arXiv subjects

Fengting Xie

Publications and source records attributed to Fengting Xie.

3 recordsLinked to original sources

arXiv2026

Detectability of secondary images from flares near Sgr A* with mock GRAVITY data

The orbital motion of near-infrared flares reported by the GRAVITY collaboration encodes information about both the dynamics of accretion matter and the underlying spacetime geometry. The centroid track of these flares, which corresponds to the flux-weighted center of light, incorporates contributions from primary, secondary and higher-order images. Thus, it potentially indicates distinctive signatures of the spacetime geometry, even when these individual multiple images remain unresolved. In this study, we explore the detectability of the secondary images from flares orbiting Sgr A* through mock data simulating future GRAVITY observations. Specifically, we compare the model in which the centroid coincides with the track of the primary images with another model in which the centroid incorporates flux-weighted contributions from both the primary and secondary images. Fitting these models to the mock data based on Bayesian framework, we quantify the conditions under which the signature of secondary images can be statistically distinguishable. We demonstrate that increasing the sample size by an order of magnitude alone could not yield strong evidence for distinguishing the secondary image. Robust detectability ($|\Delta\text{BIC}| >7.9$) is achieved when both with the improved sample size and astrometric uncertainties reduced to 40\% of current uncertainties of GRAVITY astrometric data. Unlike the primary image, which is dominated by accretion flow physics, the secondary images originate from gravitational lensing in the strong-field regime. Their detection is an essential first step toward probing higher-order images and the photon rings.

astro-ph.HE↗

arXiv2026

Exploring the statistical anisotropy of primordial curvature perturbations with pulsar timing arrays

The recent detection of a stochastic gravitational wave background by pulsar timing arrays has opened a new window in understanding supermassive black hole binaries and in probing the universe at the early time. Recently, pulsar timing array (PTA) collaborations have been further paving the way to probe anisotropies in the stochastic gravitational wave background. This study investigates dipole-type statistical anisotropy in the primordial power spectrum within a phenomenological framework. We demonstrate that the primordial dipole induces both dipolar and quadrupolar anisotropies in the energy density spectrum of scalar-induced gravitational waves (SIGWs), without generating extra polarization modes. Based on this anisotropic spectrum, we derive the corresponding PTA overlap reduction functions (ORFs), which exhibit frequency dependence, with the anisotropies enhanced on small scales. Furthermore, owing to the non-uniform distribution of millisecond pulsars over the sky in current PTA dataset, the ORFs exhibit a morphology that explicitly depends on the preferred direction of the anisotropy. However, our bayesian analysis of the NANOGrav 15-year dataset still yields no significant evidence for a preferred direction and a weak upper limit on anisotropy amplitude $(g\lesssim0.5)$. This result arises because the observational frequency band lies below the spectral peak, where our models predict suppressed anisotropic contributions. This limitation highlights the potential of future PTA observations. Specifically, datasets with broader frequency coverage are expected to tighten constraints on dipole-type anisotropy.

gr-qc↗

arXiv2025

Investigating non-Keplerian motion in flare events with astrometric data

The GRAVITY interferometer has achieved microarcsecond precision in near-infrared interferometry, enabling the tracking of flare centroid motion in the strong gravitational field near the Sgr A*. It might be promising to serve as a unique laboratory for exploring the accretion matter near black holes or testing Einstein's gravity. Recent studies debated whether there is a non-Keplerian motion of the flares in the GRAVITY dataset. This motivates us to present a comprehensive analysis based on error estimation under the Bayesian framework. This study uses astrometric flare data to investigate the possibility that the flares exhibit deviations from the circular Keplerian motion. We analyze both averaged and individual flare data, modeling the hotspot with either circular orbits parameterized by a non-Keplerian correction or planar geodesic orbits. It is confirmed that the astrometric data favor the circular orbits over non-circular ones, with the orbital circularity parameter of $\gamma = 0.99_{-0.10}^{+0.07}$. Our results show that the joint posteriors for black hole mass and non-Keplerian parameter are negatively correlated. Fixing the mass to be its established value yields a non-Keplerian parameter of $\omega/\omega_k = 1.45^{+0.35}_{-0.38}$, at approximately the 1$\sigma$ level. The statistical significance is insufficiently high, and the conclusion is found to be sensitive to the presence of correlations in the astrometric data, which might originate from the non-uniform $u$-$v$ coverage in interferometer measurements. In this sense, the current data might be insufficient to draw a definitive conclusion regarding the presence of non-Keplerian motion. Future improvements in astrometry precision might enable stronger constraints on the kinematical behavior of the flares.

astro-ph.HE↗