Hao Zhou

We present a multiwavelength study of the γ /X-ray transient EP250416a (also designated GRB 250416C), triggered by the Einstein Probe (EP) Wide-field X-ray Telescope as well as by SVOM and Konus-Wind (KW). Observations spanning the γ -ray, X-ray, and optical bands facilitated a detailed analysis of the burst’s prompt emission, afterglow evolution, and physical origin. GRB 250416C exhibits a burst duration of 30 s in the X-ray and 17.7 s in γ rays, and a joint spectral fitting of 0.5−5000 keV data yields E _peak = 342 ₋₂₃₂ ^+0.90 keV. Optical spectroscopy of the afterglow, acquired with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South, yields a redshift of z = 0.963. Accounting for the measured redshift, the isotropic energies are E _X,sio = 2.7 _−0.5 ^+0.9 × 10 ⁵⁰ erg and E _{γ ,sio} = 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 _−0.12 ^+0.10 classifies GRB 250416C as an X-ray rich GRB. The X-ray afterglow exhibits 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. This corresponds to a jet half-opening angle of θ _j = 10.6 _−1.8 ^+1.9 degrees. GRB 250416C is optically dark, as it shows only a faint r -band detection ( r = 24.16 ± 0.13 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 = 2.7 ± 0.3 mag derived from the extinction curve model.

Using deep near-infrared and optical JWST and Hubble Space Telescope imaging, we identify a new host-galaxy candidate for the short GRB 061201 at an angular offset of ∼2″ from the optical afterglow, with a best-fit photometric redshift of z ∼ 1.2. We compare it with the prior putative host candidate at z = 0.111. The new candidate has a chance-coincidence probability of P _cc = 0.18, above the classical 0.1 threshold but consistent with a physical association given the depth of the JWST imaging. In contrast, the lower P _cc = 0.11 of the z = 0.111 galaxy is driven primarily by bright-tail statistics. A high-redshift origin is favored by three lines of evidence. First, although both scenarios satisfy the Amati ( E _p,i – E _iso ) relation, the beaming-corrected energy at z = 0.111 makes GRB 061201 an outlier in the Ghirlanda ( E _p,i – E _γ ) relation. Second, near-infrared data disfavor an AT2017gfo-like kilonova at z = 0.111. Third, afterglow modeling favors the high-redshift scenario, with the Akaike information criterion (AIC) for the z = 1.2 model lower than that for z = 0.111 by ΔAIC = 16.35. Adopting the z ∼ 1.2 candidate as the host gives a physical offset of 16.4–16.9 kpc (substantially reduced from ∼42 kpc implied under the low-redshift scenario) and a stellar age of ∼2 Gyr, consistent with the general population of short gamma-ray bursts (sGRBs). Furthermore, a low-redshift origin implies a binary neutron-star merger rate of ∼1400 Gpc ⁻³ yr ⁻¹ , in tension with gravitational-wave constraints. We conclude that a moderately high-redshift host provides a more self-consistent physical framework for GRB 061201, demonstrating the capability of deep JWST imaging for historically hostless sGRBs.

We present a comprehensive multiwavelength analysis of the long-duration gamma-ray burst (GRB) 250424A. Our dataset spans from the prompt gamma-ray emission to late-time optical monitoring, including spectra obtained with the Keck 10 m telescope. We find that the afterglow light curves display a prominent, simultaneous shallow decay phase in both X-ray and optical bands, followed by an achromatic transition to a standard decay regime. The broadband spectral energy distributions are well modeled by a single power-law function, indicating a common synchrotron origin for the emission across frequencies. We interpret the afterglow evolution within the framework of a relativistic forward shock refreshed by continuous energy injection. This scenario successfully reproduces the observed temporal and spectral behavior, yielding an isotropic equivalent kinetic energy of E_K,iso ≈ 5.5 × 10⁵² erg and an injection index of q ≈ 0.34 in a constant-density circumburst environment. The shallow decay phase is consistent with sustained energy injection lasting ∼9 ks. Despite the relatively low redshift, late-time optical observations reveal no distinct supernova component; however, our derived upper limits do not strictly rule out the presence of a typical GRB-associated supernova.

