Maurizio Falanga

Modeling the observed emission from highly magnetized accreting neutron stars is essential for interpreting their X-ray spectra and variability. This task is challenging due to the dynamic, multidimensional nature of the accretion columns and the strong gravitational field. Radiation propagation outside the column, within the neutron star's gravitational field, significantly shapes the observed emission and its variability with rotational phase. Geometry -- the location of the magnetic poles and the observer’s inclination -- is one of the key factors influencing the observables due to the anisotropy of the problem. In this work, we study how visibility effects in the gravitational field of the neutron star influence the direct emission from an accretion column, highlighting their importance when interpreting plasma parameters from observed spectra. Building on the physical model from the previous paper in this series, we investigate the directly observable X-ray flux from one and two columns with fan-beam wall emission, down-boosted by the bulk flow, focusing on the impact of geometry. We find that the observed flux strongly depends on the geometrical setup, which we illustrate across various observables. Particularly notable are special geometries in which a column on the far side of the neutron star aligns with the line of sight. In these cases, shadowing and strong light bending produce a narrow intense peak in pulse profiles at soft energies and a pronounced dip at higher energies (≳ 20, ). The combination of shadowing and height dependence of emission also leads to spectral softening. Flux-derived luminosities can be overestimated by up to a factor of 10. The fundamental cyclotron resonant scattering feature (CRSF) appears predominantly in emission across geometries, and we discuss possible causes. keV Our results demonstrate that geometry has a major impact on pulse profiles, phase-averaged spectra, phase–energy maps, and the anisotropy factor relevant for luminosity estimates. Future studies should include the reflected component in addition to the direct emission considered here, and account for a more accurate treatment of resonant redistribution in the CRSF-forming region.

Accretion-powered X-ray pulsars show complex spectral and timing behavior that reflects the extreme conditions near the neutron star. Modeling these observables remains challenging, particularly at high mass-accretion rates, dot M 17 , , ^ g s -1 , where the extended accretion column plays a role in shaping anisotropic emission seen by a remote observer. Here, we present a modular framework that links radiation processes inside the column to the emission observed at infinity. The framework combines existing models for continuum and cyclotron resonant scattering feature (CRSF) formation in sidewall column emission with ray tracing, thereby bridging the gap between the rest-frame emission and the observable flux. We model the radiation from the accretion column using continuum emission from the column walls, calculated in the regime of saturated Comptonization in the presence of a radiation-dominated shock. This emission then passes through a thin outer layer with a fast-moving bulk flow, where CRSFs, derived from Monte Carlo simulations, are imprinted. Finally, a geometry-independent ray-tracing code accounts for light bending in the Schwarzschild metric, incorporating the shape and location of the emitting regions and neutron star rotation to compute the observed phase- and energy-dependent flux. We present the height-dependent anisotropic emission in the column rest frame. For the chosen model, approximately 70% of the radiation is emitted within 1, of the base of the column, predominantly at large angles to the magnetic field. Relativistic boosting in the bulk flow affects the appearance of CRSFs in the spectra, leading to a strong dependence of the line locations on the angle relative to the magnetic field. The proposed theoretical framework allows a straightforward combination of internal emission models with ray tracing to study the observed flux. Here, we focus on the joint treatment of the two-dimensional column structure and CRSF formation in the neutron star rest frame. The observed phase and spectral variations of the flux are highly sensitive to the location of the emitting regions and the observer's viewing angle, which is explored in an accompanying paper.

Understanding accretion components in neutron star low-mass X-ray binary systems is important to probe fundamental aspects of the accretion mechanism and evolution of the system, constraining its physical properties. Here, we present spectral and aperiodic timing analyses of the Neutron star Interior Composition Explorer and AstroSat data from the accretion-powered millisecond X-ray pulsar SAX J1808.4−3658 during its 2022 outburst. We find that emissions from a softer accretion disk and a harder, centrally located, compact, partially covering, Comptonizing corona explain the continuum spectra from the source throughout the outburst. The disk inner edge temperature, the coronal electron temperature, and photon index are found to be around ∼0.5−0.9 keV, a few keV, and ∼1.1–1.8, respectively, during the entire outburst. We also find an intrinsic atomic hydrogen medium in the system, which substantially and systematically evolved throughout the outburst. We detect two broadband aperiodic features (∼0.004–2 Hz; ∼10–100 Hz), with the former having a significant hard lag of ∼11 ms between 1.5–10.0 and 0.5–1.5 keV photons. We conclude that both the disk photons and the photons upscattered by the corona contributed to each aperiodic feature, with the disk and the corona contributing more to the low and high-frequency ones, respectively.

