arXiv subjects

I. El Mellah

Publications and source records attributed to I. El Mellah.

At least 19 recordsLinked to original sources

arXiv2026

Fingerprints of thermal Comptonization in accreting neutron stars. Plasma-vacuum interplay in cyclotron lines and polarisation

X-ray emission from accreting, strongly magnetised neutron stars and its pulse-phase variability probe their magnetic-field geometry, spin orientation, and emission processes. Whether the radiation emerges mainly from a hot spot or column, and whether its properties are shaped by bulk or thermal Comptonization, remain debated across luminosity regimes. We aim to disentangle intrinsic emission from visibility effects and identify observables characteristic of thermal Comptonization in hot spots and columns. We therefore derived energy-dependent beam patterns and observable signatures without assigning the model to a luminosity regime, focusing on cyclotron-resonance and polarisation effects as tracers of anisotropy. To do so, we computed angle-dependent polarised broadband spectra, including the fundamental cyclotron line, for a homogeneous, self-emitting, magnetised Comptonizing plasma over a broad parameter range. Accounting for light bending and projection, we obtained phase-dependent fluxes for different geometries and, for hot spots, observed linear polarisation. The beam patterns evolve with energy, driving pulse-profile changes. Near the cyclotron resonance, plasma-vacuum interplay produces a narrow central beam and side petals. Their visibility creates geometry-dependent dips, bumps, and M- and W-shaped structures in hot-spot pulsed fraction spectra, but only dips and bumps for columns. Thermally Comptonized cyclotron lines do not reliably trace plasma temperature; plasma-induced ellipticity and band averaging reduce observed soft-X-ray linear polarisation to 0-30%. Under typical X-ray pulsar accretion-channel conditions, thermal Comptonization leaves robust energy-dependent anisotropic signatures. Energy-resolved pulse profiles, pulsed fraction spectra, and polarisation thus provide complementary diagnostics of neutron star geometry, emission-region shape, and spectral formation.

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arXiv2026

Spectroscopic modeling of ionic structure in stellar winds of high-mass X-ray binaries

High-mass X-ray binaries (HMXBs) provide a natural laboratory to study radiatively driven stellar winds under strong external X-ray irradiation. As the compact object moves along its orbit, the wind density and ionization structure vary with orbital phase, leaving characteristic signatures in X-ray emission and absorption features. The amplitude and morphology of this variability depend on the system geometry, including the orbital inclination (via line-of-sight and occultation effects) and the orbital eccentricity (via phase-dependent changes in the orbital separation). We present a computational framework that connects phase-resolved spectroscopic variability to the three-dimensional structure of irradiated winds, and that enables inference of physically meaningful wind--irradiation parameters within a Bayesian setting. We combine photoionization calculations with a parametric wind description and orbital geometry to construct three-dimensional maps of density and ionization. From these maps we compute orbital-phase-dependent diagnostics, accounting for geometric occultation and wind inhomogeneity through a clumping prescription. We then use Bayesian inference to compare model predictions with phase-resolved observables and to quantify parameter constraints and degeneracies. The framework reproduces the main orbital-phase-dependent trends expected for irradiated winds and yields robust constraints on the system ionization balance. While combinations of wind and luminosity parameters are well constrained, individual parameters can remain partially degenerate depending on the orbital configuration and data quality. This modular and computationally efficient approach provides a route to interpret HMXB phase variability in physical terms, and offers a foundation for future extensions toward forward spectral modeling and population-level applications.

