arXiv subjects

Lijuan Dong

Publications and source records attributed to Lijuan Dong.

7 recordsLinked to original sources

arXiv2026

Gaussian-process evidence for a stochastic-variability transition in the recovering corona of 1ES 1927+654

We investigate the stochastic X-ray variability of the changing-look active galactic nucleus 1ES 1927+654 during its 2018--2024 evolution, focusing on the recovery of the X-ray corona after its 2018 collapse. Using XMM-Newton EPIC-pn light curves in the 0.3--2.0 keV and 2.0--10.0 keV bands, we model the variability with Gaussian process (GP) covariance components including Matérn-3/2, damped-random-walk (DRW), stochastically driven damped simple-harmonic-oscillator (SHO), and white-noise terms. Bayesian model comparison reveals an X-ray stochastic-variability transition during the changing-look recovery phase. In the 2019 May 5 observation, the preferred covariance changes from a Matérn-3/2-like state to a DRW-like state within a single continuous exposure. A phenomenological gated-kernel estimate localizes this transition sharply in the hard band at $t_c\simeq23.5~{\rm ks}$, while the soft band shows the same qualitative change over a broader interval. This transition occurs after the X-ray corona had reappeared but before the later pronounced hardening and brightening of the coronal emission, suggesting an early timing-domain signature of disk--corona reconfiguration. Phenomenologically, the dominant variability evolves from a smoother, finite-memory correlated process to a rougher, shorter-memory red-noise process. In the later 2022--2024 observations, SHO-like components associated with the known millihertz QPO show increasing characteristic frequency and quality factor, indicating a faster and more coherent oscillatory component during the QPO-plus-jet phase. GP-based time-domain inference therefore provides a sensitive probe of stochastic-variability changes in recovering AGN coronae.

astro-ph.HE↗

arXiv2026

Gaussian Process Inference of Stochastic Magneto-Active Dynamics and Viscosity in Swift J1727.8-1613

Linking X-ray variability to the underlying magnetohydrodynamic (MHD) dynamics of black hole X-ray binaries remains challenging. We systematically investigate the stochastic and oscillatory variability of the black hole X-ray binary candidate Swift J1727.8$-$1613 during its 2023 outburst using Gaussian process (GP) regression applied to Insight-HXMT multi-band light curves. The variability is modeled with a physically motivated composite kernel comprising one stochastically driven damped simple harmonic oscillator (SHO) and two damped random walk (DRW) components. The SHO term robustly recovers quasi-periodic oscillations (QPOs) with frequencies $ν_0 \sim 0.07$--$5$ Hz, consistent with the fundamental Alfvén mode of a contracting magnetically confined disk--coronal cavity. The quality factor rises from $Q \sim 3$ to $Q \sim 10$, suggesting increasing coherence of the magnetic cavity. We also find an anti-correlation between QPO frequency and the short DRW damping timescale, supporting our proposed stochastic magneto-active dynamics scenario. Associating the short and long DRW timescales with the local turbulent turnover and thermal adjustment timescales, respectively, we infer an effective viscosity parameter of $α\approx 0.1$, supporting a strongly magnetized accretion flow. Strikingly, near the onset of relativistic jet ejection around MJD 60206, both relaxation timescales collapse toward the 0.1 s sampling limit, suggesting a rapid reorganization of the disk internal energy balance immediately before jet launching. Our results establish GP inference as a powerful route to connecting X-ray timing observables with the dynamical state of black hole accretion flows.

astro-ph.HE↗

arXiv2025

Patterns of X-ray and $γ$-ray Flares: from Blazar to Maganetar and Sun

Using Gaussian process methods, we analyzed the light curves of three extreme solar X-ray flares observed by the RHESSI satellite. Their variability characteristics were then compared with those of HXMT-HE X-ray burst (XRB; in SGR 1935+2154) associated with fast radio burst (FRB) 200428 and blazar $γ$-ray giant flares, to investigate the origins of these extreme flaring events. The variability patterns of the solar X-ray flares follow the stochastically driven damped simple harmonic oscillator (SHO) model. The derived timescales $t_{\rm B\underline{} steep}$ and $t_{\rm B\underline{~} flat}$ (corresponding to PSD breaks) are in the range of 4-7 s and 16-53 s, respectively. The FRB-associated HXMT-HE burst has a $Q$ value near 0.3, matching those of the solar flares occurred on 23 July 2002 (flare 1) and 3 November 2003 (flare 2). By contrast, blazar $γ$-ray giant flares show $Q >$ 0.3, similar to the solar flare occurred on 25 February 2014 (flare 3). We proposed that the critically damped state of the system may be the condition triggering the association between the XRB in SGR 1935+2154 and the FRB. In this scenario, the critical damping $Q$ value of the system is around 0.3, not the theoretical 0.5. The similarity in $Q$ values might imply that the FRB-associated HXMT-HE XRB and solar X-ray flares 1 $\&$ 2 share comparable dynamic behavior, while blazar $γ$-ray flares and solar X-ray flare 3 exhibit another distinct but similar dynamic behavior. Like solar X-ray flares, these extreme flares may all be related to the magnetic reconnection process.

