He-Yang Liu

The Einstein Probe (EP) is dedicated to explore the dynamic X-ray universe and capture transient events in real time with its Wide-field X-ray Telescope (WXT). However, WXT’s unprecedentedly large instantaneous field of view—exceeding 3600 square degrees—simultaneously encompasses numerous known X-ray emitters. Distinguishing genuine novel transients from these persistent sources is a critical observational challenge. To resolve this, EP-WXT incorporates a dedicated reference catalog of known X-ray sources directly into its onboard data processing and triggering system. In this paper, we detail the compilation of this onboard catalog. By merging data from the ROSAT All-Sky Survey, the Monitor of All-sky X-ray Image source list, and a curated stellar flare candidate catalog, we constructed a robust baseline database of 9000 sources. This catalog provides coordinates, baseline count rates, and spatial veto radii for exceptionally bright emitters. Real-time cross-matching against this database effectively decouples known background sources from the transient alert stream. Recent in-orbit operations validate the high stability and efficiency of this catalog-driven trigger system. Notably, the catalog successfully masked over 6100 potential triggers from known active stars. This proves its essential role in ensuring EP’s rapid and accurate response to genuine astrophysical discoveries.

Context. Stellar flares are energetic events driven by the sudden release of magnetic energy in the stellar atmosphere. Compared to the Sun, the diversity of stellar types and environments leads to a richer variety of flaring phenomena. Studying these flares is crucial for understanding their impact on exoplanets, the circumstellar environment, and stellar evolution itself. The launch of the Einstein Probe ( EP ) offers a unique opportunity to systematically detect such events. Aims. We present a systematic analysis of the flaring activity of the active RS CVn-type binary σ Gem, utilizing the first-year monitoring data from the Wide-field X-ray Telescope (WXT) aboard EP . Our goals are to demonstrate the unique capability of EP in monitoring stellar X-ray activity and detecting flares by identifying and characterizing extreme X-ray flares on σ Gem and estimating their occurrence rate. Methods. We developed a data-processing pipeline to select and extract EP -WXT observations, producing a background-subtracted, vignetting-corrected light curve. We employed the Bayesian blocks method to detect significant flares in the long-term X-ray light curve. The key detection parameter was optimized through extensive simulations. For each identified flare, we performed light curve and spectral fitting to derive its duration, peak luminosity, total energy, and plasma properties (e.g., temperature and emission measure). Results. Between October 2024 and April 2025, WXT detected six distinct flares from σ Gem. These lasted from 21 hours to 3 days, with peak X-ray luminosities (0.5–4 keV) of 3.7 × 10 ³¹ to 7.0 × 10 ³² erg s ⁻¹ and total energies of 1.1 × 10 ³⁶ to 4.4 × 10 ³⁷ erg, placing them among the “superflare” class. Conclusions. Using σ Gem as a case study, we demonstrate an analysis process for flare detection and analysis with EP -WXT data, which has rapidly expanded the catalog of known X-ray flares for this star and provides new statistical constraints on its flaring behavior. Applying this methodology to the growing EP stellar archive promises to yield a vast sample of X-ray flares, which will significantly advance our understanding of stellar magnetic activity.

The wide-field X-ray telescope (WXT) is the primary payload of the Einstein Probe (EP) mission. Utilizing innovative lobster-eye micro-pore optics (MPO), it achieves an unprecedented combination of a large instantaneous field-of-view (FoV) and high detection sensitivity. Precise scientific data analysis relies fundamentally on a standardized and robust calibration database (CALDB). In this paper, we present the design and implementation of the WXT CALDB. The database is designed to manage calibration data for 12 independent WXT modules and 48 CMOS sensors, incorporating both extensive on ground calibration results and dynamic in-orbit updates. We detail the directory hierarchy, naming conventions, functionality, and the specific formats of key calibration products. The WXT CALDB is seamlessly integrated into the WXT data analysis pipeline, ensuring high data integrity and maximizing the scientific return of the mission.

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.

Young planets offer a unique window into the early stages of planetary evolution. AU Mic is one of the nearest (9.8 pc) pre-main-sequence stars (∼20 Myr), hosting two transiting Neptune-sized planets and a debris disk. Previous studies have shown that the rotations of the central star, the debris disk, and the inner planet b are all aligned, suggesting that the system has not undergone violent evolution. Here we report new Rossiter–McLaughlin measurements for both AU Mic b and c, which happened to transit back-to-back on 2024 August 24 and 25, using the Magellan Planet Finder Spectrograph, together with contemporaneous photometry from LCOGT and CHEOPS. We confirm the aligned orbit of AU Mic b ( λ _b = 1 ^∘ ± 12 ^∘ ) and find two possible solutions for AU Mic c: we slightly favor an aligned solution ( λ _c = −10 ^∘ ± 16 ^∘ ) but cannot rule out a polar solution ( λ c = 8 7 ∘ − 2 9 ∘ + 3 6 ∘ ). Broader considerations, including dynamical stability and transit possibility, also support the mutually aligned scenario. An unexpected stellar signal during ingress and the poor transit-timing variation predictions of AU Mic c prevent a precise constraint on its obliquity, and various attempts using chromatic spectral analyses fail to outperform simple data exclusion in mitigating stellar contamination. Our observation highlights the importance of understanding stellar activity across multiple timescales and channels when characterizing young, active systems. A robust solution for the AU Mic architecture will require either a better understanding of stellar activity or future observations fortuitously free from strong stellar contamination.

