arXiv subjects

Xiaohui Sun

Publications and source records attributed to Xiaohui Sun.

At least 37 records · Page 2Linked to original sources

arXiv2025

The Southern Twenty-centimetre All-sky Polarization Survey (STAPS): survey description and maps

We present data processing and verification of the Southern Twenty-centimetre All-sky Polarization Survey (STAPS) conducted with Murriyang, the Parkes 64-m telescope. The survey covers the sky area of -89<Dec<0 and the frequency range of 1.3-1.8 GHz split into 1-MHz channels. STAPS was observed commensally with the S-band Polarization All-Sky Survey (S-PASS). The survey is composed of long azimuth scans, which allows us to absolutely calibrate Stokes Q and U with the data processing procedure developed for S-PASS. We obtain I, Q, and U maps in both flux density scale (Jy/beam) and main beam brightness temperature scale (K), for the 301 frequency channels with sufficiently good data. The temperature scale is tied to the Global Magneto-ionic Medium Survey (GMIMS) high-band north sky survey conducted with the Dominion Radio Astrophysical Observatory 26-m telescope. All the STAPS maps are smoothed to a common resolution of 20 arcmin. The rms noise per channel ranges from about 16 mK to 8 mK for I, and from about 8 mK to 5 mK for Q and U at frequencies from 1.3 to 1.8 GHz. The rms noise in Q and U varies with declination and reaches minimum at declination of -89 degree. We also run rotation measure (RM) synthesis and RM clean to obtain peak polarized intensity and Faraday depth maps. The whole STAPS data processing is validated by comparing flux densities of compact sources, pixel flux density versus pixel flux density for Cen A, pixel temperature versus pixel temperature for the entire survey area, and RMs of extragalactic sources between STAPS and other measurements. The uncertainty of the flux density scale is less than 10%. STAPS delivers an L-band (20 cm) multi-frequency polarization view of the Galaxy, and will help advance our understanding of the Galactic magnetic field and magnetized interstellar medium.

astro-ph.GA↗

arXiv2025

Identification of 4876 Bent-Tail Radio Galaxies in the FIRST Survey Using Deep Learning Combined with Visual Inspection

Bent-tail radio galaxies (BTRGs) are characterized by bent radio lobes. This unique shape is mainly caused by the movement of the galaxy within a cluster, during which the radio jets are deflected by the intra-cluster medium. A combined method, which involves a deep learning-based radio source finder along with visual inspection, has been utilized to search for BTRGs from the Faint Images of the Radio Sky at Twenty-centimeters survey images. Consequently, a catalog of 4876 BTRGs has been constructed, among which 3871 are newly discovered. Based on the classification scheme of the opening angle between the two jets of the galaxy, BTRGs are typically classified as either wide-angle-tail (WAT) sources or narrow-angle-tail (NAT) sources. Our catalog comprises 4424 WATs and 652 NATs. Among these, optical counterparts are identified for 4193 BTRGs. This catalog covers luminosities in the range of $1.91\times10^{20} \leq L_{\rm 1.4\,GHz} \leq 1.45\times10^{28}$ ${\rm W\,Hz^{-1}}$ and redshifts from $z = 0.0023$ to $z = 3.43$. Various physical properties of these BTRGs and their statistics are presented. Particularly, by the nearest neighbor method, we found that 1825 BTRGs in this catalog belong to galaxy clusters reported in literature.

astro-ph.GA↗

arXiv2024

Latent Diffusion, Implicit Amplification: Efficient Continuous-Scale Super-Resolution for Remote Sensing Images

Recent advancements in diffusion models have significantly improved performance in super-resolution (SR) tasks. However, previous research often overlooks the fundamental differences between SR and general image generation. General image generation involves creating images from scratch, while SR focuses specifically on enhancing existing low-resolution (LR) images by adding typically missing high-frequency details. This oversight not only increases the training difficulty but also limits their inference efficiency. Furthermore, previous diffusion-based SR methods are typically trained and inferred at fixed integer scale factors, lacking flexibility to meet the needs of up-sampling with non-integer scale factors. To address these issues, this paper proposes an efficient and elastic diffusion-based SR model (E$^2$DiffSR), specially designed for continuous-scale SR in remote sensing imagery. E$^2$DiffSR employs a two-stage latent diffusion paradigm. During the first stage, an autoencoder is trained to capture the differential priors between high-resolution (HR) and LR images. The encoder intentionally ignores the existing LR content to alleviate the encoding burden, while the decoder introduces an SR branch equipped with a continuous scale upsampling module to accomplish the reconstruction under the guidance of the differential prior. In the second stage, a conditional diffusion model is learned within the latent space to predict the true differential prior encoding. Experimental results demonstrate that E$^2$DiffSR achieves superior objective metrics and visual quality compared to the state-of-the-art SR methods. Additionally, it reduces the inference time of diffusion-based SR methods to a level comparable to that of non-diffusion methods.

