White Dwarf Research Articles

Cosmological Inhomogeneities, Primordial Black Holes, and a Hypothesis on the Death of the Universe

We study the impact of the expansion of the universe on a broad class of objects, including black holes, neutron stars, white dwarfs, and others. Using metrics that incorporate primordial inhomogeneities, the effects of a hypothetical “center of the universe” on inflation are calculated. Dynamic coordinates for black holes that account for expansions or contractions with arbitrary rates are provided. We consider the possibility that the universe may be bound to evolve into an ultimate state of “total dilution”, wherein stable particles are so widely separated that physical communication among them will be impossible for eternity. This is also a scenario of “cosmic virtuality”, as no wave-function collapse would occur again. We provide classical models evolving this way, based on the Majumdar–Papapetrou geometries. More realistic configurations, instead, indicate that gravitational forces locally counteract expansion, except in the universe’s early stages. We comment on whether quantum phenomena may dictate that total dilution is indeed the cosmos’ ultimate destiny.

Bounds on ALP-mediated dark matter models from celestial objects

We have studied the signals from axion-like particles (ALPs) as dark matter mediators from celestial objects such as neutron stars, brown dwarfs or white dwarfs. We consider the accumulation of dark matter inside the celestial objects using the multiscatter capturing process. The production of ALP from the dark matter annihilation can escape the celestial object and decay into gamma-rays and neutrinos before reaching the Earth. We investigate our model using gamma-ray observations from Fermi and H.E.S.S. and neutrino observations from IceCube and ANTARES. The effective Lagrangian approach allows us to place constraints on the ALP-photon and ALP-fermion couplings. In the gamma-ray channel, our results are able to rule out the existence of ALP with mass up to ~ O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(10) GeV. On the other hand, the neutrino observations can be used to probe a higher mass range with ALP mass up to ~ O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(100) GeV.

Uncertainty of the white dwarf astrophysical gravitational wave background

The astrophysical gravitational wave background (AGWB) is a stochastic gravitational wave (GW) signal emitted by different populations of in-spiralling binary systems containing compact objects throughout the Universe . In the frequency range between 10$^ $ and 10$^ $ hertz (Hz), it will be detected by future space-based gravitational wave detectors, such as Laser Interferometer Space Antenna (LISA). In a recent work, we concluded that the white dwarf (WD) contribution to the AGWB dominates that of black holes (BHs) and neutron stars (NSs) We aim to investigate the uncertainties of the WD AGWB that arise from the use of different stellar metallicities, star formation rate density (SFRD) models, and binary evolution models. We used the code we previously developed to determine the WD component of the AGWB. We used a metallicity-dependent SFRD based on an earlier work to construct five different SFRD models. We used four different population models based on a range of common-envelope treatments and six different metallicities for each model. For all possible combinations, the WD component of the AGWB is dominant over other populations of compact objects. The effects of metallicity and population model are less significant than the effect of a (metallicity dependent) SFRD model. We find a range of about a factor of 5 in the level of the WD AGWB around a value of $ WD $ at 1 mHz and a shape that is weakly dependent on the model. We find the uncertainty for the WD component of the AGWB to be about a factor of 5. We note that there are other uncertainties that have an effect on this signal as well. We discuss whether the turnover of the WD AGWB at 10 mHz will be detectable by LISA and find it to be likely. We confirm our previous findings asserting that the WD component of the AGWB dominates over other populations, in particular, BHs.

High γ-ray escape time in 2003fg-like supernovae: a challenge to proposed models

ABSTRACT A rare subclass of Type Ia supernovae (SNe Ia), named after the prototype SN 2003fg, includes some of the brightest SNe Ia, often called ‘super Chandrasekhar-mass’ SNe Ia. We calculate the $\gamma$-ray deposition histories and the $^{56}$Ni mass synthesized in the explosion, $M_\mathrm{Ni56}$, for eight 2003fg-like SNe. Our findings reveal that the $\gamma$-ray escape time, $t_0$, for these objects is $t_0\approx 45\!-\!60 \,$ d, significantly higher than that of normal SNe Ia. 2003fg-like SNe are distinct from normal SNe Ia in the $t_0$–$M_\mathrm{Ni56}$ plane, with a noticeable gap between the two populations. The observed position of 2003fg-like SNe in this plane poses a significant challenge for theoretical explosion models. We demonstrate that the merger of two white dwarfs (WDs) and a single star exceeding the Chandrasekhar limit fail to reproduce the observed $t_0$–$M_\mathrm{Ni56}$ distribution. However, preliminary calculations of head-on collisions of massive WDs show agreement with the observed $t_0$–$M_\mathrm{Ni56}$ distribution.

