He Gao

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.

Intermediate-mass black holes (IMBHs) are the missing link in the cosmic hierarchy of black holes, bridging the gap between stellar-mass black holes and supermassive ones. They also serve as unique laboratories for testing strong-field gravity and are prime targets for future multimessenger observations. However, IMBHs are a population that has remained notoriously difficult to detect. The microlensing effect of fast radio bursts (FRBs) can serve as a clean and powerful method to probe IMBHs. In this work, we develop a pipeline to search for microlensed FRBs based on their dynamic spectra and apply it to the CHIME/FRB Catalog 2. Two microlensing signatures have been identified in two separate sources, i.e., FRB 20190131D and FRB 20211115A. The inferred lens masses for these two signatures are ∼[280–467] M _⊙ and ∼[539–609] M _⊙ , respectively. Here we interpret them as evidence for IMBHs. If there are no intervening structures—such as galaxies or clusters—along the lines of sight for these two sources, the two identified IMBHs might be isolated and of primordial origin. In that case, we obtain that primordial black holes (PBHs) within these two mass ranges would constitute ∼4% of dark matter. Moreover, if these two candidates are not genuine lensing signatures, the abundance of intermediate-mass PBHs with masses >300 M _⊙ is constrained to be ∼13% at 95% confidence level. Therefore, more comprehensive observational information for FRBs, together with a deeper understanding of whether the intrinsic emission mechanisms of FRBs can produce lensing-like signals, will be crucial for establishing this effect as a powerful tool for probing (primordial) IMBHs.

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.

Gravitational-wave astronomy has revealed that close binaries with compact companions are widespread. Long gamma-ray bursts (LGRBs) from massive star collapse face persistent challenges in achieving the rapid core rotation required by the collapsar model. Binary interaction via tidal spin-up offers a natural solution; recent population synthesis studies suggest a substantial fraction of LGRBs may originate from close binaries with a compact companion. In this scenario, supernova ejecta from the primary can be accreted by the companion, potentially launching a second relativistic jet after a delay set by the binary separation. We develop a comprehensive model for these double-jet systems, analyzing the dynamics of the second jet and its interaction with the first. The resulting observational signatures depend critically on the Lorentz factor ratio, the alignment angle, and the time delay. For aligned jets, two regimes arise: a fast second jet producing multiple gamma-ray triggers with distinct spectral/polarization evolution, and a slow second jet where its emission appears as an X-ray flare followed by an afterglow plateau from energy injection. For misaligned jets, the observed signal ranges from normal gamma-ray bursts (GRBs) with late-time radio structures to fast X-ray transients followed by off-axis rebrightening. These features have observational parallels in existing GRB data. High-resolution radio interferometry with SKA, time-resolved polarimetry with eXTP, and multiwavelength surveys with Einstein Probe and SVOM will test these predictions, providing constraints on the evolution of close massive binaries as progenitors of GRBs and gravitational-wave sources.

Einstein’s weak equivalence principle (WEP), the universality of free fall, is a fundamental component of general relativity and other metric theories of gravity. Its validity can be tested through the post-Newtonian parameter γ , which quantifies the amount of spacetime curvature due to the presence of unit rest mass. In this Letter, we use high-energy neutrino events detected by IceCube and associated with the gamma-ray blazars TXS 0506+056 and PKS 0735+178 to test the WEP via the Shapiro delay induced by the gravitational potential of Laniakea. We find that violation of the equivalence principle for neutrinos and photons is limited to an accuracy of 10 ⁻⁶ , 10 ⁻⁷ , and 10 ⁻⁸ , representing improvements of 1, 2, and 3 orders of magnitude, respectively, over previous constraints obtained from other high-energy neutrino-blazar associations and up to 6 orders of magnitude tighter compared to the constraints obtained with MeV neutrinos from SN 1987A.

This study investigates the morphological classification and drifting rate measurement of the repeating fast radio burst (FRB) source FRB 20240114A using the Five-hundred-meter Aperture Spherical Telescope. Detected on 2024 January 14, FRB 20240114A exhibited an exceptionally high burst rate, revealing unique properties. Through observational campaigns over several months, we selected a data set comprising 3203 bursts (2109 burst clusters) during a continuous monitoring session (15,780 s) on 2024 March 12. Improving upon previous work, we clarify the definitions of sub-bursts, bursts, and burst clusters. Using an average dispersion measures of 529.2 pc cm ⁻³ , we classified the burst clusters into Downward Drifting, Upward Drifting, No Drifting, No Evidence for Drifting, Not-Clear, and Complex burst clusters. Among the 978 burst clusters that exhibit drifting behavior, 233 (23.82%) show upward drifting. Additionally, if 142 upward-drifting single-component burst clusters are excluded, upward-drifting double- and multicomponent burst clusters still account for 10.89% of the 836 burst clusters exhibiting drifting behavior, equating to 91 burst clusters. Furthermore, if only upward-drifting burst clusters with consecutive time intervals (or upward-drifting bursts) are considered, only nine bursts remain. Drifting rate comparisons with other physical quantities reveal that the drifting rate increases with peak frequency for single-component burst clusters with drifting behavior. Moreover, in single-component burst clusters, those with upward drifting exhibit smaller effective widths, bandwidths, and fluxes than their downward-drifting counterparts. A Kolmogorov–Smirnov test further indicates that upward-drifting burst clusters possess longer consecutive time intervals than downward-drifting ones, suggesting distinct underlying physical mechanisms.

