Xuelei Chen

FRB 20240114A is an extremely active repeating fast radio burst for which plasma lensing has been proposed to explain its burst-rate variations, spectral evolution, and apparently “carbon-copy” burst pairs. Using Five-hundred-meter Aperture Spherical radio Telescope (FAST) data and publicly available Parkes observations, we test this interpretation with the predictions of a one-dimensional Gaussian plasma-lens model. Although the burst-rate enhancements may be fitted separately for Parkes and FAST, the corresponding magnification peaks and demagnification troughs between the two datasets are offset from each other, inconsistent with plasma-lensing predictions. The enhancement peaks also do not show consistent periodicity, disfavoring a binary plasma-lensing origin. The burst bandwidths are not systematically narrower during the proposed lensing interval than in other episodes. Most importantly, the burst energy distribution, which shows an intrinsic turnover peak at low energies, shows no significant shift of the turnover during the magnification interval, providing a model-independent test to the plasma-lensing interpretation. All these suggest that there is no strong evidence in favor of a plasma-lensing interpretation to the source. Rather, all the burst property variations are consistent with being caused by intrinsic variations in the FRB central engine.

We investigate the relative importance of active galactic nucleus (AGN) feedback and environmental processes using a large sample of H i galaxies from the Arecibo Legacy Fast ALFA Survey (ALFALFA) and FAST All Sky H i Survey (FASHI) surveys. By applying the optical spectroscopy from Sloan Digital Sky Survey DR7/DR8 and the Dark Energy Spectroscopic Instrument survey, we analyse the gas content and physical properties of AGN-hosting galaxies in group environments. Our results show that AGN-hosting galaxies exhibit significantly suppressed star formation rates and H i gas fraction, approximately one order of magnitude lower than star-forming counterparts, regardless of their group-centric position. AGN-hosting satellites exhibit a significant and persistent deficit in both gas fraction and star formation rate relative to normal satellites without an AGN, even at the halo virial radius ( R / R ₁₈₀ ≈ 1). This suggests that cold gas depletion is primarily driven by internal AGN feedback before these galaxies experience intense environmental interactions. The relatively flat radial profiles of gas fraction and specific star formation rate further indicate that the evolution of AGN-hosting satellites is governed by internal physical processes rather than environmental interactions. Moreover, the apparent increase in H i gas at R / R ₁₈₀ < 0.3 is identified as an artifact of beam confusion. We conclude that for the AGN-hosting population, internal feedback is likely the prior quenching mechanism, while environmental effects act as a secondary, subsequent process.

The Dark Ages offer a uniquely clean window on primordial physics, making the redshifted 21-cm signal a promising probe of inflation models. Forecasts for interferometric 21-cm measurements must account for foreground wedge avoidance, which removes a substantial fraction of Fourier modes, and this effect is especially severe at high redshifts. We develop a wedge-aware Fisher framework in which this mode loss is propagated directly into the variances of the anisotropic 21-cm power spectrum and reduced bispectrum. As a case study, we apply the method to forecast constraints on two inflation models with different oscillatory features, a resonant model and a step model, for a lunar far-side array including thermal noise. Since these features affect both the power spectrum and the bispectrum, we apply wedge avoidance to both forecasts and compare their constraining power on oscillatory amplitudes in these feature models. We find that wedge avoidance reduces available Fourier modes significantly in both observables, leading to much weaker constraints on inflationary features. This framework is broadly applicable to 21-cm power spectrum and bispectrum forecasts across redshifts and is particularly useful in regimes where foreground wedge effect is severe, such as the epoch of reionization and the higher-redshift Dark Ages.

We explore the joint probe on the baryonic feedback effect using the weak-lensing measurement from the upcoming Chinese Space Station Survey Telescope (CSST) photometric survey and the dispersion measure (DM) statistics of the fast radio bursts (FRBs) from next-generation radio telescopes, i.e., the Square Kilometre Array (SKA) and the Deep Synoptic Array 2000 (DSA-2000). By employing the baryonic halo model, we compute the matter, electron, and matter–electron power spectra and generate mock data considering realistic noise and systematic effects based on the designs of the telescopes. These mock data are then analyzed using the Markov Chain Monte Carlo (MCMC) method to investigate the parameter constraints. We find that CSST weak lensing alone can constrain the baryonic feedback parameter log 10 T AGN to an accuracy of 3.1%, with the sum of neutrino mass bound ∑ m _ν < 0.53 eV. When performing the 3 × 2 pt( e , m ) analysis, the inclusion of FRB DM measurements can significantly improve the precision of log 10 T AGN to 0.4% and will lead to a better constraint on ∑ m _ν with an upper limit <0.47 eV by effectively breaking the degeneracy. Our results demonstrate that the joint observation of future FRB DM and weak lensing surveys is a powerful tool for probing the baryonic feedback effect, which is helpful in obtaining robust constraints on the neutrino mass and other important cosmological parameters.