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arXiv2026

ATOMIUM: Inner circumstellar envelopes of oxygen-rich AGB stars as revealed by highly excited SiO lines

Silicon monoxide (SiO) traces the physical conditions and dynamics in the circumstellar envelopes (CSEs) of AGB stars. We present high-resolution ALMA Band 6 observations of highly excited SiO emission in 14 oxygen-rich AGB stars. We cover transitions from v = 0 to v = 8, including first detections of 28SiO v = 3, 4, 8, J = 6-5, 29SiO v = 6, J = 6-5, and 30SiO v = 4, 5, J = 6-5, some of which are masers. The v = 8 transition is the highest v-state observed in an AGB star yet. Masers in v = 0 are detected clearly in V PsA and IRC+10011 and tentatively in T Mic. R Hya exhibits the richest SiO spectrum. SiO J = 6-5 absorption is seen in R Aql, R Hya, S Pav, and T Mic, with features indicative of both infalls and outflows, and tentative detection of 28SiO v = 8, J = 6-5 absorption is found towards S Pav and R Aql. Highly excited SiO emission is often distributed in arcs or clumps with velocity gradients; components in R Hya and U Her align with predicted shock fronts. Detection rates show no significant difference between low and high mass-loss rate stars, although line overlap may affect some intensities. Maser detections appear uncorrelated with pulsation period or phase. The radius enclosing 90 per cent of compact SiO emission shows a tentative correlation with mass-loss rate. These results highlight the role of mass loss and CSE geometry in shaping high-excitation SiO emission.

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arXiv2026

A newly born spider system at the core of a radio shell: Evidence for a low-energy supernova

In a search for low surface brightness radio nebulae using the ASKAP-EMU survey, we discovered a faint radio shell, G289.6+5.8, and its central point radio source at the position of the soft gamma-ray source IGR J11187-5438. The central radio source is spatially coincident with a previously known low-mass X-ray binary (LMXB) with an M-type donor star. However, the newly determined Gaia DR3 distance of 267 pc and correspondingly low X-ray luminosity (3 x 10e31 erg/s) cast doubt on the LMXB classification. Neither radio nor X-ray pulsations are detected. Chance-alignments between radio shell, central radio source, optical star, gamma-ray, and X-ray sources appear unlikely. By combining all currently available evidence, we conclude that G289.6+5.8 is a remnant of a low-energy core-collapse explosion of an intermediate mass star (~8Msun) in a binary system with an M-type secondary, which remained bound after the explosion. In this scenario, G289.6+5.8 is a supernova remnant, while the central gamma- and X-ray source is associated with a young neutron star driving a pulsar wind interacting with its M-type stellar companion, making IGR J11187-5438 a nascent spider-type X-ray binary.

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arXiv2026

Characterizing the Post-Red Supergiant binary system AFGL 4106 and its complex nebula with SPHERE/VLT

Aims: We aim to characterize the physical and morphological properties of the binary system AFGL 4106, composed of two evolved massive stars. Understanding its mass-loss processes and circumstellar environment offers insight into the late stages of stellar evolution in massive binary systems. Methods: We obtained high-angular--resolution, high-contrast imaging using VLT/SPHERE with ZIMPOL (optical) and IRDIS (near-infrared) across multiple filters. We used aperture photometry to extract the spectral energy distributions (SEDs) of each star, and applied radiative transfer modelling to study the system and its surrounding dusty environment. Results: The observations resolve both components of the binary and unveil a complex, dusty nebula featuring asymmetric structures and cavities. SED fitting yields stellar temperatures of T$_1 = 6723\pm196$ K and T$_2 = 3394\pm264$ K, along with bolometric luminosities of L$_1 = (7.9 \pm 0.18) \times 10^4$ L$_\odot$ and L$_2 = (3.8 \pm 0.11) \times 10^4$ L$_\odot$. These values support the classification of the primary as being in a post-red supergiant (post-RSG) phase and the secondary as an active red supergiant (RSG). The luminosity ratio, combined with the inferred radii, indicates that both stars are at close yet distinct stages of their evolution. The binary is surrounded by an extended shell whose asymmetric morphology and large-scale features suggest interaction with the stellar winds and interstellar medium (ISM), and possibly the presence of a third, undetected companion. Conclusions: These observations provide the first resolved view of AFGL 4106's system and its dusty envelope. Our analysis sets constraints on the physical properties and evolutionary status of the system. This work contributes to understanding mass-loss processes in massive binaries and the shaping of nebulae around evolved stars.