Fundamental Parameters of a Binary System Consisting of a Red Dwarf and a Compact Star

TIC 157365951 has been classified as a δ Scuti type by the International Variable Star Index. Through the spectra from Large Sky Area Multi-Object Fiber Spectroscopic Telescope and its light curve, we further discovered that it is a binary system. This binary system comprises a red-dwarf star and a compact star. Through the spectral energy distribution fitting, we determined the mass of the red dwarf star as M 1 = 0.31 ± 0.01M ⊙ and its radius as R 1 = 0.414 ± 0.004R ⊙. By fitting the double-peaked Hα emission, we derived the mass ratio of q = 1.76 ± 0.04, indicating a compact star mass of M 2 = 0.54 ± 0.01M ⊙. Using Phoebe to model the light curve and radial velocity curve for the detached binary system, we obtained a red dwarf star mass of M 1 = 0.29 ± 0.02M ⊙, a radius of R 1 = 0.39 ± 0.04R ⊙, and a Roche–lobe filling factor of f = 0.995 ± 0.129, which is close to the f = 1 expected for a semidetached system. The Phoebe model gives a compact star mass M 2 = 0.53 ± 0.05M ⊙. Constraining the system to be semidetached gives M 1 = 0.34 ± 0.02M ⊙, R 1 = 0.41 ± 0.01R ⊙, and M 2 = 0.62 ± 0.03M ⊙. The consistency of the models is encouraging. The value of the Roche–lobe filling factor suggests that there might be ongoing mass transfer. The compact star mass is as massive as a typical white dwarf.

Tidal Spin-up of Subdwarf B Stars

Hot subdwarf B (sdB) stars are stripped helium-burning stars that are often found in close binaries, where they experience strong tidal interactions. The dissipation of tidally excited gravity waves alters their rotational evolution throughout the sdB lifetime. While many sdB binaries have well-measured rotational and orbital frequencies, there have been few theoretical efforts to accurately calculate the tidal torque produced by gravity waves. In this work, we directly calculate the tidal excitation of internal gravity waves in realistic sdB stellar models and integrate the coupled spin–orbit evolution of sdB binaries. We find that for canonical sdB (M sdB = 0.47 M ⊙) binaries, the transitional orbital period below which they could reach tidal synchronization in the sdB lifetime is ∼0.2 day, with weak dependence on the companion masses. For low-mass sdBs (M sdB = 0.37 M ⊙) formed from more massive progenitor stars, the transitional orbital period becomes ∼0.15 day. These values are very similar to the tidal synchronization boundary (∼0.2 day) evident from observations. We discuss the dependence of tidal torques on stellar radii, and we make predictions for the rapidly rotating white dwarfs formed from synchronized sdB binaries.

Crystallization onset in white dwarfs

We studied the thermal evolution of the central region of a $0.9 \, M_ C/O white dwarf at the initial stage of ion mixture crystallization by numerically solving the heat equation on a fine spatial and temporal grid and by including a detailed treatment of the latent heat release. The formation of two spherical shells is observed. The outer one surrounds a region where crystallization has begun. The inner one bounds a fully solidified core that has exhausted its latent heat. The region between the shells is partially liquid and partially solid. It gradually emits the latent heat generated by crystallization and also releases light elements (carbon) in the process of element redistribution, which accompanies the mixture solidification. Assuming that all released light elements cross the outer shell, we estimated the flux induced by the mixture crystallization. The resulting flux is not divergent and is much lower than the estimate derived from the growth rate of the fully crystallized core.

