arXiv subjects

Lijing Shao

Publications and source records attributed to Lijing Shao.

At least 19 recordsLinked to original sources

arXiv2026

Extremal Scalarization of Charged Black Holes: Miransky Scaling across Reissner-Nordström Extremality

We study spontaneous scalarization near and at extremality in Einstein-Maxwell-scalar theory. As Reissner-Nordström (RN) extremality is approached from below, the nonextremal scalarization existence curves collapse toward a common critical threshold $α_c=1/4$ given by the Breitenlohner-Freedman bound for the near-horizon throat (AdS$_2\times S^2$) of its extremal black hole. For a polynomial coupling, we construct extremal black holes with nonconstant scalar hair, satisfying $M^2+Q_s^2=Q^2$, which places them in the overcharged region $Q/M>1$ beyond the RN bound. A common near-region phase mechanism predicts accumulation toward the RN extremality from both sides, yielding Miransky-type scaling. In the extremal sector, the logarithmic phase interval doubles, halving the exponent which denotes the geometric spacing of successive node scales.

gr-qc↗

arXiv2026

An agentic framework for gravitational-wave counterpart association in the multi-messenger era

With the detection of gravitational waves (GWs), multi-messenger astronomy has opened a new window for advancing our understanding of astrophysics, dense matter, gravitation, and cosmology. The GW sources detected to date are from mergers of compact object binaries, which possess the potential to generate detectable electromagnetic (EM) counterparts. Searching for associations between GW signals and their EM counterparts is an essential step toward enabling subsequent multi-messenger studies. In the era of next-generation GW and EM detectors, the rapid increase in the number of events brings not only unprecedented scientific opportunities, but also substantial challenges to the existing data analysis paradigm. To help address these challenges, we develop GW-Eyes, an agentic framework powered by large language models (LLMs). For the first time, GW-Eyes integrates domain-specific tools and autonomously performs counterpart association tasks between GW and candidate EM events. It supports natural language interaction to assist human experts with auxiliary tasks such as catalog management, skymap visualization, and rapid verification. Our framework leverages the complex decision-making capabilities of LLMs and their traceable reasoning processes, offering a new perspective to the multi-messenger astronomy.

astro-ph.IM↗

arXiv2026

Constraining Ultralight Scalar Dark Matter in the Galactic Center with the S2 Orbit

The dense environment of our Galactic Center (GC) offers a unique laboratory for probing ultralight dark matter (ULDM). We explore the prospect of detecting a scalar ULDM field through its effects on the orbital dynamics of S-stars around the supermassive black hole in the GC, Sgr A$^*$. We consider both linear and quadratic couplings between the real scalar field $ϕ$ and Standard Model particles, and analyze two representative ULDM structures: the scalar gravitational atom and the spherical soliton. We first find that quadratic coupling induces a non-oscillatory perturbation, leading to a long-term secular orbital evolution. We use the observed periastron precession rate of S2 star to put stringent constraints on the total ULDM mass in the GC and the quadratic coupling constant. For the gravitational atom $|211\rangle$ state, assuming an equatorial S2 orbit, we constrain the mass ratio of ULDM to Sgr A$^*$ to $β\lesssim 10^{-3}$ at $m \sim 10^{-18}$ eV, and for the spherical soliton which extends to $\sim 0.2\,$pc, the mass ratio is limited to $β\lesssim 1$ at $m \sim 3\times10^{-20}$ eV. Notably, over part of the mass range $10^{-20}\,\mathrm{eV}\lesssim m\lesssim10^{-18}\,\mathrm{eV}$, the S2 constraints are stronger than the corresponding bounds from Cassini and MICROSCOPE.

hep-ph↗

arXiv2026

Probing an Intermediate-Mass Black Hole Companion of Sagittarius A* with Pulsar Timing

An intermediate-mass black hole (IMBH) hidden in our Galactic Center (GC) may explain the puzzling observations of the stellar distribution around Sagittarius A* (Sgr A*), the supermassive black hole (SMBH) in the GC. Future observations with the next-generation radio telescopes, such as the SKA, are promising to discover pulsars orbiting around Sgr A*, and thus provide the possibility of constraining the hidden IMBH with pulsar timing. We study the detectability of a third-body, the IMBH, in the pulsar-SMBH system based on radio timing observation. We find that the pulsar-SMBH system is very sensitive to such a third-body perturbation and can be used to put stringent constraints on the existence of the IMBH. Even under strong perturbations caused by the complex GC astrophysical environments, timing observation will still complement the existing observational constraints.

