Ultra-compact X-ray binaries (UCXBs) are a subclass of low-mass X-ray binaries (LMXBs) characterized by ultra-short orbital periods, typically less than 60-80min. They consist of a compact mass-accretor and a hydrogen-poor mass-donor, in which the mass-accretor could be a neuron star (NS) or even a black hole (BH). UCXBs play an important role in multiple areas of astrophysics. In particular, they are considered strong, continuous gravitational wave (GW) sources in the low-frequency band, making them key targets for future space-based GW observatories such as LISA, TianQin and Taiji. As the most compact binaries, the formation and evolution of UCXBs remain highly uncertain. In this article, we review four classic formation channels: the white dwarf donor channel, the He star donor channel, the evolved main-sequence donor channel, and the accretion-induced collapse channel. We also discuss recent progress in these channels, covering evolutionary scenarios, the initial parameter space for UCXB formation, and associated objects. A comparison between observed UCXBs and theoretical expectations is provided, along with a discussion on the observed BH-UCXB candidates. The origin of UCXBs can be constrained by the chemical composition of mass-donors and their locations in diagrams of mass-transfer rate and X-ray luminosity versus orbital period. We also examine the implications of UCXBs for several astrophysical fields, including GW astronomy, multi-messenger astronomy, binary evolution, and NS physics under extreme conditions. Further progress will depend on multi-wavelength observations, the discovery of more UCXB samples, and more detailed theoretical simulations.

Based on (10087±44)×10^6 J/ψ events collected with the BESIII detector, the J/ψ^0_SK^0_Sπ^0 and J/ψ^0π^0η processes are studied. The X(2370) is observed in both the K^0_SK^0_Sπ^0 and π^0π^0η invariant mass spectra, with statistical significances greater than 14σ and 20σ, respectively. By combining measurements from these processes with those from the previously reported J/ψ^0_SK^0_Sη^' process, the mass and width of the X(2370) are determined to be 2359^+13_-14 MeV/c^2 and 170^+44_-29 MeV, respectively. In addition, the decay X(2370)→ a_0(980)^0π^0 with a_0(980)^0→ π^0η is observed with a statistical significance exceeding 9σ. The properties of the X(2370)—decay pattern similarities to that of η_c, are consistent with those of a pseudoscalar glueball.

It is generally believed that the electron-capture reactions happen when the oxygen-neon (ONe) cores grow in masses close to the Chandrasekhar limit, leading to the formation of neutron stars (NSs) via electron-capture supernovae (EC-SNe). EC-SNe are predicted to be the most likely short-lived and faint optical transients, and a small ejecta mass is expected during the collapse. This kind of SNe provide a distinct channel for producing isolated NSs and NS systems, especially for the formation of X-ray binaries and double NSs. Although EC-SNe were proposed ∼45 yr ago, there are still some uncertainties for the origin of EC-SNe and their productions. In this article, we review recent studies on the two classic progenitor channels of EC-SNe, i.e., the single star channel and the binary channel. In the single star channel, EC-SNe can happen in super asymptotic giant branch stars or He stars, whereas in the binary channel EC-SNe can occur in He stars in binaries (involving He star+MS systems and NS+He star systems) or accretion-induced collapse in white dwarf binaries (involving the single-degenerate scenario and the double-degenerate scenario). Recent progress on these two progenitor channels is discussed, including the initial parameter range for EC-SNe, the evolutionary paths to EC-SNe, related objects and some observational constraints, etc. We also make some discussions on the possible candidates for EC-SNe in this article, and the impacts of EC-SNe on some research fields, e.g., the properties of NSs, double NS population and chemical products, etc. It is noting that EC-SNe show some similar properties with ultra-stripped SNe, e.g., low ejecta masses and small kicks. Accordingly, we also discuss the difference between these two types of SNe in this article. Research on EC-SNe is at a pivotal stage, with key theoretical uncertainties and observational challenges requiring integrated modeling and multi-wavelength observations for robust identification.

