Magnetic White Dwarfs Research Articles

An X-Ray-dim “Isolated” Neutron Star in a Binary?

We report the discovery of a dark companion to 2MASS J15274848+3536572 with an orbital period of 6.14 hr. Combining the radial velocity from LAMOST observations and modeling of the multiband light curve, one obtains a mass function of ≃0.131 M ⊙, an inclination of , and a mass ratio of , which demonstrate the binary nature of the dark companion with a mass of 0.98 ± 0.03M ⊙ and a main-sequence K9-M0 star of 0.62 ± 0.01 M ⊙. LAMOST optical spectra at a range of orbital phases reveal extra-peaked H α emission that suggests the presence of an accretion disk. The dark companion does not seem to be a white dwarf because of the lack of any observed dwarf nova outbursts in the long-term data archive, although a magnetic white dwarf cannot be excluded. Alternatively, we propose a scenario wherein the dark companion is a neutron star, but we have not detected radio pulsations or a single pulse from the system with the FAST (Five-hundred-meter Aperture Spherical radio Telescope), which hints at a radio-quiet compact object. If the dark companion is identified as a neutron star, it will be the nearest (∼118 pc) and lightest neutron star. Furthermore, kinematic analysis of the system’s orbit in the galaxy may suggest its supernova event is associated with the radionuclide 60Fe signal observed from deep-sea crusts. This radio-quiet and X-ray-dim nearby neutron star may resemble an XDINS (X-ray-dim isolated neutron star) but in a binary.

Discovery of magnetic fields in five DC white dwarfs

About half of white dwarfs (WDs) evolve to the DC state as they cool; the others become DQ or (temporarily?) DZ WDs. The recent magnetic survey of the local 20 pc volume has established a high frequency of magnetic fields among WDs older than 2–3 Gyr, demonstrating that in low- and average-mass WDs, the effects of magnetism become more common as they age, and the fields on average become stronger. However, the available statistics of WDs older than about 5 Gyr do not clearly establish how fields evolve beyond this age. We are carrying out a survey to clarify the occurrence of magnetism in DC-type WDs in order to better understand this late evolution. We use broadband filter polarimetry, arguably the most efficient way to detect magnetic fields in featureless WDs via continuum circular polarization. Here we report the discovery of a magnetic field in five DC WDs (of 23 observed), almost doubling the total sample of known magnetic WDs belonging to the DC spectral class.

Spectrophotometric analysis of magnetic white dwarf – I. Hydrogen-rich compositions

ABSTRACTWe present a homogeneous analysis of all DA stars labelled as magnetic in the Montreal White Dwarf Database. Our sample is restricted to almost all known magnetic white dwarf showing clear sign of splitting (B ≳ 1–2 MG) that have parallax measurements from the second Gaia data release, photometric data from diverse surveys and spectroscopic data from Sloan Digital Sky Survey or archival data from the Montreal group. We determine the atmospheric parameters (effective temperature, surface gravity, and magnetic field strength/geometry) of all objects using state-of-the-art model atmosphere/magnetic synthetic spectra, as well as reclassify many objects that were prematurely labelled as potentially magnetic. Finally, we discuss the atmospheric parameters/field properties distribution as well as the implication on our understanding of magnetic white dwarfs origin and evolution.

Spectrophotometric analysis of magnetic white dwarf – II. Helium-rich compositions

ABSTRACT We present an analysis of all single white dwarf stars known to exhibit spectroscopic signatures of neutral helium line splitting due to the presence of a strong magnetic field. Using state-of-the-art models taking into account the effects of magnetic fields on the synthetic spectra, we determine effective temperatures, surface gravities, and masses for the stars in our sample. Our analysis uses data from the second and third Gaia (early) data releases, photometric data from diverse surveys such as the Sloan Digital Sky Survey and Pan-STARRS, and archived spectroscopic data. We are able to successfully reproduce the spectra of eight objects using an offset dipole geometry while several others seem to require either a more complex geometry or a different chemical composition. We also highlight a group of hot featureless white dwarfs that are most probably highly magnetic objects whose spectra are completely smeared due to the field strength distribution across the surface.

