Huaqing Cheng

Context. S4 0954+65 (4FGL J0958.7+6534) is a TeV-detected blazar at a redshift of z = 0.3694 ± 0.0011, classified as an intermediate-synchrotron-peaked BL Lac object. In early 2025, it entered an exceptional γ -ray high state, exceeding the flux of the 2015 outburst, during which contemporaneous MAGIC TeV observations were obtained. This event offers an opportunity to investigate the radiation mechanisms, particle acceleration, and jet dynamics. Aims. We aim to investigate the origin and physical properties of the exceptional 2025 flare, and to constrain the emission processes responsible for this flaring activity. Methods. We performed a multiwavelength analysis using γ -ray, X-ray, optical/UV, radio, and 43 GHz Very Long Baseline Array (VLBA) observations. We examined multiband correlations with the z -transformed discrete correlation function (zDCF), analyzed the spectral evolution and parsec-scale jet kinematics, and modeled the broadband spectral energy distributions (SEDs). Results. The γ -ray variations lead the optical and radio emission by 3.22 days and possibly 18.26 days, respectively, while no significant correlation is found between the γ -ray and X-ray emission. The γ -ray spectra show a harder-when-brighter behavior, consistent with enhanced particle acceleration during the active state. A similar spectral trend is also observed in the X-ray band, where the photon index is anticorrelated with the flux. During the major γ -ray flare, VLBA images reveal the emergence of a new superluminal radio knot from the core, whose extrapolated ejection time is consistent with the peak of the γ -ray flare. Conclusions. The temporal and structural evolution during the 2025 flare is consistent with a shock-in-jet scenario, in which a newly emerging disturbance propagates downstream and drives the flare. Broadband SED modeling shows that a leptonic synchrotron self-Compton plus external Compton model with dusty torus seed photons reproduces the multiwavelength observations with physically plausible parameters. A lepto-hadronic interpretation remains possible but requires substantially higher jet power.

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.

Changing-look active galactic nuclei (CL-AGN) have been proposed to follow an X-ray-binary-like, V-shaped relation between the optical/ultraviolet-to-X-ray spectral slope, αOX, and the Eddington ratio, λEdd ≡ Lbol/LEdd. We test this interpretation using CL-AGN selected from the Dark Energy Spectroscopic Instrument (DESI) and control samples matched to the eROSITA all-sky survey (eRASS). We find that the apparent positive correlation between αOX and the optical-continuum-based Eddington ratio, λEdd, opt, is not unique to CL-AGN, but also appears in matched non-changing-look samples. In broad-Hα AGN, our main statistical sample, this structure is largely driven by the known dependence of αOX on ultraviolet luminosity and by the covariance between the rest-frame 2500 Å luminosity, L2500, the black-hole mass, MBH, and λEdd, opt. When an X-ray-based Eddington ratio is used instead, the relation reverses sign because the X-ray luminosity, LX, enters the two axes with opposite signs. An apparent αOX–Eddington-ratio V-shape can therefore be induced by the luminosity estimator and is not a standalone accretion-state diagnostic. As a complementary test, we examine the eRASS effective photon index, Γeff, inferred from catalogue band fluxes. In broad-line AGN, it shows a weak positive association with the X-ray-independent optical Eddington ratio. However, the weaker hard-band trend depends on the null model and does not isolate coronal softening. These results motivate further X-ray spectroscopy, combined with independently estimated Eddington ratios, to test for intrinsic changes in the disk–corona system.

