arXiv subjects

D. Li

Publications and source records attributed to D. Li.

At least 55 records · Page 3Linked to original sources

arXiv2024

AT2018fyk: Candidate Tidal Disruption Event by a (Super)massive Black Hole Binary

The tidal disruption event (TDE) AT2018fyk has unusual X-ray, UV, and optical light curves that decay over the first $\sim$600d, rebrighten, and decay again around 1200d. We explain this behavior as a one-off TDE associated with a massive black hole (BH) \emph{binary}. The sharp drop-offs from $t^{-5/3}$ power laws at around 600d naturally arise when one BH interrupts the debris fallback onto the other BH. The BH mass $M_\bullet$ derived from fitting X-ray spectra with a slim disk accretion model and, independently, from fitting the early UV/optical light curves, is smaller by two orders of magnitude than predicted from the $M_\bullet$--$σ_*$ host galaxy relation, suggesting that the debris is accreted onto the secondary, with fallback cut off by the primary. Furthermore, if the rebrightening were associated with the primary, it should occur around 5000d, not the observed 1200d. The secondary's mass and dimensionless spin is $M_{\bullet,{\rm s}}=2.7^{+0.5}_{-1.5} \times 10^5 M_\odot$ and $a_{\bullet,{\rm s}}>0.3$ (X-ray spectral fitting), while the primary's mass is $M_{\bullet,{\rm p}}=10^{7.7\pm0.4}M_\odot$ ($M_\bullet$-$σ_*$ relation). An intermediate mass BH secondary is consistent with the observed UV/optical light curve decay, i.e., the secondary's outer accretion disk is too faint to produce a detectable emission floor. The time of the first accretion cutoff constrains the binary separation to be $(6.7\pm 1.2) \times 10^{-3}~{\rm pc}$. X-ray spectral fitting and timing analysis indicate that the hard X-rays arise from a corona above the secondary's disk. The early UV/optical emission, suggesting a super-Eddington phase for the secondary, possibly originates from shocks arising from debris circularization.

astro-ph.HE↗

arXiv2024

RRAT J1913+1330: an extremely variable and puzzling pulsar

Rotating Radio Transients (RRATs) are neutron stars that emit sporadic radio bursts. We detected 1955 single pulses from RRAT J1913+1330 using the 19-beam receiver of the Five-hundred-meter Aperture Spherical Radio Telescope (FAST). These pulses were detected in 19 distinct clusters, with 49.4% of them occurring with a waiting time of one rotation period. The energy distribution of these individual pulses exhibited a wide range, spanning three orders of magnitude, reminiscent of repeating fast radio bursts (FRBs). Furthermore, we observed abrupt variations in pulse profile, width, peak flux, and fluence between adjacent sequential pulses. These findings suggest that this RRAT could be interpreted as a pulsar with extreme pulse-to-pulse modulation. The presence of sequential pulse trains during active phases, along with significant pulse variations in profile, fluence, flux, and width, should be intrinsic to a subset of RRATs. Our results indicate that J1913+1330 represents a peculiar source that shares certain properties with populations of nulling pulsars, giant pulses, and FRBs from different perspectives. The dramatic pulse-to-pulse variation observed in J1913+1330 could be attributed to unstable pair creation above the polar cap region and the variation of the site where streaming pairs emit coherently. Exploring a larger sample of RRATs exhibiting similar properties to J1913+1330 has the potential to significantly advance our understanding of pulsars, RRATs, and FRBs.

astro-ph.HE↗

arXiv2024

Global destabilization of drift-tearing mode with coupling to discretized electron drift-wave instability

The global linear behaviors of 2/1 DTM in the collisional regime are investigated based on a concisely resistive drift-MHD model. Besides DTM, extra normal modes including EDW and SAW are coupled together and destabilized in different parameter regimes by considering resistivity in this system. The EVP approach is applied for solving the eigenstate spectra with the distribution of all unstable solutions. It is found that in the small EDD frequency (omega_*e) regime, DTM growth rate agrees well with local theory that is reduced with increasing omega_*e. However, when omega_*e exceeds a critical threshold omega_*crit, the strongly linear coupling between DTM and other discretized EDW instabilities happens so that the free energies from current and pressure channels can be released together and thus enhance the DTM, of which growth rate increases with increasing omega_*e and deviates from local theory results qualitatively. Correspondingly, a cross-scale mode structure forms with mixed polarization, namely, phi perturbation is dominated by electrostatic polarized short-wavelength oscillation as EDW instability character, and A_para perturbation remains typical tearing mode solution of Alfvenic polarized macroscopic structure. Within omega_*e > omega_*crit, the additional IDD causes phi oscillating structure to shift towards small density gradient domain, which cancels the extra drive from ion channel and thus DTM growth rate is insensitive to IDD frequency. Compared to EDD effects, the IDD effect alone with zero-omega_*e only leads to the stabilization of RTM that shows agreements between global simulation and local theory, which is no longer the condition for DTM regime. These results are useful for clarifying the DTM global properties with underlying physics mechanisms, which occurs in the regime of omega_*e >> gamma_c that is relevant to nowadays tokamak discharges with hot plasmas.