Instability of magnetosonic waves in magnetized degenerate half-spin-polarized quantum plasma

Shock and solitary waves in plasmas are the main focus of many current studies of localized disturbances in both laboratory and astrophysical plasmas. Here, the linear and the nonlinear properties evolving in three dimensions quantum magnetized spin-1/2 plasma formed of spin up (↑) and spin down (↓), nonrelativistic degenerate inertialess electrons, and classical ions, are considered. The nonlinear propagation is governed by a quantum Zakharov–Kuznetsov–Burger (QZKB) and QZK equations which are derived by using the multiple scales perturbation technique. The shock and solitary wave structures are dependent on the spin polarization parameter, Bohm potential, Zeeman energy, and dissipative parameter. The first, and the second, orders instability growth rates are also derived by using the small-k perturbation expansion method. The previous parameters affect also the instability growth rates. There is a critical angle below which the first order becomes positive while the second order is negative, and over which the first order disappears and the second order becomes positive. Our present results may be useful in understanding the dynamics in the magnetic pulsars, and also in the magnetic white dwarfs and neutron stars systems where degenerate spin-up and spin-down electrons with dissipative effects can exist.

Spun-Up Rotation-Powered Magnetized White Dwarfs in Close Binaries as Possible Gamma-ray Sources: Signatures of Pulsed Modulation from AE Aquarii and AR Scorpii in Fermi-LAT Data

In this paper, the possibility of periodic pulsar-like gamma-ray emission from the white dwarfs in AE Aquarii and AR Scorpii is investigated. We show that the white dwarf magnetospheres in AE Aquarii and AR Scorpii can possibly induce potentials to accelerate charged particles to energies in excess of one tera electronvolt (TeV) with associated gamma-ray emission through processes such as curvature radiation, inverse Compton, and hadronic processes such as neutral pion decay. We report here pulsed gamma-ray signatures at or close to the spin period of white dwarfs in both AE Aquarii and AR Scorpii in the Fermi-LAT dataset. This may indicate that both these white dwarfs possibly contain a particle accelerator that can produce relativistic electrons and ions and associated high energy radiation. The possibility of pair production is also investigated, which could provide a source for relativistic e± pairs in the magnetosphere. This could possibly be a driver for other forms of lepton-induced multi-wavelength pulsar-like emission from these two systems as well, for example, to explain the recently detected pulsed radio emission from AE Aquarii and R Scorpii in MeerKAT observations at the spin period of the white dwarf. The possibility of future detection of AE Aquarii and AR Scorpii with the Cherenkov Telescope Array (CTA) is also discussed. The future Vera Rubin Observatory will make a revolutionary contribution to time-domain astrophysics, which may lead to the discovery of thousands of new transient sources, possibly also many more close binaries with highly spun-up magnetized white dwarfs such as AE Aquarii and AR Scorpii for future investigation.

Polarised line shape calculations for conditions encountered in stellar magnetic atmospheres

We present new investigations into spectral line shapes under conditions typical for magnetic stellar atmospheres. Our method for simulating line profiles takes into account the coupled Zeeman and Stark effects. The calculations presented here will ultimately help to correctly model the Stokes parameters of peculiar stellar objects such as the Magnetic White Dwarfs (MWDs), retrieving their magnetic field geometries from spectropolarimetric observations. For the magnetic analysis of this class of stars, in general only Stokes I profiles as a function of rotational phase have been used in the past, in a very few cases, also Stokes V. The present improved simulations of the full Stokes parameters are expected to provide a powerful tool for the analysis of strong magnetic fields like those found in the MWDs.

Equation of state and surface thermal emission of magnetized white dwarfs

AbstractThe observation and theoretical study of massive magnetized white dwarfs is one of the most important topics in the field of astronomy. This paper aims to obtain a coupled magneto‐thermal evolution model of magnetized white dwarfs (B‐WDs). As a fiducial model, we choose a non‐magnetized WD with a baryonic mass M0 = 1.37 M⊙ (M⊙ is the solar mass) for comparison with the magnetized cases. Firstly, we give an overview of the WD cooling, then study the equation of the state of a B‐WD. We find that the WD mass increases with magnetic field strength B, while its radius decreases with B, and the minimum of B required to reach the Chandrasecka limit is about 3.2 × 1014 G. Finally, by introducing the magnetic‐to‐thermal conversion coefficient ξ, and taking the temperature effect on the stellar radius into consideration, we calculate the luminosities of thermal photons Lph and surface effective temperature, Teff due to Ohmic dissipation for B‐WDs. According to our calculations, magnetic field decay can definitely maintain a long‐term thermal evolution for massive B‐WDs. This study will be useful for the study of internal thermal evolution and internal stability of B‐WDs in the future.

