arXiv subjects

C. Liu

Publications and source records attributed to C. Liu.

At least 19 recordsLinked to original sources

arXiv2026

SN 2024afyu interpreted as a Pair Instability Supernova

Pair-instability supernovae (PISNe) are the predicted explosions of very massive stars triggered by electron-positron pair production. Numerous transients have been proposed as PISN candidates, yet none has provided unambiguous confirmation of this explosion mechanism. The predicted strengths of nebular emission lines offer a powerful means of testing the PISN scenario. We investigate the nature of SN 2024afyu, a nearby (z = 0.0085), long-lived (trise = 85 +- 11.7 days) SN with peculiar spectral evolution, with the aim of identifying its powering mechanism. We analyse multi-band photometry and optical and near-infrared spectroscopy from shortly after explosion to the nebular phase (around 500 days past peak). Besides early appearance of [Ca II] features, we identify a number of sulfur and silicon emission lines, for which we estimate electron temperatures and elemental masses. SN 2024afyu has an inferred 56Ni mass of around 0.4 to 1.0 solar masses and an inferred sulfur mass of the order of 3 solar masses, substantially larger than expected for conventional core-collapse explosions. SN 2024afyu is photometrically similar (although fainter, M_Peak(r) = -18.9 +- 0.04 mag) but spectroscopically distinct to other proposed PISNe. Yet, existing PISN models broadly reproduce several key characteristics, including the overall spectral appearance and broad photometric evolution. SN 2024afyu is a strong PISN candidate, since alternative scenarios would struggle to explain the combination of broad light curve, large intermediate-mass-element abundance, and general spectroscopic evolution. The discrepancies between the observations and currently available theoretical models highlight the need for new PISN calculations spanning a wider range of progenitor masses, metallicities, mixing prescriptions, and circumstellar environments.

astro-ph.SR↗

arXiv2026

Peak 'Nebular' Emission and Early Flux Excesses in Ca-strong Supernovae

Calcium-strong supernovae (Ca-strong SNe) are a class of thermonuclear transient characterised by rapid photometric evolution (rise times of <= 16 d) and nebular-phase spectra dominated by strong [Ca II]7291,7324 emission, in comparison to the strength of [O I]6300,6364 emission. Despite their distinctive spectra and association with remote environments, their origins remain uncertain. Using new discoveries from surveys such as the Zwicky Transient Facility, supplemented by archival data, we compile a sample of 46 Ca-strong SNe - the largest sample to date. We present a comprehensive optical photometric and spectroscopic study of Ca-strong SNe, aimed at better constraining their diversity, progenitor systems, and explosion mechanisms. Of the 35 Ca-strong SNe with spectra near peak light, we find that 32 objects exhibit significant [Ca II] emission within 10 days of peak. Forbidden [Ca II] emission is typically expected to emerge after the ejecta have expanded, cooled, and reached a sufficiently low density for forbidden transitions to occur. The prevalence of these early features in Ca-strong SNe, however, remains unexplained by current explosion models of these objects. In several objects, we identify complex, multi-component line profiles that suggest a transition between two distinct velocity regimes. These transitional features, combined with the common presence of early forbidden emission, lead us to favour a scenario in which extended material surrounds the progenitor and pre-dates the SN itself. Our results suggest that the diverse spectroscopic behaviours of Ca-strong SNe are consistent with white dwarfs exploding in extended environments polluted by prior activity of the progenitor system.

astro-ph.HE↗

arXiv2026

Simulations of internal kink modes and sawtooth crashes for SPARC baseline-like scenarios using the M3D-C1 code

