arXiv subjects

Jörn Wilms

Publications and source records attributed to Jörn Wilms.

At least 19 recordsLinked to original sources

arXiv2026

Pulse profiles of accreting neutron stars - A review of analysis methods, observations, and theoretical models

X-ray pulsars are highly magnetized ($B\sim 10^{12}$ G) neutron stars accreting from a donor star. Their characteristic X-ray emission arises from accreted material decelerated from relativistic velocities near the magnetic poles of the neutron star. As our line of sight onto the magnetic poles changes with the rotation of the neutron star, the X-rays are periodically modulated, resulting in X-ray pulsations. The shape of the pulse profiles depends on the physics of the interaction between the bright X-rays from the magnetic poles with the infalling matter, the location of the magnetic poles on the neutron star with respect to its spin axis, and on the properties of the space-time around the neutron star. In this review we give a pedagogical introduction to the accretion mechanisms operating in the various types of accreting neutron star systems and the observational techniques used to characterize the pulse profiles. We summarize how the pulse profiles depend on X-ray luminosity and energy and discuss the attempts to connect theoretically these observables with the physical accretion mechanisms. We conclude with an outline of future observational needs and further developments for theoretical models of magnetic accretion.

astro-ph.HE↗

arXiv2026

Generation of Correlated Time Series for X-ray Astronomy Applications

Cross-spectral methods have become essential for studying accretion physics in X-ray binaries and active galactic nuclei, where coherence and phase lag measurements constrain physical models and reveal variability components invisible in power spectra alone. Recent multi-Lorentzian fitting techniques have uncovered new quasi-periodic features through joint analysis of power spectra and cross-spectra, but testing these methods requires synthetic data with realistic statistical properties. We present an algorithm for generating pairs of time series with arbitrary power spectra, coherence functions, and phase lag profiles. The method extends existing methods by decomposing the dependent time series into coherent and incoherent components, where the coherent part is constructed through a complex transfer function applied to a reference series. We derive the transfer function and normalization required to preserve target spectral shapes while achieving specified cross-spectral properties. We obtain approximate analytic expressions for the variance of coherence and phase lag estimators and construct the approximate covariance matrix relating these quantities to the underlying power and cross-spectra. When fitting models jointly to power and cross-spectra, these correlations must be incorporated into the likelihood. We apply the method to a two-Lorentzian model with component-specific phase lags and demonstrate close agreement between input models and generated power spectra, coherence function, and phase lag profile across four decades in frequency.

astro-ph.HE↗

arXiv2026

Future Detections of the Warm-Hot Intergalactic Medium using Bright Power Law Sources with NewAthena

Context. Hydrodynamical cosmological simulations based on the $Λ$-Cold Dark Matter ($Λ$CDM) model predict that $\sim$40% of the baryons in the local Universe are missing. These missing baryons are predicted to lie in low-density filamentary structures that trace connections between galaxies. This so-called warm-hot intergalactic medium (WHIM) is predicted to be observable in the far-ultraviolet (FUV) and soft X-ray regimes, and detectable in ion species with transitions in these bands. Aims. We investigate the capability of the X-ray Integral Field Unit (X-IFU) on NewAthena to detect this WHIM through O vii absorption lines imprinted on bright power-law sources in soft X-rays (0.3-1.0 keV) by examining the parameter space of different observing conditions. Methods. Through advanced simulations of the X-IFU detector, a multidimensional approach is taken to investigate the relationship between source parameters and their impacts on the significance of detection of a single absorption feature. By first studying the effect of line placement (redshift) for an observation of fixed length, followed by varying the observation length, the required counts for 3$σ$ and 5$σ$ detections are calculated for various photon indices and local Galactic absorptions. The rest-frame equivalent widths of the imprinted absorption features are then varied, and the same requirements are calculated for each line strength. Results. We discuss the detection and recovery of the imprinted WHIM features. Another method is used to derive count requirements for detecting these features at different levels of significance, which is then compared to the previously obtained requirements. Using publicly available data from the Swift archives, various non-transient sources are analysed for their suitability to detect WHIM features in absorption.

astro-ph.IM↗

arXiv2026

Energy-resolved pulse profiles of Vela X-1: cross-calibrating XMM-Newton and NuSTAR to trace spectral features