Correction to: Universal relationship for magnetized white dwarfs

Correction to: Universal relationship for magnetized white dwarfs

The Periodic Signals of Nova V1674 Herculis (2021)

We present time-series photometry during the early decline phase of the extremely fast nova V1674 Herculis. The 2021 light curve showed periodic signals at 0.152921(3) days and 501.486(5) s, which we interpret as respectively the orbital and white dwarf spin periods in the underlying binary. We also detected a sideband signal at the difference frequency between these two clocks. During the first 15 days of outburst, the spin period appears to have increased by 0.014(1)%. This increase probably arose from the sudden loss of high-angular-momentum gas (“the nova explosion”) from the rotating, magnetic white dwarf. Both periodic signals appeared remarkably early in the outburst, which we attribute to the extreme speed with which the nova evolved (and became transparent to radiation from the inner binary). After that very fast initial period increase of 71 ms, the period subsequently decreased—at 182(18) ms yr−1 in 2021, and 88(18) ms yr−1 in 2022. These rates are ∼100× faster than typically seen in intermediate polars. This could be due to high accretion torques from very high mass-transfer rates, which might be common when low-mass donor stars are strongly irradiated by a nova outburst.

The Double White Dwarf Merger Progenitors of SDSS J2211+1136 and ZTF J1901+1458

Double white dwarf (DWD) mergers are possibly the leading formation channel of massive, rapidly rotating, high-field magnetic white dwarfs (HFMWDs). However, a direct link connecting a DWD merger to any observed HFMWD is still missing. We here show that the HFMWDs SDSS J221141.80+113604.4 (hereafter J2211+1136) and ZTF J190132.9+145808.7 (hereafter J1901+1458) might be DWD merger products. J2211+1136 is a 1.27 M ⊙ white dwarf (WD) with a rotation period of 70.32 s and a surface magnetic field of 15 MG. J1901+1458 is a 1.327–1.365 M ⊙ WD with a rotation period of 416.20 s, and a surface magnetic field in the range 600–900 MG. With the assumption of single-star evolution and the currently measured WD masses and surface temperatures, the cooling ages of J2211+1136 and J1901+1458 are, respectively, 2.61–2.85 Gyr and 10–100 Myr. We hypothesize that these WDs are DWD merger products and compute the evolution of the postmerged configuration formed by a central WD surrounded by a disk. We show that the postmerger system evolves through three phases depending on whether accretion, mass ejection (propeller), or magnetic braking dominates the torque onto the central WD. We calculate the time the WD spends in each of these phases and obtain the accretion rate and disk mass for which the WD rotational age, i.e., the total time elapsed since the merger to the instant where the WD central remnant reaches the current measured rotation period, agrees with the estimated WD cooling age. We infer the mass values of the primary and secondary WD components of the DWD merger that lead to a postmerger evolution consistent with the observations.

Gaia white dwarfs within 40 pc – III. Spectroscopic observations of new candidates in the Southern hemisphere

Abstract We present a spectroscopic survey of 248 white dwarf candidates within 40 pc of the Sun; of these 244 are in the Southern hemisphere. Observations were performed mostly with the Very Large Telescope (X-Shooter) and Southern Astrophysical Research Telescope. Almost all candidates were selected from Gaia Data Release 3 (DR3). We find a total of 246 confirmed white dwarfs, 209 of which had no previously published spectra, and two main-sequence star contaminants. Of these, 100 white dwarfs display hydrogen Balmer lines, 69 have featureless spectra, and two show only neutral helium lines. Additionally, 14 white dwarfs display traces of carbon, while 37 have traces of other elements that are heavier than helium. We observe 35 magnetic white dwarfs through the detection of Zeeman splitting of their hydrogen Balmer or metal spectral lines. High spectroscopic completeness (> 97 per cent) has now been reached, such that we have 1058 confirmed Gaia DR3 white dwarfs out of 1083 candidates within 40 pc of the Sun at all declinations.

Comparison study of the energy and instability of ion-acoustic solitary waves in magnetized electron–positron–ion quantum plasma

Notably, solitary waves that emerge from the nonlinear properties of plasmas are the main focus of many current studies of localized disturbances in both laboratory and astrophysical plasmas. By applying the reductive perturbation method, we derive the nonlinear homogeneous quantum Zakharov–Kuznetsov (QZK) equation in three-component collisionless quantum plasma consisting of electrons, positrons, and ions in the presence of an external static magnetic field. The solitary wave structures are dependent on the Bohm potential, magnetic field, obliqueness, species Fermi temperatures, and densities. The soliton’s electric field and energy are also derived and investigated, which were found to be reduced as the magnetic field increases. The instability growth rate is also derived by using the small-k perturbation expansion method. The previous parameters affect the instability growth rate as well. A comparison of the energy and instability growth rate behaviour against system parameters is carried out. Large energy and large instability growth rate occur at large values of positron density or lower values of ion density. At zero or small rotation angle, both decrease as the magnetic field increases. Our findings could help us understand the dynamics of magnetic white dwarfs, pulsar magnetospheres, semiconductor plasma, and high-intensity laser-solid matter interaction experiments where e-p-i plasma exists.