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arXiv2025

Interferometric view into RT Pav's long secondary period. binary vs oscillatory convective modes

Long secondary periods (LSPs) occur in about one-third of evolved stars, yet their origin remains unclear. The leading explanations are oscillatory convective modes and a binary companion embedded in dust. We investigate the LSP of the red giant RT Pav using multi-wavelength VLTI interferometry (PIONIER, GRAVITY, MATISSE; 1.5-5.0 microns), obtained near the phase where a companion would appear most separated. These data, combined with photometry and Gaia DR3 astrometry, constrain possible companion masses, orbits, and photometric effects. We model the interferometric observables using uniform-disk, limb-darkened, ellipse, binary, and oscillatory convective dipole representations, supported by Monte Carlo simulations. Gaia limits any companion to a mass whose Roche-lobe volume is too small to hold the obscuring or scattering material required to reproduce the observed LSP modulation. While binary fits can yield low chi-squared values, the inferred positions are inconsistent across wavelength, closure phases do not increase with wavelength as dusty companions predict, and significant detections occur in only two of four bands. Theoretical estimates show that a roughly 1 percent flux companion at LSP-like separations should be consistently detectable for typical O-rich AGB dust, but is not consistently observed. In contrast, an oscillatory convective dipole with a temperature contrast of about 200 K reproduces the H-band morphology and visible-light amplitude without violating Gaia or photometric constraints, and binary-like residuals vanish when dipole models are fitted. Our results therefore favor oscillatory convective modes over a binary origin for the LSP in RT Pav. Time-resolved spectro-interferometry across the LSP cycle is a natural next step.

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arXiv2025

Pulse profile variations in the accreting X-ray pulsar Vela X-1

Vela X-1 is a well-studied accreting X-ray pulsar, with a distinctive pulse profile that has been found to be very similar in different observations spread out over decades. On the other hand, significant variations down to the timescale of individual pulses have been observed. The physical mechanisms leading to the energy-resolved pulse profile and its variations are not fully understood. Long, uninterrupted observations of Vela X-1 with XMM-Newton in 2000, 2006 and 2019 at different orbital phases allow us to study variations of the pulse properties in the soft X-ray range on all timescales in detail. We aim to characterize and quantify the variations of pulse profiles and individual pulse cycles on all timescales probed, and to identify possible factors driving the observed variations on these timescales. We generated consistent energy-resolved pulse profiles for each observation, as well as profiles built from subsets of individual pulse cycles selected by time, flux, or similarity to the mean profiles. We identified five pulsed phases based on the profile morphology and hardness, and examined the relative contributions over time. To quantify short-timescale variability, we compared individual pulse cycles with synthetic light curves derived from scaled versions of the average profiles. The pulse profile of Vela X-1, when averaged over many pulse cycles, remains remarkably stable, as expected. The most prominent variations between epochs are attributable to changes in absorption. Residual systematic differences are primarily flux-dependent, with profiles showing less variability at higher flux levels. On shorter timescales, most individual pulse cycles resemble the average profile, even though significant, sporadic deviations are also present.

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arXiv2025

Low-frequency spectra of neutron star + OB supergiant binaries: Does wind density drive persistent and flaring modes of accretion?

Neutron star high-mass X-ray binaries are well-studied in wavebands between the infrared and hard X-rays. Their low-frequency millimeter and radio properties, on the other hand, remain poorly understood. We present observations of the millimeter and radio emission of binaries where a neutron star accretes from an OB supergiant. We report ALMA and NOEMA millimeter observations of twelve systems, supplemented by VLA radio observations of six of those targets. Our targets include six Supergiant X-ray Binaries (SgXBs), four Supergiant Fast X-ray Transients (SFXTs), and two intermediate systems. Nine out of twelve targets, including all SFXTs, are detected in at least one millimeter band, while in the radio, only two targets are detected. All detected targets display inverted radio/millimeter spectra, with spectral indices in the range $α=0.6-0.8$ for those systems where accurate SED fits could be performed. We conclude, firstly, that the low-frequency SEDs of neutron star SFXTs and SgXBs are dominated by free-free emission from the OB supergiant's stellar wind, and that jet emission is unlikely to be observed unless the systems can be detected at sub-GHz frequencies. Secondly, we find that SFXTs are fainter at 100 GHz than prototypical SgXBs, probably due to systematically less dense winds in the former, as supported further by the differences in their fluorescence Fe K$α$ lines. We furthermore compare the stellar wind constraints obtained from our millimeter observations with those from IR/optical/UV studies and bow shock detections, and present evidence for long-term stellar wind variability visible in the thermal emission.

