arXiv subjects

J. Shi

Publications and source records attributed to J. Shi.

At least 19 recordsLinked to original sources

arXiv2026

The Enigmatic Type Icn Supernova 2024abvb Located ~22 kpc from Its Host Galaxy

We report multiwavelength observations of the highly offset (~22.4 kpc) SN 2024abvb, the sixth Type Icn supernova to date. With a peak magnitude of Mr = -19.55 +/- 0.11 mag, it is among the most luminous in the existing sample and shows similar colours and decline rates to other SNe Icn. The early optical spectra show a blue continuum with narrow C II features (vFWHM ~ 2000 km s^-1), consistent with a typical wind velocity of a Wolf-Rayet star. The absence of C III lambda 5696 emission at the time of explosion is consistent with a Type Ibn supernova; however, the lack of narrow He lines in both the optical and near-infrared spectra supports a SNe Icn classification. Unlike the majority of SNe Icn, we do not detect broad features in the late-time (7-21 days relative to o-band peak) spectral phase of SN 2024abvb. Semi-analytical modelling of the light curves shows that it can be reproduced by ~2.6 Msun of SN ejecta interacting with ~0.3 Msun of circumstellar material (CSM), both larger than other SNe Icn but consistent with rapidly evolving SNe Ibn. The metallicity at the SN location is significantly lower than the global metallicity of its host galaxy, suggesting that line-driven mass loss required to strip the progenitor of its H and He envelopes was likely inefficient. We estimate the star-formation-rate history at the location of SN 2024abvb and find that it lies at the bottom ~5th percentile among SESNe hosts, inconsistent with a Wolf-Rayet progenitor. Based on its spectral features, local and host environment properties, and host-galaxy offset, we favour an 8-10 Msun star stripped by a compact companion as the progenitor, with a sufficient runaway velocity to reach the observed offset.

astro-ph.HE↗

arXiv2025

University of Hawaii 88-inch Telescope Observations of the Interstellar Comet 3I/ATLAS: Spectrophotometric Blue-Sensitive Spectral Time Series Spanning Two Months from Discovery

Interstellar objects are the ejected building blocks of other solar systems. As such, they enable the acquisition of otherwise inaccessible information about nascent extrasolar systems. The discovery of the third interstellar object, 3I/ATLAS, provides an opportunity to explore the properties of a small body from another solar system and to compare it to the small bodies in our own. To that end, we present spectrophotometric observations of 3I/ATLAS taken using the SuperNova Integral Field Spectrograph on the University of Hawaii 2.2-m telescope. Our data includes the earliest $\lambda\leq3800$ A spectrum of 3I/ATLAS, obtained $\sim$12.5 hours after the discovery announcement. Later spectra confirm previously reported cometary activity, including Ni and CN emission. The data show wavelength-varying spectral slopes ($S\approx($0\%-29\%)/1000 A, depending on wavelength range) throughout the pre-perihelion ($r_h=4.4$-$2.5$ au) approach of 3I/ATLAS. We perform synthetic photometry on our spectra and find 3I/ATLAS shows mostly stable color evolution over the period of our observations, with $g-r$ colors ranging from $\sim$0.69-0.75 mag, $r-i$ colors ranging from $\sim$0.26-0.30 mag, and $c-o$ colors ranging from $\sim$0.50-0.55 mag. Ongoing post-perihelion observations of 3I/ATLAS will provide further insight into its potentially extreme composition.

astro-ph.EP↗

arXiv2025

JWST Observations of SN 2023ixf II: The Panchromatic Evolution Between 250 and 720 Days After the Explosion

We present the nebular phase spectroscopic and photometric observations of the nearby hydrogen-rich core-collapse supernova (CC-SN) 2023ixf, obtained through our JWST programs. These observations, combined with ground-based optical and near-infrared spectra, cover +252.67 - 719.96 d, creating a comprehensive, panchromatic time-series dataset spanning 0.32 - 30$\mu$m. In this second paper of the series, we focus on identifying key spectral emission features and tracking their evolution through the nebular phase. The JWST data reveal hydrogen emission from the Balmer to Humphreys series, as well as prominent forbidden lines from Ne, Ar, Fe, Co, and Ni. NIRSpec observations display strong emission from the first overtone and fundamental bands of carbon monoxide, which weaken with time as the ejecta cools and dust emission dominates. The spectral energy distribution shows a clear infrared excess emerging by +252.67 d peaking around 10.0$\mu$m, with a secondary bump at 18.0$\mu$m developing by +719.96 d. We suggest that this evolution could arises from multiple warm dust components. In upcoming papers in this series, we will present detailed modeling of the molecular and dust properties. Overall, this dataset significantly advances our understanding of the mid-infrared properties of CC-SNe, providing an unprecedented view of their late-time line, molecule, and dust emission.

