Chichuan Jin

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.

Multiwavelength observations of stellar flares trace the activity of different components of the stars’ outer atmosphere, providing insight into their interactions. In the present paper, we report a superflare from BP Tau, simultaneously observed with the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) satellite and TESS. While we attribute the X-ray flux increase to a magnetically powered flare, the optical light curve likely results from the superposition of the flare and an accretion burst. The X-ray flare has a mean flux of (1.5 _−0.04 ^+0.3 ) × 10 ⁻¹¹ erg cm ⁻² s ⁻¹ in the WXT energy band (0.5−4.0 keV), with e-folding times of 1.7 ± 1.0 ks and 14 ± 5 ks for the rise and decay phase, respectively. The corresponding time-integrated flare energy is (1.0 ± 0.2)×10 ³⁶ erg. The optical flare has an e-folding time of 0.33 ± 0.04 ks for the rise phase, but the data do not constrain the decay timescale. Assuming a decay phase equal to the rise phase, the resulting optical flare energy is (2.8 ± 0.4)×10 ³⁴ erg in the TESS band (∼6000 − ∼10 000 Å), corresponding to a bolometric energy of (1.9 ± 0.3)×10 ³⁵ erg (assuming a blackbody at 11 000 K). The Follow-up X-ray Telescope (FXT) on EP triggered an observation ∼1.5 day after the flare, with a flux of (4.6 _−0.5 ^+0.2 ) × 10 ⁻¹³ erg cm ⁻² s ⁻¹ (0.5−10.0 keV), indicating that BP Tau had returned to quiescence. This work demonstrates the potential of jointly analyzing EP and TESS data for superflares. WXT is expected to detect ∼800 superflares per year, with FXT capable of slewing to the flaring star within ∼3 − 5 minutes. The large field of view of both missions offers us the opportunity to study multiwavelength variability during energetic flares.

We present deep X-ray observations of the nearby X-ray-luminous galaxy cluster Abell 1795 obtained with the Einstein Probe Follow-up X-ray Telescope ( EP-FXT ), with a total exposure time of ∼480 ks. Exploiting the large field of view and low particle background of EP-FXT , we directly measured the radial temperature profile of A1795 out to R ₂₀₀ with full azimuthal coverage, which was shown to increase in line with the radius within 6′ and then gradually decline toward larger radii. The surface-brightness residual map and 2D thermodynamic maps reveal a clockwise spiral structure extending from the cluster core towards the south-east, which traces low-temperature and low-entropy gas, which is also consistent with sloshing-induced cold fronts. In the north-west, the surface-brightness-enhanced region exhibits an arc-like high-temperature feature and both the temperature-derived and density-derived Mach numbers support the assumption that it is a weak shock. These substructures can be explained by a binary merger scenario: the perturbing subcluster induces sloshing during its first passage past the primary core and its subsequent return passage through the ICM might drive the shock towards the north-west. Our results indicate that relaxed galaxy clusters such as A1795 might still retain signatures of dynamical activity.

We present timing and spectral analysis of the recently identified ultracompact double-degenerate (DD) white dwarf binary eRASSU J060839.5–704014 using observations from NICER and Einstein Probe (EP), together with archival XMM-Newton data. By phase-connecting the long-term XMM-Newton, NICER, and EP observations, we obtain a coherent quadratic timing solution, yielding an orbital period of 374.15013 (2) s and an orbital decay rate of P ̇ = − 4.7 ( 1 ) × 1 0 − 11 s s − 1 . This orbital decay exceeds that measured in the prototypical DD binaries HM Cnc and V407 Vul. Assuming that the observed orbital evolution is primarily driven by gravitational-wave (GW) angular momentum loss, the inferred chirp mass is ∼0.43 M _⊙ , placing the source among the most massive known systems of this class. The phase-averaged spectra of NICER and EP-Follow-up X-ray Telescope (FXT) are described by a soft thermal component with temperatures of ∼126 and ∼144 eV, respectively, confirming the supersoft nature of the source. Phase-resolved spectroscopy reveals a clear decrease in temperature across the bright phase in both instruments, indicating a structured emission region with significant temperature gradients. These results establish eRASSU J060839.5–704014 as one of the most rapidly evolving ultracompact DD binaries presently known, belonging to the rare class of direct-impact ultracompact binaries, and a promising verification source for future low-frequency GW studies.