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. 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. We construct energy-phase matrices for both instruments and derive pulsed fraction spectra after carefully accounting for instrumental and observational effects. We quantify the residual systematics in the overlapping 3-10 keV band. We then model the broadband pulsed fraction spectra phenomenologically and search for timing signatures of spectral features. 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 cyclotron resonant scattering features. Orbital-phase-resolved modeling of the EPIC-pn pulsed fraction spectrum shows that the soft-band features depend strongly on the equivalent absorption column, with emission-line signatures becoming progressively suppressed during highly absorbed intervals. The pulsed fraction spectrum serves both as a quantitative cross-calibration diagnostic and as a powerful spectro-timing diagnostic.

astro-ph.HE↗

arXiv2026

Systematic assessment of disk truncation in the black hole X-ray binary Swift J1727.8-1613 using NICER

The 2023/24 NICER monitoring campaign of the 7 Crab bright black hole X-ray binary Swift J1727.8-1613 covered the outburst in almost all accretion states. High-quality data are available in the high-Eddington-fraction hard-intermediate state, hard-to-soft transition, the soft state, and the poorly studied back-transition to the dim hard state, making it an ideal dataset to compare the accretion flow at vastly different accretion rates. We apply disk continuum fitting techniques to investigate the evolution of the inner disk radius throughout the outburst. Taking a temperature-dependent color-correction factor into account, we see evolution of the disk inner radius by a factor of a few comparing the hard states to the thermal/soft state. We tentatively detect an onset of disk truncation in the soft-to-hard transition, right after the source leaves the soft state. After accounting for model systematics, we find the disk to be more truncated in the high-luminosity bright hard state compared to the low-luminosity dim hard state.

astro-ph.HE↗

arXiv2026

A simple relation: Neutron star magnetic field strength and spectral shape at low mass accretion rates

The X-ray spectra of neutron stars with moderate magnetic fields ($B\sim 10^{12}$ G) in high-mass X-ray binaries (HMXBs) at low X-ray luminosities ($L_\mathrm{X}\lesssim 10^{35}$ erg/s) are characterized by a double humped shape. This shape has been explained either as the radiation from a two-temperature magnetized atmosphere, where thermal radiation dominates at soft X-rays below about 10 keV, and cyclotron radiation with an imprinted cyclotron line dominates at high energies, or by the complex redistribution of primary X-rays in a structured atmosphere. The theoretical explanations of the double humped structure predict the spectra to depend on the magnetic field. We aim to connect the model predictions with observations. We analyzed archival NuSTAR observations of four HMXBs consisting of a neutron star and a Be star (BeXRBs), with known magnetic fields at luminosities low enough to show the characteristic double-hump spectrum. We modeled these spectra empirically and derived a relation between the energy of the intersection of the two humps and the magnetic field strength. In a second step, we tested whether this correlation is supported by fitting synthetic spectra simulated with the physically self-consistent polcap model. We find a linear correlation between the magnetic field strength and the intersection energy for the real BeXRB NuSTAR spectra and polcap-based simulated NuSTAR spectra alike. The effect of the magnetic field on spectral formation results in an observable correlation between the field strength and spectral shape. This derived positive correlation between intersection energy and magnetic field strength also allowed us to roughly estimate the magnetic field strength. Additional observations of XRBs and dedicated modeling efforts will be necessary to determine whether this approach is valid beyond the B-field range that was tested in this work.

astro-ph.HE↗

arXiv2025

Modeling X-ray photon pile-up with a normalizing flow

The dynamic range of imaging detectors flown on-board X-ray observatories often only covers a limited flux range of extrasolar X-ray sources. The analysis of bright X-ray sources is complicated by so-called pile-up, which results from high incident photon flux. This nonlinear effect distorts the measured spectrum, resulting in biases in the inferred physical parameters, and can even lead to a complete signal loss in extreme cases. Piled-up data are commonly discarded due to resulting intractability of the likelihood. As a result, a large number of archival observations remain underexplored. We present a machine learning solution to this problem, using a simulation-based inference framework that allows us to estimate posterior distributions of physical source parameters from piled-up eROSITA data. We show that a normalizing flow produces better-constrained posterior densities than traditional mitigation techniques, as more data can be leveraged. We consider model- and calibration-dependent uncertainties and the applicability of such an algorithm to real data in the eROSITA archive.