astro-ph.SR↗

arXiv2025

JWST Observations of SN 2023ixf I: Completing the Early Multi-Wavelength Picture with Plateau-phase Spectroscopy

We present and analyze panchromatic (0.35--14 $\mu$m) spectroscopy of the Type II supernova 2023ixf, including near- and mid-infrared spectra obtained 33.6 days after explosion during the plateau-phase, with the James Webb Space Telescope (JWST). This is the first in a series of papers examining the evolution of SN 2023ixf with JWST spanning the initial 1000 days after explosion, monitoring the formation and growth of molecules and dust in ejecta and surrounding environment. The JWST infrared spectra are overwhelmingly dominated by H lines, whose profiles reveal ejecta structures, including flat tops, blue notches, and red shoulders, unseen in the optical spectra. We characterize the nature of these structures, concluding that they likely result from a combination of ejecta geometry, viewing angle, and opacity effects. We find no evidence for the formation of dust precursor molecules such as carbon-monoxide (CO), nor do we observe an infrared excess attributable to dust. These observations imply that the detections of molecules and dust in SN 2023ixf at later epochs arise either from freshly synthesized material within the ejecta or circumstellar material at radii not yet heated by the supernova at this epoch.

astro-ph.HE↗

arXiv2025

Early and Extensive Ultraviolet Through Near Infrared Observations of the Intermediate-Luminosity Type Iax Supernovae 2024pxl

We present ultraviolet (UV) through near-infrared (NIR) photometric and spectroscopic observations of the nearby SN 2024pxl, the third Type Ia supernova (SN Ia) in NGC 6384. SN 2024pxl is a Type Iax supernova (SN Iax) with an intermediate luminosity ($M_r = -16.99\pm0.32$ mag) and an average SN Iax light curve decline rate. SN 2024pxl was discovered $\sim$3 days after first light, and the rising light curve follows a single power law that is inconsistent with significant interaction with a companion star or circumstellar material. Our extensive NIR photometric coverage is comparable to that of the well-observed SNe Iax 2005hk and 2012Z, and we demonstrate that the $J-H$ colors of SNe Iax differ from normal SNe Ia and appear to be more homogeneous as a class. Spectroscopically, we report the earliest-ever NIR spectrum of a SN Iax as measured from maximum light ($t\approx-9$ days): a featureless continuum with similarities to a $\sim$9,000 K blackbody, and the line velocities are consistent with a mixed-ejecta structure, with C, Si, and Fe having similar velocities and velocity evolutions. We find a tentative correlation between the $H$-band break Co II velocity $\sim$20 days post-peak and absolute magnitude, with more luminous SNe Iax showing faster Co II velocities. Our observations suggest that SN 2024pxl resulted from the thermonuclear disruption of a CO white dwarf star that undergoes deflagration burning.

astro-ph.HE↗

arXiv2025

Seeing the Outer Edge of the Infant Type Ia Supernova 2024epr in the Optical and Near Infrared

We present optical-to-near-infrared (NIR) photometry and spectroscopy of the Type Ia supernova (SN Ia) 2024epr, including NIR spectra observed within two days of first light. The early-time optical spectra show strong, high-velocity Ca and Si features near rarely-observed velocities at $\sim$0.1$c$, and the NIR spectra show a C I "knee." Despite early-time, high-velocity features, SN 2024epr evolves into a normal SN Ia, albeit with stronger peak-light Ca absorption than other SNe Ia with the same light curve shape. Although we infer a normal decline rate, $\Delta m_{15}(B)=1.09\pm0.12$ mag, from the light-curve rise, SN 2024epr is a Branch "cool" object and has red early-time colors ($g-r\approx0.15$ mag at $-10$ days). The high velocities point to a density enhancement in the outer layers of the explosion, predicted by some models, but thick-shell He-detonation models do not match the smoothly rising light curve or apparent lack of He in our early-time NIR spectra. No current models (e.g., delayed detonation or thin He shell double detonation) appear to reproduce all observed properties, particularly the unusual early-time colors. Such constraints are only possible for SN 2024epr from the earliest optical and NIR observations, highlighting their importance for constraining SN Ia models. Finally, we identify several literature SNe Ia with intermediate mass elements at $\sim$30\,000 km s$^{-1}$ within days after the explosion that evolve into otherwise normal SNe Ia at peak light, suggesting the early-time spectra of SNe Ia may hide a broad diversity of observational characteristics.

