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A systematic search of Zwicky Transient Facility data for ultracompact binary LISA-detectable gravitational-wave sources
Abstract
Using photometry collected with the Zwicky Transient Facility (ZTF), we are conducting an ongoing survey for binary systems with short orbital periods (Pb<1hr) with the goal of identifying new gravitational-wave sources detectable by the upcoming Laser Interferometer Space Antenna (LISA). Here, we present a sample of fifteen binary systems discovered thus far, with orbital periods ranging from 6.91min to 56.35min. Of the fifteen systems, seven are eclipsing systems which do not show signs of significant mass transfer. Additionally, we have discovered two AM Canum Venaticorum (AM CVn) systems and six systems exhibiting primarily ellipsoidal variations in their light curves. We present follow-up spectroscopy and high-speed photometry confirming the nature of these systems, estimates of their LISA signal-to-noise ratios (SNR), and a discussion of their physical characteristics.
... White dwarfs are compact objects composed of degenerate matter, which can not undergo any further nuclear fusion. Usually lower-mass stars in the mass range 0.8 ≤ M ⊙ ≤ 8 end-up as white dwarfs e.g., [1][2][3]. These progenitor stars can either be solitary stars or be in a wide binary system. ...
... Janus was found during a search for periodic variability on and around the white-dwarf cooling track with ZTF 13,32-34 , which has already yielded several results, including finding numerous double white-dwarf binaries [35][36][37] and an extremely massive and magnetic white dwarf that is most likely the result of a white-dwarf merger 18 . The targets were selected using the first survey of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS PS1) source catalogue 38 , cross-matched with a white-dwarf catalogue 39 , after imposing a photometric colour selection of (g − r) < 0.2 mag and (r − i) < 0.2 mag. ...
White dwarfs, the extremely dense remnants left behind by most stars after their death, are characterized by a mass comparable to that of the Sun compressed into the size of an Earth-like planet. In the resulting strong gravity, heavy elements sink towards the centre and the upper layer of the atmosphere contains only the lightest element present, usually hydrogen or helium1,2. Several mechanisms compete with gravitational settling to change a white dwarf’s surface composition as it cools³, and the fraction of white dwarfs with helium atmospheres is known to increase by a factor of about 2.5 below a temperature of about 30,000 kelvin4–8; therefore, some white dwarfs that appear to have hydrogen-dominated atmospheres above 30,000 kelvin are bound to transition to be helium-dominated as they cool below it. Here we report observations of ZTF J203349.8+322901.1, a transitioning white dwarf with two faces: one side of its atmosphere is dominated by hydrogen and the other one by helium. This peculiar nature is probably caused by the presence of a small magnetic field, which creates an inhomogeneity in temperature, pressure or mixing strength over the surface9–11. ZTF J203349.8+322901.1 might be the most extreme member of a class of magnetic, transitioning white dwarfs—together with GD 323 (ref. ¹²), a white dwarf that shows similar but much more subtle variations. This class of white dwarfs could help shed light on the physical mechanisms behind the spectral evolution of white dwarfs.
... This search was based on newly generated forced point-spread-function photometry performed at the coordinates of all Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1) sources in ZTF images over a period range of 250 d to just 2 min. By systematically searching all 1,220,038,476 unique sources in the Pan-STARRS1 source catalogue 32 with more than 50 ZTF epochs, we eliminated biases of previous searches relying on colour-or astrometric-based selections 21 , which represents a major advancement in probing for short-period astrophysical variables across the northern sky. We searched a total of 1,461,592 trial frequencies per source, corresponding to an oversampling factor of 2. In total, given that we searched 1,220,038,476 sources, this means a total of 1.78 × 10 15 trial frequencies were searched, and each source had on average approximately 10 3 epochs. ...