astro-ph.HE↗

arXiv2026

Constraining Dipole Radiation with Multiband Gravitational Waves from Eccentric Binary Black Holes

Dipole-radiation-like deviations from general relativity are most prominent during the early inspiral of compact binaries, making space-ground multiband observations a potential probe of such effects. In the same regime, orbital eccentricity can leave a significant imprint on the waveform and is therefore essential for robust dipole-radiation constraints. For the first time we present a multiband Bayesian inference pipeline for stellar-mass binary black holes that simultaneously incorporates eccentricity and a theory-agnostic dipole-radiation correction. We find strong degeneracies among the dipole parameter, chirp mass, and eccentricity, showing that eccentricity can broaden the inferred dipole posterior by opening an additional degeneracy direction. Even so, for a GW231123-like source, one year of TianQin or LISA observation with ground-informed priors from a next-generation detector network can still constrain the dipole parameter to $|b|\lesssim\mathcal{O}(10^{-7})$ under inference with noisy data. Our results show that multiband binary black hole observations provide a promising and distinct channel for testing theory-agnostic dipole radiation, while also highlighting the need for more complete waveform modeling in future precision tests of gravity.

gr-qc↗

arXiv2026

A Realistic Pulsar -- Supermassive Black Hole Timing Model

Timing observation of pulsars orbiting around a supermassive black hole (SMBH) can measure the spacetime around the SMBH to a high precision and thus be a novel probe of the gravity theory. Future high-frequency surveys of the Galactic Centre (GC) region to be performed by the next-generation radio telescopes, such as the SKA, may discover pulsars that orbit around Sagittarius A* (Sgr A*), the SMBH dwelling in our GC. In this paper, we present a realistic pulsar-SMBH timing model based on the post-Newtonian equations of motion of the pulsar. Considering the expected timing precision in the future, we take into account several next-to-leading order light propagation time delays in the timing model. For the first time, we include the effects of proper motion of Sgr A*, which were expected to break the spin measurement degeneracy. We forecast the measurement precision of various parameters of Sgr A*, and discuss the data analysis procedure in the presence of red noise, which can be strong if the pulsar is a normal pulsar. The realistic timing model constructed in this study will serve as a useful tool in future searching and timing of pulsar-SMBH systems in the GC.

astro-ph.HE↗

arXiv2026

Improved proper motion and gravity tests with PSR J1913+1102

PSR J1913+1102 is a highly asymmetric double neutron star system and an excellent laboratory for testing scalar-tensor gravity theories, as well as a potential progenitor analogue of GW170817 that will merge in 470 Myr. We present an updated timing analysis combining 13 years of historical Arecibo observations and new FAST measurements, using two approaches to model dispersion-measure variations. The new timing solution provides precise measurements of four post-Keplerian parameters and improves the system mass estimates. Assuming general relativity and modelling the DM variation with a Gaussian process, we obtain a three-fold improvement in the total mass, m_{tot}=2.88948(20) M_\odot, and nearly four-fold improvements in the pulsar and companion masses, m_p=1.599(8) M_\odot and m_c=1.290(8) M_\odot, giving the mass ratio, q=0.807(8). We also measure an improved proper motion, μ=7.71(25) mas yr^{-1}, enabling a more accurate correction of the observed orbital-period derivative. Combined with the improved orbital-decay measurement, this yields an intrinsic orbital-period derivative \dot{P}_b^{intr}=-4.60(6)\times10^{-13} s s^{-1}, five times more precise than the previous value and fully consistent with the general-relativistic prediction for gravitational-wave damping. The improved masses and precise \dot{P}*b^{intr} place stringent constraints on dipolar gravitational-wave emission and the spontaneous-scalarisation window around 1.6 M*\odot. The refined proper motion and mass measurements also provide tighter constraints on the final helium-star mass immediately prior to its core collapse and formation of the second NS in a supernova, as well as on the magnitude and direction of the associated natal kick of the DNS system.

