Abstract
AT2019wey (SRGA J043520.9+552226, SRGE J043523.3+552234) is a transient first reported by the ATLAS optical survey in 2019 December. It rose to prominence upon detection, three months later, ...by the Spektrum-Roentgen-Gamma (SRG) mission in its first all-sky survey. X-ray observations reported in Yao et al. suggest that AT2019wey is a Galactic low-mass X-ray binary (LMXB) with a black hole (BH) or neutron star (NS) accretor. Here we present ultraviolet, optical, near-infrared, and radio observations of this object. We show that the companion is a short-period (
P
≲ 16 hr) low-mass (<1
M
⊙
) star. We consider AT2019wey to be a candidate BH system since its locations on the
L
radio
–
L
X
and
L
opt
–
L
X
diagrams are closer to BH binaries than NS binaries. We demonstrate that from 2020 June to August, despite the more than 10 times brightening at radio and X-ray wavelengths, the optical luminosity of AT2019wey only increased by 1.3–1.4 times. We interpret the UV/optical emission before the brightening as thermal emission from a truncated disk in a hot accretion flow and the UV/optical emission after the brightening as reprocessing of the X-ray emission in the outer accretion disk. AT2019wey demonstrates that combining current wide-field optical surveys and SRG provides a way to discover the emerging population of short-period BH LMXB systems with faint X-ray outbursts.
Abstract
AM Canum Venaticorum (AM CVn) systems are ultracompact binaries where a white dwarf accretes from a helium-rich degenerate or semidegenerate donor. Some AM CVn systems will be among the ...loudest sources of gravitational waves for the upcoming Laser Interferometer Space Antenna; yet the formation channel of AM CVns remains uncertain. We report the study and characterization of a new eclipsing AM CVn, SRGeJ045359.9+622444 (hereafter, SRGeJ0453), discovered from a joint Spektrum-Roentgen-Gamma (SRG) Extended Roentgen Survey with an Imaging Telescope Array (eROSITA) mission and Zwicky Transient Facility program to identify cataclysmic variables (CVs). We obtained optical photometry to confirm the eclipse of SRGeJ0453 and determine the orbital period to be
P
orb
=
55.0802
±
0.0003
min
. We constrain the binary parameters by modeling the high-speed photometry and radial-velocity curves and find
M
donor
= 0.044 ± 0.024
M
⊙
and
R
donor
= 0.078 ± 0.012
R
⊙
. The X-ray spectrum is approximated by a power-law model with an unusually flat photon index of Γ ∼ 1 previously seen in magnetic CVs with SRG/eROSITA, but verifying that the magnetic nature of SRGeJ0453 requires further investigation. Optical spectroscopy suggests that the donor star of SRGeJ0453 could have initially been a He star or a He white dwarf. SRGeJ0453 is the ninth eclipsing AM CVn system published to date, and its lack of optical outbursts have made it elusive in previous surveys. The discovery of SRGeJ0453 using joint X-ray and optical surveys highlights the potential for discovering similar systems in the near future.
Abstract
We searched the Gaia DR3 database for ultramassive white dwarfs with kinematics consistent with having escaped the nearby Hyades open cluster, identifying three such candidates. Two of these ...candidates have masses estimated from Gaia photometry of approximately 1.1 solar masses; their status as products of single-stellar evolution that have escaped the cluster was deemed too questionable for immediate follow-up analysis. The remaining candidate has an expected mass >1.3 solar masses, significantly reducing the probability of it being an interloper. Analysis of follow-up Gemini GMOS spectroscopy for this source reveals a nonmagnetized hydrogen atmosphere white dwarf with a mass and age consistent with having formed from a single star. Assuming a single-stellar-evolution formation channel, we estimate a 97.8% chance that the candidate is a true escapee from the Hyades. With a determined mass of 1.317 solar masses, this is potentially the most massive known single-evolution white dwarf and is by far the most massive with a strong association with an open cluster.
Abstract
Magnetic cataclysmic variables (CVs) are luminous Galactic X-ray sources, which have been difficult to find in purely optical surveys due to their lack of outburst behavior. The eROSITA ...telescope on board the Spektr-RG mission is conducting an all-sky X-ray survey and recently released the public eROSITA Final Equatorial Depth Survey (eFEDS) catalog. We crossmatched the eFEDS catalog with photometry from the Zwicky Transient Facility and discovered two new magnetic CVs. We obtained high-cadence optical photometry and phase-resolved spectroscopy for each magnetic CV candidate and found them both to be polars. Among the newly discovered magnetic CVs is eFEDS J085037.2+044359/ZTFJ0850+0443, an eclipsing polar with orbital period
P
orb
= 1.72 hr and WD mass
M
WD
= 0.81 ± 0.08
M
⊙
. We suggest that eFEDS J085037.2+044359/ZTFJ0850+0443 is a low magnetic field strength polar, with
B
WD
≲ 10 MG. We also discovered a non-eclipsing polar, eFEDS J092614.1+010558/ZTFJ0926+0105, with orbital period
P
orb
= 1.47 hr and magnetic field strength
B
WD
= 36–42 MG.
