arXiv subjects

S. S. Sun

Publications and source records attributed to S. S. Sun.

13 recordsLinked to original sources

arXiv2022

Deep learning for track recognition in pixel and strip-based particle detectors

The reconstruction of charged particle trajectories in tracking detectors is a key problem in the analysis of experimental data for high-energy and nuclear physics. The amount of data in modern experiments is so large that classical tracking methods, such as the Kalman filter, cannot process them fast enough. To solve this problem, we developed two neural network algorithms based on deep learning architectures for track recognition in pixel and strip-based particle detectors. These are TrackNETv3 for local (track by track) and RDGraphNet for global (all tracks in an event) tracking. These algorithms were tested using the GEM tracker of the BM@N experiment at JINR (Dubna) and the cylindrical GEM inner tracker of the BESIII experiment at IHEP CAS (Beijing). The RDGraphNet model, based on a reverse directed graph, showed encouraging results: 95% recall and 74% precision for track finding. The TrackNETv3 model demonstrated a recall value of 95% and 76% precision. This result can be improved after further model optimization.

hep-ex↗

arXiv2021

B3\,0749+460A: A New "Changing-Look" Active Galactic Nucleus Associated with X-ray Spectral Slope Variations

Here we report an identification of B3\,0749+460A as a new double-peaked local "changing-look" active galactic nucleus (CL-AGN) in terms of our multi-epoch spectroscopic analysis. By comparing our new spectra taken in 2021 with the ones taken by the Sloan Digital Sky Survey in 2004, BOSS in 2013, and MaNGA in 2016, we reveal type transitions of Seyfert (Sy) 1.9$\rightarrow$Sy1.8$\rightarrow$Sy1.9. In the transitions, the classical broad H$α$ emission fades away since 2013, and disappears in our 2021 spectrum, although the absence of broad H$β$ can be traced back to at least 2016. A follow-up observation in X-rays by the {\it Swift}/XRT reveals that (1) the X-ray emission level gradually decreases since 2005; and (2) the X-ray spectrum is soft in the optical "turn-off" state and hard in the "turn-on" state. We argue that the disappearance of the classical broad H$α$ emission can be likely explained by the disk-wind broad-line-region model, in which the CL phenomenon is sensitive to luminosity in individual AGNs.

astro-ph.HE↗

arXiv2021

Constrains on optical emission of FAST-detected FRB 20181130B with GWAC synchronized observations

Multi-wavelength simultaneous observations are essential to the constraints on the origin of fast radio bursts (FRBs). However, it is a significant observational challenge due to the nature of FRBs as transients with a radio millisecond duration, which occur randomly in the sky regardless of time and position. Here, we report the search for short-time fast optical bursts in the GWAC archived data associated with FRB 20181130B, which were detected by the Five Hundred Meter Spherical Radio Telescope (FAST) and recently reported. No new credible sources were detected in all single GWAC images with an exposure time of 10 s, including image with coverage of the expected arrival time in optical wavelength by taking the high dispersion measurements into account. Our results provide a limiting magnitude of 15.43$\pm0.04$ mag in R band, corresponding to a flux density of 1.66 Jy or 8.35 mag in AB system by assuming that the duration of the optical band is similar to that of the radio band of about 10 ms. This limiting magnitude makes the spectral index of $α<0.367$ from optical to radio wavelength. The possible existence of longer duration optical emission was also investigated with an upper limits of 0.33 Jy (10.10 mag), 1.74 mJy (15.80 mag) and 0.16 mJy (18.39 mag) for the duration of 50 ms, 10 s and 6060 s, respectively. This undetected scenario could be partially attributed to the shallow detection capability, as well as the high inferred distance of FRB 20181130B and the low fluence in radio wavelength. The future detectability of optical flashes associated with nearby and bright FRBs are also discussed in this paper.

astro-ph.HE↗

arXiv2021

Detection of Flare-associated CME Candidates on Two M-dwarfs by GWAC and Fast, Time-resolved Spectroscopic Follow-ups

The flare-associated stellar coronal mass ejection (CME) in solar-like and late type stars is quite essential for the habitability of an exoplanet. In this paper, we report detection of flare-associated CMEs in two M-dwarfs, thanks to the high cadence survey carried out by the Ground Wide-angle Camera system and the fast photometric and spectroscopic follow-ups. The flare energy in $R-$band is determined to be $1.6\times10^{35}\ \mathrm{erg}$ and $8.1\times10^{33}\ \mathrm{erg}$ based on the modeling of their light curves. The time-resolved spectroscopyic observations start at about 20 and 40 minutes after the trigger in both cases. The large projected maximum velocity of $\sim500-700\ \mathrm{km\ s^{-1}}$ suggests that the high velocity wing of their H$α$ emission lines are most likely resulted from a CME event in both stars, after excluding the possibility of chromospheric evaporation and coronal rain. The masses of the CMEs are estimated to be $1.5-4.5\times10^{19}\ \mathrm{g}$ and $7.1\times10^{18}\ \mathrm{g}$.