SRGA J144459.2−604207 is a newly confirmed accreting millisecond X-ray pulsar and type I X-ray burster. We present the broadband X-ray timing and spectral behaviors of SRGA J144459.2−604207 during its 2024 outburst. The data were collected from NICER, Einstein Probe, Imaging X-ray Polarimetry Explorer (IXPE), Insight Hard X-ray Modulation Telescope (Insight-HXMT), NuSTAR, and INTEGRAL observations. X-ray pulsations have been detected for the 1.5–90 keV energy range throughout the “ON” phase of the outburst from MJD ∼ 60355–60385. We refined the orbital and spin ephemerides assuming a circular orbit, and found that the pulsar was in a spin-up state during MJD ∼ 60361–60377 showing a significant spin-up rate ν ̇ of (3.15 ± 0.36) × 10⁻¹³ Hz s⁻¹. Around MJD ∼ 60377 a swing was detected in the spin evolution accompanied by significantly enhanced pulsed emission. We studied the pulse-profile morphology during the X-ray bursts as observed by Insight-HXMT, IXPE, and NuSTAR. During the bursts, pulsations were detected across the 2–60 keV with shapes broadly consistent with those observed for the persistent emission. We found, however, that the “burst” pulse profiles exhibit significant phase offsets relative to the pre- and postburst profiles. These offsets systematically decrease with increasing energy, Δϕ ≈ 0.15, 0.11, and 0.02 for IXPE, Insight-HXMT ME, and HE in 2–8, 5–30, and 20–60 keV, respectively, and Δϕ ≈ 0.21, 0.10, and 0.07 for NuSTAR in 3–10, 20–35, and 35–60 keV, respectively, compared to the pre- and postburst profiles. We performed a joint spectral analysis of quasi-simultaneous NICER, NuSTAR, and Insight-HXMT data for two epochs. The resulting spectra from both observations were consistent and well described by an absorbed thermal Comptonization model, nthcomp, plus relativistic reflection, relxillCp.

We analyze the emission and absorption lines during photospheric radius expansion (PRE) X-ray bursts from the ultracompact binary 4U 1820–30, observed with the Neutron Star Interior Composition Explorer (NICER). Using Monte Carlo simulations to estimate the significance, we identified a 1 kiloelectron volt (keV) emission line from 14 bursts, a 3 keV absorption line from 12 bursts, and a 1.6 keV absorption line from one burst. By coadding the burst spectra at the maximum radius phase, we detected a 1.034 keV emission line with significance of 14.2σ, along with absorption lines at 1.64 and 3 keV, with significances of 10.8σ and 11.7σ, respectively. The observed energy shifts are consistent with the prediction from the burst-driven wind model, indicating that all three spectral features are produced by the PRE wind. An analysis of the ratios between the emission and absorption line energies suggests that the 1 keV feature is a superposition of several narrower Fe L-shell lines. To evaluate the scientific capabilities of the Hot Universe Baryon Surveyor (HUBS), we simulated mock observations of multiple narrow lines near 1 keV. These results demonstrate that HUBS is well suited for detailed studies of the 1 keV emission line during bursts, offering significant potential to advance our understanding of these phenomena.

We report the superburst from 4U 1820–30 in 2021 observed by the Monitor of All-sky X-ray Image and Neutron star Interior Composition Explorer (NICER). During the tail of the superburst, we found that the NICER light curve unexpectedly increased from 1080 to 2204 counts s⁻¹ over 6.89 hr. From the time-resolved superburst spectra, we estimated the burst decay time of ≈2.5 hr, the ignition column depth of ≈0.3 × 10¹² g cm⁻², the energy release per unit mass of ≈2.4 × 10¹⁷ erg g⁻¹, the fluence of ≈4.1 × 10⁻⁴ erg cm⁻², and the total energy release of ≈3.5 × 10⁴² erg. Notably, we found a gradual increase in the Comptonization flux from 8.9 × 10⁻¹⁰ erg s⁻¹ cm⁻² to the preburst level during the superburst. This increase can be interpreted as a consequence of superburst radiation depleting the inner accretion disk, leading to a near-complete quenching of the persistent emission. As the burst radiation decayed, the inner accretion disk gradually returned to its preburst state, as evidenced by the best-fit spectral parameters. Additionally, we observed a prominent absorption line that exhibited a gravitational redshift, shifting from 4.15 to 3.62 keV during the recovery phase of persistent emission. This absorption feature likely originates from the inner accretion disk rather than from burst emission on the neutron star (NS) surface. The observed changes in the absorption line energy suggest that the inner disk approached the NS to a distance as close as ≈17 km.