We present a comprehensive temporal and spectral analysis of the long-duration gamma-ray burst GRB 110801A, utilizing multiband data from the Neil Gehrels Swift Observatory and ground-based telescopes. The gamma-ray emission exhibits a distinct two-episode (“double-burst”) structure. Rapid follow-up observations in the optical and X-ray bands provide full coverage of the second burst. The optical light curve begins to rise approximately 135 s after the trigger, significantly preceding the second emission episode observed in X-rays and gamma rays at ∼320 s. This chromatic behavior suggests different physical origins for the optical and high-energy emissions. Joint broadband spectral fitting (optical to gamma rays) during the second episode reveals that a two-component model, consisting of a power law plus a Band function, provides a superior fit compared to single-component models. We interpret the power-law component as the afterglow of the first burst (dominating the optical band), while the Band component is attributed to the prompt emission of the second burst (dominating the high-energy bands). A physical synchrotron model is also found to be a viable candidate to explain the high-energy emission. Regarding the afterglow, the early optical light curve displays a sharp transition from a rise of ∼ t ^2.5 to ∼ t ^6.5 , which is well-explained by a scenario involving both reverse shock and forward shock components. We constrain the key physical parameters of the burst, deriving an initial Lorentz factor Γ ₀ ∼ 60, a jet half-opening angle θ _j ∼ 0.09, and an isotropic kinetic energy E _k,iso ∼ 10 ^54.8 erg.

Shock breakout emission represents the first light enlightening the dying star, which carries essential information probing nature of progenitor, ambient environment, and explosion physics. For instance, compared to a spherical explosion, an aspherical explosion can leave adiscernible imprint on the early-time spectral energy distribution characterized by a delayed shockbreakout and a possibly complex thermal emission. However, such early-time emis-sion has scarcely been detected, especially for hydrogen-rich type II supernovae, and hastherefore been far less explored in detailed modeling than the more readily observed shock-coolingphase. Here we report the ultraviolet, optical and X-ray observations of a type II supernova2024ggi (at a distance of 6.7 Mpc), beginning at a few hours after the explosion. Its spectral en-ergy distribution can be described only by a hotter component with a blackbody temperature ofabout 100,000 K and a cooler component with a temperature of 10,000 K. Such a two-componentthermal emission, together with the rapid blueward color evolution and increased ionization statevariation of emission lines observed within about one day after the explosion, are consistent with ascenario in which the supernova shock partially breaks out of a dense circumstellar material (CSM).We develop a semi-analytical shock-breakout radiation model which can well reproduce the earlymultiwavelength emission of SN 2024ggi and predict a disk-like CSM configuration around the ex-ploding star. These results provide independent and crucial evidence for aspherical explosion andunstable mass ejection of massive stars.

EP241217a is an X-ray transient detected by the Einstein Probe lasting for about 100 s and without accompanying γ -ray detection. The optical spectroscopy reveals the redshift of EP241217a is 4.59. By combining the γ -ray upper limit provided by GECAM-C, there is a considerable possibility that EP241217a is a typical type II gamma-ray burst, but it is fainter than the detection threshold of any available γ -ray monitors (i.e., E _{γ ,iso} ≲ 10 ⁵³ erg). The X-ray light curve exhibits a plateau lasting for ∼5 × 10 ⁴ s. However, the joint analysis with optical data suggests the presence of an achromatic bump peaking at ∼3 × 10 ⁴ s after the trigger, indicating the actual duration of the X-ray plateau may be significantly shorter than it appears. To interpret the achromatic bump, we adopt the scenario of a mildly relativistic jet coasting in a wind-like medium and encountering a rapid density enhancement of the circumburst medium, which is likely induced by the interaction of the progenitor’s stellar wind and the interstellar medium. However, this model cannot fully explain observed data, and some issues do exist, e.g., the observed spectrum is harder than the model prediction. Consequently, we conclude that the scenario of a mildly relativistic jet coasting in the wind-like medium cannot explain all observed features of EP241217a. In addition, some alternative models commonly invoked to explain X-ray plateaus are discussed, but there are more or less issues when they were applied to EP241217a. Therefore, further theoretical modeling is encouraged to explore the origin of EP241217a.