eess.IV↗

arXiv2024

Patient-Oriented Unsupervised Learning to Unlock Patterns of Multimorbidity Associated with Stroke using Primary Care Electronic Health Records

Background: Identifying and characterising the longitudinal patterns of multimorbidity associated with stroke is needed to better understand patients' needs and inform new models of care. Methods: We used an unsupervised patient-oriented clustering approach to analyse primary care electronic health records (EHR) of 30 common long-term conditions (LTC), in patients with stroke aged over 18, registered in 41 general practices in south London between 2005 and 2021. Results: Of 849,968 registered patients, 9,847 (1.16%) had a record of stroke, 46.5% were female and median age at record was 65.0 year (IQR: 51.5 to 77.0). The median number of LTCs in addition to stroke was 3 (IQR: from 2 to 5). Patients were stratified in eight clusters. These clusters revealed contrasted patterns of multimorbidity, socio-demographic characteristics (age, gender and ethnicity) and risk factors. Beside a core of 3 clusters associated with conventional stroke risk-factors, minor clusters exhibited less common but recurrent combinations of LTCs including mental health conditions, asthma, osteoarthritis and sickle cell anaemia. Importantly, complex profiles combining mental health conditions, infectious diseases and substance dependency emerged. Conclusion: This patient-oriented approach to EHRs uncovers the heterogeneity of profiles of multimorbidity and socio-demographic characteristics associated with stroke. It highlights the importance of conventional stroke risk factors as well as the association of mental health conditions in complex profiles of multimorbidity displayed in a significant proportion of patients. These results address the need for a better understanding of stroke-associated multimorbidity and complexity to inform more efficient and patient-oriented healthcare models.

stat.AP↗

arXiv2023

New Continuum Observations of the Andromeda galaxy M31 with FAST

We present a new total intensity image of M31 at 1.248 GHz, observed with the Five-hundred-meter Aperture Spherical radio telescope (FAST) with an angular resolution of 4 arcmin and a sensitivity of about 16 mK. The new FAST image clearly reveals weak emission outside the ring due to its high sensitivity on large-scale structures. We derive a scale length of 2.7 kpc for the cosmic ray electrons and find that the cosmic ray electrons propagate mainly through diffusion by comparing the scale length at 4.8 GHz. The spectral index of the total intensity varies along the ring, which can be attributed to the variation of the spectra of synchrotron emission. This variation is likely caused by the change of star formation rates along the ring. We find that the azimuthal profile of the non-thermal emission can be interpreted by an axisymmetric large-scale magnetic field with varying pitch angle along the ring, indicating a complicated magnetic field configuration in M31.

astro-ph.GA↗

arXiv2023

Sub-second periodic radio oscillations in a microquasar

Powerful relativistic jets are one of the ubiquitous features of accreting black holes in all scales. GRS 1915+105 is a well-known fast-spinning black-hole X-ray binary with a relativistic jet, termed as a ``microquasar'', as indicated by its superluminal motion of radio emission. It exhibits persistent x-ray activity over the last 30 years, with quasi-periodic oscillations of $\sim 1-10$ Hz and 34 and 67 Hz in the x-ray band. These oscillations likely originate in the inner accretion disk, but other origins have been considered. Radio observations found variable light curves with quasi-periodic flares or oscillations with periods of $\sim 20-50$ minutes. Here we report two instances of $\sim$5 Hz transient periodic oscillation features from the source detected in the 1.05-1.45 GHz radio band that occurred in January 2021 and June 2022, respectively. Circular polarization was also observed during the oscillation phase.

astro-ph.HE↗

arXiv2023

A Catalogue of Radio Supernova Remnants and Candidate Supernova Remnants in the EMU/POSSUM Galactic Pilot Field