We report multiwavelength observations of EP240305a, an uncatalogued X-ray transient detected by the Einstein Probe on March 5, 2024. The source exhibits distinct characteristics across the X-ray, optical, near-infrared, and radio bands. The soft X-ray observations show two significant flares lasting ~100 − 250 s, accompanied by rapid flux decay in a few days, and the optical and near-infrared data reveal a faint, candidate counterpart. In contrast, the radio observations expose a long-term spectral evolution from a self-absorbed to an optically thin state within two months, implying discrete jet ejection. We compare EP240305a with known classes of X-ray transients and find that it is unlikely to be associated with long-timescale transients such as jetted tidal disruption events or X-ray binaries. Its properties also disfavor a short-timescale stellar flare origin. Although the absence of optical spectroscopy prevents a redshift determination, the source exhibits properties similar to those of gamma-ray-dark gamma-ray burst-like transients, which may be associated with relativistic jets viewed off-axis or with choked jets. The discovery of EP240305a, along with other uncataloged transients detected by the Einstein Probe, underscores the scientific potential of highly sensitive X-ray survey telescopes and rapid-response multiwavelength follow-up observations in exploring the nature of atypical astronomical transients.

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.

Wind is a key mechanism for supermassive black hole (SMBH) feedback to their host galaxies. In tidal disruption events (TDEs), black holes spend most of their time accreting at highly sub-Eddington rates, implying that feedback from persistent sub-Eddington winds could be significant. We investigate the effects of black hole mass, viscosity parameter, and stellar debris temperature on the properties of winds from hot accretion flows in TDEs. We find that more massive black holes yield a higher accreted fraction and launch faster winds, while the debris temperature has a negligible influence on the accretion flow. For α = 0.1, the mildly relativistic unbound winds (∼0.1 c ) are launched predominantly from the outside of the accretion flows along the equatorial plane, with a kinetic energy of ∼10 ⁻⁴ L _Edd . In contrast, convective bound outflows dominate for α = 0.01, which differs from the true winds typically seen in active galactic nucleus and X-ray binaries. Potential applications for explaining delayed radio brightening in TDEs at ∼10 ³ days and for searching for intermediate-mass black holes through radio and X-ray surveys are also discussed.

We report the results of a simultaneous X-ray and optical spectroscopy campaign on the Galactic black hole X-ray binary (BH XRB) V4641 Sgr, carried out with XRISM and the Seimei telescope during a low-luminosity phase toward the end of its 2024 outburst. Despite a very low X-ray luminosity of 10 ³⁴ erg s ⁻¹ , the continuum spectrum is well reproduced by a disk blackbody model with a high inner disk temperature (1.8 keV). XRISM/Resolve provides the highest resolution X-ray spectrum ever obtained from the source. Several strong, narrow emission lines have been detected, resolved, and characterized at a high significance. The continuum shape and narrow emission lines both indicate that the inner disk region is obscured by the surrounding high-density gas, while the intrinsic luminosity is at least one order of magnitude higher. In the simultaneous optical observation from the Seimei telescope, the line features are largely dominated by the optical companion. Although we detected a clear emission component in H α that could originate from a cold outflow or the disk atmosphere, there are no signs of the strong outflow signatures historically detected in this source. In X-rays, the combination of significantly redshifted (∼700 km s ⁻¹ ) and weakly blueshifted (∼ − 250 km s ⁻¹ ) components, all varying strongly on kilosecond timescales, along with a marginally significant (99.2%) highly blueshifted (∼ − 1300 km s ⁻¹ ) component, indicating a complex, inhomogeneous outflow geometry. This is corroborated by the erratic long-term evolution of the source seen in the complementary X-ray monitoring, as well as radio detections spanning several orders of magnitude.

We report the discovery and multiwavelength characterization of the Galactic transient EPJ174942.2–384834, first detected by the Einstein Probe during a faint X-ray outburst in 2025 March. Coordinated follow-up observations revealed two major outbursts and a rebrightening over a 7 month period. Broadband X-ray spectral modeling shows that the outburst emission was dominated by thermal Comptonization of very soft seed photons. The absence of a detected thermal disk component, together with the low inferred seed-photon temperature, is consistent with a cool and possibly truncated accretion disk. The X-ray spectrum remained consistently hard throughout the outburst activity, with a power-law photon index of Γ ≈ 1–2, gradually softening as the flux declined. The optical/UV counterpart brightened in tandem with the X-ray emission and exhibited a blue continuum with broad Balmer absorption features. Together with the optical/UV–X-ray luminosity correlation, this supports a disk-dominated origin of the optical/UV outburst emission, with viscous heating likely playing a major role and irradiation possibly contributing, especially in the UV. No radio counterpart was detected, implying at most very faint jet activity. Taken together, the observed properties support the classification of EPJ174942.2–384834 as a very faint X-ray transient black hole candidate. This study demonstrates the ability of the Einstein Probe to uncover and characterize the faintest accreting compact objects in the Galaxy.