astro-ph.SR↗

arXiv2021

The intra-day Optical Monitoring of BL Lacerate Object 1ES 1218+304 at Its Highest X-ray Flux Level

We here report a monitor of the BL Lac object 1ES 1218+304 in both B- and R-bands by the GWAC-F60A telescope in eight nights, when it was triggerd to be at its highest X-ray flux in history by the VERITAS Observatory and Swift follow-ups. Both ANOVA and $χ^2$-test enable us to clearly reveal an intra-day variability in optical wavelengths in seven out of the eight nights. A bluer-when-brighter chromatic relationship has been clearly identified in five out of the eight nights, which can be well explained by the shock-in-jet model. In addtion, a quasi-periodic oscilation phenomenon in both bands could be tentatively identified in the first night. A positive delay between the two bands has been revealed in three out of the eight nights, and a negative one in the other nights. The identfied minimum time delay enables us to estimate the $M_{\mathrm{BH}}=2.8\times10^7 \rm M_{\odot}$that is invalid.

astro-ph.GA↗

arXiv2020

A $ΔR\sim 9.5$ mag Super Flare of An Ultracool Star Detected by $\text{SVOM/GWAC}$ System

In this paper, we report the detection and follow-ups of a super stellar flare GWAC\,181229A with an amplitude of $ΔR\sim$9.5 mag on a M9 type star by $\text{SVOM/GWAC}$ and the dedicated follow-up telescopes. The estimated bolometric energy $E_{bol}$ is $(5.56-9.25)\times10^{34}$ ergs, which places the event to be one of the most powerful flares on ultracool stars. The magnetic strength is inferred to be (3.6-4.7) kG. Thanks to the sampling with a cadence of 15 seconds, a new component near the peak time with a very steep decay is detected in the $R$-band light curve, followed by the two-component flare template given by Davenport et al. (2014). An effective temperature of $5340\pm40$ K is measured by a blackbody shape fitting to the spectrum at the shallower phase during the flare. The filling factors of the flare are estimated to be $\sim$30\% and 19\% at the peak time and at 54 min after the first detection. The detection of the particular event with large amplitude, huge-emitted energy and a new component demonstrates that a high cadence sky monitoring cooperating with fast follow-up observations is very essential for understanding the violent magnetic activity.

astro-ph.SR↗

arXiv2020

Suspended and restored activities of A Nearby Super Massive Black Hole

The discovery of spectral type transition of active galactic nuclei (AGNs), the so-called changing-look(CL) phenomenon, challenges the widely accepted AGN paradigm, not only in the orientation based Unified Model, but also in the standard disk model. In past decades, only a couple of nearby repeat "changing-look" active galactic nuclei (CL-AGNs) have been identified. Here we report spectroscopic observations of UGC 3223 over the course of 18 years, from 2001 onwards. Combining the spectrum taken in 1987 by Stirpe, we have witnessed its type transitions from $1.5\rightarrow2.0\rightarrow1.8$ over 32 years, and captured a long-lived (at least 10 years) thorough "turn-off" state with a spectrum typical of a Seyfert 2 galaxy. The long-term thorough "turn-off" state probably suggests a once-dormant and an awakening central engine in UGC3223. We argue the (dis)appearance of the broad Balmer emission lines can be explained by the disk-wind BLR model given the evolution of the calculated Eddington ratio of accretion of the supermassive black hole.

astro-ph.CO↗

arXiv2017

High-Tc superconductivity up to 55 K under high pressure in the heavily electron doped Lix(NH3)yFe2Se2 single crystal

We report a high-pressure study on the heavily electron doped Lix(NH3)yFe2Se2 single crystal by using the cubic anvil cell apparatus. The superconducting transition temperature Tc = 44 K at ambient pressure is first suppressed to below 20 K upon increasing pressure to Pc = 2 GPa, above which the pressure dependence of Tc(P) reverses and Tc increases steadily to ca. 55 K at 11 GPa. These results thus evidenced a pressure-induced second high-Tc superconducting (SC-II) phase in Lix(NH3)yFe2Se2 with the highest Tcmax = 55K among the FeSe-based bulk materials. Hall data confirm that in the emergent SC-II phase the dominant electron-type carrier density undergoes a fourfold enhancement and tracks the same trend as Tc(P). Interesting, we find a nearly parallel scaling behavior between Tc and the inverse Hall coefficient for the SC-II phases of both Lix(NH3)yFe2Se2 and (Li,Fe)OHFeSe. The present work demonstrates that high pressure offers a distinctive means to further raising the maximum Tc of heavily electron doped FeSe-based materials by increasing the effective charge carrier concentration via a plausible Fermi surface reconstruction at Pc.