astro-ph.HE↗

arXiv2026

Asymptotically-flat Black holes in Bumblebee gravity: Exact solutions and Thermodynamics

We construct analytic solutions to the bumblebee gravity theory in static and spherically symmetric spacetimes, where the bumblebee vector field admits only a non-vanishing temporal component. In particular, we identify the parameter space that allows for asymptotically flat black hole solutions. We further investigate the thermodynamic properties of these black holes and obtained the analytic formulas for the $Y$ charge and $X$ potential, which were introduced in the prior work to ensure the Smarr relation and the first law of black hole thermodynamics. Using the new analytic results, we verify the numerical findings reported in early work and uncover multiple cases missed in the previous numerical analysis. These include: (i) an unbounded charge-mass ratio when the non-minimal coupling parameter $ξ$ is larger than $2κ$, (ii) the emergence of a traversable wormhole configuration for overcharged solutions with $ξ<0$, (iii) the non-monotonic turning behavior of the Hawking temperature as a function of the charge-mass ratio, and (iv) the presence of two divergent points in the constant-$Y$ heat capacity.

gr-qc↗

arXiv2026

Granular mass perturbations on the pulsar - supermassive black hole system

Discovery and timing observations of a radio pulsar orbiting around Sagittarius A*, the supermassive black hole (SMBH) in our Galactic Centre (GC), will provide unprecedented opportunities of studying the SMBH spacetime, testing gravity theories, and probing the astrophysical environment in the GC. However, unknown mass distributions might cause timing residuals that are much larger than the timing precision. With extensive numerical simulations, for the first time we find that the perturbations caused by a granular cusp of stellar-mass black holes in the GC lead to post-fit timing residuals of 10-100 s, contrary to traditional wisdom, even for a pulsar in a tight orbit with an orbital period $P_b=0.5\,{\rm yr}$. Such a large timing residual can lead to significant measurement bias or even prevent construction of a phase-connected timing solution for the full orbit. We revisit the idea of extracting SMBH parameters only with data around periastron where the perturbation is small. Under the realistic phase-disconnected assumption, we point out that it is vital to consider the frame-dragging effect in the light propagation, which breaks parameter degeneracy and leads to an order of magnitude improvement for the measurement precision of the SMBH spin.

astro-ph.HE↗

arXiv2026

Fundamental Physics and Cosmology with TianQin

The exploration of the surrounding world and the universe is an important theme in the legacy of humankind. The detection of gravitational waves is adding a new dimension to this grand effort. What are the fundamental physical laws governing the dynamics of the universe? What is the fundamental composition of the universe? How has the universe evolved in the past and how will it evolve in the future? These are the basic questions that press for answers. The space-based gravitational wave detector TianQin will tune in to gravitational waves in the millihertz frequency range ($10^{-4} \sim 1$ Hz, to be specific), opening a new gravitational wave spectrum window to explore many of the previously hidden sectors of the universe. TianQin will discover many astrophysical systems, populating the universe at different redshifts: some will be of new types that have never been detected before, some will have very high signal-to-noise ratios, and some will have very high parameter estimation precision. The plethora of information collected will bring us to new fronts on which to search for the breaking points of general relativity, the possible violation of established physical laws, the signature of possible new gravitational physics and new fundamental fields, and to improve our knowledge on the expansion history of the universe. In this white paper, we highlight the advances that TianQin can bring to fundamental physics and cosmology.

gr-qc↗

arXiv2026

Neutron stars more compact than black holes as a probe of strong-field gravity

Probing gravity in its strongest regime is a central goal of modern physics, as the nature of the most compact objects reflects fundamental aspects of Einstein's theory of general relativity (GR). In GR, black holes are regarded as the most compact objects in the Universe. Here, for the first time, we demonstrate that stable stellar configurations more compact than black holes can arise when neutron-star equations of state are embedded in quasi-topological gravity, a class of higher-curvature extensions of GR. We construct such ultra-compact stars, analyze their macroscopic properties, and establish their stability against radial perturbations, confirming their physical plausibility. We further identify potential observational signatures to distinguish these stars from black holes, most notably gravitational-wave echoes whose detectability could provide direct evidence of physics beyond Einstein's GR in the strong-field regime.