astro-ph.HE↗

arXiv2024

Finite-time stability of nonlinear conformable fractional-order delayed impulsive systems: Impulsive control and perturbation perspectives

This paper investigates the finite-time stability (FTS) of nonlinear conformable fractional-order delayed impulsive systems (CFODISs). Using the conformable fractional-order (CFO) derivative framework, we derive a novel FTS result by extending the existing works on continuous integer-order (IO) systems. This result highlights that the settling time of continuous CFO systems depends on the system order and plays a crucial role in discussing FTS scenarios subject to delayed impulses. We establish Lyapunov-based FTS criteria for CFODISs, considering both impulsive control and impulsive perturbation. Additionally, we estimate the settling time for both cases, revealing distinct forms compared to the IO case. We apply the theoretical results to delayed impulsive conformable fractional-order memristive neural networks (CFOMNNs) under an elaborately designed controller. We present several simulations to illustrate the validity and applicability of our results.

math.DS↗

arXiv2024

The Gaia Ultracool Dwarf Sample -- IV. GTC/OSIRIS optical spectra of Gaia late-M and L dwarfs

As part of our comprehensive, ongoing characterisation of the low-mass end of the main sequence in the Solar neighbourhood, we used the OSIRIS instrument at the 10.4 m Gran Telescopio Canarias to acquire low- and mid-resolution (R$\approx$300 and R$\approx$2500) optical spectroscopy of 53 late-M and L ultracool dwarfs. Most of these objects are known but poorly investigated and lacking complete kinematics. We measured spectral indices, determined spectral types (six of which are new) and inferred effective temperature and surface gravity from BT-Settl synthetic spectra fits for all objects. We were able to measure radial velocities via line centre fitting and cross correlation for 46 objects, 29 of which lacked previous radial velocity measurements. Using these radial velocities in combination with the latest Gaia DR3 data, we also calculated Galactocentric space velocities. From their kinematics, we identified two candidates outside of the thin disc and four in young stellar kinematic groups. Two further ultracool dwarfs are apparently young field objects: 2MASSW J1246467$+$402715 (L4$\beta$), which has a potential, weak lithium absorption line, and G 196$-$3B (L3$\beta$), which was already known as young due to its well-studied primary companion.

astro-ph.SR↗

arXiv2023

MAD: Meta Adversarial Defense Benchmark

Adversarial training (AT) is a prominent technique employed by deep learning models to defend against adversarial attacks, and to some extent, enhance model robustness. However, there are three main drawbacks of the existing AT-based defense methods: expensive computational cost, low generalization ability, and the dilemma between the original model and the defense model. To this end, we propose a novel benchmark called meta adversarial defense (MAD). The MAD benchmark consists of two MAD datasets, along with a MAD evaluation protocol. The two large-scale MAD datasets were generated through experiments using 30 kinds of attacks on MNIST and CIFAR-10 datasets. In addition, we introduce a meta-learning based adversarial training (Meta-AT) algorithm as the baseline, which features high robustness to unseen adversarial attacks through few-shot learning. Experimental results demonstrate the effectiveness of our Meta-AT algorithm compared to the state-of-the-art methods. Furthermore, the model after Meta-AT maintains a relatively high clean-samples classification accuracy (CCA). It is worth noting that Meta-AT addresses all three aforementioned limitations, leading to substantial improvements. This benchmark ultimately achieved breakthroughs in investigating the transferability of adversarial defense methods to new attacks and the ability to learn from a limited number of adversarial examples. Our codes and attacked datasets address will be available at https://github.com/PXX1110/Meta_AT.

