Melania Del Santo

We present the first X-ray polarimetric measurement of the neutron star low-mass X-ray binary system AX J1745.6−2901 conducted by the Imaging X-ray Polarimetry Explorer (IXPE) satellite. This transient source, located within ∼1 . ′ 5 of the Galactic center, was observed serendipitously during a MAXI J1744−294 observation with a duration of 150 ks. The complex nature of the region in which AX J1745.6−2901 is located poses a challenge for studying its polarization. By performing a detailed analysis of the contamination from MAXI J1744−294 and the Galactic center diffuse emission, we find the source polarization degree PD = 14.7% ± 4.0% and polarization angle PA = 122° ± 8°. The phase-resolved analysis shows increase in polarization during the eclipse phase, with PD = 34.2% ± 8.7%, suggesting that the polarization-inducing mechanisms are of scattering nature, probably originating from disk winds.

During the hard and quiescent spectral states in X-ray binaries, a non-linear correlation is observed between radio and X-ray luminosities, providing a valuable tool to probe the connection between accretion and jet production. This relation was originally thought to define a single ‘standard’ correlation that spans several orders of magnitude in X-ray luminosity, and was extended to active galactic nuclei by including a mass term. However, subsequent studies revealed a more complex picture, with some sources deviating from the standard correlation and instead populating distinct tracks. To date, all large studies of the radio:X-ray plane have combined data from multiple telescopes, introducing uncertainties due to differing instrument systematics and flux conversions between observing frequencies, thereby complicating comparisons and limiting constraints. ThunderKAT was a five-year programme on the MeerKAT radio telescope that monitored X-ray binaries in outburst, and ran alongside SwiftKAT which provided quasi-simultaneous Swift/XRT X-ray coverage. We present the full set of light curves from these programmes, comprising 948 radio and 1,029 X-ray data points. An important finding is the frequent detection of unresolved radio emission during the soft state, likely dominated by previously launched jet ejecta. Using these data, we construct the largest, observationally homogeneous X-ray binary radio:X-ray plane to date. We relate these results to the physical mechanisms proposed to drive inter-source diversity, and outline directions for future observational and theoretical work. This paper is accompanied by a public data release of the ThunderKAT and SwiftKAT measurements and a compiled radio:X-ray plane, available through an interactive website.

Context. Ultraluminous X-ray sources (ULXs) are non-nuclear X-ray binary systems that exceed the Eddington luminosity for a 10 M _⊙ black hole. The majority of these sources are thought to be stellar-mass compact objects accreting at super-Eddington rates, exhibiting powerful relativistic winds. These winds have been identified through the detection of absorption lines with a blueshift as high as 0.3 c and emission lines typically found at their laboratory wavelengths. Aims. In this work, we analysed the XMM-Newton data of the ULX NGC 5204 X-1, which has been observed to exhibit emission lines with a blueshift of about 0.3 c . The aim of this study is to examine the geometry and physical properties of the accretion disc and the relativistic outflows. In addition, we aim to explore the factors that influence the ULX spectral transitions. Methods. We undertook an observing campaign with XMM-Newton to explore the source behaviour at different luminosities. In this first paper of the series, we performed high-resolution X-ray spectroscopy, including archival data, with the Reflection Grating Spectrometer (RGS) instrument, which allowed us to resolve both emission and absorption lines. The outflows features were characterised using physical models of plasma in collisional ionisation and photoionisation equilibrium. Results. We identify collisionally ionised blueshifted and redshifted components at about 0.3 c . These findings have a high statistical significance and suggest a biconical structure for the outflow. Additionally, the analysis of the O VII line triplet observed in the spectrum enables us to infer physical properties of the low-velocity line-emitting plasma; for example, electron density ( n _e ∼ 10 ¹⁰ cm ⁻³ ) and temperature ( T _e ≥ 1.5 × 10 ⁵ K). A hybrid plasma whose ionisation balance is affected by both collisions and radiation is favoured.

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.

We present the first IXPE spectro-polarimetric observation of the black hole candidate MAXI J1744−294, a transient X-ray source observed during a bright 2025 outburst in the Galactic center region. The source has recently been identified as most likely a repeat outburst of the 2016 transient Swift J174540.2−290037. During the ∼150 ks observation, the source was detected in the soft state, and its spectrum was well described by an absorbed multicolor disk with a minor high-energy tail. We did not detect any significant polarization from the source, and hence we derived a 3 σ upper limit on the polarization degree of 1.3% in the 2–8 keV energy band. This result is consistent with previous findings for soft-state black hole binaries observed at low to intermediate inclination angles. By comparing the polarization degree upper limit with theoretical predictions for standard accretion disk emission, we constrain the disk inclination to i ≲ 38°–72°, depending on the black hole spin and the disk atmosphere albedo, consistent with inclination estimates obtained during the 2016 outburst of Swift J174540.2−290037.

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.

This work presents comprehensive multifrequency radio monitoring of the black hole low-mass X-ray binary (LMXB) Swift J1727.8−1613, which underwent its first recorded outburst after its discovery in 2023 August. Through a considerable community effort, we have coalesced the data from multiple, distinct observing programs; the light curves include ∼10 months and 197 epochs of monitoring from seven radio facilities with observing frequencies ranging from (approximately) 0.3–230 GHz. The primary purpose of this work is to provide the broader astronomical community with these light curves to assist with the interpretation of other observing campaigns, particularly nonradio observing frequencies. We discuss the phenomenological evolution of the source, which included (i) multiple radio flares consistent with the launching of discrete jet ejections, the brightest of which reached ∼1 Jy; (ii) temporally evolving radio spectral indices (α), reaching values steeper than expected for optically thin synchrotron emission (α < −1) and emission with significant radiative cooling (α < −1.5). We have published a digital copy of the data and intend for this work to set a precedent for the community to continue releasing comprehensive radio light curves of future LMXB outbursts.

MAXI J1834–021 is a new X-ray transient discovered in February 2023. We analysed the spectral and timing properties of MAXI J1834–021 using NICER, NuSTAR and Swift data collected between March and October 2023. The light curve showed a main peak followed by a second activity phase. Most of the spectra extracted from individual NICER observations could be adequately fit with a Comptonisation component alone, while some required an additional thermal component. The spectral evolution is consistent with a softening trend as the source becomes brighter in X-rays. We also analysed the broadband spectrum by combining data from simultaneous NICER and NuSTAR observations on March 10, 2023. This spectrum can be fitted with a disc component having an inner radius temperature of kT_in∼0.4 keV and a Comptonisation component with a power-law photon index of Γ∼1.8. By including a reflection component in the modelling, we obtained a 3σ upper limit for the inner disc radius of 11.4 gravitational radii. We also detected a quasi-periodic oscillation (QPO), whose central frequency varies with time (from 2 Hz to ∼0.9 Hz), and anti-correlates with the hardness ratio. Based on the observed spectral-timing properties, MAXI J1834–021 can be classified as a low-mass X-ray binary in outburst. However, we cannot draw a definitive conclusion about the nature of the accreting compact object, which could currently be a black hole or a neutron star.

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.