arXiv subjects

Nathalie Degenaar

Publications and source records attributed to Nathalie Degenaar.

At least 19 recordsLinked to original sources

arXiv2026

4U 1556-60 as a very faint neutron star X-ray binary at 700 pc with an undetected radio jet

4U 1556-60 is a low-mass X-ray binary that was discovered more than 50 years ago as a persistent X-ray source; however, very little was known about it. Recently, Gaia obtained a parallax for the optical counterpart that places 4U 1556-60 at a distance of only about 700 pc, making it one of the closest X-ray binaries known to date. This close distance drastically alters what was previously assumed about the source. We revisit 4U 1556-60 in light of the newly determined distance of 700 pc, reinterpreting its literature and presenting new X-ray and radio observations to better understand various characteristics of the system. We conclude that a scenario in which 4U 1556-60 is a candidate ultracompact neutron star X-ray binary at a distance of ~700 pc is able to explain the observed properties of the source. It resides at a persistent X-ray luminosity of ~2x10^34 erg/s, an unusual value for a typical X-ray binary, but similar to several ultracompact systems. The ratio of the X-ray to optical luminosity is very high, also suggesting a physically small accretion disk. The radio jet is undetected with a very deep upper limit of 3x10^25 erg/s, which is about 10^3 times fainter than the expected black hole jet correlation, strongly indicating a neutron star accretor. The X-ray spectrum is dominated by a power law, and the X-ray timing properties are also consistent with observations of other very low accretion rate X-ray binaries. No spin or orbital periodicity are found in the X-ray data. Future observations, especially to determine its orbital period, will further aid in understanding 4U 1556-60.

astro-ph.HE↗

arXiv2026

A multiwavelength study of the Galactic center black hole candidate MAXI J1744-294

For the first time in nearly a decade, a bright transient was detected in the central parsec (pc) of the Galaxy. MAXI J1744-294, or -- as it was known in its previous life -- Swift J174540.2-290037, was discovered in outburst by the MAXI telescope in January 2025. We present the results of a broadband, multi-wavelength study of MAXI J1744-294, including data from the NuSTAR, Chandra, XMM-Newton, Swift, and NICER X-ray telescopes, as well as complementary radio and near-infrared observations. We analyze the changing X-ray emission as the outburst evolved from the high/soft to the low/hard state. Using relativistic reflection features in the data, we estimate a spin of $a>0.92$ and viewing inclination $θ=28^{+3}_{-4}$ deg, consistent with the parameters measured for Swift J174540.2-290037. Based on the spectral and temporal characteristics of MAXI J1744-294, we reaffirm its classification as a candidate black hole (BH) low-mass X-ray binary (LMXB) -- the third candidate BH transient discovered within 20 arcsec of the Galactic supermassive black hole Sgr~A*. This work provides further evidence for a cusp of BH-LMXBs in the central pc of our Galaxy, as argued for in previous observational studies and suggested by analytical and theoretical work. Our ongoing multi-wavelength study, involving a complementary range of observatories and spanning different outburst states, can serve as a model for future time domain astrophysics research.

astro-ph.HE↗

arXiv2026

Decoding Cygnus X-2: The Critical Role of Reflection in IXPE Data

We present a spectro-polarimetric re-analysis of the first IXPE observation of Cygnus X-2 which we determine to be mainly in the normal branch, from quasi-simultaneous observations with NuSTAR, NICER, and INTEGRAL. We measure the hard X-ray polarization angle and find it to be consistent with the previously measured position angle of the radio jet. Leveraging NuSTAR's detection of both the relativistic Fe K emission line and the Compton hump, we constrain the flux contribution of the reflected emission from the inner accretion disk to be 10% of the total X-ray flux in the IXPE energy band. Unlike previous studies that modeled only the Fe K emission line, we fit the full-band reflection spectrum using a fully relativistic disk model. There is strong degeneracy between the Comptonized and reflection components. Given that the Comptonized component is not expected to be highly polarized, a polarization degree of approximately 20% for the reflection component could explain the X-ray polarization data from IXPE. We also discuss the disk inclination angle inferred from our spectro-polarimetric modeling, as well as other possible explanations for the data.

astro-ph.HE↗

arXiv2026

The Advanced X-ray Imaging Satellite (AXIS) Community Science Book

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.