A relaxed baseline case, based on the SPARC Primary Reference Discharge (PRD) design point, is used to conduct a thorough investigation for the most unstable low-$n$ MHD instabilities for the first time. The simulations use the high-fidelity 3D extended-MHD code M3D-C1. The linear simulation, by scanning over the resistivity, identifies a dominant internal kink mode at the $q=1$ surface with a toroidal mode number $n=1$. Both the current and the pressure profiles are strongly affecting the kink instability in the baseline case. The linear growth rate is sensitive to the keV-level temperature profile and the on-axis $q_0$ around unity. A simplified 1D eigenvalue solver shows a good qualitative agreement for the observed pressure effects. In 3D nonlinear simulations, the marginally unstable case gives a moderate sawtooth crash soon after $q_0$ drops below unity, likely because of the lack of stabilizing effects in our simulations, such as heating and energetic particles. When both the current and the pressure drives exist (the baseline case), a strong sawtooth is observed, which features a magnetic reconnection event and a hollowed pressure profile. This can be explained by mixing both the Kadomtsev and Wesson models. The actual sawtooth crash may occur in SPARC before $q_0$ drops far below unity due to the sensitive changes of the instability around $q_0\sim 1$. The sawtooth-like oscillations shown in low-$\beta$ simulations also provides an opportunity to investigate periodic sawtoothing timescales in SPARC. This work forms a basis for understanding particle and heat transport under the influence of MHD instabilities, which can be essential for properly assessing the performance of the SPARC tokamak and future fusion pilot plants.

physics.plasm-ph↗

arXiv2026

GOTO identification and broadband modelling of the counterpart to the SVOM GRB 250818B

Rapid localisation and follow-up of gamma-ray bursts (GRBs) increasingly rely on low-latency triggers from new missions coupled to wide-field robotic optical facilities. We present the discovery and multi-wavelength follow-up of GRB 250818B, detected by the Space Variable Objects Monitor (SVOM) and localised optically by the Gravitational-wave Optical Transient Observer (GOTO). We compile and homogenise X-ray, optical/NIR, and radio data to build broadband light curves and spectral energy distributions. The afterglow is unusually luminous for a nominal short GRB, lying on the bright end of the short-GRB population in X-rays and optical and among the most luminous high-redshift short-GRB afterglows in the radio. MeerKAT detects the source at 3.1 GHz, while ALMA provides deep higher-frequency limits. Keck/LRIS spectroscopy shows continuum and metal absorption (Fe II, Mg II, Mg I), giving $z=1.216$. Synchrotron forward-shock modelling favours a constant-density medium and strongly prefers refreshed (energy-injection) emission, well described by a two-component jet with $E_{K,iso} \sim 4\times10^{52}$ erg, $n_0 \sim 3.6$ cm$^{-3}$, $\theta_j \simeq 0.10$ rad ($\sim 5.7$ deg), and $p \simeq 1.64$. The host association is ambiguous: the nearest LS DR10 galaxy candidate ($r_{AB} \sim 24.7$) is offset by $\sim 4$ arcsec ($\sim 34$ kpc) with chance-alignment probability $P_{cc} \sim 0.2$, and current imaging does not exclude a fainter, near-coincident host. SED fitting of the candidate host suggests a low-mass galaxy. GRB 250818B highlights the power of rapid wide-field counterpart identification in the SVOM era, while host-association uncertainty can still limit offset-based interpretation.

astro-ph.HE↗

arXiv2026

The X-ray properties and structure of A3571 up to $R_{500}$

Abell 3571 is a nearby, X-ray bright galaxy cluster located in the Shapley Supercluster. Although it appears morphologically relaxed in X-ray images, multiwavelength observations reveal subtle indications of residual dynamical activity, likely associated with past merger events. Using wide-field ($1^{\circ} \times 1^{\circ}$) data from the Einstein Probe Follow-up X-ray Telescope (EP-FXT), we extend measurements of the cluster's properties beyond its $R_{500}$ radius. We detect surface-brightness excesses on both the northern and southern sides, consistent with previous XMM-Newton results. The temperature, pressure, and entropy in the northern excess region are lower than the average values, whereas those on the southern side are slightly higher. However, we find no evidence for cold fronts or shocks. These features can be interpreted as sloshing motions triggered by an off-center minor merger. Our findings suggest that, despite its symmetric appearance, A3571 is still recovering from a minor merger and is currently in a post-merger phase. This work also demonstrates the excellent capability of EP-FXT for probing the outskirts of galaxy clusters.

astro-ph.HE↗

arXiv2025

Field localisation and spin-momentum locking in zero-dimensional dissipative topological photonic interface state