astro-ph.HE↗

arXiv2025

Deep XMM-Newton observation reveals hot gaseous outflow in NGC 5746

Context. We present a deep XMM-Newton observation of the massive, edge-on galaxy NGC 5746. The total exposure time of 250 ks provides unprecedented sensitivity to study the diffuse hot gas in the halo, significantly surpassing the depth of previous observations. Aims. While the presence of hot, circumgalactic gas is well tested for starburst galaxies, detections in normal galaxies remain scarce. By studying the diffuse X-ray emission in NGC 5746, we aim to provide new insights into the evolution of star-forming galaxies and their surroundings. Methods. We create X-ray images and surface brightness profiles to quantify the distribution of extraplanar gas in the halo of NGC 5746. In addition, we isolate the diffuse emission component from point source- and background-contamination and study the spectral characteristics of the hot plasma. Results. We detect soft X-ray emission out to 40 kpc from the galactic disc. The gas distribution is reminiscent of a stellar outflow, with two bubbles extending perpendicular to the disc in a biconical shape. The spectral analysis of the halo emission yields a plasma temperature of 0.56 keV, higher than the typical values observed in spiral galaxies (0.2 keV). The disc has an even higher plasma temperature of 0.7 keV, and is dominated by non-thermal emission from unresolved X-ray binaries. The signs of a stellar outflow, bright X-ray emission, and high plasma temperatures indicate that the star-forming activity in NGC 5746 might be higher than previously thought. Conclusions. Our results demonstrate that massive spiral galaxies can host luminous X-ray halos, and support theoretical models that predict their existence. Earlier claims of a lack of hot gas around quiescent spirals might be attributed to the detection thresholds in shallower observations, stressing the need for more, deeper observations of non-starburst galaxies.

astro-ph.GA↗

arXiv2025

The most distant $γ$-ray flare to date: a multiwavelength campaign on the $z = 4.715$ blazar GB6 B1428+4217

In November 2023, the Fermi Large Area Telescope detected a $γ$-ray flare from the high-redshift blazar GB6 B1428+4217 ($z=4.715$). We initiated a multi-wavelength follow-up campaign involving Swift, NuSTAR, the Sierra Nevada and Perkins Observatories, and the Effelsberg 100-m radio telescope. This source, also known as 5BZQ J1430+4204, has shown an anomalous soft X-ray spectrum in previous observations, including possible ionized absorption features or signatures of bulk Comptonization of thermal electrons, which are also detected during the flaring episode. Simultaneous optical data revealed a polarization fraction of ${\sim}8$\% in the R band, confirming that synchrotron emission dominated over thermal emission from the accretion disk. The hard X-ray flux was enhanced during the flare. Modeling of the broadband spectral energy distribution suggests that the high-energy component is dominated by Compton scattering by external seed photons from the accretion disk. The origin of the flare is consistent with the injection of a hard-spectrum electron population in the emission region. With a $γ$-ray luminosity among the top 5% of flaring events, GB6 B1428+4217 exemplifies a prototypical MeV blazar. Its Compton-dominated SED and extreme luminosity are in line with expectations from the blazar sequence. High-redshift flares like this are critical for understanding jet physics in the early Universe and may improve detection prospects with future missions such as COSI.

astro-ph.HE↗

arXiv2025

Long term variability of Cygnus X-1. IX. A spectral-timing comparison of Cygnus X-1 and MAXI J1820+070 in the hard state