The merger fraction of ultramassive white dwarfs

ABSTRACT We search for merger products among the 25 most massive white dwarfs in the Montreal White Dwarf Database 100 pc sample through follow-up spectroscopy and high-cadence photometry. We find an unusually high fraction, 40 per cent, of magnetic white dwarfs among this population. In addition, we identify four outliers in transverse velocity and detect rapid rotation in five objects. Our results show that $56^{+9}_{-10}$ per cent of the $M\approx 1.3\, {\rm M}_{\odot }$ ultramassive white dwarfs form through mergers. This fraction is significantly higher than expected from the default binary population synthesis calculations using the α prescription (with αλ = 2), and provides further support for efficient orbital shrinkage, such as with low values of the common-envelope efficiency.

Constraining the Nature of the 18 min Periodic Radio Transient GLEAM-X J162759.5-523504.3 via Multiwavelength Observations and Magneto-thermal Simulations

We observed the periodic radio transient GLEAM-X J162759.5-523504.3 (GLEAM-X J1627) using the Chandra X-ray Observatory for about 30 ks on 2022 January 22–23, simultaneously with radio observations from the Murchison Widefield Array, MeerKAT, and the Australia Telescope Compact Array. Its radio emission and 18 min periodicity led the source to be tentatively interpreted as an extreme magnetar or a peculiar highly magnetic white dwarf. The source was not detected in the 0.3–8 keV energy range with a 3σ upper limit on the count rate of 3 × 10−4 counts s−1. No radio emission was detected during our X-ray observations either. Furthermore, we studied the field around GLEAM-X J1627 using archival European Southern Observatory and DECam Plane Survey data, as well as recent Southern African Large Telescope observations. Many sources are present close to the position of GLEAM-X J1627, but only two within the 2″ radio position uncertainty. Depending on the assumed spectral distribution, the upper limits converted to an X-ray luminosity of L X < 6.5 × 1029 erg s−1 for a blackbody with temperature kT = 0.3 keV, or L X < 9 × 1029 erg s−1 for a power law with photon index Γ = 2 (assuming a 1.3 kpc distance). Furthermore, we performed magneto-thermal simulations for neutron stars considering crust- and core-dominated field configurations. Based on our multiband limits, we conclude that (i) in the magnetar scenario, the X-ray upper limits suggest that GLEAM-X J1627 should be older than ∼1 Myr, unless it has a core-dominated magnetic field or has experienced fast cooling; (ii) in the white dwarf scenario, we can rule out most binary systems, a hot sub-dwarf, and a hot magnetic isolated white dwarf (T ≳ 10.000 K), while a cold isolated white dwarf is still compatible with our limits.

Quadratic Zeeman effect in hydrogen at 2-3 MG magnetic fields.

The Zeeman effect is used for measurement of magnetic fields in astrophysical and laboratory plasmas. Magnetic fields in atmospheres of magnetic white dwarf stars are in the range 40 kG-1 GG. The quadratic Zeeman effect results in the additional split and shift of lines for magnetic fields >2 MG. Hydrogen Balmer lines were studied in magnetic fields delivered by a 1 MA pulse power generator. The magnetic field was generated by rod loads 0.8-1 mm in diameter. A droplet of CH oil on the load center was the source of hydrogen. A low ionized oil layer was backlit by blackbody emission from the rod with a temperature of 0.5-0.6 eV. Zeeman splitting of H-alpha and H-beta absorption lines were with a grating spectrometer. A spectral shift of the central component of the triplet indicated the quadratic Zeeman effect in hydrogen lines.