gr-qc↗

arXiv2026

Probing RLVR training instability through the lens of objective-level hacking

Prolonged reinforcement learning with verifiable rewards (RLVR) has been shown to drive continuous improvements in the reasoning capabilities of large language models, but the training is often prone to instabilities, especially in Mixture-of-Experts (MoE) architectures. Training instability severely undermines model capability improvement, yet its underlying causes and mechanisms remain poorly understood. In this work, we introduce a principled framework for understanding RLVR instability through the lens of objective-level hacking. Unlike reward hacking, which arises from exploitable verifiers, objective-level hacking emerges from token-level credit misalignment and is manifested as system-level spurious signals in the optimization objective. Grounded in our framework, together with extensive experiments on a 30B MoE model, we trace the origin and formalize the mechanism behind a key pathological training dynamic in MoE models: the abnormal growth of the training-inference discrepancy, a phenomenon widely associated with instability but previously lacking a mechanistic explanation. These findings provide a concrete and causal account of the training dynamics underlying instabilities in MoE models, offering guidance for the design of stable RLVR algorithms.

cs.AI↗

arXiv2026

An Enhanced Formation Channel for Galactic Dual-Line Gravitational-Wave Sources: von Zeipel-Lidov-Kozai Effect in Triples Involving Sgr A*

The dense Galactic Center environment is expected to host compact binary inspirals detectable by future space-borne gravitational wave (GW) observatories (e.g., LISA, TianQin, Taiji) in the millihertz band. Aided by information from these facilities, next-generation ground-based GW detectors (e.g., Cosmic Explorer, Einstein Telescope) can potentially capture gravitational radiation in the hectohertz band from rapidly spinning neutron star (NS) components in such binaries. These Galactic Center systems are thus anticipated to act as dual-line (i.e., low-frequency inspiral and high-frequency spin) GW sources. However, the formation channels of these systems remain largely unexplored. In this \textit{Letter}, we propose that the von Zeipel-Lidov-Kozai (ZLK) effect can enhance the formation of dual-line GW sources in hierarchical triples involving the Galactic supermassive black hole, Sgr A*. We show that ZLK-driven oscillations in the eccentricity and inclination of the inner binary can modulate the GW emission from both the binary inspiral and the individual NS spins. This effect boosts the expected dual-line source count by a factor of $\sim 5-10$, from rare to $\mathcal{O}(1)$ in 4 years, making dual-line observations substantially more probable. Our results demonstrate that the ZLK effect may provide an important formation channel for Galactic dual-line GW sources.

astro-ph.HE↗

arXiv2026

Cracking Gravitational Wave Multiple Ringdown Modes in Space

Ringdown signals from perturbed black holes (BHs) offer a clean window into BH spacetime, strong-field gravity, and fundamental physics. Presently the quasi-normal modes of stellar-mass BH ringdowns have been successfully extracted in the ground-based gravitational wave (GW) observations. Looking ahead, the future space-borne observatories will listen to the ringdowns from massive BH binary coalescences more loudly and resolve multiple modes to unprecedented precision, which calls for efficient approaches to mitigate the sharply increasing computational burden. We develop a practical ringdown analysis pipeline for space-borne detectors by implementing FIREFLY, a novel acceleration algorithm validated in ground-based detectors, and for the first time demonstrate its compatibility and effectiveness with the time-delay interferometry (TDI) observables. With high fidelity, we achieve a $\sim 200$-fold speedup for a simulated ringdown signal including six modes, providing a viable and scalable route for multi-mode ringdown analysis in the space context. This new approach has sound statistical interpretation and is extensible to other GW sources in band.

gr-qc↗

arXiv2026

Inferring neutron-star Love-Q relations from gravitational waves in the hierarchical Bayesian framework

