Dong Xu

EP241021a is a peculiar fast X-ray transient (FXT) exhibiting a long X-ray plateau and an optical rebrightening, both challenging the traditional jet afterglow model. Whether its nature is a stellar explosion or a tidal disruption event (TDE) remains debated. Here we present late-time Hubble Space Telescope (HST)/WFC3 and Gemini-N/GMOS observations of its host galaxy at z = 0.748. After ∼300 days posttrigger, the transient emission had faded below detection, revealing a dwarf disk galaxy with an effective radius R _e = 1.7 kpc, a stellar mass M _⋆ ∼ 3 × 10 ⁸ M _⊙ , and a high star formation rate of ∼5 M _⊙ yr ⁻¹ . Combined with previous very long baseline interferometry measurements, the transient can be located in the galactic disk with a projected physical offset of 0.95 ± 0.13 kpc from the galaxy center. Host-subtracted optical light curves show that the rebrightening ceased at ∼47 days, followed by a sudden drop with a power-law index steeper than −3.3, coincident with a similar drop in the X-ray light curve. This synchronous drop in both bands is consistent with jet shutoff, and the required month-long jet activity suggests a persistently accreting compact object fed by a TDE. Among the TDE scenarios, the off-nuclear location and the starburst host galaxy slightly favor the postsupernova micro-TDE scenario in a binary system, in which a natal kick drives the newborn compact object into a tidal disruption of its companion. Nevertheless, an off-nuclear intermediate-mass black hole (IMBH) TDE or a stellar-mass black hole TDE induced by dynamical scattering in a dense stellar cluster cannot yet be excluded.

EP251023a is an extragalactic fast X-ray transient (eFXT) that was only detected by the Einstein Probe (EP) and that lacks a gamma-ray counterpart. The prompt emission consisted of a main emission with a duration T ₉₀ = 292 ± 19 s, followed by a long-lasting tail emission that persisted until the observation ended at T ₀ + 1571 s. With the upper limit of Konus–Wind, we derived a conservative upper limit on the isotropic gamma-ray energy E _{γ ,iso} of 5.7 × 10 ⁵² erg for the main emission phase. We identified a redshift of z = 2.232 ± 0.001 based on the strong absorption features in the Keck spectrum, which also indicate a relatively low host-galaxy HI column density. Based on the broadband spectral energy distribution, the late-time light curves show an achromatic plateau, followed by an extremely steep decay with a slope of 3.99 after a break at about 49 ks. This is consistent with a rapidly spinning millisecond magnetar engine. Under the isotropic wind scenario, we obtained an initial period P ₀ < 2.27 ms and a magnetic field strength B _p < 8.33 × 10 ¹⁴ G for the magnetar. If a jet collimation with a typical opening angle of 0.1 rad is considered, these constraints are relaxed to P ₀ < 32.15 ms and B _p < 1.18 × 10 ¹⁶ G. Together with GRB 070707, EP251023a might represent a rare class of optical magnetar-powered internal plateaus with little external-shock contamination. This is different from previous examples that were primarily detected in X-rays. Future discoveries of similar events will help us to clarify the relation between magnetar-powered internal emission observed in the optical band and the emission that is only detected in X-rays.

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 s, 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 ⁵⁰ erg 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 ∼0.35 c and interact with an extended circumstellar medium with radius ∼10 ¹³ cm, generating X-ray breakout emission through nonthermal 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 ⁵⁶ Ni power the late-time emission. We end by discussing the landscape of XRF-SNe within the context of EP’s recent discoveries.

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.

Gamma-ray bursts (GRBs) and superluminous supernovae (SLSNe) are the most energetic explosions known in nature. Both arise from the collapse of massive stars in low metallicity environments [1–4], and are likely driven to their extreme energetics by a black hole [5] or neutron star central engine [6, 7]. Their excep- tional brightness has enabled us to probe stellar death from the nearby to very distant Universe [8, 9]. However, the relationship, if any, between these two extreme populations has been unclear. Here we report observations with the James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) that uncover a superluminous supernova (SLSN) following a GRB (GRB 230818A) at z = 2.375. The supernova (SN) reaches a peak of MB ∼ −21 at around 50 rest-frame days and decays slowly. These observations clearly demonstrate that, at least in some cases, the same stellar collapse can drive both of the most extreme explosions in nature. A magnetar model cannot simultaneously explain both the GRB and supernova properties with consistent parameters for standard spin-down assumptions, but reasonable fits to the SN are found for both the radioactive power and circumstellar interaction. Phenomenologically, the SN is similar to SN 2007bi, suggested to be radioactively powered [10]. If GRB 230818A is radioactively powered, then massive core collapse can produce both a black hole and a very high ejecta and nickel mass. This would have substantial implica- tions for the study of SN and GRBs from first-generation stars, as well as for the production of the binary black hole population observed in gravitational waves.

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.

We present James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) observations of the counterpart of EP 240801a, at z = 1.67, the first fast X-ray transient (FXT) identified as an X-ray flash by the Einstein Probe and Fermi Gamma-Ray Burst Monitor. Our observations reveal strong photometric and spectroscopic evidence for an associated broad-lined Type Ic supernova (SN) SN 2024aihh, the most distant spectroscopically identified gamma-ray-burst (GRB)–SN to date. The SN exhibits similar luminosity and light-curve evolution to the prototype GRB-SN 1998bw with an absolute magnitude of the SN at ∼23 days rest frame of M _F140W ≈ −19 mag. The SN is located in a host galaxy with complex morphology at a large (∼6 kpc) offset in a region of relatively low surface brightness. The region around the SN has a modest star formation rate and is dominated by an intermediate mass-weighted age (1.4 ± 0.3 Gyr) population, despite the apparent presence of a young, massive broad-lined Type Ic SN progenitor. These observations demonstrate that observations with HST and JWST can greatly extend the redshift range over which the GRB/FXT-SN connection can be studied, including in relatively low-luminosity, X-ray-rich events. They demonstrate little apparent evolution in the SN properties from local examples despite EP 240801a originating from an epoch 10 Gyr ago.

We present imaging and spectroscopic observations of supernova SN 2025wny, associated with the lens candidate PS1 J0716+3821. Photometric monitoring from the Lulin and Maidanak observatories confirms multiple point-like images, consistent with SN 2025wny being strongly lensed by two foreground galaxies. Optical spectroscopy of the brightest image with the Nordic Optical Telescope and the University of Hawaii 88-inch Telescope allows us to determine the redshift to be z_s = 2.008 +- 0.001, based on narrow absorption lines originating in the interstellar medium of the supernova host galaxy. At this redshift, the spectra of SN 2025wny are consistent with those of superluminous supernovae of Type I. We find a high ejecta temperature and depressed spectral lines compared to other similar objects. We also measure, for the first time, the redshift of the fainter of the two lens galaxies (the "perturber") to be z_p = 0.375 +- 0.001, fully consistent with the DESI spectroscopic redshift of the main deflector at z_d = 0.3754. SN 2025wny thus represents the first confirmed galaxy-scale strongly lensed supernova with time delays likely in the range of days to weeks, as judged from the image separations. This makes SN 2025wny suitable for cosmography, offering a promising new system for independent measurements of the Hubble constant. Following a tradition in the field of strongly-lensed SNe, we give SN 2025wny the nickname SN Winny.