The Penning effect plays a crucial role in gas detectors by significantly enhancing ionization efficiency through optimized gas mixtures. Although this effect has been extensively studied in conventional detector configurations, its systematic characterization in Thick Gas Electron Multipliers (THGEMs) remains limited. In this work, we performed concentration-scan measurements on six Penning mixtures based on Ar and Kr (Ar/CO2, Ar/Xe, Ar/DME, Kr/DME, Kr/C3H6, and Kr/iC4H8) using a single-THGEM detector at pressures of 0.6 and 1.0 bar. The effective gain and energy resolution were systematically investigated as functions of the admixture gas concentration. The results show that, with the exception of Ar/CO2, all five remaining mixtures exhibit significant Penning effects. From the analysis of the gain curves, the admixture concentrations corresponding to the minimum THGEM operating voltage were determined for each gas mixture, ranging from 0.1% to 2%. Among them, the optimal concentration of C3H6 in the Kr/C3H6 mixture is about 0.3%, consistent with previous measurements. The degree of gain enhancement due to Penning ionization in these mixtures depends specifically on the energy matching between the first ionization energy of the admixture gas and the excitation energy levels of the noble gas molecules. Based on the monitoring of discharge frequencies near the optimal concentrations for each mixture, the effective gains achievable at the same discharge probability can be quantitatively compared. Compared with pure Ar, the effective gain of Ar/Xe is enhanced by more than a factor of 20 at the same discharge probability, while Kr/DME shows an enhancement of about a factor of 40 compared with pure Kr.

X-ray polarization observations of pulsar wind nebulae (PWNe) provide crucial insights into magnetic field structures and particle acceleration mechanisms. While the Imaging X-ray Polarimetry Explorer (IXPE) has made significant contributions to PWN studies, its limited effective area restricts observations to only the brightest sources, leaving many fainter nebulae unexplored. We evaluate the polarization capabilities of the enhanced X-ray Timing and Polarimetry mission (eXTP) for studying PWNe and establish a methodology for simulating eXTP Polarimetry Focusing Array observations using modified IXPEOBSSIM. We develop and validate a simulation framework with appropriate response functions and instrumental background models, conducting comprehensive simulations of 12 PWNe selected from the SNRcat catalogue across various evolutionary stages and brightness levels. Our simulations demonstrate that eXTP provides approximately a factor of 2 improvement in minimum detectable polarization at the 99% confidence level (MDP99) compared to IXPE. For the brightest targets (N157B, G54.1+0.3, and Mouse), 1 Ms observations achieve MDP99 values of 4%-5%. The area with significant polarization detection for extended sources like Vela PWN is nearly twice as large as achievable with IXPE. These enhanced capabilities will significantly expand the sample of PWNe with robust X-ray polarization measurements, enabling systematic studies of magnetic field structures, particle acceleration mechanisms, and PWN-environment interactions across different evolutionary phases.

As one of the key ground-based facilities of the Chinese-French SVOM mission, the main scientific objectives of the Ground-based Wide Angle Camera array (GWAC) are to detect prompt optical emission of gamma-ray bursts or other short duration astronomical transients on a second-scale temporal resolution. GWAC is located at Xinglong observatory, China, and consists of 10 mounts and 40 cameras, providing a joint field of view of about 3600 square degrees.The detection ability is 16 magnitude in 10 seconds of exposure time in the visual band under the condition of the new moon phase. Here, we give an overview of GWAC and introduce the science motivation of the project, as well as the performance of the hardware and the software. The observation strategies and the data processing are briefly presented. The early sciences in the last 5 years since the first light are summarized.

Short gamma-ray bursts (GRBs) exhibiting a plateau phase provide valuable insights into the postmerger activity of their central engines. Although the physical origin of the plateau remains uncertain, the magnetar energy injection model offers a compelling explanation that reproduces the observed temporal and luminosity features. However, previous studies relying solely on X-ray data have suffered from strong parameter degeneracies when constraining the magnetar parameters. Here we perform broadband afterglow modeling on seven short GRBs with plateau features by combining X-ray, optical, and radio observations within the framework of the magnetar energy injection model. Key model parameters are derived by using the Markov Chain Monte Carlo method. It is found that the energy injection substantially modifies the afterglow dynamics in most events. Compared with X-ray-only analyses, our broadband modeling systematically yields a lower magnetic field strength and a shorter spin period for the central magnetar, corresponding to a higher injection luminosity. The study clearly shows that incorporating multiwavelength data effectively alleviates the degeneracy between the magnetar parameters and X-ray radiative efficiency. In addition, the distribution of our short GRBs differs markedly from long GRBs when they are plotted on the initial Lorentz factor versus gamma-ray energy plane. This offset, consistent with the observed harder spectrum of short GRBs, may serve as a useful diagnostic for investigating the progenitor as larger samples are available.