Massive stars die as core-collapse supernovae, whose optical light emerges days after the implosion. Theory predicts that the initial collapse-driven shock, upon breaking through the star and dense circumstellar medium, emits a brief thermal flash of soft X-rays and ultraviolet. Yet these elusive first signals have remained largely undetected, owing to limited wide-field soft X-ray monitoring. Here we report the discovery of a soft X-ray flash, EP260321a, followed days later by a broad-lined supernova from an envelope-stripped progenitor. Its X-ray spectrum, best modeled with blackbody, establishes it as the long-sought archetypal shock breakout. The burst's duration and energetics place the breakout at a radius of 300 solar radii, tracing a dense surrounding shell and revealing abrupt mass ejection within the final month before collapse.

We present optical observations on the young type Ia supernova (SN Ia) SN 2021fxy obtained within a few days after the explosion, with a focus on its prominent high-velocity features (HVFs). It reached a B-band maximum of M_ max(B) = -19.36±0.31 mag, corresponding to a bolometric luminosity of ∼ 1.3×10^43 erg s^-1 with a synthesized ^56Ni mass of 0.58±0.14 M_⊙. The early spectra exhibit strong HVFs of intermediate-mass elements that are significantly detached from the photospheric components. In particular, the velocity of the Si II λ6355 HVFs follows a power-law evolution (β≈ 0.1), shallower than the expected photospheric velocity evolution expected for an assumed n=10 density profile (β≈ 0.22) under homologous expansion. This behavior is consistent with the HVFs forming in intrinsic ejecta structures at least partially decoupled from the bulk outer ejecta, providing a possible constraint on the explosion physics of SNe Ia.

Massive He stars are potential candidates of type Ib/c supernova (SN) progenitors. Understanding their final fates remains a key issue in astrophysics. In this work, we investigate the evolution of He stars with initial masses from 5 M⊙ to 65 M⊙, focusing on the presupernova (pre-SN) core structures to assess their explodability. Our simulations indicate that the final core structure is determined by the CO core mass and the central 12C mass fraction at the end of core He burning, affecting the properties of central C-burning and the locations of convective shells. The location of the last convective C-burning shell sets the mass of the C-free core, constraining the iron core mass and compactness. We found that the final compactness and iron core mass exhibit non-monotonic behavior with initial mass, suggesting that the boundary between neutron star and black hole formation is not a simple mass threshold. This is due to core C/Ne burning becoming neutrino dominated. This process drives stronger core contraction, ultimately increasing the iron core mass and the final compactness. In contrast, earlier core Ne/O/Si ignition and shell mergers inhibit core contraction, reducing both the iron core mass and final compactness. We also discuss the effects of metallicity and overshooting on the pre-SN core structure. These factors potentially affect the explodability of progenitors.

Streak cameras exhibit excellent temporal resolution for ultrafast optical measurements. However, under single-photon-level extremely low-light conditions, their detection performance remains limited by system noise and sensitivity bottlenecks, making it difficult to simultaneously achieve ultrafast and ultra-low-light detection. To address this challenge, this work proposes a streak-camera low-light signal extractor (SC-LLSE) and establishes an adaptive multidimensional denoising and signal reconstruction framework. The proposed method statistically models background noise, incorporates statistical feature representations of multi-frame signal-containing images, and integrates a noise-condition-aware deep learning model to achieve adaptive extraction and enhancement of weak streak signals. Experimental validation is performed using typical fluorescent samples, including Rhodamine B, Rhodamine 6G, and Erythrosin B, under light attenuation conditions ranging from LD15 to LD8. Under a 9 ns time window and a single-frame exposure time of 100 ms, the proposed method is compared with the conventional centroid method. The results show that under the LD15 intensity condition, the proposed SC-LLSE method improves the Signal-to-Noise Ratio (SNR) of Rhodamine B, Rhodamine 6G, and Erythrosin B from 6.7564 dB, 6.1358 dB, and 3.3493 dB to 32.4333 dB, 26.7901 dB, and 27.9296 dB, respectively. Under the extremely low-light LD8 condition, after 10 min of processing (approximately 6000 frames), the fluorescence lifetime measurement error of Rhodamine B decreases from 76.46 % to 12.44 %, while that of Rhodamine 6G decreases from 86.38 % to 22.47 %, with a maximum coefficient of determination (R2) of 0.9941 for fluorescence lifetime fitting. These results demonstrate that the proposed SC-LLSE method can stably extract effective weak signals under extremely low-light conditions and significantly improve the SNR and fluorescence lifetime measurement accuracy of streak cameras.

