To investigate whether apparently nonrepeating fast radio bursts (FRBs) are truly one-off transients, we conducted systematic follow-up observations of 27 out of 81 nonrepeating FRBs identified in the Parkes Transient Database. Using 59.0 hr of data from the Parkes Ultra-Wideband Low receiver and 6.3 hr from the Five-hundred-meter Aperture Spherical Telescope 19-beam receiver, we searched for repeated bursts from these sources. No additional bursts were detected from any of the 27 FRBs. Combining these nondetections with prior archival observations, we derived stringent upper limits on their repetition rates above 1 Jy under two statistical models: Poisson process constraints range from ∼10 ^−3.5 to 10 ^−1.9 hr ^−1 , while Weibull process constraints range from ∼10 ^−3.4 to 10 ^−1.5 hr ^−1 . These limits are approximately an order of magnitude stricter than those reported in previous studies. By applying consistent observational setups and analytical methodologies across all sources, the derived rate limits converge to a narrow, well-defined range. This suggests that these FRBs form a relatively homogeneous population with extremely low intrinsic activity rates.

Fast radio bursts (FRBs) and gamma-ray bursts (GRBs) are both linked to compact-object activity, yet their possible connection remains unclear. Here we perform a systematic search for spatial and temporal associations between FRBs in the second CHIME/FRB catalog and Swift GRBs. Instead of using the positional ellipses reported in the catalog, the full CHIME localization probability maps are adopted for spatial cross-matching. This yields 130 candidate pairs and increases the number of spatially consistent matches by a factor of several. A redshift consistency requirement reduces the sample to 45 pairs. Applying an additional temporal criterion, requiring long GRBs to precede FRBs and short GRBs to follow them, further reduces the sample to 26 candidates. Monte Carlo simulations show that the overall excess of associations is not statistically significant, and the distribution of matches across localization confidence levels is consistent with random expectations. However, potential associations may be diluted by localization uncertainties and a dominant background of chance coincidences. These results place constraints on any FRB-GRB connection and highlight the need for improved localization and larger samples.

The gravitational wave (GW) event S250206dm, as the first well-localized neutron star (NS) merger candidate potentially located in the mass gap, presented a unique opportunity to probe the electromagnetic signatures from such a system. Here we report a deep, multiband search with the new 2.5 m Wide Field Survey Telescope (WFST), covering similar to 64% of the localization region up to a 5 sigma limiting magnitude of 23 mag. In total, 12 potential candidates have been identified, but none of them are likely related to S250206dm. This nondetection provides the most stringent constraint to date on any associated kilonova. Crucially, an AT 2017gfo-like event at 269 Mpc can be excluded only by WFST observations. Based on ejecta mass limits, a NS-black hole with a large mass ratio (Q greater than or similar to 3.2) is disfavored. This optically derived constraint on the mass ratio reaches, for the first time, a precision comparable to that inferred from the GW signal. This work presents the best observation of this type of event until now, and demonstrates the power of rapid, deep follow-up observations to constrain the properties of compact binary progenitors, offering key insights into the constituents of the mass gap.

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.

We present constraints on the Hubble constant (H_0) derived from the observed dispersion measure (DM) distribution of unlocalized fast radio bursts (FRBs). While localized FRBs with redshift measurements have been used to investigate the Hubble tension, their sample remains limited. Here we demonstrate that unlocalized FRBs – which are far more numerous – can independently constrain H_0 without requiring redshift information, as cosmic expansion imprints itself on their DM distribution. Analyzing a selected sample of 2124 unlocalized FRBs from the CHIME Catalog II, we obtain H_0 = 73.8^+14.0_-12.3 km s^-1 Mpc^-1 at the 1σ confidence level, corresponding to an uncertainty of about 18

The rotation measure (RM) and dispersion measure (DM) of fast radio bursts (FRBs) serve as critical probes of the magnetoionic environments along the line of sight. The significant temporal evolution of RM observed in some repeating FRBs is generally attributed to the local environment of the source, since the intergalactic medium is not expected to vary on such short timescales. Recent observations of repeating FRB 20201124A and FRB 20220529 exhibit complex RM phenomenology, including large-amplitude global fluctuations and short-term substructures. Here, we attribute these short-term RM variations to the ponderomotive force exerted by inertial Alfv & eacute;n waves (IAWs). We propose that IAWs, generated via magnetic reconnection or turbulent cascades in a low-beta plasma, induce nonlinear density perturbations in the source environment. We demonstrate that the resulting plasma density redistribution can produce RM suppression consistent with observed substructures. This model presents a physically motivated mechanism for the short-term RM variability observed in active repeaters. It demonstrates that such fluctuations can arise from wave-driven density cavitation within a broad, coupled parameter space involving wave amplitude, plasma density, and temperature, thereby characterizing the localized plasma dynamics required to produce the observed RM jitters.