The Gas Microchannel Plate Pixel Detector (GMPD) is used in astrophysics for measuring X-ray polarization and serves as the prototype for the Low-Energy Polarization Detector (LPD) of the POLAR-2 project. During gamma-ray burst (GRB) observations, the incident flux can vary rapidly on short timescales, inducing charging-up effects that degrade gain stability. We characterize the chip-level charging-up behavior and quantify its impact on detector gain. To suppress this effect at the chip level, a 300 nm resistive film with a sheet resistance of 1010 Omega/sq is applied, effectively maintaining gain stability within 9% across varying operational conditions.

We inv estigat e the int eraction betw een relativistic jets and supernova (SN) ejecta as a pot ential origin of X-ray knots in radio galaxies, employing knot A in M 87 as a test case. By modelling the dynamical evolution of the int eraction, w e evaluate this scenario based on particle acceleration efficiency and spatial morphology. Our modelling indicates that the ejecta shock expands to only similar to 30 pc, which is inconsistent with the observed spatial scale of knot A ( similar to 60 pc). In contrast, the jet shock can successfully r epr oduce the observed scale after appr o ximately 3000 yr, with the ejecta being accelerated to a bulk velocity of beta ej approximate to 0 . 43 . We fit the multiw avelength spectr al energy distribution (SED) using a one-zone leptonic fr amework, at tributing the X-rays to synchr otr on radiation fr om electr ons accelerat ed up t o similar to 1 PeV at the jet shock. The derived magnetic field is appr o ximately 70 mu G in the SN ejecta rest frame, which is significantly below the equipartition value. Protons may be accelerated up to similar to EeV, supporting the hypothesis that the jets of radio galaxies may be the potential site for ultrahigh-energy cosmic-ray acceleration within the framework of the jet-ejecta interaction

Building upon our previous work, this study extends the analysis to the rotating case and systematically examines the combined effects of magnetic charge, spin, and accretion flow structure on the resulting shadow and photon ring features. We derive the photon geodesics in the rotating Hayward spacetime via a semi-analytical approach and perform detailed ray-tracing simulations under both spherical and geometrically thin disk accretion scenarios. Our results demonstrate that the presence of magnetic charge induces significant deformations in the black hole (BH) shadow, most notably the emergence of a characteristic D-shaped asymmetry at high spin values and large inclination angles. Furthermore, we analyze the redshift and intensity distributions associated with both prograde and retrograde accretion disks, revealing that the observed image morphology is strongly influenced by relativistic Doppler boosting, gravitational redshift, and frame-dragging effects. Remarkably, the predicted brightness asymmetry and centroid displacement of the shadow images are in good agreement with current VLBI observations, such as those of M87 ^* . These findings underscore the potential of BH shadow imaging as a powerful probe of deviations from the classical Kerr geometry and offer theoretical guidance for interpreting future high-resolution observations.

The standard external-shock model, assuming a homogeneous turbulent downstream, has been widely used to decipher the afterglows of gamma-ray bursts. However, such assumption is invalid when the shock encounters a density jump. In this paper, we propose a self-consistent scenario to model the external forward shock emission, involving the advection and decaying behaviors of the shock-generated magnetic fields (sgMFs) in the downstream as found in particle-in-cell simulations. In an interstellar medium, our model is almost returned to the standard model but with & varepsilon;B proportional to(1+tdyn/tB)alpha t . Here, & varepsilon;B describes the sgMFs' fraction of shock energy in the standard model, tdyn is the shock dynamic time, and tB together with alpha t depicts the sgMFs' decaying behavior in our model. The situation is the same for the wind medium but with weak deviation in the early phase. When the shock encounters the density rise/dip, a shallower/deeper decay appears in the light curve. These behaviors are obvious for high-frequency emission or at the phase after the jet break. The GeV emission in the afterglow can serve to probe the circumburst density jump. We apply our model to decipher the later afterglows of the giant flare from SGR 1806-20. It is shown that a fireball propagating into a magnetar bow shock environment can well reproduce the observed peculiar steep decay in the radio light curve at similar to 10 days from the burst.