СЕМАНТИЧЕСКАЯ БЛИЗОСТЬ АСТРОНОМИЧЕСКИХ ПОНЯТИЙ, ОБОЗНАЧАЮЩИХ НЕБЕСНЫЕ ТЕЛА

The problem of determining the semantic similarity of astronomical terms denoting celestial objects in a school astronomy course is considered. A distributive method is used, which consists in comparing the distributions of concepts in the astronomy textbook. With the help of special computer programs, punctuation marks and stop words are removed from the text, the number of mentions of various terms is deter-mined, the cosine measure of proximity and the semantic distance between concepts are calculated. This allows: 1) to reveal frequently used astronomical concepts denoting celestial bodies; 2) to obtain matrices of proximity and semantic distances between the concepts of "galaxy", "star", "Sun", "planet", "black hole", "white dwarf", etc.; 3) to create a mental map of the celestial bodies that make up the Solar system, taking into account the distances between the terms designating them 4) to build clouds of concepts "types of stars", "Earth – star – Universe". To model the semantic space of the textbook, the Graf.pas computer program is used, which randomly moves vertices in 2D or 3D space, each time calculating the potential energy W of the elastic rods connecting them and choosing such an arrangement of vertices that W is minimal. It is believed that the closely located vertices are connected by rigid rods, and the far removed ones are connected by elastic rubber threads.

Exploring faint white dwarfs and the luminosity function with Subaru HSC and SDSS in Stripe 82

Abstract We present 5,080 white dwarf (WD) candidates selected from stars matching between the multi-band imaging datasets of the Subaru Hyper Suprime-Cam (HSC) Survey and the Sloan Digital Sky Survey (SDSS) in the Stripe82 region covering about 165 deg2. We select WD candidates from the “reduced proper motion” diagram by combining the apparent magnitude in the range i = 19 – 24 and the proper motion measured from the datasets among a baseline of ∼ 14 years. We refine the WD candidates by fitting blackbody and template WD atmosphere models to HSC photometries for each candidate, enabling the estimation of distance and tangential velocity (vt). The deep HSC data allow us to identify low-temperature (<4000 K) and faint WD candidates down to absolute magnitude, Mbol ≃ 17. We evaluate the selection function of our WD candidates using a mock catalogue of spatial and kinematic distributions of WDs in the (thin and thick) disc and halo regions based on a Galactic model. We construct samples of disc and halo WD candidates by selecting WDs with tangential velocity, 40 < vt/[km s−1] < 80 and 200 < vt/[km s−1] < 500, respectively. The total number densities of the disc and halo WDs are (9.33 ± 0.89) × 10−3 pc−3 and (6.34 ± 2.90) × 10−4 pc−3. Our luminosity functions (LF) extend down to fainter absolute magnitudes compared with previous work. The faint WDs could represent the oldest generation of building blocks over the past ∼10 billion years of the assembly history of our Milky Way.

Hidden Companions to Intermediate-mass Stars. Upgraded Multiplicity 2 → 3. XXIII. Discovery of a 0.94M ⊙, 9.2 au Companion to the Ap Star Gamma Piscis Austrini A* *Based on observations collected at the European Southern Observatory, Chile, Program ID 113.26GM.001.

Gamma Piscis Austrini is a nearby ρ = 4.″1 ↔ 284 au visual binary consisting of an A0Vp primary (A) and an F5V secondary (B). Here we report that the primary is actually a closer binary based on a VLTI/GRAVITY interferometric observation. The newly discovered companion Ab has a K band flux ratio 4.8% ± 0.2% and a projected separation ρ = 143.50 mas ↔ 9.2 au relative to Aa. We estimate isochrone masses M Aa = 2.65M ⊙, M Ab = 0.94M ⊙ and M B = 1.20M ⊙. γ PsA will become a triple white dwarf system within a Hubble time, with a low enough hierarchy for possibly interesting dynamical effects with a timescale P KL ∼ 200 kyr.

Research on the Evolution and Core Structure of Low Mass Stars

This article studies the complete evolution process of low-quality stars from main sequence stars to red giants, and then to white dwarfs, and analyzes in detail the physical characteristics of the stellar core. By applying the stellar structure equation, energy generation rate formula, and degeneracy pressure formula, this paper derives the evolution laws of temperature and density of the stellar core over time, and explores the influence of these physical properties on different stages of stellar evolution. Through detailed analysis of the stellar core at various stages, this article reveals the physical evolution mechanism of low mass stars during their lifecycle.