Of more than a thousand known cataclysmic variables (CVs), where a white dwarf is accreting from a hydrogen-rich star, only a dozen have orbital periods below 75 minutes1–9. One way to achieve these short periods requires the donor star to have undergone substantial nuclear evolution before interacting with the white dwarf10–14, and it is expected that these objects will transition to helium accretion. These transitional CVs have been proposed as progenitors of helium CVs13–18. However, no known transitional CV is expected to reach an orbital period short enough to account for most of the helium CV population, leaving the role of this evolutionary pathway unclear. Here we report observations of ZTF J1813+4251, a 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a temperature comparable to that of the Sun but a density 100 times greater owing to its helium-rich composition, accreting onto a white dwarf. Phase-resolved spectra, multi-band light curves and the broadband spectral energy distribution allow us to obtain precise and robust constraints on the masses, radii and temperatures of both components. Evolutionary modelling shows that ZTF J1813+4251 is destined to become a helium CV binary, reaching an orbital period under 20 minutes, rendering ZTF J1813+4251 a previously missing link between helium CV binaries and hydrogen-rich CVs. A 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a comparable temperature to that of the Sun but a 100 times greater density, accreting onto a white dwarf is reported.
... As predicted by the Fisher matrix (see Table I), any value aboveĠ/G 0 > 2.05 × 10 −4 yr −1 will be measured with a relative precision larger than 50%. Therefore, we useĠ/G 0 = 1 × 10 −3 yr −1 , above the detection limit, and use the frequency, mass and distance parameters of ZTF J1539+5027 [46,47] to inject the signal. The resulting parameter values are: ...
Spaceborne gravitational-wave (GW) detectors observing at milli-Hz and deci-Hz frequencies are expected to detect large numbers of quasi-monochromatic signals. The first and second time-derivative of the GW frequency ($\dot f_0$ and $\ddot f_0$) can be measured for the most favourable sources and used to look for negative post-Newtonian corrections, which can be induced by the source's environment or modifications of general relativity. We present an analytical, Fisher-matrix-based approach to estimate how precisely such corrections can be constrained. We use this method to estimate the bounds attainable on the time evolution of the gravitational constant $G(t)$ with different classes of quasi-monochromatic sources observable with LISA and DECIGO, two representative spaceborne detectors for milli-Hz and deci-Hz GW frequencies. We find that the most constraining source among a simulated population of LISA galactic binaries could yield $\dot G/G_0 \lesssim 10^{-6}\text{yr}^{-1}$, while the best currently known verification binary will reach $\dot G/G_0 \lesssim 10^{-4}\text{yr}^{-1}$. We also perform Monte-Carlo simulations using quasi-monochromatic waveforms to check the validity of our Fisher-matrix approach, as well as inspiralling waveforms to analyse binaries that do not satisfy the quasi-monochromatic assumption. We find that our analytical Fisher matrix produces good order-of-magnitude constraints even for sources well beyond its regime of validity. Monte-Carlo investigations also show that chirping stellar-mass compact binaries detected by DECIGO-like detectors at cosmological distances of tens of Mpc can yield constraints as tight as $\dot G/G_0 \lesssim 10^{-11}\text{yr}^{-1}$.
... Using photometry from the Zwicky Transient Facility 31 (ZTF), we searched for short timescale periodic flux variations in 20 million objects that were underluminous relative to the main sequence (Methods) as part of an ongoing campaign to identify short orbital period binary systems 8 . During this search, we identified ZTF J1406+1222, an object which exhibits strong quasi-sinusoidal variability on a period of 62 minutes and a larger amplitude of variability in the ZTF g-band than in the ZTF ror i-bands. ...
Over a dozen millisecond pulsars are ablating low-mass companions in close binary systems. In the original "black widow", the 8-hour orbital period eclipsing pulsar PSR J1959+2048 (PSR B1957+20), high energy emission originating from the pulsar is irradiating and may eventually destroy a low-mass companion. These systems are not only physical laboratories that reveal the dramatic result of exposing a close companion star to the relativistic energy output of a pulsar, but are also believed to harbour some of the most massive neutron stars, allowing for robust tests of the neutron star equation of state. Here, we report observations of ZTF J1406+1222, a wide hierarchical triple hosting a 62-minute orbital period black widow candidate whose optical flux varies by a factor of more than 10. ZTF J1406+1222 pushes the boundaries of evolutionary models, falling below the 80 minute minimum orbital period of hydrogen-rich systems. The wide tertiary companion is a rare low metallicity cool subdwarf star, and the system has a Galactic halo orbit consistent with passing near the Galactic center, making it a probe of formation channels, neutron star kick physics, and binary evolution.
