Recent observations made with a novel instrument revealed the fastest eclipsing white dwarf binary known to date.
The instrument was developed for use in the 2.1-m (84”) telescope at the Kitt Peak National Observatory of National Science Foundation. The rapidly orbiting stars, clocking in with an orbital period of just 6.91 minutes, are anticipated to be one of the most powerful sources of gravitational waves to be detected with Laser Interferometer Space Antenna (LISA)—the unique, future space-based gravitational wave detector.
The Dense “Afterlives” of Stars
The Sun-like star will ultimately become a dense white dwarf after it expands into a red giant by the end of its life. A white dwarf is an object that has a similar mass as that of the Sun and is reduced to a size analogous to Earth.
In a similar way, as binary stars emerge, they will consume their companion in the red giant phase, spiral very close together, and then would ultimately leave behind a close white dwarf binary.
It is expected that white dwarf binaries having extremely tight orbits will be powerful sources of gravitational wave radiation. Although such systems are expected to be quite common, they have been shown to be elusive, with just a few numbers detected so far.
Record-Setting White Dwarf Binary
A recent survey of the night sky, presently ongoing at Kitt Peak National Observatory and Palomar Observatory, is altering this situation.
Caltech’s Zwicky Transient Facility (ZTF)—a survey that utilizes the 48-inch telescope at Palomar Observatory—studies the sky each night for objects that blink, move, or otherwise differ in brightness.
Potential candidates are followed up with the Kitt Peak 84-inch Electron Multiplying Demonstrator (KPED)—a new instrument developed for use in the Kitt Peak 2.1-meter telescope to detect brief period eclipsing binaries. KPED was developed to determine the varying brightness of celestial sources with excellent sensitivity and speed.
This new technique has resulted in the discovery of ZTF J1539+5027, also known as J1539. It is a white dwarf eclipsing binary that has the shortest period known so far—that is, only 6.91 minutes. The stars orbit very closely together such that the whole system can possibly fit inside the diameter of the planet Saturn.
As the dimmer star passes in front of the brighter one, it blocks most of the light, resulting in the seven-minute blinking pattern we see in the ZTF data.
Kevin Burdge, Study Lead Author and Graduate Student, Caltech
Burdge has reported the findings in the recent issue of the journal, Nature.
A Strong Source of Gravitational Waves
White dwarfs that are closely orbiting are expected to spiral together faster and closer, as the system loses its energy by releasing gravitational waves.
The orbit of J1539 is so tight that its orbital period may turn out to be significantly shorter after just a few years. Burdge’s research team successfully confirmed the prediction based on general relativity of a dwindling orbit, by comparing the latest data with archival results obtained over the past decade.
J1539 is considered a unique gem. It is one among the few known sources of gravitational waves—ripples in time and space—that will be identified by the upcoming European space mission(LISA, which is anticipated to be launched in 2034.
NASA plays a role in LISA, which will be analogous to the ground-based Laser Interferometer Gravitational-wave Observatory (LIGO) of National Science Foundation. In 2015, LIGO made history by directly detecting gravitational waves emerging from a couple of colliding black holes, for the first time.
Space’s gravitational waves will be detected by LISA at lower frequencies. J1539 suitably complements LISA; J1539’s 4.8 mHz gravitational wave frequency is proximal to the peak of LISA’s sensitivity.
Discoveries Continue for Historic Telescope
The 2.1-m telescope of Kitt Peak is the second major telescope to be built at the site. Since 1964, it has been in continuous operation and it history includes several major discoveries in astrophysics. These include the first pulsating white dwarf, the first gravitational lens by a galaxy, the Lyman-alpha forest in quasar spectra, and the first detailed study of the binary frequency of stars such as the Sun. The new result continues its esteemed track record.
We’re thrilled to see that our 2.1-meter telescope, now more than 50 years old, remains a powerful platform for discovery.
Lori Allen, Director, Kitt Peak National Observatory
Allen is also the Acting Director of NOAO.
“These wonderful observations are further proof that cutting-edge science can be done on modest-sized telescopes like the 2.1-meter in the modern era,” added Chris Davis, NSF Program Officer for NOAO.
More Thrills Ahead!
As incredible as it is, J1539 was identified with just a small amount of the information anticipated from ZTF. It was detected in the ZTF team’s initial analysis of 10 million sources, while the project will ultimately investigate over a billion stars.
Only months after coming online, ZTF astronomers have detected white dwarfs orbiting each other at a record pace. It’s a discovery that will greatly improve our understanding of these systems, and it’s a taste of surprises yet to come.
Anne Kinney, Assistant Director for Mathematical and Physical Sciences, National Science Foundation
Artist’s animation depicting the eclipsing binary ZTF J1530+5027, which is comprised of two extremely dense objects (white dwarfs) that orbit each other roughly every seven minutes. One second of time in the animation represents two minutes of real time. The smaller white dwarf is slightly larger than Earth and is the more massive of the two, with about 60% the mass of the sun. Its companion is larger but less massive, with only about 20% the mass of the sun. The orbital separation of these objects is shrinking by about 26 cm per day due to the emission of gravitational waves, depicted in green near the end of the movie. (Video credit: Caltech/IPAC)