arXiv subjects

Sijia Peng

Publications and source records attributed to Sijia Peng.

17 recordsLinked to original sources

arXiv2026

Spatially resolved spectral properties of M87* on event horizon scales

The supermassive black hole at the center of the nearby radio galaxy M87 (M87*) is a prime target for studying black hole physics. Spatially resolved spectral measurements on event-horizon scales can reveal the origin of the emission and probe the plasma and gravitational environment in the immediate vicinity of the black hole. Here, we present an analysis of spectral properties based on nearly simultaneous high-resolution images at 3.5 mm (86 GHz) and 1.3 mm (230 GHz), obtained in 2018 with the Global Millimeter VLBI Array (GMVA) including ALMA and the Greenland Telescope, and the Event Horizon Telescope (EHT). We obtain the first spatially resolved spectral-index map ($S_\nu \propto \nu^\alpha$) within the compact region ($\leq 100\,\mu$as). We further detect a robust radial gradient with a modest rise in the inner $\lesssim 20~\mu$as (slightly inside the 1.3 mm ring), followed by a systematic decline at larger radii. The spectral index transitions from positive to negative values near $\sim 30~\mu$as, close to the 3.5 mm ring radius, consistent with frequency-dependent synchrotron opacity in the innermost accretion flow. These results provide new observational constraints that can help discriminate between models of the horizon-scale emission and the launching of relativistic jets in M87*.

astro-ph.HE↗

arXiv2026

ALOHA IRDCs Molecular Line Follow-up: I. Gas properties and kinematics

Infrared Dark Clouds are ideal sites for investigating the initial conditions of massive star and cluster formation. The A Lei Of the Habitat and Assembly of Infrared Dark Clouds (ALOHA IRDCs), a James Clerk Maxwell Telescope (JCMT) Large Program, has mapped nearby IRDCs with SCUBA-2. Complementary molecular line observations are needed to characterise the physical, kinematic, and chemical properties of the dense gas. We aim to determine the thermal, kinematic, and chemical properties of clumps identified in the ALOHA IRDCs, and to assess their evolutionary status and level of star-forming activity. We performed single-pointing K-band and W-band observations towards 56 ALOHA IRDCs clumps using the Effelsberg 100-m and Yebes 40-m telescopes, respectively. We derived NH3 kinetic temperatures using the hyperfine group ratio (HFGR) method and identified infall and shock signatures from HCO+, H13CO+, SiO, and HNCO profiles. Water masers and NH2D emission were used as complementary tracers of chemical evolution and star formation. The clumps exhibit kinetic temperatures of 15-29 K. We detect NH2D emission towards 18 sources, with NH2D centroid velocities consistent with NH3, indicating both species trace the same dense gas component. More than half of the clumps display blue-asymmetric HCO+ profiles, identifying them as infall candidates. Water masers are detected in 22 sources, with prominent velocity ranges and variability. Broad SiO emission (>~20 km/s) indicates strong shocks, while narrower extents (<~6km/s) likely trace large-scale interactions or low-velocity shocks. The widespread infall signatures, shock tracers, masers, and NH2D emission suggest that relatively quiescent, chemically young material can coexist with dynamically active gas affected by early protostellar feedback, providing insight into the coupled physical and chemical evolution of massive IRDC clumps.

astro-ph.GA↗

arXiv2026

Dynamics and geometry of the inner sub-parsec-scale jet in 3C 279 observed with the Event Horizon Telescope

The 2021 Event Horizon Telescope observations resolve the innermost jet region of the blazar 3C279 with unprecedented detail. The reconstructed images consistently reveal a compact core elongated nearly orthogonal to the large-scale jet axis. This rarely observed morphology recurs across multiple epochs and from 22-230 GHz and is therefore intrinsic rather than an imaging artifact. Geometric model fitting identifies several components with apparent speeds up to 10c, requiring bulk Lorentz factors greater than 10.3 and constraining viewing angles to extremely small values (smaller than one degree). Rest-frame brightness temperatures are systematically low (between 10^9 and 10^10 K), consistent with optically thin emission at 230 GHz. These results suggest that the jet bends toward the observer on sub-parsec scales, producing strong relativistic beaming. Possible drivers of the observed jet bending and temporal evolution include the jet's interaction with the interstellar medium, kink or Kelvin--Helmholtz instabilities, magnetic reconnection near the horizon, or binary-induced precession. However, the current temporal coverage of VLBI data remains insufficient to distinguish between these mechanisms. Continued multifrequency VLBI monitoring will be essential to constraining the dynamics and geometry of the jet base in 3C279.