SRGe J214919.3+673634—a Candidate for AM Her Variables Discovered by the eROSITA Telescope onboard the Spectrum–Roentgen–Gamma Orbital Observatory

We present the results of the optical identification, classification, as well as analysis of photometric and spectral observations of the X-ray transient SRGe2149+6736 detected by the eROSITA telescope during SRG all-sky X-ray survey. Photometric observations of the optical companion of SRGe2149+6736 were carried out on 6m telescope BTA SAO RAS, 1.5m Russian-Turkish telescope RTT-150 and 2.5m telescope CMO of Moscow State University. Together with ZTF data they showed that the source is a cataclysmic variable with an orbital period $P=85\pm0.4$~min which demonstrates long-term brightness variability from $23.5$~mag (low state) to $20$~mag (high state). The high-state light curves are consistent with a model of accreting magnetic white dwarf and suggest that SRGe2149+6736 belongs to AM~Her type variables. The optical spectra obtained in the low state are consistent with a spectral energy distribution of a white dwarf with a temperature of ~24000 K.

Can we reveal the core-chemical composition of ultra-massive white dwarfs through their magnetic fields?

ABSTRACT Ultra-massive white dwarfs ($1.05\, \rm M_\odot \lesssim M_{WD}$) are particularly interesting objects that allow us to study extreme astrophysical phenomena such as type Ia supernovae explosions and merger events. Traditionally, ultra-massive white dwarfs are thought to harbour oxygen–neon (ONe) cores. However, recent theoretical studies and new observations suggest that some ultra-massive white dwarfs could harbour carbon–oxygen (CO) cores. Although several studies have attempted to elucidate the core composition of ultra-massive white dwarfs, to date, it has not been possible to distinguish them through their observed properties. Here, we present a new method for revealing the core-chemical composition in ultra-massive white dwarfs that is based on the study of magnetic fields generated by convective mixing induced by the crystallization process. ONe white dwarfs crystallize at higher luminosities than their CO counterparts. Therefore, the study of magnetic ultra-massive white dwarfs in the particular domain where ONe cores have reached the crystallization conditions but CO cores have not, may provide valuable support to their ONe core-chemical composition, since ONe white dwarfs would display signs of magnetic fields and CO would not. We apply our method to eight white dwarfs with magnetic field measurements and we suggest that these stars are candidate ONe white dwarfs.

NICER Monitoring of Supersoft X-Ray Sources

We monitored four supersoft sources—two persistent ones, CAL 83 and MR Vel, and the recent novae YZ Ret (Nova Ret 2020), and V1674 Her (Nova Her 2021)—with NICER. The two persistent supersoft X-ray sources (SSS) were observed with unvaried X-ray flux level and spectrum, respectively, 13 and 20 yr after the last observations. Short-period modulations of the SSS appear where the spectrum of the luminous central source was fully visible (in CAL 83 and V1674 Her) and were absent in YZ Ret and MR Vel, in which the flux originated in photoionized or shocked plasma, while the white dwarf (WD) was not observable. We thus suggest that the pulsations occur on, or very close to, the WD surface. The pulsations of CAL 83 were almost unvaried after 15 yr, including an irregular drift of the ≃67 s period by 2.1 s. Simulations, including previous XMM-Newton data, indicate actual variations in period length within hours, rather than an artifact of the variable amplitude of the pulsations. Large amplitude pulsations with a period of 501.53 ± 0.30 s were always detected in V1674 Her, as long as the SSS was observable. This period seems to be due to rotation of a highly magnetized WD. We cannot confirm the maximum effective temperature of (≃145,000 K) previously inferred for this nova, and discuss the difficulty in interpreting its spectrum. The WD appears to present two surface zones, one of which does not emit SSS flux.

Upper limit on the axion-photon coupling from magnetic white dwarf polarization

Polarization measurements of thermal radiation from magnetic white dwarf (MWD) stars have been proposed as a probe of axion-photon mixing. The radiation leaving the surface of the MWD is unpolarized, but if low-mass axions exist then photons polarized parallel to the direction of the MWD's magnetic field may convert into axions, which induces a linear polarization dependent on the strength of the axion-photon coupling $g_{a\gamma\gamma}$. We model this process by using the formalism of axion-photon mixing in the presence of strong-field vacuum birefringence to show that of all stellar types MWDs are the most promising targets for axion-induced polarization searches. We then consider linear polarization data from multiple MWDs, including SDSS J135141 and Grw+70$^\circ$8247, to show that after rigorously accounting for astrophysical uncertainties the axion-photon coupling is constrained to $|g_{a\gamma\gamma}| \lesssim 5.4 \times 10^{-12}$ GeV$^{-1}$ at 95% confidence for axion masses $m_a \lesssim 3 \times 10^{-7}$ eV. This upper limit puts in tension the previously-suggested explanation of the anomalous transparency of the Universe to TeV gamma-rays in terms of axions. We identify MWD targets for which future data and modeling efforts could further improve the sensitivity to axions.