physics.plasm-ph↗

arXiv2024

Discovery of Very-high-energy Gamma-ray Emissions from the Low Luminosity AGN NGC 4278 by LHAASO

The first source catalog of Large High Altitude Air Shower Observatory reported the detection of a very-high-energy gamma ray source, 1LHAASO J1219+2915. In this paper a further detailed study of the spectral and temporal behavior of this point-like source have been carried. The best-fit position of the TeV source ($\rm{RA}=185.05^{\circ}\pm0.04^{\circ}$, $\rm{Dec}=29.25^{\circ}\pm0.03^{\circ}$) is compatible with NGC 4278 within $\sim0.03$ degree. Variation analysis shows an indication of the variability at a few months level in the TeV band, which is consistent with low frequency observations. Based on these observations, we report the detection of TeV $γ$-ray emissions from this low-luminosity AGN NGC 4278. The observations by LHAASO-WCDA during active period has a significance level of 8.8\,$σ$ with best-fit photon spectral index $\varGamma=2.56\pm0.14$ and a flux $f_{1-10\,\rm{TeV}}=(7.0\pm1.1_{\rm{sta}}\pm0.35_{\rm{syst}})\times10^{-13}\,\rm{photons\,cm^{-2}\,s^{-1}}$, or approximately $5\%$ of the Crab Nebula. The discovery of VHE from NGC 4278 indicates that the compact, weak radio jet can efficiently accelerate particles and emit TeV photons.

astro-ph.HE↗

arXiv2024

Statistical analysis of pulsar flux density distribution

This study presents a comprehensive analysis of the spectral properties of 886 pulsars across a wide frequency range from 20MHz to 343.5GHz, including a total of 86 millisecond pulsars. The majority of the pulsars exhibit power-law behavior in their spectra, although some exceptions are observed. Five different spectral models, namely simple power-law, broken power-law, low-frequency turn-over, high-frequency cut-off, and double turn-over, were employed to explore the spectral behaviors. The average spectral index for pulsars modeled with a simple power-law is found to be -1.64 +/-0.80, consistent with previous studies. Additionally, significant correlations between the spectral index and characteristic parameters are observed particularly in millisecond pulsars, while no strong correlation is observed in normal pulsars. Different models show variations in the most influential characteristic parameters associated with the spectral index, indicating diverse dominant radiation mechanisms in millisecond pulsars.Finally, this study identifies 22 pulsars of the Gigahertz-peaked Spectra (GPS) type for the first time based on the Akaike information criterion.

astro-ph.HE↗

arXiv2024

LHAASO-KM2A detector simulation using Geant4

KM2A is one of the main sub-arrays of LHAASO, working on gamma ray astronomy and cosmic ray physics at energies above 10 TeV. Detector simulation is the important foundation for estimating detector performance and data analysis. It is a big challenge to simulate the KM2A detector in the framework of Geant4 due to the need to track numerous photons from a large number of detector units (>6000) with large altitude difference (30 m) and huge coverage (1.3 km^2). In this paper, the design of the KM2A simulation code G4KM2A based on Geant4 is introduced. The process of G4KM2A is optimized mainly in memory consumption to avoid memory overffow. Some simpliffcations are used to signiffcantly speed up the execution of G4KM2A. The running time is reduced by at least 30 times compared to full detector simulation. The particle distributions and the core/angle resolution comparison between simulation and experimental data of the full KM2A array are also presented, which show good agreement.

astro-ph.IM↗

arXiv2024

Characterizing the Gamma-ray Emission Properties of the Globular Cluster M5 with the Fermi-LAT