astro-ph.GA↗

arXiv2026

When Alpha Disappears: A One-Switch Benchmark for Decision-Time Leakage in Financial Backtests

We introduce When Alpha Disappears, a paired evaluation benchmark for diagnosing decision-time leakage in financial machine-learning backtests. Rather than treating leakage as a binary property, the benchmark estimates protocol-induced inflation by toggling one evaluation convention at a time around a clean $t{+}1$-open reference, while holding the data panel, walk-forward split, model family, horizon, portfolio rule, and cost convention fixed. Across two daily-OHLCV equity panels, six model families, and yearly tests from 2016--2024, we find that inflation is highly selective: centered temporal features and same-day-open execution with post-open daily-bar information cause large and stable increases in both predictive and trading metrics, whereas global normalization, future-informed graph structure, and same-day-close execution are weak in most settings. The benchmark is diagnostic rather than a claim of tradable alpha, and is intended to make evaluation assumptions, failure modes, and protocol fragility directly measurable.

q-fin.RM↗

arXiv2026

ShapeCond: Fast Shapelet-Guided Dataset Condensation for Time Series Classification

Time series data supports many domains (e.g., finance and climate science), but its rapid growth strains storage and computation. Dataset condensation can alleviate this by synthesizing a compact training set that preserves key information. Yet most condensation methods are image-centric and often fail on time series because they miss time-series-specific temporal structure, especially local discriminative motifs such as shapelets. In this work, we propose ShapeCond, a novel and efficient condensation framework for time series classification that leverages shapelet-based dataset knowledge via a shapelet-guided optimization strategy. Our shapelet-assisted synthesis cost is independent of sequence length: longer series yield larger speedups in synthesis (e.g., 29$\times$ faster over prior state-of-the-art method CondTSC for time-series condensation, and up to 10,000$\times$ over naively using shapelets on the Sleep dataset with 3,000 timesteps). By explicitly preserving critical local patterns, ShapeCond improves downstream accuracy and consistently outperforms all prior state-of-the-art time series dataset condensation methods across extensive experiments. Code is available at https://github.com/lunaaa95/ShapeCond.

cs.LG↗

arXiv2025

Orbital-resolved three-body recombination across a p-wave Feshbach resonance in ultracold $^6$Li

We report precision, orbital-resolved measurements of three-body recombination near the 159~G $p$-wave Feshbach resonance in an ultracold gas of $^{6}$Li atoms prepared in their lowest hyperfine state. Using a radio-frequency gated protocol that suppresses magnetic-field transients below the milligauss level, we resolve loss features associated with the $|m_\ell|=1$ and $m_\ell=0$ orbital projections. The measured three-body loss coefficient $L_3$ is well captured by a thermally averaged cascade-recombination model, enabling extraction of the resonance splitting $\delta B$ and effective-range parameter $k_e$. At the lowest temperature, we obtain $\delta B = 7.6(3)$~mG and $k_e = 0.151(6)\,a_0^{-1}$, both in quantitative agreement with coupled-channel theory. These results establish orbital-resolved three-body spectroscopy as a precision probe of $p$-wave scattering and provide a benchmark for microscopic models of resonant few-body loss.

cond-mat.quant-gas↗

arXiv2025

Episodic Feedback in Triple AGN Candidate SDSS J0849+1114 Revealed by Extended ionized gas

Galaxy mergers funnel gas toward the nuclei, igniting starbursts and active galactic nuclei (AGNs). The AGN feedback can reshape the host galaxy and regulate both star formation and super-massive black-hole (SMBH) accretion. Using VLT/MUSE integral-field spectroscopy, we conduct a spatially resolved study of the triple-AGN candidate SDSS J0849+1114. Extended ionized gas structures ($>10$ kpc from nucleus A) primarily associated with tidal tails are detected. Meanwhile, two distinct ionized gas outflows are revealed. One extends over $>5$ kpc around nuclei A with a kinetic power of $\dot{E}_{\rm out,A} = 3.0\times10^{42}\rm\, erg\, s^{-1}$, which might be driven by the radio jet. The other outflow extends $\sim 5.9$ kpc around nucleus C, with a kinetic power of $\dot{E}_{\rm out,C} = 2.0\times10^{40}\rm\, erg\, s^{-1}$. High [O III]/H$\alpha$ and [N II]/H$\alpha$ ratios in the tidal gas require that nucleus A radiated at a high accretion rate with $L_{\rm A,bol} \sim 0.1$--$0.5\,L_{\rm Edd,A}$ at least $\sim3$--$\times10^{4}\rm\,yr$ ago, $20$--$100$ times brighter than today. Combined with multi-wavelength constraints, we find evidence for episodic AGN feedback that expelled circumnuclear gas and rapidly quenched accretion. This triple AGN candidate demonstrates how AGN feedback can self-regulate black hole growth and impact hosts during mergers.