We report an analysis of Insight-HXMT observations of the newly discovered accreting millisecond pulsar SRGA J144459.2–604207. During the outburst, detected in 2024 February by SRG/ART-XC, the broadband persistent spectrum was well fitted by an absorbed Comptonization model. We detected 60 type I X-ray bursts in the Insight-HXMT medium energy (ME) data, and 37 were also detected with the low-energy (LE) telescope. By superimposing the Insight-HXMT/LE/ME/HE light curves of 37 bursts with similar profiles and intensities, we measured a deficit of X-rays in the 40–70 keV energy band. By analyzing the time-resolved X-ray burst spectra, we determine the mean ratio of persistent to burst flux ofα = 71 ± 7. We estimate the average hydrogen mass fraction in the fuel at ignition, as X ¯ = 0.342 ± 0.033 , and constrain the burst fuel composition as X ₀ ≲ 0.4. We found that 14 out of 60 X-ray bursts exhibited photospheric expansion, and thus we estimated the distance to the source as 10.03 ± 0.71 kpc. Combined with IXPE observations, the burst recurrence time increased from 1.55 to 8 hr as the local mass accretion rate decreased, which can be described as Δ T rec ∼ m ̇ − 0.91 ± 0.02 .

The Lunar Gravitational-wave Antenna (LGWA) is a proposed array of next-generation inertial sensors to monitor the response of the Moon to gravitational waves (GWs). Given the size of the Moon and the expected noise produced by the lunar seismic background, the LGWA would be able to observe GWs from about 1 mHz to 1 Hz. This would make the LGWA the missing link between space-borne detectors like LISA with peak sensitivities around a few millihertz and proposed future terrestrial detectors like Einstein Telescope or Cosmic Explorer. In this article, we provide a first comprehensive analysis of the LGWA science case including its multi-messenger aspects and lunar science with LGWA data. We also describe the scientific analyses of the Moon required to plan the LGWA mission.

We present our findings from the first long X-ray observation of the hard X-ray source Swift J0826.2−7033 with XMM-Newton, which has shown characteristics of magnetic accretion. The system appears to have a long orbital period (∼7.8 h) accompanied by short timescale variabilities, which we tentatively interpret as the spin and beat periods of an intermediate polar. These short- and long-timescale modulations are energy-independent, suggesting that photoelectric absorption does not play any role in producing the variabilities. If our suspected spin and beat periods are true, then Swift J0826.2−7033 accretes via disc-overflow with an equal fraction of accretion taking place via disc and stream. The XMM-Newton and Swift-BAT spectral analysis reveals that the post-shock region in Swift J0826.2−7033 has a multi-temperature structure with a maximum temperature of ∼43 keV, which is absorbed by a material with an average equivalent hydrogen column density of ∼1.6 × 10²² cm⁻² that partially covers ∼27% of the X-ray source. The suprasolar abundances, with hints of an evolved donor, collectively make Swift J0826.2−7033 an interesting target, which likely underwent a thermal timescale mass transfer phase.

We report on the broadband spectral and timing properties of the accreting millisecond X-ray pulsar IGR J17498−2921 during its April 2023 outburst. We used data from NICER (1–10 keV), NuSTAR (3–79 keV), Insight-HXMT (2–150 keV), and INTEGRAL (30–150 keV). We detected significant 401 Hz pulsations across the 0.5–150 keV band. The pulse fraction increases from ∼2% at 1 keV to ∼13% at 66 keV. We detected five type-I X-ray bursts, including three photospheric radius expansion bursts, with a rise time of ∼2 s and an exponential decay time of ∼5 s. The recurrence time is ∼9.1 h, which can be explained by unstable thermonuclear burning of hydrogen-deficient material on the neutron star surface. The quasi-simultaneous 1–150 keV broadband spectra from NICER, NuSTAR and INTEGRAL can be aptly fitted by an absorbed reflection model, relxillCp, and a Gaussian line of instrumental origin. The Comptonized emission from the hot corona is characterized by a photon index Γ of ∼1.8 and an electron temperature kT_e of ∼40 keV. We obtained a low inclination angle i ∼ 34°. The accretion disk shows properties of strong ionization, log(ξ/erg cm s⁻¹)∼4.5, over-solar abundance, A_Fe ∼ 7.7, and high density, log(n_e/cm⁻³)∼19.5. However, a lower disk density with normal abundance and ionization could also be possible. Based on the inner disk radius of R_in = 1.67R_ISCO and the long-term spin-down rate of −3.1(2)×10⁻¹⁵ Hz s⁻¹, we were able to constrain the magnetic field of IGR J17498−2921 to the range of (0.9 − 2.4)×10⁸ G.