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arXiv2025

ATOMIUM: Continuum emission and evidence of dust enhancement from binary motion

Low- and intermediate-mass stars on the asymptotic giant branch (AGB) account for a significant portion of the dust and chemical enrichment in their host galaxy. Here we present ALMA observations of the continuum emission at 1.24 mm around a sample of 17 stars from the ATOMIUM survey. From our analysis of the stellar contributions to the continuum flux, we find that the semi-regular variables all have smaller physical radii and fainter monochromatic luminosities than the Mira variables. Comparing these properties with pulsation periods, we find a positive trend between stellar radius and period only for the Mira variables with periods above 300 days and a positive trend between the period and the monochromatic luminosity only for the red supergiants and the most extreme AGB stars with periods above 500 days. We find that the continuum emission at 1.2 mm can be classified into four groups. "Featureless" continuum emission is confined to the (unresolved) regions close to the star for five stars in our sample, relatively uniform extended flux is seen for four stars, tentative elongated features are seen for three stars, and the remaining five stars have unique or unusual morphological features in their continuum maps. These features can be explained by binary companions to 10 out of the 14 AGB stars in our sample. Based on our results we conclude that there are two modes of dust formation: well established pulsation-enhanced dust formation and our newly proposed companion-enhanced dust formation. If the companion is located close to the AGB star, in the wind acceleration region, then additional dust formed in the wake of the companion can increase the mass lost through the dust driven wind. This explains the different dust morphologies seen around our stars and partly accounts for a large scatter in literature mass-loss rates, especially among semiregular stars with small pulsation periods.

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arXiv2025

The pre-eruption state of T CrB as observed with ALMA in 2024

T CrB is a nearby symbiotic binary and a recurrent nova with a period of ca. 80 years. The next eruption is expected to take place in 2025 or 2026. We present our pre-eruption observations made in ALMA frequency Bands 1, 3, 4, 6, 7, and 8 in August to November 2024 and constrain the properties of the environment into which the imminent next nova will erupt. We find that in the second half of 2024, the quiescent T CrB was a faint mm source with a spectral energy distribution well described by a powerlaw with index $α=$0.56$\pm$0.11 and a flux density of ca. 0.1 mJy at 44 GHz and 0.4 mJy at 400 GHz. There is no significant line emission. This is in agreement with expectations for free-free emission from the partially ionized wind of the red giant donor star and, in extrapolation to 35 GHz, a factor 5 fainter than the emission observed in 2016/17 during the latest high state. Comparing the spectra from that high-state between 13.5 GHz and 35 GHz with our spectrum from 2024, our spectrum is softer. The spectral index is on average lower by 0.34$\pm$0.11 . Our per-band and aggregate bandwidth images of T CrB show an unresolved point source with no evidence for extended structure. A simple model of a free-free emitting, fully-ionized stellar wind seems to describe well the 2016/17 high state of T CrB but not our 2024 ALMA measurements with their low flux and high turnover frequency suggesting that in 2024, the wind was far from fully ionized. (See the unabridged version of the abstract in the paper.)