Dark lens candidates from Gaia Data Release 3

Gravitational microlensing is a phenomenon that allows us to observe the dark remnants of stellar evolution, even if these bodies are no longer emitting electromagnetic radiation. In particular, it can be useful to observe solitary neutron stars or stellar-mass black holes, providing a unique window through which to understand stellar evolution. Obtaining direct mass measurements with this technique requires precise observations of both the change in brightness and the position of the microlensed star. The European Space Agency's satellite can provide both. Using publicly available data from different surveys, we analysed events published in the Data Release 3 ( ) microlensing catalogue. Here, we describe our selection of candidate dark lenses, where we suspect the lens is a white dwarf (WD), a neutron star (NS), a black hole (BH), or a mass-gap object, with a mass in the range between the heaviest NS and the least massive BH. We estimated the mass of the lenses using information obtained from the best-fitting microlensing models, source star, Galactic model, and the expected parameter distributions. We found eleven candidates for dark remnants: one WDs, three NSs, three mass-gap objects, and four BHs.

A Statistical, Photometric, and SED Analysis to Characterize the BSS Populations in Old Open Cluster: Berkeley 39

ABSTRACT We present a statistical, photometric, and spectral energy distribution (SED) analysis to characterize the blue straggler stars (BSSs) populations in the Galactic old open cluster Berkeley 39. Berkeley 39 is a 6.16 Gyr old open cluster located at a distance of 3.99 Kpc.Gaia DR3 astrometry data have been used to estimate the membership probabilities using ensemble-based unsupervised machine learning techniques. We identified 21 BSS candidates on the colour–magnitude diagram, with 19 of them being detected in the Swift/UVOT UVW2 filter. We analysed the radial surface density profile and examined the cluster dynamical states and mass segregation effect. The SEDs of 19 BSSs are constructed using multiwavelength data covering UV to IR wavelengths. A single-component SED is fitted successfully for 14 BSS candidates. We discovered hot companions in five BSS candidates. These hot companions have temperatures of approximately 14 000 to 23 000 K, radii ranging from 0.04 to 0.13 R$_{\odot }$, and luminosities ranging from 0.16 to 2.91 L$_{\odot }$. Among these, three are most likely extremely low-mass white dwarfs (WDs) with masses around 0.17 to 0.18 M$_{\odot }$, and two are low-mass WDs with masses around 0.18 to 0.39 M$_{\odot }$. This confirms that they are post-mass transfer (Case A or Case B) systems. We also investigated the variable characteristics of BSSs by analysing their light curves using data from TESS. Our analysis confirms that two BSSs identified as eclipsing binaries in Gaia DR3 are indeed eclipsing binaries. Additionally, one of the two eclipsing binary BSSs shows evidence of having hot companions, as indicated by the multiwavelength SEDs.

3D hydrodynamic simulations of white dwarf–main-sequence star collisions – I. Head-on collisions

ABSTRACT Recently inaugurated telescopes, such as the MeerKAT radio telescope and the upcoming Rubin Observatory Legacy Survey of Space and Time, will be able to detect millions of transient events in the night sky. Stellar collisions between white dwarfs (WDs) and main-sequence (MS) stars may be detectable among such transients. Simulations will play a key role in characterizing these events and selecting targets for follow-up. We present 3D smoothed particle hydrodynamics models of dynamical interactions between a $0.6\ {\rm M}_{\odot }$ WD and 0.3, 0.6, and $1.2\ {\rm M}_{\odot }$ MS stars within globular cluster environments. Utilizing a 34-isotope nuclear network, we investigate the energetics, gas morphologies, and mass-loss properties of these collisions for different stellar mass ratios. Our models predict an overabundance of $^{13}$C, $^{15}$N, and $^{17}$O isotopes relative to solar abundances. Moreover, we find that the time-scale of the collisions is too short and maximum temperatures too low for any significant hydrogen burning or triple-alpha reactions to occur. This combined with a negligible production of elements heavier than neon may be key signatures in distinguishing these events from other transient events with similar peak bolometric luminosities ($\sim\!\! 10^{38}\!\!-\!\!10^{41}$ erg s$^{-1}$).

