Alessio Marino

Context. 4U 1822–37 is a high-inclination eclipsing neutron-star low-mass X-ray binary and a prototype accretion-disc corona (ADC) source, in which the direct inner emission is largely obscured and the observed X-ray flux is dominated by scattered radiation. X-ray polarimetry provides a direct probe of this scattering geometry. Aims. We investigated the geometry of 4U 1822–37 through its first X-ray spectro-polarimetric observation, testing the high-inclination ADC scenario with simultaneous broadband spectroscopy and polarimetry. Methods. We analysed coordinated IXPE , XMM-Newton , NuSTAR , and Swift observations. We performed broadband spectral modelling and model-independent and spectro-polarimetric analyses of the IXPE data, including energy and orbital phase-resolved polarimetry. Results. The broadband spectrum is described by a soft thermal component, a Comptonised continuum, a hard power-law tail, and relativistically blurred reflection. The observed luminosity, L _obs ≃ 6.1 × 10 ³⁶ ergs ⁻¹ , is well below the intrinsic luminosity expected from the orbital and spin evolution, consistent with the presence of an extended, optically thin corona that scatters only a fraction of the intrinsic radiation into the line of sight. In the 2–8 keV band, IXPE measures a polarisation degree (PD) of 7.9 ± 0.6% and a polarisation angle (PA) of −24° ±2°. The PD increases with energy, while the PA remains approximately constant. Spectro-polarimetric modelling confirms the energy trend, although the relative contributions of the Comptonised continuum and reflection remain degenerate. The orbital-phase-resolved analysis suggests a lower PD during an eclipse, PD = 5.5 ± 1.7%, with no significant PA change or major broadband-continuum variation above 2 keV. Conclusions. The high polarisation degree, the stable polarisation angle, the increase in PD with energy, and the decrease in PD during an eclipse all support the view that 4U 1822–37 is observed in an extreme high-inclination, scattering-dominated regime. In this picture, the extended corona plays a central role in shaping the observed X-ray emission and its polarisation properties.

Understanding the Galactic population of isolated neutron stars within a unified framework provides key insights into their birth properties, evolutionary pathways, and the connections between different neutron star classes. In this work, we aim to reproduce both the observed radio and quiescent X-ray emission from isolated neutron stars, including radio pulsars, magnetars, and X-ray-dim isolated neutron stars (XDINSs). We develop a comprehensive population-synthesis framework that models neutron star birth properties, their dynamical, rotational, and magnetothermal evolution, as well as radio and X-ray emission, and the selection effects of corresponding surveys. To simulate realistic X-ray spectra, we account for magnetospheric resonant cyclotron scattering and interstellar absorption. Additionally, we model the observational bias introduced by magnetar outbursts by linking the outburst rate to magnetic stresses in the stellar crust. We then employ a simulation-based inference method, namely truncated sequential neural posterior estimation, to reconstruct the birth properties, such as the initial magnetic-field distribution. We find that the Galactic neutron star population can be described by a two-component log-normal distribution of birth magnetic fields with peaks at 5 × 10 ¹² G and 10 ¹⁴ G, respectively. We further find that the Galactic neutron star birth rate is around 3–4 per century. Our results help establish the contribution of neutron stars as central engines of powerful astrophysical transient phenomena, such as fast radio bursts, superluminous supernovae, and gamma-ray bursts.

Accreting millisecond pulsars (AMSPs) and transitional millisecond pulsars (tMSPs) are neutron star low-mass X-ray binaries that can evolve into recycled radio millisecond pulsars. In both types of systems, X-ray pulsations have been detected during phases of X-ray activity when matter accretion through a disc was turned on. On the other hand, when accretion stops, and these systems enter the quiescent low-luminosity X-ray state, and only tMSPs become visible as radio pulsars. Despite several attempts, radio pulsations have never been detected in quiescent AMSPs, except for IGR J18245−2452. We present the results of two observational campaigns performed on the AMSPs IGR J00291+5934 and MAXI J1957+032 with the Five-hundred-meter Aperture Spherical Telescope ( FAST ) in L band (1–1.5 GHz). The two sources have most likely been observed in quiescence, as suggested by the upper limits on their X-ray and optical flux obtained with Swift and the Las Cumbres Observatory, respectively. We performed a deep search for coherent periodicities in radio, but found no significant candidate signal at the known spin frequency of these sources or at other frequencies. Assuming a pulse duty cycle of 10%, we derive upper limits on the pulsed radio flux density of 3.3 μJy and 5.6 μJy for IGR J00291+5934 and MAXI J1957+032, respectively, which are the most stringent limits for any known persistent AMSP so far.