astro-ph.GA↗

arXiv2025

Precision Measurement of Spin-Dependent Dipolar Splitting in $^6$Li p-Wave Feshbach Resonances

The magnetic dipolar splitting of a p-wave Feshbach resonance is governed by the spin-orbital configuration of the valence electrons in the triplet molecular state. We perform high-resolution trap loss spectroscopy on ultracold 6Li atoms to resolve this splitting with sub-milligauss precision. By comparing spin-polarized (|mS| = 1) and spin-mixture (mS = 0) configurations of the triplet state, we observe a clear spin-dependent reversal in the splitting structure, confirmed via momentumresolved absorption imaging. This behavior directly reflects the interplay between electron spin projection mS and orbital angular momentum ml in the molecular states. Our results provide a stringent benchmark for dipole-dipole interaction models and lay the groundwork for controlling the pairing in p-wave superfluid systems.

cond-mat.quant-gas↗

arXiv2025

A 2D Semantic-Aware Position Encoding for Vision Transformers

Vision transformers have demonstrated significant advantages in computer vision tasks due to their ability to capture long-range dependencies and contextual relationships through self-attention. However, existing position encoding techniques, which are largely borrowed from natural language processing, fail to effectively capture semantic-aware positional relationships between image patches. Traditional approaches like absolute position encoding and relative position encoding primarily focus on 1D linear position relationship, often neglecting the semantic similarity between distant yet contextually related patches. These limitations hinder model generalization, translation equivariance, and the ability to effectively handle repetitive or structured patterns in images. In this paper, we propose 2-Dimensional Semantic-Aware Position Encoding ($\text{SaPE}^2$), a novel position encoding method with semantic awareness that dynamically adapts position representations by leveraging local content instead of fixed linear position relationship or spatial coordinates. Our method enhances the model's ability to generalize across varying image resolutions and scales, improves translation equivariance, and better aggregates features for visually similar but spatially distant patches. By integrating $\text{SaPE}^2$ into vision transformers, we bridge the gap between position encoding and perceptual similarity, thereby improving performance on computer vision tasks.

cs.CV↗

arXiv2025

Rethinking Time Encoding via Learnable Transformation Functions

Effectively modeling time information and incorporating it into applications or models involving chronologically occurring events is crucial. Real-world scenarios often involve diverse and complex time patterns, which pose significant challenges for time encoding methods. While previous methods focus on capturing time patterns, many rely on specific inductive biases, such as using trigonometric functions to model periodicity. This narrow focus on single-pattern modeling makes them less effective in handling the diversity and complexities of real-world time patterns. In this paper, we investigate to improve the existing commonly used time encoding methods and introduce Learnable Transformation-based Generalized Time Encoding (LeTE). We propose using deep function learning techniques to parameterize non-linear transformations in time encoding, making them learnable and capable of modeling generalized time patterns, including diverse and complex temporal dynamics. By enabling learnable transformations, LeTE encompasses previous methods as specific cases and allows seamless integration into a wide range of tasks. Through extensive experiments across diverse domains, we demonstrate the versatility and effectiveness of LeTE.

cs.LG↗

arXiv2024

The Faraday rotation measure of the M87 jet at 3.5mm with the Atacama Large Millimeter/submillimeter Array

Faraday rotation is an important probe of the magnetic fields and magnetized plasma around active galactic nuclei (AGN) jets. We present a Faraday rotation measure image of the M87 jet between 85.2 GHz and 101.3 GHz with a resolution of ~2" with the Atacama Large Millimeter/submillimeter Array (ALMA). We found that the rotation measure (RM) of the M87 core is $\rm (4.5\pm 0.4)\times10^{4}\ rad\ m^{-2}$ with a low linear polarization fraction of $\rm (0.88\pm 0.08)\%$. The spatial RM gradient in the M87 jet spans a wide range from $\sim -2\times10^4\rm~rad\ m^{-2}$ to $\sim 3\times10^4\rm~rad\ m^{-2}$ with a typical uncertainty of $0.3\times10^4\rm~rad\ m^{-2}$. A comparison with previous RM measurements of the core suggests that the Faraday rotation of the core may originate very close to the super massive black hole (SMBH). Both an internal origin and an external screen with a rapidly varying emitting source could be possible. As for the jet, the RM gradient indicates a helical configuration of the magnetic field that persists up to kpc scale. Combined with the kpc-scale RM measurements at lower frequencies, we found that RM is frequency-dependent in the jet. One possible scenario to explain this dependence is that the kpc-scale jet has a trumpet-like shape and the jet coil unwinds near its end.