The hyperon weak radiative decay Ξ0 → γΣ0 is measured using a sample of Ξ0 from J/ψ→Ξ^0Ξ^0 decays. The absolute branching fraction of Ξ0 → γΣ0 is determined to be (3.66 ± 0.21stat ± 0.10syst) × 10−3, based on (10.087 ± 0.044) × 109 J/ψ events collected with the BESIII detector operating at the BEPCII collider. The decay asymmetry parameter is measured, with a complete angular analysis of the cascade decay chain, to be αγ = −0.807 ± 0.095stat ± 0.011syst.

By analysing 6.1 fb−1 of data collected at centre-of-mass energies between √(s)=4.600 and 4.843 GeV with the BESIII detector at the BEPCII collider, we observe the decay Λ_c^+→ nπ^+η for the first time with a statistical significance of 9.5σ. The ratio of branching fractions ℬ(Λ_c^+→ nπ^+η)/ℬ(Λ_c^+→Λπ^+η) is measured to be 0.155 ± 0.031stat. ± 0.012syst. Taking the world average of ℬ(Λ_c^+→Λπ^+η) as reference, the absolute branching fraction is calculated to be ℬ(Λ_c^+→ nπ^+η)=(2.94±0.59_stat.±0.23_syst.±0.13_ref.)×10^-3 . The intermediate process Λ_c^+→ na_0(980)^+ is also searched for in the π+η invariant mass spectrum. Since no significant signal is found, the upper limit on ℬ(Λ_c^+→ na_0(980)^+)×ℬ(a_0(980)^+→π^+η) is set to 8.4 × 10−4 at 90

Context. Recently, a new class of supernovae with prominent narrow emission features of Si, S and Ar has been reported, i.e. SN 2021yfj-like events (SNe Ien). Their progenitor origin is still unknown. It has been suggested that a massive CO WD may evolve into a Si/S-rich WD when accreting He-rich matter at a high mass-transfer rate. If the He companion subsequently evolves into another more massive WD, the merger of this double WD system can generate Si/S-rich CSM through tidal stripping. Following the SN explosion, the interaction between the SN ejecta and the Si/S-rich CSM could produce the Si, S, and Ar emission lines characteristic of SN 2021yfj-like events. Aims. In this work, we aim to determine the initial parameter space of WD+He star systems that can lead to SN 2021yfj-like events via the double WD merger scenario, and to quantify their theoretical Galactic birthrate and delay-time distributions. Methods. We perform detailed binary evolution simulations of a large number of semidetached WD+He star systems to obtain the parameter space that leads to the formation of Si/S-rich WDs and subsequent double WD mergers. We then use binary population synthesis to calculate the Galactic birthrate and delay-time distribution of such events. Results. For the formation of SN 2021yfj-like events, we found that the initial CO WD and He companion masses must lie in the ranges of 1.0-1.2 M_sun and 2.2-2.5 M_sun, respectively. The derived merger rate for this scenario is (2.6-4.4)x10^-5 yr^-1 with delay times longer than 450 Myr, which is about 1

We present that supernovae interacting with a dense Si-and S-rich circumstellar matter like SN 2021yfj can originate from mergers of two white dwarfs. A C + O white dwarf accreting He from its non-degenerate He companion star can initiate a C-burning frame at its surface propagating inward under certain conditions. Such a burning frame synthesizes intermediate-mass elements such as Si and S, forming a hybrid white dwarf with an outer Si + S-rich layer. After the He star companion becomes a white dwarf, the two white dwarfs can eventually merge. During the merger, the outer layers of the hybrid white dwarf can be tidally stripped, forming a dense Si-and S-rich circumstellar matter. If a thermonuclear explosion is triggered in the merging white dwarfs, an explosion within a dense Si-and S-rich circumstellar matter can be realized, resulting in SN 2021yfj-like events. We argue that the properties of SN 2021yfj can be reproduced by a dense Si-and S-rich circumstellar matter having similar or equal to 0.3 M-circle dot within which an explosion with kinetic energy of similar or equal to 4 & times; 10(50) erg and ejecta mass of similar or equal to 0.3 M-circle dot occurred. These properties are consistent with the double white dwarf merger scenario. This scenario can naturally explain the existence of He observed in SN 2021yfj. Because white dwarf mergers can also lead to the formation of He and C + O dense circumstellar matter, some Type Ibn and Icn supernovae may also originate from a similar evolutionary path.

