Rosario Iaria

Context. Ultraluminous X-ray sources (ULXs) are thought to be powered, in many cases, by super-Eddington accretion onto compact objects. While soft X-ray lags, commonly interpreted as reverberation in optically thick outflows, have been detected in several ULXs, hard lags remain rare and poorly understood. Aims. We investigate the temporal and energy dependence of X-ray lags in NGC 7456 ULX-1 to constrain their physical origin and probe the structure of the super-Eddington accretion flow. Methods. We analysed the two deepest XMM-Newton observations of NGC 7456 ULX-1, taken in 2018 and 2023. Hard (1–10 keV) light curves were cross correlated with soft (0.3–1 keV) light curves using adaptive-binning techniques optimized for Poissonian low-count data. Time-resolved lag measurements were performed over consecutive 10 ks intervals, and the energy dependence of the delays was investigated. Broadband spectra were modelled using multi-component thermal and Comptonization models. Results. We detect significant hard X-ray lags in both observations, with the 1–10 keV emission delayed by ∼10 ³ s relative to the soft band during phases of rapid flux variability. The energy-resolved analysis shows that the delays are primarily driven by the lowest-energy photons. Spectral modelling indicates a Comptonization-dominated super-Eddington flow comprising a cooler, extended outer region and a hotter, compact inner flow embedded in an optically thick wind. We interpret the observed delays as the combined effect of inward propagation of accretion-rate fluctuations and photon diffusion within the dense outflow. In this scenario, fluctuations first enhance the soft-emitting outer regions and subsequently propagate towards the hotter inner flow, where photons undergo stronger Comptonization before escaping with a kilosecond delay. The relatively small inferred inner emitting radius disfavours an intermediate-mass black hole accretor, while the observed delays imply diffusion through an extended optically thick wind. Conclusions. The sign, amplitude, and energy dependence of the observed delays disfavour a standard reverberation scenario. Instead, propagation-driven variability coupled with radiative transfer in optically thick winds appears to play a major role in shaping the timing properties of super-Eddington accretion flows.

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.

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.

The High Energy Rapid Modular Ensemble of Satellites (HERMES) instrument is a compact X-/gamma-ray spectrometer operating on board the 6U (11 kg) Space Industry–Responsive–Intelligent–Thermal (SpIRIT) CubeSat. The payload is particularly well suited for the observation of cosmic transients such as gamma-ray bursts (GRBs) and bright pulsars thanks to its unique broadband sensitivity from a few keV to a few MeV and temporal resolution down to half a microsecond. We report the detection of the ∼33 ms double-peaked pulse profile of the Crab Pulsar, obtained by considering the canonical Crab ephemerides as provided by the Jodrell Bank catalog. We collected approximately 5.7 × 10 ⁴ photons from 730 s of observations in the 3 keV–2 MeV energy band during a single operation, achieving a 5 σ pulse-profile significance in the 3–11.5 keV energy band with millisecond-scale binning. These results demonstrate that SpIRIT/HERMES can achieve millisecond timing accuracy at high energies and, thanks to its wide field of view and broad energy coverage, has the potential to contribute to GRB monitoring in the near future. Such capabilities were previously the domain of flagship observatories, underscoring the performance of the HERMES instrument even with its compact form factor.