This paper reports on the performance study of a double-layer THGEM-multiplied gas detector using argon-, krypton-, and xenon-based gas mixtures. Using the test system, pressure-scanned measurements of the effective gain, energy resolution, and maximum gain were performed for 20 different gas mixtures. The operating gas pressure ranged from 0.4 to 1.2 bar, with quenchers including carbon dioxide (CO2), methane (CH4), iso-butane (i-C4H10), dimethyl ether (C2H6O, DME), and carbon tetrafluoride (CF4). To eliminate the influence of charging-up effects in the THGEM on effective gain measurements, the THGEM substrates were pre-charged prior to the tests. Measurements across the induction, transfer, and drift regions show that Xe-based mixtures require higher drift fields, while CF4 imposes stricter requirements on the transfer region. Pressure scan results indicate that the maximum gain follows an exponential decay trend with increasing pressure. Among the three noble gases, Ar/iso-butane achieves the highest effective gain, exceeding 105, which is characteristic of a strong Penning mixture. Furthermore, we observed that Ar/DME and Kr/DME mixtures require notably low THGEM operating voltages. Measurements of quencher fraction confirm that both are Penning mixtures. This can be explained by the fact that the ionization potential of DME is lower than the metastable energy levels of Ar and Kr.

Addressing the unique requirements for a wide field of view and rapid response in soft X-ray polarization measurements of transient sources such as gamma-ray bursts, this paper proposes the design of a high-reliability CubeSat payload electronics system for dual-satellite cooperative observation. The system employs a Gas Microchannel Pixel Detector (GMPD) and a Topmetal-L sensor as its core components, forming a highly integrated, low-noise payload hardware platform. It achieves a field of view of 90^∘ × 90^∘, a sensitive area of 3.69 cm^2, and a power consumption of less than 6 W, operating within an energy range of 2–10 keV. The system incorporates autonomous high-voltage (HV) ramp-up/ramp-down control and a dual-protection mechanism based on count rate and discharge events, enabling in-orbit responses to risks such as the South Atlantic Anomaly and solar particle events. It also supports single-event upset detection and recovery, as well as in-orbit firmware upgrades. The communication interface adopts a redundant primary/backup Controller Area Network (CAN) bus design, with measured channel switching times of less than 100 ms, meeting the demands for real-time command interaction in dual-satellite coordination. By utilizing a large-array pixel sensor with region-of-interest readout and integrating an in-orbit track compression algorithm, the system significantly reduces data storage and downlink transmission resource burdens. Ground tests demonstrate an equivalent noise charge of 22.35 e^-, HV monitoring linearity better than ±0.5%, and an output range extending to -5 kV. Thermal vacuum cycling tests show no performance degradation after five cycles between -5 ^∘C and 40 ^∘C. This work demonstrates the system's capability for autonomous observation, intelligent coordination, and reliable operation in complex space environments.

Nearby pulsars within 1 kpc are considered to be possible sources of 10-500 GeV cosmic-ray positron excess measured by PAMELA and AMS-02. A TeV halo around Geminga is detected by High-Altitude Water Cherenkov Observatory, and the measurements of its surface brightness profile indicate a slow particle diffusion surrounding the source. This result challenges the pulsar interpretation of the positron excess. The observations at GeV energies provide direct information on the electron/positron density in the GeV nebula, which can offer more direct constraints on the origin of the positron excess. Two previous works have performed analyses on the GeV emission of the pulsar halo but focused on the energy band above 8 GeV. In this work, we use a longer dataset from the Fermi Large Area Telescope (LAT) to reanalyze the GeV halo emission of Geminga, extending the analysis to cover the energy range of 1-1000 GeV. We find that the analysis in this wider energy range results in a low significance of the halo emission. This can be attributed to the Galactic interstellar emission model being unable to perfectly fit the background over this broader energy range and due to the low measured halo flux at <10 GeV energies leading to a mismatch between the observation and model expectation. We also derive the spectral energy distribution of the tentative halo emission, which shows a very hard spectrum in the 1-10 GeV range.