We use data from the pilot observations of the EMU/POSSUM surveys to study the "missing supernova remnant (SNR) problem", the discrepancy between the number of Galactic SNRs that have been observed and the number that are estimated to exist. The Evolutionary Map of the Universe (EMU) and the Polarization Sky Survey of the Universe's Magnetism (POSSUM) are radio sky surveys that are conducted using the Australian Square Kilometre Array Pathfinder (ASKAP). We report on the properties of 7 known SNRs in the joint Galactic pilot field, with an approximate longitude and latitude of 323$^\circ\leq$ l $\leq$ 330$^\circ$ and -4$^\circ\leq$ b $\leq$ 2$^\circ$ respectively, and identify 21 SNR candidates. Of these, 4 have been previously identified as SNR candidates, 3 were previously listed as a single SNR, 13 have not been previously studied, and 1 has been studied in the infrared. These are the first discoveries of Galactic SNR candidates with EMU/POSSUM and, if confirmed, they will increase the SNR density in this field by a factor of 4. By comparing our SNR candidates to the known Galactic SNR population, we demonstrate that many of these sources were likely missed in previous surveys due to their small angular size and/or low surface brightness. We suspect that there are SNRs in this field that remain undetected due to limitations set by the local background and confusion with other radio sources. The results of this paper demonstrate the potential of the full EMU/POSSUM surveys to uncover more of the missing Galactic SNR population.

astro-ph.GA↗

arXiv2023

Radio Sources Segmentation and Classification with Deep Learning

Modern large radio continuum surveys have high sensitivity and resolution, and can resolve previously undetected extended and diffuse emissions, which brings great challenges for the detection and morphological classification of extended sources. We present HeTu-v2, a deep learning-based source detector that uses the combined networks of Mask Region-based Convolutional Neural Networks (Mask R-CNN) and a Transformer block to achieve high-quality radio sources segmentation and classification. The sources are classified into 5 categories: Compact or point-like sources (CS), Fanaroff-Riley Type I (FRI), Fanaroff-Riley Type II (FRII), Head-Tail (HT), and Core-Jet (CJ) sources. HeTu-v2 has been trained and validated with the data from the Faint Images of the Radio Sky at Twenty-one centimeters (FIRST). We found that HeTu-v2 has a high accuracy with a mean average precision ($AP_{\rm @50:5:95}$) of 77.8%, which is 15.6 points and 11.3 points higher than that of HeTu-v1 and the original Mask R-CNN respectively. We produced a FIRST morphological catalog (FIRST-HeTu) using HeTu-v2, which contains 835,435 sources and achieves 98.6% of completeness and up to 98.5% of accuracy compared to the latest 2014 data release of the FIRST survey. HeTu-v2 could also be employed for other astronomical tasks like building sky models, associating radio components, and classifying radio galaxies.

astro-ph.IM↗

arXiv2023

Fine structures of radio bursts from flare star AD Leo with FAST observations

Radio bursts from nearby active M-dwarfs have been frequently reported and extensively studied in solar or planetary paradigms. Whereas, their sub-structures or fine structures remain rarely explored despite their potential significance in diagnosing the plasma and magnetic field properties of the star. Such studies in the past have been limited by the sensitivity of radio telescopes. Here we report the inspiring results from the high time-resolution observations of a known flare star AD Leo with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We detected many radio bursts in the two days of observations with fine structures in the form of numerous millisecond-scale sub-bursts. Sub-bursts on the first day display stripe-like shapes with nearly uniform frequency drift rates, which are possibly stellar analogs to Jovian S-bursts. Sub-bursts on the second day, however, reveal a different blob-like shape with random occurrence patterns and are akin to solar radio spikes. The new observational results suggest that the intense emission from AD Leo is driven by electron cyclotron maser instability which may be related to stellar flares or interactions with a planetary companion.

astro-ph.SR↗

arXiv2023

Plasma lensing with magnetic field and a small correction to the Faraday rotation measurement

Plasma lensing displays interesting characteristics that set it apart from gravitational lensing. The magnetised medium induces birefringence in the two polarisation modes. As the lensing deflection grows stronger, e.g. when images form near the critical curve, the geometric delay of the signal can cause rotation in linear polarisation, in addition to Faraday rotation. This rotation has a frequency dependence to the power of four. We study the geometric rotation of the lensed image in a Gaussian density model and find that it is necessary to take into account the geometric rotation when estimating magnetised media, especially in the under-dense lens. At frequencies of $\sim 1$ GHz or lower, the geometric rotation can dominate. We simulate the flux of lensed images and find that when the image forms near the lensing critical curve, the birefringence can convert the linear polarisation and un-polarisation pulse into a circular mode. The lensing magnification has the potential to increase the probability of detecting such events.

astro-ph.CO↗

arXiv2021

Discovery of Galaxy Clusters and a Head-Tail Radio Galaxy in the Direction of Globular Cluster NGC 6752