We analyzed the globular cluster M5 (NGC 5904) using 15 years of gamma-ray data from the Fermi Large Area Telescope (LAT). Using rotation ephemerides generated from Arecibo and FAST radio telescope observations, we searched for gamma-ray pulsations from the seven millisecond pulsars (MSPs) identified in M5. We detected no significant pulsations from any of the individual pulsars. Also, we searched for possible variations of the gamma-ray emission as a function of orbital phase for all the six MSPs in binary systems, but did not detect any significant modulations. The gamma-ray emission from the direction of M5 is well described by an exponentially cutoff power-law spectral model, although other models cannot be excluded. The phase-averaged emission is consistent with being steady on a time scale of a few months. We estimate the number of MSPs in M5 to be between 1 and 10, using the gamma-ray conversion efficiencies for well-characterized gamma-ray MSPs in the Third Fermi Large Area Telescope Catalog of Gamma-ray Pulsars, suggesting that the sample of known MSPs in M5 is (nearly) complete, even if it is not currently possible to rule out a diffuse component of the observed gamma rays from the cluster.

astro-ph.HE↗

arXiv2024

Narrowly-Banded Spectra with Peak Frequency Around 1 GHz of FRB 20201124A: Implications for Energy Function and Radiation Physics

The radiation physics of fast radio bursts (FRBs) remains an open question. Current observations have discovered that narrowly-banded bursts of FRB 20201124A are active in 0.4-2 GHz and their spectral peak frequency ($ν^{\rm obs}_{p}$) are mostly toward $\sim 1$ GHz. Utilizing a sample of 1268 bursts of FRB 20201124A detected with the FAST telescope, we show that the $1σ$ spectral regime of 71.4\% events (in-band bursts) is within the FAST bandpass. Their intrinsic burst energies ($E^{\rm obs}_{\rm BWe}$) and spectral widths ($σ_s^{\rm obs}$) are well measured by fitting the spectral profile with a Gaussian function. The derived $E^{\rm obs}_{\rm BWe}$ and $σ_s^{\rm obs}$ distributions are log-normal and centering at $\log E^{\rm obs}_{\rm BWe}/{\rm erg}=37.2~ (σ=0.76)$ and $\log σ_s^{\rm obs}/{\rm GHz}=-1.16~ (σ=0.17)$. Our Monte Carlo simulation analysis infers its intrinsic $ν_p$ distribution as a normal function centered at $ν_{p,c}=1.16$ GHz ($σ=0.22$) and its intrinsic energy function as $Φ(E)\propto E^{-0.60}e^{-E/E_c}$ with $E_c=9.49 \times 10^{37}$ erg. We compare these results with that of typical repeating FRBs 20121102A and 20190520B that are active over a broad frequency range at several specific frequencies and discuss possible observational biases on the estimation of the event rate and energy function. Based on these results, we argue that FRB 20201124A likely occurs in a fine-tuned plasma for maser radiations at a narrow frequency range, while FRB 20121102A and FRB 20190520B could involve clumpy plasma conditions that make maser emission around several specific frequencies in a broad range.

astro-ph.HE↗

arXiv2024

Precision premium transformation -- a high-precision astrometric solution based on the precision premium curve

Context. In Gaia era, atmospheric turbulence, which causes stochastic wander of a star image, is a fundamental limitation to the astrometric accuracy of ground-based optical imaging. However, the positional bias caused by turbulence (called turbulence error here) can be effectively reduced by measuring a target relative to another reference (a star or a fast-moving target) which locates in the range of only several tens of arcsec, since they suffer from similar turbulence errors. This phenomenon is called the precision premium and has been effectively applied to the astrometry of solar system. Further investigation for the precision premium shows that, the precision premium works at less than about 100 arcsec for two specific objects and the relative positional precision as a function of their angular seperation can be well fitted by a sigmoidal function, called the precision premium curve (PPC). Aims. We want to reduce the turbulence error of a target if it is imaged in an area of high stellar density of a ground-based observation by taking advantage of more Gaia reference stars. Methods. Based on the PPC, we proposed a high-precision astrometric solution called precision premium transformation (PPT) in this paper, which takes advantage of high similarity of turbulence errors in a small region and the dense Gaia reference stars in the region to reduce the turbulence errors on the observation, through a weighted solution. Results. Through systematic analysis, the PPT method exhibits significant advantages in terms of not only precision but also applicability when a target is imaged in an area of high stellar density. The PPT method is also applied to the determination of the proper motion of an open cluster, and the results demonstrate and quantify benefits that the PPT method bestows on ground-based astrometry.

astro-ph.IM↗

arXiv2024

Investigation of profile shifting and subpulse movement in PSR J0344-0901 with FAST