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 (MJDtopo), dispersion measure (DM), burst width (Weff), bandwidth, Faraday rotation measure (RM), linear and circular polarization degrees (L/I, V/I), and intrinsic polarization angle (PA0). We confirm a clear temporal evolution of RM: after an initial stable phase, it decreases linearly by similar to 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 sigma lower bound around 76%, while circular polarization is low, with 1157 of 17,356 bursts (6.67%) having divided by V divided by/I >= 10%. We perform a power-law fit between divided by V divided by/I and divided by RM divided by, 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 PA0 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.

High-redshift gamma-ray bursts (GRBs), putative counterparts of massive, low-metallicity population III (Pop III) stars, are a promising probe of the first stars. We assess the detectability of these Pop III GRBs using a metallicity-based progenitor criterion and cosmological N -body/hydrodynamical simulations with three distinct Pop III initial mass functions (IMFs), focusing on the capabilities of the Wide-field X-ray Telescope (WXT) aboard the Einstein Probe (EP) and the coded-mask gamma-ray imager (ECLAIRs) aboard the Space-based multiband astronomical Variable Objects Monitor (SVOM). Our population synthesis model, calibrated to Swift data, predicts the following population II/I (Pop II/I) GRB detection rates at z > 6: ∼2.4 events yr ^−1 for EP/WXT and ∼0.9 events yr ^−1 for SVOM/ECLAIRs. For the IMF with very massive first stars (100 − 500 M _⊙ ), we derive upper limits on the Pop III GRB rate at z > 6 of <0.06 events yr ^−1 (EP/WXT) and <0.13 events yr ^−1 (SVOM/ECLAIRs), based on the absence of confirmed Pop III progenitors in Swift bursts at z > 5.5. Our results indicate that while Pop III GRBs are subdominant to Pop II/I GRBs at z < 10, their fractional contribution rises significantly with redshift, reaching ∼8% (∼34%) at z > 10 and ∼28% (∼68%) at z > 16 for EP/WXT (SVOM/ECLAIRs). This trend is systematically enhanced in the other two IMF models, which adopt a lower stellar mass range of [0.1, 100] M _⊙ . We conclude that detecting Pop III GRBs at high redshifts is a realistic prospect, and any GRB detected at z > 16 is most likely of Pop III origin.

We regret that this statement “This work was supported by China’s Space Origins Exploration Program.” was omitted in the beginning of the Acknowledgements.

We present multi-wavelength study of the γ/X-ray transient EP250416a (also designated GRB 250416C), triggered by the Einstein Probe (EP) Wide-field X-ray Telescope and also by SVOM and Konus-Wind. Observations spanning the gamma-ray, X-ray, and optical bands facilitated detailed analysis of the burst's prompt emission, afterglow evolution, and physical origin. EP250416a exhibits a burst duration of 30 s in X-ray and 17.7 s in gamma-rays, with joint spectral fitting of 0.5-5000 keV data gives E_peak=342_-232^+90 keV. Optical spectroscopy of the afterglow, acquired with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South, yielded a redshift of z=0.963. Accounting for the measured redshift, the isotropic energies are E_X,iso=2.7_-0.5^+0.9×10^50 erg and E_γ,iso=7.34_-2.1^+5.1×10^51 erg, aligning with the Amati relation for long GRBs. The fluence ratio S(25-50 keV)/S(50-100 keV)=0.78_-0.15^+0.1 classifies EP250416a as an X-ray rich (XRR) GRB. The X-ray afterglow shows an initial shallow decay (α≈ -0.5) transitioning to a canonical decay phase (α≈ -1), with a very late jet break at t∼ 1.5× 10^6 s, corresponding to a jet half-opening angle of θ_j=10.6_-1.8^+1.9 degrees. EP250416a is optically dark, as it shows only a faint r-band detection (r=24.16 mag) from Gemini South-GMOS and a low optical-to-X-ray spectral index β_ OX = 0.3. This may be attributed to significant host-galaxy extinction, with a required A_V^host=5.5 mag derived from the extinction curve model.