Topological photonic systems support edge states that are robust against disorder and perturbation. Depending on the symmetry and dimensionality of the bulk systems, different edge states emulating soliton, quantum integer and quantum spin Hall effects have been realized. A major concern in photonics is how one can shape the strength and polarisation of electromagnetic fields to suit different applications. Here, we show zero-dimensional (0D) interface state arising from one-dimensional (1D) dissipative topological photonic crystals exhibit strong field localisation and spin-momentum locking thanks to its complex classical analogue Dirac mass parameter. By using spatiotemporal coupled mode theory to formulate 1D photonic crystals and their corresponding Jackiw Rebbi-like (JR) interface state, we find the interaction between two energy bands at high symmetry points plays a major role in defining not only the topological triviality of the crystals but also its complex Dirac mass parameter. More importantly, when two topological trivial and nontrivial bulk systems are brought together to form a JR state, while the real part of the Dirac mass parameter governs the spectral and spatial field localisations of the interface state, the imaginary part gives rise to a net flow of energy towards the interface and a transverse spin angular momentum, resulting in a strong spin-momentum locking. We verify our theory by 1D plasmonic crystals using finite-difference time-domain simulations as well as far-field angle-resolved spectroscopy and imaging.

physics.optics↗

arXiv2025

Next generation direct RF sampling LLRF control and monitoring system for linear accelerators

The low-level RF (LLRF) systems for linear accelerating structures are typically based on heterodyne architectures. The linear accelerators normally have many RF stations and multiple RF inputs and outputs for each station, so the complexity and size of the LLRF system grows rapidly when scaling up. To meet the design goals of being compact and affordable for future accelerators, or upgrading existing ones, we have developed and characterized the next generation LLRF (NG-LLRF) platform based on the RF system-on-chip (RFSoC) for S-band and C-band accelerating structures. The integrated RF data converters in RFSoC sample and generate the RF signals directly without any analogue mixing circuits, which significantly simplified the architecture compared with the conventional LLRF systems. We have performed high-power tests for the NG-LLRF with the S-band accelerating structure in the Next Linear Collider Test Accelerator (NLCTA) test facility at SLAC National Accelerator Laboratory and a C-band structure prototyped for Cool Cooper Collider (CCC). The NG-LLRF platform demonstrated pulse-to-pulse fluctuation levels considerably better than the requirements of the targeted applications and high precision and flexibility in generating and measuring the RF pulses. In this paper, the characterization results of the platform with different system architectures will be summarized and a selection of high-power test results of the NG-LLRF will be presented and analyzed.

physics.acc-ph↗

arXiv2025

Imaging-spectroscopy diagnosis of the giant sloshing spiral in the Virgo cluster with the Einstein Probe Follow-up X-ray Telescope

We performed deep X-ray observations of the Virgo cluster using the Einstein Probe Follow-up X-ray Telescope (EP-FXT) with a total exposure of 295 ks. Leveraging the large field of view (FoV) and low particle background of EP-FXT, the image reveals a giant spiral feature connecting the cold fronts in the northwest and southeast, forming a coherent structure consistent with earlier results from XMM-Newton and Suzaku. We also present two-dimensional maps of temperature, metallicity, and entropy across the Virgo Cluster, covering a FoV of approximately 28.5 arcmin. These maps clearly show a spiral structure with high density, low temperature, high metallicity, and low entropy. The results support a scenario where the spiral morphology arises from gas sloshing driven by a minor merger. Additionally, EP-FXT temperature measurements agree well with XMM-Newton data within uncertainties.

astro-ph.HE↗

arXiv2025

Runaway electron-induced plasma facing component damage in tokamaks

This Roadmap article addresses the critical and multifaceted challenge of plasma-facing component (PFC) damage caused by runaway electrons (REs) in tokamaks, a phenomenon that poses a significant threat to the viability and longevity of future fusion reactors such as ITER and DEMO. The dramatically increased RE production expected in future high-current tokamaks makes it difficult to avoid or mitigate REs when a plasma discharge terminates abnormally. Preventing damage from the intense localised heat loads REs can cause requires a holistic approach that considers plasma, REs and PFC damage. Despite decades of progress in understanding the physics of REs and the thermomechanical response of PFCs, their complex interplay remains poorly understood. This document aims to initiate a coordinated, interdisciplinary approach to bridge this gap by reviewing experimental evidence, advancing diagnostic capabilities, and improving modelling tools across different scales, dimensionalities and fidelities. Key topics include RE beam formation and transport, damage mechanisms in brittle and metallic PFCs, and observations in major facilities such as JET, DIII-D, WEST and EAST. The Roadmap emphasises the urgency of predictive, high-fidelity modelling validated against well-diagnosed controlled experiments, particularly in the light of recent changes in ITER's wall material strategy and the growing importance of private sector initiatives. Each section of the article is written to provide a concise overview of one area of this multidisciplinary subject, with an assessment of the status, a look at current and future challenges, and a brief summary. The ultimate goal of this initiative is to guide future mitigation strategies and design resilient components that can withstand the loads imposed by REs, thus ensuring the safe and sustainable operation of the next generation of fusion power plants.

