature. Combining with first principles calculations and angle resolved photoemission spectroscopy, we establish the Fermi surface origin of the helical SDW with intertwined spin, charge, and lattice degrees of freedom in EuAl$_4$. Our results reveal an unprecedented spontaneous chirality flipping and lays the groundwork for a new functional manipulation of chirality through momentum dependent spin-charge-lattice interactions.

cond-mat.str-el↗

arXiv2024

Origins of limited non-basal plasticity in the μ-phase at room temperature

We unveil a new non-basal slip mechanism in the μ-phase at room temperature using nanomechanical testing, transmission electron microscopy and atomistic simulation. The (1-105) planar faults with a displacement vector of 0.07[-5502] can be formed by dislocation glide. They do not disrupt the Frank-Kasper packing and therefore enable the accommodation of plastic strain at low temperatures without requiring atomic diffusion. The intersections between the (1-105) planar faults and basal slip result in stress concentration and crack nucleation during loading.

cond-mat.mtrl-sci↗

arXiv2024

Pulsed Iron line Emission from the First Galactic Ultraluminous X-ray Pulsar Swift J0243.6+6124

We report the phase-resolved spectral results of the first Galactic Pulsating Ultra-Luminous X-ray source (PULX) Swift J0243.6+6124, modeling at its 2017-2018 outburst peak using data collected by the Hard X-ray Modulation Telescope (Insight-HXMT). The broad energy coverage of Insight-HXMT allows us to obtain more accurate spectral continuum to reduce the coupling of broad iron line profiles with other components. We use three different continuum spectrum models but obtain similar iron line results. For the first time, we detected the pulse characteristics of the broad iron line in a PULX. The variation in width and intensity of this iron line with $σ\sim 1.2-1.5$\,keV has a phase offset of about 0.25 from the pulse phase. We suggest that the uneven irradiation of the thick inner disk by the accretion column produces the modulated variation of the broad iron line. In addition, the non-pulsed narrow line is suggested to come from the outer disk region.

astro-ph.HE↗

arXiv2024

Synergistic Effect of Multi-Walled Carbon Nanotubes and Ladder-Type Conjugated Polymers on the Performance of N-Type Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) have the potential to revolutionize the field of organic bioelectronics. To date, most of the reported OECTs include p-type (semi-)conducting polymers as the channel material, while n-type OECTs are yet at an early stage of development, with the best performing electron-transporting materials still suffering from low transconductance, low electron mobility, and slow response time. Here, the high electrical conductivity of multi-walled carbon nanotubes (MWCNTs) and the large volumetric capacitance of the ladder-type π-conjugated redox polymer poly(benzimidazobenzophenanthroline) (BBL) are leveraged to develop n-type OECTs with record-high performance. It is demonstrated that the use of MWCNTs enhances the electron mobility by more than one order of magnitude, yielding fast transistor transient response (down to 15 ms) and high uC* (electron mobility x volumetric capacitance) of about 1 F/cmVs. This enables the development of complementary inverters with a voltage gain of > 16 and a large worst-case noise margin at a supply voltage of < 0.6 V, while consuming less than 1 uW of power.

physics.app-ph↗

arXiv2024

Optical probe on doping modulation of magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$

The magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ is extensively investigated due to its giant anomalous Hall effect (AHE).Recent studies demonstrate that the AHE can be effectively tuned by multi-electron Ni doping.To reveal the underlying mechanism of this significant manipulation,it is crucial to explore the band structure modification caused by Ni doping. Here,we study the electrodynamics of both pristine and Ni-doped Co$_{3-x}$Ni$_x$Sn$_2$S$_2$ with $x=$0, 0.11 and 0.17 by infrared spectroscopy. We find that the inverted energy gap around the Fermi level($E_{F}$) gets smaller at $x=$0.11,which is supposed to enhance the Berry curvature and therefore increase the AHE.Then $E_{F}$ moves out of this gap at $x=$0.17. Additionally,the low temperature carrier density is demonstrated to increase monotonically upon doping,which is different from previous Hall measurement results. We also observe the evidences of band broadening and exotic changes of high-energy interband transitions caused by doping.Our results provide detailed information about the band structure of Co$_{3-x}$Ni$_x$Sn$_2$S$_2$ at different doping levels,which will help to guide further studies on the chemical tuning of AHE.

cond-mat.mes-hall↗

arXiv2023

Ab initio descriptions of $A=16$ mirror nuclei with resonance and continuum coupling

We have used an {\it ab initio} Gamow shell model to study the isospin symmetry breaking in the $A=16$ mirror nuclei of $^{16}$F, $^{16}$N, $^{16}$Ne and $^{16}$C. Starting from a chiral interaction with two-nucleon force (2NF) at N$^3$LO and three-nucleon force (3NF) at N$^2$LO, a complex-momentum ${\it psd}$-shell Hamiltonian was constructed by employing the many-body perturbation theory in the Gamow Hartree-Fock basis which includes bound, resonant and continuum states self-consistently. Such an elaborated {\it ab initio} Gamow shell model with both continuum coupling and 3NF included can properly treat the many-body correlations of weakly bound and unbound nuclei. The mirror partners of $^{16}$F and $^{16}$N exhibit different level orders in their excitation spectra, which can be well explained by the inclusion of 3NF in the calculation. The isospin asymmetry between the mirror partners $^{16}$Ne and $^{16}$C was studied in detail by insight into their configuration structures. The interplay between 3NF and the continuum coupling is discussed in the weakly bound and unbound nuclear states.

nucl-th↗

arXiv2023

From Stellar Death to Cosmic Revelations: Zooming in on Compact Objects, Relativistic Outflows and Supernova Remnants with AXIS

Compact objects and supernova remnants provide nearby laboratories to probe the fate of stars after they die, and the way they impact, and are impacted by, their surrounding medium. The past five decades have significantly advanced our understanding of these objects, and showed that they are most relevant to our understanding of some of the most mysterious energetic events in the distant Universe, including Fast Radio Bursts and Gravitational Wave sources. However, many questions remain to be answered. These include: What powers the diversity of explosive phenomena across the electromagnetic spectrum? What are the mass and spin distributions of neutron stars and stellar mass black holes? How do interacting compact binaries with white dwarfs - the electromagnetic counterparts to gravitational wave LISA sources - form and behave? Which objects inhabit the faint end of the X-ray luminosity function? How do relativistic winds impact their surroundings? What do neutron star kicks reveal about fundamental physics and supernova explosions? How do supernova remnant shocks impact cosmic magnetism? This plethora of questions will be addressed with AXIS - the Advanced X-ray Imaging Satellite - a NASA Probe Mission Concept designed to be the premier high-angular resolution X-ray mission for the next decade. AXIS, thanks to its combined (a) unprecedented imaging resolution over its full field of view, (b) unprecedented sensitivity to faint objects due to its large effective area and low background, and (c) rapid response capability, will provide a giant leap in discovering and identifying populations of compact objects (isolated and binaries), particularly in crowded regions such as globular clusters and the Galactic Center, while addressing science questions and priorities of the US Decadal Survey for Astronomy and Astrophysics (Astro2020).

astro-ph.HE↗