Science & Technology

Gravitational-wave detection hopes bolstered by fastest eclipsing White Dwarf binary

The discovery of the fastest yet eclipsing White Dwarf binary — expected to be a powerful source of gravitational waves — holds promise for this burgeoning realm of astronomy. (Caltech)
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The discovery of the fastest yet eclipsing White Dwarf binary — expected to be a powerful source of gravitational waves — holds promise for this burgeoning realm of astronomy. 

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% of the mass of the sun. The orbital separation of these objects is shrinking by about 26 centimetres per day due to the emission of gravitational waves, depicted in green near the end of the movie. (Caltech/IPAC)

The discovery of the fastest eclipsing white dwarf binary yet known has been made by researchers using a new instrument at the National Science Foundation’s Kitt Peak National Observatory.

Clocking in with an orbital period of only 6.91 minutes, the rapidly orbiting stars are expected to be one of the strongest sources of gravitational waves detectable with LISA, the future space-based gravitational wave detector.

After expanding into a red giant at the end of its life, a star like the Sun will eventually evolve into a dense white dwarf, an object with a mass like that of the Sun squashed down to a size comparable to Earth.

Similarly, as binary stars evolve, they can engulf their companion in the red giant phase and spiral close together, eventually leaving behind a close white dwarf binary.

White dwarf binaries with very tight orbits are expected to be strong sources of gravitational wave radiation. Although anticipated to be relatively common, such systems have proven elusive, with only a few identified to date.

A new survey of the night sky, currently underway at Palomar Observatory and Kitt Peak National Observatory, is changing this situation.

Each night, Caltech’s Zwicky Transient Facility (ZTF), a survey that uses the 48-inch telescope at Palomar Observatory, scans the sky for objects that move, blink, or otherwise vary in brightness. Promising candidates are followed up with a new instrument, the Kitt Peak 84-inch Electron Multiplying Demonstrator (KPED), at the Kitt Peak 2.1-meter telescope to identify short-period eclipsing binaries. KPED is designed to measure with speed and sensitivity the changing brightness of celestial sources.

This approach has led to the discovery of ZTF J1539+5027 (or J1539 for short), a white dwarf eclipsing binary with the shortest period known to date, a mere 6.91 minutes. The stars orbit in such close proximity that the entire system could fit within the diameter of the planet Saturn.

Kevin Burdge, a Caltech graduate student and the lead author of a paper reporting the discovery, explains: “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.”

The findings appear in today’s issue of the journal Nature.

A Strong Source of Gravitational Waves

Closely orbiting white dwarfs are predicted to move together closer together in a tightening orbit, moving faster and faster, as the system loses energy by emitting gravitational waves.

J1539’s orbit is so tight that its orbital period is predicted to become measurably shorter after only a few years. Burdge and his team were able to confirm the prediction from general relativity of a shrinking orbit, by comparing their new results with archival data acquired over the past ten years.

J1539 is a rare gem. It is one of only a few known sources of gravitational waves — ripples in space and time — that will be detected by the future European space mission LISA (Laser Interferometer Space Antenna), which is expected to launch in 2034.

LISA will be similar to the National Science Foundation’s ground-based LIGO (Laser Interferometer Gravitational-wave Observatory)–which made history in 2015 by making the first direct detection of gravitational waves from a pair of colliding black holes–except located in space.

LISA will detect gravitational waves from space at lower frequencies. J1539 is well matched to LISA; the 4.8 MHz gravitational-wave frequency of J1539 is close to the peak of LISA’s sensitivity.

The Kitt Peak Telescope, still making history

P. Marenfeld & NOAO/AURA/NSF
The 2.1-meter telescope at Kitt Peak National Observatory, where KPED is installed. (P. Marenfeld & NOAO/AURA/NSF)

Kitt Peak’s 2.1-meter telescope–the second major telescope to be constructed at the site–has been in continuous operation since 1964.

Its history is studded with several important discoveries in astrophysics, such as the Lyman-alpha forest in quasar spectra, the first gravitational lens by a galaxy, the first pulsating white dwarf, and the first comprehensive study of the binary frequency of stars like the Sun.

This latest result simply continues that impressive and venerable track record.

Lori Allen, Director of Kitt Peak National Observatory and Acting Director of NOAO says: “We’re thrilled to see that our 2.1-meter telescope, now more than 50 years old, remains a powerful platform for discovery.”

Chris Davis, NSF Program Officer for NOAO, says: “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.”

As remarkable as it is, J1539 was discovered with only a small portion of the data expected from ZTF. It was found in the ZTF team’s initial analysis of 10 million sources, whereas the project will eventually study more than a billion stars.

NSF Assistant Director for Mathematical and Physical Sciences, Anne Kinney, concludes: “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.”


Original research: https://www.nature.com/articles/s41586-019-1403-0

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