Context . Outflows in active galactic nuclei (AGNs) are recognized as fundamental mechanisms driving the coevolution of supermassive black holes (SMBHs) and their host galaxies by regulating the gas reservoir for SMBH growth and host star formation. Although powerful outflows are frequently detected in gas-rich, active star-forming galaxies, their existence and potential impact within gas-poor, quiescent galaxies remain poorly understood. Aims . We report the first detection of a powerful ultrafast outflow (UFO) in a nearby quiescent galaxy KUG 1208+386, providing a multi-scale analysis of AGN winds from nuclear to galactic scales. Methods . We performed Bayesian-based X-ray spectroscopy of archival XMM-Newton and NuSTAR observations to characterize the circumnuclear materials and UFOs. Using optical spectra from the Dark Energy Spectroscopic Instrument ( DESI ), we constrained the kinematics of the galactic-scale outflows and star formation history of the host galaxy. Additionally, we derived galaxy properties, such as stellar mass ( M ★ ) and star formation rate (SFR), via multiwavelength spectral energy distribution fitting of photometric data. Results . We detect a nuclear X-ray UFO with a velocity of v out ≈ −0.07 c and a kinetic power of Ė UFO = (0.8-6.5) × 10 43 erg/s. This power is sufficient to drive effective AGN feedback ( Ė UFO / L Edd = (1-8)%) and far exceeds the galactic [OIII] outflow power ∼10 40 erg/s. Host galaxy analysis reveals a massive quiescent system (specific SFR ∼3 × 10 −12 yr −1 ) quenched ∼9 Gyr ago. The central AGN is obscured by a line-of-sight column density of log( N H LOS /cm −2 ∼ 23 and the circumnuclear scattering material is Compton-thick log(N H scatter /cm −2 ) = 24.7 −0.5 +0.8 , indicating a dense nuclear environment. Conclusions . The discovery of a nuclear UFO in a long-quenched massive galaxy challenges the paradigm that UFOs are exclusive to gas-rich, star-forming systems, suggesting that they are governed by the local circumnuclear environments rather than the global gas reservoir. Our results indicate that episodic, powerful winds can maintain the quiescent state of KUG 1208+386 over gigayear timescales, supporting a wind-driven “maintenance” mode of AGN feedback that is distinct from the classical jet mode. Future deep X-ray and submillimeter observations are essential for definitively characterizing the cold gas reservoir and the coupling efficiency among multiphase and multi-scale outflows.

X-ray binary accretion disk winds can carry away a significant fraction of the matter transferred from the companion and hence strongly affect the accretion flow and the long-term evolution of the binary. However, accurate mass outflow rate measurements are challenging due to uncertainties in our understanding of the 3D wind structure. Most studies employ absorption line spectroscopy, which only gives us a single sightline through the wind streamlines. Hercules X-1 is a peculiar X-ray binary which allows us to avoid this issue, as its warped, precessing accretion disk naturally presents a range of sightlines through the vertical structure of its disk wind. Here we present the first results from a large, coordinated campaign on Her X-1 led by the new XRISM observatory (with an exposure of 210 ks) and supported by XMM-Newton, NuSTAR, and Chandra. We perform a time-resolved analysis and constrain the wind properties. With XRISM/Resolve, we directly detect the Her X-1 orbital motion with an amplitude of 170 km s-1 in the evolution of the wind velocity. After correcting for this effect, we observe an increase in wind velocity from 250 to 600 km s-1 as the wind rises to greater heights above the disk. The wind column density decreases with increasing height, as expected, but its ionization parameter log(xi /erg cm s-1) evolves only weakly from 3.65 to 3.9 as the wind expands away. Additionally, we detect a new orbital dependence of the wind properties, revealing a likely second component that appears only briefly after the eclipse by the secondary star.