We present the first blind search for OH 18 cm absorption with the Five-hundred-meter Aperture Spherical Telescope (FAST), conducted alongside the H i 21 cm absorption search. Our previous FAST blind H i absorption search identified 34 systems. In this work, we extend the search using 2024 and part of the 2025 CRAFTS and FAST All Sky H i Survey data (394.4 hr and 1622.1 deg ² ) together with FATHOMER observations, yielding four known and three new H i absorbers, for a total of 41 H i absorption systems. We search for OH absorption in 19 H i absorption systems whose OH redshifted frequencies fall within the FAST band. The known OH absorber toward PKS 1413+135 was redetected, making our survey the first blind survey to detect OH absorption. No new OH absorbers were identified. We examine the relationship between N _OH and N _HI , applying survival analysis to account for upper limits. The analysis does not provide statistically significant evidence for either an N _OH – N _HI correlation or redshift evolution of N _OH / N _HI . Finally, spectral stacking sets 3 σ OH column density upper limits of 4.93, 1.64, and 1.72 T _ex / c _f,OH × 10 ¹² cm ⁻² K ⁻¹ for associated, intervening, and combined samples, corresponding to [OH]/[H i ] ratios of −8 , −8 , and −8 , assuming T _ex = 10 K for OH and T _s = 100 K for H i . These results place the strongest constraints to date on the OH content in radio-selected H i absorbers and establish a blind-survey benchmark for future studies of molecular gas in H i -selected systems. They also demonstrate that known H i 21 cm absorbers provide an effective parent sample for systematic OH absorption searches, paving the way for future larger surveys.

Ground-based radio observations below 30 MHz are severely limited by ionospheric interference and radio-frequency interference from Earth. A lunar-orbiting radio interferometer mission, the Discovering the Sky at the Longest wavelengths (DSL, also known by its Chinese name “Hongmeng”), has been proposed to overcome these obstacles. However, for such a mission, there are new challenges, such as the nearly all-sky field of view and dynamic 3D baselines, which require a huge computational cost for interferometric image reconstruction. In this work, we present augmented Lagrangian multiplier imaging using gradient descent optimization (AMIGO), a novel imaging algorithm tailored to lunar-orbiting arrays like DSL, combining the minibatch gradient descent (MBGD) method with the augmented Lagrangian multiplier (ALM) technique. MBGD reduces the computational complexity and memory cost, enabling efficient handling of large datasets. ALM flexibly incorporates physical priors like nonnegative sky temperature and prior angular power spectrum into the imaging algorithm, with adjustable stopping criteria to quantitatively control prior strength. We validate AMIGO using mock visibility data generated under realistic DSL orbit configurations. Reconstructed sky maps at various frequencies and spatial resolutions show that this approach provides a computationally feasible framework for all-sky imaging with a lunar-orbiting array.

We study supernova (SN) classification using the Recurrent Neural Networks within the Chinese Space Station Survey Telescope Ultra-deep Field (CSST-UDF) photometric survey and explore the improvements of the cosmological constraint. We simulate Type Ia supernovae (SNe Ia) and core-collapse supernovae (CCSNe) using SNCosmo with the SALT3 SN Ia model and CCSN templates and apply the SuperNNova (SNN) program for classification. Our study indicates that the SNN combined with the Joint Light-curve Analysis cuts can enhance the purity of the CSST-UDF SN Ia sample up to over 99.5% with 2193 SNe Ia and 4 CCSNe, which can significantly increase the reliability of the cosmological constraints. The method based on the Bayesian Estimation Applied to Multiple Species with Bias Corrections framework is used to correct the SN Ia magnitude bias caused by the selection effect and CCSN contamination, and the Markov Chain Monte Carlo method is employed for cosmological constraints. We find that the accuracy of the constraints on the matter density Ω _M and the equation of state of dark energy parameter w can achieve 14% and 18%, respectively, assuming the flat w CDM model. This result is comparable to current surveys relying on spectroscopic confirmation. Our results indicate that our data analysis method is effective, and the CSST-UDF SN photometric survey is a powerful tool to explore the expansion history of the Universe.

