Finding Represents the Shortest-period Black Widow Binary Found to Date
Maunakea, Hawaiʻi – In black widow star systems, a rapidly spinning dead star, called a pulsar, blasts its lower-mass orbiting companion with radiation, slowly evaporating it. Like their namesake spiders, the pulsars take advantage of their companions before destroying them, in this case by harnessing material and energy from the doomed partner stars.
A new study published in the journal Nature, which includes data from W. M. Keck Observatory on Maunakea, Hawaiʻi, reports a new candidate black widow star system, named ZTF J1406+1222, in which the stars orbit around each other every 62 minutes—the shortest orbital period observed to date for this type of binary star system. The previous black widow record holder, PSR J1653-0158, contains stars orbiting around each other every 75 minutes.
The new candidate system was found using the Zwicky Transient Facility (ZTF) instrument at Caltech’s Palomar Observatory near San Diego. ZTF is funded by the National Science Foundation (NSF) and an international consortium of partners.
“This 62-minute orbit is remarkable because we don’t understand how the stars could get into such a tight orbit,” says Kevin Burdge (PhD ’21), a postdoctoral scholar at MIT who performed the research while at Caltech. “The process of the pulsar ablating its companion should actually drive them apart. This is pushing the boundaries of what we thought possible.”
Burdge says that upcoming observations from NASA’s Chandra X-ray Observatory should help confirm the result. “Our data indicate we are looking at a black widow binary, but it could be something entirely new.”
The first black widow star was discovered in the 1980s, and, since then, a few dozen have been found along with a similar spidery “species” of stars called redbacks, which also consist of pulsars and doomed partners. Scientists say that the rapidly spinning pulsar stars, in both redback and black widow systems, pick up energy and speed by siphoning material away from their companions.
“The lower-mass stars donate material to their partner stars but suffer the consequences,” explains Burdge.
This animation depicts a different black widow system, illustrates how a pulsar evaporates its companion over time. Credit: NASA’s Goddard Space Flight Center/Cruz deWilde
The new candidate black widow binary stars were identified by scanning millions of stars for those that rapidly blink on and off in the night sky. ZTF captures images of the entire night sky every two nights thanks to its camera’s large field of view, which covers an area of sky equivalent to a grid of 247 full moons. The sky survey enables researchers to search for objects that change in brightness or location on rapid timescales.
“ZTF has revealed a zoo of exotic stellar species,” says co-author Tom Prince, the Ira S. Bowen Professor of Physics, Emeritus, at Caltech and a co-investigator of ZTF. “Rather than being boring, static objects, a large fraction of stars exhibit dips, pulsations, or periodic brightenings that are a key to understanding their nature.”
In this case, Burdge developed an algorithm to search for ZTF objects that dramatically change in brightness on timescales of less than 80 minutes.
That led them to ZTF J1406+1222, which varies in brightness by a factor of 13 every 62 minutes. The periodic dimming and brightening of the system is caused by the companion to the pulsar, a cool failed star called a brown dwarf, rotating around on its axis. The brown dwarf, which is being blasted by the pulsar, retains heat on one side while its other side remains cooler. Telescopes cannot distinguish between the pulsar and the brown dwarf, but when the brown dwarf’s hot side rotates into our view from Earth, the overall brightness of the system goes up; conversely, when the cool side swings around, the brightness goes down.
Interestingly, this black widow system also has a third companion orbiting much farther out, at a distance of roughly 600 astronomical units, or AUs (an AU is the distance between Earth and the Sun). While the primary pulsar and brown dwarf orbit each other every 62 minutes, the third partner star orbits the tight pair every 10,000 years.
The scientists also point out that this is the first black widow system discovered using optical light. “The population of black widows we’ve been finding so far with other wavelengths of light, such as X-rays, gamma rays, and radio waves, is probably biased because we haven’t been catching all of them,” says Burdge. “Now we have a new lens through which we can identify these systems.”
Observations using Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS); the Grand Canary Telescope in the Canary Islands, Spain; the European Space Agency’s Gaia space observatory; and the Sloan Digital Sky Survey were also used to confirm the findings.
The study titled “A 62-minute orbital period black widow binary in a wide hierarchical triple,” was funded primarily by the NSF, NASA, and the MIT Pappalardo Fellowships program. Other Caltech authors include: Jim Fuller, Illaria Caiazzo, Matthew Graham, Amruta Jaodand, Shri Kulkarni, Sterl Phinney (BS ’80), Jan van Roestel, Andrew Drake, Richard Dekany (BS ’89), Dmitry Duev, Ashish Mahabal, and Reed Riddle.
The Low Resolution Imaging Spectrometer (LRIS) is a very versatile and ultra-sensitive visible-wavelength imager and spectrograph built at the California Institute of Technology by a team led by Prof. Bev Oke and Prof. Judy Cohen and commissioned in 1993. Since then it has seen two major upgrades to further enhance its capabilities: the addition of a second, blue arm optimized for shorter wavelengths of light and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe. LRIS was used in observing distant supernovae by astronomers who received the Nobel Prize in Physics in 2011 for research determining that the universe was speeding up in its expansion.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.