astro-ph.HE↗

arXiv2025

The very faint X-ray transient Swift J174610-290018 at the Galactic center

Very Faint X-ray Transients (VFXTs) are a class of X-ray binary systems that exhibit occasional outbursts with peak X-ray luminosities (L_X< 1e36 erg s^-1) much lower than typical X-ray transients. On 22nd February 2024, during its daily Galactic center monitoring, Swift-XRT detected a VFXT, 7 arcmin from Sgr A* dubbing it Swift J174610--290018. We aim to characterize the outburst that occurred in 2024, and a second, distinct outburst in 2025, to understand the nature and accretion flow properties of this new VFXT. Swift-XRT light curves are used to constrain the duration of the two events. We carried out X-ray spectral analysis exploiting XMM and NuSTAR data. We used Chandra and XMM observations of the last 25 years to constrain the quiescent luminosity of the source. During the 2024 outburst, which lasted about 50 days, the source reached a luminosity in the 2-10 keV band of L_X = 1.2e35 erg s^-1 (assuming it is located at the Galactic center). The 2025 outburst is shorter (about 5 days), and reached L_X = 9e34 erg s^-1. The spectral features of the source include an excess at 6.5-7 keV, which can be associated either with a single reflection line or with the ionized Fe XXV and XXVI lines. The same source was identified in both the XMM and Chandra catalogs of point sources (known as 4XMM J174610.7--290020). During previous detections, the source displayed luminosity levels ranging from L_X= 2e32 to L_X = 3e34 erg s^-1 between 2000 and 2010. Moreover, it exhibited a potential type I X-ray burst in 2004. The analysis of the outbursts and the potential type I burst strongly suggests the neutron star low mass X-ray binary (NS-LMXB) nature of the VFXT. The source can be described by an accretion disk corona (as has been recently proposed by the XRISM/Xtend analysis). This scenario explains the overall low luminosity of this transient and the peculiar iron lines in the spectrum.

astro-ph.HE↗

arXiv2025

An XMM long look at the accretion disk plasma in the dipping neutron star LMXB 4U1624-490

Dipping neutron star low-mass X-ray binaries (NS LMXBs) are systems that exhibit periodic drops in their X-ray light curves. These are believed to be caused by material at the impact point of the gas stream onto the accretion disk, the bulge. Dipping systems are observed at high inclination and provide exceptional opportunities to address important open questions about accretion disks, such as the physical properties of the bulge, and the connection between disk atmospheres and disk winds. We aimed to characterize the accretion disk plasmas present in the 21h-period NS LMXB 4U 1624-490, and perform a detailed spectral analysis of the material present at the impact region. We used four XMM EPIC pn observations that were specifically targeting dips, and allow us to probe dipping activity over different timescales (i.e. consecutive orbits and $\sim$6 months). We use both time- and flux-resolved spectroscopic analysis to probe the structural properties of the bulge moving along the line of sight and its homogeneity, respectively. During dipping, the primary spectrum is modulated by an ionized (log$ξ\sim$ 3.4) absorber with varying column density and covering factor, as well as a colder absorber. This suggests that the bulge is a multiphase and clumpy absorbing medium. From size scale arguments, we estimate the number of clumps in the bulge to be $>$7$\times10^{3}$. A highly ionized disk atmosphere becomes evident only when different phases of absorption are analyzed individually. This work demonstrates the feasibility of constructing a physical picture of the bulge, and highlights how future research could reveal how its properties depend on system parameters, and whether the bulge could influence the dynamics of the accretion disk.

astro-ph.HE↗

arXiv2025

Catalog of outbursts of neutron star LMXBs

Many X-ray binaries are transiently accreting. Having statistics on their recurrence times is helpful to address questions related to binary evolution and populations, as well as the physics of binary systems. We compile a catalog of known outbursts of 87 transient neutron star (identified through bursts or pulsations) low-mass X-ray binaries, until mid-2025. Most outbursts are taken from the literature, but we also identify some outbursts from public X-ray monitoring lightcurves. We find 109 outbursts not previously identified in the literature; most are from the frequent transients GRS 1747-312 and the Rapid Burster MXB 1730-335, though we suspect that two outbursts from Liller 1 may be from another transient, besides the Rapid Burster. We also find new outbursts for 10 other systems, and verify substantial quiescent intervals for XMM J174457-2850.3, XMMU J174716.1-281048, and AX J1754.2-2754. Outburst detection has been relatively efficient since 1996 for outbursts above $F_X$(2-10)$=3\times10^{-10}$ ergs/s/cm$^2$. While several systems have many known outbursts, 40 of the 87 systems we track have zero or one recorded outburst between 1996 and 2023. This suggests that many faint Galactic Center X-ray binaries may be neutron star X-ray binaries, though we cannot completely rule out the proposition that most neutron star X-ray binaries undergo frequent outbursts below all-sky monitor detection limits.