physics.plasm-ph↗

arXiv2025

The ambiguous AT2022rze: Changing-look AGN mimicking a supernova in a merging galaxy system

AT2022rze is a luminous, ambiguous transient located South-East of the geometric center of its host galaxy at redshift z = 0.08. The host appears to be formed by a merging galaxy system. The observed characteristics of AT2022rze are reminiscent of active galactic nuclei (AGN), tidal disruption events (TDEs), and superluminous supernovae (SLSNe). The transient reached a peak absolute magnitude of -20.2 +- 0.2 mag, showing a sharp rise (trise,1/e = 27.5 +- 0.6 days) followed by a slow decline (tdec,1/e = 382.9 +- 0.6). Its bumpy light curve and narrow Balmer lines indicate the presence of gas (and dust). Its light curve shows rather red colors, indicating that the transient could be affected by significant host extinction. The spectra reveal coronal lines, indicative of high-energy (X-ray/UV) emission. Archival data reveal no prior activity at this location, disfavoring a steady-state AGN, although an optical spectrum obtained prior to the transient is consistent with an AGN classification of the host. Based on this, we conclude that the transient most likely represents a Changing-look AGN at the center of the smallest component of the merging system.

astro-ph.HE↗

arXiv2025

Novel bipartite entanglement in the quantum dimer magnet Yb$_2$Be$_2$SiO$_7$

The quantum dimer magnet, with antiferromagnetic intradimer and interdimer Heisenberg exchange between spin-1/2 moments, is known to host an up/down - down/up singlet ground state when the intradimer exchange is dominant. Rare-earth-based quantum dimer systems with strong spin-orbit coupling offer the opportunity for tuning their magnetic properties by using magnetic anisotropy as a control knob. Here, we present bulk characterization and neutron scattering measurements of the quantum dimer magnet Yb$_2$Be$_2$SiO$_7$. We find that the Yb$^{3+}$ ions can be described by an effective spin-1/2 model at low temperatures and the system does not show signs of magnetic order down to 50 mK. The magnetization, heat capacity, and neutron spectroscopy data can be well-described by an isolated dimer model with highly anisotropic exchange that stabilizes a singlet ground state with a wavefunction up/up - down/down or up/up + down/down. Our results show that strong spin-orbit coupling can induce novel entangled states of matter in quantum dimer magnets.

cond-mat.str-el↗

arXiv2025

Study of polarization-dependent band inversions and edge states from two-dimensional square lattice plasmonic crystals

When two topologically trivial and nontrivial systems are brought together, a localized energy state is formed at the interface. For crystalline quantum and classical systems, their topology can be determined by studying the eigenmode symmetries at high symmetry points (HSPs) in the Brillouin zone. As electromagnetic systems are usually leaky, their radiations retain the eigenmode information, thus providing a means for probing the band topology. Here, we formulate the far-field characteristics of the eigenmodes at HSPs in 2D square lattice plasmonic systems and reveals, unlike the conventional tight-binding model, several polarization-dependent band inversions occur at the{\Gamma}and X points, rendering subtle changes in their band topology. In particular, by carefully tuning the system geometry to facilitate trivial and non-trivial phases, polarization selective 0D and 1D edge states that possess distinct field symmetries are realized. The evolution of 0D or 1D edge state depends on the degree of bulk-edge overlapping. We perform angle- and polarization-resolved diffraction spectroscopy on 2D Au plasmonic nanohole arrays to verify the theory and observe such states. Our study demonstrates the applicability of simple far-field technique for diagnosing the band topology of various crystalline classical systems in electromagnetics and acoustics.