eess.IV↗

arXiv2023

Deep Synoptic Array Science: First FRB and Host Galaxy Catalog

Fast Radio Bursts (FRBs) are a powerful and mysterious new class of transient that are luminous enough to be detected at cosmological distances. By associating FRBs to host galaxies, we can measure intrinsic and environmental properties that test FRB origin models, in addition to using them as precise probes of distant cosmic gas. The Deep Synoptic Array (DSA-110) is a radio interferometer built to maximize the rate at which it can simultaneously detect and localize FRBs. Here, we present the first sample of FRBs and host galaxies discovered by the DSA-110. This sample of 11 FRBs is the largest, most uniform sample of localized FRBs to date, as it is selected based on association to host galaxies identified in optical imaging by Pan-STARRS1. These FRBs have not been observed to repeat and their radio properties (dispersion, temporal scattering, energy) are similar to that of the known non-repeating FRB population. Most host galaxies have ongoing star formation, as has been identified before for FRB hosts. Two hosts of the new sample are massive, quiescent galaxies. The distribution of star-formation history across this host-galaxy sample shows that the delay-time distribution is wide, with a powerlaw model that spans from $\sim100$\,Myr to $\gtrsim2$\,Gyr. This requires the existence of one or more progenitor formation channels associated with old stellar populations, such as the binary evolution of compact objects.

astro-ph.HE↗

arXiv2022

Tunable Coupler for Mediating Interactions between a Two-Level System and a Waveguide from a Decoupled State to the Ultra-Strong Coupling Regime

Two-level systems (TLS) coupled to waveguides are a fundamental paradigm for light-matter interactions and quantum networks. We introduce and experimentally demonstrate a method to tune the interaction between a TLS, implemented as a flux qubit, and a transmission line waveguide from a decoupled state to a coupling strength that is a significant fraction of the TLS transition frequency, near the ultra-strong coupling regime. The coupling, controlled via magnetic flux, is described by a normalized coupling strength $\alpha$ that is measured to range between $6.2\times10^{-5}$ and $2.19\times10^{-2}$, with larger attainable maximum values predicted by a circuit model of the device. This system enables future investigations in the dynamics of the spin-boson model, microwave photonics, and relativistic quantum information.

quant-ph↗

arXiv2021

Band-selective Holstein polaron in Luttinger liquid material A0.3MoO3 (A = K, Rb)

(Quasi-)one-dimensional systems exhibit various fascinating properties such as Luttinger liquid behavior, Peierls transition, novel topological phases, and the accommodation of unique quasiparticles (e.g., spinon, holon, and soliton, etc.). Here we study molybdenum blue bronze A0.3MoO3 (A = K, Rb), a canonical quasi-one-dimensional charge-density-wave material, using laser-based angle-resolved photoemission spectroscopy. Our experiment suggests that the normal phase of A0.3MoO3 is a prototypical Luttinger liquid, from which the charge-density-wave emerges with decreasing temperature. Prominently, we observe strong renormalizations of band dispersions, which is recognized as the spectral function of Holstein polaron derived from band-selective electron-phonon coupling in the system. We argue that the strong electron-phonon coupling plays a dominant role in electronic properties and the charge-density-wave transition in blue bronzes. Our results not only reconcile the long-standing heavy debates on the electronic properties of blue bronzes but also provide a rare platform to study novel composite quasiparticles in Luttinger liquid materials.

cond-mat.str-el↗

arXiv2021

Predicting Future Cognitive Decline with Hyperbolic Stochastic Coding

Hyperbolic geometry has been successfully applied in modeling brain cortical and subcortical surfaces with general topological structures. However such approaches, similar to other surface based brain morphology analysis methods, usually generate high dimensional features. It limits their statistical power in cognitive decline prediction research, especially in datasets with limited subject numbers. To address the above limitation, we propose a novel framework termed as hyperbolic stochastic coding (HSC). Our preliminary experimental results show that our algorithm achieves superior results on various classification tasks. Our work may enrich surface based brain imaging research tools and potentially result in a diagnostic and prognostic indicator to be useful in individualized treatment strategies.

eess.IV↗

arXiv2020

Direct Observation of Coherent Longitudinal and Shear Acoustic Phonons in TaAs Using Ultrafast X-ray Diffraction