astro-ph.HE↗

arXiv2024

Investigating the Ultra-Compact X-ray Binary Candidate SLX 1735-269 with NICER and NuSTAR

We present two simultaneous NICER and NuSTAR observations of the ultra-compact X-ray binary (UCXB) candidate SLX 1735-269 while the source was in two different spectral states. Using various reflection modeling techniques, we find that xillverCO, a model used for fitting X-ray spectra of UCXBs with high carbon and oxygen abundances is an improvement over relxill or relxillns, which instead contains solar-like chemical abundances. This provides indirect evidence in support of the source being ultra-compact. We also use this reflection model to get a preliminary measurement of the inclination of the system, $i = 57^{+23}_{-7}$ degrees. This is consistent with our timing analysis, where a lack of eclipses indicates an inclination of $i<80^{\circ}$. The timing analysis is otherwise inconclusive, and we can not confidently measure the orbital period of the system.

astro-ph.HE↗

arXiv2024

Thermonuclear explosions on neutron stars reveal the speed of their jets

Relativistic jets are observed from accreting and cataclysmic transients throughout the Universe, and have a profound affect on their surroundings. Despite their importance, their launch mechanism is not known. For accreting neutron stars, the speed of their compact jets can reveal whether the jets are powered by magnetic fields anchored in the accretion flow or in the star itself, but to-date no such measurements exist. These objects can display bright explosions on their surface due to unstable thermonuclear burning of recently accreted material, called type-I X-ray bursts, during which the mass accretion rate increases. Here, we report on bright flares in the jet emission for a few minutes after each X-ray burst, attributed to the increased accretion rate. With these flares, we measure the speed of a neutron star compact jet to be $v=0.38^{+0.11}_{-0.08}$c, much slower than those from black holes at similar luminosities. This discovery provides a powerful new tool in which we can determine the role that individual system properties have on the jet speed, revealing the dominant jet launching mechanism.

astro-ph.HE↗

arXiv2023

Simultaneous NICER and NuSTAR Observations of the Ultra-compact X-ray Binary 4U 0614+091

We present the first joint NuSTAR and NICER observations of the ultra-compact X-ray binary (UCXB) 4U 0614+091. This source shows quasi-periodic flux variations on the timescale of ~days. We use reflection modeling techniques to study various components of the accretion system as the flux varies. We find that the flux of the reflected emission and the thermal components representing the disk and the compact object trend closely with the overall flux. However, the flux of the power-law component representing the illuminating X-ray corona scales in the opposite direction, increasing as the total flux decreases. During the lowest flux observation, we see evidence of accretion disk truncation from roughly 6 gravitational radii to 11.5 gravitational radii. This is potentially analogous to the truncation seen in black hole low-mass X-ray binaries, which tends to occur during the low/hard state at sufficiently low Eddington ratios.

astro-ph.HE↗

arXiv2023

A catalogue of unusually long thermonuclear bursts on neutron stars

Rare, energetic (long) thermonuclear (Type I) X-ray bursts are classified either as intermediate-duration or superbursts, based on their duration. Intermediate-duration bursts lasting a few to tens of minutes are thought to arise from the thermonuclear runaway of a relatively thick (10^10 g/cm2) helium layer, while superbursts lasting hours are attributed to the detonation of an underlying carbon layer. We present a catalogue of 84 long thermonuclear bursts from 40 low-mass X-ray binaries, and defined from a new set of criteria distinguishing them from the more frequent short bursts. The three criteria are: (1) a total energy release larger than 10^40 erg, (2) a photospheric radius expansion phase longer than 10 s, and (3) a burst time-scale longer than 70 s. This work is based on a comprehensive systematic analysis of 70 bursts found with INTEGRAL, RXTE, Swift, BeppoSAX, MAXI, and NICER, as well as 14 long bursts from the literature that were detected with earlier generations of X-ray instruments. For each burst, we measure its peak flux and fluence, which eventually allows us to confirm the distinction between intermediate-duration bursts and superbursts. Additionally, we list 18 bursts that only partially meet the above inclusion criteria, possibly bridging the gap between normal and intermediate-duration bursts. With this catalogue, we significantly increase the number of long-duration bursts included in the MINBAR and thereby provide a substantial sample of these rare X-ray bursts for further study.