... Using photometry from the Zwicky Transient Facility 7 (ZTF), we searched for short-timescale periodic flux variations in 20 million objects that were underluminous relative to the main sequence (Methods) as part of an ongoing campaign to identify short-orbital-period binary systems 8 . During this search, we identified ZTF J1406+1222, an object that exhibits strong quasi-sinusoidal variability on a period of 62 min and a larger amplitude of variability in the ZTF g band than in the ZTF r or i bands. ...
Over a dozen millisecond pulsars are ablating low-mass companions in close binary systems. In the original ‘black widow’, the eight-hour orbital period eclipsing pulsar PSR J1959+2048 (PSR B1957+20)¹, high-energy emission originating from the pulsar² is irradiating and may eventually destroy³ a low-mass companion. These systems are not only physical laboratories that reveal the interesting results of exposing a close companion star to the relativistic energy output of a pulsar, but are also believed to harbour some of the most massive neutron stars⁴, allowing for robust tests of the neutron star equation of state. Here we report observations of ZTF J1406+1222, a wide hierarchical triple hosting a 62-minute orbital period black widow candidate, the optical flux of which varies by a factor of more than ten. ZTF J1406+1222 pushes the boundaries of evolutionary models⁵, falling below the 80-minute minimum orbital period of hydrogen-rich systems. The wide tertiary companion is a rare low-metallicity cool subdwarf star, and the system has a Galactic halo orbit consistent with passing near the Galactic Centre, making it a probe of formation channels, neutron star kick physics⁶ and binary evolution.
... White dwarfs represent the last stage of evolution of stars with mass less than about eight times that of the Sun and, like other stars, are often found in binaries 1,2 . If the orbital period of the binary is short enough, energy losses from gravitational-wave radiation can shrink the orbit until the two white dwarfs come into contact and merge 3 . ...
White dwarfs represent the last stage of evolution of stars with mass less than about eight times that of the Sun and, like other stars, are often found in binaries. If the orbital period of the binary is short enough, energy losses from gravitational-wave radiation can shrink the orbit until the two white dwarfs come into contact and merge. Depending on the component masses, the merger can lead to a supernova of type Ia or result in a massive white dwarf. In the latter case, the white dwarf remnant is expected to be highly magnetised because of the strong magnetic dynamo that should arise during the merger, and be rapidly spinning from the conservation of the orbital angular momentum. Here we report observations of a white dwarf, ZTF J190132.9+145808.7, that exhibits these properties, but to an extreme: a rotation period of 6.94 minutes, a magnetic field ranging between 600 megagauss and 900 megagauss over its surface, and a stellar radius of about 2,100 km, slightly larger than the radius of the Moon. Such a small radius implies that the star's mass is close to the maximum white-dwarf mass, or Chandrasekhar mass. ZTF J190132.9+145808.7 is likely to be cooling through the Urca processes (neutrino emission from electron capture on sodium) because of the high densities reached in its core.
... Our search for periodicity in massive white dwarfs was part of a broader search for periodic variability on and around the white dwarf cooling track with ZTF, which has already yielded several results, including finding numerous double white dwarf binaries 2,34,35 . The targets were selected using the Pan-STARRS (PS1) source catalogue 18 , cross-matched with a white dwarf catalogue 36 , after imposing a photometric colour selection of (g − r) < 0.2 and (r − i) < 0.2. ...