Using femtosecond time-resolved X-ray diffraction, we investigated optically excited coherent acoustic phonons in the Weyl semimetal TaAs. The low symmetry of the (112) surface probed in our experiment enables the simultaneous excitation of longitudinal and shear acoustic modes, whose dispersion closely matches our simulations. We observed an asymmetry in the spectral lineshape of the longitudinal mode that is notably absent from the shear mode, suggesting a time-dependent frequency chirp that is likely driven by photoinduced carrier diffusion. We argue on the basis of symmetry that these acoustic deformations can transiently alter the electronic structure near the Weyl points and support this with model calculations. Our study underscores the benefit of using off-axis crystal orientations when optically exciting acoustic deformations in topological semimetals, allowing one to transiently change their crystal and electronic structures.

cond-mat.mtrl-sci↗

arXiv2020

A survey on Kornia: an Open Source Differentiable Computer Vision Library for PyTorch

This work presents Kornia, an open source computer vision library built upon a set of differentiable routines and modules that aims to solve generic computer vision problems. The package uses PyTorch as its main backend, not only for efficiency but also to take advantage of the reverse auto-differentiation engine to define and compute the gradient of complex functions. Inspired by OpenCV, Kornia is composed of a set of modules containing operators that can be integrated into neural networks to train models to perform a wide range of operations including image transformations,camera calibration, epipolar geometry, and low level image processing techniques, such as filtering and edge detection that operate directly on high dimensional tensor representations on graphical processing units, generating faster systems. Examples of classical vision problems implemented using our framework are provided including a benchmark comparing to existing vision libraries.

cs.CV↗

arXiv2020

Characterization of Control in a Superconducting Qutrit Using Randomized Benchmarking

We characterize control of a qutrit implemented in the lowest three energy levels of a capacitively-shunted flux-biased superconducting circuit. Randomized benchmarking over the qutrit Clifford group yields an average fidelity of 98.89 $\pm$ 0.05%. For a selected subset of the Clifford group, we perform quantum process tomography and observe the behaviour of repeated gate sequences. Each qutrit gate is generated using only two-state rotations via a method applicable to any unitary. We find that errors are due to decoherence primarily and have a significant contribution from level shifts. This work demonstrates high-fidelity qutrit control and outlines avenues for future work on optimal control of superconducting qudits.

quant-ph↗

arXiv2020

Characterization of multi-level dynamics and decoherence in a high-anharmonicity capacitively shunted flux circuit

We present the design and characterization of a three-Josephson-junction superconducting loop circuit with three large shunt capacitors. Used as a qubit, the circuit shows long energy-relaxation times, of the order of 40 ${ \unicode[Times]{x3BC}}$s and a spin-echo dephasing time of 9.4 ${ \unicode[Times]{x3BC}}$s. The circuit has high anharmonicity, of 2$\pi$$\times$3.69 GHz. We extract the multilevel relaxation and dephasing rates of the circuit used as a qutrit and discuss the possible sources for the decoherence. The high anharmonicity allows for fast qubit control with nanosecond-range gate durations and a measured average gate fidelity of 99.92%, characterized by randomized benchmarking. These results demonstrate interesting potential use for fast nanosecond-time-scale two-qubit gates and multilevel quantum logic.

quant-ph↗

arXiv2020

Bypassing the structural bottleneck in the ultrafast melting of electronic order

The emergent properties of quantum materials, such as symmetry-broken phases and associated spectral gaps, can be effectively manipulated by ultrashort photon pulses. Impulsive optical excitation generally results in a complex non-equilibrium electron and lattice dynamics that involves multiple processes on distinct timescales, and a common conception is that for times shorter than about 100 fs the gap in the electronic spectrum is not seriously affected by lattice vibrations. Here, we directly monitor the photo-induced collapse of the spectral gap in a canonical charge-density-wave material, blue bronze Rb0.3MoO3. We find that ultra-fast (about 60 fs) vibrational disordering due to efficient hot-electron energy dissipation quenches the gap significantly faster than the typical structural bottleneck time corresponding to one half-cycle oscillation (about 315 fs) of the coherent charge-density-wave amplitude mode. This result not only demonstrates the importance of incoherent lattice motion in the photo-induced quenching of electronic order, but also resolves the perennial debate about the nature of the spectral gap in a coupled electron-lattice system.

cond-mat.str-el↗