Cygnus X-1 is a persistent, high-mass black hole X-ray binary (BHXRB) which in the hard state shows many similar properties to transient BHXRBs, along with intriguing differences, such as the lack of quasi-periodic oscillations. Here, we compare for the first time the detailed spectral-timing properties of Cyg X-1 with a transient BHXRB, MAXI J1820+070, combining data from XMM-Newton and NICER with contemporaneous INTEGRAL data to study the power spectra, rms spectra and time-lags over a broad 0.5 - 200 keV range. We select bright hard state MAXI J1820+070 data with similar power-spectral shapes to the Cyg X-1 data, to compare the source behaviours while accounting for the evolution of spectral-timing properties, notably the lags, through the hard state. Cyg X-1 shows no evidence for soft lags in the 1 - 10 Hz frequency range where they are clearly detected for MAXI J1820+070. Furthermore the low-frequency hard lags and rms-spectra evolve much more strongly during the hard state of Cyg X-1 than for MAXI J1820+070. We argue that these differences cannot be explained by the different black hole masses of these systems, but may be related to their different accretion rates and corresponding locations on the hardness-intensity diagram. We conjecture that there is a significant luminosity-dependence of coronal geometry in the hard state of BHXRBs, rather than an intrinsic difference between Cyg X-1 and transient BHXRBs. This possibility has also been suggested to explain a common time-lag feature that appears in the hard intermediate states of Cyg X-1 and transient BHXRBs.

astro-ph.HE↗

arXiv2025

A hidden quasi-periodic oscillation in Cygnus X-1 revealed by NICER

Cygnus X-1 is a high-mass black hole binary extensively studied since its discovery in 1964. Its rapid X-ray variability provides insights into accretion physics. Unlike other black hole X-ray binaries, its power spectra are generally featureless and modeled with two broad Lorentzians, without requiring narrow quasi-periodic oscillations. We investigate the possibility that some undetected variability components in power spectra may appear in the imaginary part of the cross spectra and the coherence function. Using NICER observations up to Cycle 6, we study the power, cross, and lag spectra, along with the coherence function, searching for these "imaginary" components. We simultaneously fit the power spectra in two energy bands, 0.3-2 keV and 2-12 keV, and the real and imaginary parts of the cross-spectrum with a multi-Lorentzian model. Assuming each Lorentzian is coherent between the two bands but incoherent with others, we predict intrinsic coherence and phase lags. he intrinsic coherence shows a narrow dip at a frequency increasing from ~1 Hz to ~6 Hz as the power-law index of the Comptonized component increases from ~1.8 to ~2.4. Simultaneously, the phase lags exhibit a steep increase (the "cliff") at the same frequencies. These features vanish when using energy bands similar to RXTE (e.g., 3-5 keV and 5-12 keV). A narrow Lorentzian component with low fractional rms and large phase lag is required to reproduce the coherence drop. Its rms and phase-lag spectra evolve systematically in the hardness-intensity diagram. This "imaginary" QPO behaves like a type-C QPO despite being undetectable in power spectra alone. Similar features in MAXI J1348-630 and MAXI J1820+070 support this interpretation, suggesting this may be the first detection of a type-C QPO in Cygnus X-1.

astro-ph.HE↗

arXiv2024

A gamma-ray flare from TXS 1508+572: characterizing the jet of a $z=4.31$ blazar in the early Universe

Blazars can be detected from very large distances due to their high luminosity. However, the detection of $γ$-ray emission of blazars beyond $z=3$ has only been confirmed for a small number of sources. Such observations probe the growth of supermassive black holes close to the peak of star formation in the history of galaxy evolution. As a result from a continuous monitoring of a sample of 80 $z>3$ blazars with Fermi-LAT, we present the first detection of a $γ$-ray flare from the $z=4.31$ blazar TXS 1508+572. This source showed high $γ$-ray activity from February to August 2022, reaching a peak luminosity comparable to the most luminous flares ever detected with Fermi -LAT. We conducted a multiwavelength observing campaign involving XMM-Newton, Swift, the Effelsberg 100-m radio telescope and the Very Long Baseline Array. In addition, we make use of the monitoring programs by the Zwicky Transient Facility and NEOWISE at optical and infrared wavelengths, respectively. We find that the source is particularly variable in the infrared band on daily time scales. The spectral energy distribution collected during our campaign is well described by a one-zone leptonic model, with the $γ$-ray flare originating from an increase of external Compton emission as a result of a fresh injection of accelerated electrons.

astro-ph.HE↗

arXiv2024

Long term variability of Cygnus X-1. VIII. A spectral-timing look at low energies with NICER