Narrow, highly ionized X-ray emission lines in black hole low-mass X-ray binaries (BH-LMXBs) are rare and have been observed in only a few sources, during unusual, heavily obscured accretion states. We report on a detailed high-resolution spectral analysis of emission line features from the first XRISM observation of a BH-LMXB candidate in a bright soft state, MAXI J1744–294/Swift J174540.2-290037, in the central parsec region of our galaxy. The source was observed as part of an extensive, coordinated multiwavelength campaign on its recurring X-ray outburst in early 2025. Following the careful modeling of the contributions from multiple point sources and diffuse emission within the XRISM/Resolve field of view in M. Parra et al. (2026a; Paper I), and combining these data with broadband X-ray coverage from XMM-Newton and NuSTAR), we now identify a narrow ( σ ∼ 500–1000 km s ⁻¹ ) static emission component intrinsic to the system. This component likely arises from a highly ionized (log ξ ≳ 5.5) photoionized plasma in the inner disk atmosphere and is accompanied by a weak, narrow Fe I K α line at 6.4 keV. We also detect at least three narrow emission features at atypical energies between 6.7 and 7.1 keV. The lack of corresponding rest-frame atomic transitions points toward highly ionized blueshifted Fe lines with outflow velocities of −1300 to −6000 km s ⁻¹ , which we model with multiple layers of photoionized or collisional plasma. We explore scenarios in which these unprecedented features are produced by multiple phases in a jet and/or a disk wind, and discuss potential similarities between MAXI J1744-294 and the exotic microquasar SS 433.

The Galactic center is a complex and crowded region hosting the supermassive black hole Sgr A*, numerous accreting compact objects, and diffuse X-ray emission. This paper presents the first in a series of studies analyzing the XRISM observation of the X-ray transient MAXI J1744-294/Swift J174540.2−290037, located ∼18″ from Sgr A*. The observation, conducted in 2025 March, along with XMM-Newton and NuSTAR coverage, aimed to investigate the Fe emission features of MAXI J1744-294 during its outburst. However, the region surrounding the source is heavily contaminated by X-ray emission from various diffuse and point sources, including strong line contributions from the supernova remnant Sgr A East and the Galactic center X-ray emission. Additionally, the nearby neutron star low-mass X-ray binary (NS-LMXB) AX J1745.6−2901 was also in outburst during the XRISM observation, further complicating the spectral analysis. This study focuses on disentangling the contributions of these overlapping sources by robustly modeling the background contamination and spatial–spectral mixing. We describe the methodologies, region selection, and data reduction techniques applied to the different instruments. Two complementary approaches—empirical and physical modeling—are employed to characterize diffuse emission and point-source contributions. The results provide a foundation for the detailed spectral analysis of MAXI J1744-294, AX J1745.6-2901, and the surrounding interstellar medium, which will be presented in subsequent papers. This study highlights the challenges and robust solutions for analyzing XRISM/Resolve data from crowded regions in conjunction with other X-ray telescope data.

Context. Gamma-ray burst GRB 241030A ( z = 1.411) exhibited a particularly bright afterglow (similar to the ‘BOAT’, GRB 221009A), detected across gamma-ray, X-ray, UV, and optical bands. The extensive, multi-wavelength observations of this remarkable event provide a valuable opportunity to advance our understanding of GRB afterglow physics. Aims. We aim to constrain the physical properties of the jet, its microphysics, and the characteristics of the circumburst environment in the context of forward-shock emission. Methods. We compiled multi-wavelength observations spanning from a minute to a week after the prompt emission, processing the data through a unified photometry pipeline. Leveraging this comprehensive dataset, we analysed the observations both analytically and using Bayesian inference with two independent models. Our models assume that the afterglow emission arises from the strong forward shock of a laterally structured jet, with possible contributions from synchrotron self-Compton (SSC) scatterings. Results. We find that our models do reproduce the afterglow observations accurately, from the X-rays to the optical, favouring a jet propagating into a constant-density interstellar medium, with a viewing angle within the jet core. However, both analyses – with and without the inclusion of SSC scatterings – require parameter values that are extreme compared to expectations from standard theory. In particular, our results imply extremely energetic jets despite regular prompt energy, leading to a very inefficient prompt emission. Furthermore, the jets are particularly inefficient at accelerating particles, with low ϵ _e and ϵ _B , leading to significant SSC emission. Finally, our analyses indicate that the jets have large opening angles and propagate in high-density media. Conclusions. If the afterglow is indeed powered by radiation emitted behind a strong forward shock, our results place GRB 241030A within a sub-class of GRBs characterised by extreme kinetic energies, large jet opening angles, and very low prompt emission efficiencies, below 10 ⁻³ , with strong SSC radiation. These predictions are difficult to reconcile with typical expectations from other GRBs. We therefore suggest that the afterglow of GRB 241030A is not solely powered by forward shock emission, and we discuss other options such as a long-lasting reverse-shock contribution.