cond-mat.supr-con↗

arXiv2017

Robust short-range-ordered nematicity in FeSe evidenced by high-pressure NMR

We report high-pressure $^{77}$Se NMR studies on FeSe single crystals that reveal a prominent inhomogeneous NMR linewidth broadening upon cooling, with the magnetic field applied along the tetragonal [110] direction. The data indicate the existence of short-range-ordered, inhomogeneous electronic nematicity, which has surprisingly long time scales over milliseconds. The short-range order survives temperatures up to $8$ times the structural transition temperature, and remains robust against pressure, in contrast to the strong pressure-dependence of the orbital ordering, structural transition, and the ground state magnetism. Such an extended region of static nematicity in the ($P$,$T$) space of FeSe indicates an enormously large fluctuating regime, and provide fresh insights and constraints to the understanding of electronic nematicity in iron-based superconductors.

cond-mat.supr-con↗

arXiv2016

Pressure Induced Stripe-order Antiferromagnetism and First-order Phase Transition in FeSe

To elucidate the magnetic structure and the origin of the nematicity in FeSe, we perform a high-pressure $^{77}$Se NMR study on FeSe single crystals. We find a suppression of the structural transition temperature with pressure up to about 2 GPa from the anisotropy of the Knight shift. Above 2 GPa, a stripe-order antiferromagnetism that breaks the spatial four-fold rotational symmetry is determined by the NMR spectra under different field orientations and with temperatures down to 50 mK. The magnetic phase transition is revealed to be first-order type, implying the existence of a concomitant structural transition via a spin-lattice coupling. Stripe-type spin fluctuations are observed at high temperatures, and remain strong with pressure. These results provide clear evidences for strong coupling between nematicity and magnetism in FeSe, and therefore support a universal scenario of magnetic driven nematicity in iron-based superconductors.

cond-mat.supr-con↗

arXiv2016

Monte-Carlo study of the MRPC prototype for the upgrade of BESIII

A GEANT4-based simulation is developed for the endcap time of flight (ETOF) upgrade based on multi-gap resistive plate chambers (MRPC) for the BESIII experiment. The MRPC prototype and the simulation method are described. Using a full Monte-Carlo simulation, the influence of high voltage and threshold on time resolution and detection efficiency are investigated. The preliminary results from simulation are presented and are compared with the experimental data taken with the prototype MRPC modules.

physics.ins-det↗

arXiv2015

Magnetism and a field-cycling induced effect in staircase Kagomé antiferromagnet PbCu$_3$TeO$_7$ revealed by NMR

We report $^{125}$Te and $^{63,65}$Cu nuclear magnetic resonance (NMR) studies on single crystals of staircase Kagomé antiferromagnet PbCu3TeO7 ($T_{N1}\approx$ 36 K). A Curie constant as large as $Θ$~140 K is obtained by a Curie-Weiss fitting of the high-temperature Knight shift. The frustration factor f (= $Θ/T_N $)$\approx$ 4 implies intermediate frustration in this system. From the high-temperature Knight shift data of $^{125}$Te, its hyperfine coupling constant is estimated to be $^{125}A_{hf}$ = -67 kOe/$μ_B$, which suggests a strong interlayer coupling bridging the neighboring Kagomé layers. At $T$= 2 K, we find two types of zero-field NMR signals. One has no dependence on the history of the field treatment, which is consistent with Cu NMR signals from inequivalent Cu sites with different internal hyperfine fields. The other one is only seen after a field-cycling process, which is consistent with domain wall contributions in this frustrated antiferromagnet.

cond-mat.str-el↗

arXiv2008

Physics at BES-III

This physics book provides detailed discussions on important topics in $τ$-charm physics that will be explored during the next few years at \bes3 . Both theoretical and experimental issues are covered, including extensive reviews of recent theoretical developments and experimental techniques. Among the subjects covered are: innovations in Partial Wave Analysis (PWA), theoretical and experimental techniques for Dalitz-plot analyses, analysis tools to extract absolute branching fractions and measurements of decay constants, form factors, and CP-violation and \DzDzb-oscillation parameters. Programs of QCD studies and near-threshold tau-lepton physics measurements are also discussed.

hep-ex↗