The Neutron Star Interior Composition Explorer (NICER) monitoring campaign of Cyg X-1 allows us to study its spectral-timing behavior at energies ${<}1$ keV across all states. The hard state power spectrum can be decomposed into two main broad Lorentzians with a transition at around 1 Hz. The lower-frequency Lorentzian is the dominant component at low energies. The higher-frequency Lorentzian begins to contribute significantly to the variability above 1.5 keV and dominates at high energies. We show that the low- and high-frequency Lorentzians likely represent individual physical processes. The lower-frequency Lorentzian can be associated with a (possibly Comptonized) disk component, while the higher-frequency Lorentzian is clearly associated with the Comptonizing plasma. At the transition of these components, we discover a low-energy timing phenomenon characterized by an abrupt lag change of hard (${\gtrsim}2$ keV) with respect to soft (${\lesssim}1.5$ keV) photons, accompanied by a drop in coherence, and a reduction in amplitude of the second broad Lorentzian. The frequency of the phenomenon increases with the frequencies of the Lorentzians as the source softens and cannot be seen when the power spectrum is single-humped. A comparison to transient low-mass X-ray binaries shows that this feature does not only appear in Cyg X-1, but that it is a general property of accreting black hole binaries. In Cyg X-1, we find that the variability at low and high energies is overall highly coherent in the hard and intermediate states. The high coherence shows that there is a process at work which links the variability, suggesting a physical connection between the accretion disk and Comptonizing plasma. This process fundamentally changes in the soft state, where strong red noise at high energies is incoherent to the variability at low energies.

astro-ph.HE↗

arXiv2024

An in-depth analysis of the variable cyclotron lines in GX 301$-$2

Context. The High-Mass X-ray Binary (HMXB) system GX 301$-$2 is a persistent source with a well-known variable cyclotron line centered at 35 keV. Recently, a second cyclotron line at 50 keV has been reported with a presumably different behavior than the 35 keV line. Aims. We investigate the presence of the newly discovered cyclotron line in the phase-averaged and phase-resolved spectra at higher luminosities than before. We further aim to determine the pulse-phase variability of both lines. Methods. We analyze a NuSTAR observation of GX 301$-$2 covering the pre-periastron flare, where the source luminosity reached its peak of ${\sim} 4 \times 10^{37}\,\mathrm{erg}\,\mathrm{s}^{-1}$ in the 5-50 keV range. We analyze the phase-averaged spectra in the NuSTAR energy range from 3.5-79 keV for both the complete observation and three time segments of it. We further analyze the phase-resolved spectra and the pulse-phase variability of continuum and cyclotron line parameters. Results. We confirm that the description of the phase-averaged spectrum requires a second absorption feature at $51.5^{+1.1}_{-1.0}$ keV besides the established line at 35 keV. The statistical significance of this feature in the phase-averaged spectrum is $>99.999\%$. We further find that the 50 keV cyclotron line is present in three of eight phase bins. Conclusions. Based on the results of our analysis, we confirm that the detected absorption feature is very likely to be a cyclotron line. We discuss a variety of physical scenarios which could explain the proposed anharmonicity, but also outline circumstances under which the lines are harmonically related. We further present the cyclotron line history of GX 301$-$2 and evaluate concordance among each other. We also discuss an alternative spectral model including cyclotron line emission wings.

astro-ph.HE↗

arXiv2023

Stellar wind variability in Cygnus X-1 from high-resolution excess variance spectroscopy with Chandra

Stellar winds of massive stars are known to be driven by line absorption of UV photons, a mechanism which is prone to instabilities, causing the wind to be clumpy. The clumpy structure hampers wind mass-loss estimates, limiting our understanding of massive star evolution. The wind structure also impacts accretion in high-mass X-ray binary (HMXB) systems. We analyse the wavelength-dependent variability of X-ray absorption in the wind to study its structure. Such an approach is possible in HMXBs, where the compact object serves as an X-ray backlight. We probe different parts of the wind by analysing data taken at superior and inferior conjunction. We apply excess variance spectroscopy to study the wavelength-dependent soft X-ray variability of the HMXB Cygnus X-1 in the low/hard spectral state. Excess variance spectroscopy quantifies the variability of an object above the statistical noise as a function of wavelength, which allows us to study the variability of individual spectral lines. As one of the first studies, we apply this technique to high-resolution gratings spectra provided by Chandra, accounting for various systematic effects. The frequency dependence is investigated by changing the time binning. The strong orbital phase dependence we observe in the excess variance is consistent with column density variations predicted by a simple model for a clumpy wind. We identify spikes of increased variability with spectral features found by previous spectroscopic analyses of the same data set, most notably from silicon in over-dense clumps in the wind. In the silicon line region, the variability power is redistributed towards lower frequencies, hinting at increased line variability in large clumps. In prospect of the microcalorimetry missions that are scheduled to launch within the next decade, excess variance spectra present a promising approach to constrain the wind structure.