astro-ph.HE↗

arXiv2025

Collective Enhancement of Photon Blockade via Two-Photon Interactions

Analogous to Coulomb blockade for electrons, photon blockade is a key quantum optical effect in which the presence of one photon prevents the transmission of subsequent ones through a nonlinear medium. Beyond its fundamental interest, photon and multi-photon blockade are actively studied as mechanisms for generating technologically-relevant quantum states of light. Although photon blockade typically requires achieving strong light-matter coupling, increasing the number of atoms fails to enhance antibunching. Here, we analyze the optical transmission properties of a quantum resonator that embeds a two-photon-coupled ensemble of emitters, combining an approximate analytical approach with full quantum numerical simulations. We show that when light and matter are coupled via a two-photon interaction, both single- and multi-photon blockade can benefit from a collective enhancement. We propose different driving schemes in which the second or third-order correlation functions are strongly suppressed with increasing atom number. Differently from established methods, this collective enhancement of non-classical properties occurs with unitary transmission and is ultimately constrained only by decoherence. This demonstrates that collective two-photon couplings are a powerful mechanism for realizing photon blockade even in platforms where individual strong coupling is not achievable.

quant-ph↗

arXiv2025

Detection of the cosmological evolution of the Doppler factor for blazars jets

The Doppler factor ($δ$) is a fundamental quality for the relativistic jets from active galactic nuclei (AGNs). It is also fundamental to assessing the number of the entire population of the jetted AGN at high redshift ($z$), and therefore is important for tracing the growth of supermassive black holes (SMBHs) across cosmic time. Here we present the identification of the positive cosmic evolution of the Doppler factor in {\it Fermi}-detected bright $γ-$ray blazars. The redshift dependence of the Doppler factor, $δ\propto(1+z)^{0.8}$, is measured from the observed characteristic energies in the gamma-ray spectra of 141 blazars. Moreover, the analysis of the characteristic timescales derived from modeling the long-term optical light curves of 89 blazars with Gaussian process regression gives $δ\propto(1+z)^{1.1}$, but with a larger scatter. \textbf{Note that each index is derived from the entire sample, representing an average evolution. Interestingly, the index itself also appears to evolve, with low-luminosity sources showing stronger evolution, as indicated by a larger index.} This detection straightly suggests that relativistic jets from AGNs are much more common at high redshifts than what is previously estimated.

astro-ph.HE↗

arXiv2021

Omnidirectional nonreciprocal absorber realized by the magneto-optical hypercrystal

Photonic bandgap design is one of the most basic ways to effectively control the interaction between light and matter. However, the traditional photonic bandgap is always dispersive (blueshift with the increase of the incident angle), which is disadvantageous to the construction of wide-angle optical devices. Hypercrystal, that the photonic crystal with layered hyperbolic metamaterials (HMMs), can strongly modify the bandgap properties based on the anomalous wavevector dispersion of the HMM. Here, based on phase variation compensation between HMM and isotropic dielectric layers, we propose for the first time to design nonreciprocal and flexible photonic bandgaps using magneto-optical HMMs in one-dimensional photonic crystals. Especially for the forward and backward incident light, the blueshift and dispersionless of the forward and backward cavity modes are designed respectively to realize the interesting omnidirectional nonreciprocal absorber. Our results show high (low) absorption about 0.99 (0.25) in an angle range of 20-75 degrees for the forward (backward) incident light at the wavelength of 367 nm. The nonreciprocal omnidirectional cavity mode not only facilitates the design of perfect unidirectional optical absorbers working in a wide-angle range, but also possesses significant applications for all-angle reflectors and filters.

physics.optics↗

arXiv2021

Phase-adaptive dynamical decoupling methods for robust spin-spin dynamics in trapped ions

Quantum platforms based on trapped ions are main candidates to build a quantum hardware with computational capacities that largely surpass those of classical devices. Among the available control techniques in these setups, pulsed dynamical decoupling (pulsed DD) revealed as a useful method to process the information encoded in ion registers, whilst minimising the environmental noise over them. In this work, we incorporate a pulsed DD technique that uses random pulse phases, or correlated pulse phases, to significantly enhance the robustness of entangling spin-spin dynamics in trapped ions. This procedure was originally conceived in the context of nuclear magnetic resonance for nuclear spin detection purposes, and here we demonstrate that the same principles apply for robust quantum information processing in trapped-ion settings.

quant-ph↗