A distinct thermal or quasi-thermal spectral component is occasionally observed in gamma-ray burst (GRB) prompt emission spectra. Taking GRB 090902B as a case study, we investigate its origin within a structured jet framework, in which the outflow consists of an ultra-relativistic uniform core surrounded by a structured cocoon. In the weak-scattering regime with inefficient shear acceleration, electrons pre-energized in the thin jet-cocoon interaction layer are further heated in the mixed jet-cocoon (MJC) region, forming a quasi-thermal electron distribution. Parameterizing the radial temperature profile of electrons as a power law with index q_T, we demonstrate that both the peak flux and spectral width of the thermal component are sensitive to maximum temperature T_max and q_T. Combined with the synchrotron emission of shock-accelerated electrons in the jet core, our model reproduces both the quasi-thermal component in the keV-MeV range and the broadband non-thermal emission observed in the time-integrated and time-resolved spectra of GRB 090902B. A comparative analysis of GRB 240825A within a shear-acceleration dominated (strong-scattering) scenario shows that shear-accelerated electrons produce broader spectra than thermalized electrons in the weak-scattering regime. These results indicate that GRB spectral diversity likely arises from the additional emission component originating in the MJC region under different physical conditions.

The search for dark matter focuses now on hypothetical light particles with masses ranging from MeV to GeV (refs. 1-12). These particles would leave very faint signals experimentally. A potential avenue for enhancing experimental sensitivity to light matter relies on the Migdal effect13-15, which involves the detectable ejection of electrons following the instantaneous accelerations of atoms colliding with neutral dark matter. However, although the Migdal effect could be equally generated in controlled experiments with neutral projectiles, a direct experimental observation of this effect is missing, casting doubt on the reliability of detection experiments relying on this effect. Here we report the direct observation of the Migdal effect in neutron-nucleus collisions, achieving a statistical significance of 5 standard deviations, which rests on 6 candidate events selected out of almost 106 recorded events. Our experiments have determined the ratio of the Migdal cross-section to the nuclear recoil cross-section to be 4.9 - 1.9 + 2.6 × 10 - 5 , in which nuclear recoils exceed 35 keVee and electron recoils span 5-10 keV. These findings are consistent with theoretical predictions. This work resolves a long-standing gap in experimental validation, which not only strengthens the theoretical foundation of the Migdal effect but also paves the way for its application in light dark matter detection.

In this analysis, we investigate the polarization radiation imaging of Kerr-Newman black holes, with a particular focus on the impact of black hole charge on photon propagation and polarization characteristics. By extending the traditional Walker-Penrose method, which is limited by its reliance on specific symmetric structures and Killing tensors, we overcome these limitations by constructing an ordinary differential equations (ODEs) numerical framework that combines the photon orbit equation with the polarization parallel transport equation. This allows for the self-consistent evolution of photon trajectories and polarization states in any spacetime backgrounds without relying on specific symmetries. Using this framework, we analyze the effects of black hole spin and charge on the polarization characteristics of radiation from both prograde and retrograde accretion disks. Our results show that black hole charge can significantly modify photon trajectories and polarization patterns: increasing charge compresses and distorts the EVPA structure on photon-ring scales, inducing localized rotations and asymmetries that may provide a potential diagnostic of a nonzero black hole charge.

GRB 161117A is a long-duration gamma-ray burst with three main overlapping peaks. By analyzing the time-resolved spectra of its data observed with the Gamma-Ray Burst Monitor on board the Fermi mission, we find that the spectral evolution shows a transition from thermal (single blackbody, hereafter BB) to hybrid (power-law, hereafter PL, +BB), and finally to nonthermal (Band and cutoff PL) emissions. Such a transition suggests that the jet composition of GRB 161117A should be changed from a fireball to a Poynting-flux-dominated jet. The bulk Lorentz factor (Gamma ph), radii (Rph and R0), magnetization factor at the central engine (sigma 0), and dimensionless entropy (eta) of the outflow can be inferred by invoking the observed quasi-thermal component within two models (e.g., pure fireball and hybrid). It is found that Gamma ph seems to be tracking with the light curve, and R0 remains a constant at similar to 108 cm. The low magnetization (1 + sigma 0 similar to 1) and high dimensionless entropy (eta >> 1) during the first seven time-intervals suggest it to be a pure fireball outflow. Moreover, we also estimate the lower limit of magnetization parameters at the photosphere radius (sigma ph similar to 1.4 and 0.75) for late phase via the nonthermal spectra, and it indicates that the particle acceleration mechanism is dominated by internal shocks rather than magnetic dissipation processes. Finally, the nu nu annihilation mechanism of NDAF model to explain the thermal emission of GRB 161117A is also discussed.