physics.optics↗

arXiv2025

EP 250108a/SN 2025kg: Observations of the most nearby Broad-Line Type Ic Supernova following an Einstein Probe Fast X-ray Transient

With a small sample of fast X-ray transients (FXTs) with multi-wavelength counterparts discovered to date, the progenitors of FXTs and their connections to gamma-ray bursts (GRBs) and supernovae (SNe) remain ambiguous. Here, we present photometric and spectroscopic observations of SN 2025kg, the supernova counterpart to the FXT EP 250108a. At $z=0.17641$, this is the closest known SN discovered following an Einstein Probe (EP) FXT. We show that SN 2025kg's optical spectra reveal the hallmark features of a broad-lined Type Ic SN. Its light curve evolution and expansion velocities are also comparable to those of GRB-SNe, including SN 1998bw, and several past FXT SNe. We present JWST/NIRSpec spectroscopy taken around SN 2025kg's maximum light, and find weak absorption due to He I $\lambda 1.0830, \lambda 2.0581$ $\mu$m and a broad, unidentified feature at $\sim$ 4-4.5 $\mu$m. Further, we observe clear evidence for broadened H$\alpha$ in optical data at 42.5 days that is not detected at other epochs, indicating interaction with hydrogen-rich material. From its light curve, we derive a $^{56}$Ni mass of 0.2 - 0.6 $M_{\odot}$. Together with our companion paper (Eyles-Ferris et al. 2025), our broadband data of EP 250108a/SN 2025kg are consistent with a trapped or low energy ($\lesssim 10^{51}$ ergs) jet-driven explosion from a collapsar with a zero-age main sequence mass of 15-30 $M_{\odot}$. Finally, we show that the sample of EP FXT SNe support past rate estimates that low-luminosity jets seen through FXTs are more common than successful (GRB) jets, and that similar FXT-like signatures are likely present in at least a few percent of the brightest Ic-BL SNe.

astro-ph.HE↗

arXiv2025

Dissociation and radiative stabilization of the indene cation: The nature of the C-H bond and astrochemical implications

Indene (C$_9$H$_8$) is the only polycyclic pure hydrocarbon identified in the interstellar medium to date, with an observed abundance orders of magnitude higher than predicted by astrochemical models. The dissociation and radiative stabilization of vibrationally-hot indene cations is investigated by measuring the time-dependent neutral particle emission rate from ions in a cryogenic ion-beam storage ring for up to 100~ms. Time-resolved measurements of the kinetic energy released upon hydrogen atom loss from C$_9$H$_8^+$, analyzed in view of a model of tunneling through a potential energy barrier, provides the dissociation rate coefficient. Master equation simulations of the dissociation in competition with vibrational and electronic radiative cooling reproduce the measured dissociation rate. We find that radiative stabilization arrests one of the main C$_9$H$_8$ destruction channels included in astrochemical models, helping to rationalize its high observed abundance.

physics.chem-ph↗

arXiv2025

Coupled 2-D MHD and runaway electron fluid simulations of SPARC disruptions

Runaway electrons (REs) generated during disruption events in tokamaks can carry mega-Ampere level currents, potentially causing damage to plasma-facing components. Understanding RE evolution during disruption events is important for evaluating strategies to mitigate RE damage. Using two-dimensional toroidally symmetric magnetohydrodynamic (MHD) simulations in M3D-C1, which incorporates a fluid RE model evolved self-consistently with the bulk MHD fluid, we examine the seeding and avalanching of REs during disruptions in the SPARC tokamak - a compact high-field high-current device designed to achieve a fusion gain Q > 2 in deuterium-tritium plasmas. The M3D-C1 simulations of unmitigated disruptions demonstrate RE plateau formation and peaking of the final current density, which agree well with the results of lower-fidelity reduced RE fluid models. This work provides the first systematic comparison and benchmarking of different primary sources, including activated tritium beta decay and Compton scattering, in SPARC disruption simulations with self-consistent MHD and RE coupling.

physics.plasm-ph↗

arXiv2025

A Compact Low-level RF Control System for Advanced Concept Compact Electron Linear Accelerator