MASTER OT J030227.28+191754.5: An unprecedentedly energetic dwarf nova outburst

Abstract We present a detailed study of the MASTER OT J030227.28$+$191754.5 outburst in 2021–2022, which reached an amplitude of $10.2\:$mag and a duration of $60\:$d. The detections of (1) the double-peaked optical emission lines, and (2) the early and ordinary superhumps, established that MASTER OT J030227.28$+$191754.5 is an extremely energetic WZ Sge-type dwarf nova (DN). Based on the superhump observations, we obtained its orbital period and mass ratio as $0.05986(1)\:$d and 0.063(1), respectively. These values are within a typical range for low-mass-ratio DNe. According to the binary parameters derived based on the thermal–tidal instability model, our analyses showed that (1) the standard disk model requires an accretion rate $\simeq\!\! 10^{20}\:$g$\:$s$^{-1}$ to explain its peak optical luminosity, and (2) large mass was stored in the disk at the outburst onset. These factors cannot be explained solely by the impact of its massive ($\gtrsim\!\! 1.15\, M_{\odot }$) primary white dwarf implied by Kimura et al. (2023, ApJ, 951, 124). Instead, we propose that the probable origin of this enormously energetic DN outburst is the even lower quiescence viscosity than other WZ Sge-type DNe. This discussion is qualitatively valid for most possible binary parameter spaces unless the inclination is low enough ($\lesssim\!\! 40^\circ$) for the disk to be bright, explaining the outburst amplitude. Such low inclinations, however, would not allow detectable amplitude of early superhumps in the current thermal–tidal instability model. The optical spectra at outburst maximum showed strong emission lines of the Balmer, He i, and He ii series, the core of which is narrower than $\sim \! 800\:$km$\:$s$^{-1}$. Considering its binary parameters, a Keplerian disk cannot explain this narrow component, but the presumable origin is disk winds.

Most extremely low mass white dwarfs with non-degenerate companions are inner binaries of hierarchical triples

Abstract Extremely-low-mass white dwarfs (ELM WDs) with non-degenerate companions are believed to originate from solar-type main-sequence binaries undergoing stable Roche lobe overflow mass transfer when the ELM WD progenitor is at (or just past) the termination of the main-sequence. This implies that the orbital period of the binary at the onset of the first mass transfer phase must have been ≲ 3 − 5 d. This prediction in turn suggests that most of these binaries should have tertiary companions since ≈90 per cent of solar-type main-sequence binaries in that period range are inner binaries of hierarchical triples. Until recently, only precursors of this type of binaries have been observed in the form of EL CVn binaries, which are also known for having tertiary companions. Here, we present high-angular-resolution images of TYC 6992-827-1, an ELM WD with a sub-giant (SG) companion, confirming the presence of a tertiary companion. Furthermore, we show that TYC 6992-827-1, along with its sibling TYC 8394-1331-1 (whose triple companion was detected via radial velocity variations), are in fact descendants of EL CVn binaries. Both TYC 6992-827-1 and TYC 8394-1331-1 will evolve through a common envelope phase, which depending on the ejection efficiency of the envelope, might lead to a single WD or a tight double WD binary, which would likely merge into a WD within a few Gyr due to gravitational wave emission. The former triple configuration will be reduced to a wide binary composed of a WD (the merger product) and the current tertiary companion.

Analyzing Supervised Machine Learning Models for Classifying Astronomical Objects Using Gaia DR3 Spectral Features

In this paper, we present an analysis of the effectiveness of various machine learning algorithms in classifying astronomical objects using data from the third release (DR3) of the Gaia space mission. The dataset used includes spectral information from the satellite’s red and blue spectrophotometers. The primary goal is to achieve reliable classification with high confidence for symbiotic stars, planetary nebulae, and red giants. Symbiotic stars are binary systems formed by a high-temperature star (a white dwarf in most cases) and an evolved star (Mira type or red giant star); their spectra varies between the typical for these objects (depending on the orbital phase of the object) and present emission lines similar to those observed in PN spectra, which is the reason for this first selection. Several classification algorithms are evaluated, including Random Forest (RF), Support Vector Machine (SVM), Artificial Neural Networks (ANN), Gradient Boosting (GB), and Naive Bayes classifier. The evaluation is based on different metrics such as Precision, Recall, F1-Score, and the Kappa index. The study confirms the effectiveness of classifying the mentioned stars using only their spectral information. The models trained with Artificial Neural Networks and Random Forest demonstrated superior performance, surpassing an accuracy rate of 94.67%.