We report the discovery of CXOU J191100-595621 and CXOU J191012-595619, two galaxy clusters serendipitously detected in the direction of globular cluster NGC 6752, based on archival {\it Chandra} observations with a total exposure time of $\sim 344$ ks. The deep {\it Chandra} X-ray data enabled us to measure properties of both systems, which result in a redshift of $z=0.239\pm0.013$ and $z=0.375\pm0.016$, a temperature of $kT=3.32^{+0.57}_{-0.46}$ keV and $kT=3.71^{+1.18}_{-0.86}$ keV, an iron abundance of $Z_{\rm Fe}=0.64^{+0.34}_{-0.29}Z_{\rm Fe\odot}$ and $Z_{\rm Fe}=1.29^{+0.97}_{-0.65}Z_{\rm Fe\odot}$, and a rest-frame full band (0.5-7 keV) luminosity of $L_{\rm X}=9.2^{+1.2}_{-1.1}\times 10^{43} {\rm \, erg\, s^{-1}}$ and $L_{\rm X}=9.9^{+2.7}_{-2.2}\times 10^{43} {\rm \, erg\, s^{-1}}$ for CXOU J191100-595621 and CXOU J191012-595619, respectively. The temperature profile of CXOU J191100-595621 is found to decreases with decreasing radius, indicating a cool core in this cluster. The hydrostatic equilibrium estimation suggests the clusters are moderately weighted, with $M_{500}=(1.3\pm0.4)\times 10^{14}\, M_{\odot}$ and $M_{500}=(2.0\pm1.5)\times 10^{14}\, M_{\odot}$, respectively. We search for optical and radio counterparts of X-ray point sources in the clusters. Three active galactic nuclei are found, among which one is identified with a narrow-angle-tail radio galaxy, and one is found to associated with the brightest central galaxy (BCG) of CXOU J191100-595621.

astro-ph.HE↗

arXiv2021

New continuum and polarization observations of the Cygnus Loop with FAST

We report on the continuum and polarization observations of the Cygnus Loop supernova remnant (SNR) conducted by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). FAST observations provide high angular resolution and high sensitivity images of the SNR, which will help to disentangle its nature. We obtained Stokes I, Q and U maps over the frequency range of 1.03 - 1.46 GHz split into channels of 7.63 kHz. The original angular resolution is in the range of ~3 arcmin - ~3.8 arcmin, and we combined all the data at a common resolution of 4 arcmin. The temperature scale of the total intensity and the spectral index from the in-band temperature-temperature plot are consistent with previous observations, which validates the data calibration and map-making procedures. The rms sensitivity for the band-averaged total-intensity map is about 20 mK in brightness temperature, which is at the level of confusion limit. For the first time, we apply rotation measure (RM) synthesis to the Cygnus Loop to obtain the polarization intensity and RM maps. The rms sensitivity for polarization is about 5 mK, far below the total-intensity confusion limit. We also obtained RMs of eight extra-galactic sources, and demonstrate that the wide-band frequency coverage helps to overcome the ambiguity of RM determinations.

astro-ph.GA↗

arXiv2020

Long, depolarising H$α$-filament towards the Monogem ring

In soft X-rays, the Monogem ring is an object with a diameter of 25° located in the Galactic anti-centre. It is believed to be a faint, evolved, local supernova remnant. The ring is also visible in the far-ultraviolet, and a few optical filaments are related. It is not seen at radio wavelengths, as other large supernova remnants are. We study a narrow about 4.5° long, faint H$α$-filament, G203.7+11.5,that is seen towards the centre of the Monogem ring. It causes depolarisation and excessive Faraday rotation of radio polarisation data. Polarisation observations at $λ$11\ cm and $λ$21\ cm with the Effelsberg 100-m telescope were analysed. in addition to $WMAP$ data, extragalactic rotation measures, and H$α$ data. A Faraday-screen model was applied. From the analysis of the depolarisation properties of the H$α$ filament, we derived a line-of-sight magnetic field, $B_{||}$, of 26$\pm5μ$G for a distance of 300~pc and an electron density, $n_\mathrm{e}$, of 1.6~cm$^{-3}$. The absolute largest rotation measure of G203.7+11.5 is -86$\pm3$~rad~ m$^{-2}$, where the magnetic field direction has the opposite sign from the large-scale Galactic field. We estimated the average synchrotron emissivity at $λ$21\ cm up to 300~pc distance towards G203.7+11.5 to about 1.1~K $T_\mathrm{b}$/kpc, which is higher than typical Milky Way values. The magnetic field within G203.7+11.5 is unexpected in direction and strength. Most likely, the filament is related to the Monogem-ring shock, where interactions with ambient clouds may cause local magnetic field reversals. We confirm earlier findings of an enhanced but direction-dependent local synchrotron emissivity.