Following its launch on 2024 January 9, the Einstein Probe (EP) telescope has detected hundreds of fast X-ray transients (FXTs), yet their physical origins remain elusive. Understanding their luminosity function and formation rate is crucial for elucidating their nature. Recently, the EP team has provided the latest catalog of EP-detected FXTs. Based on this catalog, we present a model-independent nonparametric approach to derive the luminosity function and formation rate of FXTs. Our analysis reveals significant cosmological luminosity evolution, characterized by a scaling relationship of (1 + z ) ^3.58 . After accounting for this evolution, we establish that the local luminosity function is best represented by a broken power law, with a break luminosity of (4.17 ± 0.34) × 10 ⁴⁶ erg s ⁻¹ . The formation rate exhibits a broken power law as ρ ( z ) ∝ (1 + z ) ^−4.25 at z ⪅ 0.9 and ρ ( z ) ∝ (1 + z ) ^−0.26 at z ⪆ 0.9, yielding a local rate of approximately 153 . 8 − 95.1 + 249.4 Gpc ⁻³ yr ⁻¹ . This rate is higher than that of long gamma-ray bursts (LGRBs). Our findings indicate that a component of FXTs is associated with LGRBs.

We have developed a lightweight tool, RapidGBM , featuring a web-based interface and capabilities of rapid calculation of Fermi Gamma-ray Burst Monitor (GBM) visibilities and performance of basic data analysis. It has two key features: (1) it can immediately check the visibility of Fermi-GBM for new transients, and (2) it can check the light curve and perform spectral analysis after the hourly Time-Tagged Event data are released. The visibility check and the response matrix generation required for spectral analysis can be achieved through the historical pointing file after the orbit calculation, even when the real-time pointing file is not yet available. As a case study, we apply the tool to EP240617a, an X-ray transient triggered by Einstein Probe (EP). We demonstrate the workflow of visibility checking, data processing, and spectral analysis for this event. The results suggest that EP240617a can be classified as an X-ray-rich gamma-ray burst and confirm the feasibility of using historical pointing files for rapid analysis. Further, we discuss possible physical interpretations of such events, including implications for jet launching and progenitor scenarios. Therefore, RapidGBM is expected to assist EP transient advocates, Space-based multiband astronomical Variable Objects Monitor burst advocates, and other members of the community in cross checking high-energy transients. Based on prompt emission parameter relations (e.g., E _p – E _{γ ,iso} ), it can also help identify peculiar GRBs (e.g., long-short burst, magnetar giant flare, etc.) and provide useful references (e.g., more accurate T ₀ ) for scheduling follow-up observations.

Gamma-ray bursts are expected to be generated by structured jets, whose profiles significantly impact their afterglow emission. Previously, we developed a numerical code jetsimpy, to model the afterglow of jets with arbitrary angular profiles. In this study, we extend the code to incorporate a stratified radial profile, enabling it to model jets with arbitrary axisymmetric two-dimensional structures. The radial profile leads to the formation of a reverse shock. We modeled the shock system using an energy conservation prescription, which differs from the pressure balance approach. This leads to remarkably different predictions for reverse shock emission. In particular, we find that the reverse shock emission in the thin shell case is significantly overestimated in analytic models. We also explore the off-axis reverse shock emission from structured jets, where the cores belong to thick shell cases and the wings belong to thin shell cases. We have confirmed the prediction that off-axis observers may see a thin-to-thick transition, but we find that the light curve morphology is hard to distinguish from pure thin or thick shell cases. A radial profile also introduces hydrodynamic energy injection. As such, our code can naturally apply to refreshed shock cases, where the modeling of kilonova afterglows is demonstrated as an example. To validate our method, we fit the optical flash of GRB 990123, showing good agreement with the data. The upgraded jetsimpy provides unprecedented flexibility in modeling the afterglow emission of jets with various profiles, including those derived from general relativistic magnetohydrodynamic simulations.