Context. MAXI J1957+032 is an accreting millisecond X-ray pulsar that shows brief, recurrent outbursts within an ultra-compact ≈1 h orbit. Aims. We characterised the X-ray timings and spectral and optical properties during the 2025 outburst and measured the long-term spin evolution relative to its previous 2022 outburst. Methods. We analysed X-ray observations from XMM-Newton , Swift , and NuSTAR , together with contemporaneous optical photometry obtained with LCO during the 2025 outburst. X-ray timing analysis included standard epoch-folding and coherent searches, while energy-resolved pulse profiles were studied through harmonic decomposition. Spectral fits used absorbed thermal–Comptonisation models complemented with a soft blackbody component, whose emission radius suggests that the blackbody emission originates from a hotspot on the neutron star surface. Results. Coherent pulsations were detected at ν ≈ 313.6 Hz, with no measurable frequency derivative within the XMM-Newton exposure. Via comparisons with the 2022 outburst, we find a long-term spin-down of ⟨ ν ˙⟩ ∼ −2 × 10 ⁻¹⁴ Hz s ⁻¹ , consistent with magnetic dipole braking during quiescence. The pulse shape is almost sinusoidal, showing significant power at the fundamental, second, and fifth harmonics. The fractional amplitude decreases with increasing flux and exhibits soft lags extending to a few keV. The X-ray spectrum between 0.5 and 10 keV is well reproduced by a thermal–Comptonised continuum with photon index Γ ≈ 2.4, plus a cool blackbody with kT ≈ 0.23 keV. No reflection or Fe-line features are detected. Assuming R _m ≲ R _co , the magnetic field is limited to B _s ≈ (0.5–3)×10 ⁸ G (for d = (5 ± 2) kpc and truncation factor ξ = 0.3–0.5), lower than the upper limit implied by the secular spin-down ( B _p ≲ 10 ⁹ G), possibly indicating a mildly leaky propeller regime. The optical and infrared (OIR) emission follows the neutron-star branch of the L _OIR – L _X relation, consistent with X-ray reprocessing in a compact accretion disc. The optical spectral energy distributions are broadly flat, supporting irradiation-dominated disc emission, and an early red excess suggests a jet contribution during the initial hard X-ray phase. A delayed optical peak relative to the X-rays may reflect the outward propagation of a heating front through the disc, consistent with rapid disc evolution in short-lived outbursts.

A mysterious absorption feature at approximately 3.8 keV has been identified in the Neutron star Interior Composition Explorer (NICER) spectrum of the low-mass X-ray binary system 4U 1820-30. We interpret this feature as a gravitationally redshifted iron absorption line. This interpretation is supported by the temporal proximity of the NICER observation to the detection of a carbon superburst—a long and intense thermonuclear flash on the neutron star’s surface—by the X-ray monitor MAXI, suggesting that the presence of the line is associated with this rare and extreme event. From the inferred redshift of the absorption line, the compactness of the neutron star can be derived. Fitting this feature with a photoionization absorption model, we measure a redshift of 1 + z ≃ 1.72, which implies a neutron star compactness of R / M = 4.46 ± 0.13 km/ M _⊙ or 3.02 ± 0.09 in dimensionless units. This unique feature highlights the importance of further observations and detailed modelling, offering promising insights into the equation of state of matter under extreme density conditions.

A mysterious absorption feature at approximately 3.8 keV has been identified in the NICER spectrum of the low-mass X-ray binary system 4U 1820-30. We interpret this feature as a gravitationally redshifted iron absorption line. This interpretation is supported by the temporal proximity of the NICER observation to the detection of a carbon superburst by the X-ray monitor MAXI, suggesting that the presence of the line is associated with this rare and extreme event. From the inferred redshift of the absorption line, the compactness of the neutron star can be derived. Using a photoionization absorption model, we measure a gravitational redshift of about 1.72, which corresponds to a compactness R/M of 4.46 \pm 0.13 km per solar mass, or 3.02 \pm 0.09 in dimensionless units. This unique feature highlights the importance of further observations and detailed modelling, offering promising insights into the equation of state of matter under extreme density conditions.