1ES 1927+654 is an extreme active galactic nucleus (AGN) that has defied our canonical expectations for how AGN appear across the electromagnetic spectrum and how they vary on short timescales. In 2022, this source began showing a X-ray quasi-periodic oscillation (QPO) at mHz frequencies, along with a newly launched radio jet. Unlike the handful of other known AGN QPOs, the QPO in 1ES 1927+654 showed a significant frequency evolution, spanning from 0.9-2.4 mHz from 2022-2024. In this work, we present the last 1.5 years of monitoring with XMM-Newton (250 ks) up to January 2026, which reveals that the QPO persists but has plateaued at a constant frequency of approximately 2.5 mHz. We perform detailed spectral-timing analyses on this exquisite dataset, consisting of over 900 QPO cycles, more than any AGN QPO to date. Our main findings are: (1) the stacked XMM-Newton power spectra shows no significant second harmonic, (2) a soft (reverberation-like) lag is observed at all frequencies and remains remarkably stable even as the QPO frequency evolved from 2022-2024, and (3) extreme X-ray jumps on the QPO period (up to 80

The study of X-ray pulsar accretion columns helps us characterize accretion physics in this extreme regime of strong gravity and strong magnetic fields. Previous observations of the X-ray pulsar Hercules X-1 revealed a highly broadened Fe K emission line, associated with Doppler motions exceeding 0.1c, suggesting its origin in the accretion column. We obtained a high-spectral resolution view of the Fe K energy band of Hercules X-1 thanks to a 200 ks observation with the XRISM observatory. The XRISM/Resolve microcalorimeter spectra allow us to separate the different spectral components and accurately model them with phenomenological models. We confirm the presence of a broad line near 6.5 keV with a typical 1σ width of 1 keV. Performing a pulse-phase-resolved analysis, we find that the feature is strongly variable with Her X-1 pulse phase. This is consistent with the proposed origin due to collisional recombination or by reprocessing of the primary X-ray emission in the accretion column, where strong variability with pulse phase is expected due to the rotation of the columns alongside with the neutron star. Additionally, the Fe K line pulsation pattern evolves with the 35-day cycle of Hercules X-1, supporting the scenario that the neutron star and its accretion columns undergo precession, in agreement with recent polarimetric results from the IXPE observatory. We discuss the future applications of modeling of this broad line in X-ray pulsars with physical spectral models. This could be used to detect and track neutron star precession, advancing our understanding of neutron star interiors.

Context. Pulse profiles probe the emission geometry of accreting X-ray pulsars, but their observed shapes may depend on instrumental response and observational setup. The pulsed fraction spectrum provides a compact spectro-timing observable that can both trace localized spectral features and serve as a quantitative cross-calibration diagnostic. Aims. We assess the consistency of energy-resolved pulse profiles obtained with simultaneous XMM-Newton/EPIC-pn and NuSTAR/FPM observations of Vela X-1 and investigate the broadband pulsed fraction spectrum as a diagnostic of spectral features from 1 to 70 keV. Methods. We constructed energy–phase matrices for both instruments and derived pulsed fraction spectra after carefully accounting for instrumental and observational effects. We quantified the residual systematics in the overlapping 3–10 keV band. We then modeled the broadband pulsed fraction spectra phenomenologically and searched for timing signatures of spectral features. Results. After correcting for instrumental effects, the pulsed fraction spectra derived strictly over the common exposure intervals of the two instruments agree within 5% in their overlapping 3–10 keV range. Remaining discrepancies larger than 5% are confined to the iron-line region and can be attributed to the different energy resolutions of the two instruments. The broadband pulsed fraction spectrum reveals significant localized features corresponding to known emission lines in the soft band and to the fundamental and harmonic cyclotron resonant scattering features at ∼25 and ∼55 keV. An orbital-phase-resolved modeling of the pulsed fraction spectrum of EPIC-pn shows that the soft band features strongly depend on the value of the equivalent absorption column, with emission line signatures becoming progressively suppressed during highly absorbed intervals. Conclusions. The pulsed fraction spectrum serves both as a quantitative cross-calibration diagnostic and as a powerful spectro-timing diagnostic. Its modeling provides independent constraints on spectral features, complementing traditional phase-averaged spectroscopy.

Transient X-ray obscuration in Seyfert 1 galaxies likely arises from clumpy accretion-disk winds near the broad-line region (BLR), but the wind structure and short-timescale variability are difficult to measure because high-resolution spectra are often suppressed during deep low states. We analyse a coordinated XMM-Newton/NuSTAR campaign on Mrk 335 in June 2021, with long-term Swift monitoring, capturing the source in an intermediate-flux state with strong RGS absorption features. We model the broadband SED to determine the ionising continuum for self-consistent photoionisation modelling of the RGS spectra. The stacked RGS spectrum requires three photoionised absorbers with log xi 3.69, 2.97, and 1.91, outflowing at |v_out| 5800, 3200, and 2100 km/s, respectively. Their properties are consistent with the three-phase obscurer reported in 2009, indicating that a similar multi-phase obscuring wind can persist over decade timescales. Using five consecutive RGS observations, we track the wind evolution on day timescales and find strong variability in column density and ionisation in all phases, together with smaller but coherent velocity changes. During a flare, the low-ionisation phase shows a significant drop in opacity, while in the subsequent epoch all phases show increased outflow velocities, suggesting a possible connection between continuum variability and changes in the line-of-sight absorber. The high-ionisation phase responds most directly to changes in ionising luminosity, while the lowest-ionisation phase shows at most a delayed response. Order-of-magnitude constraints place the obscurer at BLR scales, 10^3-10^5 Rg, with kinetic power potentially reaching the percent level of L_bol for plausible assumptions on geometry and clumpiness.