We propose a data-driven approach to reconstruct the all-sky distribution of the dispersion measure contribution from the Galactic halo (DM _halo ) through a spherical harmonic expansion, enabling an investigation of its possible anisotropies. Based on the NE2001 model and using 92 localized and 574 unlocalized nonrepeating fast radio bursts (FRBs) at Galactic latitudes ∣ b ∣ > 15°, we find a significant dipole anisotropy in DM _halo , pointing toward ( l = 130°, b = +5°) with a 1 σ uncertainty of approximately 28°. The DM _halo value in this direction is 63 ± 9 pc cm ^−3 , exceeding the all-sky mean by about 2.6 σ . This result is not significantly affected by the choice of Galactic interstellar medium models. Furthermore, even when using a refined sample of 62 localized FRBs (excluding CHIME detections, repeaters, and unlocalized events), the dipole anisotropic structure persists, with a direction of ( l = 141°, b = +51°) and a larger 1 σ uncertainty of ∼44°. Model comparisons using the Akaike information criterion and Bayesian evidence yield consistent preferences, and together they suggest that current FRB data slightly favor the existence of a dipole structure in DM _halo . If this feature is not a statistical fluctuation or systematic error, its physical origin requires further investigation. Future FRB samples with larger sizes and more complete sky coverage will be essential to confirm or refute this possible anisotropic structure.

Approximately 20%–50% of the gamma-ray burst (GRB) X-ray afterglows exhibit shallow decay features. Two popular energy-injection models have been proposed to interpret such observational phenomena, the relativistic wind bubble (RWB) model with a Poynting-flux injection and the structured ejecta (SE) model with a dynamical energy injection. Polarization predictions of the two models have been investigated and can be used as a test of the two models. However, the impacts of the parameters on the model predictions were not studied, and the comparisons with the detection ability of the forthcoming mission, enhanced X-ray Timing and Polarimetry (eXTP), has not been discussed. We considered the above issues and found that influences of the model parameters on the predicted polarizations of the two models are very limited. To perform a feasible polarization detection during the plateau phase, a priority target-of-opportunity (ToO) response is required. The detection probability of the GRB plateau phase is about 1/3 for one pointing under a priority ToO. The polarization detection probability would depend on the ratio between the Poynting-flux injection to the dynamical energy injection, which is unclear currently. The predicted flux density and polarization degree (PD) of the RWB model could be well above the threshold flux and minimal detectable PD of the Polarimetry Focusing Array (PFA) on board eXTP, while the predicted PDs of the SE model would be difficult to be detected by eXTP/PFA. Therefore, a detection of a significant polarization signal during the GRB plateau phase would prefer the RWB model, and the injected energy would be in the form of the Poynting flux, while a no detection of the polarized signal would indicate a dynamical energy injection of the SE model.

We present the discovery of EP250827b/SN 2025wkm, an X-ray Flash (XRF) discovered by the Einstein Probe (EP), accompanied by a broad-line Type Ic supernova (SN Ic-BL) at z = 0.1194. EP250827b possesses a prompt X-ray luminosity of ∼ 10^45 erg s^-1, lasts over 1000 seconds, and has a peak energy E_p < 1.5 keV at 90% confidence. SN 2025wkm possesses a double-peaked optical light curve (LC), though its bolometric luminosity plateaus after its initial peak for ∼ 20 days, consistent with a central engine injecting additional energy into the explosion. Its spectrum transitions from a blue to red continuum with clear blueshifted broad absorption features consistent with a SN Ic-BL classification. We do not detect any transient radio emission and rule out the existence of an on-axis, energetic jet ≳ 10^50erg assuming a typical LGRB circumburst constant density (n ≈ 10^-3–10^-1 cm^-3) and microphysical parameters (ε_ e = 0.1 and ε_ B = 0.01). In the model we invoke, the collapse gives rise to a long-lived magnetar, potentially surrounded by an accretion disk. Magnetically–driven winds from the magnetar and the disk mix together and break out with a velocity ∼ 0.35c and interact with an extended circumstellar medium with radius ∼ 10^13 cm, generating X-ray breakout emission through non-thermal free-free processes. The disk outflows and magnetar winds power blackbody photospheric emission as they cool adiabatically and thermalize, producing the first SN peak. The spin-down luminosity of the magnetar and radioactive decay of ^56Ni powers the late-time emission. We end by discussing the landscape of XRF-SNe within the context of EP's recent discoveries.