The evolutionary processes of galaxy clusters influence the properties of their member galaxies. We present a joint X-ray–optical analysis of the galaxy cluster WHY J050106.2+013714 at z _c = 0.151. X-ray observations with the Einstein Probe Follow-up X-ray Telescope indicate that the cluster is dynamically young. The cluster displays an average X-ray temperature of 2.8 _−0.3 ^+0.4 keV and a total luminosity of 9.4 ± 0.3 × 10 ⁴³ erg s ⁻¹ , consistent with the scaling relation of typical disturbed clusters. Remarkably, the cluster hosts a multi-merging brightest cluster galaxy (BCG) system composed of six massive galaxies, with a total stellar mass of 1.16 × 10 ¹² M _⊙ . We detected a well-defined intracluster light component extending to a size of 310 kpc. A systematic search for merging BCGs in the DESI Legacy Surveys reveals that this sextuple-merging BCG is extremely rare in the local Universe. Additionally, other merging BCGs are also likely to form in moderately disturbed clusters, which provides valuable insights into the formation of BCGs.

We present a multiepoch study of the extreme X-ray variability of the type 1 quasar SDSS J000532.84+200717.4 using archival observations from XMM-Newton, Swift/XRT, EP-FXT, and ROSAT, together with new optical spectroscopy and multiwavelength photometry. The 0.2–10 keV X-ray flux exhibits a transition from a high state to a subsequent low state, declining by more than an order of magnitude and placing the source in the X-ray–weak regime (Δ α _ox ≲ −0.3). Significant variability on timescales of days to weeks persists within the low state. In contrast, the optical and mid-infrared emission remain stable over decade-long timescales, while the ultraviolet continuum varies only mildly and broadly tracks the X-ray evolution. Multiepoch optical spectroscopy shows no significant long-term changes in either the continuum shape or the broad emission-line profiles. The Mg ii emission is relatively weak compared with typical quasars, suggesting similarities to weak-line quasars. The pronounced wavelength-dependent variability indicates that the accretion disk remains largely intact while the X-ray emission undergoes dramatic changes. The spectral hardening in the low state and the viability of ionized partial-covering models are consistent with variable, largely dust-free absorbing gas, possibly associated with clumpy inner disk winds, although intrinsic coronal variations cannot be excluded. SDSS J0005+200717.4 therefore provides evidence that extreme X-ray weakness can arise as a transient phase in otherwise normal quasars.

Transient low-mass X-ray binaries (LMXBs) are usually first detected by all-sky X-ray monitors when they enter new outbursts. These detections typically occur at X-ray luminosities above ∼10 ³⁶ erg/s. Observations of these sources during the early rise of the outbursts have therefore been very limited so far. However, the launch of the Einstein Probe (EP) has greatly improved our ability to detect fainter X-ray activity, unlocking access to the early rise of the outburst. In September 2024, EP detected the early onset of a new outburst from the well-known neutron star LMXB Aql X-1, catching the source at a luminosity below 10 ³⁵ erg/s. We present results from a comprehensive multiwavelength campaign of this event, combining data from EP, NICER , NuSTAR , and Swift and Las Cumbres Observatory covering the full outburst from its early rise through its return to quiescence. By comparing X-ray and optical light curves obtained by the Las Cumbres Observatory during the initial rise, we show that the start of the X-ray emission lagged the optical rise by 13 days at most. We did not observe the exact moment at which the source changed from (optical or X-rays) quiescence to rise, so that other lags cannot be completely ruled out, however. Time-resolved X-ray spectroscopy revealed the evolution of the geometry and the physical properties of the accretion flow during this early stage of the outburst, as well as at higher luminosities as the source transitioned through the canonical X-ray spectral states, the hard, intermediate, and soft state. These data show that the source underwent a very rapid transition of about 12 h from the hard to the soft state about two weeks after the optical onset of the outburst. At the state transition, peculiar trends are observed for the temperature and physical sizes of the inner disk region and a blackbody near the surface of the neutron star, which might suggest that at this stage, a geometrically thicker inner disk emerges. We discuss these results in the context of timescales for outburst evolution and state transitions in accreting neutron stars and black holes.