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arXiv2025

An accreting dwarf star orbiting the S-type giant star pi1 Gru

Aims. We aim to characterize the properties of the inner companion of the S-type AGB star pi1 Gru and to identify plausible future evolutionary scenarios for this triple system. Methods. We observed pi1 Gru with ALMA and VLT/SPHERE. In addition, we collected archival photometry data and used the Hipparcos-Gaia proper motion anomaly. We derive the best orbital parameters from Bayesian inference. Results. In June-July 2019, the inner companion, pi1 Gru C, was located at 37.4 +/- 2.0 mas from the primary (a projected separation of 6.05 +/- 0.55 au at 161.7 +/- 11.7 pc). The best orbital solution yields a companion mass of 0.86 (+0.22/-0.20) Msun (using the derived mass of the primary) and a semi-major axis of 7.05(+0.54/-0.57) au, corresponding to an orbital period of 11.0 (+1.7/-1.5) yr. The preferred solution is an elliptical orbit with eccentricity e = 0.35(+0.18/-0.17), although a circular orbit cannot be fully excluded. The close companion could be either a K1V (F9.5V to K7V) star or a white dwarf (WD). Ultraviolet and millimeter continuum photometry are consistent with the presence of an accretion disk around the close companion. The ultraviolet emission may originate from hot spots in an overall cooler disk, or from a hot disk if the companion is a WD. Conclusions. Although the close companion and the AGB star are interacting and an accretion disk is observed around the companion, the mass-accretion rate is too low to trigger a Type Ia supernova, but it could produce novae every ~900 yr. Short-wavelength, spatially resolved observations are required to further constrain the nature of the C companion. Searches for close-in companions similar to this system will improve our understanding of the physics of mass and angular momentum transfer, as well as orbital evolution during late evolutionary stages.

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arXiv2025

Don't torque like that. Measuring compact object magnetic fields with analytic torque models

Context. Changes of the rotational period observed in various magnetized accreting sources are generally attributed to the interaction between the in-falling plasma and the large-scale magnetic field of the accretor. A number of models have been proposed to link these changes to the mass accretion rate, based on different assumptions on the relevant physical processes and system parameters. For X-ray binaries with neutron stars, with the help of precise measurements of the spin periods provided by current instrumentation, these models render a way to infer such parameters as the strength of the dipolar field and a distance to the system. Often, the obtained magnetic field strength values contradict those from other methods used to obtain magnetic field estimates. Aims. We want to compare the results of several of the proposed accretion models. To this end an example application of these models to data is performed. Methods. We reformulate the set of disk accretion torque models in a way that their parametrization are directly comparable. The application of the reformulated models is discussed and demonstrated using Fermi/GBM and Swift/BAT monitoring data covering several X-ray outbursts of the accreting pulsar 4U 0115+63. Results. We find that most of the models under consideration are able to describe the observations to a high degree of accuracy and with little indication for one model being preferred over the others. Yet, derived parameters from those models show a large spread. Specifically the magnetic field strength ranges over one order of magnitude for the different models. This indicates that the results are heavily influenced by systematic uncertainties.

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arXiv2024

Variable structures in the stellar wind of the HMXB Vela X-1

Strong stellar winds are an important feature in wind-accreting high-mass X-ray binary (HMXB) systems, providing insights into stellar evolution and their impact on surrounding environments. However, the long-term evolution and temporal variability of these winds are not fully understood. This work probes the archetypal wind-accreting HMXB Vela X-1 using MAXI observations over 14 years, focusing on orbit-to-orbit absorption variability in the 2-10 keV band. Additionally, the relation between hardness ratio trends in binary orbits and neutron star spin states is investigated. We calculate hardness ratios to track absorption variability, comparing flux changes across energy bands, as the effect of absorption on the flux is energy-dependent. Variability is analyzed by comparing hardness ratio trends across binary orbits to the MAXI long-term averaged evolution. The long-term averaged hardness ratio evolution displays a stable pattern. Yet, individual binary orbits reveal different hardness ratio evolutions between consecutive orbits with no evident periodicity. Less than half of the binary orbits align with the long-term evolution. Moreover, neutron star spin-up episodes exhibit harder-than-average hardness trends compared to spin-down episodes, although their distributions overlap considerably. The long-term averaged hardness ratio dispersion is consistent with absorption column densities reported in literature from shorter observations, suggesting that heterogeneous wind structures, including accretion wakes and wind clumps, drive observed variations. The orbit-to-orbit variability indicates that pointed X-ray observations provide limited insight into wind structure. The link between neutron star spin states and hardness trends underscores the influence of accretion on absorption, with variability tied to stellar wind density fluctuations.