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 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. The Galactic black hole candidate MAXI J1810−222 was recently reported to exhibit a notable absorption spectral feature at around 1 keV in low-resolution X-ray spectra from detectors resembling charge-coupled devices. The feature is typically correlated with the spectral state of the source being stronger in the soft states, as often occurs in the typical Fe K winds of X-ray binaries (XRBs). However, the results have hinted at rather extreme wind velocities of up to ∼0.1 c . Aims. We requested and obtained an observation with XMM-Newton to take advantage of the ten-fold higher spectral resolution ( R = λ /Δ λ ∼ 200 − 400) provided by the RGS detector to resolve the lines and break the degeneracy between different models and interpretations. Methods. We applied state-of-the-art models of plasma in photoionisation equilibrium and multi-phase interstellar medium (ISM). We performed further comparisons with a re-analysis of NICER and NuSTAR data. Results. The XMM-Newton /RGS spectrum is consistent with the presence of a mildly relativistic wind, confirming the earlier indications obtained with NICER; however, it places tighter constraints on the outflow properties, with the lines being intrinsically broad. The data would then favour magnetically driven winds, although thermal effects might still contribute to mass loading. NuSTAR and XMM-Newton (EPIC) show a further hotter component, indicating a stratified or multi-phase outflow. Fe K spectra taken with calorimetric detectors (e.g. Resolve on XRISM) will enable a high-resolution view of the complex extreme outflow in this source and shed new light on outflow processes in XRBs.

Ultraluminous X-ray sources represent extreme super-Eddington accretion regimes, and a subset is now known to host highly magnetized neutron stars. However, direct observational probes of their surface magnetic fields remain scarce. In this Letter, we report the detection of a narrow X-ray absorption feature at 3.29 ± 0.02 keV in the XMM–Newton /EPIC-pn spectrum of NGC 4656 ULX-1. The source exhibits a hard ultraluminous state, while our timing analysis reveals a candidate pulsation at ∼0.9736 Hz, with a local significance of 5.5 σ and a pulsed fraction of ∼11%. The feature is robust against changes in continuum modeling and data-selection criteria, retaining a statistical significance of ≳3 σ in Monte Carlo simulations. Interpreting the absorption as a proton cyclotron resonant scattering feature implies a local magnetic field of B ∼ (6 − 7)×10 ¹⁴ G in the line-forming region. This value is consistent with strong magnetic fields anchored near the neutron star surface, even if the large-scale dipole is substantially weaker. Although we discuss electron cyclotron features and atomic transitions as possible alternatives, they appear to be less consistent with the observed phenomenology.

Context. Winds are an important ingredient in the evolution of X-ray binary (XRB) systems, particularly those at high accretion rates such as ultra-luminous X-ray sources (ULXs), because they may regulate the accretion of matter onto the compact object. Aims. Our aim is to understand the properties of ULX winds and their link with the source spectral and temporal behaviour. Methods. We performed high-resolution X-ray spectroscopy of the variable source NGC 55 ULX-1 to resolve emission and absorption lines as observed with XMM-Newton at different epochs. Optically thin plasma models were used to characterise the wind. Results. We confirmed and thoroughly strengthened previous evidence of outflows in NGC 55 ULX-1. The presence of radiative recombination signatures and the ratios of the fluxes of the emission lines favours photoionisation balance and low to moderate densities, which confirm that the lines originate from classical XRB disc winds. An in-depth parameter space exploration shows line emission from a slowly moving, cool, and variable plasma perhaps associated with a thermal wind. Mildly relativistic Doppler shifts (about −0.15 c ) associated with the absorption lines confirm, at higher confidence, the presence of powerful radiatively driven winds. Conclusions. The comparison between the results obtained at different epochs revealed that the wind responds to the variability of the underlying continuum. These variations may be used to understand the actual accretion regime and the nature of the source.