Accretion onto supermassive black holes (SMBHs) powers active galactic nuclei (AGNs) and drives feedback that shapes galaxy evolution. Constraining AGN accretion disk structure is therefore essential for understanding black hole growth and feedback processes. However, direct constraints on disk size remain rare—particularly from long-term, multiseason spectroscopic reverberation mapping (RM), which is critical for isolating the intrinsic disk response from the broad-line region (BLR). We present results from an intensive multiwavelength RM campaign of NGC 4151 during its brightest state in nearly two decades. This represents the third high-cadence monitoring over the past decade, capturing accretion states spanning the transitional regime between thin and thick disks, making NGC 4151 the only AGN with continuum RM observations across such a wide range in accretion states. Combining spectroscopy from the Lijiang 2.4 m telescope with coordinated Swift UV/X-ray monitoring, we measure interband continuum lags from UV to optical. The wavelength-dependent lags follow a tight τ ∝ λ ^4/3 relation, consistent with reprocessing in a thin disk, but exceed theoretical predictions by a factor of 6.6. Our lag spectrum reveals clear excesses near the Balmer and possibly Paschen jumps, confirming diffuse continuum (DC) contamination from the BLR. By comparing the three campaigns, we discover a nonmonotonic lag–luminosity trend (>3 σ ), which cannot be explained by DC emission alone. We propose that the lags reflect combined disk and BLR contributions, and present the first evidence that the DC component follows an intrinsic Baldwin effect. These results offer new insights into SMBH mass measurements and theoretical models of AGN inner structure.

Transient X-ray obscuration in Seyfert 1 galaxies likely arises from clumpy accretion-disk winds near the broad-line region (BLR), but the wind structure and short-timescale variability are difficult to measure because high-resolution spectra are often suppressed during deep low states. We analyse a coordinated XMM-Newton + NuSTAR campaign on Mrk 335 in 2021 June, with long-term Neil Gehrels Swift Observatory monitoring, capturing the source in an intermediate-flux state with strong Reflection Grating Spectrometer (RGS) absorption features. We model the broadband spectral energy distribution to determine the ionizing continuum for self-consistent photoionization modelling of the RGS spectra. The stacked RGS spectrum requires three photoionized absorbers with $\mathrm{log}\xi \simeq 3.69$ , 2.97, and 1.91, outflowing at ∣ v _out ∣ ≃ 5800, 3200, and 2100 km s ^−1 , respectively. Their properties are consistent with the three-phase obscurer reported in 2009, indicating that a similar multiphase obscuring wind can persist over decade timescales. Using five consecutive RGS observations, we track the wind evolution on day timescales and find strong variability in column density and ionization in all phases, together with smaller but coherent velocity changes. During a flare, the low-ionization phase shows a significant drop in opacity, while in the subsequent epoch all phases show increased outflow velocities, suggesting a possible connection between continuum variability and changes in the line-of-sight absorber. The high-ionization phase responds most directly to changes in ionizing luminosity, while the lowest-ionization phase shows at most a delayed response. Order-of-magnitude constraints place the obscurer at BLR scales ∼ 10 ^3 –10 ^5 R _g , with kinetic power potentially reaching the percent level of L _bol for plausible assumptions on geometry and clumpiness.

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 similar to 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 similar to 10 - 15 degrees 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.

Quasi-periodic eruptions (QPEs) are a recently identified class of X-ray transient associated with tidal disruption events by supermassive black holes, and for which there are multiple possible explanations. In this paper, we present a simple model which requires the black hole be spinning, be misaligned with the accretion flow (both conditions of which are almost certainly met), and that the accretion rate is a few times the Eddington limit. We speculate that the resulting Lense-Thirring torques force the disc and entrained outflows to precess, leading to increased X-ray flux when the wind-cone is oriented at lower inclinations to the observer. We test the range of parameters for which this model could explain the period and brightness of the QPE events discovered thus far, and make qualitative comparisons between the observed X-ray spectra and light curves to those extracted from general relativistic radiation magnetohydrodynamic simulations. Overall, we find some areas of promising concordance, and identify challenges related to the details of current simulations.