astro-ph.HE↗

arXiv2023

High-Resolution Spectroscopy of X-ray Binaries

X-ray binaries, as bright local sources with short variability timescales for a wide range of accretion processes, represent ideal targets for high-resolution X-ray spectroscopy. In this chapter, we present a high-resolution X-ray spectral perspective on X-ray binaries, focusing on black holes and neutron stars. The majority of the chapter is devoted to observational and theoretical signatures of mass ejection via accretion disk winds: we discuss their appearance (including an overview of photoionization and thermodynamic processes that determine their visibility in X-ray spectra) and their life cycles (including efforts to constrain their time-dependent mass loss rates), and we provide a broad overview of the primary accretion disk wind driving mechanisms that have been considered in the literature: (1) radiation pressure, where radiation accelerates a wind by scattering off electrons or atoms in the disk or its atmosphere; (2) thermal driving, where Compton heating of the outer accretion disk causes gas thermal velocities to exceed the local escape speed; and (3) magnetohydrodynamic processes, where gas may be ejected from the disk via magnetic pressure gradients or magnetocentrifugal effects. We then turn to spectroscopic constraints on the geometry of accreting systems, from relativistically blurred emission lines to dipping sources, clumpy, structured stellar winds, and baryonic jets. We conclude with discussions of measurements of the interstellar medium and the potential of next-generation high-resolution X-ray spectroscopy for X-ray binaries.

astro-ph.HE↗

arXiv2022

Low-Mass X-ray Binaries

A large fraction of X-ray sources in our Galaxy are low-mass X-ray binaries, containing a black hole or a neutron star accreting from a gravitationally bound low-mass ($\leq$1 M$_\odot$) companion star. These systems are among the older population of stars and accreting systems in the Galaxy, and typically have long accretion histories. Low-mass X-ray binaries are categorized into various sub-classes based on their observed properties such as X-ray variability and brightness, nature of the companion star and/or the compact object, and binary configuration. In this Chapter, we review the phenomenology of sub-classes of these systems and summarize observational finding regarding their characteristics, populations, and their distribution in the Galaxy.

astro-ph.HE↗

arXiv2022

On the recurrence times of neutron star X-ray binary transients and the nature of the Galactic Centre quiescent X-ray binaries

The presence of some X-ray sources in the Galactic Centre region which show variability, but do not show outbursts in over a decade of monitoring has been used to argue for the presence of a large population of stellar mass black holes in this region. A core element of the arguments that these objects are accreting black holes is the claim that neutron stars (NSs) in low mass X-ray binaries (LMXBs) do not have long transient recurrence times. We demonstrate in this paper that about half of the known transient LMXBs with clear signatures for NS primaries have recurrence times in excess of a decade for outbursts at the sensitivity of MAXI. We furthermore show that, in order to reconcile the expected total population of NS LMXBs with the observed one and with the millisecond radio pulsar (MSRP) population of the Galaxy, systems with recurrence times well in excess of a century for outbursts detectable by instruments like MAXI must be the dominant population of NS LMXBs, and that few of these systems have yet been discovered.

astro-ph.HE↗

arXiv2022

A "Hyperburst" in the MAXI J0556-332 Neutron Star: Evidence for a New Type of Thermonuclear Explosion

The study of transiently accreting neutron stars provides a powerful means to elucidate the properties of neutron star crusts. We present extensive numerical simulations of the evolution of the neutron star in the transient low-mass X-ray binary MAXI J0556--332. We model nearly twenty observations obtained during the quiescence phases after four different outbursts of the source in the past decade, considering the heating of the star during accretion by the deep crustal heating mechanism complemented by some shallow heating source. We show that cooling data are consistent with a single source of shallow heating acting during the last three outbursts, while a very different and powerful energy source is required to explain the extremely high effective temperature of the neutron star, ~350 eV, when it exited the first observed outburst. We propose that a gigantic thermonuclear explosion, a "hyperburst" from unstable burning of neutron rich isotopes of oxygen or neon, occurred a few weeks before the end of the first outburst, releasing 10^44 ergs at densities of the order of 10^11 g/cm^3. This would be the first observation of a hyperburst and these would be extremely rare events as the build up of the exploding layer requires about a millennium of accretion history. Despite its large energy output, the hyperburst did not produce, due to its depth, any noticeable increase in luminosity during the accretion phase and is only identifiable by its imprint on the later cooling of the neutron star.