EP J171159.4−333253 is a new neutron star low-mass X-ray binary discovered in outburst by the Einstein Probe (EP) on 2025 June 23, exhibiting clocked type I X-ray bursts, eclipses, and dips. In this paper, we report on the results of the X-ray spectral and timing analyses for EP J171159.4−333253 using data collected by EP and NuSTAR during the first 21 days of the outburst. The X-ray burst recurrence time can be characterized over a subset of nine bursts spanning 1.6 days around the NuSTAR observation, and the result is t _rec = 8196 ± 177 s, with indications of a possible decreasing trend. From the X-ray eclipse events, the binary orbital period and the eclipse duration are estimated to be P _orb = 6.48301 ± 0.00003 hr and D ⋆ , X = 1245 . 5 − 6.5 + 6.9 s, respectively. These enable an estimate of the mass and radius of the companion star and the binary inclination, which are M ₂ ≈ 0.6–0.8 M _⊙ , R ₂ ≈ 0.7–0.8 R _⊙ , and i ≈ 73°–75°, respectively. We also report on joint ULTRACAM and EP observations on 2025 July 21–22, detecting the source optical counterpart and covering an eclipse in both X-ray and optical bands. The optical eclipse is wavelength dependent and broader than in X-rays, indicating that part of the optical emission arises from an extended region in the accretion flow. Despite a moderate variation in the source flux, the properties of the persistent X-ray emission are typical of a hard spectral state. We further evaluated the ratio of the accretion energy to the thermonuclear energy to be 120–130, implying helium bursts with the accreted hydrogen being depleted in between bursts.

Context . Detecting faint and cross-shaped sources in long-exposure images from lobster-eye X-ray telescopes is particularly challenging due to the low signal-to-noise ratio (S/N), complex noise, and unique point spread function (PSF) of these instruments. Conventional deep learning networks struggle in such conditions as they fail to effectively incorporate astrophysical priors such as photon arrival time, energy, and PSF morphology. Furthermore, the significant flux-scale differences between faint and bright sources complicate feature extraction in the standard feature pyramid network (FPN). Aims . The goal is to propose a PSF-guided multi-scale detection framework that integrates prior physical features and is specifically designed for the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP). Methods . Our approach began by constructing three-channel inputs, combining conventional grayscale images with photon energy and arrival time data. We then introduced a PSF-guided morphological convolution module (PSFConv) and a multi-kernel multi-scale FPN (MKMS-FPN) to enhance the extraction of morphological features and improve multi-scale target perception. Additionally, we integrated high-level convolutional features with prior physical statistics within a multibranch binary classifier of our detection network to boost both detection sensitivity and interpretability. Results . Using simulated long-exposure WXT data and in-orbit EP observations, our framework achieves 90% precision with recall rates of 0.85 and 0.95 for sources with flux >2 mCrab (i.e. 3.21 × 10 ⁻¹¹ erg cm ⁻² s ⁻¹ , unabsorbed, 0.5-4 keV) and flux >3 mCrab, respectively. At moderate precision levels, the framework further doubles the number of detectable sources with flux < 1 mCrab compared to conventional CNNs and the SExtractor pipeline. Conclusions . The integration of physical priors with deep convolutional features demonstrates a robust detection performance across diverse flux regimes. The proposed framework offers a practical reference for data processing in future lobster-eye X-ray missions, promising enhanced sensitivity for detecting faint sources in challenging astrophysical observations.