Abstract
We report the discovery of ZTF J0127+5258, a compact mass-transferring binary with an orbital period of 13.7 minutes. The system contains a white dwarf accretor, which likely originated as a ...post–common envelope carbon–oxygen (CO) white dwarf, and a warm donor (
T
eff,donor
= 16,400 ± 1000 K). The donor probably formed during a common envelope phase between the CO white dwarf and an evolving giant that left behind a helium star or white dwarf in a close orbit with the CO white dwarf. We measure gravitational wave–driven orbital inspiral with ∼51
σ
significance, which yields a joint constraint on the component masses and mass transfer rate. While the accretion disk in the system is dominated by ionized helium emission, the donor exhibits a mixture of hydrogen and helium absorption lines. Phase-resolved spectroscopy yields a donor radial velocity semiamplitude of 771 ± 27 km s
−1
, and high-speed photometry reveals that the system is eclipsing. We detect a Chandra X-ray counterpart with
L
X
∼ 3 × 10
31
erg s
−1
. Depending on the mass transfer rate, the system will likely either evolve into a stably mass-transferring helium cataclysmic variable, merge to become an R CrB star, or explode as a Type Ia supernova in the next million years. We predict that the Laser Space Interferometer Antenna (LISA) will detect the source with a signal-to-noise ratio of 24 ± 6 after 4 yr of observations. The system is the first LISA-loud mass-transferring binary with an intrinsically luminous donor, a class of sources that provide the opportunity to leverage the synergy between optical and infrared time domain surveys, X-ray facilities, and gravitational-wave observatories to probe general relativity, accretion physics, and binary evolution.
The effect of vacuum birefringence is one of the first predictions of quantum electrodynamics (QED): the presence of a charged Dirac field makes the vacuum birefringent when threaded by magnetic ...fields. This effect, extremely weak for terrestrial magnetic fields, becomes important for highly magnetized astrophysical objects, such as accreting black holes. In the X-ray regime, the polarization of photons traveling in the magnetosphere of a black hole is not frozen at emission but is changed by the local magnetic field. We show that, for photons traveling along the plane of the disk, where the field is expected to be partially organized, this results in a depolarization of the X-ray radiation. Because the amount of depolarization depends on the strength of the magnetic field, this effect can provide a way to probe the magnetic field in black-hole accretion disks and to study the role of magnetic fields in astrophysical accretion in general.
In the next decade, x-ray polarimetry will open a new window on the high-energy Universe, as several missions that include an x-ray polarimeter are currently under development. Observations of the ...polarization of x rays coming from the accretion disks of stellar-mass and supermassive black holes are among the new polarimeters’ major objectives. In this paper, we show that these observations can be affected by the quantum electrodynamic (QED) effect of vacuum birefringence: after an x-ray photon is emitted from the accretion disk, its polarization changes as the photon travels through the accretion disk’s magnetosphere, as a result of the vacuum becoming birefringent in the presence of a magnetic field. We show that this effect can be important for black holes in the energy band of the upcoming polarimeters and has to be taken into account in a complete model of the x-ray polarization that we expect to detect from black-hole accretion disks, both for stellar mass and for supermassive black holes. We find that, for a chaotic magnetic field in the disk, QED can significantly decrease the linear polarization fraction of edge-on photons, depending on the spin of the hole and on the strength of the magnetic field. This effect can provide, for the first time, a direct way to probe the magnetic field strength close to the innermost stable orbit of black-hole accretion disks and to study the role of magnetic fields in astrophysical accretion in general.
ABSTRACT
It has been recently suggested that white dwarfs generate magnetic fields in a process analogous to the Earth. The crystallization of the core creates a compositional inversion that drives ...convection, and combined with rotation, this can sustain a magnetic dynamo. We reanalyse the dynamo mechanism, arising from the slow crystallization of the core, and find convective turnover times tconv of weeks to months – longer by orders of magnitude than previously thought. With white dwarf spin periods P ≪ tconv, crystallization-driven dynamos are almost always in the fast-rotating regime, where the magnetic field B is at least in equipartition with the convective motion and is possibly further enhanced by a factor of B ∝ (tconv/P)1/2, depending on the assumed dynamo scaling law. We track the growth of the crystallized core using mesa and compute the magnetic field B(Teff) as a function of the white dwarf’s effective temperature Teff. We compare this prediction with observations and show that crystallization-driven dynamos can explain some – but not all – of the ∼MG magnetic fields measured for single white dwarfs, as well as the stronger fields measured for white dwarfs in cataclysmic variables, which were spun up by mass accretion to short P. Our B(Teff) curves might also explain the clustering of white dwarfs with Balmer emission lines around Teff ≈ 7500 K.
ABSTRACT
We determine the distribution of cooling ages of massive Gaia EDR3 white dwarfs identified with over 90 per cent probability within 200 pc and with mass in the range 0.95–1.25 M⊙. Using ...three sets of publicly available models, we consider sub-samples of these white dwarfs sorted into three equally spaced mass bins. Under the assumption of a constant white dwarf formation rate, we find an excess of white dwarfs, both along the Q branch and below it, corresponding respectively to stars that are in the process of freezing and those that are completely frozen. We compare the cooling age distributions for each of these bins to the recently determined time-varying star formation rate of Gaia DR2 main sequence stars. For white dwarfs in the two lightest mass bins, spanning the mass range 0.95–1.15 M⊙, we find that the cumulative cooling age distribution is statistically consistent with the expectation from the star formation rate. For white dwarfs in the heaviest mass bin, 1.15–1.25 M⊙, we find that their cumulative distribution is inconsistent with the star formation rate for all of the models considered; instead, we find that their cooling age distribution is well fitted by a linear combination of the distribution expected for single stellar evolution products and the distribution expected for double white dwarf merger products when approximately 40–50 per cent of the 1.15–1.25 M⊙ white dwarfs that formed over the past 4 Gyr are produced through double white dwarf mergers.
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