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arXiv2024

Characterisation of the stellar wind in Cyg X-1 via modelling of colour-colour diagrams

Cygnus X-1 is a high mass X-ray binary where accretion onto the black hole is mediated by the stellar wind from the blue supergiant companion star HDE 226868. Depending on the position of the black hole along the orbit, X-ray observations can probe different layers of the stellar wind. Deeper wind layers can be investigated at superior conjunction (i.e. null orbital phases). We aim at characterising the stellar wind in the Cyg X-1/HDE 226868 system analysing one passage at superior conjunction covered by XMM-Newton during the CHOCBOX campaign via modelling of colour-colour diagrams. Since X-ray absorption is energy-dependent, colour indices provide information on the parameters of the stellar wind, such as the column density $N_{H,w}$ and the covering factor $f_c$. We fitted colour-colour diagrams with models that include both a continuum and a stellar wind component. We used the KDE method to infer the unknown probability distribution of the data points in the colour-colour diagram, and selected the model corresponding to the highest likelihood. In order to study the temporal evolution of the wind around superior conjunction, we extracted and fitted time-resolved colour-colour diagrams. We found that the model that best describes the shape of the colour-colour diagram of Cyg X-1 at superior conjunction requires the wind to be partially ionised. The shape of the colour-colour diagram strongly varies during the analysed observation, as due to concurrent changes of the mean $N_{H,w}$ and the $f_c$ of the wind. Our results suggest the existence of a linear scaling between the rapid variability amplitude of $N_{H,w}$ (on time scales between 10 s and 11 ks) and its long term variations (on time scales 11>ks). Using the inferred best-fit values, we estimated the stellar mass loss rate to be $\sim 7\times10^{-6} {\rm M_{\odot}yr^{-1}}$ and the clumps to have a mass of $\sim10^{17}$ g.

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arXiv2023

ATOMIUM: Molecular inventory of 17 oxygen-rich evolved stars observed with ALMA

The dusty winds of cool evolved stars are a major contributor of the newly synthesised material enriching the Galaxy and future generations of stars. However, the details of the physics and chemistry behind dust formation and wind launching have yet to be pinpointed. Recent spatially resolved observations show the importance of gaining a more comprehensive view of the circumstellar chemistry, but a comparative study of the intricate interplay between chemistry and physics is still difficult because observational details such as frequencies and angular resolutions are rarely comparable. Aiming to overcome these deficiencies, ATOMIUM is an ALMA Large Programme to study the physics and chemistry of the circumstellar envelopes of a diverse set of oxygen-rich evolved stars under homogeneous observing conditions at three angular resolutions between ~0.02"-1.4". Here we summarize the molecular inventory of these sources, and the correlations between stellar parameters and molecular content. Seventeen oxygen-rich or S-type asymptotic giant branch (AGB) and red supergiant (RSG) stars have been observed in several tunings with ALMA Band 6, targeting a range of molecules to probe the circumstellar envelope and especially the chemistry of dust formation close to the star. We systematically assigned the molecular carriers of the spectral lines and measured their spectroscopic parameters and the angular extent of the emission of each line from integrated intensity maps. Across the ATOMIUM sample, we detect 291 transitions of 24 different molecules and their isotopologues. This includes several first detections in oxygen-rich AGB/RSG stars: PO v=1, SO2 v1=1 and v2=2, and several high energy H2O transitions. We also find several first detections in S-type AGB stars: vibrationally excited HCN v2=2,3 and SiS v=4,5,6, as well as first detections of the molecules SiC, AlCl, and AlF in W Aql...