astro-ph.HE↗

arXiv2023

NICER/NuSTAR Characterization of 4U 1957+11: A Near Maximally Spinning Black Hole Potentially in the Mass Gap

4U 1957+11 is a black hole candidate system that has been in a soft X-ray spectral state since its discovery. We present analyses of recent joint NICER and NuSTAR spectra, which are extremely well-described by a highly inclined disk accreting into a near maximally spinning black hole. Owing to the broad X-ray coverage of NuSTAR the fitted spin and inclination are strongly constrained for our hypothesized disk models. The faintest spectra are observed out to 20 keV, even though their hard tail components are almost absent when described with a simple corona. The hard tail increases with luminosity, but shows clear two track behavior with one track having appreciably stronger tails. The disk spectrum color-correction factor is anti-correlated with the strength of the hard tail (e.g., as measured by the Compton $y$ parameter). Although the spin and inclination parameters are strongly constrained for our chosen model, the mass and distance are degenerate parameters. We use our spectral fits, along with a theoretical prior on color-correction, an observational prior on likely fractional Eddington luminosity, and an observational prior on distance obtained from Gaia studies, to present mass and distance contours for this system. The most likely parameters, given our presumed disk model, suggest a 4.6 $\mathrm{M_\odot}$ black hole at 7.8 kpc observed at luminosities ranging from $\approx 1.7\%$--$9\%$ of Eddington. This would place 4U 1957+11 as one of the few actively accreting sources within the `mass gap' of ${\approx} 2$--$5\,\mathrm{M_\odot}$ where there are few known massive neutron stars or low mass black holes. Higher mass and distance, however, remain viable.

astro-ph.HE↗

arXiv2022

X-ray detection of a nova in the fireball phase

Novae are caused by runaway thermonuclear burning in the hydrogen-rich envelopes of accreting white dwarfs, which results in the envelope to expand rapidly and to eject most of its mass. For more than 30 years, nova theory has predicted the existence of a "fireball" phase following directly the runaway fusion, which should be observable as a short, bright, and soft X-ray flash before the nova becomes visible in the optical. Here we present the unequivocal detection of an extremely bright and very soft X-ray flash of the classical Galactic nova YZ Reticuli 11 hours prior to its 9 mag optical brightening. No X-ray source was detected 4 hours before and after the event, constraining the duration of the flash to shorter than 8 hours. In agreement with theoretical predictions, the source's spectral shape is consistent with a black body of $3.27^{+0.11}_{-0.33}\times 10^5$ K ($28.2^{+0.9}_{-2.8}$ eV), or a white dwarf atmosphere, radiating at the Eddington luminosity, with a photosphere that is only slightly larger than a typical white dwarf. This detection of the expanding white dwarf photosphere before the ejection of the envelope provides the last link of the predicted photospheric lightcurve evolution and opens a new window to measure the total nova energetics.

astro-ph.HE↗

arXiv2022

Orbital decay in M82 X-2

M82 X-2 is the first pulsating ultraluminous X-ray source (PULX) discovered. The luminosity of these extreme pulsars, if isotropic, implies an extreme mass transfer rate. An alternative is to assume a much lower mass transfer rate, but with an apparent luminosity boosted by geometrical beaming. Only an independent measurement of the mass transfer rate can help discriminate between these two scenarios. In this Paper, we follow the orbit of the neutron star for seven years, measure the decay of the orbit ($\dot{P}_{orb}/{P}_{orb}\approx-8\cdot10^{-6}\mathrm{yr}^{-1}$), and argue that this orbital decay is driven by extreme mass transfer of more than 150 times the mass transfer limit set by the Eddington luminosity. If this is true, the mass available to the accretor is more than enough to justify its luminosity, with no need for beaming. This also strongly favors models where the accretor is a highly-magnetized neutron star.

astro-ph.HE↗