Recently, the Lobster Eye Imager for Astronomy detected the longest-lasting and most energetic stellar X-ray flare event from HD 251108. In this work, we redetermined the atmospheric parameters of HD 251108 using three spectroscopic observations obtained with the 2.4 m Lijiang Telescope. Combined with the stellar radius derived from spectral energy distribution fitting, we found that HD 251108 contains a K-type giant with a mass of approximately 1.3 M _⊙ . Long-term photometric monitoring over 12 yr reveals a modulation suggestive of a stellar activity cycle, but inconclusive given the limited time span to date. Light-curve fitting indicates that the variations in both amplitude and shape are primarily driven by the evolution of a large spot. The fitting further indicates that the spot migrated from low latitudes toward the pole between 2014 and 2020, and began to recede from the pole after 2022. Using spot parameters from light-curve fitting, we found that the observed radial velocity variations arise from both the spot-induced distortions and the Keplerian orbital motion of the giant star. Additionally, we detect a possible M dwarf companion with a mass of approximately 0.25 M _⊙ . Our findings suggest a notable effect on the radial velocity caused by stellar magnetic activity.

The Lobster Eye Imager for Astronomy (LEIA), as a pathfinder of the Wide-field X-ray Telescope (WXT) on board the Einstein Probe ( EP ) satellite, is the first lobster-eye-focusing X-ray telescope with a considerably large field of view (FoV) to ever be flown. During the two and a half years of operations, a series of calibration observations were performed to fully characterize its in-orbit performance and calibrate the instrumental properties. In this paper, we present the results of the in-flight calibration campaign of LEIA , focusing on the properties of the point spread function (PSF), source positional accuracy, effective area, energy response, and the instrumental background. The calibration sources used are the Crab Nebula, Scorpius X-1, and the Cassiopeia A supernova remnant. Specifically, it is found that the spatial resolution (represented by the full width at half maximum of the elliptical PSF focal spot in the direction of the long axis) remains almost unchanged compared to the prelaunch values, ranging from 3.6′-9.3′ with a median of 5.9′. The post-calibration source positional accuracy is found to be ∼2′ (at the 90% confidence level). The Crab spectra can be well reproduced by an absorbed power-law model with the best-fit parameters largely in agreement with the literature values, indicating that the inorbit effective area is overall consistent with the model predictions and ground measurements. The effective area exhibits a systematic uncertainty of ≲10% (at the 68% C.L.), and a mild deterioration of ∼15% at the lower-energy end after one year of operation. The Cas A spectral analysis shows that the energy scale and spectral resolution of the detectors are generally consistent with ground values. The instrumental background is found to be largely consistent among the four detectors, with strong modulations by geomagnetic activity and the spectrum qualitatively consistent with our previous simulations. These instrumental performances meet the design requirements well. This work paves the way for the in-orbit calibration of the EP -WXT.

Extragalactic fast X-ray transients (eFXTs) represent a rapidly growing class of high-energy phenomena, whose physical origins remain poorly understood. With its wide-field, sensitive all-sky monitoring, the Einstein Probe (EP) has greatly increased the discovery rate of eFXTs. The search for and identification of the optical counterparts of eFXTs are vital for understanding their classification and constraining their physical origin. Yet, a considerable fraction of eFXTs still lack secure classifications due to the absence of timely follow-up observations. We carry out a systematic search of publicly available optical survey data and transient databases (including the Zwicky Transient Facility and the Transient Name Server) for optical counterparts to eFXT candidates detected by EP. In this paper, we describe our ongoing program and report the first results. Specifically, we identified the eFXT EP240506a to be associated with a UV/optical counterpart, AT 2024ofs. Spectroscopy of its host galaxy with the Very Large Telescope yields a redshift of z = 0.120 ± 0.002. By combining archival survey data with early-time multiwavelength observations, we find that the luminosity and light-curve evolution of AT 2024ofs are consistent with a core-collapse supernova origin. From detectability simulations, we estimate a local event rate density ρ 0 = 8. 8 − 3.9 + 21.2 yr − 1 Gpc − 3 for EP240506a-like events, and completeness-corrected rate of about 36–78 yr ⁻¹ Gpc ⁻³ for EP-detected X-ray transients associated with supernovae. Our results demonstrate the potential of EP to uncover prompt high-energy emission from core-collapse supernovae and underscore the critical importance of timely follow-up of future eFXT events.

The physical origin of fast radio bursts (FRBs) remains uncertain. Although multiwavelength observations have been widely conducted, only Galactic FRB 20200428D is associated with an X-ray burst from the magnetar SGR J1935+2154. Here we present multiwavelength follow-up observations of the nearby bright FRB 20250316A, including the Five-hundred-meter Aperture Spherical radio Telescope (FAST), Einstein Probe (EP) X-ray mission, Chandra X-ray Observatory, Wide Field Survey Telescope (WFST), and Space Variable Objects Monitor/Visible Telescope (SVOM/VT). The 13.08 hr FAST follow-up campaign without pulse detection requires an energy distribution flatter than those of well-known repeating FRBs, suggesting that this burst is likely a one-off event. A prompt EP follow-up and multiepoch observational campaign totaling >100 ks led to the detection of an X-ray source within the angular resolution of its Follow-up X-ray Telescope (FXT; 10″). A subsequent Chandra observation revealed this source to be offset by 7″ from the FRB position and established a 0.5–10 keV flux upper limit of 7.6 × 10 ⁻¹⁵ erg cm ⁻² s ⁻¹ at the FRB position, corresponding to ∼10 ³⁹ erg s ⁻¹ at the 40 Mpc distance of the host galaxy NGC 4141. These results set one of the most stringent limits on X-ray emission from a nonrepeating FRB, disfavoring ultraluminous X-ray sources as counterparts of apparently one-off FRBs and offering critical insights into afterglow models. Our study suggests that an arcsecond localization of both the FRB and its potential X-ray counterpart is essential for exploring the X-ray counterpart of an FRB.