A compact low-level RF (LLRF) control system based on RF system-on-chip (RFSoC) technology has been designed for the Advanced Concept Compact Electron Linear-accelerator (ACCEL) program, which has challenging requirements in both RF performance and size, weight and power consumption (SWaP). The compact LLRF solution employs the direct RF sampling technique of RFSoC, which samples the RF signals directly without any analogue up and down conversion. Compared with the conventional heterodyne based architecture used for LLRF system of linear accelerator (LINAC), the elimination of analogue mixers can significantly reduce the size and weight of the system, especially with LINAC requires a larger number of RF channels. Based on the requirements of ACCEL, a prototype LLRF platform has been developed, and the control schemes have been proposed. The prototype LLRF system demonstrated magnitude and phase fluctuation levels below 1% and 1 degree, on the flat top of a 2 microseconds RF pulse. The LLRF control schemes proposed for ACCEL are implemented with a prototype hardware platform. This paper will introduce the new compact LLRF solution and summarize a selection of experimental test results of the prototype itself and with the accelerating structure cavities designed for ACCEL.

physics.acc-ph↗

arXiv2025

High-Power Test of a C-band Linear Accelerating Structure with an RFSoC-based LLRF System

Normal conducting linear particle accelerators consist of multiple rf stations with accelerating structure cavities. Low-level rf (LLRF) systems are employed to set the phase and amplitude of the field in the accelerating structure, and to compensate the pulse-to-pulse fluctuation of the rf field in the accelerating structures with a feedback loop. The LLRF systems are typically implemented with analogue rf mixers, heterodyne based architectures and discrete data converters. There are multiple rf signals from each of rf station, so the number of rf channels required increases rapidly with multiple rf stations. With many rf channels, the footprint, component cost and system complexity of the LLRF hardware increase significantly. To meet the design goals to be compact and affordable for future accelerators, we have designed the next generation LLRF (NG-LLRF) with higher integration level based on RFSoC technology. The NG-LLRF system samples rf signals directly and performs the rf mixing digitally. The NG-LLRF has been characterized in a loopback mode to evaluate the performance of the system and tested with a standing-wave accelerating structure, a prototype for the Cool Copper Collider (C3) with peak rf power up to 16.45 MW. The loopback test demonstrated amplitude fluctuation below 0.15% and phase fluctuation below 0.15 degree, which are considerably better than the requirements of C3. The rf signals from the different stages of accelerating structure at different power levels are measured by the NG-LLRF, which will be critical references for the control algorithm designs. The NG-LLRF also offers flexibility in waveform modulation, so we have used rf pulses with various modulation schemes which could be useful for controlling some of rf stations in accelerators. In this paper, the high-power test results at different stages of the test setup will be summarized, analyzed and discussed.

physics.acc-ph↗

arXiv2025

ZTF SN Ia DR2: High-velocity components in the Si II $\lambda$6355

The ZTF SN Ia Data Release 2 provides a perfect opportunity to perform a thorough search for, and subsequent analysis of, high-velocity components in the Si II $\lambda$6355 feature in the pre-peak regime. The source of such features remains unclear, with potential origins in circumstellar material or density/abundance enhancements intrinsic to the SN ejecta. Therefore, they may provide clues to the elusive progenitor and explosion scenarios of SNe Ia. We employ a MCMC fitting method followed by BIC testing to classify single and double Si II $\lambda$6355 components in the DR2. The detection efficiency of our classification method is investigated through the fitting of simulated features, allowing us to place cuts upon spectral quality required for reliable classification. These simulations were also used to perform an analysis of the recovered parameter uncertainties and potential biases in the measurements. Within the 329 spectra sample that we investigate, we identify 85 spectra exhibiting Si II $\lambda$6355 HVFs. We find that HVFs decrease in strength with phase relative to their photospheric counterparts - however, this decrease can occur at different phases for different objects. HVFs with larger velocity separations from the photosphere are seen to fade earlier leaving only the double components with smaller separations as we move towards maximum light. Our findings suggest that around three quarters of SN Ia spectra before -11 d show high-velocity components in the Si II $\lambda$6355 with this dropping to around one third in the six days before maximum light. We observe no difference between the populations of SNe Ia that do and do not form Si II $\lambda$6355 HVFs in terms of SALT2 light-curve parameter x1, peak magnitude, decline rate, host mass, or host colour, supporting the idea that these features are ubiquitous across the SN Ia population.

astro-ph.HE↗