We analyse 77 Fermi sources and their potential low-energy counterparts previously proposed in the literature. These sources were classified as active galactic nuclei (AGNs), mainly blazars, based on optical spectroscopy. The main goals of this work are to examine these associations, classify the blazars based on their multiwavelength spectral energy distributions (SEDs), and identify potential masquerading BL Lac objects. Through SED analysis, we assess whether the multiwavelength emission follows the characteristic double-peaked curve of blazars. Additionally, we propose the region of origin of the emission at different wavelengths, investigate the correlation between gamma-ray and lower energy emission, and classify objects as low-, intermediate-, high-, or extreme high-synchrotron peaked (LSP, ISP, HSP, E-HSP) blazars. We search for masquerading BL Lacs, a class of flat-spectrum radio quasars where broad emission lines are swamped by non-thermal jet emission. The multiwavelength analysis revealed that the 64 radio-loud sources in our sample exhibit an SED with a double-peaked structure, typically ascribed to jet activity. Based on the synchrotron peak, 46 are HSP, 11 are ISP, and seven are LSP. We also found 9-18 masquerading BL Lac candidates (approximate to 15-30 per cent of the radio-loud sample). For the 13 radio-quiet unassociated gamma-ray sources, the SEDs do not exhibit the double-peaked structure typical of jetted AGNs. Further analysis ruled out star formation as the origin of the observed gamma-ray emission, making its reconciliation with lower energy emission challenging. We explored alternative counterparts, identifying low-energy matches for seven sources, with no plausible counterparts found for the others.

We searched for the multi-wavelength (X-ray, optical, and radio) counterparts to the unassociated gamma-ray sources (UGS) of the Fermi 4FGL-DR4 catalogue. The main goal was to identify new blazars and/or new active galactic nuclei that emit at GeV energies such as (narrow-line) Seyfert-1 and radio galaxies We focused on sky regions that were observed by the Swift satellite and that overlap with the reported positions of the UGSs. Our primary interest are extragalactic sources, and we therefore focused on UGSs that are located outside the Galactic plane (|b|>10^∘). Because of the large number of sources (about 1800 UGS), we developed a pipeline to automatise the search for counterparts and significantly reduce the computational time for the analysis. Our association process began by identifying potential X-ray counterparts for each UGS. When one was found, we further searched for corresponding radio and optical counterparts in the X-ray counterpart error box to minimise ambiguities. Out of the 1284 UGSs in the 4FGL-DR4 catalogue, 714 were observed at least once by Swift /X-ray Telescope ( Swift /XRT). We detected at least one X-ray source within the Fermi error box with a significance of ≥ 3σ for 274 of these γ-ray emitters. Of these, 193 UGSs have a single potential X-ray counterpart (referred to as UGS1), while 81 have multiple potential X-ray counterparts within the Fermi error box (referred to as UGS2). Of the UGS2, 54 have two X-ray counterparts, 11 have three, and the remaining 16 have more than three counterparts. Each UGS1 has an optical counterpart, and 113 might also be associated with a radio counterpart. We compared the properties of the possible counterpart with those of the γ-ray emitters identified by Fermi with the aim to assess the goodness of our associations.

The Athena mission entered a redefinition phase in July 2022, driven by the imperative to reduce the mission cost at completion for the European Space Agency below an acceptable target, while maintaining the flagship nature of its science return. This notably called for a complete redesign of the X-ray Integral Field Unit (X-IFU) cryogenic architecture towards a simpler active cooling chain. Passive cooling via successive radiative panels at spacecraft level is now used to provide a 50 K thermal environment to an X-IFU owned cryostat. 4.5 K cooling is achieved via a single remote active cryocooler unit, while a multi-stage Adiabatic Demagnetization Refrigerator ensures heat lift down to the 50 mK required by the detectors. Amidst these changes, the core concept of the readout chain remains robust, employing Transition Edge Sensor microcalorimeters and a SQUID-based Time-Division Multiplexing scheme. Noteworthy is the introduction of a slower pixel. This enables an increase in the multiplexing factor (from 34 to 48) without compromising the instrument energy resolution, hence keeping significant system margins to the new 4 eV resolution requirement. This allows reducing the number of channels by more than a factor two, and thus the resource demands on the system, while keeping a 4' field of view (compared to 5' before). In this article, we will give an overview of this new architecture, before detailing its anticipated performances. Finally, we will present the new X-IFU schedule, with its short term focus on demonstration activities towards a mission adoption in early 2027.