astro-ph.GA↗

arXiv2024

Mamba or Transformer for Time Series Forecasting? Mixture of Universals (MoU) Is All You Need

Time series forecasting requires balancing short-term and long-term dependencies for accurate predictions. Existing methods mainly focus on long-term dependency modeling, neglecting the complexities of short-term dynamics, which may hinder performance. Transformers are superior in modeling long-term dependencies but are criticized for their quadratic computational cost. Mamba provides a near-linear alternative but is reported less effective in time series longterm forecasting due to potential information loss. Current architectures fall short in offering both high efficiency and strong performance for long-term dependency modeling. To address these challenges, we introduce Mixture of Universals (MoU), a versatile model to capture both short-term and long-term dependencies for enhancing performance in time series forecasting. MoU is composed of two novel designs: Mixture of Feature Extractors (MoF), an adaptive method designed to improve time series patch representations for short-term dependency, and Mixture of Architectures (MoA), which hierarchically integrates Mamba, FeedForward, Convolution, and Self-Attention architectures in a specialized order to model long-term dependency from a hybrid perspective. The proposed approach achieves state-of-the-art performance while maintaining relatively low computational costs. Extensive experiments on seven real-world datasets demonstrate the superiority of MoU. Code is available at https://github.com/lunaaa95/mou/.

cs.LG↗

arXiv2024

Observation of a broad state-to-state spin-exchange collision near a p-wave Feshbach resonances of $^6$Li atoms

The study of state-to-state spin-exchange collisions in the vicinity of $p$-wave Feshbach resonances offer great opportunities to explore many-body interactions and novel quantum phases. Here, we report the observation of a spin-exchange collision near a $p$-wave Feshbach resonance within a mixture of the lowest and third-lowest hyperfine states of $^6$Li atoms. The spin-exchange interaction is observed over a range of ten gausses and produces a pair of atoms in the second-lowest hyperfine states that are captured by a deep optical dipole trap. We apply a coupled-channel method to calculate the scattering properties of this system. We find that the $p$-wave resonance exhibits a low inelastic collision rate and a broad resonance profile, which is due to the modification by the accompanying spin-exchange collisions. These findings open up new possibilities for the creation of long-lived, strongly interacting $p$-wave Fermi gases.

cond-mat.quant-gas↗

arXiv2023

Searching for Radio Outflows from M31* with VLBI Observations

As one of the nearest and most dormant supermassive black holes (SMBHs), M31* provides a rare but promising opportunity for studying the physics of black hole accretion and feedback at the quiescent state. Previous Karl G. Jansky Very Large Array (VLA) observations with an arcsec resolution have detected M31* as a compact radio source over centimeter wavelengths, but the steep radio spectrum suggests optically-thin synchrotron radiation from an outflow driven by a hot accretion flow onto the SMBH. Aiming to probe the putative radio outflow, we have conducted milli-arcsec-resolution very long baseline interferometric (VLBI) observations of M31* in 2016, primarily at 5 GHz and combining the Very Long Baseline Array, Tianma-65m and Shanghai-25m Radio Telescopes. Despite the unprecedented simultaneous resolution and sensitivity achieved, no significant ($\gtrsim 3σ$) signal is detected at the putative position of M31* given an RMS level of $\rm 5.9~μJy\ beam^{-1}$, thus ruling out a point-like source with a peak flux density comparable to that ($\sim30~μJy\ beam^{-1}$) measured by the VLA observations taken in 2012. We disfavor the possibility that M31* has substantially faded since 2012, in view that a 2017 VLA observation successfully detected M31* at a historically-high peak flux density ($\sim75~μJy\ beam^{-1}$ at 6 GHz). Instead, the non-detection of the VLBI observations is best interpreted as the arcsec-scale core being resolved out at the milli-arcsec-scale, suggesting an intrinsic size of M31* at 5 GHz larger than $\sim300$ times the Schwarzschild radius. Such extended radio emission may originate from a hot wind driven by the weakly accreting SMBH.