Extragalactic Fast X-ray Transients (EFXTs) represent an emerging class of high-energy phenomena characterized by X-ray outbursts lasting from tens to hundreds of seconds. However, for more than half of the EFXTs, their physical origins remain elusive. In this Letter, we report the discovery of EP250302a, a luminous EFXT detected by the Einstein Probe (EP) at a redshift of z = 1.131. The multi-wavelength light curves of EP250302a reveal remarkable temporal features that distinguish it from the previously known EP-detected EFXT population, most notably a needle-like X-ray flare accompanied by smooth optical rebrightening during the afterglow phase. We suggest that the distinct X-ray and optical behaviors constitute the first observed instance of late-time violent collision of two relativistic shells in an EFXT. Drawing on insights from GRB studies, such a collision process strongly indicates the reactivation of a central engine, making EP250302a-like transients a unique laboratory for probing the late-time activity and jet physics of EFXT central engines.

Fast blue optical transients (FBOTs) represent one of the most exotic astrophysical transients, exhibiting unusually strong emission across X-ray, optical, and radio wavelengths. Their physical origins remain highly debated, with proposed explanations ranging from stellar explosion to tidal disruption event (TDE). Here we report observations of the most luminous FBOT, AT 2024wpp whose post-peak luminosity rebrightens in X ray and becomes flattening in optical in a manner follows the decay rate characteristic of TDEs (L_ bol∝ t^-5/3). This invokes energy contribution of accretion by a central compact object, getting further corroborations from hardening of X-ray spectral index and detection of outflow inferred from the emission lines at similar phase. Detailed modeling of luminsoity evolution favors a coalesce explosion of a 34 M_⊙ Wolf-Rayet star with a 15 M_⊙ black hole (BH), demonstrating that some FBOTs may be associated with TDE of a stellar blackhole.

Stars getting close enough to black holes (BHs) can be torn apart by strong tidal forces, producing electromagnetic flares. To date, more than 100 tidal disruption events (TDEs) have been observed, each involving invariably normal gaseous stars whose debris falls onto the BH, sustaining the flares over years. White dwarfs (WDs), which are the most prevalent compact stars and a million times denser-and therefore tougher-than gaseous stars, can only be disrupted by intermediate-mass black holes (IMBHs) of 102-105 solar masses. WD-TDEs are considered to generate more powerful and short-lived flares, but their evidence has been lacking. Here we report observations of a fast and luminous X-ray transient EP250702a detected by Einstein Probe. Its one-day-long X-ray peak as luminous as 1047-49ergs-1showed strong recurrent flares with hard spectra extending to several tens of MeV gamma-rays, as detected by Fermi/GBM and Konus-Wind, indicating relativistic jet emission. The jet's X-rays dropped sharply from 3×1049ergs-1 to around 1044ergs-1within 20 days (10 days in the source rest frame). These characteristics are inconsistent with any previously known transient phenomena. We suggest that this fast-evolving event over the unprecedentedly short timescale arises likely from disruption of a WD by an IMBH. At late times, a soft component progressively dominates the X-ray spectrum, reaching a luminosity as high as 1044 erg s-1, which is consistent with being extreme super-Eddington emission from an accretion disk expected to form in an IMBH-WD TDE. WD-TDEs open a new window for investigating the elusive IMBHs and their surrounding stellar environments, and they are prime sources of gravitational waves in the band of space-based interferometers.

Observations of fast X-ray transients (FXRTs) with the Einstein Probe have successfully led to the discovery of some unusual extragalactic optical transients. EP241021a is a newly discovered FXRT that was featured by a significant bump at around 10 days in both optical and X-ray bands. This timescale and the exceptionally high peak bolometric luminosity up to ∼10 ^44 erg s ^−1 of the optical bump make it somewhat similar to fast blue optical transients but still distinctive from them by its relatively red color. We then suggest that the multiwavelength bump of EP241021a could represent an explosion-type transient, while the underlying power-law decaying component of the optical and X-ray emission as well as the total radio emission are produced by a moderately relativistic jet. By fitting the observed multiwavelength light curves, it is found that the explosion ejecta that produce the thermal optical emission can have a mass of ∼0.03 M _⊙ , an expanding velocity of ∼0.25 c , and an optical opacity of ∼12 cm ^2 g ^−1 , which was continuously powered by a rapidly rotating and highly magnetized neutron star (NS; i.e., a magnetar). In addition to heating the explosion ejecta, the magnetar also provided the dominant contribution to the observed X-ray rebrightening through the nonthermal emission of its wind. These properties suggest that the explosion may result from a catastrophic collapse/merger of a compact star system, which led to the formation of a millisecond magnetar, and the possible progenitor could be an accreting white dwarf (WD) or a binary consisting of a double WD, a double NS, or a WD and an NS.