Despite the large uncertainties in the equation of state for neutron stars (NSs), a tight universal ``Love-Q'' relation exists between their dimensionless tidal deformability, $Λ$, and the dimensionless quadrupole moment, $Q$. However, this relation has not yet been directly measured through observations. Gravitational waves (GWs) emitted from binary NS (BNS) coalescences provide an avenue for such a measurement. In this study, we adopt a hierarchical Bayesian framework and combine multiple simulated GW events to measure the Love-Q relation. We simulate 1000 GW sources and select 20 events with the highest signal-to-noise ratios and NS spins for the analysis. By inspecting four parameterization models of the Love-Q relation, we observe strong correlations between the model parameters. We verify that a linear relation between $\lnΛ$ and $\ln Q$ is practically sufficient to describe the Love-Q relation with the precision expected from next-generation GW detectors. Furthermore, we utilize the inferred Love-Q relation to test modified gravity. Taking the dynamical Chern-Simons gravity as an example, our results suggest that the characteristic length can be constrained to $10\, \mathrm{km}$ or less with future GW observations.

gr-qc↗

arXiv2026

Novae breves from magnetar giant flares: Potential probes of neutron star crusts

Matter ejected from the magnetar crust during giant flares (GFs) may undergo $r$-process nucleosynthesis, producing short-lived optical transients termed "novae breves". Although intrinsically much fainter than kilonovae from compact binary mergers, novae breves may occur within or near the Galaxy, making them promising observational targets. We aim to investigate how the neutron star (NS) equation of state (EOS) and the mass of the central magnetar affect the ejecta properties following GFs and the resulting nova brevis emission. We employ a semi-analytical ejecta model combined with nuclear reaction network calculations to compute nucleosynthesis yields and multi-band light curves for different EOSs and magnetar masses, and assess their detectability with current and future facilities. We find that variations in the EOS and magnetar mass modify the ejecta mass and its density and velocity distributions, etc., leading to observable differences in nova brevis light curves. In particular, both the peak luminosity and the characteristic peak timescale are EOS-dependent. Assuming a fixed Galactic magnetar mass of 1.4 solar mass and taking the $u$ band as an example, we find that the minimum apparent AB magnitudes range from 7 mag (H4 EOS) to 8.5 mag (WFF EOS) with peak timescales of 100-1000 s. A more massive magnetar produces fainter emission with a shorter peak timescale. For a magnetar mass of 1.4 solar mass, novae breves associated with known magnetars may reach peak luminosities of 1e37-1e39 erg/s, enabling targeted searches, particularly following high-energy GF alerts. Moreover, a detection horizon of 10 Mpc or beyond is achievable with current and future facilities, allowing searches for novae breves from previously unknown magnetars in the Local Volume. Although challenging, detection of such rapidly evolving transients is feasible.

astro-ph.HE↗

arXiv2026

Linear analysis of I-C-Love universal relations for neutron stars

Neutron stars (NSs) are excellent laboratories for testing gravity theories as they are strongly self-gravitating bodies and have rich observational phenomena. However, strong-field gravity effects in NS could be degenerate with their equation of state (EOS) which is largely unknown. Fortunately, there exist the so-called universal relations among the NS macroscopic quantities that are found to be insensitive to the underlying EOS. Studying the origin of these relations can lead to a better understanding of NSs and the gravitational interaction. We develop a new perspective of view to analyze the I-C and I-Love universal relations for NSs. At the linear order, we separate the deviation of the universal relations into two factors that are multiplied together. One is the EOS difference while the other factor only depends on the background star structure. The smallness of the second factor that does not depend on the EOS difference then indicates the origin of the universality. We discuss the validity of our linear approximation when considering the difference among realistic EOSs. Our study can be regarded as a new frame for quantitative representation of the universality and may provide new insights to the universal relations of NSs.

gr-qc↗

arXiv2026

Bumblebee cosmology: The FLRW solution and the CMB temperature anisotropy

We put into test the idea of replacing dark energy by a vector field against the cosmic microwave background (CMB) observation using the simplest vector-tensor theory, where a massive vector field couples to the Ricci scalar and the Ricci tensor quadratically. First, a remarkable Friedmann-Lemaître-Robertson-Walker (FLRW) metric solution that is completely independent of the matter-energy compositions of the universe is found. Second, based on the FLRW solution as well as the perturbation equations, a numerical code calculating the CMB temperature power spectrum is built. We find that though the FLRW solution can mimic the evolution of the universe in the standard $Λ$CDM model, the calculated CMB temperature power spectrum shows unavoidable discrepancies from the CMB power spectrum measurements.

gr-qc↗