We report two phenomena detected in PSR J0344$-$0901 from two observations conducted at frequency centered at 1.25 GHz using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The first phenomenon manifests as shifting in the pulse emission to later longitudinal phases and then gradually returns to its original location. The event lasts for about 216 pulse periods, with an average shift of about $0.7^\circ$ measured at the peak of the integrated profile. Changes in the polarization position angle (PPA) are detected around the trailing edge of the profile, together with an increase in the profile width. The second phenomenon is characterized by the apparent movement of subpulses, which results in different subpulse track patterns across the profile window. For the first time in this pulsar, we identify four emission modes, each with unique subpulse movement, and determine the pattern periods for three of the emission modes. Pulse nulling was not detected. Modeling of the changes in the PPA using the rotating vector model gives an inclination angle of $75.12^\circ \pm 3.80^\circ$ and an impact parameter of $-3.17^\circ \pm 5.32^\circ$ for this pulsar. We speculate that the subpulse movement may be related to the shifting of the pulse emission.

astro-ph.HE↗

arXiv2024

Galaxy Clusters Discovered via the Thermal Sunyaev-Zel'dovich Effect in the 500-square-degree SPTpol Survey

We present a catalog of 689 galaxy cluster candidates detected at significance $ξ>4$ via their thermal Sunyaev-Zel'dovich (SZ) effect signature in 95 and 150 GHz data from the 500-square-degree SPTpol survey. We use optical and infrared data from the Dark Energy Camera and the Wide-field Infrared Survey Explorer (WISE) and \spitzer \ satellites, to confirm 544 of these candidates as clusters with $\sim94\%$ purity. The sample has an approximately redshift-independent mass threshold at redshift $z>0.25$ and spans $1.5 \times 10^{14} < M_{500c} < 9.1 \times 10^{14}$ $M_\odot/h_{70}$ \ and $0.03 1$. We use external radio data from the Sydney University Molonglo Sky Survey (SUMSS) to estimate contamination to the SZ signal from synchrotron sources. The contamination reduces the recovered $ξ$ by a median value of 0.032, or $\sim0.8\%$ of the $ξ=4$ threshold value, and $\sim7\%$ of candidates have a predicted contamination greater than $Δξ= 1$. With the exception of a small number of systems $(<1\%)$, an analysis of clusters detected in single-frequency 95 and 150 GHz data shows no significant contamination of the SZ signal by emission from dusty or synchrotron sources. This cluster sample will be a key component in upcoming astrophysical and cosmological analyses of clusters. The SPTpol millimeter-wave maps and associated data products used to produce this sample are available at https://pole.uchicago.edu/public/data/sptpol_500d_clusters/index.html, and the NASA LAMBDA website. An interactive sky server with the SPTpol maps and Dark Energy Survey data release 2 images is also available at NCSA https://skyviewer.ncsa.illinois.edu.

astro-ph.CO↗

arXiv2024

Pulse Jitter and Single-pulse Variability in Millisecond Pulsars

Understanding the jitter noise resulting from single-pulse phase and shape variations is important for the detection of gravitational waves using pulsar timing array. We presented measurements of jitter noise and single-pulse variability of 12 millisecond pulsars that are part of the International Pulsar Timing Array sample using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We found that the levels of jitter noise can vary dramatically among pulsars. A moderate correlation with a correlation coefficient of 0.57 between jitter noise and pulse width is detected. To mitigate jitter noise, we performed matrix template matching using all four Stokes parameters. Our results revealed a reduction in jitter noise ranging from 6.7\% to 39.6\%. By performing longitude-resolved fluctuation spectrum analysis, we identified periodic intensity modulations in 10 pulsars. In PSR J0030+0451, we detected single-pulses with energies more than 10 times the average pulse energy, suggesting the presence of giant pulses. We also observed a periodic mode-changing phenomenon in PSR J0030+0451. We examined the achievable timing precision by selecting a sub-set of pulses with a specific range of peak intensity, but no significant improvement in timing precision is achievable.

astro-ph.HE↗

arXiv2023

Discovery of four pulsars in a pilot survey at intermediate Galactic latitudes with FAST

We present the discovery and timing results of four pulsars discovered in a pilot survey at intermediate Galactic latitudes with the Five-hundred Aperture Spherical Telescope (FAST). Among these pulsars, two belong to the category of millisecond pulsars (MSPs) with spin periods of less than 20 ms. The other two fall under the classification of "mildly recycled" pulsars, with massive white dwarfs as companions. Remarkably, this small survey, covering an area of 4.7 $deg^2$ , led to the discovery of four recycled pulsars. Such success underscores the immense potential of future surveys at intermediate Galactic latitudes. In order to assess the potential yield of MSPs, we conducted population simulations and found that both FAST and Parkes new phased array feed surveys, focusing on intermediate Galactic latitudes, have the capacity to uncover several hundred new MSPs.