We present optical photometric and spectroscopic observations of the low-luminosity (LL) Type IIP supernova SN 2024abfl. The distance to its host galaxy is highly uncertain, with independent estimates of 9.5-2.4+2.3 Mpc and 15.0-1.9+8.9 Mpc. Even adopting the larger distance, the inferred plateau luminosity is only similar to 1041 erg s-1, placing SN 2024abfl at the extreme faint end of the supernovae (SNe) IIP population. Its light curve exhibits a long-lasting plateau of approximately 110 days. The spectra show exceptionally low expansion velocities, with a Fe ii velocity of similar to 1200 km s-1 at 50 days after the explosion, significantly lower than the typical values of similar to 2000-5500 km s-1 observed in SNe IIP, placing SN 2024abfl among the slowest-expanding LL SNe IIP. Bolometric modeling yields a synthesized 56Ni mass of similar to 0.002-0.004 M circle dot, though this estimate remains subject to significant uncertainty owing to the poorly constrained distance. Considering the plateau color and duration, the magnitude drop from plateau to tail, and the progenitor luminosity, we favor a low-mass core-collapse origin for SN 2024abfl.

Using ( 10087 ± 44 ) × 10 6 J / ψ events collected with the BESIII detector at the BEPCII collider, we report the first observation of the decay η c → Ξ 0 Ξ ¯ 0 . The interference between J / ψ → γ η c → γ Ξ 0 Ξ ¯ 0 and J / ψ → γ Ξ 0 Ξ ¯ 0 | non − resonance is considered in the Ξ 0 Ξ ¯ 0 mass spectrum fits. The branching fractions are measured to be B ( η c → Ξ 0 Ξ ¯ 0 ) = ( 1.33 ± 0.03 ± 0.18 ) × 10 − 3 for the constructive interference and B ( η c → Ξ 0 Ξ ¯ 0 ) = ( 1.63 ± 0.04 ± 0.21 ) × 10 − 3 for the destructive interference, where the first uncertainties are statistical and the second are systematic.

The Chinese Space Station Survey Telescope (CSST) is an upcoming Stage-IV sky survey telescope, distinguished by its large field of view (FoV), high image quality, and multi-band observation capabilities. It can simultaneously conduct precise measurements of the Universe by performing multi-color photometric imaging and slitless spectroscopic surveys. The CSST is equipped with five scientific instruments, i.e., Multi-band Imaging and Slitless Spectroscopy Survey Camera (SC), Multi-Channel Imager (MCI), Integral Field Spectrograph (IFS), Cool Planet Imaging Coronagraph (CPI-C), and THz Spectrometer (TS). Using these instruments, CSST is expected to make significant contributions and discoveries across various astronomical fields, including cosmology, galaxies and active galactic nuclei (AGN), the Milky Way and nearby galaxies, stars, exoplanets, Solar System objects, astrometry, and transients and variable sources. This review aims to provide a comprehensive overview of the CSST instruments, observational capabilities, data products, and scientific potential.

We present a photometric and spectroscopic study of the superluminous Type Ia supernova SN 2022erq. Its early spectra, dominated by iron-group elements with weak intermediate-mass features, might indicate highly efficient nuclear burning, broadly similar to that inferred for some overluminous SNe Ia. The rapid emergence and persistence of narrow Balmer emission lines superposed on this iron-rich spectrum provide clear evidence of long-lived interaction with a hydrogen-rich circumstellar medium (CSM), establishing SN 2022erq as a member of the rare Ia-CSM class. SN 2022erq reached a peak bolometric luminosity of about 8 x 10^43 erg/s and exhibited an exceptionally slow post-peak decline, indicating that its light curve is dominated by long-duration ejecta-CSM interaction. By combining H-alpha diagnostics with bolometric light-curve modeling, we reconstruct the pre-explosion mass-loss history of the progenitor. The mass-loss rate escalated by one order of magnitude over the final decades, rising from about 0.04 to about 0.6 solar masses per year. This surge produced a massive, extended CSM shell of about 3 solar masses out to about 3.5 x 10^16 cm. The young stellar environment (about 100 Myr) together with this substantial, extensive CSM points to a progenitor system consisting of a white dwarf and an intermediate-mass companion that underwent increasing mass loss prior to explosion.