This paper presents simulations of the synchrotron self-Compton (SSC) spectrum within the Internal-Collision-induced Magnetic Reconnection and Turbulence (ICMART) model. We investigate how key parameters like the magnetization σ ₀ shape the broadband spectral energy distribution by regulating the electron distribution and magnetic field strength. The overall spectrum typically comprises two components: synchrotron radiation peaking at E _p with a low-energy spectral index α between −1 and −1.5 and an SSC component peaking at E _ssc . At high energies, Klein–Nishina (KN) suppression causes an exponential cutoff. The flux ratio Y between these components is critical: when Y is small, the SSC peak can be suppressed. Spectral features of the synchrotron component reveal the underlying physical conditions: harder spectra with α ∼ −1 indicate a large Y parameter and strong KN suppression. We find a positive correlation between Y and σ ₀ , contrasting with internal shock model predictions. Applied to GRB 221009A, our model suggests σ ₀ ≤ 20 can reproduce the MeV–TeV observations. This study underscores the value of combined MeV–TeV observations in probing gamma-ray burst emission mechanisms.

The recent launches of the Einstein Probe and the Space Variable Objects Monitor mission have led to the detection of a growing number of long gamma-ray bursts (GRBs) with significant, early soft X-ray flux during their gamma-ray emission, prompting the question of whether their multiband prompt emission shares a common origin in region and mechanism. To address this, we utilize the 20-year Swift archival data, which provides a substantial sample of joint soft X-ray and gamma-ray observations, enabling a systematic joint spectral study. We resolve 110 temporal pulses from 46 GRBs and find that a single power-law model with a low-energy break or cutoff adequately describes the prompt spectra from 150 keV down to 0.5 keV. More than half of the sample pulses require a break around a few keV, with average spectral indices 〈 α ₁ 〉 = −0.88 and 〈 α ₂ 〉 = −1.46 consistent with synchrotron radiation in a marginally fast-cooling regime. The observed spectral evolution and the distribution of indices support a single-emission-region origin, where the varying spectral shapes are largely governed by the evolution of the synchrotron cooling frequency ν _c and the effect of finite emission width. The observed differences in the temporal behavior between X-ray and gamma-ray light curves can be naturally explained by this spectral evolution across the broad band.

A newly born millisecond magnetar has been proposed as one possible central engine of some long gamma-ray bursts (LGRBs) with X-ray plateau emission. In this work, we used a universal correlation between the initial spin period ( P ₀ ) and the surface magnetic field ( B _p ) of the newborn magnetar based on an LGRB sample in L. Lan et al. (2025) to explore the propeller properties of accreting magnetars with R / I evolutionary effects. We found that the B _p – P ₀ relation is approximately consistent with B p ∝ P eq 7 / 6 . Here P _eq is the equilibrium spin period in the magnetic propeller model, where the electromagnetic dipole radiation and the propeller mechanism jointly modulate the spin evolution of a newborn magnetar. The B _p – P ₀ relation indicates that P ₀ constrained by X-ray plateau data may not be the true initial spin period of a newborn magnetar but had reached an equilibrium spin period via fallback accretion in the propeller model. The magnetar accretion rate in our LGRBs is in the range of M ̇ ∼ 1 0 − 5 to 1 0 − 2 M ⊙ s − 1 by incorporating R / I evolutionary effects and using the transition relation between gravitational mass M _g and baryonic mass M _b in different equations of state and X-ray radiation efficiencies. Such accretion rates ensure that the accreting magnetars in our sample survive until reaching the equilibrium spin period, and the accretion rate is one order of magnitude lower compared to the statistical results in G. Stratta et al. (2018) and W. L. Lin et al. (2020), which used the constant R / I / M _g scenario. The fallback rate of progenitor envelope materials onto the magnetar accretion disk for our LGRBs is compatible with the theoretical mass fallback rate of some low-metallicity progenitors. We suggested that adopting a constant R / I / M _g scenario for modeling the propeller regime in accreting magnetars results in a higher mass accretion rate, which may impair our understanding of the physical nature of an accreting magnetar and its surroundings, and that the low-metallicity progenitors can provide enough material to satisfy the accretion requirements of the newborn accreting magnetar in LGRBs.