Gamma-ray bursts (GRBs) are a promising probe of the high-redshift Universe, but their detection remains observationally challenging. In this work, we explore the detectability of high- z GRBs by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) and the coded-mask gamma-ray imager (ECLAIRs) on board the Space-based multi-band astronomical Variable Objects Monitor (SVOM). Using a population synthesis model calibrated to Swift GRB observations, we develop a tool to estimate high- z GRB detection rates for instruments with specific energy bands and sensitivities. Our results indicate that EP/WXT could detect ∼ 5 . 1 − 2.4 + 3.4 (with 68% confidence level) GRBs annually at z > 6, compared to ∼ 0 . 7 − 0.4 + 1.0 events per year at z > 6 for SVOM/ECLAIRs. While EP cannot independently determine redshifts (requiring optical/near-infrared, NIR, follow-up), its assumed ∼30% follow-up efficiency yields ∼ 1 . 5 − 0.7 + 1.0 confirmed z > 6 GRBs annually. SVOM, equipped with dedicated follow-up telescopes, will promptly identify high- z candidates deserving deep NIR spectroscopy to ensure robust confirmation of high- z GRBs. We anticipate that EP and SVOM will open new avenues for utilizing enlarged samples of high- z GRBs to explore the early Universe. Moreover, EP will assemble a substantial sample of soft, low-luminosity GRBs at low-to-intermediate redshifts, providing critical insights into the structure of GRB jets.

The physical origin of fast radio bursts (FRBs) remains uncertain. Although multiwavelength observations have been widely conducted, only Galactic FRB 20200428D is associated with an X-ray burst from the magnetar SGR J1935+2154. Here we present multiwavelength follow-up observations of the nearby bright FRB 20250316A, including the Five-hundred-meter Aperture Spherical radio Telescope (FAST), Einstein Probe (EP) X-ray mission, Chandra X-ray Observatory, Wide Field Survey Telescope (WFST), and Space Variable Objects Monitor/Visible Telescope (SVOM/VT). The 13.08 hr FAST follow-up campaign without pulse detection requires an energy distribution flatter than those of well-known repeating FRBs, suggesting that this burst is likely a one-off event. A prompt EP follow-up and multiepoch observational campaign totaling >100 ks led to the detection of an X-ray source within the angular resolution of its Follow-up X-ray Telescope (FXT; 10″). A subsequent Chandra observation revealed this source to be offset by 7″ from the FRB position and established a 0.5–10 keV flux upper limit of 7.6 × 10 ^−15 erg cm ^−2 s ^−1 at the FRB position, corresponding to ∼10 ^39 erg s ^−1 at the 40 Mpc distance of the host galaxy NGC 4141. These results set one of the most stringent limits on X-ray emission from a nonrepeating FRB, disfavoring ultraluminous X-ray sources as counterparts of apparently one-off FRBs and offering critical insights into afterglow models. Our study suggests that an arcsecond localization of both the FRB and its potential X-ray counterpart is essential for exploring the X-ray counterpart of an FRB.

Fast radio bursts (FRBs) are mysterious millisecond-duration radio transients of extragalactic origin. Some of them repeat, while others apparently do not. Investigations of periodic activity in repeating FRB have been conducted to probe their origins. While periodicity in the burst rate has been reported, studies of periodicities in other properties, such as dispersion measure (DM) and rotation measure (RM), are sparse. FRB 20220529 was monitored by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) for nearly three years, providing an opportunity to investigate periodicity in its observed properties. Here we report a possible period of ∼ 200 days in the RM evolution, with a significance of 4.1 σ estimated via the Lomb-Scargle algorithm and 3.1 σ with the phase-folding method. Periodicity in the burst rate was also investigated. It may indicate that the FRB progenitor is in a binary system, which is consistent with the significant RM increase and prompt recovery of this FRB on a week-timescale. Other scenarios, such as a system with an intermediate-mass black hole, are also explored.