Motivated by horizon-scale polarization observations of M87*, we investigate polarized emission from rotating black holes (BHs) immersed in a perfect fluid dark matter (PFDM) background. Fully relativistic ray tracing is utilized to examine how the polarization structure jointly depends on magnetic-field geometry, higher-order imaging, and the PFDM intensity parameter k. We show that, for a given k value, magnetic configurations with a polar component naturally produce a continuous spiral pattern in the electric vector position angle (EVPA), while the large-scale EVPA morphology remains primarily controlled by the magnetic-field topology. Variations in k affect light propagation and polarization transport near the horizon, leading to corresponding changes in EVPA deflection and polarized intensity around the photon ring. A decomposition of the total polarized image further reveals that higher-order images provide localized yet non-negligible corrections near the ring. Compared with the polarization characteristics of M87* inferred by the EHT, image-domain quantities such as mnet and arg beta 2 indicate that PFDM induces systematic shifts relative to the pure Kerr BH case, lowering the net linear polarization fraction and modifying the large-scale polarization phase. Overall, PFDM acts as an additional strong-gravity ingredient that systematically reorganizes the horizon-scale polarization structure, suggesting that surrounding dark matter can leave observable imprints on black hole polarization signatures.

A comprehensive analysis of the energy and waiting time distributions of the bursts from FRB 20240114A detected by the Five-hundred-meter Aperture Spherical Radio Telescope between 2024 January 28 and August 29 is presented. For the full sample, its energy distribution cannot be fitted with the simple power-law, bent power-law (BPL), thresholded power-law (TPL) or Band function models, and its waiting time distribution excluding intervals shorter than 0.5 s cannot be fitted with the Poisson or Weibull models. Nevertheless, for the subsamples with more than 50 bursts in single-day observations, their energy distributions can be fitted with the BPL or TPL models, and their waiting time distributions are better described by a Weibull model. It is noted that the best-fitting BPL parameter beta is approximately invariant within the epochs before and after 2024 March 21 with an average of beta & strns;(b )= 1 . 006 +/- 0 . 074 and beta & strns;(a) = 1 . 236 +/- 0 . 183 (one standard deviation), respectively. Most subsamples from the later epoch have a smaller burst rate parameter r in the Weibull model than those from the earlier epoch. The majority of bursts with E > 10(39) erg occurred in the earlier epoch. The energy distributions in the high-energy range (> 6 & times; 10(37) erg) differ significantly between the two epochs, and power-law fits to dN/dE yield indices of-1.97(-0.02)(+0.02) and-2.34(-0.06)(+0.06) , respectively. The median of the waiting time distribution of the later epoch is larger than that in the earlier epoch. These results suggest that the two epochs may be dominated by different types of bursts, possibly attributed to changes in the physical properties of the emission region.

How do magnetic fields shape the way young stars gather gas from their birth clouds? Using high-resolution Atacama Large Millimeter/submillimeter Array observations of a young triple protostellar system HOPS-182, we identify an elongated stream of gas, or accretion streamer, that extends over several thousand astronomical units (1 astronomical unit is the Earth-Sun distance) and carries a substantial flow of material toward the system. The gas speeds along this filament increase toward the star in a way consistent with gravitational free fall, while the streamer's shape closely follows the magnetic field threading the region. By comparing the strengths of gravity and magnetic tension and measuring how the gas rotates compared with the local magnetic field, we show that the field is strong enough to help confine and guide the infalling gas and efficiently remove the angular momentum. These results suggest that a substantial fraction of the material falling onto young protostellar systems can be funneled through elongated, magnetically structured accretion streamers.

This paper primarily investigates the optical properties of two minimal deformations of the Schwarzschild black hole-the Kazakov-Solodukhin and Ghosh-Kumar black holes-under different accretion models. The event horizon, photon sphere, and critical impact parameter of the former increase relative to the Schwarzschild case, whereas those of the latter decrease. Data from the Event Horizon Telescope Collaboration are used to constrain the parameter ranges of the two black holes. Under spherical accretion, the quantum correction of the Kazakov-Solodukhin black hole enlarges the black hole shadow and reduces the integrated intensity, while the shadow of the magnetically charged Ghosh-Kumar black hole shrinks and the integrated intensity increases. The black hole's shadow radius is independent of the choice of spherical accretion model. For an optically and geometrically thin accretion disk, the integrated intensity is dominated by direct emission, with photon-ring and lensed-ring contributions being negligible. In addition, the photon and lensed rings of the Kazakov-Solodukhin black hole are narrower, whereas those of the Ghosh-Kumar black hole are broader. Whereas the Kazakov-Solodukhin black hole is brighter, the Ghosh-Kumar black hole is dimmer. Additionally, bringing the disk closer to the black hole yields a smaller shadow radius. This paper proposes a method to distinguish different black holes within a specific thin-disk model.