Common envelope evolution is a critical but still poorly understood phase in binary evolution. It plays a key role in forming close binaries such as hot subdwarfs, double white dwarfs (WDs), X-ray binaries, and double neutron stars. However, its outcomes remain highly uncertain. Depending on the efficiency of envelope ejection, a system may either survive as a close binary or undergo a complete merger. In this work, we investigate the postmerger evolution of systems where a CO WD merges with the core of an asymptotic giant branch (AGB) star. A grid of merger remnant models with various core and envelope masses is constructed. At the onset of evolution, the CO core contracts and undergoes off-center carbon ignition, producing an inwardly propagating carbon flame. For remnants with a relatively low mass of CO core, the flame phase is followed by core contraction and subsequent H shell burning. For more massive CO cores, the carbon flame reaches the center and is soon followed by off-center neon burning, which is expected to eventually lead to core-collapse supernovae. The merger remnants occupy nearly the same region on the H-R diagram as ordinary AGB or super-AGB stars, exhibiting similar surface properties. Although their surface abundance may differ slightly from those of normal AGB stars, depending on the initial core and envelope masses, these differences are strongly reduced once mass loss is taken into account. We suggest that some giant-like stars, including candidates for Thorne-& Zdot;ytkow objects (e.g., HV 2112), might alternatively be explained as AGB-WD merger remnants.

Abstract A search for a massless beyond-standard-model particle is performed in the decay Ξ0 → Λ + invisible using (1.0087 ± 0.0044) × 1010 J/ψ events collected with the BESIII detector at the BEPCII collider. No significant signal is observed and the upper limit on the branching fraction ℬ(Ξ0 → Λ + invisible) is set to be 2.3 × 10 −4 at the 90% confidence level. This is the first search for a flavor-changing neutral current process with missing energy in Ξ0 decays. Throughout this paper, charge-conjugate processes are always implied.

Based on 8.5 fb^-1 of e^+e^- collision data collected at center-of-mass energies between 4.42 and 4.95 GeV with the BESIII detector at the BEPCII storage ring, we investigate the process e^+e^-→ π^+π^-D_s^+D_s^-. With no significant signal observed, upper limits on the Born cross sections of e^+e^-→ π^+π^-D_s^+D_s^- at each energy value are determined at the 90

This work presents a laboratory prototype of an Electron-Bombarded Active Pixel Sensor (EBAPS) to investigate the effects of passivation film thickness on electron energy loss and bombardment gain. Through combined experimental characterization and numerical simulations, we systematically examine the correlations among accelerating voltage, passivation layer thickness, electron gain, and dead-layer energy dissipation. Experimental results show that the fabricated EBAPS achieves a spatial resolution of 18 lp/mm at an accelerating voltage of 8000 V. Reducing the passivation layer thickness from 70 nm to 30 nm decreases dead-layer energy loss from 2000 eV to 1000 eV. An optimized Monte Carlo model is developed to simulate electron penetration behaviors under different thicknesses and voltages, and its predictions are in good agreement with experimental data. This study confirms that thinning the passivation layer effectively lowers the required bombardment voltage and improves the long-term operational reliability of EBAPS devices. These findings offer both experimental evidence and theoretical guidance for substrate thinning and surface modification strategies in EBAPS development.

Using 7.33 fb−1 of e+e− collision data samples collected with the BESIII detector at center-of-mass energies between 4.128 and 4.226 GeV, we perform a search for the radiative decay D_s^+→γK^∗(892)^+ via K*(892)+ → K+π0 and K^∗(892)^+→K_S^0π^+ for the first time. No significant signals are observed. The upper limit on the branching fraction of D_s^+→γK^∗(892)^+ is set to be 2.3 × 10−4 at the 90