Quasiperiodic eruptions (QPEs) are high-amplitude, soft X-ray bursts recurring every few hours, associated with supermassive black holes. Many interpretations for QPEs were proposed since their recent discovery in 2019, including extreme mass ratio inspirals and accretion disk instabilities. But, as of today, their nature still remains debated. We perform the first high-resolution X-ray spectral study of a QPE source using the Reflection Grating Spectrometers' gratings on board XMM-Newton, leveraging nearly 2 Ms of exposure on GSN 069, the first discovered source of this class. We resolve several absorption and emission lines including a strong line pair near the N vii rest-frame energy, resembling the P-Cygni profile. We apply photoionization spectral models and identify the absorption lines as an outflow blueshifted by 1700-2900 km s-1, with a column density of about 1022 cm-2 and an ionization parameter log(xi /erg cm s-1) of 3.9-4.6. The emission lines are instead redshifted by up to 2900 km s-1, and likely originate from the same outflow that imprints the absorption features, and covers the full 4 pi sky from the point of view of GSN 069. The column density and ionization are comparable to the outflows detected in some tidal disruption events, but this outflow is significantly faster and has a strong emission component. The outflow is more highly ionized when the system is in the phase during which QPEs are present, and from the limits, we derive on its location, we conclude that the outflow is connected to the recent complex, transient activity of GSN 069, which began around 2010.

The AXIS Community Science Book represents the collective effort of 592 scientists worldwide to define the transformative science enabled by the Advanced X-ray Imaging Satellite (AXIS), a next-generation X-ray mission selected by NASA's Astrophysics Probe Program for Phase A study. AXIS will advance the legacy of high-angular-resolution X-ray astronomy with 1.5” imaging over a wide 24' field of view and an order of magnitude greater collecting area than Chandra in the 0.3-12 keV band. Combining sharp imaging, high throughput, and rapid response capabilities, AXIS will open new windows on virtually every aspect of modern astrophysics, exploring the birth and growth of supermassive black holes, the feedback processes that shape galaxies, the life cycles of stars and exoplanet environments, and the nature of compact stellar remnants, supernova remnants, and explosive transients. This book compiles 138 community-contributed science cases developed by five Science Working Groups focused on AGN and supermassive black holes, galaxy evolution and feedback, compact objects and supernova remnants, stellar physics and exoplanets, and time-domain and multi-messenger astrophysics. Together, these studies establish the scientific foundation for next-generation X-ray exploration in the 2030s and highlight strong synergies with facilities of the 2030s, such as JWST, Roman, Rubin/LSST, SKA, ALMA, ngVLA, and next-generation gravitational-wave and neutrino networks.

Aims. We aim to investigate the energy-resolved pulse profile changes of the accreting X-ray pulsar V 0332+53 focusing in the cyclotron line energy range, using the full set of available NuSTAR observations. Methods. We applied a tailored pipeline to study the energy dependence of the pulse profiles and to build the pulsed fraction spectra (PFS) for the different observations. We also studied the profile changes using cross-correlation and lag spectra. We re-analysed the energy spectra to search for links between the local features observed in the PFS and spectral emission components associated with the shape of the fundamental cyclotron line. Results. In the PFS data, with sufficiently high statistics, we observe a consistent behaviour around the cyclotron line energy. Specifically, two Gaussian-shaped features appear symmetrically on either side of the putative cyclotron line. These features exhibit minimal variation with source luminosity, and their peak positions consistently remain on the left and right of the cyclotron line energy. Associated with the cyclotron line-forming region, we interpret them as evidence for the resonant cyclotron absorption line wings, as predicted by theoretical models of how the cyclotron line profile should appear along the observer's line of sight. A phase-resolved analysis of the pulse in the energy bands surrounding these features enables us to determine both the spectral shape and the intensity of the photons responsible for these peaks in the PFS. Assuming these features correspond to a spectral component, we used their shapes as priors for the corresponding emission components, finding a statistically satisfactory description of the spectra. To explain these results, we propose that our line of sight is close to the direction of the spin axis, while the magnetic axis is likely orthogonal to it.