We introduce a native tensor-based framework for foreground mitigation in 21 cm intensity mapping (IM), utilizing the Oriented Singular Value Decomposition (O-SVD) algorithm. While 21 cm IM is a powerful probe of the large-scale structure of the Universe, its efficacy is severely limited by astrophysical foregrounds that are orders of magnitude brighter than the cosmological signal. Traditional mitigation strategies often necessitate flattening multidimensional data cubes into 2D matrices, a process that potentially compromises the intrinsic spatial–spectral correlations by treating distinct spatial pixels as independent samples. By treating multifrequency sky maps and angular power spectra as third-order tensors, the O-SVD method performs decomposition directly on the multilinear manifold, preserving the underlying physical topology and leveraging the distinct coherence properties of astrophysical foregrounds across different dimensions. We demonstrate the performance and versatility of the O-SVD framework through its application to high-fidelity simulations from the Square Kilometre Array Science Data Challenge 3a and real-world observational data from the Tianlai Cylinder Pathfinder Array. Our results indicate that O-SVD provides a robust and universal approach for foreground subtraction, achieving high-fidelity signal recovery while offering superior performance compared to conventional matrix-based SVD methods.

This study forecasts the constraints on the properties of primordial black holes (PBHs) as a cold dark matter component using the galaxy clustering, weak lensing, and galaxy-galaxy lensing (i.e. 3×2 pt) measurements from the upcoming Chinese Space Station Survey Telescope (CSST) photometric survey. Since PBHs formed via gravitational collapse in the early Universe, they can additionally affect the formation and evolution of the cosmic large-scale structure (LSS) through “Poisson” effect. We compute the angular power spectra for PBH-ΛCDM cosmology, and generate mock data based on the CSST instrumental and survey design. The Markov Chain Monte Carlo (MCMC) method is employed to constrain the free parameters, such as the product of the PBH fraction and mass f _PBH m _PBH and other cosmological parameters. The systematic parameters are also included in the fitting process, such as the parameters of baryonic effect, intrinsic alignment, galaxy bias, photometric redshift (photo- z ) calibration, shear calibration, and noise terms. We find that the CSST 3×2 pt analysis can achieve tight constraints on f _PBH m _PBH , with 68% and 95% confidence levels (CLs) reaching < 10 ^3.9 M _⊙ and < 10 ^4.7 M _⊙ , respectively. Additionally, the cosmological parameters, e.g. Ω_m, σ_8 and w, can be constrained with the precisions of 3.3%, 1.7%, 13%, respectively. This indicates that the CSST 3×2 pt analysis is a powerful tool to advance the PBH dark matter studies in the near future.

Polarization measurements of fast radio bursts (FRBs) probe the magnetized plasma surrounding their central engines. FRB 20240114A is an exceptionally active repeating source, with 17,356 bursts detected between 2024 January 28 and 2025 May 30 by FAST, enabling studies of the temporal evolution of its polarization properties. In this work, we present a polarimetric catalog of 6131 bright bursts (with a signal-to-noise ratio (S/N) ≥ 20, 35.3% of the total sample), including arrival time (MJD _topo ), dispersion measure (DM), burst width ( W _eff ), bandwidth, Faraday rotation measure (RM), linear and circular polarization degrees ( L / I , V / I ), and intrinsic polarization angle (PA ₀ ). We confirm a clear temporal evolution of RM: after an initial stable phase, it decreases linearly by ∼200 rad m ⁻² over 200 days, forming a bimodal distribution, whereas DM remains stable at 529.3 ± 1.2 pc cm ⁻³ . The linear polarization fraction is generally high, with the 3 σ lower bound around 76%, while circular polarization is low, with 1157 of 17,356 bursts (6.67%) having ∣ V ∣/ I ≥ 10%. We perform a power-law fit between ∣ V ∣/ I and ∣RM∣, which yields an index of −2.98 ± 0.80. It is found that the combined 2D distribution of L / I versus V / I remains stable, implying that the emission mechanism is largely invariant. Our PA ₀ measurements show a broad, nonuniform distribution, implying a complex emission geometry. These results suggest that FRB 20240114A resides in a dynamically evolving magneto-ionic environment. This catalog provides a foundation for studies of repeating FRB progenitors and their environments.