astro-ph.HE↗

arXiv2023

Universal orbital and magnetic structures in infinite-layer nickelates

We conducted a comparative study of the rare-earth infinite-layer nickelates films, RNiO2 (R = La, Pr, and Nd) using resonant inelastic X-ray scattering (RIXS). We found that the gross features of the orbital configurations are essentially the same, with minor variations in the detailed hybridization. For low-energy excitations, we unambiguously confirm the presence of damped magnetic excitations in all three compounds. By fitting to a linear spin-wave theory, comparable spin exchange coupling strengths and damping coefficients are extracted, indicating a universal magnetic structure in the infinite-layer nickelates. Interestingly, while signatures of a charge order are observed in LaNiO2 in the quasi-elastic region of the RIXS spectrum, it is absent in NdNiO2 and PrNiO2. This prompts further investigation into the universality and the origins of charge order within the infinite-layer inickelates.

cond-mat.str-el↗

arXiv2023

Spatiotemporal control of high-intensity laser pulses with a plasma lens

Spatiotemporal control encompasses a variety of techniques for producing laser pulses with dynamic intensity peaks that move independently of the group velocity. This controlled motion of the intensity peak offers a new approach to optimizing laser-based applications and enhancing signatures of fundamental phenomena. Here, we demonstrate spatiotemporal control with a plasma optic. A chirped laser pulse focused by a plasma lens exhibits a moving focal point, or "flying focus," that can travel at an arbitrary, predetermined velocity. Unlike currently used conventional or adaptive optics, a plasma lens can be located close to the interaction region and can operate at an orders of magnitude higher, near-relativistic intensity.

physics.plasm-ph↗

arXiv2023

Matter ejections behind the highs and lows of the transitional millisecond pulsar PSR J1023+0038

Transitional millisecond pulsars are an emerging class of sources that link low-mass X-ray binaries to millisecond radio pulsars in binary systems. These pulsars alternate between a radio pulsar state and an active low-luminosity X-ray disc state. During the active state, these sources exhibit two distinct emission modes (high and low) that alternate unpredictably, abruptly, and incessantly. X-ray to optical pulsations are observed only during the high mode. The root cause of this puzzling behaviour remains elusive. This paper presents the results of the most extensive multi-wavelength campaign ever conducted on the transitional pulsar prototype, PSR J1023+0038, covering from the radio to X-rays. The campaign was carried out over two nights in June 2021 and involved 12 different telescopes and instruments, including XMM-Newton, HST, VLT/FORS2 (in polarimetric mode), ALMA, VLA, and FAST. By modelling the broadband spectral energy distributions in both emission modes, we show that the mode switches are caused by changes in the innermost region of the accretion disc. These changes trigger the emission of discrete mass ejections, which occur on top of a compact jet, as testified by the detection of at least one short-duration millimetre flare with ALMA at the high-to-low mode switch. The pulsar is subsequently re-enshrouded, completing our picture of the mode switches.

astro-ph.HE↗

arXiv2023

Muon Collider Forum Report

A multi-TeV muon collider offers a spectacular opportunity in the direct exploration of the energy frontier. Offering a combination of unprecedented energy collisions in a comparatively clean leptonic environment, a high energy muon collider has the unique potential to provide both precision measurements and the highest energy reach in one machine that cannot be paralleled by any currently available technology. The topic generated a lot of excitement in Snowmass meetings and continues to attract a large number of supporters, including many from the early career community. In light of this very strong interest within the US particle physics community, Snowmass Energy, Theory and Accelerator Frontiers created a cross-frontier Muon Collider Forum in November of 2020. The Forum has been meeting on a monthly basis and organized several topical workshops dedicated to physics, accelerator technology, and detector R&D. Findings of the Forum are summarized in this report.

hep-ex↗

arXiv2023

An Algorithm for the Constrained Longest Common Subsequence and Substring Problem

Let $Σ$ be an alphabet. For two strings $X$, $Y$, and a constrained string $P$ over the alphabet $Σ$, the constrained longest common subsequence and substring problem for two strings $X$ and $Y$ with respect to $P$ is to find a longest string $Z$ which is a subsequence of $X$, a substring of $Y$, and has $P$ as a subsequence. In this paper, we propose an algorithm for the constrained longest common subsequence and substring problem for two strings with a constrained string.

cs.DS↗