Understanding the nature of the accretion disk, its interplay with the X-ray corona, and assessing black hole spin demographics remain open challenges in astrophysics. In this paper, we examine the predictions of the standard α-disk model, origin of the puzzling soft X-ray excess, and measure the black hole spin parameter by applying an updated high-density disk reflection model to the XMM-Newton/NuSTAR broadband (0.3-78 keV) X-ray spectra of a sample of 11 Type-1 AGN. Our Bayesian analysis confirms that a variable-density relativistic disk reflection model with a broken power-law emissivity profile can simultaneously fit the soft X-ray excess, broad iron K line emission, and Compton hump in 3 out of 11 AGN. For the remaining sources, a distinct warm Comptonization component is still required, which supports a hybrid origin for the soft X-ray excess. The measured temperature and optical depth of the warm corona span nearly the entire theoretically allowed range, with median values of 0.43_-0.18^+0.40 keV and 12.5_-3.9^+3.1, respectively. Our first systematic calculation of the disk-to-corona power transfer fraction reveals that the fraction of power released from the accretion disk into the hot corona spans a wide range, with a sample median of 0.68_-0.25^+0.25. The sample median values for the hot coronal plasma temperature and optical depth are 54_-12^+11 keV and 0.98_-0.28^+0.22, respectively. Finally, through both hard X-ray (3-78 keV) and broadband (0.3-78 keV) relativistic reflection spectroscopy, we systematically constrain the black hole spin parameter across the mass scales of log(M_ BH/M_⊙) ∼ 5.5-9.0, thereby increasing or refining the available spin measurements in the AGN population by ∼20

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.

Recent discoveries from time-domain surveys are defying our expectations for how matter accretes onto supermassive black holes (SMBHs). The increased rate of short-timescale, repetitive events around SMBHs, including the recently discovered quasi-periodic eruptions1-5, are garnering further interest in stellar-mass companions around SMBHs and the progenitors to millihertz-frequency gravitational-wave events. Here we report the discovery of a highly significant millihertz quasi-periodic oscillation (QPO) in an actively accreting SMBH, 1ES 1927+654, which underwent a major optical, ultraviolet and X-ray outburst beginning in 20186,7. The QPO was detected in 2022 with a roughly 18-minute period, corresponding to coherent motion on a scale of less than 10 gravitational radii, much closer to the SMBH than typical quasi-periodic eruptions. The period decreased to 7.1 minutes over 2 years with a decelerating period evolution ( P ¨ greater than zero). To our knowledge, this evolution has never been seen in SMBH QPOs or high-frequency QPOs in stellar-mass black holes. Models invoking orbital decay of a stellar-mass companion struggle to explain the period evolution without stable mass transfer to offset angular-momentum losses, and the lack of a direct analogue to stellar-mass black-hole QPOs means that many instability models cannot explain all of the observed properties of the QPO in 1ES 1927+654. Future X-ray monitoring will test these models, and if it is a stellar-mass orbiter, the Laser Interferometer Space Antenna (LISA) should detect its low-frequency gravitational-wave emission.

We present a comprehensive spectral analysis of the ultraluminous X-ray source Holmberg II X-1 using broadband and high-resolution X-ray spectra taken with the XMM-Newton satellite over a period of 19 years benefiting from a recent campaign. We tested several models for the broadband spectra among which a double thermal component provided a reasonable description for the continuum between 0.3-10 keV and enabled us to constrain the properties of the accretion disc. The Luminosity-Temperature trends of the inner and outer disc components broadly agree with the expectations for a thin disc, although the exact values of the slopes are slightly sensitive to the adopted model. However, all tested models show L-T trends which deviate from a power law above a bolometric luminosity of about 5 $\times \ 10^{39} $erg/s, particularly for the hot thermal component associated to the inner accretion flow. Assuming that such deviations are due to the accretion rate exceeding its Eddington limit or, most likely, the super-critical rate, a compact object with a mass 16-36 Msun, i.e. a stellar-mass black hole, is inferred. The time-averaged (2021) high resolution spectra present narrow emission lines at 1 keV primarily from Ne IX-X and a very strong at 0.5 keV from N VII, which indicate Ne-N-rich gas with non-Solar abundances. This favours a nitrogen-rich donor star, such as a blue/red supergiant, which has escaped from its native stellar cluster characterised by a low-metallicity environment.