White dwarfs represent the last stage of evolution of stars with mass less than about eight times that of the Sun and, like other stars, are often found in binaries1,2. If the orbital period of the binary is short enough, energy losses from gravitational-wave radiation can shrink the orbit until the two white dwarfs come into contact and merge³. Depending on the component masses, the merger can lead to a supernova of type Ia or result in a massive white dwarf⁴. In the latter case, the white dwarf remnant is expected to be highly magnetized5,6 because of the strong magnetic dynamo that should arise during the merger, and be rapidly spinning from the conservation of the orbital angular momentum⁷. Here we report observations of a white dwarf, ZTF J190132.9+145808.7, that exhibits these properties, but to an extreme: a rotation period of 6.94 minutes, a magnetic field ranging between 600 megagauss and 900 megagauss over its surface, and a stellar radius of 2140−230+160\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${2140}_{-230}^{+160}$$\end{document} kilometres, only slightly larger than the radius of the Moon. Such a small radius implies that the star’s mass is close to the maximum white dwarf mass, or Chandrasekhar mass. ZTF J190132.9+145808.7 is likely to be cooling through the Urca processes (neutrino emission from electron capture on sodium) because of the high densities reached in its core.
Binary evolution theory predicts that the second common envelope ejection can produce low-mass (0.32–0.36 M⊙) subdwarf B (sdB) stars inside ultrashort-orbital-period binary systems, as their helium cores are ignited under nondegenerate conditions. With the orbital decay driven by gravitational-wave (GW) radiation, the minimum orbital periods of detached sdB binaries could be as short as ∼20 min. However, only four sdB binaries with orbital periods below an hour have been reported so far, and none of them has an orbital period approaching the above theoretical limit. Here we report the discovery of a 20.5-min-orbital-period ellipsoidal binary, TMTS J052610.43+593445.1, in which the visible star is being tidally deformed by an invisible carbon–oxygen white dwarf companion. The visible component is inferred to be an sdB star with a mass ∼0.33 M⊙ approaching the helium-ignition limit, although a He-core white dwarf cannot be completely ruled out. In particular, the radius of this low-mass sdB star is only 0.066 R⊙, about seven Earth radii. Such a system provides a key clue in mapping the binary evolution scheme from the second common envelope ejection to the formation of AM CVn stars having a helium-star donor. It may also serve as a crucial verification binary of space-borne GW observatories such as LISA and TianQin in the future.
Binary evolution theory predicts that the second common envelope (CE) ejection can produce low-mass (0.32-0.36 M⊙) subdwarf~B (sdB) stars inside ultrashort-orbital-period binary systems as their helium cores are ignited under nondegenerate conditions. With the orbital decay driven by radiation of gravitational waves, the minimum orbital period that an sdB binary formed in this channel can reach before the Roche-lobe overflow is ~20 minutes. However, only four sdB binaries with orbital periods below an hour have been reported so far, while none of them has an orbital period approaching the above theoretical limit. This leads to an unclear role of the second CE ejection in forming ultracompact sdB binaries and a blocked link between detached helium-burning-star binaries and gravitational-wave sources, AM CVn stars. Here we report the discovery of a 20.5-minute-orbital-period ellipsoidal binary, TMTS J052610.43+593445.1, in which the visible helium-burning sdB star is being tidally deformed by an invisible carbon-oxygen white dwarf (WD) companion. This sdB star is inferred to have a mass of ~0.33 M⊙, approaching that of the helium-ignition limit. Owing to the low-mass sdB star, WD companion, and extremely short orbital period, this system provides the first clear case to justify the formation of sdB binaries through the nondegenerate channel of second CE ejection. In particular, the radius of this low-mass sdB star is only 0.066 R⊙, about seven Earth radii, representing the most compact nondegenerate star ever known. As the shortest-orbital-period single-degenerate detached binary discovered up to date, TMTS J052610.43+593445.1 provides a crucial bridge to map the binary evolution scheme from the second CE ejection to the formation of AM CVn stars having a helium-star donor, and will serve as a crucial verification binary of space-borne gravitational-wave detectors in the future.