astro-ph.HE↗

arXiv2021

Revisiting the archetypical wind accretor Vela X-1 in depth -- A case study of a well-known X-ray binary and the limits of our knowledge

Context: Vela X-1 is one of the best studied X-ray binaries. Frequently though, specific values for its parameters have been used in subsequent studies without considering alternatives. Aims: We aim to provide a robust compilation and synthesis of the accumulated knowledge about Vela X-1 as a solid baseline for future studies and identify specific avenues of possible future research. Methods: We explore the literature for Vela X-1 and on modelling efforts, describing the evolution of the system knowledge. We also add information derived from public data, especially the Gaia EDR3 release. Results: We update the distance to Vela X-1, the spectral classification for HD 77518 and find that the supergiant may be very close to filling its Roche lobe. Constraints on the clumpiness of the stellar wind have improved. The orbit is very well determined, but the uncertain inclination limits information on the neutron star mass. Estimates for the stellar wind have evolved towards lower velocities, supporting the idea of transient wind-captured disks around the neutron star. Hydrodynamic models and observations are consistent with an accretion wake trailing the neutron star. Conclusions: Vela X-1 is an excellent laboratory, but a lot of room remains to improve. Well-coordinated multi-wavelength observations and campaigns addressing the intrinsic variability are required. New opportunities will arise through new instrumentation. Models of the stellar wind should account for the orbital eccentricity and the non-spherical shape of HD 77581. Realistic multi-dimensional models of radiative transfer in the UV and X-rays are needed, but remain very challenging. Improved MHD models covering a wide range of scales would be required to improve understanding of the plasma-magnetosphere coupling. A full characterization of the accretion column remains another open challenge. (Abbreviated for arXiv)

astro-ph.HE↗

arXiv2021

Exploring the tilted accretion disc of AQ Men with TESS

AQ Men is a nova-like variable which is presumed to have a tilted, precessing accretion disc. Grazing eclipses in this system have been speculated to be useful in exploring the geometry of its accretion disc. In this work we analysed TESS observations of AQ Men, which provide the best light curve of this object thus far. We show that the depths of the eclipses are changing with the orientation of the accretion disc, which means that they can serve as a direct test of the tilted accretion disc models. The precession period of the accretion disc is increasing during the TESS observations. However, it is still shorter than the period determined in the previous studies. The amplitude of the variability related to the precession of the accretion disc varies, and so does the shape of this variability. Moreover, we have detected a positive superhump that was previously unseen in AQ Men. Interestingly, the positive superhump has a strongly non-sinusoidal shape, which is not expected for a nova-like variable.

astro-ph.SR↗

arXiv2021

Discovery of UV millisecond pulsations and moding in the low mass X-ray binary state of transitional millisecond pulsar J1023+0038

PSR J1023+0038 is a rapidly-spinning neutron star with a low-mass-binary companion that switches between a radio pulsar and low-luminosity disk state. In 2013, it transitioned to its current disk state accompanied by brightening at all observed wavelengths. In this state, PSR J1023+0038 now shows optical and X-ray pulsations and abrupt X-ray luminosity switches between discrete 'low' and 'high' modes. Continuum radio emission, denoting an outflow, is also present and brightens during the X-ray low modes. Here, we present a simultaneous optical, ultraviolet (UV) and X-ray campaign comprising Kepler ($400-800$ nm), Hubble Space Telescope ($180-280$ nm), XMM-Newton ($0.3-10$ keV) and NuSTAR ($3 - 79$ keV). We demonstrate that low and high luminosity modes in the UV band are strictly simultaneous with the X-ray modes and change the UV brightness by a factor of $\sim25$\% on top of a much brighter persistent UV component. We find strong evidence for UV pulsations (pulse fraction of $0.82\pm0.19$\%) in the high-mode, with a similar waveform as the X-ray pulsations making it the first known UV millisecond pulsar. Lastly, we find that the optical mode changes occur synchronously with the UV/X-ray mode changes, but optical modes are inverted compared to the higher frequencies. There appear to be two broad-band emission components: one from radio to near-infrared/optical that is brighter when the second component from optical to hard X-rays is dimmer (and vice-versa). We suggest that these components trace switches between accretion into the neutron star magnetosphere (high-energy high-mode) versus ejection of material (low-energy high-mode). Lastly, we propose that optical/UV/X-ray pulsations can arise from a shocked accretion flow channeled by the neutron star's magnetic field.

astro-ph.HE↗