The timing analysis of transient events allows for investigating numerous still open areas of modern astrophysics. The article explores all the mathematical and physical tools required to estimate delays and associated errors between two Times of Arrival (ToA) lists, by exploiting Cross Correlation Function (CCF) techniques. The CCF permits the establishment of the delay between two observed signals and is defined on two continuous functions. A detector does not directly measure the intensity of the electromagnetic signal (interacting with its material) but rather detects each photon ToA through a probabilistic process. Since the CCF is defined on continuous functions, the crucial step is to obtain a continuous rate curve from a list of ToA. This step is treated in the article and the constructed rate functions are light curves that are continuous functions. This allows, in principle, the estimation of delays with any desired resolution. Due to the statistical nature of the measurement process, two independent detections of the same signal yield different photon times. Consequently, light curves derived from these lists differ due to Poisson fluctuations, leading the CCF between them to fluctuate around the true theoretical delay. This article describes a Monte Carlo technique that enables reliable delay estimation by providing a robust measure of the uncertainties induced by Poissonian fluctuations. GRB data are considered as they offer optimal test cases for the proposed techniques. The developed techniques provide a significant computational advantage and are useful analysis of data characterized by low-count statistics (i.e., low photon count rates in c/s), as they allow overcoming the limitations associated with traditional fixed bin-size methods.

Aims. We aim to study the energy-dependent pulse profile of the X-ray accreting pulsar 4U 1538–52 and its phase-dependent spectral variability, with a particular emphasis on the behavior around the cyclotron resonant scattering feature at E_cyc ∼ 21 keV. Methods. We analyzed all available NuSTAR observations of 4U 1538–52. We decomposed the energy-resolved pulse profiles into Fourier harmonics to study their energy dependence. Specifically, we computed the pulsed fraction spectra, cross-correlation, and lag spectra, identifying discontinuities and linking them to features in the phase-averaged spectra. We performed both phase-averaged and phase-resolved spectral analyses to probe spectral variability and its relation to pulse profile changes. Finally, we interpreted our findings based on a physical modeling of the energy- and angle-dependent pulse profile emission, performing radiative transfer in a homogeneous slab-like atmosphere under conditions relevant to 4U 1538–52. The emission is projected onto the observer’s sky plane to derive the expected observables. Results. In contrast to the dips in pulsed fraction spectra observed in other sources (e.g., Her X-1), we find a broad bump near the cyclotron resonance energy in 4U 1538–52. This increase is driven primarily by phase-dependent spectral variability, especially by strong variations in cyclotron line depth across different phase intervals. We interpreted the observed contrast between dips and bumps in various sources as arising from phase-dependent variations of cyclotron line depth relative to the phase-modulated flux. We modeled the X-ray emission from an accreting neutron star and found that our simulations indicate high values of both the observer’s inclination and the magnetic obliquity, along with a ∼10 − 15° asymmetry between the locations of the magnetic poles. Assuming this geometry, we were able to adequately reproduce the observed pulse profiles and introduce general trends in the observables resulting from the system’s geometry.

We study the energy-dependent pulse profile of 4U 1538-52 and its phase-dependent spectral variability, with emphasis on the behavior around the cyclotron resonant scattering feature at around 21 keV. We analyze all available NuSTAR observations of 4U 1538-52. We decompose energy-resolved pulse profiles into Fourier harmonics to study their energy dependence. Specifically, we compute pulsed fraction spectra, cross-correlation and lag spectra, identifying discontinuities and linking them to features in the phase-averaged spectra. We perform phase-averaged and phase-resolved spectral analyses to probe spectral variability and its relation to pulse profile changes. Finally, we interpret our findings via physical modeling of energy- and angle-dependent pulse profile emission, performing radiative transfer in a homogeneous slab-like atmosphere under conditions relevant to 4U 1538-52. The emission is projected onto the observer's sky plane to derive expected observables. In contrast to the dips in pulsed fraction spectra observed in other sources (e.g., Her X-1), we find a broad bump near the cyclotron resonance energy in 4U 1538-52. This increase is driven primarily by phase-dependent spectral variability, especially by strong variations in cyclotron line depth across different phase intervals. We interpret the observed contrast between dips and bumps in various sources as arising from phase-dependent variations of cyclotron line depth relative to the phase-modulated flux. We model the X-ray emission from an accreting neutron star and find that our simulations indicate high values of both the observer's inclination and the magnetic obliquity, along with a 10-15 degrees asymmetry between the locations of the magnetic poles. Assuming this geometry, we satisfactorily reproduce the observed pulse profiles and introduce general trends in the observables resulting from the system's geometry.