astro-ph.GA↗

arXiv2022

Very Large Array Multi-band Radio Imaging of the Triple AGN Candidate SDSS J0849+1114

Kpc-scale triple active galactic nuclei (AGNs), potential precursors of gravitationally-bound triple massive black holes (MBHs), are rarely seen objects and believed to play an important role in the evolution of MBHs and their host galaxies. In this work we present a multi-band (3.0, 6.0 10.0, and 15.0 GHz), high-resolution radio imaging of the triple AGN candidate, SDSS J0849+1114, using the Very Large Array. Two of the three nuclei (A and C) are detected at 3.0, 6.0, and 15 GHz for the first time, both exhibiting a steep spectrum over 3--15 GHz (with a spectral index $-0.90 \pm 0.05$ and $-1.03 \pm 0.04$) consistent with a synchrotron origin. Nucleus A, the strongest nucleus among the three, shows a double-sided jet, with the jet orientation changing by $\sim20^{\circ}$ between its inner 1" and the outer 5.5" (8.1 kpc) components, which may be explained as the MBH's angular momentum having been altered by merger-enhanced accretion. Nucleus C also shows a two-sided jet, with the western jet inflating into a radio lobe with an extent of 1.5" (2.2 kpc). The internal energy of the radio lobe is estimated to be $\rm 5.0 \times 10^{55}$ erg, for an equipartition magnetic field strength of $\rm \sim 160\ μG$. No significant radio emission is detected at all four frequencies for nucleus B, yielding an upper limit of 15, 15, 15, and 18 $\rm μJy\ beam^{-1}$ at 3.0, 6.0, 10.0, and 15.0 GHz, based on which we constrain the star formation rate in nucleus B to be $\lesssim 0.4~\rm M_{\odot}~yr^{-1}$.

astro-ph.GA↗

arXiv2020

Resolving the Nuclear Radio Emission from M32 with Very Large Array

The Local Group dwarf elliptical galaxy M32 hosts one of the nearest and most under-luminous super-massive black holes (SMBHs) ever known, offering a rare opportunity to study the physics of accreting SMBHs at the most quiescent state. Recent Very Large Array (VLA) observations have detected a radio source at the nucleus of M32, which is suggested to be the radio counterpart of the SMBH. To further investigate the radio properties of this nuclear source, we have conducted follow-up, high-resolution VLA observations in four epochs between 2015--2017, each with dual frequencies. At 6 GHz, the nuclear source is resolved under an angular resolution of $\sim$0\farcs4, exhibiting a coreless, slightly lopsided morphology with a detectable extent of $\sim$2.5 \arcsec ($\sim$10 parsec). No significant variability can be found among the four epochs. At 15 GHz, no significant emission can be detected within the same region, pointing to a steep intrinsic radio spectrum (with a 3\,$σ$ upper limit of -1.46 for the spectral index). We discuss possible scenarios for the nature of this nuclear source and conclude that a stellar origin, in particular planetary nebulae, X-ray binaries, supernova remnants or diffuse ionized gas powered by massive stars, can be ruled out.Instead, the observed radio properties can be explained by synchrotron radiation from a hypothetical wind driven by the weakly accreting SMBH.

astro-ph.GA↗

arXiv2019

A Trio of Massive Black Holes Caught in the Act of Merging

We report the discovery of SDSS J0849+1114 as the first known triple Type 2 Seyfert nucleus. It represents three active black holes that are identified from new spatially resolved optical slit spectroscopy using the Dual Imaging Spectrograph on the 3.5 m telescope at the Apache Point Observatory. We also present new complementary observations including the Hubble Space Telescope Wide Field Camera 3 U- and Y-band imaging, Chandra Advanced CCD Imaging Spectrometer S-array X-ray 0.5--8 keV imaging spectroscopy, and NSF Karl G. Jansky Very Large Array radio 9.0 GHz imaging in its most extended A configuration. These comprehensive multiwavelength observations, when combined together, strongly suggest that all three nuclei are active galactic nuclei. While they are now still at kiloparsec-scale separations, where the host-galaxy gravitational potential dominates, the black holes may evolve into a bound triple system in $\lesssim$2 Gyr. These triple merger systems may explain the overly massive stellar cores that have been observed in some elliptical galaxies such as M87, which are expected to be unique gravitational wave sources. Similar systems may be more common in the early universe, when galaxy mergers are thought to have been more frequent.

astro-ph.GA↗