Yilong Wang

We report on the detection of polarized X-ray emission from the new Galactic magnetar EP J223759.5+531421, discovered by the Einstein Probe Wide-field X-ray Telescope on 2026 June 28. The Imaging X-ray Polarimetry Explorer (IXPE) follow-up observation started on 2026 July 6 and detected the source at a (absorbed) flux of ≈4.5 × 10 ⁻¹¹ erg cm ⁻² s ⁻¹ (2–7.5 keV range), confirming the presence of two thermally emitting regions on the star surface, with temperatures ≈0.5 and ≈1 keV. A significant (at >4 σ confidence level) phase- and energy-integrated polarization degree of ≈7% was detected with the polarization angle ≈−2 ^∘ East of the celestial North. The two thermal components exhibit quite different polarization properties. The difference is larger in the phase-folded data, with the polarization degree reaching ∼63% in the range 4–7.5 keV, in correspondence with the secondary peak of the pulse profile, and never exceeding ∼30% at lower energies. The polarization angle continuously oscillates from −90 ^∘ to +90 ^∘ over one rotational cycle and is well fit by the rotating vector model, with an inclination of the line of sight and of the magnetic dipole axis relative to the star spin axis of ≈28 ^∘ and ≈109 ^∘ , respectively. The data point to an emission geometry in which the warm thermal component could originate from two antipodal caps, possibly in a magnetically condensed state, while the hot one could come from a smaller region covered by an atmosphere.

Long-period transients (LPTs) challenge our knowledge of the mechanism producing radio periodic pulsations in compact objects. Some LPTs have been associated with systems hosting a white dwarf and a low-mass star in a detached binary. Recently, a new LPT (ASKAP J174508.9-505149) has been classified as an accreting cataclysmic variable (CV). In the present Letter, we report on the detailed study of the X-ray variability of ASKAP J174508.9-505149 as observed by XMM-Newton and Einstein Probe between September 2025 and May 2026. Simultaneous optical and radio observations are also presented. We studied the timing variability of the source, and estimated an X-ray periodicity of P = 4868(22) s, consistent with radio and optical periods. We also observe the same periodicity in the hardness ratio extracted from the XMM-Newton observation, peaking at the minimum of the modulation. A long-term modulation is also present in the X-rays and in the B-band photometry, but it is poorly constrained by the current dataset. Spectral X-ray analysis shows the presence of a black-body component (∼0.1 keV), a collisionally ionised plasma (∼15 keV), and an absorption feature at 0.77 keV (possibly due to Oxygen-VII). This is the third LPT detected in the X-ray band, the second with a detected X-ray periodicity and variable X-ray emission, and the first conclusively recognised as an accreting magnetic CV.

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.

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.

Accretion in black hole X-ray binaries is commonly believed to be supplied by the Roche lobe overflow or the stellar wind. The former is thought to form a geometrically thin disc while the diffuse wind could form a geometrically thick hot accretion flow. In this paper, we instead consider a more generalized case, i.e. accretion with both cold and hot gas supplies, which feed a disc and a corona, respectively. We investigate the interaction of disc and corona by analysing the energy coupling and matter exchange, i.e. corona condensation/disc evaporation, with a semi-analytical method. It is found that the accretion geometry in the radial direction and the resultant emission spectrum depend strongly on both the total gas supply rate and the ratio of cold and hot gases. For gas supply rates of a few per cent of the Eddington value, diverse geometries and spectral shapes are possible, depending on the fraction of cold gas supply. This provides an interpretation for the various spectra observed in intermediate states. However, at higher accretion rates, regardless of the form of the feeding gas, the inner accretion flow is always disc-dominated, implying an inevitable transition to the soft state, while at very low gas supply rates, hard state spectrum dominated by the hot flow is expected. We also present the predicted hardness–intensity correlation of Cygnus X-1, and constrain the value of the viscosity parameter of the accretion flow to the range of 0.25–0.35 by comparing our results with MAXI observations.

Accretion in black hole X-ray binaries is commonly believed to be supplied by the Roche lobe overflow or the stellar wind. The former is thought to form a geometrically thin disc while the diffuse wind could form a geometrically thick hot accretion flow. In this paper, we instead consider a more generalised case, i.e., accretion with both cold and hot gas supplies, which feed a disc and a corona respectively. We investigate the interaction of disc and corona by analysing the energy coupling and matter exchange, i.e. corona condensation/disc evaporation, with a semi-analytical method. It is found that the accretion geometry in the radial direction and the resultant emission spectrum depend strongly on both the total gas supply rate and the ratio of cold and hot gases. For gas supply rates of a few percent of the Eddington value, diverse geometries and spectral shapes are possible, depending on the fraction of cold gas supply. This provides an interpretation for the various spectra observed in intermediate states. However, at higher accretion rates, regardless of the form of the feeding gas, the inner accretion flow is always disc-dominated, implying an inevitable transition to the soft state, while at very low gas supply rates, hard state spectrum dominated by the hot flow is expected. We also present the predicted hardness-intensity correlation of Cygnus X-1, and constrain the value of the viscosity parameter of the accretion flow to the range of 0.25--0.35 by comparing our results with MAXI observations.