White dwarfs, the extremely dense remnants left behind by most stars after their death, are characterised by a mass comparable to that of the Sun compressed into the size of an Earth-like planet. In the resulting strong gravity, heavy elements sink toward the centre, and the upper layer of the atmosphere contains only the lightest element present, usually hydrogen, or, if the hydrogen content is low, helium. Helium-atmosphere white dwarfs account for about 20% of all white dwarfs; however, several mechanisms can compete with gravitational settling to change their surface composition as they cool, and the fraction of white dwarfs with helium atmospheres is not constant at all temperatures. This fraction is known to increase by a factor ~2.5 below a temperature of about 30,000 K; therefore, some white dwarfs that appear to have a hydrogen-dominated atmosphere above that temperature are bound to transition to a helium-dominated atmosphere as they cool below it. Here we report observations of ZTF J203349.8+322901.1, or ``Janus'', a white dwarf with two faces: one side of its atmosphere is dominated by hydrogen and the other one by helium. The peculiar double-faced nature of Janus is most likely caused by a small magnetic field, which creates an inhomogeneity in temperature, pressure, or mixing strength over the surface. We appear to have caught a white dwarf as it is undergoing the transition from a hydrogen-dominated to a helium-dominated atmosphere. If this is the case, Janus might be the most extreme member of a class of magnetic transitioning white dwarfs and could help shed light on the physical mechanisms behind white dwarf spectral evolution.
White dwarfs represent the last stage of evolution for low and intermediate-mass stars (below about 8 times the mass of our Sun), and like their stellar progenitors, they are often found in binaries. If the orbital period of the binary is short enough, energy losses from gravitational wave radiation can shrink the orbit until the two white dwarfs come into contact and merge. Depending on the masses of the coalescing white dwarfs, the merger can lead to a supernova of type Ia, or it can give birth to a massive white dwarf. In the latter case, the white dwarf remnant is expected to be highly magnetised due to the strong dynamo that may arise during the merger, and rapidly rotating due to conservation of the orbital angular momentum of the binary. Here we report the discovery of a white dwarf, ZTF J190132.9+145808.7, which presents all these properties, but to an extreme: a rotation period of 6.94 minutes, one of the shortest measured for an isolated white dwarf, a magnetic field ranging between 600 MG and 900 MG over its surface, one of the highest fields ever detected on a white dwarf, and a stellar radius of 1810 km, slightly larger than the radius of the Moon. Such a small radius implies the star’s mass is the closest ever detected to the white dwarf maximum mass, or Chandrasekhar mass. In fact, as the white dwarf cools and its composition stratifies, it may become unstable and collapse due to electron capture, exploding into a thermonuclear supernova or collapsing into a neutron star. Neutron stars born in this fashion could account for ∼10% of their total population.
White dwarfs represent the last stage of evolution for low and intermediate-mass stars (below about 8 times the mass of our Sun), and like their stellar progenitors, they are often found in binaries. If the orbital period of the binary is short enough, energy losses from gravitational wave radiation can shrink the orbit until the two white dwarfs come into contact and merge. Depending on the masses of the coalescing white dwarfs, the merger can lead to a supernova of type Ia, or it can give birth to a massive white dwarf. In the latter case, the white dwarf remnant is expected to be highly magnetised due to the strong dynamo that may arise during the merger, and rapidly rotating due to conservation of the orbital angular momentum of the binary. Here we report the discovery of a white dwarf, ZTF J190132.9+145808.7, which presents all these properties, but to an extreme: a rotation period of 6.94 minutes, one of the shortest measured for an isolated white dwarf, a magnetic field ranging between 600 MG and 900 MG over its surface, one of the highest fields ever detected on a white dwarf, and a stellar radius of 1810 km, slightly larger than the radius of the Moon. Such a small radius implies the star's mass is the closest ever detected to the white dwarf maximum mass, or Chandrasekhar mass. In fact, as the white dwarf cools and its composition stratifies, it may become unstable and collapse due to electron capture, exploding into a thermonuclear supernova or collapsing into a neutron star. Neutron stars born in this fashion could account for 10% of their total population.
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