astro-ph.GA↗

arXiv2020

A polarization study of the supernova remnant CTB 80

We present a radio polarization study of the supernova remnant CTB 80 based on images at 1420 MHz from the Canadian Galactic plane survey, at 2695 MHz from the Effelsberg survey of the Galactic plane, and at 4800 MHz from the Sino-German 6cm polarization survey of the Galactic plane. We obtained a rotation measure (RM) map using polarization angles at 2695 MHz and 4800 MHz as the polarization percentages are similar at these two frequencies. RM exhibits a transition from positive values to negative values along one of the shells hosting the pulsar PSR B1951+32 and its pulsar wind nebula. The reason for the change of sign remains unclear. We identified a partial shell structure, which is bright in polarized intensity but weak in total intensity. This structure could be part of CTB 80 or part of a new supernova remnant unrelated to CTB 80.

astro-ph.GA↗

arXiv2020

A doubly stochastic block Gauss-Seidel algorithm for solving linear equations

We propose a simple doubly stochastic block Gauss--Seidel algorithm for solving linear systems of equations. By varying the row partition parameter and the column partition parameter of the coefficient matrix, we recover the Landweber algorithm, the randomized Kaczmarz algorithm, the randomized Gauss--Seidel algorithm, and the doubly stochastic Gauss--Seidel algorithm. For general (consistent or inconsistent) linear systems, we show the exponential convergence of the {\it norms of the expected iterates} via exact formulas. For consistent linear systems, we prove the exponential convergence of the {\it expected norms of the error and the residual}. Numerical experiments are given to illustrate the efficiency of the proposed algorithm.

math.NA↗

arXiv2020

Randomized extended block Kaczmarz for solving least squares

Randomized iterative algorithms have recently been proposed to solve large-scale linear systems. In this paper, we present a simple randomized extended block Kaczmarz algorithm that exponentially converges in the mean square to the unique minimum $\ell_2$-norm least squares solution of a given linear system of equations. The proposed algorithm is pseudoinverse-free and therefore different from the projection-based randomized double block Kaczmarz algorithm of Needell, Zhao, and Zouzias. We emphasize that our method works for all types of linear systems (consistent or inconsistent, overdetermined or underdetermined, full-rank or rank-deficient). Moreover, our approach can utilize efficient implementations on distributed computing units, yielding remarkable improvements in computational time. Numerical examples are given to show the efficiency of the new algorithm.

math.NA↗

arXiv2020

Magnetism Science with the Square Kilometre Array

The Square Kilometre Array (SKA) will answer fundamental questions about the origin, evolution, properties, and influence of magnetic fields throughout the Universe. Magnetic fields can illuminate and influence phenomena as diverse as star formation, galactic dynamics, fast radio bursts, active galactic nuclei, large-scale structure, and Dark Matter annihilation. Preparations for the SKA are swiftly continuing worldwide, and the community is making tremendous observational progress in the field of cosmic magnetism using data from a powerful international suite of SKA pathfinder and precursor telescopes. In this contribution, we revisit community plans for magnetism research using the SKA, in the light of these recent rapid developments. We focus in particular on the impact that new radio telescope instrumentation is generating, thus advancing our understanding of key SKA magnetism science areas, as well as the new techniques that are required for processing and interpreting the data. We discuss these recent developments in the context of the ultimate scientific goals for the SKA era.

astro-ph.GA↗

arXiv2019

Large-Scale Online Experimentation with Quantile Metrics

Online experimentation (or A/B testing) has been widely adopted in industry as the gold standard for measuring product impacts. Despite the wide adoption, few literatures discuss A/B testing with quantile metrics. Quantile metrics, such as 90th percentile page load time, are crucial to A/B testing as many key performance metrics including site speed and service latency are defined as quantiles. However, with LinkedIn's data size, quantile metric A/B testing is extremely challenging because there is no statistically valid and scalable variance estimator for the quantile of dependent samples: the bootstrap estimator is statistically valid, but takes days to compute; the standard asymptotic variance estimate is scalable but results in order-of-magnitude underestimation. In this paper, we present a statistically valid and scalable methodology for A/B testing with quantiles that is fully generalizable to other A/B testing platforms. It achieves over 500 times speed up compared to bootstrap and has only $2\%$ chance to differ from bootstrap estimates. Beyond methodology, we also share the implementation of a data pipeline using this methodology and insights on pipeline optimization.

stat.AP↗