The evolutionary history of GD 1400AB, a white dwarf–brown dwarf binary

ABSTRACT GD1400AB was one of the first known white dwarf $+$ brown dwarf binaries and is the only one of these systems where the white dwarf is a ZZ Ceti pulsator. Here, we present both radial velocity measurements and time-series photometry, analysing both the white dwarf pulsations and the effects of irradiation on the brown dwarf. We find that the brightness temperatures of 1760 $\pm $ 10 K for the nightside and 1860 $\pm$ 10 K for the dayside indicate that the brown dwarf is hotter than spectra have previously suggested, although brightness temperatures calculated using a larger radius for the brown dwarf are consistent with previously determined spectral types. We also discuss the likely evolutionary pathway of this binary and put its common envelope phase into context with the other known systems.

Blue large-amplitude pulsators formed from the merger of low-mass white dwarfs

Blue large-amplitude pulsators (BLAPs) are a recently discovered group of hot stars pulsating in radial modes. Their origin needs to be explained, and several scenarios for their formation have already been proposed. We investigate whether BLAPs can originate as the product of a merger of two low-mass white dwarfs (WDs) and estimate how many BLAPs can be formed in this evolutionary channel. We used the Modules for Experiments in Stellar Astrophysics (MESA) code to model the merger of three different double extremely low-mass (DELM) WDs and the subsequent evolution of the merger product. We also performed a population synthesis of Galactic DELM WDs using the COSMIC code. We find that BLAPs can be formed from DELM WDs provided that the total mass of the system ranges between 0.32 and 0.7\,M$_ BLAPs born in this scenario either do not have any thermonuclear fusion at all or show off-centre He burning. The final product evolves to hot subdwarfs and eventually finishes its evolution either as a cooling He WD or a hybrid He/CO WD. The merger products become BLAPs only a few thousand years after coalescence, and it takes them 20\,--\,70 thousand years to pass the BLAP region. We found the instability of the fundamental radial mode to be in fair agreement with observations, but we also observed instability of the radial first overtone. The calculated evolutionary rates of period change can be both positive and negative. From the population synthesis, we found that up to a few hundred BLAPs born in this scenario can exist at present in the Galaxy. Given the estimated number of BLAPs formed in the studied DELM WD merger scenario, there is a good chance to observe BLAPs that originated through this scenario. Since strong magnetic fields can be generated during mergers, this scenario could lead to the formation of magnetic BLAPs. This fits well with the discovery of two likely magnetic BLAPs whose pulsations can be explained in terms of the oblique rotator model.

Discovery of persistent quasi-periodic oscillations in accreting white dwarfs: a new link to X-ray binaries

ABSTRACT Almost all accreting black hole and neutron star (NS) X-ray binary systems (XRBs) exhibit prominent brightness variations on a few characteristic time-scales and their harmonics. These quasi-periodic oscillations (QPOs) are thought to be associated with the precession of a warped accretion disc, but the physical mechanism that generates the precessing warp remains uncertain. Relativistic frame dragging (Lense–Thirring precession) is one promising candidate, but a misaligned magnetic field is an alternative, especially for NS XRBs. Here, we report the discovery of five accreting white dwarf systems (AWDs) that display strong optical QPOs with characteristic frequencies and harmonic structures that suggest they are the counterpart of the QPOs seen in XRBs. Since AWDs are firmly in the classical (non-relativistic) regime, Lense–Thirring precession cannot account for these QPOs. By contrast, a weak magnetic field associated with the white dwarf can drive disc warping and precession in these systems, similar to what has been proposed for NS XRBs. Our observations confirm that magnetically driven warping is a viable mechanism for generating QPOs in disc-accreting astrophysical systems, certainly in AWDs and possibly also in NS XRBs. Additionally, they establish a new way to estimate magnetic field strengths, even in relatively weak-field systems where other methods are not available.