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arXiv2023

The first mm detection of a neutron star high-mass X-ray binary

Neutron stars accreting from OB supergiants are often divided between persistently and transiently accreting systems, called Supergiant X-ray Binaries (SgXBs) and Supergiant Fast X-ray Transients (SFXTs). This dichotomy in accretion behaviour is typically attributed to systematic differences in the massive stellar wind, binary orbit, or magnetic field configuration, but direct observational evidence for these hypotheses remains sparse. To investigate their stellar winds, we present the results of pilot 100-GHz observations of one SFXT and one SgXB with the Northern Extended Millimetre Array. The SFXT, IGR J18410-0535, is detected as a point source at $63.4 \pm 9.6$ $μ$Jy, while the SgXB, IGR J18410-0535 remains undetected. Radio observations of IGR J18410-0535 imply a flat or inverted low-frequency spectrum, arguing for wind emission and against non-thermal flaring. Due to the uncertain SFXT distance, however, the observations do not necessarily imply a difference between the wind properties of the SFXT and SgXB. We compare the mm constraints with other HMXBs and isolated OB supergiants, before considering how future mm campaigns can constrain HMXB wind properties by including X-ray measurements. Specifically, we discuss caveats and future steps to successfully measure wind mass loss rates and velocities in HMXBs with coordinated mm, radio, and X-ray campaigns.

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arXiv2023

Reconnection-driven flares in 3D black hole magnetospheres -- A scenario for hot spots around Sagittarius A*

Low-luminosity supermassive and stellar-mass black holes (BHs) may be embedded in a collisionless and highly magnetized plasma. They show non-thermal flares indicative of efficient dissipative processes in the vicinity of the BH. During NIR flares from the supermassive BH Sagittarius A* (Sgr A*), GRAVITY detected circular motion and polarization evolution which suggest the presence of transient synchrotron-emitting hot spots moving around the BH. We study 3D reconnecting current layers in the magnetosphere of spinning BHs to determine whether plasma-loaded flux ropes formed near the event horizon could reproduce the hot spot observations and help constraining the BH spin. We perform global 3D particle-in-cell simulations in Kerr spacetime of a pair plasma embedded in a strong large-scale magnetic field originating in a disk in prograde Keplerian rotation. A cone-shaped current layer develops which surrounds the twisted open magnetic field lines threading the event horizon. Magnetic field lines coupling the disk to the BH inflate and reconnect a few gravitational radii above the disk. Particles accelerate and accumulate in a few rotating macroscopic flux ropes. Once flux ropes detach, they propagate in the current layer following what appears as a rapidly opening spiral when seen face-on. A single flux rope carries enough relativistic particles to emit synchrotron radiation at levels suitable to reproduce the flares' peak-luminosity of Sgr A* but it quickly fades away as it flows away. Our kinematic analysis of flux ropes' motion favors a BH spin of 0.65 to 0.8 for Sgr A*. The flares' duration require that the underlying magnetic loop seeded in the disk mid-plane has a finite lifetime and azimuthal extent. In this scenario, the hot spot corresponds to a spinning arc along which multiple reconnection sites power the net emission as flux ropes episodically detach.

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arXiv2023

Observing the onset of the accretion wake in Vela X-1

High-Mass X-ray Binaries (HMXBs) offer a unique opportunity for the investigation of accretion onto compact objects and of wind structure in massive stars. A key source for such studies is the bright neutron star HMXB Vela X-1 whose convenient physical and orbital parameters facilitate the analysis and in particular enable studies of the wind structure in HMXBs. Here, we analyse simultaneous XMM-Newton and NuSTAR observations at $ϕ_{\mathrm{orb}} \approx$ 0.36-0.52 and perform time-resolved spectral analysis down to the pulse period of the neutron star, based on our previous NuSTAR-only results. For the first time, we are able to trace the onset of the wakes in a broad 0.5-78 keV range with a high-time resolution of $\sim$283 s and compare to theoretical predictions. We observe a clear rise of the absorption column density of the stellar wind $N_{\mathrm{H,1}}$ starting at orbital phase $\sim$0.44, corresponding to the wake structure entering our line of sight towards the neutron star, together with local extrema throughout the observation possibly associated with clumps or other structures in the wind. Periods of high absorption reveal the presence of multiple fluorescent emission lines of highly ionised species, mainly in the soft X-ray band between 0.5 and 4 keV, indicating photoionisation of the wind.

astro-ph.HE↗